Prognostic significance and functional activity of voltage-gated sodium channels in breast cancer

Voltage gated sodium channels (VGSCs), specifically the cardiac isoform Nav1.5, have previously been shown to increase invasion and metastasis in breast cancer. In this thesis, results from immunohistochemical analysis of 1480 breast tumours showed that Nav1.5 expression correlates with worse prognosis, increased metastasis and higher grade and stage cancer. Breast cancer tissue from patients and primary breast cancer cells were assessed using patch clamp recording for the presence of Na+ currents through VGSCs and some cells showed small inward currents consistent with VGSCs.
VGSCs have previously been shown to increase cancer cell invasion through increased H+ efflux from cancer cells through the Na+/H+ exchanger NHE1. This acidifies the extracellular fluid, thereby aiding enzymes which degrade the extracellular matrix. In this project, the mechanism by which VGSCs increase H+ efflux was investigated and evidence is presented suggesting that VGSCs increase the rate of glycolysis and therefore H+ production, via increasing activity of the Na+/K+ ATPase. Gene expression changes dependent on VGSCs in breast cancer were explored in an RNAseq analysis of Nav1.5 knock-down in MDA-MB-231 xenograft tumours. As well as downregulation of invasion- and migration-related genes with Nav1.5 knock-down, there were changes related to pH control, Ca2+ signalling and immune function. Gene set enrichment analysis revealed that the differentially expressed genes were important in cancer.
Previously an antiepileptic drug and VGSC inhibitor, phenytoin was shown to slow breast tumour growth and metastasis in MDA-MB-231 xenograft tumours. This project assessed a newer antiepileptic VGSC inhibitor with an improved safety profile, eslicarbazepine acetate. This drug inhibited transient and persistent Na+ current through Nav1.5 so it could be used in vivo to assess its activity against breast cancer.
In conclusion, evidence is presented confirming the prognostic significance of VGSCs in breast cancer and showing new insight into the mechanism by which VGSCs increase metastasis.