A novel heterozygous large deletion of MSH6 gene in a Chinese family with Lynch syndrome.

Lynch syndrome (LS) is a common cancer syndrome that is inherited in an autosomal dominant manner. Its pathogenesis is thought to be closely related to germline mutations of mismatch repair (MMR) genes such as the MLH1, MSH2, PMS2 and MSH6 genes. This study identifies a Chinese family with LS clinically diagnosed according to the Amsterdam II criteria. In these patients, immuno-histochemical staining showed negative MSH6 expressions but positive MLH1, MSH2, and PMS2 expressions. In order to further explore the molecular biology of this LS family, we used targeted next-generation sequencing (NGS) and Multiplex ligation dependent probe amplification (MLPA) to identify the mutation and verify the authenticity of the mutation in 15 family members. For NGS, two panels have been used, one is of MLH1, MSH2, PMS2 and MSH6 genes, the other one is of 139 cancer genetic susceptibility genes. And for the large deletions/duplications can also be identified by NGS panel, an adjusted data analysis strategy of NGS has been used. As a result, we identified a novel heterozygous large deletion in MSH6 gene that was found to be co-segregated among affected family members. This deletion results in the loss of a 3246 bp-sized fragment in MSH6 gene exons 5-9 which represents the coding regions of the MSH6 ATPase domain. This novel mutation has yet to be documented in the International Society for Gastrointestinal Hereditary Tumours (InSiGHT) database. This mutation resulted in MSH6 protein losing gene mismatch repair function, and further caused the microsatellite instable. We speculate that this mutation may significantly impact MMR function through impaired ATP domain function. Theoretically, this proband would likely benefit from PD-1 immune check-point blockade therapy, but conversely, we observed that tumors appeared to rapidly progress after 4 sessions of anti-PD-1 treatment. Further studies to validate the effectiveness of anti-PD-1 treatments in carriers of this mutation are necessary.