Inherited peripheral neuropathies are a genetically heterogeneous group of disorders characterized by distal muscle weakness and sensory loss. Mutations in genes encoding aminoacyl-tRNA synthetases have been implicated in peripheral neuropathies, suggesting that these tRNA charging enzymes are uniquely important for the peripheral nerve. Recently, a mutation in histidyl-tRNA synthetase (HARS) was identified in a single patient with a late-onset, sensory-predominant peripheral neuropathy; however, the genetic evidence was lacking, making the significance of the finding unclear. Here, we present clinical, genetic, and functional data that implicate HARS mutations in inherited peripheral neuropathies. The associated phenotypic spectrum is broad and encompasses axonal and demyelinating motor and sensory neuropathies, including four young patients presenting with pure motor axonal neuropathy. Genome-wide linkage studies in combination with whole-exome and conventional sequencing revealed four distinct and previously unreported heterozygous HARS mutations segregating with autosomal dominant peripheral neuropathy in four unrelated families (p.Thr132Ile, p.Pro134His, p.Asp175Glu and p.Asp364Tyr). All mutations cause a loss of function in yeast complementation assays, and p.Asp364Tyr is dominantly neurotoxic in a Caenorhabditis elegans model. This study demonstrates the role of HARS mutations in peripheral neuropathy and expands the genetic and clinical spectrum of aminoacyl-tRNA synthetase-related human disease.

Although mutations in more than 90 genes are known to cause CMT, the underlying genetic cause of CMT remains unknown in more than 50% of affected individuals. The discovery of additional genes that harbor CMT2-causing mutations increasingly depends on sharing sequence data on a global level. In this way-by combining data from seven countries on four continents-we were able to define mutations in ATP1A1, which encodes the alpha1 subunit of the Na,K-ATPase, as a cause of autosomal-dominant CMT2. Seven missense changes were identified that segregated within individual pedigrees: c.143T>G (p.Leu48Arg), c.1775T>C (p.Ile592Thr), c.1789G>A (p.Ala597Thr), c.1801_1802delinsTT (p.Asp601Phe), c.1798C>G (p.Pro600Ala), c.1798C>A (p.Pro600Thr), and c.2432A>C (p.Asp811Ala). Immunostaining peripheral nerve axons localized ATP1A1 to the axolemma of myelinated sensory and motor axons and to Schmidt-Lanterman incisures of myelin sheaths. Two-electrode voltage clamp measurements on Xenopus oocytes demonstrated significant reduction in Na current activity in some, but not all, ouabain-insensitive ATP1A1 mutants, suggesting a loss-of-function defect of the Na,K pump. Five mutants fall into a remarkably narrow motif within the helical linker region that couples the nucleotide-binding and phosphorylation domains. These findings identify a CMT pathway and a potential target for therapy development in degenerative diseases of peripheral nerve axons.

Mutations in the HINT1 gene were recently discovered as being the major cause of autosomal recessive axonal neuropathy with neuromyotonia. This combination was clinically recognized and described previously in a few reports but is generally unknown. We aimed to establish the importance of HINT1 mutations as the cause of hereditary neuropathy and particularly hereditary motor neuropathy/axonal Charcot-Marie-Tooth (HMN/CMT2) among Czech patients. Overall, mutations in the HINT1 gene seem to be a surprisingly frequent cause of inherited neuropathy in our group of patients. Biallelic pathogenic mutations were found in 21 patients from 19 families. The prevalent mutation in the Czech population is the p.R37P (95% of pathogenic alleles). Clinically, all patients with biallelic mutations presented with early onset of symptoms at the end of the first decade. Foot/toe extension weakness to plegia was present in almost all patients. Neuromyotonia was present in all but two patients. However, it had been properly recognized in only three patients prior to molecular genetic diagnosis. HINT1 mutations seem to be one of the most frequent causes of inherited neuropathy and are probably the most frequent cause of HMN in Czech patients. We suggest all HMN/CMT2 patients be tested for the presence of the prevalent mutation, the p.R37P.

Pontocerebellar hypoplasia type 1 (PCH1) is characterized by cerebellar and anterior horn motor neuron degeneration and loss, signs of spinal muscular atrophy plus. Patients manifest severe perinatal weakness, hypotonia, and respiratory insufficiency, causing death frequently before the age of 1 year. Recently, causative mutations in EXOSC3 were reported in a majority of PCH1 patients, but the detailed clinical phenotype caused by EXOSC3 mutations, genotype-phenotype correlations, and prevalent mutations in specific ethnic groups is not yet known. Three unrelated Czech Roma patients with PCH1 were investigated clinically, electrophysiologically, neuroradiologically, and neuropathologically (patients 1 and 2). The entire coding region of the EXOSC3 gene, including the adjacent intron sequences, was sequenced in all three patients. The same mutation c.92G→C, p.G31A in EXOSC3 was found in all three affected patients in homozygous state and in heterozygous state in the parents from two of the families. Haplotype analysis with four flanking microsatellite markers showed identical haplotype in 9 out of 11 haplotypes carrying the c.92G→C, p.G31A mutation. Furthermore, four heterozygotes for this mutation were found in anonymous DNA samples from 90 unrelated Roma individuals. All four of these samples shared the same haplotype. No heterozygous sample was found among 120 anonymous DNA samples from Czech non-Roma individuals with no familial relation. It may therefore be concluded that EXOSC3 c.92G→C, p.G31A mutation is a founder mutation with high prevalence among the Czech Roma causing a similar and particularly severe phenotype of PCH1. These observations from the Czech Roma may have consequences also for other Roma from other countries. PCH1 caused by EXOSC3 founder mutation c.92G→C, p.G31A extends the list of autosomal recessive disorders rare among the general population but more frequent among Roma at least in the Czech Republic.

BackgroundInherited peripheral neuropathies (IPN) are the most common inherited neurological condition. It represents a highly heterogeneous group, both clinically and genetically.Targeted disease specific gene panel massively parallel sequencing (MPS) seems to be a useful tool in diagnosis of disorders with high genetic heterogeneity.MethodsIn our study, we have designed, validated and updated our own custom gene panel of all known genes associated with IPN. One hundred and ninety-eight patients have been tested so far. Only patients in whom mutations in more common causes or relevant genes have already been excluded were enrolled. Five consecutive panel designs were prepared according to recent literature search, the last one covering ninety-three genes. Each patient was tested only once. All data were evaluated with at least two different pipelines.ResultsIn summary, causative mutation has been found in fifty-one patients (26 %). The results were inconclusive in thirty-one (16 %) patients. No variants of likely significance to IPN were found in one hundred and sixteen (58 %) patients.ConclusionMPS gene panel enables testing of all known IPN causes at once with high coverage and at an affordable cost making it truly a method of choice also in IPN. Gene panel testing results in several interesting results and findings.Electronic supplementary materialThe online version of this article (doi:10.1186/s13023-016-0500-5) contains supplementary material, which is available to authorized users.