DICER1 syndrome: clarifying the diagnosis, clinical features and management implications of a pleiotropic tumour predisposition syndrome.

Abstract

Constitutional DICER1 mutations were recently reported to cause familial pleuropulmonary blastoma (PPB).
To investigate the contribution and phenotypic spectrum of constitutional and somatic DICER1 mutations to cancer.
The authors sequenced DICER1 in constitutional DNA from 823 unrelated patients with a variety of tumours and in 781 cancer cell lines. Constitutional DICER1 mutations were identified in 19 families including 11/14 with PPB, 2/3 with cystic nephroma, 4/7 with ovarian Sertoli-Leydig-type tumours, 1/243 with Wilms tumour (this patient also had a Sertoli-Leydig tumour), 1/1 with intraocular medulloepithelioma (this patient also had PPB), 1/86 with medulloblastoma/infratentorial primitive neuroectodermal tumour, and 1/172 with germ cell tumour. The inheritance was investigated in 17 families. DICER1 mutations were identified in 25 relatives: 17 were unaffected, one mother had ovarian Sertoli-Leydig tumour, one half-sibling had cystic nephroma, and six relatives had non-toxic thyroid cysts/goitre. Analysis of eight tumours from DICER1 mutation-positive patients showed universal retention of the wild-type allele. DICER1 truncating mutations were identified in 4/781 cancer cell lines; all were in microsatellite unstable lines and therefore unlikely to be driver mutations.
Constitutional DICER1 haploinsufficiency predisposes to a broad range of tumours, making a substantial contribution to PPB, cystic nephroma and ovarian Sertoli-Leydig tumours, but a smaller contribution to other tumours. Most mutation carriers are unaffected, indicating that tumour risk is modest. The authors define the clinical contexts in which DICER1 mutation testing should be considered, the associated tumour risks, and the implications for at-risk individuals. They have termed this condition 'DICER1 syndrome'. ACCESSION NUMBERS: The cDNA Genbank accession number for the DICER1 sequence reported in this paper is NM_030621.2.

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DICER1 syndrome – clarifying the diagnosis, clinical features and management
implications of a pleiotropic tumor predisposition syndrome

Ingrid Slade,1 Chiara Bacchelli,1 Helen Davies,2 Anne Murray,1 Fatemeh
Abbaszadeh,1 Sandra Hanks,1 Rita Barfoot,1 Amos Burke,3 Julia Chisholm,4 Martin
Hewitt,5 Helen Jenkinson,6 Derek King,7 Bruce Morland,6 Barry Pizer,8 Katrina
Prescott,9 Anand Saggar,10 Lucy Side,11 Heidi Traunecker,12 Sucheta Vaidya,13 Paul
Ward,14 P. Andrew Futreal,2 Gordan Vujanic,15 Andrew G Nicholson,16 Neil Sebire,17
John R Priest,18 Kathryn Pritchard-Jones,13* Richard Houlston,1 Clare Turnbull,1
Charles Stiller,19 Michael R Stratton,1,2 Jenny Douglas,1 Nazneen Rahman1

1Section of Cancer Genetics and 13Section of Pediatrics, Institute of Cancer
Research and Royal Marsden Hospital, Sutton, Surrey, United Kingdom; 2The
Wellcome Trust Sanger Institute, Hinxton, Cambridge, United Kingdom; 3Pediatric
Oncology, Addenbrooke’s Hospital, Cambridge, United Kingdom; 4Department of
Pediatric Oncology, 11Department of Clinical Genetics and 17Department of
Histopathology and Pediatric Laboratory Medicine, Great Ormond Street Hospital,
London, United Kingdom; 5Pediatric Oncology, Nottingham University Hospital,
Nottingham, United Kingdom; 6Pediatric Oncology, Birmingham Children’s Hospital,
Birmingham, United Kingdom; 7Department of Haematology and Oncology, Royal
Aberdeen Children’s Hospital, Aberdeen, Scotland, United Kingdom; 8Department of
Pediatric Oncology, Alder Hey Children’s Hospital, Liverpool, United Kingdom;
9Department of Clinical Genetics, Chapel Allerton Hospital, Leeds, United Kingdom;
10Department of Clinical Genetics, St. George’s Hospital, London, United Kingdom;
12Pediatric Oncology, Children’s Hospital for Wales, Cardiff, United Kingdom;
14Department of Pediatrics, Derriford Hospital, Plymouth, United Kingdom;
15Department of Histopathology, School of Medicine, Cardiff University, Cardiff,
United Kingdom; 16Department of Histopathology, Royal Brompton Hospital and the
National Heart and Lung Institute, Imperial College, London, United Kingdom;
18International PPB Registry, Children’s Hospitals and Clinics of Minnesota,
Minneapolis, USA; 19Childhood Cancer Research Group, Department of Pediatrics,
University of Oxford, Oxford, United Kingdom.
*current affiliation Molecular Haematology and Cancer Biology, Institute of Child
Health, London, United Kingdom.

Succinct title: DICER1 syndrome
Key words: DICER1, microRNA, cancer syndrome, tumor predisposition
Word count: 2850 words
Research support: This work was supported by Cancer Research UK (grants
C8620_A9024 and C8620_A8857) and the Institute of Cancer Research (UK).
Corresponding author: Professor Nazneen Rahman MD PhD, Professor of Human
Genetics, The Institute of Cancer Research; 15 Cotswold Road; Sutton SM2 5NG;
United Kingdom. Email: nazneen.rahman@icr.ac.uk
Tel: +44 (0)20 8722 4145 Fax: +44 (0)20 8722 4026
peer-00603432, version 1 - 25 Jun 2011
Author manuscript, published in "Journal of Medical Genetics 48, 4 (2011) 273"
DOI : 10.1136/jmg.2010.083790

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ABSTRACT

Background: Constitutional DICER1 mutations were recently reported to cause
familial pleuropulmonary blastoma (PPB). We sought to investigate the contribution
and phenotypic spectrum of constitutional and somatic DICER1 mutations to cancer.

Methods and Results: We sequenced DICER1 in constitutional DNA from 823
unrelated individuals with a variety of tumors and in 781 cancer cell lines. We
identified constitutional DICER1 mutations in 19 families including 11/14 with PPB,
2/3 with cystic nephroma, 4/7 with ovarian Sertoli-Leydig type tumors, 1/243 with
Wilms tumor (this individual also had Sertoli-Leydig tumor), 1/1 with intra-ocular
medulloepithelioma (this individual also had PPB), 1/86 with
medulloblastoma/infratentorial PNET and 1/172 with germ cell tumor. The
inheritance was investigated in 17 families. DICER1 mutations were identified in 25
relatives, 17 were unaffected, one mother had ovarian Sertoli-Leydig tumor, one half-
sibling had cystic nephroma and six relatives had non-toxic thyroid cysts/goiter.
Analysis of eight tumors from DICER1 mutation-positive individuals demonstrated
universal retention of the wild-type allele. We identified DICER1 truncating mutations
in 4/781 cancer cell lines; all were microsatellite unstable lines and hence these are
unlikely to be driver mutations.

Conclusion: Constitutional DICER1 haploinsufficiency predisposes to a broad range
of tumors, making a substantial contribution to PPB, cystic nephroma and ovarian
Sertoli-Leydig tumors, but a small contribution to other tumors. Most mutation
carriers are unaffected indicating that the risk of tumors is modest. We define the
clinical contexts in which DICER1 mutation testing should be considered, the
associated tumor risks and the implications for at-risk individuals. We have termed
this condition ‘DICER1 syndrome’.
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INTRODUCTION
DICER1 is an RNase endonuclease important in the production of microRNAs
(miRNA) which are non-protein-coding small RNAs that are estimated to regulate the
expression of over 30% of protein-coding genes at the post transcriptional level.[1, 2]
miRNAs are transcribed as long precursors, known as pri-miRNAs, which are
processed in the nucleus to produce pre-miRNAs.[3] The pre-miRNAs are exported
to the cytoplasm where DICER1 processing generates a double-strand miRNA
duplex.[4] The duplex is unwound to generate the final miRNA which interacts with
mRNA to regulate gene expression, typically through translational repression or
mRNA degradation. [2, 5, 6]

Over 900 human miRNAs are currently recognised.[7, 8] These have been implicated
in a wide range of biological processes including metabolism, morphogenesis, cell
fate determination, cell proliferation and apoptosis.[9, 10] There is increasing
evidence implicating dysregulation of miRNAs in several human diseases, including
cancer.[11, 12] Widespread alteration of miRNA levels is seen in cancers and miRNA
profiles characteristic for cancer type and stage are increasingly recognised.[12, 13]
Furthermore, global downregulation of miRNAs due to abrogation of miRNA
processing has been shown to promote tumorigenesis.[14]

Recently, germline inactivating DICER1 mutations were shown to cause familial
pleuropulmonary blastoma (PPB, OMIM #601200), a rare malignant lung tumor,
primarily affecting children before age 6.[15, 16] By linkage analysis Hill et al mapped
a familial PPB gene to chromosome 14q. They considered DICER1 to be a promising
candidate and identified pathogenic mutations in 11 families.[15] This important
finding raises a number of questions. Firstly, what is the contribution of DICER1
mutations to non-familial, sporadic PPB? Mutations in some cancer predisposition
genes contribute appreciably to both familial and sporadic forms of disease, whereas
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for others the contribution to non-familial cases is small. Secondly, do constitutional
DICER1 mutations predispose to tumors other than PPB? The International PPB
Registry has collected information from over 200 PPB families and a variety of
different tumors have been reported in PPB cases and/or their relatives [17, 18]
However, it is unknown which tumor types are genuinely associated with PPB, which
are related to DICER1 mutations, and which reflect ascertainment. Thirdly, do
somatic DICER1 mutations contribute to cancer? This is of particular interest as it
has been proposed that somatic 14q loss, which has been reported in many cancers,
may be targeted at DICER1.[19, 20, 21]

To address these questions we have conducted exhaustive DICER1 sequencing, in
>1,600 patient samples, including constitutional DNAs from 823 individuals with a
broad range of tumors, but particularly focussing on tumors that have been proposed
to be associated with PPB (http://www.ppbregistry.org/pdf/Doc_D.pdf) (Table 1). We
also sequenced DICER1 in DNA from 781 cancer cell lines to assess the impact of
somatic DICER1 mutation on cancer development (Supplementary Table 1).






















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Table 1. Tumors in individuals screened for constitutional DICER1 mutations
Tumor Type
athis patient also had ovarian Sertoli-Leydig cell tumor
bthis patient also had pleuropulmonary blastoma
Number of
cases

14
1
Number with
DICER1 mutation

11

Lung tumors
Pleuropulmonary blastoma
Congenital lung cyst

Sex-cord stromal tumors
Testicular Sertoli-Leydig/Sertoli/Leydig tumor
Ovarian Sertoli-Leydig tumor
Ovarian granulosa cell tumor
Gonadal stromal cell tumor unclassified
Mixed stromal cell tumor
Stromal sex-cord tumor unclassified

Renal tumors
Wilms tumor
Cystic nephroma
Clear cell sarcoma

CNS tumors
Medulloblastoma/Infratentorial PNET

Eye tumors
Medulloepithelioma (dictyoma)

Germ cell tumors
Seminoma
Non-seminoma (testis)
Mixed testicular cancer
Embryonal carcinoma
Testicular cancer unspecified
Teratoma
Dysgerminoma/Germinoma
Yolk-sac tumor
Gonadoblastoma

Neuroblastoma

Thyroid carcinoma
Papillary thyroid carcinoma
Follicular thyroid carcinoma
Thyroid cancer unspecified
Anaplastic thyroid carcinoma

Soft tissue and other extraosseous sarcomas
Rhabdomyosarcoma
Fibrosarcoma
Other soft tissue sarcomas

Malignant bone tumors
Ewings sarcoma
Osteosarcoma
Chondrosarcoma

Hepatic tumors
Hepatoblastoma
Hepatocellular carcinoma


11
6
6
5
1
1


3



1

243
3
3

1a
2

84

1

1

1b

71
52
14
13
12
10
8
4
1

1
73

51
18
15
4


19
7
49


8
7
4


2
2


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METHODS
Samples
Constitutional DNA was extracted from EDTA-venous blood samples and collected
through the Factors Associated with Childhood Tumors (FACT) study, the Royal
Marsden Hospital (RMH) cancer collections, and the Institute of Cancer Research
UK-wide testicular germ cell tumor collections, all of which have been approved by
an appropriate Ethics Board. All samples were obtained with full informed consent.
The research was undertaken as part of the Factors Associated with Childhood
Tumors Study (FACT study) which was approved by the London Multicentre
Research Ethics Committee (05/MRE02/17). The FACT study aims to identify
genetic factors that predispose to the development of childhood tumors
(www.icr.ac.uk/fact). The National Registry of Childhood Tumors (NRCT) was
utilised to identify the total number (indicated in brackets) of cases nationally for PPB
(20, 5 of whom were deceased), cystic nephroma (15, none deceased) and Sertoli-
Leydig cell tumors (7, none deceased) that had been registered since its inception in
1962. The clinicians were contacted to request that they recruit these patients to the
FACT study. In addition, a small minority of patients referred to our Clinical Genetics
service with these tumors were recruited directly. Tumor DNA from DICER1
mutation-positive paraffin-embedded tissues was extracted using QIAamp DNA
FFPE Tissue kit (QIAGEN) according to manufacturer instructions. We analysed
whole genome amplified DNA from 781 cancer cell lines as part of the Cancer
Genome Project, Cell Line Project (Supplementary Table 1).

DICER1 Sequencing
For analysis of the constitutional DNA we designed PCR primers to amplify the 26
coding exons and intron-exon boundaries of DICER1 in a multiplex PCR reaction
(Supplementary Table 2). Products were sequenced by capillary sequencing using
the BigDye Terminator Cycle Sequencing Kit and an ABI3730 Genetic Analyzer
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(Applied Biosystems, Foster City CA). Sequences were analysed using Mutation
Surveyor software v3.20 (SoftGenetics, USA). We only included samples in which at
least 90% or more of the coding sequence was successfully screened in subsequent
analyses. For the cancer cell lines, PCR primers that amplify 500bp PCR products
encompassing the 26 coding DICER1 exons and intron-exon boundaries were
designed and sequenced as described above. Sequence traces were analyzed using
AutoCSA software[22], followed by manual inspection of putative variants. All
putative variants were confirmed by bidirectional sequencing of a second
independently amplified PCR product. Matched normal cell lines were available for
forty cell lines. The somatic status of variants identified in these forty cell lines was
determined by sequencing DNA from the corresponding normal. In the remaining cell
lines we assumed that cell line variants that were also identified in the constitutional
DNA screen were not somatic. We evaluated the likely pathogenicity of sequence
variants using Polyphen, SIFT and NNSplice software.

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RESULTS
Germline DICER1 mutation analysis
We identified pathogenic mutations in 19/823 index individuals (Table 2 and
Supplementary Figure 1). Seventeen mutations lead to premature protein truncation
as a result of frameshift, nonsense or consensus splice-site mutations. Two
mutations are missense alterations for which there is substantial evidence of
pathogenicity. Firstly, they are the only two missense alterations in the 3214
chromosomes screened that are predicted to be pathogenic by SIFT and Polyphen.
Secondly, both target highly conserved residues in the RNase III domain. Thirdly,
they are in the vicinity of a missense variant identified by Hill et al, which resulted in a
similar DICER1 histochemical profile to truncating mutations.[15] We also identified
several non-pathogenic variants including nine missense variants, 23 synonymous
variants and five intronic variants (Supplementary Table 3).

Pleuropulmonary blastoma
We identified DICER1 mutations in 10 individuals with PPB and in the mother of a
child that had died from PPB but from whom no sample was available. One child
developed an intra-ocular medulloepithelioma four years after PPB. Three-
generational pedigrees were available for most cases and no relative had PPB. One
sibling died from neuroblastoma; her DICER1 status is not known, but a mutation
was present in her mother. There were three children with PPB in whom we did not
identify a DICER1 mutation.

Cystic nephroma
The commonest reported association of PPB is cystic nephroma[17], a rare benign
renal tumor that typically presents as a multicystic renal mass without solid nodules.
It has a bimodal incidence with 50% occurring in children less than 4 years and 30%
in the 5th and 6th decades.[23] We had DNA from three unilateral childhood cases
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and in two we identified truncating DICER1 mutations. One of the children had a half-
sibling with cystic nephroma who also has the mutation. The second DICER1
mutation-positive case was recently found to have a small lung cyst which is being
monitored, but has not had histological evaluation.

Ovarian Sertoli-Leydig tumors
We analysed DNA from 30 individuals with sex cord tumors of which six were ovarian
Sertoli-Leydig tumors, which are sex cord-tumors that exhibit testicular
differentiation.[24] The age range of diagnosis is between 2-75 years but ~75%
present in the second or third decades.[24] We identified truncating DICER1
mutations in four individuals; three had young-onset bilateral ovarian Sertoli-Leydig
tumors, and one had a unilateral ovarian sex cord tumor that could not be further
classified because of necrosis. One of the bilateral cases previously had Wilms tumor
(see below). The mother of one case had also developed a Sertoli-Leydig tumor at
21 years and carried the DICER1 mutation. She has subsequently had melanoma at
50 years, endometrial cancer at 62 years and breast cancer at 68 years.

Wilms tumor
Wilms tumor is an embryonal cancer of the kidney that affects ~1 in 10,000 children,
usually before the age of 6 years.[25, 26] We analysed DNA from 243 individuals
with Wilms tumor. We identified one truncating DICER1 mutation, in a child who
developed Wilms tumor of atypical histology at the unusually late age of 8 years.
Four years after treatment the child developed bilateral ovarian Sertoli-Leydig cell
tumors.

Medulloblastoma/Infratentorial PNET
Medulloblastoma is a primitive neuro-ectodermal tumor that arises in the posterior
fossa.[27] We analysed 84 childhood medulloblastoma/infratentorial PNET cases and
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identified one truncating mutation in a child of 13 years. No other information or
samples were available.

Seminoma
We analysed DNA from 185 individuals with germ cell tumor, of which 128 had a
family history of testicular cancer. We identified one missense DICER1 mutation,
Q1580H. A maternal first cousin once removed of this proband developed testicular
cancer at 27 years but the proband’s mother does not carry the DICER1 mutation. It
is not possible to conclusively establish whether this mutation is pathogenic on the
available evidence.

Intra-ocular medulloepithelioma
Intra-ocular medulloepithelioma, also known as dictyoma, is a very rare embryonal
tumor usually originating in the ciliary body of the eye that most commonly occurs
during childhood.[28] One child with PPB also developed a dictyoma, and has a
DICER1 mutation.

Thyroid non-toxic goiters / cysts
In the PPB Registry thyroid cancers and thyroid hyperplasia are reported in both
probands and relatives of PPB cases. We analysed DNA from 88 individuals with
thyroid cancer but did not identify any mutations. However, one proband and six
relatives of DICER1 mutation-positive individuals developed thyroid cysts /
multinodular colloid goiter between the ages of 9-30 years. All were non-toxic,
associated with normal thyroid function and non-malignant. Thyroidectomy was
required in four individuals because of recurrent disease.

Other tumors/cysts
We did not identify mutations in any of the other tumor types as detailed in Table 1.
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Table 2. Probands and relatives with constitutional DICER1 mutations
ID Phenotype Age at
diagnosis (yrs)
Current age
(yrs)
Family 1 - c.4403_4406delCTCT
proband
parent
uncle
grandparent none
Family 2 - c.1716delT
proband
parent none
Family 3 - c.1196_1197dupAG
proband
parent
aunt
grandparent
none
Family 4 - c.3505delT
proband
sibling
parent none
Family 5 - c.1966C>T p.R656X
proband
parent none
Family 6 - c.2268_2271delTTTG
proband
parent none
Family 7 - c.3665delT
proband
parent none
Family 8 - c.3583_3584delGA
proband

parent none
Family 9 - c.2040+1 G>C
proband
parent thyroid cysts unclassified
Family 10 - c.3726C>A p.Y1242X
proband
parent thyroid cysts unclassified
bFamily 11 - c.5465A>T p.D1822V
proband pleuropulmonary blastoma
pleuropulmonary blastoma
none
none
1.5



5
40
44
68
pleuropulmonary blastoma 0.8

2
36
pleuropulmonary blastoma
none
none
4



5
30
26
62
pleuropulmonary blastoma
none
3

3
12
36
pleuropulmonary blastoma 7

12
52
pleuropulmonary blastoma 0.9

1
38
pleuropulmonary blastomaa 4.2 died 5.2
36
pleuropulmonary blastoma
intra-ocular medulloepithelioma
1.3
6
7

29
pleuropulmonary blastoma 3
25
13
38
pleuropulmonary blastoma 4
30
15
37
1.8 9
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aindividual deceased, no sample available
bthis family includes a sibling with neuroblastoma, no sample available






parent
Family 12 - c.3288_3289insTTTC
proband
half-sibling
parent multinodular colloid goitre
Family 13 - c.328_338dupGTGTCAGCTGT
proband
parent

grandparent
great-aunt
none
Family 14 - c.5122_5128delGGAGATG
proband

parent


endometrial cancer
breast cancer
Family 15 - c.1966C>T_p.R656X
proband

parent none
Family 16 - c.3793delA
proband
parent none
Family 17 - c.2988-2_2988-1delAGinsCT
proband



parent none
Family 18 - c.1153delC
proband medulloblastoma/infratentorial PNET 13 18
Family 19 - c.4740G>T p.Q1580H
proband seminoma
thyroid cysts unclassified 20 40
cystic nephroma
cystic nephroma
1.5
0.8
25
11
5
39
cystic nephroma
multinodular colloid goitre
thyroglossal duct cyst
thyroid cysts unclassified
3
20
30
7

5
35

66
59
ovarian Sertoli-Leydig cell tumor L
ovarian Sertoli-Leydig cell tumor R
ovarian Sertoli-Leydig cell tumor R
melanoma
17
27
21
50
62
68
35

71
ovarian Sertoli-Leydig cell tumor R
ovarian Sertoli-Leydig cell tumor L
12
14

17

50
ovarian sex cord stromal tumor R 6

9
29
Wilms tumor
multinodular thyroid goiter unclassified
ovarian Sertoli-Leydig cell tumor R
ovarian Sertoli-Leydig cell tumor L
8
9
12
12

15



40
32 46
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Family studies in DICER1 mutation-positive individuals
We had samples from both parents in 17 families in which a DICER1 mutation had
been identified in an index individual. In each, the mutation had been inherited (Table
2). We had grandparental samples for five families. In two families, the mutation was
absent in the respective grandparents indicating that the mutation had arisen de novo
in the parent. In the other three families, the mutation was present in a grandparent.

We analysed DNA from 51 relatives and we identified 25 relatives with DICER1
mutations. Of these, as described above, one mother had a Sertoli-Leydig tumor, one
half-sibling had cystic nephroma and six relatives had thyroid cysts / goiter. The
remaining 17 individuals did not have clinical features or symptoms likely to be
related to the DICER1 mutation; one relative having muscular dystrophy and another
Wegener’s granulomatosis.

Analysis of tumors from DICER1 mutation-positive individuals
We obtained eight tumors from six DICER1 mutation-positive individuals. This
included three PPB, four Sertoli-Leydig tumors and one cystic nephroma. We
analysed each tumor for the relevant mutation. Each was heterozygous for the
mutation, i.e. the tumor showed a similar mutational profile as in the blood and there
was no loss of the wild-type allele in any tumor.

DICER1 mutation analysis in cancer cell lines
We screened 781 cancer cell lines (Supplementary Table 1) for DICER1 mutations.
These represent an extensive cross-section of cancers, but do not include many of
the cancers in which we identified germline DICER1 mutations. 206 of the cell lines
have previously been shown to have loss of heterozygosity of 14q encompassing
DICER1.

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We identified 4 truncating mutations in the 781 cell lines and these were in
microsatellite unstable lines. We identified 22 non-synonymous variants that were
either proven to be somatic or in which normal DNA was not available for evaluation.
None were predicted to be deleterious (Supplementary Table 4).

DISCUSSION
In 2009, germline DICER1 mutations were identified in familial pleuropulmonary
blastoma, adding to the list of rare familial cancer syndromes that have yielded
critical evidence linking essential biological processes with cancer causation.[15] In
this study we have expanded knowledge of the link between DICER1 and cancer.
Firstly, we demonstrate that germline DICER1 mutations are the major cause of PPB.
In the UK, 20 cases of PPB have been registered in the National Registry of
Childhood Tumors over the last 35 years. We were able to include 14 in the current
study, none of whom had a family history of PPB. We identified DICER1 mutations in
11 of these cases. [15] It is uncertain whether cryptic DICER1 mutations account for
any of the negative cases, whether mutations in another gene can also cause PPB,
or whether the remaining cases are not due to germline predisposition genes.
However, it is clear that germline DICER1 mutations are the major cause of both
familial and non-familial PPB.

We have also demonstrated that DICER1 mutations cause a range of phenotypes;
not all families include PPB and a high proportion of mutation carriers are clinically
well. In view of this we suggest that ‘DICER1 syndrome’ is a preferable term to ‘PPB
familial tumor syndrome’ which has previously sometimes been used. The range of
different tumors that can occur in individuals with DICER1 mutations is broad and it is
likely that more associated tumors will be identified as further mutation testing is
undertaken. The contribution of DICER1 mutations to different tumors is very
variable. The major tumors that occur in DICER1 syndrome appear to be PPB, cystic
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nephroma and ovarian Sertoli-Leydig tumor. Evidence for this comes from our
mutational data, our clinical and mutational investigation of relatives of mutation-
positive probands and from the spectrum of tumors that have been documented in
relatives of PPB cases in the PPB Registry.[17, 18] The contribution of DICER1
mutations to different tumors is also very variable. Whilst germline DICER1 mutations
may contribute significantly to cystic nephroma, ovarian Sertoli-Leydig tumor and
intra-ocular medulloepithelioma they are unlikely to have a major impact on the
incidence of Wilms tumor, medulloblastoma/infratentorial PNET or neuroblastoma.

In addition to tumors, DICER1 mutations also appear to confer a risk of thyroid cysts.
One proband and six DICER1 mutation-positive relatives developed thyroid cysts in
childhood or young adulthood. While histology was only available for two cases, both
were non-toxic multinodular colloid goiters. This is of particular interest as the gene
for familial non-toxic multinodular thyroid goiter has been previously shown to localise
to 14q.[29]

The mutation analyses in cancer cell lines suggest that somatic DICER1 mutations
do not make a substantial contribution to cancer. This is in contrast to recent reports
hypothesising that the 14q hemizygosity observed in 206/761 cell lines is targeted at
DICER1.[19, 20, 21] In the great majority of the cell lines the 14q loss of
heterozygosity extends over a very large area and includes many genes. If loss of
DICER1 were the main driver, one would anticipate that somatic DICER1 mutations
would occur in at least some of the cell lines with normal 14q chromosomes.
However, we only identified four truncating mutations and these were in microsatellite
unstable lines and therefore are unlikely to be driver mutations. Thus the somatic
mutational profile of DICER1 appears to differ from that of other ubiquitously
expressed, critically important genes such as TP53 and RB1 that also predispose to
rare childhood cancers when mutated in the germline.
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