Bias in Detection of Instability of the (C)8 Mononucleotide Repeat of MSH6 in Tumours from HNPCC Patients.

Recently, we and others reported instability in the (C)8 repeat in exon 5 of MSH6 as a preferential target for somatic mutations in tumours from MSH6 germline mutation carriers. Here, we report that in 45% of tumours from MLH1 , 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 frequency of instability (50–86%) of the (C)8 repeat was found, but using a modified PCR reverse primer, accomplishing modulation of non-templated addition of adenine during in vitro PCR amplification by the Taq polymerase, a markedly lower frequency of instability was found in tumours from MLH1 , MSH2 and MSH6 mutation carriers (6, 13 and 40%, respectively). Furthermore, a significant difference of the frequency of instability of the (C)8 repeat in tumours from MSH6 mutation carriers was found compared to MLH1 , MSH2 mutation carriers. These results might have important implications for the detection of instability of other short mononucleotide repeats, e.g. TGFβRII , BAX , IGFRII , PTEN , BRCA2 .