Transgenerational inheritance of a reversible germline epimutaiton of MLH1 predisposing to cancer. Free

LB-379

Hereditary non-polyposis colorectal cancer (HNPCC) is characterised by the development of microsatellite unstable tumors of the colorectum, endometrium and other sites at a young age. This autosomal dominant cancer predisposition syndrome is classically caused by heterozygous germline sequence mutations of the DNA mismatch repair genes, most frequently MLH1 and MSH2. In rare cases, an HNPCC-like phenotype has been associated with an epimutation of MLH1 that has arisen in the germline. Individuals with a ‘germline epimutation’ of MLH1 have a genetically normal gene sequence, but show soma-wide uniparental hypermethylation of the promoter, which is associated with transcriptional silencing of the affected allele. Epigenetic reprogramming of the genome at various stages of embryonic development should nullify the risk of transmission of germline epimutations from parent to offspring. Consistent with this, germline epimutations are not found in cases with a strong family history of HNPCC, but tend to arise sporadically. Here we show that germline epimutations are heritable in a stochastic manner through the maternal germline. We identified two new females with germline MLH1 epimutations from among 24 individuals with a clinical suspicion of HNPCC, but in whom no sequence mutations of the mismatch repair genes were identified. Both individuals had multiple primary cancers demonstrating microsatellite instability and loss of MLH1 protein expression at an early age of onset, but no family history of cancer. The first-degree relatives of both probands were studied for potential inheritance of the MLH1 epimutation by analyzing the inheritance patterns of the genetic alleles at the MLH1 locus, and the methylation status of the MLH1 promoter in their somatic tissues. One family showed transmission of the MLH1 epimutation from the proband to one of her three sons who had inherited the affected allele. This was evidenced by methylation and allelic silencing of the affected allele in the son’s somatic tissues, placing him at considerable risk of developing cancer. However, there was no trace of methylation in the son’s spermatozoa, and MLH1 was biallelically expressed, indicating erasure of the epimutation in his germline. In the two siblings who had also inherited the affected maternal, this had reverted to the unmethylated state, with concomitant allelic reactivation. In the second family, one son had also inherited the affected allele, but this had also reverted to the normal functional state. The case reported here is consistent with trans-generational inheritance of a ‘reversible’ epigenetic error at a single gene locus, providing a novel pattern of non-Mendelian inheritance that confers disease susceptibility.