Abstract 6085: Clonal evolution of mammary epithelial cells into breast cancers

Abstract [Introduction] Previous studies suggested that breast cancers (BCs) and some adjacent atypical proliferative lesions evolved from the same ancestral cell. However, the clonal structure of normal epithelial cells and their clonal history during evolution to cancer are poorly understood. In this study, we analyzed genetic profiles of normal epithelia and proliferative lesions in the cancer-borne breast to illustrate the clonal evolution of cancer from a normal epithelial cell. [Methods] Single cell-derived normal epithelial organoids (n=47) were established from breast milk of 4 healthy women aged 22-36 and surgical specimens of 15 BC patients aged 29-83 to evaluate somatic mutation rate. Multiple normal lobules or proliferative lesions together with cancer cells were collected using laser-capture micro-dissection (LCM) from fresh frozen (n=3) or formalin-fixed paraffin-embedded (n=6) surgical specimens in 9 premenopausal BC patients. Somatic mutations and copy number alterations were evaluated using whole-genome sequencing. [Results] On the basis of single cell-derived mammary gland-derived organoids, somatic mutations increased in mammary gland cells at a constant rate of 19.4/genome/year before menopause, which then decreased to 6.9/genome/year after menopause. Parity was negatively associated with mutation number (-49.3 per life birth). In total, we analyzed 113 LCM samples, including those from 66 normal lobules, 28 proliferative lesions, and 14 non-invasive and 5 invasive cancer samples. Phylogenetic analysis showed that most of the neighboring normal lobules (53/66) from 3 BC patients shared no somatic mutations with each other and therefore were clonally independent. In other 13 normal lobules, shared mutations including a PIK3CA mutation were observed across multiple adjacent samples, although they were still confined to <10 mm regions. By contrast, we found a large expansion of proliferative lesions sharing a substantial number of somatic mutations with cancer in 5 out of the 6 remaining BC cases. They were expanded over a distance of 25-85 mm, sharing tens to hundreds of mutations including those in BC-related driver genes, such as PIK3CA, AKT1, GATA3, CBFB and PTEN. Of interest, Tumors in 4 out of these 5 cases was Luminal-A type and characterized in common by the presence of 1q gain and 16q loss (1q+/16q-) in both cancer and proliferative lesions. Phylogenetic analysis adapted with the mutation rate in normal cells predicted that 1q+/16q- had been acquired during adolescence to mid-20s, where the clonal expansion had occurred years to decades before the onset of cancer. [Conclusions] Our results suggest that in some BC cases, particularly in those with 1q+/16q-, the clones with the founder driver mutations expanded macroscopically long before the onset of cancer, in which further clonal evolutions recursively occur multi-focally, giving rise to multiple proliferative lesions and ultimately, invasive cancers. Citation Format: Tomomi Nishimura, Nobuyuki Kakiuchi, Kenichi Yoshida, Yasuhide Takeuchi, Hirona Maeda, Yusuke Shiozawa, Masahiro M. Nakagawa, Ryunosuke Saiki, Yotaro Ochi, Tomonori Hirano, Yukiko Kawata, Kosuke Aoki, Masahiro Hirata, Tatsuki R. Kataoka, Takaki Sakurai, Satoko Baba, Yuichi Shiraishi, Kenichi Chiba, Kengo Takeuchi, Hironori Haga, Satoru Miyano, Masakazu Toi, Seishi Ogawa. Clonal evolution of mammary epithelial cells into breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6085.