Extended microsatellite analysis in microsatellite stable, MSH2 and MLH1 mutation-negative HNPCC patients: Genetic reclassification and correlation with clinical features

Background: Hereditary nonpolyposis colorectal cancer (HNPCC) is an
autosomal dominant disorder predisposing to predominantly
colorectal cancer (CRC) and endometrial cancer frequently due to
germline mutations in DNA mismatch repair (MMR) genes, mainly MLH1,
MSH2 and also MSH6 in families seen to demonstrate an excess of
endometrial cancer. As a consequence, tumors in HNPCC reveal
alterations in the length of simple repetitive genomic sequences
like poly-A, poly-T, CA or GT repeats (microsatellites) in at least
90% of the cases. Aim of the Study: The study cohort consisted of
25 HNPCC index patients ( 19 Amsterdam positive, 6 Bethesda
positive) who revealed a microsatellite stable (MSS) - or low
instable (MSI-L) - tumor phenotype with negative mutation analysis
for the MMR genes MLH1 and MSH2. An extended marker panel (BAT40,
D10S197, D13S153, D18S58, MYCL1) was analyzed for the tumors of
these patients with regard to three aspects. First, to reconfirm
the MSI-L phenotype found by the standard panel; second, to find
minor MSIs which might point towards an MSH6 mutation, and third,
to reconfirm the MSS status of hereditary tumors. The
reconfirmation of the MSS status of tumors not caused by mutations
in the MMR genes should allow one to define another entity of
hereditary CRC. Their clinical features were compared with those of
150 patients with sporadic CRCs. Results: In this way, 17 MSS and 8
MSI-L tumors were reclassified as 5 MSS, 18 MSI-L and even 2 MSI-H
( high instability) tumors, the last being seen to demonstrate at
least 4 instable markers out of 10. Among all family members, 87
malignancies were documented. The mean age of onset for CRCs was
the lowest in the MSI-H-phenotyped patients with 40.5 +/- 4.9 years
(vs. 47.0 +/- 14.6 and 49.8 +/- 11.9 years in MSI-L- and
MSS-phenotyped patients, respectively). The percentage of CRC was
the highest in families with MSS-phenotyped tumors (88%), followed
by MSI-L-phenotyped ( 78%) and then by MSI-H-phenotyped (67%)
tumors. MSS tumors were preferentially localized in the distal
colon supposing a similar biologic behavior like sporadic CRC. MSH6
mutation analysis for the MSI-L and MSI-H patients revealed one
truncating mutation for a patient initially with an MSS tumor,
which was reclassified as MSI-L by analyzing the extended marker
panel. Conclusion: Extended microsatellite analysis serves to
evaluate the sensitivity of the reference panel for HNPCC detection
and permits phenotype confirmation or upgrading. Additionally, it
confirms the MSS status of hereditary CRCs not caused by the common
mutations in the MMR genes and provides hints to another entity of
hereditary CRC. Copyright (C) 2004 S. Karger AG, Basel.