Abstract
AIMS: Various inherited and acquired alterations affecting the genes
and gene products of the WNT pathway appear to be involved in the
different molecular routes leading to colorectal cancer (CRC). This
study was initiated to investigate the prevalence of somatic mutations
in the beta-catenin gene (CTNNB1) and the associated pathology in
CRC with defective DNA mismatch repair. METHODS: Paraffin and/or
frozen sections of 33 primary CRC (including any liver and lymph
node metastases present) with high-grade microsatellite instability
(MSI-H; i.e. with > or = 5 unstable microsatellite markers of 10
tested) were polytopically fractionated by microdissection. Genomic
and c-DNA samples were sequenced across exons 2-4 of CTNNB1 and the
expression patterns of beta-catenin (beta-C) analyzed by immunohistology
and Western blotting. RESULTS: Seven somatic mutations affecting
phosphorylation sites of exon 3 (2 deletions also encompassing parts
of either intron 2 or exon 4 delta X2/3 bzw. delta X3/4 and 5 missense
mutations 2 x T41A, 2 x S45F, S45P) were identified. Two mutations
(delta X3/4 and S45F) were concordantly present in CRC primaries
and their respective metastases whereas the S45P mutation was restricted
to a hepatic metastasis. In the delta X2/3 CRC primary only a shortened
66 kD CTNNB1 gene product was present while its associated liver
metastasis showed a total loss of beta-C expression. CONCLUSIONS:
Both exon 3 and the entire locus coding for beta-C are somatically
altered in approximately 20% of CRC with MSI-H at different stages
of tumor progression. Thus CTNNB1 appears to be a genomic target
for complex oncogenic mutations and deletional processes in a substantial
fraction of this molecular subset of CRC.
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