Human pain channelopathies.

There has been significant progress in our understanding of the molecular basis by which nociceptors transduce and transmit noxious (tissue damaging) stimuli. This is dependent on ion channels, many of which are selectively expressed in nociceptors. Mutations in such proteins have recently been linked to inherited pain disorders in humans. An exemplar is the voltage-gated sodium channel (VGSC) Na1.7. Loss of function mutations in Na1.7 result in congenital inability to experience pain while gain-of-function mutations can cause a number of distinct neuropathic pain disorders, including erythromelalgia, paroxysmal extreme pain disorder, and small-fiber neuropathy. Furthermore, variants in the VGSCs 1.8 and 1.9 have also been linked to human pain disorders. There is a correlation between the impact of mutations on the biophysical properties of the ion channel and the severity of the clinical phenotype. Pain channelopathies are not restricted to VGSCs: a mutation in the ligand-gated ion channel TRPA1, (which responds to environmental irritants) causes a familial episodic pain disorder. Ion channel variants have also been linked to more common neuropathic pain disorders such as painful diabetic neuropathy. Not only do these ion channels present targets for novel analgesics, but stratification based on genotype may improve treatment selection of existing analgesics.