Abstract Recent studies have reported cancer-associated mutations in normal endometrium. Mutations in eutopic endometrium may lead to endometriosis and endometriosis-associated ovarian cancer. We investigated PIK3CA mutations ( PIK3CA m) for three hotspots (E542K, E545K, H1047R) in eutopic endometrium in patients with ovarian cancer and endometriosis from formalin-fixed paraffin-embedded specimens by laser-capture microdissection and droplet digital PCR. The presence of PIK3CA m in eutopic endometrial glands with mutant allele frequency ≥15% were as follows: ovarian clear cell carcinoma (OCCC) with PIK3CA m in tumors, 20/300 hotspots in 11/14 cases; OCCC without PIK3CA m, 42/78 hotspots in 11/12 cases; high-grade serous ovarian carcinoma, 8/45 hotspots in 3/5 cases; and endometriotic cysts, 5/63 hotspots in 5/6 cases. These rates were more frequent than in non-cancer non-endometriosis controls (7/309 hotspots in 5/17 cases). In OCCC without PIK3CA m, 7/12 (58%) cases showed multiple hotspot mutations in the same eutopic endometrial glands. In 3/54 (5.6%) cases, PIK3CA m was found in eutopic endometrial stroma. Multi-sampling of the OCCC tumors with PIK3CA m showed intratumor heterogeneity in three of eight cases. In two cases, PIK3CA m was detected in the stromal component of the tumor. Homogenous PIK3CA m in the epithelial component of the tumor matched the mutation in eutopic endometrial glands in only one case. Eutopic endometrial glands in ovarian cancer and endometriosis show high frequency of PIK3CA m that is not consistent with tumors, and multiple hotspot mutations are often found in the same glands. Our results suggest that most PIK3CA m in eutopic endometrial glands are passenger rather than driver mutations.

Frequent PIK3CA mutations in eutopic endometrium of patients with ovarian clear cell carcinoma