Exome sequencing reveals HINT1 mutations as a cause of distal hereditary motor neuropathy

Novel gene-intergenic fusion involving ubiquitin E3 ligase UBE3C causes distal hereditary motor neuropathy

Distal hereditary motor neuropathies (dHMNs) are a group of inherited diseases involving the progressive, length-dependent axonal degeneration of the lower motor neurons. There are currently 29 reported causative genes and four disease loci implicated in dHMN. Despite the high genetic heterogeneity, mutations in the known genes account for less than 20% of dHMN cases, with the mutations identified predominantly being point mutations or indels. We have expanded the spectrum of dHMN mutations with the identification of a 1.35 Mb complex structural variation (SV) causing a form of autosomal dominant dHMN (DHMN1 OMIM %182906). Given the complex nature of SV mutations and the importance of studying pathogenic mechanisms in a neuronal setting, we generated a patient-derived DHMN1 motor neuron model harbouring the 1.35 Mb complex insertion. The DHMN1 complex insertion creates a duplicated copy of the first 10 exons of the ubiquitin-protein E3 ligase gene (UBE3C) and forms a novel gene-intergenic fusion sense transcript by incorporating a terminal pseudo-exon from intergenic sequence within the DHMN1 locus. The UBE3C intergenic fusion (UBE3C-IF) transcript does not undergo nonsense-mediated decay and results in a significant reduction of wild-type full-length UBE3C (UBE3C-WT) protein levels in DHMN1 iPSC-derived motor neurons. An engineered transgenic Caenorhabditis elegans model expressing the UBE3C-IF transcript in GABA-ergic motor neurons shows neuronal synaptic transmission deficits. Furthermore, the transgenic animals are susceptible to heat stress, which may implicate defective protein homeostasis underlying DHMN1 pathogenesis. Identification of the novel UBE3C-IF gene-intergenic fusion transcript in motor neurons highlights a potential new disease mechanism underlying axonal and motor neuron degeneration. These complementary models serve as a powerful paradigm for studying the DHMN1 complex SV and an invaluable tool for defining therapeutic targets for DHMN1.

Case report: A novel homozygous histidine triad nucleotide-binding protein 1 mutation featuring distal hereditary motor-predominant neuropathy with rimmed vacuoles

Introduction

Recessive mutations in the gene encoding the histidine triad nucleotide-binding protein 1 (HINT1) are associated with axonal motor-predominant Charcot-Marie-Tooth (CMT) disease with neuromyotonia. A total of 24 HINT1 gene mutations have been reported so far. Some of these cases had mild to moderate elevations of creatinine kinase with no earlier reports of muscle biopsy findings in these cases. In this study, we describe a patient with axonal motor-predominant neuropathy and myopathy with rimmed vacuoles, likely due to a novel HINT1 gene mutation.

Case report

A 35-year-old African American man presented with insidious onset and progressive symmetric distal leg weakness followed by hand muscle atrophy and weakness since the age of 25. He had no muscle cramps or sensory complaints. His 38-year-old brother developed similar symptoms beginning in his early 30 s. On neurologic examination, the patient had distal weakness and atrophy in all limbs, claw hands, pes cavus, absent Achilles reflexes, and normal sensory examination. Electrodiagnostic studies revealed absent/reduced compound motor action potential amplitudes distally with normal sensory responses with no neuromyotonia. His sural nerve biopsy showed a chronic non-specific axonal neuropathy, and a biopsy of the tibialis anterior muscle demonstrated myopathic features and several muscle fibers harboring rimmed vacuoles without inflammation in addition to chronic denervation changes. A homozygous variant, p.I63N (c.188T > A), in the HINT1 gene was found in both brothers.

Conclusion

We describe a novel, likely pathogenic, HINT1 pI63N (c.188T > A) homozygous variant associated with hereditary axonal motor-predominant neuropathy without neuromyotonia in two African American brothers. The presence of rimmed vacuoles on muscle biopsy raises the possibility that mutations in the HINT1 gene may also cause myopathy.

Early onset hereditary neuronopathies: an update on non-5q motor neuron diseases

Hereditary motor neuropathies (HMN) were first defined as a group of neuromuscular disorders characterized by lower motor neuron dysfunction, slowly progressive length-dependent distal muscle weakness and atrophy, without sensory involvement. Their cumulative estimated prevalence is 2.14/100 000 and, to date, around 30 causative genes have been identified with autosomal dominant, recessive,and X-linked inheritance. Despite the advances of next generation sequencing, more than 60% of patients with HMN remain genetically uncharacterized. Of note, we are increasingly aware of the broad range of phenotypes caused by pathogenic variants in the same gene and of the considerable clinical and genetic overlap between HMN and other conditions, such as Charcot-Marie-Tooth type 2 (axonal), spinal muscular atrophy with lower extremities predominance, neurogenic arthrogryposis multiplex congenita and juvenile amyotrophic lateral sclerosis. Considering that most HMN present during childhood, in this review we primarily aim to summarize key clinical features of paediatric forms, including recent data on novel phenotypes, to help guide differential diagnosis and genetic testing. Second, we describe newly identified causative genes and molecular mechanisms, and discuss how the discovery of these is changing the paradigm through which we approach this group of conditions.

HINT1 neuropathy: Expanding the genotype and phenotype spectrum

Inherited peripheral neuropathy (IPN) is a heterogeneous group of disorders due to pathogenic variation in more than 100 genes. In 2012, the first cases of IPN associated with HINT1 pathogenic variations were described in 33 families sharing the same phenotype characterized by an axonal neuropathy with neuromyotonia and autosomal recessive inheritance (NMAN: OMIM #137200). Histidine Triad Nucleotide Binding Protein 1 regulates transcription, cell-cycle control, and is possibly involved in neuropsychiatric pathophysiology. Herein, we report seven French patients with NMAN identified by Next Generation Sequencing. We conducted a literature review and compared phenotypic and genotypic features with our cohort. We identified a new HINT1 pathogenic variation involved in NMAN: c.310G>C p.(Gly104Arg). This cohort is comparable with literature data regarding age of onset (7,4yo), neuronal involvement (sensorimotor 3/7 and motor pure 4/7), and skeletal abnormalities (scoliosis 3/7, feet anomalies 6/7). We expand the phenotypic spectrum of HINT1-related neuropathy by describing neurodevelopmental or psychiatric features in six out of seven individuals such as generalized anxiety disorder (GAD), obsessive-compulsive disorder (OCD), mood disorder and attention deficit hyperactivity disorder (ADHD). However, only 3/128 previously described patients had neuropsychiatric symptomatology or neurodevelopmental disorder. These features could be part of HINT1-related disease, and we should further study the clinical phenotype of the patients.

A Novel Mutation of HINT1 Gene in an Adolescent Female with Axonal Neuropathy and Neuromyotonia

HINT1 gene mutations cause an axonal neuropathy with some specific findings including presence of neuromyotonia, autosomal recessive inheritance, onset in the first decade, and primary motor involvement. In this case report, we described an 18-year-old female patient who presented to the clinic with gait instability and muscle stiffness. A homozygous novel c.180_181delAT (p.Ser61Profs*8) variant in the HINT1 gene was found by clinical exome analysis. Parents were heterozygous for the same variant. The patient was diagnosed with autosomal recessive axonal neuropathy with neuromyotonia. The presence of neuromyotonia must be evaluated in patients with hereditary axonal neuropathies as this can help the diagnosis prior to genetic testing.

The expanding genetic landscape of hereditary motor neuropathies

Hereditary motor neuropathies are clinically and genetically diverse disorders characterized by length-dependent axonal degeneration of lower motor neurons. Although currently as many as 26 causal genes are known, there is considerable missing heritability compared to other inherited neuropathies such as Charcot-Marie-Tooth disease. Intriguingly, this genetic landscape spans a discrete number of key biological processes within the peripheral nerve. Also, in terms of underlying pathophysiology, hereditary motor neuropathies show striking overlap with several other neuromuscular and neurological disorders. In this review, we provide a current overview of the genetic spectrum of hereditary motor neuropathies highlighting recent reports of novel genes and mutations or recent discoveries in the underlying disease mechanisms. In addition, we link hereditary motor neuropathies with various related disorders by addressing the main affected pathways of disease divided into five major processes: axonal transport, tRNA aminoacylation, RNA metabolism and DNA integrity, ion channels and transporters and endoplasmic reticulum.

Genetic Fine Mapping and Genomic Annotation Defines Causal Mechanisms at A Novel Colorectal Cancer Susceptibility Locus in Han Chinese

Genome-wide association studies of colorectal cancer (CRC) have identified two risk SNPs. The characterization of these risk regions in diverse racial groups with different linkage disequilibrium structure would aid in localizing the causal variants. Herein, fine mapping of the established CRC loci was carried out in 1,508 cases and 1,482 controls obtained from the Han Chinese population. One distinct association signal was identified at these loci, where fine mapping implicated rs1010208 as a functional locus. Next, the candidate target genes of functional SNP rs1010208 were analyzed using data from TCGA databases by expression quantitative trait loci analysis method; the data from Peking University People's Hospital were utilized for verification. The dual-luciferase reporter system analysis confirmed that rs1010208 is a regulatory region that can be mutated to decrease the expression of HINT1, resulting in proliferation and invasiveness of CRC.

HINT1 gene pathogenic variants: the most common cause of recessive hereditary motor and sensory neuropathies in Russian patients

Pathogenic variants in the HINT1 gene lead to hereditary axonopathy with neuromyotonia. However, many studies show that neuromyotonia may remain undiagnosed, while axonopathy is the major clinical finding. The most common cause of neuromyotonia and axonopathy, especially in patients of Slavic origin, is a c.110G>C (p.Arg37Pro) pathogenic variant in homozygous or compound heterozygous state. In this study, we analyzed a peripheral neuropathy caused by pathogenic variants in the HINT1 gene and evaluated its contribution to the hereditary neuropathy structure. The studied group included 1596 non-related families diagnosed with hereditary motor and sensory neuropathy (HMSN). The results show that HINT1 gene pathogenic variants make a significant contribution to the hereditary neuropathy epidemiology in Russian patients. They account for at least 1.9% of all HMSN cases and 9% of axonopathy cases. The most common HINT1 pathogenic variant in Russian patients is the c.110G>C (p.Arg37Pro) substitution. Its allelic frequency is 0.2% (95% CI 0.19-0.21%), carrier frequency is 1 in 250 people in Russian Federation, and the estimated disease incidence is 1 in 234,000 individuals. It was determined that the cause of this pathogenic variant's prevalence is the founder effect.

Whole exome sequencing reveals a broader variant spectrum of Charcot-Marie-Tooth disease type 2

Charcot-Marie-Tooth disease type 2 (CMT2) is a clinically and genetically heterogeneous inherited neuropathy. Although new causative and disease-associated genes have been identified for CMT2 in recent years, molecular diagnoses are still lacking for a majority of patients. We here studied a cohort of 35 CMT2 patients of Chinese descent, using whole exome sequencing to investigate gene mutations and then explored relationships among genotypes, clinical features, and mitochondrial DNA levels in blood as assessed by droplet digital PCR. We identified pathogenic variants in 57% of CMT2 patients. The most common genetic causes in the cohort were MFN2 mutations. Two patients with typical CMT phenotype and neuromyotonia were detected to harbor compound heterozygous variations in the HINT1 gene. In conclusion, our work supports that the molecular diagnostic rate of CMT2 patients can be increased via whole exome sequencing, and our data suggest that assessment of possible HINT1 mutations should be undertaken for CMT2 patients with neuromyotonia.

The role of the Hint1 protein in the metabolism of phosphorothioate oligonucleotides drugs and prodrugs, and the release of H2S under cellular conditions

Phosphorothioate oligonucleotides (PS-oligos) containing sulfur atom attached in a nonbridging position to the phosphorus atom at one or more internucleotide bond(s) are often used in medicinal applications. Their hydrolysis in cellular media proceeds mainly from the 3'-end, resulting in the appearance of nucleoside 5'-O-phosphorothioates ((d)NMPS), whose further metabolism is poorly understood. We hypothesize that the enzyme responsible for (d)NMPS catabolism could be Hint1, an enzyme that belongs to the histidine triad (HIT) superfamily and is present in all organisms. We previously found that (d)NMPS were desulfurated in vitro to yield (d)NMP and H₂S in a Hint1-assisted reaction. Here, we demonstrate that AMPS/GMPS/dGMPS introduced into HeLa/A549 cells are intracellularly converted into AMP/GMP/dGMP and H₂S. The level of the released H₂S was relative to the concentration of the compounds used and the reaction time. Using RNAi technology, we have shown decreased levels of AMPS/GMPS desulfuration in HeLa/A549 cells with reduced Hint1 levels. Finally, after transfection of a short Rp-d(A_PSA_PSA) oligomer into HeLa cells, the release of H₂S was observed. These results suggest that the metabolic pathway of PS-oligos includes hydrolysis into (d)NMPS (by cellular nucleases) followed by Hint1-promoted conversion of the resulting (d)NMPS into (d)NMP accompanied by H₂S elimination. Our observations may be also important for possible medicinal applications of (d)NMPS because H₂S is a gasotransmitter involved in many physiological and pathological processes.

A novel VRK1 mutation associated with recessive distal hereditary motor neuropathy

Vaccinia‐related kinase 1 (VRK1) mutations can cause motor phenotypes including axonal sensorimotor neuropathy, distal hereditary motor neuropathy (dHMN), spinal muscular atrophy, and amyotrophic lateral sclerosis. Here, we identify a novel homozygous VRK1 p.W375X mutation causing recessive dHMN. The proband presented with juvenile onset of weakness in the distal lower extremities, slowly progressing to the distal upper limbs, with bilateral pes cavus and no upper motor or sensory neuron involvement. Nerve conduction studies showed a pure motor axonal neuropathy. Our findings extend the ethnic distribution of VRK1 mutations, indicating that these mutations should be included in genetic diagnostic testing for dHMN.

Novel mutations in HINT1 gene cause the autosomal recessive axonal neuropathy with neuromyotonia

Autosomal recessive axonal neuropathy with neuromyotonia (ARAN-NM) is a rare form of hereditary neuropathy. Mutations in HINT1 gene have been identified to be the cause of this disorder. We report two unrelated patients who presented gait impairment, progressive distal muscle weakness and atrophy, neuromyotonia and foot deformities. Electrophysiological studies showed axonal motor neuropathy and neuromyotonic discharges. Using Next-generation sequencing, we identified two homozygous mutations, NM_005340.6: c.112T > C; p.(Cys38Arg) and NM_005340.6: c.289G > A; p.(Val97Met) in HINT1 gene. Based on the clinical presentation and molecular genetic analyses, ARAN-NM was diagnosed in both patients and NM_005340.6: c.112T > C; p.(Cys38Arg) and NM_005340.6: c.289G > A; p.(Val97Met) in HINT1 gene were believe to be causative for the disorder.

Structure and Functional Characterization of Human Histidine Triad Nucleotide-Binding Protein 1 Mutations Associated with Inherited Axonal Neuropathy with Neuromyotonia

Inherited peripheral neuropathies are a group of neurodegenerative disorders that clinically affect 1 in 2500 individuals. Recently, genetic mutations in human histidine nucleotide-binding protein 1 (hHint1) have been strongly and most frequently associated with patients suffering from axonal neuropathy with neuromyotonia. However, the correlation between the impact of these mutations on the hHint1 structure, enzymatic activity and in vivo function has remained ambiguous. Here, we provide detailed biochemical characterization of a set of these hHint1 mutations. Our findings indicate that half of the mutations (R37P, G93D and W123*) resulted in a destabilization of the dimeric state and a significant decrease in catalytic activity and HINT1 inhibitor binding affinity. The H112N mutant was found to be dimeric, but devoid of catalytic activity, due to the loss of the catalytically essential histidine; nevertheless, it exhibited high affinity to AMP and a HINT1 inhibitor. In contrast to the active-site mutants, the catalytic activity and dimeric structure of the surface mutants, C84R and G89V, were found to be similar to the wild-type enzyme. Taken together, our results suggest that the pathophysiology of inherited axonal neuropathy with neuromyotonia can be induced by conversion of HINT1 from a homodimer to monomer, by modification of select surface residues or by a significant reduction of the enzyme's catalytic efficiency.

Novel mutations in HINT1 gene cause autosomal recessive axonal neuropathy with neuromyotonia in two cases of sensorimotor neuropathy and one case of motor neuropathy

Autosomal recessive axonal neuropathy with neuromyotonia (ARANNM) is a rare disease caused by mutations of histidine triad nucleotide binding protein 1 (HINT1) gene. ARANNM has been reported mainly in European countries but little reported so far in China. We describe novel mutations of HINT1 in three Chinese patients with ARANNM from unrelated families. Patient 1 was a 14-year-old girl who presented with progressive distal weakness of upper limbs at two years of age. After that, she reported weakness of both feet, and difficulty in muscle relaxation after making a fist. Patient 2 was an 18-year-old boy, who presented with progressive distal weakness of all limbs with foot drop at the age of ten with loss of ambulation at age 15. Patient 3 was a 26-year-old man who had been afflicted with weakness and atrophy of distal lower limbs since the age of 16 complaining about muscle stiffness of the lower limbs when standing and walking, and contraction of finger flexion muscles when releasing a forced grip. Electrodiagnostic testing revealed an axonal motor or sensorimotor neuropathy with or without myokymic discharges. Sural biopsy showed no pathological changes in patient 1 and mild axonal neuropathies with demyelination in patients 2 and 3. Genetic analysis revealed HINT1 with novel compound heterozygous c.112T > C (p.C38R) and c.171G > C (p.K57N) mutations in patient 1, homozygous c.112T > C (p.C38R) mutation in patient 2, as well as compound heterozygous c.112T > C (p.C38R) and c.98T > C (p.F33S) mutations in patient 3. Our study, for the first time, confirms ARANNM in the Chinese population. These genetic findings can help expand the genotypic spectrum of HINT1 mutations.

HINT1 in Neuropsychiatric Diseases: A Potential Neuroplastic Mediator

Although many studies have investigated the functions of histidine triad nucleotide-binding protein 1 (HINT1), its roles in neurobiological processes remain to be fully elucidated. As a member of the histidine triad (HIT) enzyme superfamily, HINT1 is distributed in almost every organ and has both enzymatic and nonenzymatic activity. Accumulating clinical and preclinical evidence suggests that HINT1 may play an important role as a neuroplastic mediator in neuropsychiatric diseases, such as schizophrenia, inherited peripheral neuropathies, mood disorders, and drug addiction. Though our knowledge of HINT1 is limited, it is believed that further research on the neuropathological functions of HINT1 would eventually benefit patients with neuropsychiatric and even psychosomatic diseases.

Axonal neuropathy with neuromyotonia: there is a HINT

Recessive mutations in the gene encoding the histidine triad nucleotide binding protein 1 (HINT1) were recently shown to cause a motor-predominant Charcot–Marie–Tooth neuropathy. About 80% of the patients exhibit neuromyotonia, a striking clinical and electrophysiological hallmark that can help to distinguish this disease and to guide diagnostic screening. HINT1 neuropathy has worldwide distribution and is particularly prevalent in populations inhabiting central and south-eastern Europe. With 12 different mutations identified in more than 60 families, it ranks among the most common subtypes of axonal Charcot–Marie–Tooth neuropathy. This article provides an overview of the present knowledge on HINT1 neuropathy with the aim to increase awareness and spur interest among clinicians and researchers in the field. We propose diagnostic guidelines to recognize and differentiate this entity and suggest treatment strategies to manage common symptoms. As a recent player in the field of hereditary neuropathies, the role of HINT1 in peripheral nerves is unknown and the underlying disease mechanisms are unexplored. We provide a comprehensive overview of the structural and functional characteristics of the HINT1 protein that may guide further studies into the molecular aetiology and treatment strategies of this peculiar Charcot–Marie–Tooth subtype.

Update on Charcot-Marie-Tooth disease

PURPOSE OF REVIEW

Charcot-Marie-Tooth disease (CMT) is the common terminology used to describe the hereditary neuropathies. This update reviews advances in the past year in our understanding of these diseases, including some important earlier references.

RECENT FINDINGS

In the past year, advances in next-generation sequencing continued to increase the number of genes associated with CMT. The connection between genotype and phenotype has become more complicated. New insights into the pathogenesis of the diseases are reviewed. Treatment and clinical trial updates coming from these new insights, as well as use of high-throughput screening to match potential treatments with targets, are moving the field forward. There is a discussion of potential next steps, including the use of patient-derived induced pluripotent stem cells, to enhance our understanding of individual genotypes and phenotypes.

SUMMARY

The use of high-throughput screens, and techniques such as RNAi and induced pluripotent stem cell continue to push forward other therapies for specific genetic forms of CMT and are potentially more generalizable to peripheral neuropathies. These developments, along with the development of improved outcome measures and longitudinal natural history data, advance CMT, making the future for finding treatments and/or cures closer than it has ever been.

A case of neuromyotonia and axonal motor neuropathy: A report of a HINT1 mutation in the United States

INTRODUCTION

HINT1 mutations cause an autosomal recessive distal hereditary motor axonal neuropathy with neuromyotonia. This is a case report of a HINT1 mutation in the United States.

METHODS

A 30-year-old man of Slovenian heritage and no significant family history presented with scoliosis as a child and later developed neuromyotonia and distal weakness. Electrodiagnostic testing revealed an axonal motor neuropathy and neuromyotonic discharges. Previous diagnostic work-up, including testing for Cx32, MPZ, PMP-22, NF-L, EGR2, CLCN1, DM1, DM2, SMN exon 7/8, emerin, LMNA, MPK, SCNA4, acid maltase gene, paraneoplastic disorder, and a sural nerve biopsy, was negative.

RESULTS

Genetic testing for a HINT1 mutation was performed and revealed a homozygous mutation at p.Arg37Pro.

CONCLUSION

This entity should be distinguished clinically and genetically from myotonic dystrophy and channelopathies with the clinical features of neuromyotonia and an axonal neuropathy. This case illustrates the importance of identifying the correct phenotype to avoid unnecessary and costly evaluations.

MORC2 mutations cause axonal Charcot-Marie-Tooth disease with pyramidal signs

OBJECTIVE

To use linkage analysis and whole exome sequencing to identify the genetic mutation in a multigenerational Australian family with Charcot-Marie-Tooth disease type 2 (CMT2) and pyramidal signs.

METHODS

Genome-wide linkage analysis was performed to map the locus. Whole exome sequencing was undertaken on selected individuals (3 affected, 1 normal), and segregation analysis and mutation screening were carried out using high-resolution melt analysis. The GEM.app database was queried to identify additional families with mutations.

RESULTS

Significant linkage (2-point LOD score ≥ +3) and haplotype analysis mapped a new locus for CMT2 and pyramidal signs to a 6.6Mb interval on chromosome 22q12.1-q12.3. Whole exome sequencing identified a novel mutation (p.R252W) in the microrchidia CW-type zinc finger 2 (MORC2) gene mapping within the linkage region. The mutation fully segregated with the disease phenotype in the family. Screening additional families and querying unsolved CMT2 exomes, we identified the p.R252W mutation in 2 unrelated early onset CMT2 families and a second mutation p.E236G in 2 unrelated CMT2 families. Both the mutations occurred at highly conserved amino acid residues and were absent in the normal population.

INTERPRETATION

We have identified a new locus in which MORC2 mutations are the likely pathogenic cause of CMT2 and pyramidal signs in these families. MORC2 encodes the human CW-type zinc finger 2 protein, which is a chromatin modifier involved in the regulation of DNA repair as well as gene transcription.

Absence of HINT1 mutations in a UK and Spanish cohort of patients with inherited neuropathies

Biallelic mutations in the HINT1 gene were recently identified as the cause of axonal neuropathy with neuromyotonia. It has been suggested that HINT1 mutations may indeed account for 11% of all inherited neuropathies with autosomal recessive inheritance. However, 81% of patients HINT1-related neuropathies reported to date are originally from five European countries and the global prevalence of the disorder is still unknown. In our study, we aimed to determine the frequency of HINT1 mutations by direct sequencing in a cohort of 152 patients with inherited neuropathies from the UK and Spain, where no cases have been described to date. We failed to identify patients with clinical myotonia, neuromyotonia or pathogenic mutations in HINT1. Our results support that HINT1-related neuropathies are not homogeneously distributed among European populations, which may be explained by founder effects. This geographical variability also underlines the importance of considering the ethnic background when screening for mutations in neuropathy-related genes.

A SIGMAR1 splice-site mutation causes distal hereditary motor neuropathy

OBJECTIVE

To identify the underlying genetic cause in a consanguineous Chinese family segregating distal hereditary motor neuropathy (dHMN) in an autosomal recessive pattern.

METHODS

We used whole-exome sequencing and homozygosity mapping to detect the genetic variant in 2 affected individuals of the consanguineous Chinese family with dHMN. RNA analysis of peripheral blood leukocytes and immunofluorescence and immunoblotting of stable cell lines were performed to support the pathogenicity of the identified mutation.

RESULTS

We identified 3 shared novel homozygous variants in 3 shared homozygous regions of the affected individuals. Sequencing of these 3 variants in family members revealed the c.151+1G>T mutation in SIGMAR1 gene, which located in homozygous region spanning approximately 5.3 Mb at chromosome 9p13.1-p13.3, segregated with the dHMN phenotype. The mutation causes an alternative splicing event and generates a transcript variant with an in-frame deletion of 60 base pairs in exon 1 (c.92_151del), and results in an internally shortened protein σ1R(31_50del). The proteasomal inhibitor treatment increased the intracellular amount of σ1R(31_50del) and led to the formation of nuclear aggregates. Stable expressing σ1R(31_50del) induced endoplasmic reticulum stress and enhanced apoptosis.

CONCLUSION

The homozygous c.151+1G>T mutation in SIGMAR1 caused a novel form of autosomal recessive dHMN in a Chinese consanguineous family. Endoplasmic reticulum stress may have a role in the pathogenesis of dHMN.

"Genotype-first" approaches on a curious case of idiopathic progressive cognitive decline

BACKGROUND

In developing countries, many cases with rare neurological diseases remain undiagnosed due to limited diagnostic experience. We encountered a case in China where two siblings both began to develop idiopathic progressive cognitive decline starting from age six, and were suspected to have an undiagnosed neurological disease.

METHODS

Initial clinical assessments included review of medical history, comprehensive physical examination, genetic testing for metabolic diseases, blood tests and brain imaging. We performed exome sequencing with Agilent SureSelect exon capture and Illumina HiSeq2000 platform, followed by variant annotation and selection of rare, shared mutations that fit a recessive model of inheritance. To assess functional impacts of candidate variants, we performed extensive biochemical tests in blood and urine, and examined their possible roles by protein structure modeling.

RESULTS

Exome sequencing identified NAGLU as the most likely candidate gene with compound heterozygous mutations (chr17:40695717C > T and chr17:40693129A > G in hg19 coordinate), which were documented to be pathogenic. Sanger sequencing confirmed the recessive patterns of inheritance, leading to a genetic diagnosis of Sanfilippo syndrome (mucopolysaccharidosis IIIB). Biochemical tests confirmed the complete loss of activity of alpha-N-acetylglucosaminidase (encoded by NAGLU) in blood, as well as significantly elevated dermatan sulfate and heparan sulfate in urine. Structure modeling revealed the mechanism on how the two variants affect protein structural stability.

CONCLUSIONS

Successful diagnosis of a rare genetic disorder with an atypical phenotypic presentation confirmed that such "genotype-first" approaches can particularly succeed in areas of the world with insufficient medical genetics expertise and with cost-prohibitive in-depth phenotyping.

Mutations in HINT1 are one of the most frequent causes of hereditary neuropathy among Czech patients and neuromyotonia is rather an underdiagnosed symptom

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.

Lack of neuropathy-related phenotypes in hint1 knockout mice

Mutations in HINT1, the gene encoding histidine triad nucleotide-binding protein 1 (HINT1), cause a recessively inherited peripheral neuropathy that primarily involves motor dysfunction and is usually associated with neuromyotonia (i.e. prolonged muscle contraction resulting from hyperexcitability of peripheral nerves). Because these mutations are hypothesized to cause loss of function, we analyzed Hint1 knockout mice for their relevance as a disease model. Mice lacking Hint1 appeared normal and yielded normal behavioral test results or motor performance, although they moved more slowly and for a smaller fraction of time in an open-field arena than wild-type mice. Muscles, neuromuscular junctions, and nodes of Ranvier were anatomically normal and did not show evidence of degeneration or regeneration. Axon numbers and myelination in peripheral nerves were normal at ages 4 and 13 months. Axons were slightly smaller than those in wild-type mice at age 4 months, but this did not cause a decrease in conduction velocity, and no differences in axon diameters were detected at 13 months. With electromyography, we were unable to detect neuromyotonia even after using supraphysiologic stimuli and stressors such as reduced temperature or 3,4-diaminopyridine to block potassium channels. Therefore, we conclude that Hint1 knockout mice may be useful for studying the biochemical activities of HINT1, but these mice do not provide a disease model or a means for investigating the basis of HINT1-associated neuropathy and neuromyotonia.