Abstract 1614: Macroscopic clonal expansion with driver mutations in normal human endometrium

Abstract Introduction: The epithelial cells of the endometrium undergo rapid proliferation and shedding each month under the influence of estrogen and progesterone. Recent studies have demonstrated pervasive driver mutations in the endometrium with ageing. However, previous observations have been limited to the microscopic scale, and it is still unclear how clonal expansion occurs over the entire endometrium and what driver mutations are responsible therein. To address these issues, we performed multisampling of the endometrium with extensive spatial mapping. Methods: We collected multiple endometrial glands from normal endometrium in a 5mm lattice in 19 patients aged 30 to 85 years who underwent hysterectomy (3-31 samples per case), followed by DNA extraction, detected somatic mutations by whole exome sequencing, to identify large clones that spread across samples based on shared mutations. Results: In total, 213 bulk endometrial glands were analyzed. The number of detected somatic mutations per sample ranged from 4 to 81 (mean±SD: 32. 5±14. 1), with at least one known driver gene mutation detected in 204 of 213 samples (mean±SD: 3. 03±1. 78). The mutational signature analysis revealed the predominance of the age-related SBS1 and SBS5 signatures. dN/dS analysis identified 20 genes that were positively selected in normal endometrium, including PIK3CA, PIK3R1, KRAS, PPP2R1A, ARHGAP35, and FBXW7, most of which are also known as driver genes for endometrial cancer. The number of mutations and their frequency for each driver gene did not differ by background diseases: endometriosis, endometriosis-associated ovarian cancer and other non-endometrial related diseases. In contrast to the high frequency of driver gene mutations, copy-number alterations were found in only 3 samples, such as +1q, +7, +10 and +12. We found clones showing a large expansion involving multiple samples in 8 cases, most of which harbored one or more driver mutations, such as PIK3CA and KRAS. The size of the largest clone carrying a KRAS mutation, which was found in a 32-year-old woman with cervical cancer, spanned as long as 10 mm in diameter. This suggests that clonal expansion in the endometrium could be very rapid even in healthy women, which contrasts to the observations that clonal expansion in other organs is usually seen in aged individuals. Conclusion: We confirmed the high frequency of driver mutations in the normal endometrium, most of which overlapped to those found in endometrial cancers, suggesting that these driver genes are involved in the early phase of the development in the endometrial cancer. Some of the driver-mutated clones expanded in macroscopic size. Our findings provide an interesting insight into the clonal structure and underlying genetic variation in the normal endometrium. Citation Format: Koichi Watanabe, Nobuyuki Kakiuchi, Kosuke Ieiri, Hirona Maeda, Tomonori Hirano, Sachiko Kitamura, Mana Taki, Koji Yamanoi, Ryusuke Murakami, Ken Yamaguchi, Junzo Hamanishi, Hiroko Tanaka, Satoru Miyano, Masaki Mandai, Seishi Ogawa. Macroscopic clonal expansion with driver mutations in normal human endometrium abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 1614.