From 2 prospective studies of the efficacy of screening for Lynch syndrome, we identified patients with colorectal and endometrial tumors who had 2 or more somatic (but not germline) mutations in genes encoding MMR proteins (double somatic). We determined the frequencies of tumor mutations in <em>PIK3CAem>, <em>BRAFem>, <em>KRASem>, <em>NRASem>, and <em>PTENem> by targeted next-generation sequencing and used logistic-regression models to compare them with those from patients with Lynch syndrome, <em>MLH1em>-hypermethylated, or microsatellite-stable tumors. We validated our findings using independent data sets from The Cancer Genome Atlas.
Among colorectal cancer cases, we found that 14 of 21 (67%) patients with double somatic tumors also had <em>PIK3CAem> mutations, compared with 4 of 18 (22%) tumors from patients with Lynch syndrome, 2 of 10 (20%) tumors with <em>MLH1em> hypermethylation, and 12 of 78 (15%) tumors with microsatellite stability (<em>Pem> < .0001 for patients with double somatic tumors vs other subgroups). Mutations in <em>PIK3CAem> were detected in all 13 patients with double somatic endometrial cancers (<em>Pem> = .04 compared with other subgroups). We did not detect <em>BRAFem> mutations in patients with double somatic colorectal tumors or Lynch syndrome. We found highly similar results in a validation cohort from The Cancer Genome Atlas (113 patients with colorectal tumors, 178 endometrial tumors); 100% of double somatic cases had a somatic mutation in <em>PIK3CAem> (<em>Pem> < .0001 compared with other subgroups).
Most patients with colorectal or endometrial tumors with 2 or more somatic (but not germline) mutations in MMR proteins also have mutations in <em>PIK3CAem>; mutations in <em>PIK3CAem> are detected at substantially higher frequencies in these double somatic tumors than in other microsatellite-instability subgroups. <em>PIK3CAem> mutation status might be used to identify a specific group of colorectal tumors, and to select treatment or determine prognosis.