Congenital insensitivity to pain: novel SCN9A missense and in-frame deletion mutations

Abstract

SCN9A encodes the voltage-gated sodium channel Nav1.7, a protein highly expressed in pain-sensing neurons. Mutations in SCN9A cause three human pain disorders: bi-allelic loss of function mutations result in Channelopathy-associated Insensitivity to Pain (CIP), whereas activating mutations cause severe episodic pain in Paroxysmal Extreme Pain Disorder (PEPD) and Primary Erythermalgia (PE). To date, all mutations in SCN9A that cause a complete inability to experience pain are protein truncating and presumably lead to no protein being produced. Here, we describe the identification and functional characterization of two novel non-truncating mutations in families with CIP: a homozygously-inherited missense mutation found in a consanguineous Israeli Bedouin family (Nav1.7-R896Q) and a five amino acid in-frame deletion found in a sporadic compound heterozygote (Nav1.7-ΔR1370-L1374). Both of these mutations map to the pore region of the Nav1.7 sodium channel. Using transient transfection of PC12 cells we found a significant reduction in membrane localization of the mutant protein compared to the wild type. Furthermore, voltage clamp experiments of mutant-transfected HEK293 cells show a complete loss of function of the sodium channel, consistent with the absence of pain phenotype. In summary, this study has identified critical amino acids needed for the normal subcellular localization and function of Nav1.7. © 2010 Wiley-Liss, Inc.

Matrix Metalloproteinase (MMP) Proteolysis of the Extracellular Loop of Voltage-gated Sodium Channels and Potential Alterations in Pain Signaling

“…A non-truncating Nav1.7-R896Q mutation, however, has been found in a consanguineous Israeli Bedouin family (6). So far, this is the only missense mutation in Nav1.7 that causes CIP.…”

“…Naturally, it is intriguing that the single missense Nav1.7-R896Q mutation resulted in the severe loss of function phenotype consistent with CIP in humans. In addition, the Nav1.7-R896Q mutation caused a significant reduction in membrane localization of the mutant protein as compared with the wild type (6).…”

“…Intriguingly, a non-truncating homozygously inherited missense mutation found in a consanguineous Israeli Bedouin family with CIP (Nav1.7-R907Q in the exposed pore S5-S6 region) has been recently reported (6). This mutation led to both a significant reduction in membrane localization of the R907Q mutant as compared with the wild type and a complete loss of function of Nav1.7, consistent with the absence of pain phenotype.…”

“…Mutations in SCN9A, encoding the ␣-subunit of the voltage-gated sodium channel Nav1.7, the predominant voltage-gated sodium channel in peripheral neurons (4), are a cause of autosomal recessive CIP. This disorder is characterized by the inability to perceive any form of pain, in any part of the body, but with intact motor, cognitive, sympathetic, or gastrointestinal and sensory modalities (5)(6)(7)(8). Affected individuals cannot normally respond to the environment and are frequently traumatized, manifesting permanent damage and high mortality.…”

Intrathecal Studies on Animal Pain Models

“…This accumulation may be due to a loss of myelin inhibition or target determined transfer of Na + channels (Aurilio et al, 2008). Loss of Nav1.7 function may lead to complete insensitivity to pain in humans (Cox et al, 2010). Compounds possessing Nav1.7 blocking effects have been reported to reverse nerve injury-induced mechanical allodynia (Tyagarajan et al, 2010).…”