T118M Variant of PMP22 Gene Presents with Painful Peripheral Neuropathy and Varying Charcot-Marie-Tooth Features: A Case Series and Review of the Literature

How T118M peripheral myelin protein 22 predisposes humans to Charcot-Marie-Tooth disease

Data from gnomAD indicate that a missense mutation encoding the T118M variation in human peripheral myelin protein 22 (PMP22) is found in roughly one of every 75 genomes of western European lineage (1:120 in the overall human population). It is unusual among PMP22 variants that cause Charcot-Marie-Tooth (CMT) disease in that it is not 100% penetrant. Here, we conducted cellular and biophysical studies to determine why T118M PMP22 predisposes humans to CMT, but with only incomplete penetrance. We found that T118M PMP22 is prone to mistraffic but differs even from the WT protein in that increased expression levels do not result in a reduction in trafficking efficiency. Moreover, the T118M mutant exhibits a reduced tendency to form large intracellular aggregates relative to other disease mutants and even WT PMP22. NMR spectroscopy revealed that the structure and dynamics of T118M PMP22 resembled those of WT. These results show that the main consequence of T118M PMP22 in WT/T118M heterozygous individuals is a reduction in surface-trafficked PMP22, unaccompanied by formation of toxic intracellular aggregates. This explains the incomplete disease penetrance and the mild neuropathy observed for WT/T118M CMT cases. We also analyzed BioVU, a biobank linked to deidentified electronic medical records, and found a statistically robust association of the T118M mutation with the occurrence of long and/or repeated episodes of carpal tunnel syndrome. Collectively, our results illuminate the cellular effects of the T118M PMP22 variation leading to CMT disease and indicate a second disorder for which it is a risk factor.

Mild Late-Onset Sensory Neuropathy Associated with Heterozygous Missense GDAP1 Variants

This study presents the clinical and electrophysiological findings of four subjects with a pathogenic heterozygous GDAP1 variant causing Charcot-Marie-Tooth disease 2K (CMT2K) and one additional subject with an uncertain GDAP1 variant and clinical findings of CMT 2K. The study evaluated these five subjects using clinical, laboratory, electrophysiological, and genetic testing. The findings showed that clinical features demonstrated no pes cavus, no significant weakness in the hands or feet, normal reflexes in four out of the five subjects, and mild to normal electrodiagnostic findings. The variant was associated with painful and numb feet with diminished sensation to pinprick. This study suggests that GDAP1 variants may be associated with very mild, predominantly sensory Charcot-Marie-Tooth disease, warranting continuing research for this type of the disease.

OPRM1, OPRK1, and COMT genetic polymorphisms associated with opioid effects on experimental pain: a randomized, double-blind, placebo-controlled study

Genetic polymorphisms have been shown to affect opioid requirement for pain relief. However, true genetic effect is often difficult to assess due to underlying pain conditions and placebo effects. The goal of this study was to understand how common polymorphisms affect opioid effects while controlling for these factors. A randomized, double-blind, placebo-controlled study was implemented to assess how opioid effects are modulated by COMT (rs6269, rs4633, rs4848, rs4680), OPRM1 (A118G), and OPRK1 (rs1051660, rs702764, rs16918875). One hundred and eight healthy subjects underwent experimental pain testing before and after morphine, butorphanol, and placebo (saline). Association analysis was performed between polymorphisms/haplotypes and opioid response, while correcting for race, gender, placebo effects, and multiple comparisons. Pressure pain was significantly associated with rs6269 and rs4633 following butorphanol. The AA genotype of rs4680 or A_T_C_A/ A_T_C_A (rs6269_rs4633_ rs4818_rs4680) diplotype of COMT, combined with the AG genotype of OPRM1 A118G, showed significantly increased pressure pain threshold from butorphanol. Opioid effects on pressure, ischemic, heat pain, and side effects were nominally associated with several SNPs and haplotypes. Effects were often present in one opioid but not the other. This indicates that these polymorphisms affect pain relief from opioids, and that their effects are opioid and pain modality specific.

Late onset CMT2A in a Family with an MFN2 Variant: c.2222T>G (p.Leu741Trp)

Mutations in MFN2 cause a range of Charcot-Marie-Tooth disease (CMT) phenotypes with different inheritance patterns and underlying pathogenic mechanisms. Recently, a family with a dominantly inherited CMT harboring c.2222T>G (p.Leu741Trp) mutation in MFN2 has been reported for the first time. Here, we report a second family also with a dominantly inherited CMT harboring the same mutation, thereby confirming the pathogenicity of this mutation. Interestingly, the disease onset of this second family is much later than the previously reported cases.