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. 2026 Mar 19;12(2):e200367. doi: 10.1212/NXG.0000000000200367

Nav1.8 Variant I206M as a Latent Susceptibility Factor in Postaxotomy Ocular Pain

Mohammad-Reza Ghovanloo 1,2,3, Philip R Effraim 2,3,4, Sidharth Tyagi 1,2,3, Alecia M Aldrich 1,2,3, Jun-Hui Yuan 1,2,3, Betsy R Schulman 1,2,3, Deborah S Jacobs 5, Sulayman D Dib-Hajj 1,2,3, Stephen G Waxman 1,2,3,✉
PMCID: PMC13004580  PMID: 41868096

Abstract

Background and Objectives

A small proportion of patients develop persistent ocular pain after corneal refractive surgery, which injures the distal axons of trigeminal ganglia neurons innervating the eye. In this study, we investigate the role of a low-frequency Nav1.8 variant, c.618A > G (p.I206M), in the pathogenesis of persistent ocular pain after corneal refractive surgery.

Methods

Whole-exome sequencing in a cohort of patients with post-LASIK ocular pain identified c.618A > G (p.I206M) in SCN10A. We characterized its functional effects using patch-clamp electrophysiology, computer simulations, and multielectrode array recordings of transfected rat trigeminal neurons.

Results

p.I206M produced a hyperpolarizing shift in activation of ∼5 mV without significant effects on current density, inactivation, persistent currents, or recovery kinetics. Modeling predicted a small increase in excitability, attenuated under simulated heterozygous conditions. MEA recordings indicated a significant increase in firing frequency in trigeminal neuron firing at 33 and 37°C for p.I206M vs reference (wild-type) neurons.

Discussion

Although subtle, the p.I206M activation shift may lower the excitability threshold of trigeminal neurons. In the context of axotomy-induced remodeling of the electrogenisome, such variants may act as latent susceptibility factors for chronic ocular pain. Together with prior findings of Nav1.7, TRPV1, and TRPM8 variants in this same cohort, these results support a multihit model, in which rare gain-of-function ion channel variants combine with nerve injury to drive persistent pain after injury, in this instance a second injury, to distal trigeminal axons. Given recent clinical validation of Nav1.8 as an analgesic target, this work further highlights the translational significance of Nav1.8 in human pain disorders.

Introduction

Despite extensive investigation, molecular mechanisms underlying pain after axonal injury remain incompletely understood.1-3 Corneal refractive surgery provides a reproducible human model of distal axonal injury.1-3 During laser-assisted in situ keratomileusis (LASIK), axons of trigeminal ganglion neurons innervating the cornea are transected.1-3 Although most patients recover from LASIK with minimal and transient discomfort, a subset develop persistent ocular pain.4-7 This clinical scenario offers a unique opportunity to investigate why some individuals remain pain-free after nerve injury, whereas others experience chronic pain.

To explore the genetic basis of this variability, we previously performed whole-exome sequencing (WES) in a cohort of patients with persistent ocular pain after LASIK.4 This analysis identified multiple rare variants in genes comprising the electrogenisome,8,9 the ensemble of ion channels and receptors that regulate sensory neuronal excitability. We previously showed that Nav1.7 variant, p.P610T, impairs slow inactivation, producing increased firing of trigeminal neurons.5 A TRPV1 variant, p.V527M, increased excitability by enhancing sensitivity to bradykinin and protons, while reducing capsaicin-induced desensitization.7 Similarly, the TRPM8 variants, p.D665N and p.V915M, increased channel activity, with p.D665N shifting hyperpolarizing activation and both variants enhancing menthol sensitivity, thereby augmenting neuronal excitability.6 Collectively, these findings suggest that rare proexcitatory ion channel variants, in the context of axonal injury, can act as molecular substrates of persistent postaxotomy pain.

Although multiple ion channels contribute to sensory signaling, Nav1.8 (encoded by SCN10A) occupies a particularly critical role in the pain pathway.10-16 Nav1.8, a tetrodotoxin-resistant sensory neuron–specific sodium channel predominantly expressed in primary sensory neurons, generates a slowly inactivating sodium current that has been shown to underlie aberrant repetitive firing in peripheral sensory axons in both rodent models of postinjury neuropathic pain17 and human peripheral neuropathy.18 Pharmacologic and genetic evidence further underscore its importance: the selective-Nav1.8 inhibitor VX-548 (suzetrigine/Journavx) recently received FDA approval for the treatment of acute postoperative pain, validating Nav1.8 as a clinical target.11,19 From a biophysical perspective, Nav1.8 differs from Nav1.7 (the main tetrodotoxin-sensitive Nav channel in nociceptor sensory neurons) in that it activates at more depolarized potentials and contributes substantially to the upstroke of the action potential in sensory neurons, where it drives repetitive firing.14,16 Gain-of-function variants in Nav1.8 have been described in painful peripheral neuropathies, further supporting its key role in human pain.20

In this study, we describe a missense SCN10A variant, c.618A > G (p.I206M; NM_001293307.2) in the voltage-sensing domain (VSD) I (VSD-I; the VSDs are the functional domains of the channel that sense changes in membrane potential during excitability events21,22), identified in our cohort of patients with persistent ocular pain after corneal refractive surgery. Given the location of I206M in VSD-I (first VSD, out of 4), we hypothesized that it alters gating properties predisposing trigeminal ganglion neurons to hyperexcitability. To test this hypothesis, we conducted unbiased automated patch-clamp electrophysiology comparing reference (wild-type) and p.I206M-variant channels in DRG-derived ND7/23 cells, followed by multielectrode array (MEA) recordings in trigeminal neurons expressing either reference or p.I206M variant. Our results demonstrate that p.I206M hyperpolarizes the voltage dependence of activation and induces hyperexcitability of trigeminal neurons. These findings provide additional evidence that gain-of-function variants in ion channels within the electrogenisome, when combined with an insult such as axonal injury, can contribute to chronic ocular pain.

Methods

Cell Culture

ND7/23 cells were used for automated patch-clamp experiments. ND7/23 cells are a hybrid neuronal cell line generated by the fusion of mouse neuroblastoma (N18 tg2) and rat DRG neurons. They are an immortalized, adherent neuronal cell line commonly used for heterologous expression and electrophysiologic studies of voltage-gated ion channels. ND7/23 cells used in this study were obtained from Sigma-Aldrich (catalog no. 92090903), which distributes the line under license from the European Collection of Authenticated Cell Cultures. The cells were transiently transfected with either reference or the p.I206M human Nav1.8 variant with lipofectamine. All cells were incubated at 37°C/5% CO2 for 48 hours before conducting voltage-clamp recordings.

Automated Patch Clamp

Automated patch-clamp recording was used for all ND7 experiments, as described in 6,23,24. Sodium currents were measured in the whole-cell configuration using a Qube-384 (Sophion A/S, Copenhagen, Denmark) automated voltage-clamp system.

Standard electrophysiologic protocols were used. Details on electrophysiology and modeling are available in eMethods.

Data Analysis and Statistics

Our data analysis and statistical testing were conducted based on our previous studies.5,6 Data were normalized to account for variability in sodium channel expression and inward current amplitude and to permit fitting with Boltzmann functions for voltage-dependent relationships or exponential and biexponential functions for inactivation kinetics. The Sophion Qube-384 is an automated electrophysiology platform in which cell selection and recording are performed in a blinded and randomized manner. Subsequent data filtering and analysis were applied uniformly to the entire data set from each Qube-384 run using predefined automated criteria, thereby minimizing bias. Curve fitting and graphing were performed using Prism 9 software (GraphPad Software Inc., San Diego, CA; RRID:SCR_005375), unless otherwise specified. Statistical comparisons between 2 groups were performed using two-tailed t tests. A significance threshold of α = 0.05 was applied. Data are reported as mean ± SEM or, where appropriate, as errors of fit, with n denoting the number of independent recordings or samples used for statistical analysis.

Standard Protocol Approvals, Registrations, and Patient Consents

Written consents were obtained from all participants and approved by Yale Human Investigation Committee (0608001728).6 Animal studies were conducted in accordance with protocols approved by the Institutional Animal Care and Use Committee of the Department of Veterans Affairs Connecticut Healthcare System, West Haven.5,6

Data Availability

All information needed to assess the findings of the study is contained within the article. Additional data can be obtained from the corresponding author on request.

Results

Patient Characteristics

The p.I206M variant was identified in a Caucasian female patient (C21).4 This patient was one of the 21 individuals identified in our previous WES study.4 She underwent bilateral LASIK at the age of 45, followed by an enhancement procedure in both eyes 1 year later. The patient had no history of painful disorders. Ocular pain began after the enhancement procedure, initially in the left eye and later involving both eyes (Figure 1A).

Figure 1. Patient Characteristics and Location of the p.I206M Variant.

Figure 1

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(A) The Ocular Surface Disease Index (OSDI) score of 75 of the patient carrying the c.618A > G (p.I206M) variant in SCN10A. The patient reported persistent ocular pain after corneal refractive surgery, characterized by burning and sandpaper-like sensations, exacerbated by environmental stimuli such as moving air. (B) Schematic of the Nav1.8 channel showing the location of the p.I206M variant at the distal end of the Domain I S3 segment, within the voltage-sensing domain.

In addition to pain, the patient reported a sensation of pronounced ocular dryness. Topical lubricants provided only transient relief, with symptoms recurring within minutes. She described the pain as a burning, sandpaper-like sensation located deep within the eye. Environmental triggers such as moving air or wind exacerbated the pain, whereas closing the eyes, applying warm compresses, or taking a shower provided partial relief. At the time of study, her Ocular Surface Disease Index score was 75.

DNA analysis revealed the c.618A > G (p.I206M) variant in SCN10A, the gene encoding Nav1.8, located at the distal end of the Domain I S3 segment of Nav1.8. This region contributes to VSD-I,25 which is an important component to channel response to membrane depolarization during channel activation (Figure 1B). According to the gnomAD database, the variant has an allele frequency of 1.891% (30,520/1,613,802), including 716 homozygotes. Considering the population frequency, and the uncertain inheritance and penetrance in our patient, this variant is classified as likely benign according to the ACMG guidelines.26

Nav1.8 p.I206M Variant Hyperpolarizes Activation

We examined the effect of the p.I206M variant on Nav1.8 gating using whole-cell recordings on an automated high-throughput patch-clamp platform.5,27 Reference and p.I206M channels were evaluated in a head-to-head manner, with both groups cultured, transfected, and recorded in parallel under identical conditions and an unbiased cell selection.

Activation properties were assessed by measuring peak sodium conductance across test potentials from −100 to +45 mV (Figure 2A). The p.I206M variant produced a hyperpolarizing shift of ∼5 mV in the activation curve, with the midpoint (V1/2) significantly shifted to more negative voltages compared with reference (p = 0.0474). These gating changes lower the threshold for channel opening, making trigeminal neurons more likely to fire in response to depolarizing stimuli. In the context of distal trigeminal axon injury after refractive surgery, such a shift could increase susceptibility to abnormal evoked or sustained firing in affected ophthalmic-division neurons.

Figure 2. Activation and Conductance.

Figure 2

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(A) Voltage dependence of activation in reference and p.I206M Nav1.8 channels measured using patch-clamp recordings. The p.I206M variant caused a hyperpolarizing shift in the midpoint (V1/2) of activation (p = 0.0474). G/Gmax indicates normalized conductance, which is a measure for activation. mV indicates the units for measuring midpoint of activation curves. (B and C) Comparison of absolute conductance and current density as a function of membrane potential. Although there was a trend toward increased values in p.I206M channels, differences did not reach significance (p > 0.05). pS is the unit for measuring absolute channel conductance, and pA/pF are the units for measuring current density. (D) Representative current traces for reference and p.I206M channels. Data are shown as individual values with mean ± SEM (n = 13–20).

The small activation shift is consistent with the absence of pain prior to ocular surgery in the patient carrying this pathogenic channel. A similar pattern of pain after ocular surgery was noted in our previous study of the Nav1.7 p.P610T variant, where relatively subtle gating changes were not linked to pain preoperatively but were sufficient to increase neuronal excitability.5

Impact of p.I206M on Absolute Conductance and Current Density

Next, we investigated the difference between reference and p.I206M channels with respect to peak absolute conductance as a function of membrane potential, as well as current density calculated from the ratio of peak current amplitude to membrane capacitance (pA/pF), plotted against membrane potential (Figure 2, B and C). Our results showed that although there was a trend toward increased conductance and current density in p.I206M compared with reference Nav1.8, these effects did not reach statistical significance ( > 0.05).

These findings suggest that if the p.I206M variant produces an increase in current flow through the channel, the effect is very small, which is consistent with the phenotype of the patient who reported no eye pain history prior to the insults associated with a second surgical procedure on each eye. Representative current traces are shown in Figure 2D.

p.I206M Does Not Alter the Voltage-Dependent Properties of Nav1.8 Inactivation

The availability or inactivation curve of Nav channels represents an important biophysical property of the channel because it defines how the probability of channels entering the inactivated state varies with membrane potential. To evaluate this in the context of our mutant channel, we measured the voltage dependence of steady-state inactivation (SSI) at 2 prepulse durations, 200 ms and 500 ms (Figure 3, A and B), and plotted normalized current amplitudes as a function of the prepulse voltage.

Figure 3. Steady-State Inactivation and Persistent Current.

Figure 3

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(A and B) Steady-state inactivation (SSI) measured with prepulse durations of 200 ms (fast inactivation) and 500 ms (intermediate inactivation). Inactivation curves for reference and I206M were nearly identical, with no significant differences (p > 0.05). Data are shown as individual values with mean ± SEM (200 ms: n = 11–22; 500 ms: n = 11–22). I/Imax represents normalized current, a measure for channel inactivation/availability at the indicated pulse durations. (C) Percentage of persistent current (% of peak) quantified as the ratio of current between 90 and 96 ms to peak current. p.I206M did not significantly alter persistent current (p > 0.05). Data are shown as individual values with mean ± SEM (n = 9–18). (D) Representative current trace of persistent currents.

Prepulse intervals on the scale of a few hundred milliseconds are generally regarded as more reflective of fast inactivation, whereas longer prepulses can also engage slower inactivation mechanisms.28,29 Thus, 200 ms is primarily indicative of fast inactivation, while 500 ms is likely to capture an intermediate (intermediate inactivation refers to time course in which one population of channels are fast inactivated, while the other population is slow inactivated28) level of inactivation.

Our results demonstrate that p.I206M channels do not differ from reference Nav1.8 in their voltage dependence of inactivation at either prepulse duration. The SSI curves showed near-complete overlap between reference and mutant channels, with no statistically significant difference (p > 0.05) (Figure 3, A and B). These results indicate that while p.I206M slightly enhances the voltage dependence of activation, once channels enter the activated state, their inactivation behavior remains indistinguishable from reference. Consequently, the mutant variant does not alter the fraction of Nav1.8 channels that can participate in firing within the neuronal environment.

p.I206M Does Not Alter Persistent Currents and Does Not Alter Open-State Fast Inactivation or Use-Dependent Inactivation

Total sodium current can be divided into 2 components: the peak current and the late or persistent current.30 The peak current reflects the maximal sodium flux during channel opening, while the smaller persistent current is thought to arise from destabilized fast inactivation and has been implicated in several channelopathies. To quantify this component, we calculated the ratio of current amplitude measured between 90 ms and 96 ms to the peak current. The percentage of late current was not significantly different between reference and p.I206M over a 100-ms test pulse (p > 0.05) (Figure 3, C and D), consistent with the idea that I206M does not influence fast inactivation.

We also evaluated the rate of open-state inactivation at +10 mV, sometimes referred to as true fast inactivation,31 by fitting current traces with an exponential function.5 The time constant did not differ between reference and p.I206M channels (p > 0.05) (eFigure 1A), providing further evidence that the mutant variant does not alter fast inactivation.

Nav1.8 plays an essential role in supporting repetitive firing through its minimal use-dependent inactivation (rooted in its unique gating kinetics).10,32 Because Nav1.8 channels typically remain available at resting membrane potential (RMP; RMP is the membrane potential at which there are no active action potentials in progress), we measured use-dependent inactivation kinetics for reference and p.I206M channels using a train of square pulses from −100 mV (to ensure full availability) to 0 mV at 20 Hz. Again, the results showed no difference between reference and p.I206M, indicating that the mutant variant does not alter the repetitive firing capacity of Nav1.8 (p > 0.05) (eFigure 1B).

p.I206M Variant Does Not Alter Recovery From Inactivation Kinetics

The rate at which Nav channels recover from the inactivated state is a critical determinant of their ability to sustain repetitive firing.30 To test whether p.I206M affects this process, we first held channels at −100 mV to ensure full availability. Channels were then pulsed to 0 mV for either 30 ms, to induce fast inactivation, or 500 ms, to further engage intermediate inactivation. Recovery was measured by varying the duration of the subsequent return to −100 mV, allowing recovery to be plotted as a function of time (Figure 4, A and B).

Figure 4. Recovery From Inactivation and Slow Inactivation.

Figure 4

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(A and B) Recovery from inactivation measured after depolarizing pulses of 30 ms (fast inactivation) or 500 ms (intermediate inactivation). Data are shown as individual values with mean ± SEM (30 ms: n = 17–30; 500 ms: n = 16–31). Recovery kinetics of p.I206M overlapped with reference (p > 0.05). (C and D) Steady-state slow inactivation measured using 5-second and 10-second depolarizing pulses. After recovery at −100 mV for 100 ms, test pulses revealed no difference between reference and p.I206M channels, indicating that slow inactivation was unaffected by the variant. Data are shown as individual values with mean ± SEM (5 seconds: n = 10–25; 10 seconds: n = 9–23). I/Imax represents normalized current.

The comparison between reference and p.I206M revealed no significant difference in the kinetics of recovery from inactivation (p > 0.05). These findings indicate that the p.I206M variant does not alter the recovery behavior of Nav1.8 channels.

p.I206M Does Not Affect Steady-State Slow Inactivation

We next assessed steady-state slow inactivation using conditioning durations of 5 seconds and 10 seconds (Figure 4, C and D). Channels were initially held at −100 mV, then subjected to depolarizing pulses for either 5 or 10 seconds. This was followed by a 100 ms hyperpolarizing step back to −100 mV to allow recovery of channels that had entered fast inactivation. The choice of 100 ms was based on our recovery data (Figure 4A), which showed that currents were fully recovered within this interval. Finally, channel availability was tested with a depolarizing pulse to 0 mV (Figure 4, C and D).

Consistent with other inactivation-related gating and kinetic parameters, the p.I206M variant did not affect slow inactivation from either duration, as no differences were observed between reference and mutant channels (p > 0.05).

p.I206M—Impact on Simulated Neuronal Excitability in a Hodgkin-Huxley–Based Model

To assess the potential impact of the p.I206M variant on neuronal excitability, we used a modified Hodgkin-Huxley model of a sensory neuron that incorporated both TTX-S and TTX-R sodium currents.24,33,34 Three sets of simulations were conducted: reference channels, p.I206M channels (with a hyperpolarized activation curve), and a heterozygous condition in which 50% of the Nav1.8 current reflected the p.I206M phenotype and 50% represented reference (Figure 5).

Figure 5. Computational Modeling of Excitability With Nav1.8 p.I206M.

Figure 5

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(A and B) Hodgkin-Huxley–based simulations incorporating either reference or p.I206M activation curves. Stepwise current injections revealed a modest increase of 1–2 additional action potentials in the p.I206M condition. (C and D) In the heterozygous model, with 50% reference and 50% p.I206M channels, the effect was attenuated, lying between reference and variant. These simulations highlight the subtle nature of the p.I206M gain-of-function effect.

In these simulations, neurons were subjected to stepwise 100-ms (the last step was 200 ms to determine effect for a prolonged stimulus) current injections of increasing intensity, followed by a recovery phase of 50 ms in which no current was applied. For the p.I206M condition, the hyperpolarizing shift in activation produced a modest increase in excitability, with 1–2 additional action potentials generated starting from the third depolarization step compared with reference as stimulus intensity was raised (Figure 5, A and B). In the heterozygous condition, the effect was attenuated, falling between reference and p.I206M, and was, therefore, even more subtle (Figure 5, C and D). The simulations did not suggest any other changes (e.g., in RMP) between the conditions.

These results suggest that the p.I206M variant confers only a modest change in excitability, which is consistent with the clinical phenotype of the patient carrying this variant.

MEA Recordings of Nav1.8 p.I206M in Trigeminal Neurons

To evaluate the impact of p.I206M on the excitability of intact neurons, we performed MEA recordings from rat trigeminal neurons transfected with either reference or mutant Nav1.8. This approach avoids dialysis of the intracellular environment by patch pipettes and allows parallel assessment of firing properties. Reference and p.I206M conditions were prepared and recorded in a head-to-head manner, with all transfections conducted simultaneously to minimize temporal confounds.

Recordings were performed at 33°C and 37°C, representing skin/peripheral and core body temperatures, respectively. At both temperatures, neurons expressing p.I206M fired significantly more frequently than those expressing reference channels (33°C: p = 0.0038; 37°C: p = 0.0103) (Figure 6, A and B). The increase in firing induced by p.I206M is consistent with the gain-of-function pattern observed in other electrogenisome variants we have described in this patient cohort.5,6 Increased spontaneous and stimulus-evoked firing in these neurons provides a functional correlate of how a modest change in excitability may become clinically relevant only after corneal axotomy, consistent with the patient's development of pain after LASIK but not before.

Figure 6. MEA Recordings of Trigeminal Neurons Expressing Nav1.8 p.I206M.

Figure 6

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(A) Representative heatmaps showing spontaneous firing activity of rat trigeminal neurons transfected with reference or p.I206M Nav1.8 at 33°C and 37°C. Each circle represents an active electrode in a 4 × 4 electrode array (interelectrode distance ∼350 µm). Warmer colors indicate higher firing intensity. A representative firing trace from a single electrode is shown below each heatmap. (B) Quantification of firing frequency (Hz). Conditions shown on the x-axis are reference 33°C, p.I206M 33°C, reference 37°C, and p.I206M 37°C. Each point represents a single biological replicate (3 replicates per condition); mean ± SEM are shown. Across replicates, 152 and 145 neurons were recorded at 33°C for reference and p.I206M, respectively, and 160 and 171 neurons at 37°C. p.I206M-expressing neurons showed significantly increased firing frequency compared with reference (33°C: p = 0.0038; 37°C: p = 0.0103). *Indicates statistical significance. MEA = multielectrode array.

Although trigeminal ganglion neurons comprise heterogeneous peripheral targets, neurons projecting via the ophthalmic division innervate ocular tissues and maintain somatotopic organization within the ganglion and central trigeminal pathways. It is important to note that trigeminal neurons necessarily undergo axotomy during isolation and culture, thereby modeling a core biological feature of post–refractive surgery ocular pain.35

Taken together, the biophysical and neurophysiologic findings indicate that p.I206M creates an increase in excitability that may become clinically relevant in the setting of corneal nerve injury, as occurred after LASIK in this patient.

Discussion

The molecular basis of chronic pain after axonal injury remains incompletely defined. Refractive surgery provides a unique clinical model of reproducible transection of the distal axons of trigeminal ganglion neurons: most patients recover uneventfully, but a subset develop persistent ocular pain. In our cohort of 21 such patients, WES has revealed rare variants within the electrogenisome that share a common feature: gain-of-function effects in channels that regulate excitability.4-6 It is important to note that these individuals reported chronic eye pain that arose only after refractive surgery, suggesting that these variants act as latent susceptibility factors after axonal transection.

In this study, we describe a previously uncharacterized Nav1.8 variant, p.I206M, and demonstrate that it induces a ∼5-mV hyperpolarizing shift in channel activation without altering current density, inactivation, persistent current, or use dependence. The variant V208E in human Nav1.2, which is located at the corresponding position to the Nav1.8-I206M (seq alignment; eFigure 2), has been reported in several members of a Swedish family with benign neonatal-infantile epilepsy.36 Seizures started at the age of 3 months and stopped after the age of 15 months. Of interest, the V208E substitution conferred a ∼5-mV hyperpolarizing shift of activation and no other reported changes in current density or other tested gating parameters. It is noteworthy that the effect of the 2 variants on channel activation was comparable, although there could have been channel isoform-specific effects of the different amino acids at the same position (Ile vs Val) and the substitution in the affected individuals (Met vs Glu). However, the clinical observation of hyperexcitability disorders (severe neuropathic pain, epilepsy) in carriers of variants at this position in the DI-S3-4 linker in the 2 channels supports the conclusion that these variants are pathogenic.

Computational modeling predicted that this modest biophysical change translates into only a few additional action potentials during stepwise current injection. This limited effect is consistent with the clinical profile of the patient carrying p.I206M, who developed pain only after surgery, and in the case of this modest increase in excitability, only after the second surgical insult to the corneal axons. Indeed, our MEA recordings, in which neurons necessarily undergo axonal transection during culture preparation, further recapitulated the variant-induced increase in excitability.

Comparison across variants from the same cohort provides important context. The Nav1.7 p.P610T and Nav1.8 p.I206M variants both produce relatively small gain-of-function effects,5 but they act on different gating processes: slow inactivation for Nav1.7 vs activation for Nav1.8. This distinction is noteworthy in the context of RMP. At ∼ -60 mV, most Nav1.7 channels reside in an inactivated state, making inactivation properties the dominant determinant of their role in excitability, at least from rest.24,37-39 By contrast, Nav1.8 remains largely available at the same potential, rendering activation properties its primary determinant of neuronal excitability. Thus, even subtle perturbations in activation gating of Nav1.8 can disproportionately influence neuronal readiness to fire, particularly under conditions of injury-induced stress.

The TRP channel variants described in this cohort illustrate a complementary theme.6,7 TRPV1 p.V527M increased responsiveness to inflammatory ligands, while TRPM8 p.D665N and p.V915M shifted activation and enhanced menthol sensitivity. In the case of these variants, particularly p.V915M, altered ligand sensitivity rather than voltage dependence appeared to be the critical driver of enhanced excitability at physiologic temperatures.6,7 Together, these findings underscore a heterogeneity of proexcitatory mechanisms across ion channel families, while reinforcing the unifying principle that modest channel perturbations can become clinically relevant in the context of axonal injury.

The concept that emerges from these studies is a multihit model of chronic pain: genetic predisposition provided by rare gain-of-function variants functions as a substrate that interacts synergistically with axotomy-induced remodeling of the electrogenisome.5-9 Nerve injury is known to cause accumulation of Nav1.7 and Nav1.8 in transected axons,39-41 upregulation of Nav1.3,42,43 and downregulation of stabilizing Kv channels, all of which bias neurons toward ectopic firing.8,9,42-44 In patients carrying variants such as p.I206M, these injury-induced changes, and particularly for this modest increase in excitability, a second injury, may be sufficient to push excitability past the threshold required for persistent pain, whereas patients without such variants may remain below that threshold (Figure 7). This model should be experimentally investigated in future studies.

Figure 7. Summary Cartoon: Multihit Model of Ocular Pain Susceptibility.

Figure 7

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Schematic illustration of the clinical and mechanistic findings. (Top) In most patients, corneal axon transection during LASIK is followed by normal recovery without persistent pain. (Bottom) In patients carrying the p.I206M Nav1.8 variant, the activation curve is shifted to more negative potentials, lowering the threshold for activation. After axonal injury, this subtle gain-of-function effect combines with injury-induced remodeling of the electrogenisome8,9 to predispose trigeminal ganglion neurons to hyperexcitability, resulting in persistent ocular pain. Kv indicates voltage-gated potassium channel. Created in BioRender. Ghovanloo, M-R. (2026) BioRender.com/3pvf732. LASIK = laser-assisted in situ keratomileusis.

While this study does not selectively isolate ophthalmic-division trigeminal neurons, the observed gain-of-function effects were measured in neurons subjected to axotomy, a defining feature of refractive surgical injury. At present, there are no validated trigeminal neuron preparations that permit reliable selection by peripheral target (e.g., corneal vs noncorneal) for electrophysiologic or MEA-based assays. With emerging single cell and cross species trigeminal atlases,45 it is plausible that projection-specific and modality-specific models can be developed, providing an avenue to further refine ocular-specific pain mechanisms.

Nav1.8 is particularly notable in this regard. Originally identified as the sensory neuron–specific sodium channel,12,13 Nav1.8 plays a unique role in nociceptors by supporting repetitive firing and contributing to action potential upstroke in sensory neurons.14 Its importance in pain signaling has been validated by genetic studies linking Nav1.8 variants to painful neuropathies20,46 and by pharmacologic evidence: VX-548 (suzetrigine), a selective Nav1.8 inhibitor, has recently received FDA approval for acute postoperative pain.11,19,20 The identification of p.I206M reinforces the clinical and translational significance of Nav1.8 as a substrate in human pain disorders.

In conclusion, the p.I206M Nav1.8 variant expands the spectrum of electrogenisome variants identified in patients with persistent ocular pain. While the effect of this variant on channel gating is modest, its importance lies in illustrating how even subtle alterations, when combined with axonal injury (and reinjury) and injury-driven electrogenisome remodeling, can predispose its carriers to chronic pain. Together with our prior work on Nav1.7, TRPV1, and TRPM8, this study supports a multihit framework in which rare gain-of-function variants and secondary insults converge to drive persistent postoperative pain (Figure 7). By situating Nav1.8 within this framework, and against the backdrop of its clinical validation, these findings further highlight the Nav1.8 channel as both a mechanistic substrate and a therapeutic target in chronic pain.

Acknowledgment

The Center for Neuroscience and Regeneration Research is a Collaboration of the Paralyzed Veterans of America with Yale University.

Glossary

Kv

voltage-gated potassium channel

LASIK

laser-assisted in situ keratomileusis

MEA

multielectrode array

Nav

voltage-gated sodium channel

reference

wild-type channel

RMP

resting membrane potential

TRPM8

transient receptor potential melastatin 8

TRPV1

transient receptor potential vanilloid 1

VSD

voltage-sensing domain

WES

whole-exome sequencing

Author Contributions

M-R. Ghovanloo: drafting/revision of the manuscript for content, including medical writing for content; major role in the acquisition of data; study concept or design; analysis or interpretation of data. P.R. Effraim: major role in the acquisition of data; analysis or interpretation of data. S. Tyagi: major role in the acquisition of data. A.M. Aldrich: major role in the acquisition of data. J-H. Yuan: study concept or design. B.R. Schulman: major role in the acquisition of data; study concept or design. D.S. Jacobs: drafting/revision of the manuscript for content, including medical writing for content; study concept or design; analysis or interpretation of data. S.D. Dib-Hajj: drafting/revision of the manuscript for content, including medical writing for content; analysis or interpretation of data. S.G. Waxman: drafting/revision of the manuscript for content, including medical writing for content; study concept or design; analysis or interpretation of data.

Study Funding

This work was supported by grants from the U.S. Department of Veterans Affairs Rehabilitation Research and Development Service, by grants from The Erythromelagia Association, and by the Flaherty Endowment.

Disclosure

M.-R. Ghovanloo, P.R. Effraim, S. Tyagi, A.M. Aldrich, J.H. Yuan, B.R. Schulman, D.S. Jacobs, and S.D. Dib-Hajj report no conflicts of interest. During the past 48 months, S.G. Waxman has served as an advisor to Site1 Therapeutics, Navega Therapeutics, Channel-Chromocell, OliPass Pharma, Latigo Therapeutics, Sangamo, Third Rock Ventures, Foresite Labs, Exicure, Arrowhead Pharmaceuticals, GenEP Therapeutics, Enveda Bioscience, Spark Therapeutics, Jazz Therapeutics, Alnylam, Population Health Partners, Medtronics, and Vertex Pharmaceuticals. Go to Neurology.org/NG for all disclosures.

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Data Availability Statement

All information needed to assess the findings of the study is contained within the article. Additional data can be obtained from the corresponding author on request.

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