Familial periodic paralysis associated with a rare KCNJ5 variant that supposed to have incomplete penetrance

Drug repurposing in skeletal muscle ion channelopathies

Skeletal muscle ion channelopathies are rare genetic diseases mainly characterized by myotonia (muscle stiffness) or periodic paralysis (muscle weakness). Here, we reviewed the available therapeutic options in non-dystrophic myotonias (NDM) and periodic paralyses (PP), which consists essentially in drug repositioning to address stiffness or weakness attacks. Empirical use followed by successful randomized clinical trials eventually led to the orphan drug designation and marketing authorization granting of mexiletine for NDM and dichlorphenamide for PP. Yet, these treatments neither consider the genetic cause of the diseases nor address the individual variability in drug response. Thus, ongoing research aims at the identification of repurposed drugs alternative to mexiletine and dichlorphenamide to allow personalization of treatment. This review highlights how drug repurposing may represent an efficient strategy in rare diseases, allowing reduction of drug development time and costs in a context in which the return on investment may be particularly challenging.

A novel de novo TMEM63A variant in a patient with severe hypomyelination and global developmental delay

BACKGROUND

Heterozygous variants in TMEM63A have been recently identified as the cause of infantile-onset transient hypomyelination. To date, four TMEM63A variants have been reported in five patients. These patients exhibited favorable clinical course, developmental progress, and completion of myelination.

CASE REPORT

The patient was a 5-year-old girl with severe global developmental delay, absent speech, no turning over, no gazing, hypotonia, and daily episodes of autonomic seizures. Brain MRI showed hypomyelination of deep and subcortical white matter that appeared hyperintense in T2-weighted imaging from 2 months of age and that showed no change at 4 years of age. Exome sequencing of the patient and her parents revealed a novel de novo missense variant, NM_014698.3:c.1658G>T, p.(Gly553Val), in the TMEM63A gene, which was confirmed by Sanger sequencing. The variant has not been registered in public databases, and it substitutes a highly conserved glycine residue located in a pore-lining transmembrane helix. No other candidate variants were identified.

CONCLUSIONS

Although TMEM63A variants are generally thought to cause transient hypomyelination with favorable developmental progress, identification of a de novo TMEM63A variant in our patient suggests that the TMEM63A-related clinical spectrum is broad and includes severe developmental delay with seizures.

Ion Channel Gene Mutations Causing Skeletal Muscle Disorders: Pathomechanisms and Opportunities for Therapy

Skeletal muscle ion channelopathies (SMICs) are a large heterogeneous group of rare genetic disorders caused by mutations in genes encoding ion channel subunits in the skeletal muscle mainly characterized by myotonia or periodic paralysis, potentially resulting in long-term disabilities. However, with the development of new molecular technologies, new genes and new phenotypes, including progressive myopathies, have been recently discovered, markedly increasing the complexity in the field. In this regard, new advances in SMICs show a less conventional role of ion channels in muscle cell division, proliferation, differentiation, and survival. Hence, SMICs represent an expanding and exciting field. Here, we review current knowledge of SMICs, with a description of their clinical phenotypes, cellular and molecular pathomechanisms, and available treatments.