GERMLINE BIALLELIC MUTATIONS IN MCM8 ARE ASSOCIATED WITH EARLY-ONSET LYNCH-LIKE SYNDROME

GOLUBICKI, MARIANO; LAIA BONJOCH ; JOSÉ G. ACUÑA-OCHOA; MARCOS DIAZ-GAY; JENNIFER MUÑOZ; MIRIAM CUATRECASAS; TERESA OCAÑA; ROBBIO, JUAN; ROCA, ENRIQUE; CASTELLS, ANTONI; BALAGUER, FRANCESC; ANTELO, MARINA; CASTELLVI-BELL, SERGI

París

Congreso; European Human Genetics Virtual Conference; 2020

The European Society of Human Genetics

Purpose: The most frequent cause of hereditary colorectal cancer (CRC) is Lynch syndrome, accounting for 3% of all CRC cases. The underlying germline predisposition is a mutation in any of the four mismatch repair (MMR) genesM (LH1, MSH2, MSH6, PMS2), while microsatellite instability(MSI) is a hallmark of these tumors. However, CRC cases with MSI, noM LH1 somatic hypermethylation and absence of MMR germline mutationaccount for up to 60% of cases of MMR-defective tumors. They have been termed Lynch-like syndrome (LLS) and treatment management decisionsare complicated due to unconfirmed suspicions of hereditary cancer. The aim of the present study was to investigate whether LLS cases in patientsunder the age of 40 carry potentially pathogenic germline variants in new genes for CRC predisposition causing a MMR-defective tumor phenotype.Methods: We performed whole-exome sequencing (WES) in the germline and tumoral DNA of 16 early-onset LLS patients. We filtered for recessivegermline variants in genes involved in DNA repair. An exhaustive functional evaluation for M2 CM8 genetic variants detected in one patient wasperformed. The CRISPR/Cas9 system was used to generateM CM8ko clones in a cell model (DLD-1). Site-directed mutagenesis produced the geneticvariants for ectopic expression in the MCM8ko model. Functional characterization included the comet assay, which detected DNA damage induced byoxaliplatin in the transfected cells. MCM8ko cells were evaluated for MSI after 30, 60 and 90 days of culture, and mutational signatures were assessedwith SigProfiler. Results: The MCM8 variants c.351_354delAAAG (p.Lys118GlufsTer5, p.K118fs) and c.414A>G (p.Ile138Met, p.I138M) were identified in one early-onset LLS patient with biallelic somatic inactivation of MLH1. Manual evaluation of WES data showed that germliMneC M8 variants were intrans. In the comet assay, both MCM8 variants revealed higher sensitivity to oxaliplatin and more DNA damage compared to the wild-type control.One of the MCM8KO clones showed MSI after 30 days of sub-culturing. After 120 daysM, CM8KO clones had acquired a significant contribution of theSBS20 mutational signature, which is associated with defective MMR DNAC. onclusions: We postulate that MCM8 is a new germline CRC predisposinggene in early-onset LLS, which may be involved in both MMR and double-strand break repair. We further provide evidence that in some LLS CRCcases, especially in young patients, the biallelic somatic MMR inactivation may be caused by new CRC germline predisposing genes. Additionalmutational screening is warranted for such cases.