The myotonic dystrophies: Molecular, clinical, and therapeutic challenges

Article (PDF Available)inThe Lancet Neurology 11(10):891-905 · October 2012with183 Reads
DOI: 10.1016/S1474-4422(12)70204-1 · Source: PubMed
Abstract
Myotonic dystrophy is the most common type of muscular dystrophy in adults and is characterised by progressive myopathy, myotonia, and multiorgan involvement. Two genetically distinct entities have been identified. Myotonic dystrophy type 1 (also known as Steinert's disease) was first described more than 100 years ago, whereas myotonic dystrophy type 2 was identified only 18 years ago, after genetic testing for type 1 disease could be applied. Both diseases are caused by autosomal dominant nucleotide repeat expansions. In patients with myotonic dystrophy type 1, a (CTG)(n) expansion is present in DMPK, whereas in patients with type 2 disease, there is a (CCTG)(n) expansion in CNBP. When transcribed into CUG-containing RNA, mutant transcripts aggregate as nuclear foci that sequester RNA-binding proteins, resulting in a spliceopathy of downstream effector genes. The prevailing paradigm therefore is that both disorders are toxic RNA diseases. However, research indicates several additional pathogenic effects take place with respect to protein translation and turnover. Despite clinical and genetic similarities, myotonic dystrophy type 1 and type 2 are distinct disorders requiring different diagnostic and management strategies.

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www.thelancet.com/neurology Vol 11 October 2012
891
Review
Lancet Neurol 2012; 11: 891–905
Neuromuscular Research Unit,
Tampere University and
University Hospital,
Tampere,
Finland (B Udd MD); and
Department of Genetics,
University of Texas MD
Anderson Cancer Center,
Houston, TX, USA (R Krahe PhD)
Correspondence to:
Dr Bjarne Udd, Neuromuscular
Research Unit, Tampere
University and University
Hospital, 33520 Tampere,
Finland
bjarne.udd@netikka.fi
Dr Ralf Krahe, Department of
Genetics, University of Texas
MD Anderson Cancer Center,
Houston, TX 77030, USA
rkrahe@mdanderson.org
The myotonic dystrophies: molecular, clinical, and
therapeutic challenges
Bjarne Udd, Ralf Krahe
Myotonic dystrophy is the most common type of muscular dystrophy in adults and is characterised by progressive
myopathy, myotonia, and multiorgan involvement. Two genetically distinct entities have been identifi ed. Myotonic
dystrophy type 1 (also known as Steinert’s disease) was fi rst described more than 100 years ago, whereas myotonic
dystrophy type 2 was identifi ed only 18 years ago, after genetic testing for type 1 disease could be applied. Both
diseases are caused by autosomal dominant nucleotide repeat expansions. In patients with myotonic dystrophy type 1,
a (CTG)
n
expansion is present in DMPK, whereas in patients with type 2 disease, there is a (CCTG)
n
expansion in
CNBP. When transcribed into CUG-containing RNA, mutant transcripts aggregate as nuclear foci that sequester
RNA-binding proteins, resulting in a spliceopathy of downstream eff ector genes. The prevailing paradigm therefore
is that both disorders are toxic RNA diseases. However, research indicates several additional pathogenic eff ects take
place with respect to protein translation and turnover. Despite clinical and genetic similarities, myotonic dystrophy
type 1 and type 2 are distinct disorders requiring diff erent diagnostic and management strategies.
Introduction
The myotonic dystrophies are the subject of extensive
research because of their clinical importance and intri-
guing molecular biology. Progressive muscle degen-
eration leading to disabling weakness and wasting with
myotonia, in combination with multisystem involve-
ment, are the main characteristics of myotonic
dystrophy type 1 (also known as Steinert’s disease)
1
and
myotonic dystrophy type 2.
2–4
Myotonic dystrophy type 1
was fi rst recognised clinically more than 100 years ago.
However, type 2 disease was recognised only 18 years
ago, after genetic testing for myotonic dystrophy type 1
could be applied in clinical practice. Originally referred
to as proximal myotonic myopathy
2,3
or proximal
myotonic dystrophy,
4
the current nomenclature of
type 1 and type 2 disease was adopted after genetic
mapping of the myotonic dystrophy type 2 locus to
chromosome 3q21.
5
Repeat expansions are the mutations underlying both
types of myotonic dystrophy: type 1 disease is caused by a
(CTG)
n
microsatellite repeat expansion in the untrans-
lated 3 region of DMPK in chromosome 19q13.3,
6–8
whereas type 2 disease is due to a (CCTG)
n
expansion in
intron 1 of CNBP in chromosome 3q21.3.
9,10
Both
mutations lead to formation of transcript aggregates in
the nucleus, so-called foci, which interfere with proteins
that play a part in RNA metabolism, including members
of the muscleblind (MBNL) family of RNA-binding
proteins.
11
Both diseases are characterised by missplicing
of several downstream eff ector genes, which are thought
to account, at least in part, for multiorgan involvement.
12
Despite clinical and genetic similarities, myotonic
dystrophy types 1 and 2 are clearly diff erent disorders
with distinct characteristics, requiring their own
diagnostic and management strategies.
13
Identifi cation of toxic CUG RNA repeats in type 1
disease or CCUG repeats in type 2 disease was the
rst major pathomechanism found to underlie these
spliceopathies. The past 5 years of research have
provided new insights and contributed to our
understanding of the molecular complexity of these
diseases. Increased understanding of the psychosocial
eff ects of myotonic dystrophy type 1 and the overall
burden of the disease has enlarged the clinical scope and
refi ned therapeutic outcome measures. Despite raised
awareness of the disease, myotonic dystrophy type 2
remains largely underdiagnosed. Our Review covers the
current state of the fi eld, introduces new aspects of
disease charac ter istics, and outlines developments in
therapeutic eff orts.
Epidemiology
Before identifi cation of the distinct genetic mutations,
the combined prevalence of the myotonic dystrophies
was estimated at 1 in 8000 (12·5/100 000), based on
clinical ascertainment.
1
However, prevalence estimates
vary widely for diff erent populations. High prevalence
has been reported in northern Sweden, the Quebec
region in Canada, and the Basque region of Spain.
1
Findings of a population genetics study in Finland
14
showed that the frequency of the myotonic dystrophy
type 2 mutation (1/1830) can be much higher than that
for type 1 mutations (1/2760) in the same population.
14
However, it is unknown whether mutation frequencies
in the Finnish population refl ect those for popu lations
elsewhere of European descent and if the mutation is
100% penetrant in all circumstances. Considering the
generally earlier onset of symptoms in myotonic
dystrophy type 1, these data suggest a prevalence in
Finland of about 20 in 100 000 for both myotonic
dystrophy type 1 and type 2 disease.
14
Many patients in
older generations with myotonic dystrophy type 1 or
type 2 with milder symptoms are clearly undiagnosed.
It is noteworthy that recessive mutations in the
chloride channel gene CLCN1, which have a high
frequency in the general population, can act as
modifi ers in patients with type 2 disease by
amplifi cation of their myotonia.
15
892
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Review
Clinical phenotypes
Patients with myotonic dystrophy type 1 can present
with four diff erent forms on initial examination: adult-
onset, congenital, childhood-onset, and late-onset
oligosymptomatic.
1,16
Adult-onset disease is the most
prevalent form. In myotonic dystrophy type 2, there are
no distinct clinical subgroups; congenital and
childhood-onset forms have not been described, and
clinical presentation comprises a continuum ranging
from early adult-onset severe forms to very late-onset
mild forms that are diffi cult to diff erentiate from
normal ageing. The diff erent clinical manifestations in
the two diseases are summarised in table 1.
Adult-onset myotonic dystrophy type 1
Diagnostic eff orts are usually initiated because of muscle
weakness, myotonia, or cataracts, the three cardinal
symptoms of myotonic dystrophy type 1. A family history
of type 1 disease combined with minor symptoms is a
common starting point for diagnostic examinations.
However, if the main symptom is muscle weakness,
referral for assessment frequently happens at a late
disease stage, when symptoms and fi ndings could be
quite advanced. Myotonia is either reported as stiff ness
by the patient or their parents from school age to the
third decade of life or identifi ed at a later time during
clinical or electro physiological examinations. However,
in early adulthood, muscle weakness can be totally
absent. C
ataracts lead to ophthalmological examinations
and surgical removal but do not usually initiate further
diagnostic considerations for myotonic dystrophy type 1,
at least when diagnosed at an older age.
Skeletal muscle weakness, leading to immobility,
respiratory insuffi
ciency, dysarthria, and dysphagia, is the
major cause of severe disability and death at late stages of
adult-onset myotonic dystrophy type 1.
18
Muscle weakness
develops in facial, neck, and distal limb muscles in
parallel with muscle wasting. Atrophy in the temporal
muscles together with ptosis contributes to a characteristic
myopathic facial appearance, which is underscored by
frontal balding in men. Myotonia is invariably present in
adult-onset myotonic dystrophy type 1 on both clinical
examination and electromyog raphy,
1
but can occasionally
be relatively subtle clinically. The most common sign is
percussion myotonia in the thenar muscle and, less
consistently, grip myotonia on activation. Myotonia can
be relieved by repeat activation (warm-up).
19
Cardiac conduction defects and tachyarrhythmias
might lead to early heart spells and cardiac death.
20
The
underlying pathological fi nding is fi brosis in the
conduction system and sinoatrial node.
20
Clinically
manifest dilated or hypertrophic cardiomyopathy leading
to heart failure is not consistently part of the disease
spectrum of adult-onset myotonic dystrophy type 1, even
if subclinical changes can be recorded.
Cataracts are iridescent posterior subcapsular opacities
and are almost pathognomonic for adult-onset myotonic
dystrophy type 1.
21
Too often the exact type of cataract is
not defi ned before surgical removal. Occurrence of
cataracts before age 50 years should alert the clinician
to consider myotonic dystrophy. Retinal degenerative
changes have been documented pathologically.
1,22
Brain abnormalities in adult-onset myotonic dystrophy
type 1 are both structural and functional. The most
characteristic neuropsychiatric feature is a personality
mode consisting of avoidance and reduced perception of
disease symptoms and signs, mild cognitive impairment,
and later apathy. Daytime sleepiness is invariably present
at the stage of physical disability and is only infrequently
due to obstructive apnoea.
23,24
Together, these changes
might lead to low education levels, reduced professional
activity, and socioeconomic disadvantage.
25
On brain
MRI scans, diff use white-matter changes are more
evident than atrophy.
26
Gastrointestinal symptoms are frequent complaints in
patients with adult-onset myotonic dystrophy type 1,
ranging from constipation to diarrhoea and incon-
tinence.
27
Occurrence of gallbladder disorders is in-
creased,
1,22
and dysphagia can become a major concern at
the disability stage, with aspirations and pneumonia.
1,22
Endocrine functions are impaired in several ways in
adults with myotonic dystrophy type 1. Insulin resistance
and susceptibility for diabetes are well documented.
1
Hypothyroidism has been shown to worsen and mask
symptoms of myotonic dystrophy.
1
Male hypogonadism
is a regular feature, and male infertility and miscarriages
are common in these patients.
1
In adult-onset type 1 disease, skin can be aff ected. Early
frontal balding is more typical in men than women.
Pilomatricomas and epitheliomas are not uncommon,
but are frequently unrecognised or misdiagnosed.
1
Congenital myotonic dystrophy type 1
The most severe form of congenital myotonic dystrophy
type 1 presents prenatally by reduced fetal movements,
polyhydramnios, and various deformities detected on
ultrasound examination.
1,22
At birth, babies have severe
hypotonia in limb, trunk, respiratory, facial, and bulbar
muscles, leading to respiratory failure and feeding
diffi culties.
1,22
This extreme muscle weakness is not
caused by degenerative changes but by develop mental
defects. Mental retardation is also seen. With intensive
care, infants survive and no longer need assisted ven-
tilation. Even if delayed, they can achieve developmental
milestones and independent walking.
1,22
In advanced
disease, myotonia can be manifest and so can degen er-
ative muscle processes in the second decade.
Childhood-onset myotonic dystrophy type 1
This form of type 1 disease has long been neglected
because of symptoms uncharacteristic for a muscular
dystrophy. Children do not present with muscle weak-
ness, wasting, or myotonia. Instead, they have diffi culties
at school that generally initiate a search by paediatric
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893
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neurologists for causes of mental retardation.
28,29
In many
cases, the true nature of the brain disorder is not
understood until a parent, usually the mother, is diagnosed
with adult-onset myotonic dystrophy type 1. As with
congenital type 1 disease, children with childhood-onset
myotonic dystrophy type 1 will develop muscle symptoms
at an older age, causing physical disabilities comparable
with severe adult-onset type 1 disease.
28,29
Adult-onset myotonic dystrophy
type 1
Myotonic dystrophy type 2
Genetics
Inheritance Autosomal dominant Autosomal dominant
Anticipation Pronounced Exceptionally rare
Congenital form Yes No
Chromosome 19q13.3 3q21.3
Locus
DMPK CNBP
Expansion mutation
(CTG)
n
(CCTG)
n
Location of the expansion
3 untranslated region
Intron 1
Core features
Clinical myotonia Typical in adult onset Present in less than 50%
Myotonia on electromyography Generally present Absent and variable in many patients; needs detailed investigation
Muscle w eakness Disability often by age 30–50 years Disability at age 60–85 years
Cataracts Generally present Present in a few patients at diagnosis
Localisation of muscle weakness
Face or jaw Generally present Usually absent
Ptosis Often present Rare, mild, or moderate
Bulbar (dysphagia) Generally present later in life Not present
Respiratory muscles Generally present later in life Exceptionally rare cases
Distal limb muscle Generally prominent Flexor digitorum profundus in some patients
Proximal limb muscle Can be absent for many years Main disability in most patients, late onset
Sternocleidomastoid muscle Generally prominent Prominent in few patients
Muscular symptoms
Myalgic pain Absent or moderate Most disabling symptom in many patients
Muscle strength variations Occasional Can be considerable
Visible muscle atrophy Face, temporal, distal hands, and legs Usually absent
Calf hypertrophy Absent Present in at least 50%
Laboratory fi ndings
Concentration of creatine kinase in serum Normal-to-moderate increase Normal-to-moderate increase
Muscle biopsy fi ndings
Fibre atrophy Smallness of type 1 fi bres Highly atrophic type 2 fi bres
Nuclear clump fi bres In late stage only Scattered early before weakness
Sarcoplasmic masses Very frequent in distal muscles Very rare
Ring fi bres Frequent May occur
Internal nuclei Massive in distal muscle Variable and mainly in type 2 fi bres
Cardiac symptoms
Conduction defects Common Highly variable, absent to severe
Other neurological symptoms
Tremors Absent Prominent in many patients
Behavioural changes Common Not apparent
Hypersomnia Prominent Infrequent
Other features
Manifest diabetes Occasional Infrequent
Frontal balding in men Generally present Exceptional
Incapacity (work and activities of daily living) Typically after age 30–35 years Rarely younger than 60 years, unless severe pain
Life expectancy Reduced Normal
Modifi ed from Vihola and colleagues,
17
with permission of Springer-Verlag.
Table 1: Clinical manifestations in the myotonic dystrophies
894
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Review
Late-onset oligosymptomatic myotonic dystrophy type 1
In many families with adult-onset type 1 disease, the
ancestor who transmitted the mutation in earlier
generations might have had very mild symptoms, such
as cataracts treated surgically or late moderate muscle
atrophy.
1
This mild expression in previous generations
combined with severe disease in later generations
(usually the third) was the basis for the characterisation
of genetic anticipation in myotonic dystrophy type 1 long
before genetic testing could be applied.
1
The milder
disorder has been correlated loosely with shorter mutant
expansion alleles.
16
Myotonic dystrophy type 2
Patients with myotonic dystrophy type 2 may present in
many diff erent ways. The clinical phenotype is highly
variable, ranging from disabilities at age 40 years onwards
or early cardiac death, to mild proximal weakness that is
hardly recognisable, to slightly raised concentrations of
creatine kinase in elderly patients. The fi rst subjective
muscle symptom is usually either proximal lower limb
weakness, causing diffi culties with climbing stairs, or
myalgic pains. Cardinal features of myotonic dystrophy
type 1, such as myotonia, can be absent in patients with
type 2 disease, even on electromyography, and cataracts
are present in few individuals at the time of diagnosis
(table 1). Generally, by contrast with adult-onset myotonic
dystrophy type 1, muscle weakness in type 2 disease
begins at a later stage, the clinical course is more
favourable, and life expectancy is almost normal.
13,30
The
severe congenital or childhood-onset forms that arise in
type 1 disease have not been noted in families aff ected by
myotonic dystrophy type 2.
13
Abnormalities in social
skills and cognitive abilities are typically mild or absent.
Furthermore, no prominent late weakness of the
respiratory, facial, and bulbar muscles is seen (table 1).
13
In myotonic dystrophy type 2, manual skills remain
largely intact, and patients are spared from complications
of general anaesthesia. However, severe variants of type 2
disease can occur, including fatal arrhythmic cardiac
complications, severe respiratory failure, progressive
muscular atrophy, and disability.
4,13
In a comparison of
patients with type 1 or 2 myotonic dystrophy, the
frequency of severe cardiac conduction disturbances was
lower in patients with type 2 disease than in patients with
type 1.
13
By contrast to these overall milder symptoms, patients
with myotonic dystrophy type 2 can have severe myalgic
pain as the major cause of dysfunction, aff ecting
professional performance (table 1).
31–33
The pain is not
distinguishable from fi bromyalgia and does not respond
well to conventional pain treatment, although about 24%
of patients use continuous pain medication.
13,32,33
By
contrast with type 1 disease, patients with myotonic
dystrophy type 2 frequently show hypertrophy of calf
muscles.
13
Studies of gastrointestinal symptoms, dys-
phagia, and sleep suggest that these manifestations are
frequently not addressed in type 2 disease. Daytime
sleepiness does not occur, despite reported fatigue, but
sleep disturbances of other types are common and
frequently caused by myalgic pains, leading to increased
sleep latency.
13
Molecular genetics
In patients with myotonic dystrophy type 1, (CTG)
n
expansions range from 51 repeats to several thousands,
whereas healthy individuals carry 5–37 repeats.
22
Repeat
lengths of 38–50 are considered premutation alleles,
whereas 51–100 repeats are protomutations, both of
which show increased instability towards expansion.
Carriers of premutations or protomutations present no
or few mild symptoms, such as cataracts. Patients with
adult-onset myotonic dystrophy type 1 carry more than
100 repeats, and those with congenital type 1 disease have
more than 1000 repeats.
In myotonic dystrophy type 1, repeat expansion length
is predictive of clinical severity and age of onset.
Progenitor allele length is the major modifi er of age of
onset and is altered itself by the level of instability in
somatic tissues (termed somatic instability), which
seems to be highly heritable and could be related to
individual-specifi c trans-acting modifi ers that might
contribute to the development of cancer and ageing.
34
Although signifi cant correlations have been reported
between genotype and some phenotypic aspects (eg, age
of onset),
16,35,36
other geno type-phenotype correlations are
unclear, possibly due to imprecision in phenotypic data
and diff ering levels of instability of mutant alleles in
various somatic tissues.
22,34,37–40
Variant repeats within the
repeat tracteg, (CCG)
n
and (GGC)
n
repeats, part of the
overall (CTG)
n
repeat arraycan greatly alter both
mutational dynamics and phenotypic manifestations.
41
Such variation is pres ent in up to 4% of unrelated
individuals with myotonic dystrophy type 1
41
but might
have gone undetected in patients presenting with atypical
manifestations. Tissue-specifi c changes of the myotonic
dystrophy type 1 repeat with complex non-CTG repeat
insertions might also aff ect phenotypic manifestations.
42
The tendency of the repeat tract to expand or contract
seems to be a function of its primary sequence, which
enables formation of secondary hairpin structures, and
its genomic location, including fl anking sequences and
the distance from the origin of replication.
43
Data from
population-based mathematical modelling of mutant
myotonic dystrophy type 1 repeat alleles in DNA extracted
from peripheral blood leucocytes suggest that the bias
towards expansion is the cumulative eff ect of many
expansion and contraction events (possibly as frequently
as every other day) that seem to be coupled to DNA repair
or transcription rather than be dependent on DNA
replication.
40
Germ-cell instability is possibly the major determining
factor underlying the pronounced anticipation in
myotonic dystrophy type 1.
22
The expansion seems to
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increase through successive generations, and there is a
modest inverse correlation between repeat size and age
of onset.
22
However, in as many as 6% of parent-child
pairs, repeat contraction has been noted with paternal
transmissions (rarely even to the normal range), whereas
there is a tendency towards expansion in maternal
transmission.
44,45
Paternal alleles seem more unstable,
but children with congenital myotonic dystrophy type 1
are born almost exclusively to aff ected mothers,
indicative of sex-specifi c diff erences in germline repeat
instability.
22,38,44,46
By contrast to the (CTG)
n
repeat in myotonic dystrophy
type 1, the (CCTG)
n
expansion in type 2 disease is part of
a complex repeat, with many, usually polymorphic, tracts
in the confi guration (TG)
n
(TCTG)
n
(CCTG)
n
(NCTG)
n
(CCTG)
n
.
9,10,47,48
Healthy alleles contain fewer than
30 copies of the (CCTG)
n
repeat. The 3 (CCTG)
n
portion
of the overall tract, when not interrupted by cryptic
repeats (ie, the fi rst nucleotide of the CCTG unit can be
any other nucleotide), becomes unstable and shows
some of the highest mutation rates reported for
polymorphic microsatellites and unstable repeat expan-
sions.
48
The smallest recorded alleles associated with a
clinical phenotype are (CCTG)
55–75
, some of which can be
mosaic for larger repeats (ie, have one predominant
allele and many additional mutant alleles of various
sizes), and expansions as large as 11 000 have been noted,
making diagnostic sizing a challenge.
9,10,47–49
Although, in
general, type 2 expansions are clinically less severe, they
are, on average, signifi cantly larger than those seen in
type 1 disease.
30,50
On the basis of scant data (since sizing
is rarely attempted nowadays), the average (CCTG)
n
repeat length in myotonic dystrophy type 2 is about
5000.
10,30
Because of the extreme somatic instability, the
threshold size of the disease-causing mutation remains
to be determined. Similar to type 1 disease, a premutation
and a protomutation range might exist. Uninterrupted
alleles of (CCTG)
22–33
in European populations show
augmented instability.
14,48
Continued somatic instability is common to both
myotonic dystrophy type 1 and type 2, and it gives rise to
intra-tissue, inter-tissue, and cell-type variability and
somatic mosaicism over a patient’s lifetime.
9,10,37,38
Inter-
generational transmission of the two mutations, how-
ever, seems to be diff erent. While the myotonic dystrophy
type 1 mutation tends to increase in size over successive
generations, resulting in the characteristic pronounced
anticipation,
22
the type 2 mutation shows both expansions
and contractions.
10,30
This fi nding could explain the lack
of a congenital form of type 2 disease, the later onset of
symptoms, and the overall lack of anticipation in
myotonic dystrophy type 2. No apparent correlation is
seen between expansion size and age of onset or
phenotypic severity in type 2 disease.
30
Diagnostic genetic
testing generally uses DNA from peripheral blood
leucocytes. However, somatic variation is considerable
both in type 1 and type 2 disease, and repeat sizes in
aff
ected tissues such as muscle or brain are much larger
in myotonic dystrophy type 1.
2,51
Myotonic dystrophy type 1 is most common in
populations of European descent, is present in Japan at
about half the frequency, and is rarer still in India.
52–54
A
single kindred of myotonic dystrophy type 1 from sub-
Saharan Africa has also been identifi ed,
55
but, in general,
type 1 disease is conspicuously absent in African
populations.
53,56
Similarly, myotonic dystrophy type 2
seems to be mainly a disease of European populations:
most patients are of northern and eastern European
descent, but single kindreds of Afghan
47,57
and Japanese
58
origin have been identifi ed. Extensive linkage disequi lib-
rium of the type 1 and type 2 mutations suggests a single
or a few founder mutations.
47,48,59
Both mutations occur
on a disease haplotype that, except for the expansions,
resembles most common normal haplotypes.
47,48,59
Whether the extensive linkage disequilibrium is refl ective
of positive selection or the presence of a nearby cis
element predisposing to increased instability remains to
be determined.
10,38,48
Both mutations are believed to have
occurred after migration out of Africa, between
120 000 and 60 000 years ago.
50,53
The age of the myotonic
dystrophy type 2 founder mutation has been estimated at
4000–12 000 years (about 200–540 generations).
10
Molecular pathomechanisms
The pathophysiological mechanisms that underlie the
myotonic dystrophies are considerably more complex
than previously anticipated.
60–64
However, most of these
seem to converge on RNA toxicity (fi gure 1).
RNA toxicity and aberrant splicing
The transcription of mutant repeats into RNA appears
to be necessary and suffi cient to cause myotonic
dystrophy.
65,66
Mutant CUG-containing RNA strands
interfere with trans-acting RNA-binding proteins leading
to increased amounts of CUGBP/Elav-like family
member 1 (CELF1) and reduced MBNL activity.
62,67,68
In
the nucleus, the activity of MBNL proteins is diminished
because of their sequestration in ribonuclear foci.
11,12,69,70
CELF1 steady-state concentrations are upregulated by
hyperphosphorylation via diff erent signalling kinases,
including protein kinase C (PKC),
71
v-akt murine
thymoma viral oncogene homolog 1 (AKT1), cyclin D3
(CCND3), cyclin-dependent kinase 4 (CDK4),
72
glycogen
synthase kinase 3 beta (GSK3B),
73
and double-stranded
RNA-dependent protein kinase (PKR).
74
All but PKR have
been linked mechanistically to the phosphorylation of
CELF1 in disease cells. In the nucleus, CELF1 and MBNL
proteins function as splice factors, and dysregulation of
both proteins (due to sequestration or inappropriate
phosphorylation) results in aberrant expression of
embryonic splice isoforms in adult tissues. To date, more
than 30 misspliced eff ector genes have been identifi ed as
targets of MBNL1, CELF1, or both. Mis splicing and loss
of function of these gene products is generally thought to
896
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Review
account for the multisystemic phenotype in myotonic
dystrophy.
67,75,76
For most mis spliced genes identifi ed so
far, patients with types 1 and 2 disease show comparable
splicing abnormalities, sug gesting that qualitative
diff erences in splicing do not account for phenotypic
diff erences. However, whereas missplicing of CLCN1 (by
giving rise to a non-functional transcript) can account for
myotonia,
77–80
a clear pathogenic role for most other
misspliced genes has not been shown and evidence
remains circumstantial.
Missplicing of exon 11 of the amphiphysin gene (BIN1),
mediated by MBNL1 loss, was linked to tubular defects in
both myotonic dystrophy types 1 and 2.
81
Missplicing was
more common in type 1 disease than type 2 and was seen
only in aff ected muscles, with secondary changes of
muscle tubulation and defective phosphoinositide sig-
nalling. These results were replicated in mouse models,
and the noted changes might be associated with central
nucleation and developmental weakness and hypotonia
in congenital myotonic dystrophy type 1. The rise in
internal nuclei in muscle biopsy samples from adults
with myotonic dystrophy types 1 and 2 could also be
related to this missplicing. Similarly, exon 29 missplicing
of the CaV1.1 calcium channel, involving MBNL1 and
CELF1, was linked functionally to muscle weakness and
altered C
+
gating.
82
Missplicing in several sarcomeric
muscle proteins, such as LIM domain binding 3 (encoded
by LDB3), myomesin (MYOM1), and myosin heavy
chain 14 non-muscle (MYH14), has also been noted in
patients with myotonic dystrophy type 1 or type 2.
12,17,83,84
Data from several animal models suggest that splicing,
foci, and muscle pathological features are separable
events (table 2).
85–87
Mbnl1
–/–
and Mbnl2
–/–
knockout
mice,
85,97
and the Cugbp1 transgenic mouse overexpressing
CELF1,
87,102
show aberrant splicing without foci or toxic
mutant RNA. Transgenic mice expressing (CUG)
n
repeats (known as HSA
LR
) and Mbnl1
–/–
knockout mice
both show the same pattern of missplicing as that seen
Transcription factor sequestration:
transcription dysregulation
CELF1 dysregulation:
dysregulation of translation
CNBP RNA processing defect:
reduced protein, dysregulation
of translation
(C/CUG)
n
double-strand RNA:
activation of stress pathways,
dysregulation of translation
DMPK RNA sequestration:
reduced protein
(CUG)/(CAG) small interfering RNA:
RNA interference pathways
RNA translation:
proteotoxicity
CELF1 upregulation:
mRNA missplicing
mRNA stability dysregulation
MBNL sequestration:
mRNA missplicing (CLCN1→myotonia),
transcription dysregulation,
microRNA dysregulation
Cytoplasm
Nucleus
↓DMPK
p-CELF1
p-CELF1
CELF1
eIF2A
(CUG)
n
(C/CUG)
n
(CTG)
n
(CCTG)
n
DM1
(GAC)
asDMPK
Activated PKR
p-eIF2A
Translation
Q
Q
Q
Q
Q
DMPK SIX5 CNBP
↓CNBP
TF
MBNL
MBNL
MBNL
Figure 1: Postulated pathological mechanisms underlying myotonic dystrophy types 1 and 2
Most mechanisms are consistent with the prevailing toxic RNA gain-of-function model and aff ect multiple cellular aspects in the nucleus (blue boxes), the cytoplasm
(grey boxes), or both (blue and grey boxes), including reduced protein function of the mutated genes, missplicing, dysregulation of transcription, protein translation
and turnover, and activation of cellular stress pathways. Both mutant (CTG)
n
and (CCTG)
n
expansions in non-coding regions of DMPK and CNBP give rise to
C/CUG-containing transcripts (red) that form stable secondary structures detectable as RNA foci (red circles) in the nucleus and the cytoplasm, and result in reduced
amounts of DMPK and CNBP protein because of DMPK mRNA sequestration or a CNBP pre-mRNA processing defect. Members of the MBNL (green) protein family,
such as MBNL1, are sequestered in ribonuclear foci leading to loss of function and dysregulation of MBNL splice and transcription targets and microRNA metabolism.
Hyperphosphorylation of nuclear and cytoplasmic CELF1 (p-CELF1; salmon) by several protein kinases, including RNA-dependent protein kinase (PKR), leads to
stabilisation of diff erent isoforms, which in turn aff ects alternative splicing, translation, and protein turnover. Inactivation of the translation initiation factor eIF2A
leads to general attenuation of translation. Sequestration of transcription factors and other nuclear factors also contributes to dysregulation of gene expression.
Generation of small interfering RNA from sense (s) and antisense (as) DMPK transcripts might activate RNA interference pathways that lead to wider dysregulation of
mRNA and protein amounts. sDMPK and asDMPK transcripts are subject to repeat-associated non-ATG translation in the polyglutamine (poly[Q]
n
) reading frame and
possibly others, giving rise to toxic homopolymeric polypeptides that accumulate in the cytoplasm. Although some mechanisms have only been reported in
myotonic dystrophy type 1, theoretically they could also operate in type 2 disease. TF=transcription factor.
www.thelancet.com/neurology Vol 11 October 2012
897
Review
in patients,
12,76
although neither mouse model develops
features associated with myotonic dystrophy type 1.
66,85
MBNL1 complementation in HSA
LR
mice fails to rescue
the histological abnormalities, but prevents myotonia
and restores adult-splicing patterns,
106
suggesting that
muscle degeneration might not be due to MBNL1 loss
alone. The fact that myotonia is less severe in myotonic
dystrophy type 2 compared with type 1 disease, despite a
similar degree of sequestered MBNL1 and misspliced
CLCN1, suggests that spliceopathy is not the only
pathomechanism in the myotonic dystrophies.
Investigations of alternative splicing in myotonic
dystrophy type 1 and type 2, and in other neuromuscular
disorders (Becker, Duchenne, and tibial muscular
dystrophy) indicate that missplicing is not unique to
myotonic dystrophy.
107
The myocyte enhancer proteins
MEF2A and MEF2C showed missplicing of exons 4
and 5, respectively, in myotonic dystrophy patients
showing the embryonic isoform. However, similar
splicing diff erences were seen in other neuromuscular
disorders, suggesting that some mechanisms of aberrant
splicing could be compensatory and not primary events,
even in myotonic dystrophy. Similar fi ndings were made
in diff erent mouse models of muscular dystrophy and
muscle injury (including myotonic dystrophy type 1).
108
The nuclear and cytoplasmic functions of CELF1 and
MBNL beyond splicing are becoming increasingly clear,
and they include RNA stability and traffi cking and
translational initiation.
CELF1 was the fi rst RNA-binding protein implicated
in the pathogenesis of myotonic dystrophy type 1, by
disruption of the splicing of TNNT2, that encodes
troponin T type 2.
109
Although it is now commonly
accepted that CELF1 is hyperphosphorylated and
upregulated in myotonic dystrophy type 1,
71,98
the issue
remains controversial for type 2 disease.
63
In some
reports no changes are indicated,
12,110
whereas in other
studies increased steady-state concentrations of CELF1
are seen in patients with myotonic dystrophy type 2,
including in human myoblasts, C2C12 mouse myoblasts
expressing (CCUG)
300
, and liver and skeletal muscle of
myotonic dystrophy type 2 transgenic mice.
111,112
Although MBNL drives splicing towards the adult iso-
form, CELF1 promotes embryonic isoforms of the
aff ected mRNA transcripts. Thus, in myotonic dystrophy
type 1 and type 2, loss of function of MBNL by
sequestration, combined with CELF1 overexpression,
leads to reprogramming of RNA splicing towards an
embryonic state.
75
Overexpression of CELF1 in adult
skeletal muscle and cardiac tissue of transgenic mice is
suffi cient to recapitulate molecular, histopathological,
and functional changes typical of type 1 disease.
103
Furthermore, CELF1 functions as a key regulator of
mRNA decay
113
and has a role in translation initiation in
Phenotype Comments References
Knockout models
Dmpk
Overtly normal; aged mice eventually develop altered ion
homoeostasis
Dmpk haploinsuffi ciency not primary pathomechanism
88–90
Six5
Cataracts, reduced male fertility, cardiac dysfunction
Six5 haploinsuffi ciency not primary pathomechanism
91–94
Cnbp
Myotonia, myopathy, cardiac conduction defects
Cnbp haploinsuffi ciency recapitulates major aspects of myotonic dystrophy
type 2, can be rescued by Cnbp complementation, no missplicing
95
Mbnl1
Myotonia, myopathy, cardiomyopathy, missplicing
Complete defi ciency of Mbnl1 recapitulates major aspects of myotonic
dystrophy type 1, missplicing and misregulation patterns similar to HSA
LR
76,85,96
Mbnl2
Myotonia, myopathy, missplicing
Complete defi ciency of Mbnl2 recapitulates specifi c aspects of myotonic
dystrophy type 1, missplicing and misregulation of MBNL target genes
97
Transgenic models
HSA
LR
Myotonia, myopathy, muscle wasting, (CUG)
n
foci sequester MBNL,
missplicing
Expression of (CUG)
250
RNA recapitulates major aspects of myotonic dystrophy
type 1, missplicing and misregulation of MBNL target genes
12,66,96
EpA960 Myotonia, myopathy, muscle wasting, cardiomyopathy, cardiac
conduction defects, (CUG)
n
foci sequester MBNL1/stabilise CELF1,
missplicing
Expression of (CUG)
960
RNA recapitulates major aspects of myotonic dystrophy
type 1, missplicing of MBNL and CELF1 target genes
71,98,99
DM300 and DMSXL
Myotonia, myopathy, (CUG)
n
foci, missplicing Expression of (CUG)
n
RNA results in mild myotonic dystrophy type 1
phenotype in dose-dependent manner, mild splicing abnormalities
100
GFP-DMPK-(CTG)
5
Myotonia, myopathy, cardiac conduction defects, missplicing Overexpression of (CUG)
5
RNA elicits myotonic dystrophy type 1 phenotype
without foci, with CELF1 upregulation and modest missplicing
86
DM2-HSAtg-(CCTG)
121
Myotonia, myopathy, multisystem pathology, (CCUG)
n
foci Expression of (CCUG)
121
RNA recapitulates aspects of myotonic dystrophy
type 2 without missplicing
13,101
CELF1 (CUGBP1) Myopathy, cardiomyopathy, cardiac conduction defects, missplicing Overexpression of CELF1 recapitulates aspects of myotonic dystrophy ,
missplicing of CELF1 target genes
87,98,102–104
Knockin models
Dmpk-(CTG)
84
No apparent myotonic dystrophy pathology Increased somatic instability dependent on mismatch repair gene background 105
Cnbp-(CCTG)
189
Myotonia, myopathy, multisystem pathology, (CCUG)
n
foci Expression of (CCUG)
189
RNA recapitulates aspects of myotonic dystrophy
type 2 without missplicing
13,101
Table 2: Mouse models of myotonic dystrophy
898
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Review
the cytoplasm.
61
Upregu lation of CELF1 increases
translation of proteins needed for myogenesis, especially
MEF2A and p21 (cyclin-dependent kinase inhibitor 1A).
114
Altered transcriptional regulation
Transcriptome-wide changes in gene expression in
muscle biopsy samples from patients with myotonic
dystrophy type 1 or type 2 have been recorded in various
studies.
13,17,115,116
Overall, types 1 and 2 disease had highly
concordant profi les, suggestive of a mutual patho-
physiology.
13,115
However, dysregulation of many of the
same genes was shared with other neuromuscular
disorders, indicating that some of the noted changes are
caused by secondary compensatory mechanisms.
Similarly, studies in HSA
LR
and Mbnl1
–/–
mice also showed
large-scale, shared transcriptional dysregulation, mainly
due to MBNL1 loss.
76,96
Inappropriate redistribution (leaching) of the tran s-
cription factor SP1 seems to result in transcriptional
downregulation of CLCN1, among other genes.
117
Mislocalisation of the transcription factor SPEN (also
known as SHARP) has been linked to changes in steady-
state concentrations of several mRNA molecules.
118
In
myotonic dystrophy type 1, impaired binding to the
CCCTC-binding factor due to methylation of CTCF sites
anking the (CTG)
n
repeats leads to antisense
transcription of the gene DMPK from the regulatory
region of the closest gene, SIX5.
39,119
MicroRNA dysregulation and RNA interference
Several muscle-specifi c microRNA molecules (miRs)
have been implicated in myotonic dystrophy type 1 and
type 2.
120,121
Molecules that are both downregulated (miR-1,
miR-29b, -miR 29c, and miR-33) and upregulated (miR-
206 and miR-335) have been identifi ed and probably
contribute to the overall myotonic dystrophy phenotype.
Downregulation of miR-1 in cardiac muscle of patients
with type 1 or type 2 disease results in dysregulation of
gap junction protein alpha 1 (GJA1) and the calcium
channel protein CACNA1C.
121
Mutant CUG-containing
RNA molecules form double-stranded hairpins which, in
myotonic dystrophy type 1, are subject to cleavage by the
ribonuclease DICER1.
122
Although controversial, some
studies have found that processed short (CUG)
7
repeats
downregulate RNAs with complementary (CAG)
n
by
activation of RNA interference pathways.
96,122
Cytoplasmic pathomechanisms
In myotonic dystrophy type 1, repeat-associated non-ATG
(RAN) translation of the DMPK antisense transcript
gives rise to homopolymeric polyglutamine (poly[Q]
n
)
peptides in patients’ cells, including skeletal muscle
cells, and in mouse models of type 1 disease. The
contribution of RAN-translated poly(Q)
n
peptides to the
overall type 1 disease phenotype is currently unclear.
123
Double-stranded (C/CUG)
n
hairpins are recognised by
PKR
124
and activate several cellular stress pathways,
74
including the innate immune response and interferon
signalling in cataracts.
116
PKR activation leads to
inactivation of eukaryotic translation initiation factor 2A
(eIF2A) by phosphorylation at serine 51, which in turn
globally inhibits initiation of translation.
125
In muscle of
patients with myotonic dystrophy type 1, a marked
reduction in protein synthesis in vivo has been noted,
suggesting a functional link between reduced protein
translation and muscle wasting,
126
and a global trans-
lational defect in type 2 disease has also been seen.
74,127,128
A possible consequence of attenuated translation is the
accumulation of untranslated mRNA molecules and
protein-folding intermediates. PKR-dependent forma-
tion of stress granules is induced in CUG-expressing
cells, and translation of the CELF1 target MRG15 is
attenuated.
74
MBNL1 might also accumulate in stress
granules by colocalising with the binding protein YB1.
129
Prolonged activation of PKR and subsequent eIF2A
inactivation leads to stress in the endoplasmic reticulum,
which has been detected in myotonic dystrophy type 1.
130
Endoplasmic reticulum stress results in a block of
translation initiation or apoptosis, both of which might
lead to muscle wasting and weakness.
Roles of DMPK and CNBP
In myotonic dystrophy type 1 and type 2, the expanded
repeats are in untranslated regions of their respective
genes, DMPK and CNBP. Selective eff ects of the
expansions on steady-state mRNA concentrations of the
mutant DMPK allele,
131
resulting in reduced amounts of
DMPK protein due to nuclear retention, have been
reported.
132,133
Although Dmpk knockout mice do not
initially develop a multisystemic phenotype mimicking
myotonic dystrophy, aged mice eventually develop
specifi c abnormalities.
88–90
By contrast, heterozygous Cnbp
+/–
mice develop
myotonia, cataracts, and muscle pathology without
missplicing of Clcn1, which can be rescued by Cnbp
complementation.
95
These data suggest that many
features of myotonic dystrophy can be elicited by
haploinsuffi ciency of CNBP alone. Findings of initial
studies in patients with type 2 disease, mostly done by
in-vitro culture of myoblasts or lymphocytes, suggested
that CNBP mRNA and protein expression are not
aff ected by haploinsuffi ciency.
134,135
However, in more
recent studies, evidence indicates reduced concentrations
of CNBP mRNA and protein.
110,112,127,128,136
The noted
reductions were attributable to inappropriate processing
of mutant allele pre-mRNA transcripts, suggesting a
processing defect in cis.
136
CNBP is an activator of cap-independent translation. It
binds to the internal ribosome entry site (IRES) within
the 5 untranslated region of target mRNAs and asso-
ciates with actively translating ribosomes. Translation of
ornithine decarboxylase (ODC1) mRNA in myotonic
dystrophy type 2 myoblasts is diminished.
128
Over-
expression of CNBP rescues ODC1 translation mediated
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899
Review
by the IRES site in human patient cells. CNBP also
interacts with terminal oligopyr imidine (TOP) tracts in
the 5 untranslated regions of mRNA molecules.
127
Binding of CNBP to mRNA molecules containing TOP-
tracts was reduced in myotonic dystrophy type 2 muscle,
and a generalised reduction of translation could be
rescued by ectopic expression of CNBP.
Animal models
Table 2 summarises several important mouse models
of myotonic dystrophy; assessment of 20 diff erent
mouse models has been done by Gomes-Pereira and
colleagues.
64
Transgenic mouse models expressing
diff erent lengths of (CUG)
n
RNA molecules have
established the toxic RNA paradigm as the molecular
culprit in myotonic dystrophy type 1. Although no one
model completely recapitulates all aspects of the
multisystemic phenotype in type 1 or type 2 disease,
diff erent models have helped to clarify distinct aspects of
the overall pathophysiology.
Diagnosis and laboratory fi ndings
Blood tests
Activity of creatine kinase in serum is usually slightly or
moderately raised in patients with myotonic dystrophy
type 1 or type 2, although normal measurements of this
enzyme are frequent in type 2 disease. Increased con-
centrations of liver enzymes, in particular γ-glutamyl-
transferase, are common fi ndings, as is IgG
hypogamma globulinaemia, both for unknown reasons.
Patients with myotonic dystrophy type 2 have augmented
amounts of positive rheumatological serological markers
and high lipid profi les.
13
As part of the male
hypogonadism, luteinising hormone and follicle-
stimulating hormone concentrations are frequently
increased, even in sub clinical hypogonadism.
1
Electrophysiological studies
Before diagnostic genetic testing was available, electro-
myography showing the combination of myotonia and
myopathic changes was pathognomonic for diagnosis.
1
However, in mildly aff ected, young adult patients the
myopathic component might be missing, leading to
considerations of myotonia congenita. Findings of nerve
conduction studies are normal. In myotonic dystrophy
type 2, the less prominent and frequently missing, or
conspicuously absent, myotonic component can easily
mislead diag nostic eff orts towards polymyositis.
13
Muscle histopathology
Muscle biopsy fi ndings in myotonic dystrophy type 1 are
well established and are more pronounced in distal than
proximal muscle, including a highly increased number
of internal nuclei, sarcoplasmic masses, ring fi bres, and
moderate atrophy of type 1 fi bres in clinically weak
muscles (fi gure 2).
1
Histo pathological features in type 2
disease are very diff erent, despite early reports describing
similarities.
137
The most characteristic feature of myotonic
dystrophy type 2 is the combination of scattered nuclear
clump fi bres and highly atrophic fi bres consisting of a
subpopulation of type 2A fi bres (fi gure 2).
138
Highly
atrophic fi bres are present even before clinical weakness
in proximal lower limb muscles, and initially they were
not recognised because the conventional histochemical
ATPase staining technique to separate fi bres types was
not appropriate for their identifi cation.
138,139
Diagnostic DNA testing
In myotonic dystrophy type 1, Southern blot analysis is
the most common technique used to evaluate the
expansion mutation, which has the added advantage of
estimating repeat size. However, this procedure has a
small false-negative rate because of reduced sensitivity
in cases of extreme somatic heterogeneity. In myotonic
dystrophy type 2, a combination of genotyping assays is
used across the genomic sequence of the expected
expansion repeat; if two normal-sized alleles are
detected, the diagnosis of type 2 disease can be excluded.
If just one normal-sized allele is detected, the sample
either contains a myotonic dystrophy type 2 mutation
on the other allele that is resistant to PCR amplifi cation
or two normal alleles of the same size are present, a
nding that arises in about 10% of the healthy
A
B
C D
Figure 2: Diff erential histopathological features on muscle biopsy
In myotonic dystrophy type 1, the characteristic feature detected on conventional haematoxylin and eosin stain
(A) is fi bre size variation and an increased number of internal nuclei in the fi bres (arrow) in distal muscles. On
immunohistochemical fi bre typing (B), slow type 1 fi bres (brown) are on average smaller than fast type 2 fi bres
(blue). In myotonic dystrophy type 2, the early fi nding at diagnosis on haematoxylin and eosin staining (C) is an
increased number of small atrophic fi bres containing, almost exclusively, nuclei (so-called nuclear clump fi bres),
even in asymptomatic proximal muscles (arrow). On immunohistochemical fi bre typing (D), a subpopulation of
fast type 2 fi bres (brown) is highly atrophic (arrow). Both nuclear clump fi bres and other type 2 fi bres are atrophic.
Original magnifi cation ×400.
900
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Review
population. In cases of an apparently homozygous
normal allele, testing is continued with repeat-primed
PCR specifi c for the type 2 disease expansion repeat.
10,30,49
Used together, these techniques provide high sensitivity
and specifi city. Diagnostic laboratories need to have
access to more than one methodological approach
because some samples will show inconclusive fi ndings
with just one method. Complementary techniques to
clarify equivocal results include Southern blots and
uorescence in situ hybrid isation (FISH) or
chromogenic in situ hybridisation (CISH). If leucocyte
DNA samples give inconclusive results, then DNA from
muscle tissue might be a good alternative.
13
Imaging of muscle and brain
In both types of myotonic dystrophy, fatty degenerative
changes in muscles, detectable by MRI, develop grossly
in parallel with clinical muscle weakness and atrophy.
13
However, the pattern of more pronounced involvement
of lower limb muscles in myotonic dystrophy type 1,
versus less involvement in type 2 disease, is clearly
diff erent (fi gure 3). In type 1 disease, initial changes are
seen in the soleus and medial gastrocnemius; these
muscles in the lower leg are replaced completely by fatty
infi ltration (degenerative changes), sometimes even
before major changes are noted in the upper leg near the
thigh. In myotonic dystrophy type 2, early muscular
changes develop in the anterior vastus group of thigh
muscles, with relative sparing of the rectus femoris. At
this stage, the lower leg muscle might appear completely
normal by imaging.
13
In congenital, childhood-onset, and advanced adult-
onset myotonic dystrophy type 1, widespread white-
matter changes are present in the brain, with grey-matter
alterations and atrophy seen less frequently and to a
lesser extent. In patients with type 2 disease, brain MRI
ndings can be entirely normal. However, in advanced
stages or more severe cases, diff use white-matter changes
might be less pronounced than and diff erent to those in
myotonic dystrophy type 1.
13,26,140
Clinical complications and management
The delay between onset of fi rst symptoms and correct
diagnosis is usually very long, on average more than
5 years for myotonic dystrophy type 1 and greater than
14 years for patients with type 2 disease.
13
The long delay
with diagnosis of myotonic dystrophy type 2 is mainly
attributable to scant awareness of the disease by
clinicians, which causes unnecessary diffi culties for
patients trying to manage their lives, mental stress, and
anguish with uncertainty of prognosis and treatment.
Cardiac arrhythmias are a major cause of mortality in
myotonic dystrophy type 1 and, thus, are an important
feature that needs monitoring and treatment.
18,20
How-
ever, in myotonic dystrophy type 2, cardiac deaths occur
with low frequency.
141
Prompt referral for cardiology
consultation and therapeutic intervention are indicated
when signs suggest cardiac dysfunction, measures
show prolonged PR interval or QRS, and atrial
brillation or fl utter is recorded. Serial electro-
cardiogram (ECG) monitoring is needed.
142
Cardio-
myopathy is infrequent, whereas coronary heart disease
A B
Figure 3: Muscle imaging
(A) CT sections of lower legs (upper) and thighs (lower) in a 32-year-old man with myotonic dystrophy type 1. He also had frontal baldness, weak fi ngers, grip
myotonia, and thin lower legs. The most evident changes after muscle weakness were fi rst noted clinically in the medial gastrocnemius and the soleus muscles
(arrows). (B) MRI sections of lower leg (upper) and thigh (lower) in a 61-year-old woman with myotonic dystrophy type 2. She also had myalgia, proximal weakness
since age 55 years (grade 4), a mild increase in creatine kinase to 380 IU/L, and myotonia on electromyography, but no clinical myotonia and no cataracts. No changes
are seen in the lower leg muscles; instead, fatty degenerative changes are present in the vastus lateralis muscles (arrows) in the thighs. Note also the overall reduced
volume of thigh muscles.
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901
Review
is a common problem in patients with myotonic
dystrophy type 2.
Cataracts need conventional surgical treatment.
Anaesthetic risk is increased in patients with myotonic
dystrophy type 1, with prolonged post-anaesthetic
respiratory recovery and increased frequency of
pneumonia.
143,144
Again, type 2 disease is diff erent and no
increased risk has been encountered.
145
Myotonia rarely restricts activities of daily life. However,
myotonic stiff ness is usually mixed with myalgic pains
and, therefore, trials with mexiletine or fl ecainide (two
doses of 50–100 mg) or propafenone (two doses of
150–300 mg) can be considered.
13,142
Hormonal changes
in pregnancy can worsen myotonia, myalgic stiff ness,
and cataracts, although the reasons are uncertain, and
these eff ects are reported more consistently in myotonic
dystrophy type 2.
13
Fatigue and daytime sleepiness can be severe in
patients with myotonic dystrophy type 1, and the eff ects
of treatment have been modest.
In patients with myotonic dystrophy types 1 or 2,
monitoring of lipid profi les and assessment of oral glucose
tolerance are advised.
142,146
Atherosclerosis is frequent in
individuals with type 2 disease, and an increased frequency
of adverse reactions to statin treatment is a challenge, not
least because a statin-related reaction can be masked in
patients with myotonic dystrophy type 2 and myalgia.
13,146
Some individuals with type 2 disease tolerate statins
without severe adverse reactions, but if side-eff ects
develop, treatment has to be discontinued; moderate rises
in cholesterol (<8 mmol/L) in older patients with myotonic
dystrophy might not need treatment.
13
Medical treatment of muscle pain has been largely
unsuccessful. Patients with severe disabling pain can
be referred to a pain specialist. Hypothyroidism and
gonadal failure are treated with hormone replacement
when needed. If drugs for erectile dysfunction are
considered, concentrations of testosterone should be
checked fi rst. Mildly raised amounts of liver enzymes
(eg, γ-glutamyltransferase) are common in patients with
myotonic dystrophy and further liver investigations are
usually not needed.
146
Cancer risk in patients with myotonic dystrophy is
uncertain. Epitheliomas and pilomatricomas have been
reported in patients with type 1 disease;
1
however, only in
recent studies of large registries has an average overall
twofold increase in relative risk for various cancers been
confi rmed for patients with a clinical diagnosis of
myotonic dystrophy; for some cancers (endometrium,
ovary, brain, and colon) risk is as high as sevenfold.
147
This augmented risk seems to be associated with female
sex in patients with myotonic dystrophy type 1 and is not
correlated with (CTG)
n
expansion size.
148
Experimental treatments
At present, no eff ective treatment is available for myo-
tonic dystrophy types 1 and 2. Experimental approaches
target elimination of toxic repeat transcripts in type 1
disease models.
64,149
Powerful and stable antisense
oligonucleotides are available—usually complementary
to the (CUG)
n
expansion in myotonic dystrophy type 1—
that reduce transcripts from the mutant DMPK allele,
leading to a decrease in ribonuclear foci without ne-
cessarily aff ecting the expression of DMPK protein.
150,151
Binding of antisense oligonucleotides to the expansion
repeat induces degradation of RNA. In mouse models
of type 1 disease, local delivery of antisense
oligonucleotides to muscle tissue has been completed,
with good results, but optimisation of treatment is
needed because current procedures allow effi cient
delivery of antisense oligo nucleotides in the liver or the
kidney, but they cannot deliver effi cient amounts in
muscle tissue. Morpholino injections into the tail vein
in a mouse model eff ectively reduced myotonia within
4–5 weeks.
150
Alternative approaches
64,149
targeting down-
stream changes include upregulation of MBNL
activity,
152
downregulation of CELF1 activity,
71,153
and
reversal of specifi c missplicing events.
80
Since only a few
other genes in the human genome have long CCTG
stretches, antisense oligonucleotides directed at
(CCUG)
n
expan sion repeats in myotonic dystrophy
type 2 should be fairly easy to design if procedures in
type 1 disease are successful.
Conclusions
20 years have passed since the (CTG)
n
repeat expansion
mutation was discovered in patients with myotonic
dystrophy type 1, and 11 years ago the (CCTG)
n
mutation
was identifi ed in type 2 disease. Although much has
been learned within this period, many challenges
remain. Emerging data indicate that molecular
pathomechanisms are much more complex than could
have been envisioned when the respective mutations
were fi rst identifi ed. Promising experimental treat-
ments are emerging. However, for ultimately eff ective
thera peutic strategies, a better understanding of the
patho physiology and mechanisms of DNA instability is
essential. RNA toxicity clearly has a major role, yet
spliceopathy alone does not seem to fully account for all
Search strategy and selection criteria
We searched PubMed with the terms “myotonic dystrophy”,
“myotonic dystrophy type 1”
, and “myotonic dystrophy
type 2” for papers published from 1992 to May, 2012, with
focus on advances made over the past 10 years. We also
manually searched our own fi les and reference lists of
published work to identify further relevant papers. Only
papers published in English were reviewed. Because of space
constraints we were unable to include all pertinent papers
published on this topic. The nal reference list was generated
on the basis of originality and relevance to the broad scope of
this Review.
902
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Review
aspects of the multisystemic myotonic dystrophy
pheno type. The resident genes (containing the
mutations), especially for type 2 disease, might also
have an important role. Other pathomechanisms,
consistent with the toxic RNA model, probably entail
regulation of gene expression and translation, and
various cellular stress pathways, and extend beyond the
nucleus to the cytoplasm. Going forward, understanding
the extent to which these mechanisms account for the
overall myotonic dystrophy phenotype will be
important. If we have learned anything from myotonic
dystrophy, it is to keep an open mind because, in the
words of Peter Harper, “anything that can go wrong
does go wrong in myotonic dystrophy”.
1
Contributors
BU initiated the project, but otherwise both authors contributed equally
to the literature search, preparation of fi gures, and writing and revising
of the paper.
Confl icts of interest
We declare that we have no confl icts of interest.
Acknowledgments
Research in BU’s group has been supported by the following
foundations: Medicinska Undestödsföreningen Liv och Hälsa rs; Medical
Foundations of Vasa Central Hospital and Pirkanmaan Hospital District;
and Folkhälsan Foundation rs. RK thanks Linda Bachinski and other
members of the Krahe lab, and Andrew Link for fruitful discussions.
Research in RK’s group has been supported by MDA, NIAMS, and the
Kleberg Foundation for Genetics Research.
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    • "The EMG is usually not diagnostic at birth, but it may give a positive result subsequently. Multiple RNA splicing defects have been recognized, leading to classify this disorder as a " spliceopathy " [46, 47]. For the congenital myasthenic syndrome, two forms are classically recognized: 1) Myasthenia Gravis is the transient, acquired neonatal myasthenia present at birth or within the first 3 days of life. "
    [Show abstract] [Hide abstract] ABSTRACT: Congenital muscular dystrophies (CMDs) are a wide group of muscular disorders that manifest with very early onset of muscular weakness, sometime associated to severe brain involvement. The histologic pattern of muscle anomalies is typical of dystrophic lesions but quite variable depending on the different stages and on the severity of the disorder. Recent classification of CMDs have been reported most of which based on the combination of clinical, biochemical, molecular and genetic findings, but genotype/phenotype correlation are in constant progression due to more diffuse utilization of the molecular analysis. In this article, the Authors report on CMDs belonging to the group of dystroglycanopathies and in particular on the most severe forms represented by the Fukuyama CMD, Muscle-Eye-Brain disease and Walker Walburg syndrome. Clinical diagnosis of infantile hypotonia is particularly difficult considering the different etiologic factors causing the lesions, the difficulty in localizing the involved CNS area (central vs. peripheral) and the limited role of the diagnostic procedures at this early age. The diagnostic evaluation is not easy mainly in differentiating the various types of CMDs, and represents a challenge for the neonatologists and pediatricians. Suggestions are reported on the way to reach a correct diagnosis with the appropriate use of the diagnostic means.
    Full-text · Article · Dec 2016
    • "The mutation causes missplicing of mRNA species which can affect many cellular processes in different organs and tissues. There is not always a clear correlation between phenotype and genetic repeat size [4]. In general, longer CTG repeat expansions are associated with an earlier age of onset and more disabling disease. "
    [Show abstract] [Hide abstract] ABSTRACT: Background Myotonic dystrophy type 1 (Steinert’s disease or DM1), the most common form of autosomal dominant muscular dystrophy in adults, is a multisystem disorder, affecting skeletal muscle as well as eyes, heart, gastrointestinal tract, endocrine system, and central nervous system, finally responsible of increasing disabilities and secondary social consequences. To date, DM1-related brain involvement represents a challenging field of research. It is well known that DM1 patients frequently present neuropsychological disturbances and psychiatric comorbidities among which reduced awareness of disease burden and its progression, also defined as anosognosia, is common in clinical practice, this leading to secondary misattribution of symptoms, delay in timely diagnostic procedures and low compliance to treatment. Methods Here we present an observational cross sectional study in which disease-related cognitive dysfunctions and quality of life were assessed by a protocol finally designed to estimate the prevalence of disease awareness in a sample of 65 adult-onset DM1 patients. Results Our analysis showed that in DM1 patients several cognitive functions, including executive and mnesic domains with visuo-spatial involvement, were affected. The assessment of anosognosia revealed that a high percentage (51.6 %) of DM1 subjects was disease unaware. The reduced illness awareness occurs across different physical and life domains, and it appears more prominent in Activities and Independence domains investigated by the Individualized Neuromuscular Quality Of Life (INQoL) questionnaire. Moreover, the unawareness resulted significantly related (at p <0.05 and p < 0.01) to the performance failure in cognitive tests, specifically in the domains of visuo-spatial memory, cognitive flexibility and conceptualization. Conclusions The obtained data confirm, by a systematic analysis, what’s the common clinical perceiving of disease unawareness in Steinert’s disease, this related to the already known cognitive-behavioural impairment of frontal type in affected patients. We believe that a deep knowledge of this aspect will be useful for medical practice in the management of patients with DM1, also for guidance in occupational and social interventions, definition of outcome measures and in preparation of trial readiness.
    Full-text · Article · Dec 2016
    • "Moreover DMPK, which has been localized to NMJ, is expressed at low levels in DM1, implying that haploinsufficiency may play a role in neuromuscular transmission defect of DM1 [44,45]. A limitation of this study is the small number of investigated patients and the fact that we cannot exclude that the documented abnormalities are related both to processes of denervation and reinnervation and to fiber muscle damage at neuromuscular junction level; however, standard nerve conduction studies did not show findings consistent with a peripheral nerve defect in our patients and the characteristic histologic muscle features of DM1 patients are not consistent with either inflammatory or dystrophic processes , that are believed responsible for jitter abnormalities [4]. In conclusion, our results appear to indicate a possible involvement of the NMJ in determining the symptoms of DM1 patients, such as weakness and fatigue. "
    [Show abstract] [Hide abstract] ABSTRACT: Objectives: Weakness and fatigue are frequent symptoms in myotonic dystrophy type 1 (DM1), mainly as a result of muscle impairment. However, neuromuscular junction (NMJ) abnormalities could play an additional role in determining these manifestations. We aimed to document the possible NMJ involvement in DM1. Patients and methods: In order to substantiate this hypothesis we performed low rate repetitive nerve stimulation (RNS) and single fiber electromyography (SFEMG), in 14 DM1 subjects. Results: RNS resulted abnormal in four patients while SFEMG revealed a pathological jitter in ten. A significative correlation was found between jitter values and decrementing response (p<0.000311; r=0.822). Conclusion: These results suggest a possible involvement of NMJ in DM1.
    Full-text · Article · Aug 2016
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