Peking University

A stepwise approach for the management of primary erythromelalgia: A prospective single-arm study.

Gain-of-function mutation of a voltage-gated sodium channel Na1.7 associated with peripheral pain and impaired limb development.

Dominant mutations in voltage-gated sodium channel Na1.7 cause inherited erythromelalgia, a debilitating pain disorder characterized by severe burning pain and redness of the distal extremities. Na1.7 is preferentially expressed within peripheral sensory and sympathetic neurons. Here, we describe a novel Na1.7 mutation in an 11-year-old male with underdevelopment of the limbs, recurrent attacks of burning pain with erythema, and swelling in his feet and hands. Frequency and duration of the episodes gradually increased with age, and relief by cooling became less effective. The patient's sister had short stature and reported similar complaints of erythema and burning pain, but with less intensity. Genetic analysis revealed a novel missense mutation in Na1.7 (2567G>C; p.Gly856Arg) in both siblings. The G856R mutation, located within the DII/S4-S5 linker of the channel, substitutes a highly conserved non-polar glycine by a positively charged arginine. Voltage-clamp analysis of G856R currents revealed that the mutation hyperpolarized (-11.2 mV) voltage dependence of activation and slowed deactivation but did not affect fast inactivation, compared with wild-type channels. A mutation of Gly-856 to aspartic acid was previously found in a family with limb pain and limb underdevelopment, and its functional assessment showed hyperpolarized activation, depolarized fast inactivation, and increased ramp current. Structural modeling using the Rosetta computational modeling suite provided structural clues to the divergent effects of the substitution of Gly-856 by arginine and aspartic acid. Although the proexcitatory changes in gating properties of G856R contribute to the pathophysiology of inherited erythromelalgia, the link to limb underdevelopment is not well understood.

A new Nav1.7 mutation in an erythromelalgia patient.

Gain-of-function missense mutations of SCN9A gene, which encodes voltage-gated sodium channel Nav1.7, alter channel's biophysical properties causing painful disorders which are refractory to pharmacotherapy in the vast majority of patients. Here we report a novel SCN9A mutation (ca.T3947C) in exon 20 in a 9 year old patient, not present in 200 ethnically-matched control alleles; the mutation substitutes the invariant valine 1316 residue within DIII/S5 by alanine (V1316A). Voltage-clamp studies show that Nav1.7 V1316A mutation hyperpolarizes activation (-9 mV), and enhances response to ramp stimuli (3-fold), changes that are predicted to cause hyperexcitability of DRG neurons. V1316A also hyperpolarizes steady-state slow-inactivation (-9.9 mV), which is predicted to attenuate the effect of this mutation on DRG neuron firing. These changes are consistent with previously characterized Erytheromelalgia associated mutations of Nav1.7.

Long-term remission of primary erythermalgia with R1150W polymorphism in SCN9A after chemical lumbar sympathectomy.

Primary erythermalgia (PEM) is recalcitrant and long-term remission is difficult to achieve. Favorable results of treatment using carbamazepine or mexiletine have been identified in some PEM patients with SCN9A gene mutations. However, no therapeutic studies regarding patients without pathogenic SCN9A gene mutation have been reported. Here we present a PEM case with R1150W polymorphism in SCN9A and a five-year remission was achieved by chemical lumbar sympathectomy (CLS). A 15-year-old girl with severe PEM attacks in both feet and lower legs was treated with CLS and followed up for five years. The encoding exons and their flanking sequences in the SCN9A gene were amplified and sequenced. A 50% immediate pain reduction was achieved after CLS. Burning pain, erythema and swelling in the lower legs disappeared in four days, and all ulceration healed in a month. The patient resumed normal exercise five months after CLS. There were no relapses in the following five years. R1150W polymorphism in SCN9A was detected in the patient and her healthy father. Long-term remission was achieved after CLS in this PEM case with R1150W polymorphism in SCN9A. The effectiveness of CLS and phenotype/genotype of PEM should be further studied in larger samples.

A case of primary erythermalgia with prurigo.

Mexiletine-responsive erythromelalgia due to a new Na(v)1.7 mutation showing use-dependent current fall-off.

Inherited erythromelalgia (IEM), characterized by episodic burning pain and erythema of the extremities, is produced by gain-of-function mutations in sodium channel Na(v)1.7, which is preferentially expressed in nociceptive and sympathetic neurons. Most patients do not respond to pharmacotherapy, although occasional reports document patients as showing partial relief with lidocaine or mexiletine. A 7-year-old girl, with a two-year history of symmetric burning pain and erythema in her hands and feet, was diagnosed with erythromelalgia. Treatment with mexiletine reduced the number and severity of pain episodes. We report here a new IEM Na(v)1.7 mutation in this patient, and its response to mexiletine. SCN9A exons from the proband were amplified and sequenced. We identified a single nucleotide substitution (T2616G) in exon 15, not present in 200 ethnically-matched control alleles, which substitutes valine 872 by glycine (V872G) within DII/S5. Whole-cell patch-clamp analysis of wild-type and mutant Na(v)1.7 channels in mammalian cells show that V872G shifts activation by -10 mV, slows deactivation, and generates larger ramp currents. We observed a stronger use-dependent fall-off in current following exposure to mexiletine for V872G compared to wild-type channels. These observations suggest that some patients with IEM may show a favorable response to mexiletine due to a use-dependent effect on mutant Na(v)1.7 channels. Continued relief from pain, even after mexiletine was discontinued in this patient, might suggest that early treatment may slow the progression of the disease.

Genetics and molecular pathophysiology of Na(v)1.7-related pain syndromes.

SCN9A, the gene which encodes voltage-gated sodium channel Na(v)1.7, is located on human chromosome 2 within a cluster of other members of this gene family. Na(v)1.7 is present at high levels in most peripheral nociceptive neurons in dorsal root ganglion (DRG) and in sympathetic neurons. In addition to its focal tissue-specific expression, Na(v)1.7 is distinguished by its ability to amplify small depolarizations, thus acting as a threshold channel and modulating excitability. Dominantly inherited gain-of-function mutations in SCN9A have been linked to two familial painful disorders: inherited erythromelalgia (IEM) and paroxysmal extreme pain disorder (PEPD). One set of mutations leads to severe episodes of pain in the feet and hands in patients with IEM, and a different set of mutations causes pain in a perirectal, periocular, and mandibular distribution in patients with PEPD. These mutations allow mutant channels to activate in response to weaker stimuli, or to remain open longer in response to stimulation. The introduction of mutant channels into DRG neurons alters electrogenesis and renders these primary sensory neurons hyperexcitable. Mutant Na(v)1.7 channels lower the threshold for single action potentials and increase the number of action potentials that neurons fire in response to suprathreshold stimuli. In contrast, recessively inherited loss-of-function mutations in SCN9A, which cause a loss of function of Na(v)1.7 in patients, lead to indifference to pain with sparing of motor and cognitive abilities. The central role of Na(v)1.7 in these disorders, and the apparently limited consequences of loss of this channel in humans make it an attractive target for treatment of pain.

Temperature dependence of erythromelalgia mutation L858F in sodium channel Nav1.7.

The disabling chronic pain syndrome erythromelalgia (also termed erythermalgia) is characterized by attacks of burning pain in the extremities induced by warmth. Pharmacological treatment is often ineffective, but the pain can be alleviated by cooling of the limbs. Inherited erythromelalgia has recently been linked to mutations in the gene SCN9A, which encodes the voltage-gated sodium channel Nav1.7. Nav1.7 is preferentially expressed in most nociceptive DRG neurons and in sympathetic ganglion neurons. It has recently been shown that several disease-causing erythromelalgia mutations alter channel-gating behavior in a manner that increases DRG neuron excitability. Here we tested the effects of temperature on gating properties of wild type Nav1.7 and mutant L858F channels. Whole-cell voltage-clamp measurements on wild type or L858F channels expressed in HEK293 cells revealed that cooling decreases current density, slows deactivation and increases ramp currents for both mutant and wild type channels. However, cooling differentially shifts the midpoint of steady-state activation in a depolarizing direction for L858F but not for wild type channels. The cooling-dependent shift of the activation midpoint of L858F to more positive potentials brings the threshold of activation of the mutant channels closer to that of wild type Nav1.7 at lower temperatures, and is likely to contribute to the alleviation of painful symptoms upon cooling in affected limbs in patients with this erythromelalgia mutation.

[Hypertension and erythromelalgia as prominent manifestations of mercury intoxication].

Mercury intoxication is a rare cause of severe hypertension. A case of mercury intoxication presented with severe hypertension and erythromelalgia was reported. A 10-year- and -5-month-old girl presented with recurrent rash and painful hands for 2 months, with seizure attack and episodic loss of consciousness for one hand half months. The girl was found to have red painful hands, a blood pressure 170/120 mm Hg(1 mm Hg=0.133 kPa), tachycardia and hypokalemia (2.83-3.25 mmol/L, reference value 3.5-5.5 mmol/L). An extensive investigation ensued. Elevated renin-angiotensin and aldosterone were demonstrated in plasma. Cranial MRI T2 weighed images showed widespread white matter signal abnormalities, which particularly involved parietal, occipital and frontal lobes. With hypertension controlled, white matter signal abnormalities weakened. Other symptoms included insomnia, nausea and paroxysmal abdominal pain. The girl was found to have a raised concentration of mercury in urine (0.171 mg/L, reference value< 0.01 mg/L), and she had been exposed to elemental mercury for several days. After chelating therapy, the girl's blood pressure returned to normal, erythromelalgia ameliorated, all other symptoms disappeared. So, mercury intoxication should be considered in the differential diagnosis of hypertension with erythromelalgia.

Mutation hotspots of SCN9A in primary erythermalgia.