Abstract P3-06-04: Clonal evolution of non-malignant proliferative lesions into breast cancers
Authors: Nishimura T , Yoshida K , Kawata Y , Takeuchi Y , Kakiuchi N , Shiozawa Y , Aoki K , Hirata M , Kataoka TR , Sakurai T , Baba S , Shiraishi Y , Chiba K , Takeuchi K , Haga H , Miyano S , Toi M , Ogawa S 
Published: May 2019

[Introduction] Non-malignant proliferative lesions in the breast have been implicated in the development of invasive breast cancer. Previous studies showed that adjacent atypical proliferative lesions and 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 (with and without pathogenic germline mutations) of normal mammary ducts, non-malignant 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 different proliferative lesions within 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 34 samples from 5 premenopausal females carrying estrogen receptor-positive cancers, where the samples were obtained from normal ducts (N = 6), non-atypical (N = 1) and atypical (N = 8) proliferative lesions, and non-invasive (N = 16) and invasive (N = 3) cancers. The number of somatic mutations per sample ranged from 1 to 276 and increased with disease progression, regardless of the germline mutation status. 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 three unilateral cases, no pathogenic germline mutations were detected, but all proliferative lesions, which were separated by a distance of 7-25 mm, shared one or more driver alterations, such as an AKT1 mutation (UID: KU01), concurrent 1q gain and 16q loss (der(1;16)) (UID: KU02), and a GATA3 mutation and der(1;16) (UID: KU03), while harboring private mutations and/or CNAs of their own. The phylogenetic analysis based on the number of shared mutations predicted an early origin of these founder mutations, 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. These clones are originated from a common ancestor that acquired a founder mutation long before the onset of cancer, followed by branching evolution of multiple clones that acquired additional driver mutations of their own, from which an invasive cancer ultimately develops. In hereditary cases, this process is thought to be substantially promoted multi-focally from within the entire breasts by a germline mutation shared by all mammary cells, frequently resulting in bilateral and/or multifocal breast cancers. Our findings provide unique insight into the early development of breast cancer.

Citation Format: Nishimura T, Yoshida K, Kawata Y, Takeuchi Y, Kakiuchi N, Shiozawa Y, Aoki K, Hirata M, Kataoka TR, Sakurai T, Baba S, Shiraishi Y, Chiba K, Takeuchi K, Haga H, Miyano S, Toi M, Ogawa S. Clonal evolution of non-malignant proliferative lesions into breast cancers [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-06-04.