Bias in detection of instability of the (C)8 mononucleotide repeat of MSH6in tumours from HNPCC patients

Recently, we and others reported instability in the (C)8 repeat in exon 5 o f MSH6 as a preferential target for somatic mutations in tumours from MSH6 germline mutation carriers. Here, we report that in 45% of tumours from MLH 1, MSH2 and MSH6 germline mutation carriers no sequence change in the (C)8 repeat of MSH6 was found upon DNA sequencing analysis of PCR products with a shift in electrophoresis mobility. Using 'standard' PCR primers a high fr equency of instability (50-86%) of the (C)8 repeat was found, but using a m odified PCR reverse primer, accomplishing modulation of non-templated addit ion of adenine during in vitro PCR amplification by the Taq polymerase, a m arkedly lower frequency of instability was found in turnours from MLH1, MSH 2 and MSH6 mutation carriers (6, 13 and 40%, respectively). Furthermore, a significant difference of the frequency of instability of the (C)8 repeat i n tumours from MSH6 mutation carriers was found compared to MLH1, MSH2 muta tion carriers. These results might have important implications for the dete ction of instability of other short mononucleotide repeats, e.g. TGF beta R II, BAX, IGFRII, PTEN, BRCA2.