To Örebro University

Human epidermal growth factor receptor 2 (HER2) overexpressing breast cancers (HER2+ breast cancer) are associated with an aggressive disease course. This thesis is focused on improving the understanding of the biological signature of HER2+ breast cancer.

In Paper I, we identified a common deletion spanning the SLC25A43 gene which codes for a mitochondrial transport protein. Loss of heterozygosity in this gene was confirmed in an extended cohort of HER2+ breast cancer and in other types of cancers. Protein expression analysis of SLC25A43using immunohistochemistry (IHC) in HER2+ breast cancers showed that tumours with negative or low expression of SLC25A43 had lower S-phase fraction compared to tumours with medium or high expression, indicating its possible role in cell proliferation. Absence of mutations in this gene in HER2+ breast cancers led to Paper II where DNA methylation in the SLC25A43 gene was interrogated using Pyrosequencing. HER2+ breast cancer with no deletion in the SLC25A43 gene showed higher methylation in the CpG island (CGI), suggesting methylation in the CGI as an alternate mechanism for SLC25A43 gene inactivation. Methylation in the CGI and in the adjacent shores of the SLC25A43 gene was associated with negative oestrogen receptor status and positive lymph node status. In Paper III, genome-wide DNA methylation analysis of HER2+ breast cancer and normal breast tissue revealed hypermethylation in HER2+ breast cancer affecting particularly the homeobox gene family when compared to normal. We identified a total of 73 candidate genes showing differential methylation in HER2+ breast cancer and external validation of gene expression in a selected group of these genes revealed lowered mean expression in HER2+ breast cancer, warranting future clinical studies of these candidate genes. In Paper IV, we investigated expression and localisation of phosphorylated (p) Akt and FOXO3a and FOXG1 in HER2+ breast cancer using IHC. Cytoplasmic expression of pFOXO3a was associated with sentinel node metastasis while cytoplasmic expression of FOXG1 was correlated to negative progesterone receptor status. This indicates the biological and prognostic value of these proteins in HER2+ breast cancer.

Thus, this thesis identified changes at the genetic, epigenetic and protein levels which add new information and improve our understanding of HER2+ breast cancer.