Dominant spinal muscular atrophy with lower extremity predominance: linkage to 14q32

Cargo specificity, regulation, and therapeutic potential of cytoplasmic dynein

Intracellular retrograde transport in eukaryotic cells relies exclusively on the molecular motor cytoplasmic dynein 1. Unlike its counterpart, kinesin, dynein has a single isoform, which raises questions about its cargo specificity and regulatory mechanisms. The precision of dynein-mediated cargo transport is governed by a multitude of factors, including temperature, phosphorylation, the microtubule track, and interactions with a family of activating adaptor proteins. Activating adaptors are of particular importance because they not only activate the unidirectional motility of the motor but also connect a diverse array of cargoes with the dynein motor. Therefore, it is unsurprising that dysregulation of the dynein-activating adaptor transport machinery can lead to diseases such as spinal muscular atrophy, lower extremity, and dominant. Here, we discuss dynein motor motility within cells and in in vitro, and we present several methodologies employed to track the motion of the motor. We highlight several newly identified activating adaptors and their roles in regulating dynein. Finally, we explore the potential therapeutic applications of manipulating dynein transport to address diseases linked to dynein malfunction.

DYNC1H1 variants associated with infant-onset epilepsy without neurodevelopmental disorders

OBJECTIVES

The DYNC1H1 variants are associated with abnormal brain morphology and neuromuscular disorders that are accompanied by epilepsy. This study aimed to explore the relationship between DYNC1H1 variants and epilepsy.

MATERIALS AND METHODS

Trios-based whole-exome sequencing was performed on patients with epilepsy. Previously reported epilepsy-related DYNC1H1 variants were systematically reviewed to analyse genotype-phenotype correlation.

RESULTS

The DYNC1H1 variants were identified in four unrelated cases of infant-onset epilepsy, including two de novo and two biallelic variants. Two patients harbouring de novo missense variants located in the stem and stalk domains presented with refractory epilepsies, whereas two patients harbouring biallelic variants located in the regions between functional domains had mild epilepsy with infrequent focal seizures and favourable outcomes. One patient presented with pachygyria and neurodevelopmental abnormalities, and the other three patients presented with normal development. These variants have no or low frequencies in the Genome Aggregation Database. All the missense variants were predicted to be damaging using silico tools. Previously reported epilepsy-related variants were monoallelic variants, mainly de novo missense variants, and all the patients presented with severe epileptic phenotypes or developmental delay and malformations of cortical development. Epilepsy-related variants were clustered in the dimerization and stalk domains, and generalized epilepsy-associated variants were distributed in the stem domain.

CONCLUSION

This study suggested that DYNC1H1 variants are potentially associated with infant-onset epilepsy without neurodevelopmental disorders, expanding the phenotypic spectrum of DYNC1H1. The genotype-phenotype correlation helps to understand the underlying mechanisms of phenotypic variation.

Genetic Primary Microcephalies: When Centrosome Dysfunction Dictates Brain and Body Size

Primary microcephalies (PMs) are defects in brain growth that are detectable at or before birth and are responsible for neurodevelopmental disorders. Most are caused by biallelic or, more rarely, dominant mutations in one of the likely hundreds of genes encoding PM proteins, i.e., ubiquitous centrosome or microtubule-associated proteins required for the division of neural progenitor cells in the embryonic brain. Here, we provide an overview of the different types of PMs, i.e., isolated PMs with or without malformations of cortical development and PMs associated with short stature (microcephalic dwarfism) or sensorineural disorders. We present an overview of the genetic, developmental, neurological, and cognitive aspects characterizing the most representative PMs. The analysis of phenotypic similarities and differences among patients has led scientists to elucidate the roles of these PM proteins in humans. Phenotypic similarities indicate possible redundant functions of a few of these proteins, such as ASPM and WDR62, which play roles only in determining brain size and structure. However, the protein pericentrin (PCNT) is equally required for determining brain and body size. Other PM proteins perform both functions, albeit to different degrees. Finally, by comparing phenotypes, we considered the interrelationships among these proteins.

The Frequency of SMN1, SMN2 Copy Numbers in 246 Turkish Cases Analyzed with MLPA Method

This study aimed to define the copy numbers of SMN1 and SMN2 genes and the diagnosis rate and carrier frequency of spinal muscular atrophy (SMA) in the Thrace region of Turkey. In this study, the frequency of deletions in exons 7 and 8 in the SMN1 gene and SMN2 copy numbers were investigated. A total of 133 cases with the preliminary diagnosis of SMA and 113 cases with the suspicion of being an SMA carrier from independent families were analyzed by multiplex ligation-dependent probe amplification method for SMN1 and SMN2 gene copy numbers. SMN1 homozygous deletions were detected in 34 patients (25.5%) of 133 cases with the suspicion of SMA. Cases diagnosed with SMA type I was 41.17% (14/34), 29.4% (10/34) with type II, 26.4% (9/34) with type III, and 2.94% (1/34) with type IV. The SMA carrier rate was 46.01% in 113 cases. In 34 SMA cases, SMN2 copy numbers were: two copies - 28 cases (82.3%), three copies - 6 cases (17.6%). SMN2 homozygous deletions were detected in 15% (17/113) of carrier analysis cases. The consanguinity rate of the parents was 23.5% in SMA diagnosed cases. In this study, we had a 25.5% of SMA diagnosis rate and 46% SMA carrier frequency. The current study also showed the relatively low consanguinity rate of the Thrace region, with 23.5% according to the east of Turkey.

DYNC1H1-related epilepsy: Genotype-phenotype correlation

AIM

To explore the phenotypic spectrum and refine the genotype-phenotype correlation of DYNC1H1-related epilepsy.

METHOD

The clinical data of 15 patients with epilepsy in our cohort and 50 patients with epilepsy from 24 published studies with the DYNC1H1 variants were evaluated.

RESULTS

In our cohort, 13 variants were identified from 15 patients (seven males, eight females). Twelve variants were de novo and seven were new. Age at seizure onset ranged from 3 months to 4 years 5 months (median age 1 year). Common seizure types were epileptic spasms, focal seizures, tonic seizures, and myoclonic seizures. Mild-to-severe developmental delay was present in all patients. Six patients were diagnosed with West syndrome and one was diagnosed with epileptic encephalopathy with continuous spikes and waves during slow sleep (CSWS). Collectively, in our cohort and published studies, 17% had ophthalmic diseases, 31% of variants were located in the stalk domain, and 92% patients with epilepsy had a malformation of cortical development (MCD).

INTERPRETATION

The phenotypes of DYNC1H1-related epilepsy included multiple seizure types; the most common epileptic syndrome was West syndrome. CSWS is a new phenotype of DYNC1H1-related epilepsy. One-third of the variants in patients with epilepsy were located in the stalk domain. Most patients had a MCD and developmental delay.

WHAT THIS PAPER ADDS

Nearly 40% of patients with DYNC1H1 variants had epilepsy. Ninety-two percent of patients with DYNC1H1-related epilepsy had malformation of cortical development. More than 10% of patients with DYNC1H1-related epilepsy were diagnosed with West syndrome. Continuous spikes and waves during slow sleep could be a new phenotype of DYNC1H1 variants. One-third of the variants in patients with epilepsy were located in the stalk domain.

Muscle and bone characteristics of a Chinese family with spinal muscular atrophy, lower extremity predominant 1 (SMALED1) caused by a novel missense DYNC1H1 mutation

BACKGROUND

Spinal muscular atrophy, lower extremity predominant (SMALED) is a type of non-5q spinal muscular atrophy characterised by weakness and atrophy of lower limb muscles without sensory abnormalities. SMALED1 can be caused by dynein cytoplasmic 1 heavy chain 1 (DYNC1H1) gene variants. However, the phenotype and genotype of SMALED1 may overlap with those of other neuromuscular diseases, making it difficult to diagnose clinically. Additionally, bone metabolism and bone mineral density (BMD) in patients with SMALED1 have never been reported.

METHODS

We investigated a Chinese family in which 5 individuals from 3 generations had lower limb muscle atrophy and foot deformities. Clinical manifestations and biochemical and radiographic indices were analysed, and mutational analysis was performed by whole-exome sequencing (WES) and Sanger sequencing.

RESULTS

A novel mutation in exon 4 of the DYNC1H1 gene (c.587T > C, p.Leu196Ser) was identified in the proband and his affected mother by WES. Sanger sequencing confirmed that the proband and 3 affected family members were carriers of this mutation. As leucine is a hydrophobic amino acid and serine is hydrophilic, the hydrophobic interaction resulting from mutation of amino acid residue 196 could influence the stability of the DYNC1H1 protein. Leg muscle magnetic resonance imaging of the proband revealed severe atrophy and fatty infiltration, and electromyographic recordings showed chronic neurogenic impairment of the lower extremities. Bone metabolism markers and BMD of the proband were all within normal ranges. None of the 4 patients had experienced fragility fractures.

CONCLUSION

This study identified a novel DYNC1H1 mutation and expands the spectrum of phenotypes and genotypes of DYNC1H1-related disorders. This is the first report of bone metabolism and BMD in patients with SMALED1.

Generation of FOUR iPSC lines (CRICKi004-A; CRICKi005-A; CRICKi006-A, CRICKi007-A) from Spinal muscle atrophy patients with lower extremity dominant (SMALED) phenotype

Spinal muscular atrophy with lower extremity dominant (SMALED) is a hereditary neuromuscular disorder characterized by degeneration of spinal cord motor neurons resulting in lower limbs muscle weakness and paralysis. Mutations in DYNC1H1, which encodes BICD2, a multifunctional adaptor for microtubule motor proteins, cause the disorder. Here, we generated four induced pluripotent stem cell (iPSC) lines from patients with SMALED. Dermal fibroblasts were obtained from the MRC neuromuscular disease biobank and reprogrammed using non-integrating mRNA-based protocol. Characterization of the four iPSC lines included karyotyping and Sanger sequencing, while the expression of associated markers confirmed pluripotency and differentiation potential.

Expanding the Phenotypic and Genetic Spectrum of Neuromuscular Diseases Caused by DYNC1H1 Mutations

Objectives

Spinal muscular atrophy with lower extremity predominance 1 (SMALED1) and Charcot–Marie-Tooth diseasetype 2O (CMT2O) are two kinds of hereditary neuromuscular diseases caused by DYNC1H1 mutations. In this study, we reported two patients with SMALED1 caused by DYNC1H1 mutations. The genotype–phenotype correlations were further analyzed by systematically reviewing previous relevant publications.

Materials and Methods

Two patients' with SMALED1 and their parents' clinical data were collected, and detailed clinical examinations were performed. WES was then applied, which was confirmed by Sanger sequencing. PubMed, Web of Science, CNKI, and Wanfang Data were searched, and all publications that met the inclusion criteria were carefully screened. Any individual patient without a detailed description of clinical phenotypes was excluded.

Results

The two patients manifested delayed motor milestones and muscle wasting of both lower extremities. The diagnosis was further confirmed as SMALED1. Genetic testing revealed heterozygous DYNC1H1 mutations c.1792C>T and c.790C>G; the latter is a novel dominant mutation. Genotype–phenotype analysis of DYNC1H1 variants and neuromuscular diseases revealed that mutations in the DYN1 region of DYNC1H1 protein were associated with a more severe phenotype, more complicated symptoms, and more CNS involvement than the DHC_N1 region.

Conclusion

Our study potentially expanded the knowledge of the phenotypic and genetic spectrum of neuromuscular diseases caused by DYNC1H1 mutations. The genotype–phenotype correlation may reflect the pathogenesis underlying the dyneinopathy caused by DYNC1H1 mutations.

De novo DYNC1H1 mutation causes infantile developmental and epileptic encephalopathy with brain malformations

Background

The human dynein cytoplasmic 1 heavy chain 1 (DYNC1H1) gene encodes a large subunit of the cytoplasmic dynein complex. DYNC1H1 mutations are associated with various neurological diseases involving both the peripheral and central nervous systems.

Methods

The clinical characteristics and genetic data of an infant carrying the de novo DYNC1H1 variant identified by trio exome sequencing were analyzed. Patients with epilepsy with DYNC1H1 mutations were summarized by reviewing the literature.

Results

We first identified an infant presenting with epileptic spasms harboring a de novo missense mutation in DYNC1H1 (c.874C>T; p. Arg292Trp), once reported in an adult case, and further summarized another 54 patients with seizures or epilepsy caused by DYNC1H1 pathogenic variants in the literature. Refractory epilepsy, intellectual disability, and cortical developmental malformations are crucial characteristics of patients with developmental and epileptic encephalopathy (DEE) caused by DYNC1H1 variants. Notably, epileptic spasms in this case were resistant to multiple anti‐seizure medications, corticosteroids, ketogenic diet, and vagus nerve stimulation treatment. The child also showed cortical gyrus malformation and global developmental delay.

Conclusion

DYNC1H1 variants can cause infantile developmental and epileptic encephalopathy, in which Arg292Trp is a mutation hotspot of the DYNC1H1 gene. Epileptic seizures in this type of DYNC1H1‐related DEE are mostly resistant to multiple antiepileptic strategies and need to explore optimized treatments.

De Novo Variants in the DYNC1H1 Gene Associated With Infantile Spasms

Objective: The DYNC1H1 gene is related to a variety of diseases, including spinal muscular atrophy with lower extremity-predominant 1, Charcot-Marie-Tooth disease type 2O, and mental retardation, autosomal dominant13 (MRD13). Some patients with DYNC1H1 variant also had epilepsy. This study aimed to detect DYNC1H1 variants in Chinese patients with infantile spasms (ISs). Methods: We reviewed clinical information, video electroencephalogram (V-EEG), and neuroimaging of a newly identified cohort of five patients with de novo DYNC1H1gene variants. Results: Five patients with four DYNC1H1variants from four families were included. All patients had epileptic spasms (ESs), the median age at seizure onset was 7.5 months (range from 5 months to 2 years 7 months), and the interictal V-EEG results were hypsarrhythmia. Four of five patients had brain magnetic resonance imaging (MRI) abnormalities. Four de novo DYNC1H1 variants were identified, including two novel variants (p.N1117K, p.M3405L) and two reported variants (p.R1962C, p.F1093S). As for the variant site, two variants are located in the tail domain, one variant is located in the motor domain, and one variant is located in the stalk domain. All patients had tried more than five kinds of antiepileptic drugs. One patient has been controlled well by vigabatrin (VGB) for 4 years, and another patient by VGB and steroids for 1.5 years. The other three patients still had frequent ESs. All patients had severe intellectual disability and development delays. Significance: IS was one of the phenotypes of DYNC1H1 variants. Most patients had non-specific brain MRI abnormality. Two of four DYNC1H1 variants were novel, expanding the variant spectrum. The IS phenotype was related to the variant's domains of DYNC1H1 variant sites. All patients were drug-refractory and showed development delays.

Missense mutation in DYNC1H1 gene caused psychomotor developmental delay and muscle weakness: A case report

BACKGROUND

The DYNC1H1 gene encodes a part of the dynamic protein, and the protein mutations may further affect the growth and development of neurons, resulting in degeneration of anterior horn cells of the spinal cord, and a variety of clinical phenotypes finally resulting in axonal Charcot-Marie-Tooth disease type 20 (CMT20), mental retardation 13 (MRD13) and spinal muscular atrophy with lower extremity predominant 1 (SMA-LED). The incidence of the disease is low, and it is difficult to diagnose, especially in children. Here, we report a case of DYNC1H1 gene mutation and review the related literature to improve the pediatrician’s understanding of DYNC1H1 gene-related disease to make an early correct diagnosis and provide better services for children.

CASE SUMMARY

A 4-mo-old Chinese female child with adducted thumbs, high arch feet, and epileptic seizure presented slow response, delayed development, and low limb muscle strength. Electroencephalogram showed abnormal waves, a large number of multifocal sharp waves, sharp slow waves, and multiple spasms with a series of attacks. High-throughput sequencing and Sanger sequencing identified a heterozygous mutation, c.5885G>A (p.R1962H), in the DYNC1H1 gene (NM_001376) of the proband, which was not identified in her parents. Combined with the clinical manifestations and pedigree of this family, this mutation is likely pathogenic based on the American Academy of Medical Genetics and Genomics guidelines. The child was followed when she was 1 year and 2 mo old. The magnetic resonance imaging result was consistent with the findings of white matter myelinated dysplasia and congenital giant gyrus. The extensive neurogenic damage to the extremities was considered, as the results of electromyography showed that the motor conduction velocity and sensory conduction of the nerves of the extremities were not abnormal, and the degree of fit of the children with severe contraction was poor. At present, the child is 80 cm in length and 9 kg in weight, with slender limbs and low muscle strength, and still does not raise her head. She cannot sit or speak. Speech, motor, and mental development was significantly delayed. There is still no effective treatment for this disease.

CONCLUSION

We herein report a de novo variant of DYNC1H1 gene, c.5885G>A (p.R1962H), leading to overlapping phenotypes (seizure, general growth retardation, and muscle weakness) of CMT20, MRD13, and SMA-LED, but there is no effective treatment for such condition. Our case enriches the DYNC1H1 gene mutation spectrum and provides an important basis for clinical diagnosis and treatment and genetic counseling.

A novel BICD2 mutation of a patient with Spinal Muscular Atrophy Lower Extremity Predominant 2

The bicaudal D homolog 2 (BICD2) gene encodes a protein required for the stable complex of dynein and dynactin, which functions as a motor protein working along the microtubule cytoskeleton. Both inherited and de novo variants of BICD2 are reported with autosomal dominant spinal muscular atrophy with lower extremity predominance (SMALED2). Here, we report a male patient with a novel mutation in the BICD2 gene caused by a heterozygous substitution of arginine with cysteine at residue 162 (Arg162Cys); inherited from his asymptomatic mother. The patient showed typical clinical symptoms of SMALED2, which was genetically confirmed by sequencing. The Arg162Cys mutant clusters with four previously reported variants (c.361C>G, p.Leu121Val; c.581A>G, p.Gln194Arg; c.320C>T, p.Ser107Leu; c.565A>T, p.Ile189Phe) in a region that binds to the dynein-dynactin complex (DDC). The BICD2 domain structures were predicted and the Arg162Cys mutation was localized in the N-terminus coiled-coil segment 1 (CC1) domain. Protein modeling of BICD2's CC1 domain predicted that the Arg162Cys missense variant disrupted interactions with dynein cytoplasmic 1 heavy chain 1 within the DDC. The mutant did this by either changing the electrostatic surface potential or making a broader hydrophobic unit with the neighboring residues. This hereditary case supports the complex and broad genotype-phenotype correlation of BICD2 mutations, which could be explained by incomplete penetrance or variable expressivity in the next generation.

The clinical-phenotype continuum in DYNC1H1-related disorders-genomic profiling and proposal for a novel classification

Mutations in the cytoplasmic dynein 1 heavy chain gene (DYNC1H1) have been identified in rare neuromuscular (NMD) and neurodevelopmental (NDD) disorders such as spinal muscular atrophy with lower extremity dominance (SMALED) and autosomal dominant mental retardation syndrome 13 (MRD13). Phenotypes and genotypes of ten pediatric patients with pathogenic DYNC1H1 variants were analyzed in a multi-center study. Data mining of large-scale genomic variant databases was used to investigate domain-specific vulnerability and conservation of DYNC1H1. We identified ten patients with nine novel mutations in the DYNC1H1 gene. These patients exhibit a broad spectrum of clinical findings, suggesting an overlapping disease manifestation with intermixed phenotypes ranging from neuropathy (peripheral nervous system, PNS) to severe intellectual disability (central nervous system, CNS). Genomic profiling of healthy and patient variant datasets underlines the domain-specific effects of genetic variation in DYNC1H1, specifically on toleration towards missense variants in the linker domain. A retrospective analysis of all published mutations revealed domain-specific genotype–phenotype correlations, i.e., mutations in the dimerization domain with reductions in lower limb strength in DYNC1H1–NMD and motor domain with cerebral malformations in DYNC1H1–NDD. We highlight that the current classification into distinct disease entities does not sufficiently reflect the clinical disease manifestation that clinicians face in the diagnostic work-up of DYNC1H1-related disorders. We propose a novel clinical classification for DYNC1H1-related disorders encompassing a spectrum from DYNC1H1–NMD with an exclusive PNS phenotype to DYNC1H1–NDD with concomitant CNS involvement.

Burden of Rare Variants in ALS and Axonal Hereditary Neuropathy Genes Influence Survival in ALS: Insights from a Next Generation Sequencing Study of an Italian ALS Cohort

Although the genetic architecture of amyotrophic lateral sclerosis (ALS) is incompletely understood, recent findings suggest a complex model of inheritance in ALS, which is consistent with a multistep pathogenetic process. Therefore, the aim of our work is to further explore the architecture of ALS using targeted next generation sequencing (NGS) analysis, enriched in motor neuron diseases (MND)-associated genes which are also implicated in axonal hereditary motor neuropathy (HMN), in order to investigate if disease expression, including the progression rate, could be influenced by the combination of multiple rare gene variants. We analyzed 29 genes in an Italian cohort of 83 patients with both familial and sporadic ALS. Overall, we detected 43 rare variants in 17 different genes and found that 43.4% of the ALS patients harbored a variant in at least one of the investigated genes. Of note, 27.9% of the variants were identified in other MND- and HMN-associated genes. Moreover, multiple gene variants were identified in 17% of the patients. The burden of rare variants is associated with reduced survival and with the time to reach King stage 4, i.e., the time to reach the need for percutaneous endoscopic gastrostomy (PEG) positioning or non-invasive mechanical ventilation (NIMV) initiation, independently of known negative prognostic factors. Our data contribute to a better understanding of the molecular basis of ALS supporting the hypothesis that rare variant burden could play a role in the multistep model of disease and could exert a negative prognostic effect. Moreover, we further extend the genetic landscape of ALS to other MND-associated genes traditionally implicated in degenerative diseases of peripheral axons, such as HMN and CMT2.

Overlapping spectrums: The clinicogenetic commonalities between Charcot-Marie-Tooth and other neurodegenerative diseases

Charcot-Marie-Tooth (CMT) disease is a progressive and heterogeneous inherited peripheral neuropathy. A myriad of genetic factors have been identified that contribute to the degeneration of motor and sensory axons in a length-dependent manner. Emerging biological themes underlying disease include defects in axonal trafficking, dysfunction in RNA metabolism and protein homeostasis, as well deficits in the cellular stress response. Moreover, genetic contributions to CMT can have overlap with other neuropathies, motor neuron diseases (MNDs) and neurodegenerative disorders. Recent progress in understanding the molecular biology of CMT and overlapping syndromes aids in the search for necessary therapeutic targets.

VRK1 functional insufficiency due to alterations in protein stability or kinase activity of human VRK1 pathogenic variants implicated in neuromotor syndromes

Very rare polymorphisms in the human VRK1 (vaccinia-related kinase 1) gene have been identified in complex neuromotor phenotypes associated to spinal muscular atrophy (SMA), pontocerebellar hypoplasia (PCH), microcephaly, amyotrophic lateral sclerosis (ALS) and distal motor neuron dysfunctions. The mechanisms by which these VRK1 variant proteins contribute to the pathogenesis of these neurological syndromes are unknown. The syndromes are manifested when both of these rare VRK1 polymorphic alleles are implicated, either in homozygosis or compound heterozygosis. In this report, to identify the common underlying pathogenic mechanism of VRK1 polymorphisms, we have studied all human VRK1 variants identified in these neurological phenotypes from a biochemical point of view by molecular modeling, protein stability and kinase activity assays. Molecular modelling predicted that VRK1 variant proteins are either unstable or have an altered kinase activity. The stability and kinase activity of VRK1 pathogenic variants detected two groups. One composed by variants with a reduced protein stability: R133C, R358X, L195V, G135R and R321C. The other group includes VRK1variants with a reduced kinase activity tested on several substrates: histones H3 and H2AX, p53, c-Jun, coilin and 53BP1, a DNA repair protein. VRK1 variants with reduced kinase activity are H119R, R133C, G135R, V236M, R321C and R358X. The common underlying effect of VRK1 pathogenic variants with reduced protein stability or kinase activity is a functional insufficiency of VRK1 in patients with neuromotor developmental syndromes. The G135 variant cause a defective formation of 53BP1 foci in response to DNA damage, and loss Cajal bodies assembled on coilin.

Novel insights into SMALED2: BICD2 mutations increase microtubule stability and cause defects in axonal and NMJ development

Bicaudal D2 (BICD2) encodes a highly conserved motor adaptor protein that regulates the dynein-dynactin complex in different cellular processes. Heterozygous mutations in BICD2 cause autosomal dominant lower extremity-predominant spinal muscular atrophy-2 (SMALED2). Although, various BICD2 mutations have been shown to alter interactions with different binding partners or the integrity of the Golgi apparatus, the specific pathological effects of BICD2 mutations underlying SMALED2 remain elusive. Here, we show that the fibroblasts derived from individuals with SMALED2 exhibit stable microtubules. Importantly, this effect was observed regardless of where the BICD2 mutation is located, which unifies the most likely cellular mechanism affecting microtubules. Significantly, overexpression of SMALED2-causing BICD2 mutations in the disease-relevant cell type, motor neurons, also results in an increased microtubule stability which is accompanied by axonal aberrations such as collateral branching and overgrowth. To study the pathological consequences of BICD2 mutations in vivo, and to address the controversial debate whether two of these mutations are neuron or muscle specific, we generated the first Drosophila model of SMALED2. Strikingly, neuron-specific expression of BICD2 mutants resulted in reduced neuromuscular junction size in larvae and impaired locomotion of adult flies. In contrast, expressing BICD2 mutations in muscles had no obvious effect on motor function, supporting a primarily neurological etiology of the disease. Thus, our findings contribute to the better understanding of SMALED2 pathology by providing evidence for a common pathomechanism of BICD2 mutations that increase microtubule stability in motor neurons leading to increased axonal branching and to impaired neuromuscular junction development.

Neurochondrin interacts with the SMN protein suggesting a novel mechanism for spinal muscular atrophy pathology

Spinal muscular atrophy (SMA) is an inherited neurodegenerative condition caused by a reduction in the amount of functional survival motor neuron (SMN) protein. SMN has been implicated in transport of mRNA in neural cells for local translation. We previously identified microtubule-dependent mobile vesicles rich in SMN and SNRPB, a member of the Sm family of small nuclear ribonucleoprotein (snRNP)-associated proteins, in neural cells. By comparing the interactomes of SNRPB and SNRPN, a neural-specific Sm protein, we now show that the essential neural protein neurochondrin (NCDN) interacts with Sm proteins and SMN in the context of mobile vesicles in neurites. NCDN has roles in protein localisation in neural cells and in maintenance of cell polarity. NCDN is required for the correct localisation of SMN, suggesting they may both be required for formation and transport of trafficking vesicles. NCDN may have potential as a therapeutic target for SMA together with, or in place of the targeting of SMN expression.

This article has an associated First Person interview with the first author of the paper.

Highlighted Article: The essential neural protein neurochondrin interacts with the spinal muscular atrophy (SMA) protein SMN in cell lines and in mice. This might be relevant to the molecular pathology of SMA.

A missense mutation in DYNC1H1 gene causing spinal muscular atrophy - Lower extremity, dominant

Spinal muscular atrophy (SMA) is a hereditary neuromuscular disorder, which causes progressive muscle weakness and in severe cases respiratory failure and death. Although the majority of the SMA cases are autosomal recessive, there is an autosomal dominant variant of SMA that primarily affects the lower extremities, known as 'spinal muscular atrophy - lower extremity, dominant' (SMALED). Mutations in the Dynein Cytoplasmic 1 Heavy Chain 1 (DYNC1H1) gene were the first to be associated with SMALED. Here we report a family with SMALED caused by a pathogenic heterozygous missense c.1809 A>T, p.glu603Asp mutation in DYNC1H1. The main clinical features were congenital hip displacement, talipes, delayed motor development, wasting and weakness in lower limbs with relative sparing of upper extremities and very slow disease progression. SMALED is extremely rare and only a handful of families have been reported. Over the years other phenotypes including Charcot Marie Tooth type 2 and hereditary mental retardation with cortical neural migration defects have also been reported to be caused by DYNC1H1 mutations. This report aims to increase our awareness of SMALED and various other phenotypes associated with mutations in this gene.

Inherited Paediatric Motor Neuron Disorders: Beyond Spinal Muscular Atrophy

Paediatric motor neuron diseases encompass a group of neurodegenerative diseases characterised by the onset of muscle weakness and atrophy before the age of 18 years, attributable to motor neuron loss across various neuronal networks in the brain and spinal cord. While the genetic underpinnings are diverse, advances in next generation sequencing have transformed diagnostic paradigms. This has reinforced the clinical phenotyping and molecular genetic expertise required to navigate the complexities of such diagnoses. In turn, improved genetic technology and subsequent gene identification have enabled further insights into the mechanisms of motor neuron degeneration and how these diseases form part of a neurodegenerative disorder spectrum. Common pathophysiologies include abnormalities in axonal architecture and function, RNA processing, and protein quality control. This review incorporates an overview of the clinical manifestations, genetics, and pathophysiology of inherited paediatric motor neuron disorders beyond classic SMN1-related spinal muscular atrophy and describes recent advances in next generation sequencing and its clinical application. Specific disease-modifying treatment is becoming a clinical reality in some disorders of the motor neuron highlighting the importance of a timely and specific diagnosis.

Expanding the phenotypic spectrum associated with mutations of DYNC1H1

Autosomal dominant mutations of DYNC1H1 cause a range of neurogenetic diseases, including mental retardation with cortical malformations, hereditary spastic paraplegia and spinal muscular atrophy. Using SNP array, linkage analysis and next generation sequencing, we identified two families and one isolated proband sharing a known spinal muscular atrophy, lower extremity predominant (SMALED) causing mutation DYNC1H1 c.1792C>T, p.Arg598Cys, and another family harbouring a c.2327C>T, p.Pro776Leu mutation. Here, we present a detailed clinical and pathological examination of these patients, and show that patients with DYNC1H1 mutations may present with a phenotype mimicking a congenital myopathy. We also highlight features that increase the phenotypic overlap with BICD2, which causes SMALED2. Serial muscle biopsies were available for several patients, spanning from infancy and early childhood to middle age. These provide a unique insight into the developmental and pathological origins of SMALED, suggesting in utero denervation with reinnervation by surrounding intact motor neurons and segmental anterior horn cell deficits. We characterise biopsy features that may make diagnosis of this condition easier in the future.

Exome Sequencing Identifies De Novo DYNC1H1 Mutations Associated With Distal Spinal Muscular Atrophy and Malformations of Cortical Development

Exome sequencing has become a formidable tool for identifying potential de novo variants in causative genes of human diseases, such as neurodegenerative disorders. This article describes a 16-month-old girl with spinal muscular atrophy with lower extremity predominance and a 13-month-old girl with malformations of cortical development. Exome sequencing identified a novel de novo heterozygous missense mutation c.3395G>A (p.Gly1132Glu) and a previously reported de novo heterozygous missense mutation c.10151G>A (p.Arg3384Gln) in the DYNC1H1 gene. Bioinformatics predictions for c.3395G>A and c.10151G>A indicated pathogenicity of the mutations. DYNC1H1 is a pivotal component of cytoplasmic dynein complex, which is a microtubule-related motor involved in retrograde transport. Previous studies indicated that mutant dynein showed decreased run-length of the motor proteins and diminished retrograde transport, which were clearly associated with neuronal death and neurologic diseases. The present findings expand the mutational spectrum of the DYNC1H1 gene, reemphasizing the significance of the DYNC1H1 protein in the functioning of neurons.

DYNC1H1 mutations associated with neurological diseases compromise processivity of dynein-dynactin-cargo adaptor complexes

Mutations in the human DYNC1H1 gene are associated with neurological diseases. DYNC1H1 encodes the heavy chain of cytoplasmic dynein-1, a 1.4-MDa motor complex that traffics organelles, vesicles, and macromolecules toward microtubule minus ends. The effects of the DYNC1H1 mutations on dynein motility, and consequently their links to neuropathology, are not understood. Here, we address this issue using a recombinant expression system for human dynein coupled to single-molecule resolution in vitro motility assays. We functionally characterize 14 DYNC1H1 mutations identified in humans diagnosed with malformations in cortical development (MCD) or spinal muscular atrophy with lower extremity predominance (SMALED), as well as three mutations that cause motor and sensory defects in mice. Two of the human mutations, R1962C and H3822P, strongly interfere with dynein's core mechanochemical properties. The remaining mutations selectively compromise the processive mode of dynein movement that is activated by binding to the accessory complex dynactin and the cargo adaptor Bicaudal-D2 (BICD2). Mutations with the strongest effects on dynein motility in vitro are associated with MCD. The vast majority of mutations do not affect binding of dynein to dynactin and BICD2 and are therefore expected to result in linkage of cargos to dynein-dynactin complexes that have defective long-range motility. This observation offers an explanation for the dominant effects of DYNC1H1 mutations in vivo. Collectively, our results suggest that compromised processivity of cargo-motor assemblies contributes to human neurological disease and provide insight into the influence of different regions of the heavy chain on dynein motility.

Impairments in dendrite morphogenesis as etiology for neurodevelopmental disorders and implications for therapeutic treatments

Dendrite morphology is pivotal for neural circuitry functioning. While the causative relationship between small-scale dendrite morphological abnormalities (shape, density of dendritic spines) and neurodevelopmental disorders is well established, such relationship remains elusive for larger-scale dendrite morphological impairments (size, shape, branching pattern of dendritic trees). Here, we summarize published data on dendrite morphological irregularities in human patients and animal models for neurodevelopmental disorders, with focus on autism and schizophrenia. We next discuss high-risk genes for these disorders and their role in dendrite morphogenesis. We finally overview recent developments in therapeutic attempts and we discuss how they relate to dendrite morphology. We find that both autism and schizophrenia are accompanied by dendritic arbor morphological irregularities, and that majority of their high-risk genes regulate dendrite morphogenesis. Thus, we present a compelling argument that, along with smaller-scale morphological impairments in dendrites (spines and synapse), irregularities in larger-scale dendrite morphology (arbor shape, size) may be an important part of neurodevelopmental disorders' etiology. We suggest that this should not be ignored when developing future therapeutic treatments.

Spontaneous activity in electromyography may differentiate certain benign lower motor neuron disease forms from amyotrophic lateral sclerosis

There is limited data on electromyography (EMG) findings in other motor neuron disorders than amyotrophic lateral sclerosis (ALS). We assessed whether the distribution of active denervation detected by EMG, i.e. fibrillations and fasciculations, differs between ALS and slowly progressive motor neuron disorders. We compared the initial EMG findings of 43 clinically confirmed, consecutive ALS patients with those of 41 genetically confirmed Late-onset Spinal Motor Neuronopathy and 14 Spinal and Bulbar Muscular Atrophy patients. Spontaneous activity was more frequently detected in the first dorsal interosseus and deltoid muscles of ALS patients than in patients with the slowly progressive motor neuron diseases. The most important observation was that absent fibrillations in the first dorsal interosseus muscle identified the benign forms with sensitivities of 66%-77% and a specificity of 93%. The distribution of active denervation may help to separate ALS from mimicking disorders at an early stage.

Spinal muscular atrophies

Spinal muscular atrophies (SMAs) are hereditary degenerative disorders of lower motor neurons associated with progressive muscle weakness and atrophy. Proximal 5q SMA is caused by decreased levels of the survival of motor neuron (SMN) protein and is the most common genetic cause of infant mortality. Its inheritance pattern is autosomal recessive, resulting from mutations involving the SMN1 gene on chromosome 5q13. Unlike other autosomal recessive diseases, the SMN gene has a unique structure (an inverted duplication) that presents potential therapeutic targets. Although there is currently no effective treatment of SMA, the field of translational research in this disorder is active and clinical trials are ongoing. Advances in the multidisciplinary supportive care of children with SMA also offer hope for improved life expectancy and quality of life.

The Genetics of Spinal Muscular Atrophy: Progress and Challenges

Spinal muscular atrophies (SMAs) are a group of inherited disorders characterized by motor neuron loss in the spinal cord and lower brainstem, muscle weakness, and atrophy. The clinical and genetic phenotypes incorporate a wide spectrum that is differentiated based on age of onset, pattern of muscle involvement, and inheritance pattern. Over the past several years, rapid advances in genetic technology have accelerated the identification of causative genes and provided important advances in understanding the molecular and biological basis of SMA and insights into the selective vulnerability of the motor neuron. Common pathophysiological themes include defects in RNA metabolism and splicing, axonal transport, and motor neuron development and connectivity. Together these have revealed potential novel treatment strategies, and extensive efforts are being undertaken towards expedited therapeutics. While a number of promising therapies for SMA are emerging, defining therapeutic windows and developing sensitive and relevant biomarkers are critical to facilitate potential success in clinical trials. This review incorporates an overview of the clinical manifestations and genetics of SMA, and describes recent advances in the understanding of mechanisms of disease pathogenesis and development of novel treatment strategies.

Novel mutations expand the clinical spectrum of DYNC1H1-associated spinal muscular atrophy

OBJECTIVE

To expand the clinical phenotype of autosomal dominant congenital spinal muscular atrophy with lower extremity predominance (SMA-LED) due to mutations in the dynein, cytoplasmic 1, heavy chain 1 (DYNC1H1) gene.

METHODS

Patients with a phenotype suggestive of a motor, non-length-dependent neuronopathy predominantly affecting the lower limbs were identified at participating neuromuscular centers and referred for targeted sequencing of DYNC1H1.

RESULTS

We report a cohort of 30 cases of SMA-LED from 16 families, carrying mutations in the tail and motor domains of DYNC1H1, including 10 novel mutations. These patients are characterized by congenital or childhood-onset lower limb wasting and weakness frequently associated with cognitive impairment. The clinical severity is variable, ranging from generalized arthrogryposis and inability to ambulate to exclusive and mild lower limb weakness. In many individuals with cognitive impairment (9/30 had cognitive impairment) who underwent brain MRI, there was an underlying structural malformation resulting in polymicrogyric appearance. The lower limb muscle MRI shows a distinctive pattern suggestive of denervation characterized by sparing and relative hypertrophy of the adductor longus and semitendinosus muscles at the thigh level, and diffuse involvement with relative sparing of the anterior-medial muscles at the calf level. Proximal muscle histopathology did not always show classic neurogenic features.

CONCLUSION

Our report expands the clinical spectrum of DYNC1H1-related SMA-LED to include generalized arthrogryposis. In addition, we report that the neurogenic peripheral pathology and the CNS neuronal migration defects are often associated, reinforcing the importance of DYNC1H1 in both central and peripheral neuronal functions.

Low dietary protein content alleviates motor symptoms in mice with mutant dynactin/dynein-mediated neurodegeneration

Mutations in components of the molecular motor dynein/dynactin lead to neurodegenerative diseases of the motor system or atypical parkinsonism. These mutations are associated with prominent accumulation of vesicles involved in autophagy and lysosomal pathways, and with protein inclusions. Whether alleviating these defects would affect motor symptoms remain unknown. Here, we show that a mouse model expressing low levels of disease linked-G59S mutant dynactin p150(Glued) develops motor dysfunction >8 months before loss of motor neurons or dopaminergic degeneration is observed. Abnormal accumulation of autophagosomes and protein inclusions were efficiently corrected by lowering dietary protein content, and this was associated with transcriptional upregulations of key players in autophagy. Most importantly this dietary modification partially rescued overall neurological symptoms in these mice after onset. Similar observations were made in another mouse strain carrying a point mutation in the dynein heavy chain gene. Collectively, our data suggest that stimulating the autophagy/lysosomal system through appropriate nutritional intervention has significant beneficial effects on motor symptoms of dynein/dynactin diseases even after symptom onset.

Clinical and genetic diversity of SMN1-negative proximal spinal muscular atrophies

Peeters et al. review current knowledge regarding the phenotypes, causative genes, and disease mechanisms associated with proximal SMN1-negative spinal muscular atrophies (SMA). They describe the molecular and cellular functions enriched among causative genes, and discuss the challenges facing the post-genomics era of SMA research.

Hereditary spinal muscular atrophy is a motor neuron disorder characterized by muscle weakness and atrophy due to degeneration of the anterior horn cells of the spinal cord. Initially, the disease was considered purely as an autosomal recessive condition caused by loss-of-function SMN1 mutations on 5q13. Recent developments in next generation sequencing technologies, however, have unveiled a growing number of clinical conditions designated as non-5q forms of spinal muscular atrophy. At present, 16 different genes and one unresolved locus are associated with proximal non-5q forms, having high phenotypic variability and diverse inheritance patterns. This review provides an overview of the current knowledge regarding the phenotypes, causative genes, and disease mechanisms associated with proximal SMN1-negative spinal muscular atrophies. We describe the molecular and cellular functions enriched among causative genes, and discuss the challenges in the post-genomics era of spinal muscular atrophy research.

Novel dynein DYNC1H1 neck and motor domain mutations link distal spinal muscular atrophy and abnormal cortical development

DYNC1H1 encodes the heavy chain of cytoplasmic dynein 1, a motor protein complex implicated in retrograde axonal transport, neuronal migration, and other intracellular motility functions. Mutations in DYNC1H1 have been described in autosomal-dominant Charcot-Marie-Tooth type 2 and in families with distal spinal muscular atrophy (SMA) predominantly affecting the legs (SMA-LED). Recently, defects of cytoplasmic dynein 1 were also associated with a form of mental retardation and neuronal migration disorders. Here, we describe two unrelated patients presenting a combined phenotype of congenital motor neuron disease associated with focal areas of cortical malformation. In each patient, we identified a novel de novo mutation in DYNC1H1: c.3581A>G (p.Gln1194Arg) in one case and c.9142G>A (p.Glu3048Lys) in the other. The mutations lie in different domains of the dynein heavy chain, and are deleterious to protein function as indicated by assays for Golgi recovery after nocodazole washout in patient fibroblasts. Our results expand the set of pathological mutations in DYNC1H1, reinforce the role of cytoplasmic dynein in disorders of neuronal migration, and provide evidence for a syndrome including spinal nerve degeneration and brain developmental problems.

Morphological changes and altered expression of antioxidant proteins in a heterozygous dynein mutant; a mouse model of spinal muscular atrophy

OBJECTIVE

There is increased evidence that oxidative stress is involved in exacerbations of neurodegenerative diseases and spinal muscular atrophies.

METHODS

We examined changes in morphology and expression of antioxidant proteins and peroxiredoxins in motor neurons of lumbar spinal cord, dorsal root ganglion sensory neurons, macroglial cells and quadriceps muscles of newborn heterozygous Loa/+ mice ("legs at odd angles"), a mouse model for early onset of the spinal muscular atrophy with lower extremity predominance (SMA-LED).

RESULTS

Our data indicate that newborn Loa-mice develop: neuroinflammation of the sensory and motor neurons; muscular inflammation with atrophic and denervated myofibers; increased expression of neuronal mitochondrial peroxiredoxins (Prxs) 3, 5 and cytoplasmic Prx 6 in motor and sensory neurons, myofibers, fibroblasts of perimysium and chondrocytes of cartilage; and decreased expression of Prx 6 by glial cells and in extracellular space surrounding motor neurons.

CONCLUSION

The decrease in expression of Prx 6 by glial cells and extracellular Prx 6 secretion in early stages of the pathological conditions is consistent with the hypothesis that chronic oxidative stress may lead to neurodegeneration of motor neurons and exacerbation of the pathology.

Cytoplasmic dynein heavy chain: the servant of many masters

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The cytoplasmic dynein complex is the main retrograde motor in all eukaryotic cells.

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This complex is built around a dimer of cytoplasmic dynein heavy chains (DYNC1H1).

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Mouse DYNC1H1 mutants have sensory defects, but motor defects have been controversial.

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Now human DYNC1H1 mutations with sensory, motor, and cognitive deficits are being found.

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The study of these mutations will give us new insight into DYNC1H1 function in the nervous system.

Cytoplasmic dynein is the main retrograde motor in all eukaryotic cells. This complex comprises different subunits assembled on a cytoplasmic dynein heavy chain 1 (DYNC1H1) dimer. Cytoplasmic dynein is particularly important for neurons because it carries essential signals and organelles from distal sites to the cell body. In the past decade, several mouse models have helped to dissect the numerous functions of DYNC1H1. Additionally, several DYNC1H1 mutations have recently been found in human patients that give rise to a broad spectrum of developmental and midlife-onset disorders. Here, we discuss the effects of mutations of mouse and human DYNC1H1 and how these studies are giving us new insight into the many critical roles DYNC1H1 plays in the nervous system.

Molecular defects in the motor adaptor BICD2 cause proximal spinal muscular atrophy with autosomal-dominant inheritance

The most common form of spinal muscular atrophy (SMA) is a recessive disorder caused by deleterious SMN1 mutations in 5q13, whereas the genetic etiologies of non-5q SMA are very heterogeneous and largely remain to be elucidated. In a Bulgarian family affected by autosomal-dominant proximal SMA, we performed genome-wide linkage analysis and whole-exome sequencing and found a heterozygous de novo c.320C>T (p.Ser107Leu) mutation in bicaudal D homolog 2 (Drosophila) (BICD2). Further analysis of BICD2 in a cohort of 119 individuals with non-5q SMA identified a second de novo BICD2 mutation, c.2321A>G (p.Glu774Gly), in a simplex case. Detailed clinical and electrophysiological investigations revealed that both families are affected by a very similar disease course, characterized by early childhood onset, predominant involvement of lower extremities, and very slow disease progression. The amino acid substitutions are located in two interaction domains of BICD2, an adaptor protein linking the dynein molecular motor with its cargo. Our immunoprecipitation and localization experiments in HeLa and SH-SY5Y cells and affected individuals' lymphoblasts demonstrated that p.Ser107Leu causes increased dynein binding and thus leads to accumulation of BICD2 at the microtubule-organizing complex and Golgi fragmentation. In addition, the altered protein had a reduced colocalization with RAB6A, a regulator of vesicle trafficking between the Golgi and the endoplasmic reticulum. The interaction between p.Glu744Gly altered BICD2 and RAB6A was impaired, which also led to their reduced colocalization. Our study identifies BICD2 mutations as a cause of non-5q linked SMA and highlights the importance of dynein-mediated motility in motor neuron function in humans.

Microtubule-based transport - basic mechanisms, traffic rules and role in neurological pathogenesis

Microtubule-based transport is essential for neuronal function because of the large distances that must be traveled by various building blocks and cellular materials. Recent studies in various model systems have unraveled several regulatory mechanisms and traffic rules that control the specificity, directionality and delivery of neuronal cargos. Local microtubule cues, opposing motor activity and cargo-adaptors that regulate motor activity control microtubule-based transport in neurons. Impairment of intracellular transport is detrimental to neurons and has emerged as a common factor in several neurological disorders. Genetic approaches have revealed strong links between intracellular transport processes and the pathogenesis of neurological diseases in both the central and peripheral nervous system. This Commentary highlights recent advances in these areas and discusses the transport defects that are associated with the development of neurological diseases.

Autosomal dominant congenital spinal muscular atrophy: a true form of spinal muscular atrophy caused by early loss of anterior horn cells

Autosomal dominant congenital spinal muscular atrophy is characterized by predominantly lower limb weakness and wasting, and congenital or early-onset contractures of the hip, knee and ankle. Mutations in TRPV4, encoding a cation channel, have recently been identified in one large dominant congenital spinal muscular atrophy kindred, but the genetic basis of dominant congenital spinal muscular atrophy in many families remains unknown. It has been hypothesized that differences in the timing and site of anterior horn cell loss in the central nervous system account for the variations in clinical phenotype between different forms of spinal muscular atrophy, but there has been a lack of neuropathological data to support this concept in dominant congenital spinal muscular atrophy. We report clinical, electrophysiology, muscle magnetic resonance imaging and histopathology findings in a four generation family with typical dominant congenital spinal muscular atrophy features, without mutations in TRPV4, and in whom linkage to other known dominant neuropathy and spinal muscular atrophy genes has been excluded. The autopsy findings in the proband, who died at 14 months of age from an unrelated illness, provided a rare opportunity to study the neuropathological basis of dominant congenital spinal muscular atrophy. There was a reduction in anterior horn cell number in the lumbar and, to a lesser degree, the cervical spinal cord, and atrophy of the ventral nerve roots at these levels, in the absence of additional peripheral nerve pathology or abnormalities elsewhere along the neuraxis. Despite the young age of the child at the time of autopsy, there was no pathological evidence of ongoing loss or degeneration of anterior horn cells suggesting that anterior horn cell loss in dominant congenital spinal muscular atrophy occurs in early life, and is largely complete by the end of infancy. These findings confirm that dominant congenital spinal muscular atrophy is a true form of spinal muscular atrophy caused by a loss of anterior horn cells localized to lumbar and cervical regions early in development.

Autosomal dominant late-onset spinal motor neuronopathy is linked to a new locus on chromosome 22q11.2-q13.2

Spinal muscular atrophies (SMAs) are hereditary disorders characterized by degeneration of lower motor neurons. Different SMA types are clinically and genetically heterogeneous and many of them show significant phenotypic overlap. We recently described the clinical phenotype of a new disease in two Finnish families with a unique autosomal dominant late-onset lower motor neuronopathy. The studied families did not show linkage to any known locus of hereditary motor neuron disease and thus seemed to represent a new disease entity. For this study, we recruited two more family members and performed a more thorough genome-wide scan. We obtained significant linkage on chromosome 22q, maximum LOD score being 3.43 at marker D22S315. The linked area is defined by flanking markers D22S686 and D22S276, comprising 18.9 Mb. The region harbours 402 genes, none of which is previously known to be associated with SMAs. This study confirms that the disease in these two families is a genetically distinct entity and also provides evidence for a founder mutation segregating in both pedigrees.

Mutations in the tail domain of DYNC1H1 cause dominant spinal muscular atrophy

OBJECTIVE

To identify the gene responsible for 14q32-linked dominant spinal muscular atrophy with lower extremity predominance (SMA-LED, OMIM 158600).

METHODS

Target exon capture and next generation sequencing was used to analyze the 73 genes in the 14q32 linkage interval in 3 SMA-LED family members. Candidate gene sequencing in additional dominant SMA families used PCR and pooled target capture methods. Patient fibroblasts were biochemically analyzed.

RESULTS

Regional exome sequencing of all candidate genes in the 14q32 interval in the original SMA-LED family identified only one missense mutation that segregated with disease state-a mutation in the tail domain of DYNC1H1 (I584L). Sequencing of DYNC1H1 in 32 additional probands with lower extremity predominant SMA found 2 additional heterozygous tail domain mutations (K671E and Y970C), confirming that multiple different mutations in the same domain can cause a similar phenotype. Biochemical analysis of dynein purified from patient-derived fibroblasts demonstrated that the I584L mutation dominantly disrupted dynein complex stability and function.

CONCLUSIONS

We demonstrate that mutations in the tail domain of the heavy chain of cytoplasmic dynein (DYNC1H1) cause spinal muscular atrophy and provide experimental evidence that a human DYNC1H1 mutation disrupts dynein complex assembly and function. DYNC1H1 mutations were recently found in a family with Charcot-Marie-Tooth disease (type 2O) and in a child with mental retardation. Both of these phenotypes show partial overlap with the spinal muscular atrophy patients described here, indicating that dynein dysfunction is associated with a range of phenotypes in humans involving neuronal development and maintenance.