286 HORMAD1 Contributes to the Development of Squamous Cell Carcinomas in Melanoma Patients

Abstract

BRAF inhibitor (BRAFi) therapies are effective treatments for metastatic melanoma. However, BRAFi monotherapies are associated with a significant risk of squamous cell carcinoma (SCC) in melanoma patients. Consequently, BRAF inhibition results in an activation of the MAPK pathway in cells that harbour wildtype BRAF. Fortunately, a combination approach with both a BRAFi and the selective MAPK inhibitor (MEKi), trametinib, attenuates BRAFi-induced risk of SCCs. Despite the wide use of this therapy, the mechanisms associated with SCC induction remain poorly understood. HORMAD1, a meiosis gene, has been shown to have an impact on genomic instability and proliferation in the development of SCCs. Recent work has shown that HORMAD1 expression is regulated by the MAPK pathway in SCC cell lines. Therefore, we sought to investigate a potential role of HORMAD1 in the risk and development of SCCs that is attenuated with the administration of the MEKi, trametinib. We used BRAFi, vemurafenib and the MEKi, trametinib to evaluate the expression of HORMAD1 in melanoma and normal keratinocyte cell lines. To determine the effects of the two inhibitors, a cell proliferation assay was conducted followed by the evaluation of various components of cell cycle regulation and genomic instability. We found that HORMAD1 protein expression increased following treatment with vemurafenib in normal keratinocytes but not in a melanoma cells. In contrast, HORMAD1 expression decreased following treatment with trametinib in both HaCaT and A375. Cell proliferation assays depict a vulnerability to both vemurafenib and trametinib in both normal and melanoma cell lines, however, HaCaT cells appear to be more susceptible to increased levels of genomic instability following treatment with vemurafenib. Our results suggest that HORMAD1 likely plays a key role in the development of SCCs by increasing genomic instability and proliferation in normal keratinocytes following BRAFi monotreatment. The increased risk of SCC formation is attenuated by dampening HORMAD1 levels with the co-administration of the MEKi, trametinib.