A novel mutational mechanism

1. A novel mutational mechanism

2. Hypermethylation • Methylation is known as a method of gene silencing • Hypermethylation can occur somatically in MLH1 and MSH2 tumours • Some v. rare cases of heritable MSH2 hypermethylation described, cause unknown • Thought that any hypermethylation imprint would normally be erased during gametogenesis – mechanism currently unknown

3. The initial cases • Dutch family with MSI high colorectal cancer and loss of MSH2/MSH6 on IHC • No mutation identified despite comprehensive mutation scanning • Routine MLPA identified a deletion 16kb upstream of MSH2 in exon 9 of the TACSTD1 gene • The deletion leaves the MSH2 promoter intact • Screening of further Dutch families with unexplained HNPCC revealed four further cases with the same deletion • Haplotype analysis indicate a founder mutation • Mosaic promoter hypermethylation present

4. Further cases • A Chinese family previously reported with heritable methylation of MSH2 also showed a deletion of exons 6-9 at the 3’ end of TACSTD1 • Screening of further Chinese HNPCC cases showed a further family with a TACSTD1 deletion and MSH2 methylation • The 2 Chinese families had the same deletion but haplotype analysis indicated different origins

5. Conclusions • TACSTD1 deletion removes the TACSTD1 polyadenylation signal • Normally the TACSTD1 transcript terminates no more than 35bp 3’ of the polyadenylation signal • Transcripts from Dutch and Chinese families show hybrid TACSTD1/MSH2 mRNA • Transcript read through then causes the MSH2 CpG island to become hypermethylated • This inactivates MSH2 leading to a phenotype of HNPCC • A novel mutational mechanism • This effect may occur in other genes and also from transcript read through on opposite DNA strand