Pathogenic germline variants in patients with early-onset colorectal cancer according to phenotype.

We assessed retrospectively the prevalence of pathogenic germline variants (PGV) in 268 French adult patients diagnosed with colorectal cancer (CRC) before age 41, stratified by phenotype. APC, BMPR1A, CDH1, EPCAM, MLH1, MSH2, MSH3, MSH6, MUTYH, NTHL1, POLE, POLD1, PTEN, PMS2, SMAD4, STK11 and TP53 were analyzed. Overall, 21.6% of cases carried a PGV. A high prevalence was observed in Mismatch Repair-deficient (MMRd) CRC (60.1%, MMR genes) and polyposis-associated CRC (48%, APC, MUTYH and MSH3-biallelic, POLE). Only 2.3% of patients with MMR proficient and without polyposis carried a PGV. The genes involved in this third group were POLE and MSH2, and three out of four cases had either two synchronous CRC or a CRC family history. Phenotypic features should be taken into account for testing decision. Evaluating the cost-effectiveness of testing all CRC cases < 41 years, as well as how it aligns with the constraints of various healthcare systems, is warranted.