Pathological and genetic aggressiveness of left-sided breast cancer.

Abstract

e12579 Background: Breast Cancer in women consistently occurs more frequently in the left breast, with the ratio of left to right sided breast cancer cases ranging from 1.05 to 1.26. In spite of the difference in frequency, prior studies have failed to show any significant difference in clinical characteristics between left sided and right sided cancer. Methods: Genomic and clinical features were collected from The Cancer Genome Atlas breast cancer project. LVI status, mitotic rate, nuclear score and tubular score were collected from pathology reports in TIES client 5.8. Fisher's exact test was used for group comparison and survival analysis was performed with Cox regression. Cytolytic activity (CYT) indicates anti-cancer immune response and was quantified from gene expression data. Hallmark gene-sets were used for gene set enrichment analysis (GSEA). Results: Among the 1081 women with unilateral invasive breast cancer, 561 had tumor on the left side compared to 520 on the right. Our results didn’t show any significant differences between left and right side with regards to tumor location, histology, race, and tumor characteristics including stage, tumor size, nodal status and receptor status. No statistical significant differences were observed in mitotic rate, LVI status and tubular score, however, the tumor grade was significantly higher in the left side. Moreover, there were no significant differences in mutation count, CYT and overall survival between both sides. GSEA revealed cell-cycle related gene sets like G2M checkpoint, Mitotic spindle, E2F targets and MYC targets which were significantly enriched in left sided tumor. Furthermore, out of the 865 genes which were highly expressed on the left side, we identified specific genes including BRCA1, BRCA2, BRIP1, CHEK2, FANCC, PALB2, TP53 and MSH6 which are associated primarily with breast cancer genesis and mostly have established clinical management guidelines. Conclusions: Our results suggest a more aggressive nature to left sided breast cancer with a higher pathological grade perhaps requiring more aggressive treatment. Such a hypothesis needs further study to confirm or refute its validity. If confirmed, it may have a major impact with regard to biology of breast cancer and its subsequent management.