Accumulation of somatic mutation and clonal architecture of fallopian tube epithelium

Background: High-grade serous ovarian carcinoma (HGSOC) is the most common subtype of epithelial ovarian cancer, characterized by somatic TP53 mutations (≈ 96 %) and by germline or somatic BRCA1 / 2 mutations in 15~20 % of cases. Mounting evidence suggests that the fallopian tube fimbria, rather than the ovary, serves as the predominant cell-of-origin for the majority of HGSOC. Despite this evidence, a direct genomic comparison between ampullary and fimbrial epithelia has been lacking. Although somatic mutations accumulate gradually, it remains poorly defined due to challenges in reconstructing clonal lineages how these processes shape clonal architecture in normal fallopian tube epithelium. Methods: Fallopian tube epithelial organoids were derived from surgically resected tissues and cultured in 3D matrigel. We established single cell-derived clonal organoids from anatomically distinct fallopian tube epithelial regions (ampulla and fimbria) obtained from BRCA1/2 germline mutation carriers ( BRCA1 n=3; BRCA2 n=1) and one BRCA wild-type individual. Low-input DNA libraries were sequenced on the Illumina NovaSeqX (~15×) to identify somatic single-base substitutions (SBS), small insertions and deletions (indels), and structural variants (SVs), followed by mutational signature extraction and phylogenetic reconstruction to reveal clonal expansion dynamics. Results: Across the five patients analyzed, somatic mutation burden showed only a weak and statistically imprecise correlation with age, with point estimates indicating annual increases of 7.12 SBS (95% CI, -9.6 to 23.9) and 0.04 indels (95% CI, -2.65 to 2.73). Total SBS and indels burdens were comparable between ampullary and fimbrial organoids and largely unrelated to germline BRCA1/2 status, parity, or menopausal age, although these null associations should be interpreted cautiously given the limited sample size. No SVs were detected in either region. BRCA1/2 germline carriers did not exhibit HRD-associated signatures, suggesting absence of somatic-level haploinsufficiency in normal epithelium. Phylogenetic analyses identified region-specific clonal dynamics: fimbrial organoids frequently showed late clonal expansion events emerging during adolescence or later. Cancer-related variants (e.g., TP53, MET, CSMD3 ) were detected in both regions, though additional clonally expanded samples will be required to evaluate region-specific enrichment. Conclusions: Somatic mutations accumulate with age in the fallopian tube epithelium, but BRCA haploinsufficiency alone does not confer homologous recombination deficiency. Region-specific late clonal expansion in the fimbria, potentially influenced by its constrained, finger-like architecture, may underlie the disproportionately higher incidence of HGSOC originating from this anatomical site. Ongoing sample expansion is underway to refine evolutionary models.