Mutations in Nav1.9 Voltage-Gated Sodium Channel Found in Inherited Pain Condition

by Stephani Sutherland on 11/5/2013

Voltage-gated sodium channels drive action potential firing in pain-sensing neurons, and mutations in the genes for Nav1.7 and Nav1.8, two channels critical for nociceptor function, have been linked to human pain disorders. In a report published online October 24 in the American Journal of Human Genetics, a group led by Jing Yu Liu and Jing Yao at Huazhong University in Wuhan, China, describes mutations in a third channel, Nav1.9, as the cause of an episodic pain disorder in two Chinese families.

Stephen Waxman, who studies Nav channels at Yale University, New Haven, Connecticut, US, and was not involved in the study, called it “a solid story, with a strong genetic substrate.”

The team, including co-first authors Xiang Yang Zhang, Jingmin Wen, and Wei Yang, studied two large families spanning five generations with many members affected by similar episodic pain symptoms, inherited in an autosomal-dominant pattern. Afflicted members described periods of intense pain that came on suddenly primarily in the lower extremities and then relapsed a number of times over several weeks before abating. In both families, pain was exacerbated by fatigue and relieved by anti-inflammatory medications.

Gain-of-function mutations in sodium channels Nav1.7 or Nav1.8 have been previously linked to rare hereditary pain conditions and more common painful neuropathies (reviewed in Dib-Hajj et al., 2012; also see Faber et al., 2012; and PRF related news story), so the researchers first looked to the genes encoding those channels—SCN9A and SCN10A—for sequence differences in the affected family members. When no mutations turned up there, they expanded the search with a genomewide linkage scan and haplotype analysis. The results pointed to the same locale in both families, in the region of SCN11A, the gene encoding Nav1.9.

DNA sequencing of the region revealed that members of one family carried a missense mutation in SCN11A that resulted in an arginine-to-cysteine substitution in the S4 transmembrane segment of the domain I region in Nav1.9. In the second family, the researchers found an alanine-to-glycine substitution in the S6 segment of domain II. The two mutations segregated with the pain symptoms, suggesting they were responsible for the disorder. A screen of over 1,000 healthy controls found no similar mutations.

In order to understand the electrophysiological consequences of the mutations, the researchers measured currents through wild-type and the two mutant versions of human Nav1.9 channels. To do that, the team isolated small neurons from the dorsal root ganglia of mice and transfected them with one of the three human channels. The electrophysiologists bathed the cells in a cocktail of drugs and various ions in order to block other sodium, potassium, and calcium channels, and then filled the recording pipette with fluoride, which reportedly shifts the voltage dependence of Nav1.9 but not Nav1.8 (Coste et al., 2004). “By combining these conditions, we were able to record Nav1.9 currents individually,” Jing Yao wrote in an email to PRF.

In voltage-clamp experiments, Nav1.9 channels carrying either mutation passed more excitatory current than wild-type channels, but the voltage dependence of activation and inactivation of the channels appeared unchanged by the mutation. The increased currents carried by the mutant channels could be explained by expression of more functional channels, a higher probability that each channel opens, greater conductance through individual channels, or some combination thereof, the authors reported.

Importantly, under physiological recording conditions, neurons expressing the mutant channels fired more spontaneous and stimulus-induced action potentials compared to wild-type, suggesting the mutations influenced cell excitability. These experiments point to a gain-of-function mutation in the Nav1.9 channel that could underlie the familial pain disorder, Waxman told PRF. Resting membrane potential and action potential thresholds were unchanged by the mutations.

The new findings stand in contrast to a recent report (Leipold et al., 2013) that a different Nav1.9 gain-of-function mutation led to pain insensitivity in two patients (see PRF related news story).

The report provides more information about these sodium channels and makes a case for Nav1.9 as yet another potential pain therapy target. Waxman said of the findings, “Given the strong contribution of Nav1.9 to the regulation of nociceptors’ excitability, I am not surprised.” He added, “I expect there will soon be other cases linking Nav1.9 to pain in humans.”

Stephani Sutherland, PhD, is a neuroscientist, yogi, and freelance writer in Southern California.

Image credit: Reprinted from The American Journal of Human Genetics, Zhang et al., in press, Copyright 2013, with permission from The American Society of Human Genetics.