LETTER TO EDITOR Pain insensitivity: distal S6-segment mutations in Na V 1.9 emerge as critical hotspot Margaret K. King 1,2 & Enrico Leipold 3 & Jessica M. Goehringer 4 & Ingo Kurth 5 & Thomas D. Challman 4 Received: 15 January 2017 /Revised: 28 February 2017 /Accepted: 2 March 2017 # Springer-Verlag Berlin Heidelberg 2017 To the editor: Gain of function mutations in the sensory neuron- specific voltage-gated sodium channel Na V 1.9/SCN11A lead to entirely opposite pain phenotypes, i.e., congenital pain insensitivity or severe and episodic neuropathic pain [1, 2]. The explanation for the discrepancy in the clinical outcome, such as the location of the mutation within the ion-channel, remains elusive. We report a 16-month-old boy with a developmental delay and self-biting of the tongue, lower lip, and finger- tips. The patient did not appear sensitive to painful stimuli and was not bothered by the ulcerated tongue and lip areas (Fig. 1a). Muscle tone was low in the axial and appendicular musculature. The patient did not have a his- tory of fractures. Weight gain during infancy was slow, and he demonstrated aversion to swallowing solid foods. The patient exhibited gastrointestinal symptoms including constipation. Magnetic resonance imaging of the brain was normal. Whole exome analysis was completed using the Agilent Clinical Research Exome Kit to target the ex- onic regions and flanking splice junctions of the ge- nome. The targeted regions were sequenced simulta- neously by NextGen sequencing. Potentially pathogenic variants were confirmed by capillary sequencing [6]. Testing revealed a de novo heterozygous variant in SCN11A which had not been reported in genomic vari- ation databases (c.1187 T > C; p.L396P) (Fig. 1b). No other pathogenic or likely pathogenic variants were identified, specifically, none were identified in other channel genes. No mitochondrial disorders were identi- fied on sequence analysis and deletion testing of the mitochondrial genome nor were any secondary findings identified. To examine the consequences of mutation p.L396P for the function of Na V 1.9, we applied whole-cell voltage- clamp electrophysiology to isolated murine SCN11A knockout dorsal root ganglion (DRG) neurons transfected with DNA constructs encoding either human Na V 1.9 or p.L396P channels (Fig. 1c–f) according to a procedure described earlier [5]. While p.L396P did not affect the voltage-dependence of activation of Na V 1.9 (Fig. 1d), the mutation slowed down the channel deactivation (Fig. 1e). This effectively increased the channel’ s open probability and reduced the voltage-dependence of steady-state inac- tivation, resulting in an increase of the availability of mutant channels (Fig. 1f). Both slowed deactivation ki- netics and increased channel availability represent gain- of-function features, which are predicted to increase the basal activity of Na V 1.9. To analyze the impact of p.L396P mutant channels on the resting membrane volt- age of nociceptive neurons, we performed current-clamp experiments on small diameter (<25 μm in diameter) DRG neurons obtained from wild-type mice [ 5 ] * Margaret K. King maggie.king@novartis.com 1 Autism & Developmental Medicine Institute, Geisinger Health System, Lewisburg, PA, USA 2 Present address: Novartis Pharmaceuticals Corporation, East Hanover, NJ, USA 3 Department of Biophysics, Center for Molecular Biomedicine, Friedrich Schiller University Jena & Jena University Hospital, 07745 Jena, Germany 4 Autism & Developmental Medicine Institute, Geisinger Health System, Lewisburg, PA, USA 5 Institute of Human Genetics, Medical Faculty, RWTH Aachen University, 52074 Aachen, Germany Neurogenetics DOI 10.1007/s10048-017-0513-9