Medulloblastoma exome sequencing uncovers subtype-specific somatic mutations

PMID: 22820256 · DOI: 10.1038/nature11329 · Journal: Nature (2012)

TL;DR

Whole-exome sequencing of 92 medulloblastoma/normal pairs identified 12 significantly mutated genes across molecular subtypes (WNT, SHH, Group 3, Group 4). The study discovered recurrent mutations in the RNA helicase DDX3X (often concurrent with CTNNB1 in WNT tumors) and in nuclear co-repressor complex genes (GPS2, BCOR, LDB1). Functional assays demonstrated that mutant DDX3X potentiates mutant beta-catenin transcriptional activity, implicating DDX3X as a component of pathogenic WNT signaling in medulloblastoma.

Cohort & data

• 92 primary medulloblastoma/normal pairs sequenced by whole-exome hybrid capture (193,094 exons from 18,863 genes; median 106X coverage).
• Cancer type: MBL (medulloblastoma).
• Subtypes represented: 6 WNT, 23 SHH, 33 Group 3, 30 Group 4.
• Dataset: mbl_broad_2012.
• Methods: whole-exome-seq, MutSig for significance testing.
• Validation by microfluidic PCR (Fluidigm) + single-molecule real-time sequencing (Pacific Biosciences); 19/20 mutations confirmed.
• Samples from Children’s Hospital Boston, Hospital for Sick Children Toronto, and Children’s Oncology Group/CHTN.

Key findings

• Medulloblastomas have a low mutation rate: median 0.35 non-silent mutations per megabase (median 12 non-silent mutations per tumor).
• 12 genes mutated at statistically significant frequency (q < 0.1): CTNNB1, PTCH1, KMT2D (MLL2), DDX3X, GPS2, TP53, KDM6A, BCOR, SMARCA4, LDB1, CTDNEP1, CSNK2B.
• Significantly mutated genes were absent from Group 3 and Group 4 tumors with extensive somatic copy number alteration, suggesting these subtypes are driven primarily by structural variation rather than point mutations.
• DDX3X mutated in 7 tumors (half of WNT subgroup, p = 0.005), with mutations clustering in helicase ATP-binding and C-terminal domains.
• Histone methyltransferase gene set enriched for somatic mutation: 21 tumors with predominantly loss-of-function HMT mutations (q = 5.8 x 10^-9).
• N-CoR complex genes (GPS2, BCOR, LDB1) recurrently mutated — a novel finding in medulloblastoma.
• Older patients (16-31 years) had higher mutation frequency (p = 7.7 x 10^-5, Wilcoxon rank-sum test).

Genes & alterations

Gene	Alteration	Subtype	Finding
CTNNB1	Missense (activating)	WNT	Found in all WNT tumors; concurrent with chr6 loss
PTCH1	Inactivating (+ 9q LOH)	SHH	Exclusive to SHH subgroup
DDX3X	Missense (helicase domains)	WNT, SHH	Potentiates mutant beta-catenin transactivation
KMT2D	Inactivating	Multiple	Recurrent loss-of-function; histone modification
KMT2C	Inactivating	Group 4	Subtype-specific MutSig hit (q = 0.039)
SMARCA4	Missense (helicase domains)	Group 3, WNT	SWI/SNF complex disruption
TP53	Inactivating	WNT	Co-occurs with CTNNB1 mutations
GPS2	Missense (aa 53-90)	Group 3	N-CoR complex; NCOR2 interaction domain
BCOR	Frameshift, nonsense	SHH	X-linked; hemizygous loss-of-function
LDB1	Missense, nonsense	SHH	N-CoR complex assembly; hemizygous via 10q loss
KDM6A	Inactivating	Group 4	Exclusive to i17q-only tumors (p = 0.0023)
CTDNEP1	Frameshift, splice site	Group 3	Phosphatase; hemizygous via i17q
SUFU	Splice site, germline LOF	SHH	Hedgehog pathway tumor suppressor
NCOR2	Nonsense	SHH	Nuclear co-repressor complex
CSNK2B	Recurrent	WNT	Co-occurs with CTNNB1

Clinical implications

• Molecular subtyping identifies biologically distinct medulloblastoma groups with different mutational landscapes, supporting subtype-directed therapeutic strategies.
• DDX3X as a component of pathogenic WNT/beta-catenin signaling suggests potential therapeutic vulnerability in WNT-subgroup tumors.
• Histone methyltransferase and N-CoR complex disruption across subtypes may indicate sensitivity to epigenetic therapies.
• Germline PTCH1 and SUFU variants with somatic LOH in SHH tumors have implications for genetic counseling and cancer predisposition screening.

Limitations & open questions

• Discovery cohort of 92 samples limits power for rare mutations, especially within individual subtypes.
• Group 3 and Group 4 tumors showed few significantly mutated genes — their drivers (likely structural variants) remain incompletely characterized.
• Functional validation of DDX3X limited to TCF reporter and cell viability assays in one medulloblastoma cell line (D425) and HeLa; in vivo studies not performed.
• The mechanism by which mutant DDX3X potentiates beta-catenin signaling remains unclear (helicase-dependent vs. -independent).
• Whether DDX3X mutations in other cancers (CLL, head and neck) similarly cooperate with WNT pathway activation requires investigation.

Citations from this paper used in the wiki

• “We identified twelve genes mutated at statistically significant frequencies, including previously known mutated genes in medulloblastoma such as CTNNB1, PTCH1, MLL2, SMARCA4 and TP53.” (Abstract)
• “Recurrent somatic mutations were identified in an RNA helicase gene, DDX3X, often concurrent with CTNNB1 mutations, and in the nuclear co-repressor (N-CoR) complex genes GPS2, BCOR, and LDB1, novel findings in medulloblastoma.” (Abstract)
• “Overall, medulloblastomas exhibit low mutation rates consistent with other pediatric tumors, with a median of 0.35 non-silent mutations per megabase.” (Abstract)
• “In combination with mutant beta-catenin (S33Y substitution), the majority of DDX3X point mutants in our cohort potentiated reporter activity (Figure 3b; p < 0.05).” (Page 5)
• “Strikingly, these genes were not mutated in c5 (Group 3) and c4 (Group 4) tumors with extensive somatic copy number alteration, suggesting these subtypes are driven primarily by structural variation, rather than base mutation.” (Page 3)

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