Pathological exon skipping in an HNPCC proband with MLHI splice acceptor site mutation

Abstract

One of the most commonly mutated mismatch repair genes in human nonpolyposis colorectal cancer (HNPCC) is MLHI. We identified a splice site mutation in MLHI in a colorectal cancer proband (T-to-A at position - II of intron I splice acceptor) and investigated its functional consequences by RT-PCR, using lymphocyte mRNA from the proband, two noncarrying siblings, and one unrelated individual. Subcloning of PCR products followed by sequencing of individual clones revealed increased transcript heterogeneity in the mutation carrier, attributable to the presence of a variety of mRNA forms lacking exon 2, or combinations of exons 2, 4, 6, 9, and 10. The full-length transcript subcloned from the mutation carrier was detected with a much reduced frequency, suggesting that only the wild-type allele produced functional MLHI mRNA. The three noncarriers expressed some previously described transcripts and several novel variants, but none that lacked exon 2. The results are consistent with the hypothesis that this splice site mutation causes skipping of MLHI exon 2 in a large proportion of mRNA transcripts derived from the mutated allele. Such an observation strengthens the case for identifying the mutation as pathogenic in this HNPCC family, which is of interest given the rarity of exon skipping defects resulting from splice acceptor site mutations outside the invariant AG dinucleotide. (C) 2000 Wiley-Liss, Inc.