Genotyping and Phenotyping of Male Breast Cancer

Male breast cancer is a rare disease and most of the knowledge has been extrapolated from females, although these entities are likely different. A better understanding of male breast carcinogenesis is crucial for developing novel targets suitable for personalized treatment. A major problem in studying male breast cancer is its rareness and the lack of large series. For the present thesis a large multi-institutional cohort of male breast cancer patients was collected to study phenotype and genotype of male breast cancer. A total 134 male breast cancer patients were collected, which represents one of the largest groups of male breast cancers published until now. In the present thesis several biomarkers of aggressive male breast cancer were identified: high mitotic count, high grade, luminal type B, fibrotic focus, HER2 overexpression, high Ki67, p21, p53, Glut-1 and HIF-1α, low progesterone receptor and Bcl-2 expression. Progesterone receptor, p53 and HIF-1α were the most promising prognostic markers, as they were independent markers for patient survival. Copy number gain of several genes was found to be involved in male breast cancer of which CCND1, TRAF4, CDC6 and MTDH copy number gain (involved in >40% of cases) seem to be the most important of the 25 breast cancer related genes tested. The number of amplified genes and several single amplified genes were associated with high grade, but only CCND1 amplification was an independent predictor of adverse survival. Promoter hypermethylation in male breast cancer had not been studied before. Promoter hypermethylation of the genes MSH6, WT1, PAX5, CDH13, GATA5 and PAX6 was seen in >50% of cases, while being uncommon or absent in normal male breast tissue, indicating that promoter hypermethylation of these genes is important in the carcinogenesis of male breast cancer. High overall methylation status was also an independent predictor of poor survival. The role of gynecomastia in male breast cancer was debated. We provided evidence, based on immunohistochemistry and analogous to the morphology of ductal hyperplasia usual type in females, that gynecomastia is not a precursor lesion of male breast cancer. In the present thesis additional proof was provided that male and female breast cancer are in fact different in many ways. This is important, as most male breast cancer patients are currently treated based on principles derived from female breast cancer. The distribution of molecular subtypes was clearly different between the two. In male breast cancer HER-driven, basal-like and unclassifiable cancers were very rare, whereas these subtypes are not uncommon in female breast cancer. Copy number gain of the genes CCND1 and EGFR were more frequently found in luminal type male compared to luminal type female breast cancers, whereas in females EMSY and CPD were more often involved and more frequent amplifications of TRAF4 and EMSY were found. At epigenetic level, the most frequently hypermethylated genes were similar. However, many of the studied genes were less frequently methylated in male breast cancer. Our results emphasize the importance of identifying specific biomarkers and therapeutic targets in male breast cancer itself.