A Novel SCN9A Mutation (F826Y) in Primary Erythromelalgia Alters the Excitability of Nav1.7

BACKGROUND

Primary erythromelalgia (PE) is a dominant inherited disorder characterized by recurrent pain, redness, and warmth of the extremities that is caused by gain-of-function mutations in Nav1.7 encoding gene SCN9A. Most of the PE-causing mutations of Nav1.7 have been shown to be able to render Nav1.7-expressing cells hyperexcitable, however in most PE cases the symptoms are refractory to treatment with sodium channel blockers and the mechanism underlying the intractability has not been clearly clarified.

OBJECTIVE

To identify the mutation of SCN9A in a Chinese Han family with typical symptoms of PE and study the electrophysiological effect of the identified mutation.

METHODS

A Chinese Han family with typical symptoms of PE was collected and the proband's response to treatment was recorded. All the exons and flanking intronic sequences of SCN9A were amplified with PCR and sequenced. Several online programs were used to predict the damaging effect of variants. The functional effect of variants was studied by voltage-clamp analysis in CHO-K1 cells.

RESULTS

The PE symptoms of the proband are refractory to all kinds of reported medications. Sequence analysis of SCN9A showed that a novel c.2477T>A (p. F826Y) mutation co-segregated with the disease phenotype. Several online programs predicted that the F826Y mutation has a deleterious effect on the gene product. Voltage-clamp analysis showed that while compared with the wild-type channel, activation of the F826Y mutant channel was shifted by 7.7 mV in a hyperpolarizing direction, whereas steadystate inactivation was shifted by 4.3 mV in a depolarizing direction.

CONCLUSION

A novel disease-causing SCN9A Mutation (F826Y) was identified in a Chinese family with typical PE symptoms refractory to treatment. F826Y of Nav1.7 could render DRG neurons hyperexcitable, contributing to the pathogenesis of PE.

[Erythromelalgia: Diagnosis and therapeutic approach]

Erythromelalgia is a rare intermittent vascular acrosyndrome characterized by the combination of recurrent burning pain, warmth and redness of the extremities. It is considered in its primary form as an autosomal dominant neuropathy related to mutations of SCN9A, the encoding gene of a voltage-gated sodium channel subtype Nav1.7. Secondary erythromelalgia is associated with myeloproliferative disorders, drugs (bromocriptine, calcium channel blockers), or clinical conditions such as rheumatic diseases or viral infection. Primary familial erythromelalgia include genetics and sporadic forms associated with small fibers neuropathy. Aspirin is a useful treatment of erythromelagia associated with myeloproliferative disorders. Treatment of primary erythromelalgia is difficult, individualized, with sodium channel blockers such as lidocaine, carbamazepine and mexiletine.