the possible eVects of shared ancestry and exercise on the
clinical expression of a specific mutation, especially in
subpopulation groups with known founder eVects.
Although these examples of divergent phenotypic
expression in kindred 101a and the twins in pedigree 138
are based on small numbers, they lend support to the
notion that HCM is not a simple monogenic disorder and
that both genetic and environmental factors are modifiers
of the disease phenotype. A strategy followed in studies of
disease phenotypes with multifactorial aetiology is to
reduce the complexity of analysis by investigating geneti-
cally homogeneous subjects. The presence of the founder
MYH7 A797T mutation suggests that the families
harbouring it share a degree of common ancestry. We
therefore propose that the presence of this HCM causing
mutation with incomplete penetrance, in a substantial
group of related people, provides an opportunity to inves-
tigate the role of additional factors involved in the develop-
ment of the disease phenotype.Only when these factors are
known will the puzzling variability in the clinical expression
which is a feature of HCM mutations, and the true patho-
physiology of this disease, be understood.
We wish to thank the patients and their families for participating in this study,as
well as the physicians who referred patients and collected samples.The work was
supported by the South African Medical Research Council and the Harry and
Doris Crossley Fund of the University of Stellenbosch.
WILLEM DE LANGE*
*US/MRC Centre for Molecular and Cellular Biology,Department of
Medical Biochemistry,University of Stellenbosch Medical School,PO Box
19063,Tygerberg 7505,South Africa
†Department of Internal Medicine,University of Stellenbosch Medical
School and Tygerberg Hospital,Tygerberg,South Africa
Correspondence to: Dr Corfield, VCl@gerga.sun.ac.za
1 Maron BJ, Gardin JM, Flack JM, Gidding SS, Kurosaki TT, Bild DE.
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2 McKenna WJ, Watkins HC. Hypertrophic cardiomyopathy. In: Scriver CR,
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ited disease. Vol III, 7th ed. New York: McGraw-Hill Health Professions
3 Moolman JC, Corfield VA, Posen B, Ngumbela K, Seidman C, Brink PA,
Watkins H. Sudden death due to troponin T mutations. J Am Coll Cardiol
4 Watkins H, McKenna WJ, Thierfelder L, Suk HJ, Anan R, O’Donoghue A,
Spirito P, Matsumori A, Moravec C, Seidman JG, Seidman CE. Mutations
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Seidman JG. Characteristics and prognostic implications of myosin
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Germline and somatic mosaicism in
EDITOR—We describe a sib recurrence in achondroplasia
with parents of normal stature. Both aVected oVspring
carry the same causal mutation (G1138C) in the fibroblast
growth factor receptor 3 (FGFR3) gene. Despite having no
clinical features of achondroplasia, a proportion of the
mother’s peripheral blood leucocytes also contained the
mutant FGFR3 allele. We conclude she is a germline and
somatic mosaic for achondroplasia and that both children
have inherited the condition from her. To our knowledge,
this is the first confirmed case of germline mosaicism in
Achondroplasia is the commonest form of short limbed
dwarfism (birth incidence estimated at between 1:10 000
and 1:70 000)1and is transmitted as an autosomal
dominant trait. As is often the case among dominant traits,
a high proportion of cases are new mutations but
achondroplasia is unusual in that the great majority are
caused by one of two mutations at the same nucleotide in
the transmembrane domain of the FGFR3 gene (G1138A
transition and G1138C transversion).1In common with
other FGFR3 mutations which cause skeletal dysplasia,the
pathogenic eVect of the achondroplasia mutations is
thought to be altered mitogenesis and/or diVerentiation
owing to constitutive activation of the receptor.2There is a
marked paternal age eVect in achondroplasia and it has
recently been shown that new mutations in achondroplasia
are almost exclusively of paternal origin.3We received
peripheral blood DNA from a family with two children
with achondroplasia; both parents were of normal stature.
They had a total of four children of whom the second and
fourth were aVected. The mother was 27 years of age and
the father 53 years at the birth of their second aVected
child. Our first thoughts in this case, taking into account
the age of the father, were that the aVected sibs were the
result of two independent new mutations in the paternal
germline, as would be expected to occur by chance as a
very rare event.
To determine the FGFR3 mutation(s) in the aVected
oVspring, blood was collected from the aVected children
and from both parents. DNA was extracted and exon 10 of
the FGFR3 gene was amplified and products were digested
with the restriction enzymes BfmI and MspI. The G1138A
transition creates a restriction site for BfmI whereas the
G1138C transversion creates a restriction site for MspI.
Analysis showed that both children were heterozygous for
the rare G1138C transversion. The father did not have the
mutation in his blood but, surprisingly, the mother did.
However, the ratio of the G1138C allele compared to the
wild type allele was less than the 1:1, which would be
expected for a straightforward heterozygote (fig 1). The
relative proportion of the G1138C allele in the mother’s
blood leucocytes was determined using primer extension4
(fig 1) followed by densitometry. The proportion of the
mutant allele in the mother was found to be 28%. She has
a height of 169 cm, span of 171 cm, upper segment/lower
segment 0.09, left hand 17.6 cm, and head circumference
of 58 cm. Apart from her slightly larger head size and mild
obesity her appearance is normal.
We conclude that, despite her normal appearance, the
mother is a germline and somatic mosaic for the G1138C
mutation and both her aVected children have inherited the
mutant allele from her. Given the mother’s relatively high
proportion of mutant alleles, her lack of phenotypic
expression is surprising; a hypochondroplasia-like pheno-
type, which is less severe than achondroplasia, might have
been expected. The most likely explanation for this is the
tissue specific distribution of the mosaicism, although the
mutant allele is present in 28% of her peripheral blood
leucocytes it may be at lower levels in her chondrocytes.
Germline and somatic mosaicism are both reasonably
common features of genetic disorders. For example, in
Duchenne muscular dystrophy and osteogenesis imper-
fecta, 15% and 6% of cases, respectively, inherit the con-
dition from a detectably mosaic parent.5Germline mosai-
cism results from a mutation in gamete precursors which
then continue to divide, whereas combined germline and
somatic mosaicism arises when the mutation occurs very
early in development before the germline and somatic lin-
eages have separated. As achondroplasia is a common
condition which arises from a highly mutable nucleotide,
high frequencies of mosaicism might have been expected.
Surprisingly, the frequency of germline mosaicism as evi-
denced by sib recurrence is very low. A few cases of recur-
rence have been reported,5 6but so infrequently that it has
been calculated that they could be accounted for by inde-
pendent mutations alone. Clinical reports of somatic
mosaicism in achondroplasia are also extremely rare.7For
some reason, somatic and germline mosaics occur much
more rarely in achondroplasia than in many other
dominant traits. One possibility is that for reasons as yet
unclear, FGFR3 nucleotide 1138 is only hypermutable in
the male germline. Alternatively, there could be somatic
selection against cells carrying the mutant allele. Interest-
ingly, Apert syndrome, which is mainly caused by either of
two point mutations in FGFR2, also seems to have low
incidence of somatic mutation is at variance with recent
findings that somatic activating mutations of FGFR3 are
relatively common in multiple myeloma8and carcinomas.9
However, all the FGFR3 mutations so far identified in
these malignant neoplasms are identical to activating
mutations that cause thanatophoric dysplasia. The greater
severity of this phenotype in comparison to achondro-
plasia is thought to be a reflection of the more strongly
activating nature of the thanatophoric dysplasia muta-
tions.10That only these highly activating FGFR3 muta-
tions have so far been found in neoplasms may suggest
that the achondroplasia mutations, when they occur in
somatic cells, do not activate the receptor to a level that it
This is the first confirmed report of germline and
somatic mosaicism for an achondroplasia mutation.
FGFR3 nucleotide 1138 appears to be highly mutable in
the male germline, but somatic mutations resulting in
mosaicism are rare. The reasons for this discrepancy are
unknown but are clearly of importance to the understand-
ing of mutagenesis. The observation that the mother has a
normal appearance, despite a high proportion of the
achondroplastic allele in her somatic tissues, exemplifies
the fine balance that the fibroblast growth factors play in
product with MspI.Lanes 1,father;2,mother;3,first aVected child;4,
second aVected child.Primers used were
5'-GCGCGTGCTGAGGTTCTGAG-3'.(Lower panel) Primer
extension was carried out with primer
5'-GATGAACAGGAAGAAGCCCA-3' (which binds 4 bp downstream
of nucleotide 1138) using methods described in Loughlin et al.4The primer
was end labelled with ?32P dATP and the extension mix contained dA,dT,
dC,and ddG.The 20mer primer was extended by 4 bp for the
achondroplastic G1138C allele and by 5 bp for the wild type allele as
shown below (added nucleotides are shown underlined,nucleotide 1138 is
shown in bold).
Wild type template: 5'-CGGGGTGGGCTTCTTCCTGTTCATC-3'
G1138C template: 5'-CCGGGTGGGCTTCTTCCTGTTCATC-3'
The products were then separated by electrophoresis through a 15%
denaturing polyacrylamide gel.The relative intensity of the 24mer and
25mer products was used to calculate the proportion of achondroplastic to
wild type allele.Lane order and PCR primers as above.
(Upper panel).Digestion of 320 bp FGFR3 genomic PCR
BRUCE BENNETTS† Download full-text
*Cellular Genetics Group,Oxford University,Institute of Molecular
Medicine,The John RadcliVe,Oxford OX3 9DS,UK
†Department of Clinical Genetics,The New Children’s Hospital,Sydney,
Correspondence to: Dr Henderson, email@example.com
1 Bellus GA, HeVeron TW, Rosa I, Luna O, Hecht JT, Horton WA, Machado
M, Kaitila I, McIntosh I, Francomano CA. Achondroplasia is defined by
recurrent G380R mutations of FGFR3. Am J Hum Genet 1995;56:368-73.
2 Webster MK, Donoghue J. FGFR activation in skeletal disorders: too much
of a good thing. Trends Genet 1997;13:178-82.
3 Wilkin DJ, Szabo JK, Cameron R, Henderson S, Bellus GA, Mack ML,
Kaitila I, Loughlin J, Munnich A, Sykes B, Bonaventure J, Francamano
CA. Mutations in fibroblast growth factor receptor 3 in sporadic cases of
achondroplasia occur exclusively on the paternally derived chromosome.
Am J Hum Genet 1998;63:711-16.
4 Loughlin J, Irven C, Athanosou N, Carr A, Sykes B. DiVerential allelic
expression of the type II collagen gene (COL2A1) in osteoarthritic
cartilage. Am J Hum Genet 1995;56:1186-93.
5 Zlotogora J. Germ line mosaicism. Hum Genet 1998;102:381-6.
6 Reiser CA, Pauli RM, Hall JG. Achondroplasia: unexpected familial recur-
rence. Am J Med Genet 1984;19:245-50.
7 Rimoin DL, McKusick VA. Somatic mosaicism in an achondroplastic
dwarf. Birth Defects 1969;5:17-19.
8 Chesi M, Nardini E, Brents LA, Schrock E, Ried T, Kuehl WM, Bergsagel
PL. Frequent translocation t(4;14)(p16.3;q32.3) in multiple myeloma is
associated with increased expression and activating mutations of fibroblast
growth factor receptor 3. Nat Genet 1997;16:260-3.
9 Capellen D, De Oliveira C, Ricol D, Gill Diez de Medina S, Bourdin J,
Sastre-Garau X, Chopin D, Thiery JP, Radanyi F. Frequent activating
mutations of FGFR3 in human bladder and cervix carcinomas. Nat Genet
10 Naski MC, Wang Q, Xu J, Ornitz DM. Graded activation of fibroblast
growth factor receptor 3 by mutations causing achondroplasia and
thanatophoric dysplasia. Nat Genet 1996;13:233-7.
J Med Genet 2000;37:958–959
Achondroplasia with the FGFR3
1138g→a (G380R) mutation in two
sibs sharing a 4p haplotype derived
from their unaVected father
EDITOR—The study of achondroplasia, the most frequent
skeletal dysplasia in man, has contributed several impor-
tant insights into both developmental biology and human
genetics, such as the recognition of the paternal age eVect
for dominant mutations,1 2the first indication of the
importance of FGFR molecules in growth and develop-
ment,3and the identification of the nucleotide with the
highest mutation rate known so far in man, nucleotide
1138 of the FGFR3 gene.4Most cases of achondroplasia
are associated with the g→a transition at nucleotide 1138
In spite of the frequency of achondroplasia, the birth of
two or more children with achondroplasia to unaVected
parents is surprisingly rare, with only a few examples pub-
lished.5 6One instance of half sibs with achondroplasia
born to the same father has been reported.7In contrast,
observations of achondroplasia in people more distantly
related are relatively more common.8–10
uncertain whether instances of achondroplasia in sibs born
to unaVected parents are caused by somatic mosaicism (as
suggested by the observation of three aVected sibs from
normal parents6 7) or by independent chance events.8
achondroplasia must be orders of magnitude rarer than for
osteogenesis imperfecta or other dominant conditions.11
Approximately, 90% of cases of achondroplasia are
caused by de novo mutations, and all de novo achondro-
plasia mutations studied so far were found to have
occurred on paternal chromosomes.12
achondroplasia with characteristic clinical and radio-
graphic signs in a brother and sister born to parents of nor-
mal stature, lacking any clinical sign of either hypochon-
droplasia or achondroplasia, aged 28 years (mother) and
25 years (father) at the time of birth of the first child (fig 1).
The family agreed to have the molecular mechanism of
recurrence investigated and consented to venepuncture
and buccal smears. Both children were heterozygous (in
leucocyte DNA) for the g1138a (G380R) FGFR3
mutation, while that mutation was not found in parental
leucocyte or buccal smear DNA by either SSCP analysis or
direct sequencing of PCR products. This made parental
somatic mosaicism unlikely.
Thus, it is
To investigate the origin of the mutation shared by the
two aVected sibs, inheritance of VNTR alleles on chromo-
some 4p was studied (fig 2). The aVected children had two
diVerent maternal haplotypes but shared a paternal 4p
haplotype encompassing the FGFR3 locus. As the FGFR3
g1138a mutation occurs exclusively on paternal chromo-
somes, and the aVected children had two diVerent
maternal 4p haplotypes, the most likely explanations for
these findings would be either two independent mutational
events occurring by chance on the same paternal
haplotype, or mosaicism at the spermatogonial level
(before meiosis I) in the father. Paternal sperm was not
available and the hypothesis of gonadal mosaicism could
not be further substantiated.
We conclude that recurrence of achondroplasia in this
family was associated with de novo mutational event(s)
occurring in the paternal germline, as is the case in
but could not distinguish between
paternal gonadal mosaicism or the chance occurrence of
two independent mutation events.8The apparent rarity of
achondroplasia aged 12 and 10 years,respectively.
Clinical appearance of the brother (right) and sister (left) with