Recent genomic research has identified variants in the SCN9A gene as a key factor in inherited erythromelalgia (IEM) pathogenesis. In this review, data are collected and analyzed to identify SCN9A sequence variants associated with IEM, demographic patterns, and treatment responses among patients. We describe a spectrum of SCN9A variants associated with IEM. Sex and age distributions among patients with IEM show a predominance of cases with clinical onset before the age of 10 years, whereas sex differences are not pronounced. Some patients with specific pathogenic variants respond to pharmacotherapy, such as p.L858F/H (mexiletine) and V400M, S241T, and I234T (carbamazepine), suggesting potential for personalized therapeutic approaches.
Erythromelalgia (EM) is a debilitating neurovascular disease that is refractory to numerous treatment modalities. The present study reported the case of a 72-year-old woman diagnosed with EM secondary to polycythemia vera (PV), who was effectively treated with the use of patient-controlled epidural analgesia (PCEA) and interferon α-2b therapy. The study aimed to provide clinical information on this condition based on the findings of the present case and previously-reported cases. PCEA containing local anesthetics and dexamethasone or fentanyl resulted in nearly complete relief of symptoms. Furthermore, continuous use of interferon α-2b for the treatment of the PV completely relieved the EM symptoms. In conclusion, PCEA and interferon α-2b may be promising treatments for EM secondary to PV.
Estacion M, Choi JS, Eastman EM, Lin Z, Li Y , et al.
The Journal of physiology •
Ion channel missense mutations cause disorders of excitability by changing channel biophysical properties. As an increasing number of new naturally occurring mutations have been identified, and the number of other mutations produced by molecular approaches such as in situ mutagenesis has increased, the need for functional analysis by patch-clamp has become rate limiting. Here we compare a patch-clamp robot using planar-chip technology with human patch-clamp in a functional assessment of a previously undescribed Nav1.7 sodium channel mutation, S211P, which causes erythromelalgia. This robotic patch-clamp device can increase throughput (the number of cells analysed per day) by 3- to 10-fold. Both modes of analysis show that the mutation hyperpolarizes activation voltage dependence (8 mV by manual profiling, 11 mV by robotic profiling), alters steady-state fast inactivation so that it requires an additional Boltzmann function for a second fraction of total current (approximately 20% manual, approximately 40% robotic), and enhances slow inactivation (hyperpolarizing shift--15 mV by human,--13 mV robotic). Manual patch-clamping demonstrated slower deactivation and enhanced (approximately 2-fold) ramp response for the mutant channel while robotic recording did not, possibly due to increased temperature and reduced signal-to-noise ratio on the robotic platform. If robotic profiling is used to screen ion channel mutations, we recommend that each measurement or protocol be validated by initial comparison to manual recording. With this caveat, we suggest that, if results are interpreted cautiously, robotic patch-clamp can be used with supervision and subsequent confirmation from human physiologists to facilitate the initial profiling of a variety of electrophysiological parameters of ion channel mutations.
Han C, Dib-Hajj SD, Lin Z, Li Y, Eastman EM , et al.
Brain : a journal of neurology •
Inherited erythromelalgia (IEM), an autosomal dominant disorder characterized by severe burning pain in response to mild warmth, has been shown to be caused by gain-of-function mutations of sodium channel Na(v)1.7 which is preferentially expressed within dorsal root ganglion (DRG) and sympathetic ganglion neurons. Almost all physiologically characterized cases of IEM have been associated with onset in early childhood. Here, we report the voltage-clamp and current-clamp analysis of a new Na(v)1.7 mutation, Q10R, in a patient with clinical onset of erythromelalgia in the second decade. We show that the mutation in this patient hyperpolarizes activation by only -5.3 mV, a smaller shift than seen with early-onset erythromelalgia mutations, but similar to that of I136V, another mutation that is linked to delayed-onset IEM. Using current-clamp, we show that the expression of Q10R induces hyperexcitability in DRG neurons, but produces an increase in excitability that is smaller than the change produced by I848T, an early-onset erythromelalgia mutation. Our analysis suggests a genotype-phenotype relationship at three levels (clinical, cellular and molecular/ion channel), with mutations that produce smaller effects on sodium channel activation being associated with a smaller degree of DRG neuron excitability and later onset of clinical signs.