Rho GTPase transcriptional activity and breast cancer risk: A Mendelian randomization analysis

Background: Rho GTPases are a family of 20 intracellular signalling proteins that influence cytoskeletal dynamics, cell migration and cell cycle progression. Rho GTPases are implicated in breast cancer progression but their role in breast cancer aetiology is unknown. As aberrant Rho GTPase activity could be associated with breast cancer, we aimed to determine the potential for a causal role of Rho GTPase gene expression in breast cancer risk, using two-sample Mendelian randomization (MR). Methods: MR was undertaken in 122,977 breast cancer cases and 105,974 controls, including 69,501 estrogen receptor positive (ER+) cases and 105,974 controls, and 21,468 ER negative (ER-) cases and 105,974 controls. Single nucleotide polymorphisms (SNPs) underlying expression quantitative trait loci (eQTLs) obtained from normal breast tissue, breast cancer tissue and blood were used as genetic instruments for Rho GTPase expression. Colocalisation was performed as a sensitivity analysis to examine whether findings reflected shared causal variants or genomic confounding. Results: We identified genetic instruments for 14 of the 20 human Rho GTPases. Using eQTLs obtained from normal breast tissue and normal blood, we identified evidence of a causal role of RHOD in overall and ER+ breast cancers (overall breast cancer: odds ratio (OR) per standard deviation (SD) increase in expression level 1.06; (95% confidence interval (CI): 1.03, 1.09) and OR 1.22 (95% CI: 1.11, 1.35) in normal breast tissue and blood respectively). The direction of association was consistent for ER- breast cancer, although the effect-estimate was imprecisely estimated. Using eQTLs from breast cancer tissue and normal blood there was some evidence that CDC42 was inversely associated with overall and ER+ breast cancer risk. The evidence from colocalization analyses strongly supported the MR results particularly for RHOD. Conclusions: Our study suggests a potential causal role of increased RHOD gene expression, and a potential protective role for CDC42 gene expression, in overall and ER+ breast cancers. These finding warrant validation in independent samples and further biological investigation to assess whether they may be suitable targets for drug targeting.