Abstract 2366: Germline biallelic mutations in MCM8 are associated with early-onset Lynch-like syndrome
Authors: Golubicki Mariano , Bonjoch Laia , Acuña-Ochoa José G. , Diaz-Gay Marcos , Muñoz Jennifer , Cuatrecasas Miriam , Ocaña Teresa , Robbio Juan , Roca Enrique L. , Castells Antoni , Balaguer Fracesc , Antelo Marina , Castellvi-Bel Sergi 
Published: September 2020

Abstract
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) genes (MLH1, MSH2, MSH6, PMS2), while microsatellite instability (MSI) is a hallmark of these tumors. However, CRC cases with MSI, no MLH1 somatic hypermethylation and absence of MMR germline mutation account for up to 60% of cases of MMR-defective tumors. They have been termed Lynch-like syndrome (LLS) and treatment management decisions are complicated due to unconfirmed suspicions of hereditary cancer. The aim of the present study was to investigate whether LLS cases in patients under 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 recessive germline variants in genes involved in DNA repair. An exhaustive functional evaluation for 2 MCM8 genetic variants detected in one patient was performed. The CRISPR/Cas9 system was used to generate MCM8ko clones in a cell model (DLD-1). Site-directed mutagenesis produced the genetic variants for ectopic expression in the MCM8ko model. Functional characterization included the comet assay, which detected DNA damage induced by oxaliplatin in the transfected cells. MCM8ko cells were evaluated for MSI after 30, 60 and 90 days of culture, and mutational signatures were assessed with SigProfiler.
Results: The MCM8 variants c.351_354delAAAG (p.Lys118GlufsTer5, p.K118fs) and c.414A&gt;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 germline MCM8 variants were in trans. 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 days, MCM8KO clones had acquired a significant contribution of the SBS20 mutational signature, which is associated with defective MMR DNA.
Conclusions: We postulate that MCM8 is a new germline CRC predisposing gene in early-onset LLS, which may be involved in both MMR and double-strand break repair. We further provide evidence that in some LLS CRC cases, especially in young patients, the biallelic somatic MMR inactivation may be caused by new CRC germline predisposing genes. Additional mutational screening is warranted for such cases.
Citation Format: Mariano Golubicki, Laia Bonjoch, José G. Acuña-Ochoa, Marcos Diaz-Gay, Jennifer Muñoz, Miriam Cuatrecasas, Teresa Ocaña, Juan Robbio, Enrique L. Roca, Antoni Castells, Fracesc Balaguer, Marina Antelo, Sergi Castellvi-Bel. Germline biallelic mutations in MCM8 are associated with early-onset Lynch-like syndrome [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2366.