Pharmacogenetic study of Fc gamma receptor and HER2 genes in breast cancer

Abstract

Breast cancer is a complex set of diseases with different biological and clinical characteristics. An important contribution to this diversity is provided by germ-line genetic variations. The HER2-positive breast cancers have been extensively studied with particular regard to their biology and targeted treatments. However, the influence of pharmacogenetic (PG) factors on these aspects remains largely unexplored. This research focused on the possible effects of common single nucleotide polymorphisms (SNPs) on specific aspects of HER2-positive disease. Initially we analysed two coding SNPs in the HER2 gene (Ile655Val and Ala1170Pro) in breast cancer patients and evaluated their potential association with HER2 expression in tumour samples. The proline variant of the Ala1170Pro SNP was associated (odds ratio = 1.7, p = 0.01) with HER2 over-expression/amplification in over 360 breast cancer patients. In contrast, Ile655Val was not associated with HER2 over-expression/amplification. Bioinformatics tools predict that Ala1170Pro might affect the structure or function of the HER2 protein. The same variants were explored in the context of DNA extracted from the patients’ primary tumours in 241 patients. We hypothesized that the proline allele of Ala1170Pro could undergo allele-specific amplification during the development of HER2-positive tumours. This hypothesis, however, was not confirmed. Although the association of the proline allele of Ala1170Pro with HER2 positivity is intriguing, the role of the two SNPs in HER2 over-expression/amplification remains to be elucidated. Trastuzumab has radically changed the treatment of HER2-positive breast cancer. However, resistance to treatment and toxicity can limit its effectiveness. The second objective of this project was the analysis of PG, biomarker and pharmacokinetic (PK) parameters in trastuzumab-treated patients. Fc Gamma Receptors (FcRs) are key proteins in the trastuzumab-induced Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC) and two coding SNPs in these genes (FCGR2A His131Arg and FCGR3A Phe158Val) were analysed. The measurement of trastuzumab in plasma was made possible by the development of a novel cell-based ELISA. Only 28 patients with advanced disease treated with trastuzumab were recruited. However, we observed a possible association of the valine allele of the FCGR3A Phe158Val SNP with a longer time to progression (p = 0.03). Cardiac toxicity was assessed in a group of 139 patients treated with adjuvant trastuzumab. Although a role of germ-line genetic variants could not be demonstrated, the analysis highlighted the challenges and limitations encountered in the conduct of an observational pharmacogenetic study. This project leaves a legacy archive composed of germ-line DNA samples, tumour DNA samples, plasma samples and tumour FFPE blocks from over 360 breast cancer patients. These samples and data are available for the exploration of further potential factors which might influence the biology of the disease and/or its response to treatment.