Abstract P4-05-15: Clonal evolution of proliferative lesions into breast cancers

Abstract

Abstract [Introduction] Proliferative lesions in the breast have been implicated in the development of invasive breast cancer. Previous studies showed that atypical proliferative lesions and adjacent breast cancers shared common genetic alterations, suggesting that these evolved from the same ancestral cell. However, the clonal structure of atypical proliferative lesions and their clonal dynamics during progression to cancer are poorly understood. In this study, we compared genetic profiles of proliferative lesions and cancer tissues from the same patients to illustrate the clonal evolution of cancer from a non-malignant epithelial cell. [Methods] Multiple samples were collected from various lesions with different disease-level located in the cancer-borne breast, including invasive cancers, using micro-dissection from formalin-fixed, paraffin-embedded surgical specimens. Somatic mutations and copy number alterations (CNAs) were then evaluated by whole exome sequencing. [Results] We analyzed a total of 39 samples from 6 premenopausal females carrying estrogen receptor-positive cancers, where the samples were obtained from non-atypical (N = 1) and atypical (N = 8) proliferative lesions, and non-invasive (N = 21) and invasive (N = 4) cancers. The number of somatic mutations per sample ranged from 2 to 311 and increased with disease progression. Two cases with bilateral cancers had a pathogenic germline mutation of either BRCA2 or TP53, where no somatic mutations or CNAs were shared by individual proliferative lesions, suggesting multifocal independent cancerous evolutions. By contrast, in the remaining four unilateral cases, no pathogenic germline mutations were detected, but all proliferative lesions and cancers, which were separated by a distance of 7- 33 mm, shared one or more driver alterations, such as an AKT1 mutation (UID: KU01), a GATA3 mutation (UID: KU03 and KU06), a CBFB mutation (UID: KU06) and concurrent 1q gain and 16q loss (UID: KU02, KU03 and KU06), while harboring private mutations/CNAs of their own. The phylogenetic analysis based on the number of shared mutations predicted an early origin of these founder genetic alterations, which frequently predated decades before the onset of cancer. [Conclusions] Our results suggest that early breast cancer development is shaped by the evolution of multiple precancerous clones. In hereditary cases, this process is thought to be substantially promoted multi-focally within the entire breasts, due to innate genomic instability in each mammary epithelial cell for pathogenic germline mutations. By contrast, in sporadic cases, the ancestral cell which has acquired a founder genetic alteration expands macroscopically long before the onset of cancer, most likely in early adolescent. These expanded clones work as the niche predisposed to additional mutations, in which branching evolution occurs multi-focally and raises up multiple proliferative lesions, from which invasive cancer finally evolves. Our findings provide unique insight into the early development of breast cancer. Citation Format: Tomomi Nishimura, Kenichi Yoshida, Nobuyuki Kakiuchi, Yasuhide Takeuchi, Yukiko Kawata, Hirona Maeda, Yusuke Shiozawa, 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 proliferative lesions into breast cancers [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P4-05-15.