The genetic and clinical characteristics of breast tumors with germline variants, including their association with biallelic inactivation through loss-of-heterozygosity (LOH) and second somatic mutations, remain elusive. We analyzed germline variants of 11 breast cancer susceptibility genes for 1,995 Japanese breast cancer patients, and identified 101 (5.1%) pathogenic variants, including 62 BRCA2 and 15 BRCA1 mutations. Genetic analysis of 64 BRCA1/2-mutated tumors including TCGA dataset tumors, revealed an association of biallelic inactivation with more extensive deletions, copy neutral LOH, gain with LOH and younger onset. Strikingly, TP53 and RB1 mutations were frequently observed in BRCA1- (94%) and BRCA2- (9.7%) mutated tumors with biallelic inactivation. Inactivation of TP53 and RB1 together with BRCA1 and BRCA2, respectively, involved LOH of chromosomes 17 and 13. Notably, BRCA1/2 tumors without biallelic inactivation were indistinguishable from those without germline variants. Our study highlights the heterogeneity and unique clonal selection pattern in breast cancers with germline variants.

Recent studies have documented frequent evolution of clones carrying common cancer mutations in apparently normal tissues, which are implicated in cancer development1–3. However, our knowledge is still missing with regard to what additional driver events take place in what order, before one or more of these clones in normal tissues ultimately evolve to cancer. Here, using phylogenetic analyses of multiple microdissected samples from both cancer and non-cancer lesions, we show unique evolutionary histories of breast cancers harbouring der(1;16), a common driver alteration found in roughly 20% of breast cancers. The approximate timing of early evolutionary events was estimated from the mutation rate measured in normal epithelial cells. In der(1;16)(+) cancers, the derivative chromosome was acquired from early puberty to late adolescence, followed by the emergence of a common ancestor by the patient’s early 30s, from which both cancer and non-cancer clones evolved. Replacing the pre-existing mammary epithelium in the following years, these clones occupied a large area within the premenopausal breast tissues by the time of cancer diagnosis. Evolution of multiple independent cancer founders from the non-cancer ancestors was common, contributing to intratumour heterogeneity. The number of driver events did not correlate with histology, suggesting the role of local microenvironments and/or epigenetic driver events. A similar evolutionary pattern was also observed in another case evolving from an AKT1-mutated founder. Taken together, our findings provide new insight into how breast cancer evolves.

Predicting pathogenic germline variants (PGVs) in breast cancer patients is important for selecting optimal therapeutics and implementing risk reduction strategies. However, PGV risk factors and the performance of prediction methods in the Japanese population remain unclear. We investigated clinicopathological risk factors using the Tyrer‐Cuzick (TC) breast cancer risk evaluation tool to predict BRCA PGVs in unselected Japanese breast cancer patients (n = 1,995). Eleven breast cancer susceptibility genes were analyzed using target‐capture sequencing in a previous study; the PGV prevalence in BRCA1, BRCA2, and PALB2 was 0.75%, 3.1%, and 0.45%, respectively. Significant associations were found between the presence of BRCA PGVs and early disease onset, number of familial cancer cases (up to third‐degree relatives), triple‐negative breast cancer patients under the age of 60, and ovarian cancer history (all P < .0001). In total, 816 patients (40.9%) satisfied the National Comprehensive Cancer Network (NCCN) guidelines for recommending multigene testing. The sensitivity and specificity of the NCCN criteria for discriminating PGV carriers from noncarriers were 71.3% and 60.7%, respectively. The TC model showed good discrimination for predicting BRCA PGVs (area under the curve, 0.75; 95% confidence interval, 0.69‐0.81). Furthermore, use of the TC model with an optimized cutoff of TC score ≥0.16% in addition to the NCCN guidelines improved the predictive efficiency for high‐risk groups (sensitivity, 77.2%; specificity, 54.8%; about 11 genes). Given the influence of ethnic differences on prediction, we consider that further studies are warranted to elucidate the role of environmental and genetic factors for realizing precise prediction.

[Introduction] Proliferative lesions in the breast have been implicated in the development of breast cancer. Previous studies showed that some proliferative lesions and adjacent breast cancers shared common genetic alterations, suggesting that these originated from the same ancestral cell. However, the clonal structure of normal epithelia 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 organoids (n=47) were established from breast milk of 4 healthy women aged 22–36 and normal breast tissue of 15 breast cancer patients aged 29–83 to evaluate somatic mutation rate in normal epithelial cells. Multiple normal lobules and proliferative lesions together with cancer lesions were collected using laser-capture micro-dissection (LCM) from fresh frozen (n=3) or formalin-fixed paraffin-embedded (n=5) surgical specimens in 9 premenopausal breast cancer patients. Somatic mutations and copy number alterations were evaluated using whole-genome sequencing. [Results] The mutation profile of single cell-derived organoids suggests that somatic mutations accumulate in normal mammary epithelial cells at a constant rate of 19.4/genome/year before menopause, and the mutation rate decreases to 6.9/genome/year after menopause. Parity was negatively associated with mutation number (-49.3 per life birth). In total, we analyzed 143 LCM samples, including those from 72 normal lobules, 43 proliferative lesions, and 19 non-invasive and 9 invasive cancer samples. Five cases showed a large expansion of proliferative lesions sharing a substantial number of somatic mutations with cancer. These lesions expanded over a distance of 35-90 mm, sharing tens to hundreds of mutations including those in breast cancer-related driver genes, such as PIK3CA, AKT1, GATA3, CBFB and PTEN, while harboring private mutations or copy number alterations of their own. Of interest, the cancers in 4 out of these 5 cases was luminal-A type invasive ducal carcinoma or ER-positive HER2-negative ductal carcinoma in situ, and characterized in common by the presence of der(1;16), concurrent whole-arm 1q gain and 16q loss, in both cancer and proliferative lesions. Phylogenetic analysis adapted with the mutation rate in normal cells predicted that der(1;16) had been acquired between puberty and early 20’s, and the common ancestors of non-cancerous and cancerous lesions emerged by early 30’s, &gt;10 years earlier than at the time of cancer diagnosis. By contrast, analysis of non-cancerous lobules unrelated to cancer showed that der(1;16)-negative non-cancer clones that had emerged after puberty stayed within a single lobule or spatially confined to adjacent lobules and rarely expanded to a large area as observed for those carrying der(1;16), even if the clones had acquired mutations in driver genes such as PIK3CA and PIK3R1, which highlighted the role of der(1;16) in wide clonal expansion. [Conclusions] Our results suggest that in some breast cancer cases, particularly in those with der(1;16), a highly recurrent translocation accounting for the major subset of Luminal A breast cancer, the clones with the funder driver alterations 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. Our findings provide new insight into the early development of breast cancer. Citation Format: Tomomi Nishimura, Nobuyuki Kakiuchi, Kenichi Yoshida, Takaki Sakurai, Tatsuki R. Kataoka, Eiji Kondoh, Yoshitsugu Chigusa, Masahiko Kawai, Morio Sawada, Takuya Inoue, Yasuhide Takeuchi, Hirona Maeda, Satoko Baba, Yusuke Shiozawa, Ryunosuke Saiki, Masahiro M. Nakagawa, Yasuhito Nannya, Yotaro Ochi, Tomonori Hirano, Yukiko Inagaki-Kawata, Kosuke Aoki, Masahiro Hirata, Eiji Suzuki, Masahiro Takada, Masahiro Kawashima, Kosuke Kawaguchi, Kenichi Chiba, Yuichi Shiraishi, Junko Takita, Satoru Miyano, Masaki Mandai, Kengo Takeuchi, Hironori Haga, Masakazu Toi, Seishi Ogawa. Clonal evolution of mammary epithelial cells into breast cancers [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-13-04.

The aldehyde degrading function of the ALDH2 enzyme is impaired by Glu504Lys polymorphisms (rs671, termed A allele), which causes alcohol flushing in east Asians, and elevates the risk of esophageal cancer among habitual drinkers. Recent studies suggested that the ALDH2 variant may lead to higher levels of DNA damage caused by endogenously generated aldehydes. This can be a threat to genome stability and/or cell viability in a synthetic manner in DNA repair‐defective settings such as Fanconi anemia (FA). FA is an inherited bone marrow failure syndrome caused by defects in any one of so far identified 22 FANC genes including hereditary breast and ovarian cancer (HBOC) genes BRCA1 and BRCA2 . We have previously reported that the progression of FA phenotypes is accelerated with the ALDH2 rs671 genotype. Individuals with HBOC are heterozygously mutated in either BRCA1 or BRCA2 , and the cancer‐initiating cells in these patients usually undergo loss of the wild‐type BRCA1/2 allele, leading to homologous recombination defects. Therefore, we hypothesized that the ALDH2 genotypes may impact breast cancer development in BRCA1/2 mutant carriers. We genotyped ALDH2 in 103 HBOC patients recruited from multiple cancer centers in Japan. However, we were not able to detect any significant differences in clinical stages, histopathological classification, or age at clinical diagnosis across the ALDH2 genotypes. Unlike the effects in hematopoietic cells of FA, our current data suggest that there is no impact of the loss of ALDH2 function in cancer initiation and development in breast epithelium of HBOC patients.