INTRODUCTION/AIMS:Giant axonal neuropathy (GAN) is a hereditary neurodegenerative disease due to the absence or loss of function of the gigaxonin gene. Pathologic findings in GAN are those of "dying-back" axonal degeneration, in which the distal axon degenerates but the more proximal axon and neuronal cell body remain intact. Aims of this study were to (1) document imaging abnormalities that may occur in the spinal cords of GAN patients; and (2) assess histologically the spinal cords of GAN rodent models. METHODS:A clinical trial of intrathecal (IT) scAAV9/JeT- GAN gene transfer provided a cohort of GAN patients for study. We examined spinal magnetic resonance imaging (MRI) studies from a subset of pretreatment GAN patients ages 6-14 years. For radiologic-pathologic correlation, we examined histologically spinal cords from GAN rodent models with pathological features of human GAN. RESULTS:Of 10 GAN-patient spinal MRIs, 7 showed cervical or diffuse cord atrophy. Five MRIs additionally showed hyperintense, T2-signal abnormalities bilaterally in the cervical gracile tracts. Microscopy of GAN-rodent spinal cords revealed many actively degenerating axons in the cervical gracile tracts but few degenerating axons elsewhere in the cord. DISCUSSION:The localization of spinal T-2 signal abnormalities to the cervical gracile tracts in GAN patients mirrors the localization of active dying-back axonal degeneration in GAN rodent models and suggests that these T2-signal abnormalities may be used as a surrogate marker of active axonal degeneration in the long tracts of the spinal cord in GAN and possibly other dying-back neurodegenerative diseases involving the spinal cord.

Background: Congenital myopathies are a group of neuromuscular disorders that typically present at birth or early childhood with hypotonia and non-progressive or slowly progressive muscle weakness. They are classically subclassified by characteristic structural changes and histopathological findings in skeletal muscle. Variants in over 40 genes have been described to date in patients with various forms of congenital myopathy with overlapping phenotypic and histological features, which poses a challenge for laboratories and clinicians in interpreting genetic findings. Objective: The purpose of this study was to evaluate the evidence supporting each gene-disease relationship and provide an expert-reviewed classification for the clinical validity of genes involved in congenital myopathies. Methods: The ClinGen Neurological Disorders Clinical Domain Working Group assembled the Congenital Myopathies Gene Curation Expert Panel (CongenMyopathy-GCEP), a group of clinicians and geneticists with expertise in congenital myopathies tasked to perform evidence-based curation of 50 gene-disease relationships using the ClinGen semiquantitative framework to assign clinical validity. Results: Our curation effort resulted in 35 (70%) Definitive, eight (16%) Moderate, six (12%) Limited, and one (2%) Disputed disease relationship classifications. The summary of each curation is made publicly available on the ClinGen website. Conclusions: Expert-reviewed assignment of gene-disease relationships by the CongenMyopathy-GCEP facilitates accurate molecular diagnoses for congenital myopathies and can allow genetic testing to focus on genes with a validated role in disease.

Collagen VI-related dystrophies manifest with a spectrum of clinical phenotypes, ranging from Ullrich congenital muscular dystrophy (UCMD), presenting with prominent congenital symptoms and characterized by progressive muscle weakness, joint contractures and respiratory insufficiency, to Bethlem muscular dystrophy, with milder symptoms typically recognized later and at times resembling a limb girdle muscular dystrophy, and intermediate phenotypes falling between UCMD and Bethlem muscular dystrophy. Despite clinical and muscle pathology features highly suggestive of collagen VI-related dystrophy, some patients had remained without an identified causative variant in COL6A1, COL6A2 or COL6A3. With combined muscle RNA sequencing and whole-genome sequencing, we uncovered a recurrent, de novo deep intronic variant in intron 11 of COL6A1 (c.930+189C>T) that leads to a dominantly acting in-frame pseudoexon insertion. We subsequently identified and have characterized an international cohort of 44 patients with this COL6A1 intron 11 causative variant, one of the most common recurrent causative variants in the collagen VI genes. Patients manifest a consistently severe phenotype characterized by a paucity of early symptoms followed by an accelerated progression to a severe form of UCMD, except for one patient with somatic mosaicism for this COL6A1 intron 11 variant who manifests a milder phenotype consistent with Bethlem muscular dystrophy. Partial amelioration of the disease phenotype in this individual provides a strong rationale for the development of our pseudoexon skipping therapy to successfully suppress the pseudoexon insertion, resulting in normal COL6A1 transcripts. We have previously shown that splice-modulating antisense oligomers applied in vitro effectively decreased the abundance of the mutant pseudoexon-containing COL6A1 transcripts to levels comparable to the in vivo scenario of the somatic mosaicism shown here, indicating that this therapeutic approach carries significant translational promise for ameliorating the severe form of UCMD caused by this common recurrent COL6A1 variant.

OBJECTIVE:Charcot-Marie-Tooth (CMT) disease is a heterogeneous group of genetic neuropathies, with >90 genes identified. Several aminoacyl-tRNA synthetases have been linked to CMT. DARS2, encoding the mitochondrial aspartyl-tRNA synthetase, has been typically associated with leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation. This study aimed to investigate the association between biallelic DARS2 variants and axonal CMT. METHODS:We investigated 5 individuals from 3 unrelated families with axonal CMT and biallelic DARS2 variants. Functional studies in fibroblasts assessed their effects on DARS2 expression, localization, and mitochondrial function. Enzymatic activity was evaluated in HEK293 cells. RESULTS:The 5 individuals, including 4 adults, presented with childhood-onset progressive axonal CMT. None had leukoencephalopathy, but one showed central nervous system involvement, with intellectual disability and epilepsy. Genetic analysis identified compound heterozygous DARS2 variants: family A, p.Ser238Phe and p.Arg336Cys; family B, p.Ser238Phe and p.Ile25Thrfs*38; family C, c.492+2T>C and p.Pro503Leu. Functional studies revealed reduced DARS2 protein levels, mitochondrial network abnormalities, and impaired mitochondrial function. p.Ser238Phe behaves as a hypomorphic allele, whereas p.Pro503Leu reduced DARS2 enzymatic activity by 75%. INTERPRETATION:Our findings expand the DARS2-related disease spectrum, establishing a novel association with axonal CMT. Hypomorphic variants, such as p.Ser238Phe, when paired with more deleterious variants, result in isolated axonal CMT, whereas more severe combinations-although not as deleterious as those seen in leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation-result in axonal CMT with central nervous system involvement, albeit without leukoencephalopathy. These observations raise the possibility that DARS2-associated diseases form a continuum rather than representing strictly distinct central or peripheral nervous system disorders. ANN NEUROL 2025;98:1335-1351.

What is this summary about? This summary describes the results of a research study (clinical trial) called ASPIRO that was published in the Lancet Neurology in 2023. This study looked at an investigational gene therapy called resamirigene bilparvovec (also known as AT132) as a possible treatment for children with a disease called X-linked myotubular myopathy (abbreviated as XLMTM).

Variants in the mitochondrial genome (mtDNA) cause a diverse collection of mitochondrial diseases and have extensive phenotypic overlap with Mendelian diseases encoded on the nuclear genome. The mtDNA is not always specifically evaluated in patients with suspected Mendelian disease, resulting in overlooked diagnostic variants. Here, we analyzed a cohort of 6,660 rare disease families (5,625 genetically undiagnosed [84%]) from the Genomics Research to Elucidate the Genetics of Rare diseases (GREGoR) Consortium, as well as other rare disease cohorts. Using dedicated pipelines to address the technical challenges posed by the mtDNA-circular genome, variant heteroplasmy, and nuclear misalignment-we called single nucleotide variants, small insertions/deletions, and large mtDNA deletions from exome and/or genome sequencing data, in addition to RNA sequencing data when available. Diagnostic mtDNA variants were identified in 10 previously genetically undiagnosed families (1 large deletion, 8 reported pathogenic variants, and 1 previously unreported likely pathogenic variant), as well as candidate diagnostic variants in a further 11 undiagnosed families. In one additional undiagnosed proband, detection of >900 heteroplasmic variants provided functional evidence of pathogenicity to a de novo variant in the nuclear gene POLG (DNA polymerase gamma), responsible for mtDNA replication and repair. Overall, mtDNA variant calling from data generated by exome and genome sequencing-primarily for nuclear variant analysis-resulted in a genetic diagnosis for 0.2% of undiagnosed families affected by a broad range of rare diseases, as well as the identification of additional promising candidates in 0.2%.

Collagen VI-related disorders (COL6-RDs) are a group of rare muscular dystrophies caused by pathogenic variants in collagen VI genes (COL6A1, COL6A2, and COL6A3). Collagen type VI is a heterotrimeric, microfibrillar component of the muscle extracellular matrix (ECM), predominantly secreted by resident fibroadipogenic precursor cells in skeletal muscle. The absence or mislocalizatoion of collagen VI in the ECM underlies the non-cell autonomous dysfunction and dystrophic changes in skeletal muscle with an as of yet elusive direct mechanistic link between the ECM and myofiber dysfunction. Here, we conduct a comprehensive natural history and outcome study in a novel mouse model of COL6-RDs (Col6a2-/- mice) using standardized (Treat-NMD) functional, histological, and physiologic parameter. Notably, we identify a conspicuous dysregulation of the TGFβ pathway early in the disease process and propose that the collagen VI deficient matrix is not capable of regulating the dynamic TGFβ bioavailability at baseline and also in response to muscle injury. Thus, we propose a new mechanism for pathogenesis of the disease that links the ECM regulation of TGFβ with downstream skeletal muscle abnormalities, paving the way for developing and validating therapeutics that target this pathway.

ABSTRACT Objectives To investigate the consequences of a pathogenic missense variant (c.838C>T; p.L280F) and a 5′‐UTR regulatory variant (c.‐122G>T) in BCS1L on disease pathogenesis and to understand how regulatory variants influence disease severity and clinical presentation. Methods Deep phenotyping, research‐based whole genome sequencing, biochemical characterization of identified variants, and studies in patient‐derived fibroblast cultures were applied to uncover the underlying genetic cause and molecular defects in siblings with a genetically uncharacterized complex neurologic condition. Results Genome sequencing identified a paternally inherited missense variant (c.838C>T; p.L280F) and a maternally inherited 5′‐UTR variant (c.‐122G>T) in BCS1L in two affected siblings. Although the missense variant disrupts complex III assembly, the 5′‐UTR variant allows residual wild‐type BCS1L expression, likely mitigating disease severity. Biochemical studies in patient‐derived fibroblasts confirmed the pathogenicity of both variants and demonstrated a moderate in vitro response to a coenzyme Q10 analog. Interpretation This study expands the clinical spectrum of BCS1L ‐related disorders to include a comparatively milder phenotype with central and peripheral nervous system involvement. Our findings demonstrate that the 5′‐UTR variant modulates disease severity by enabling residual wild‐type BCS1L expression, partially mitigating the pathogenic effects of the missense variant. These insights underscore the importance of evaluating both protein coding and regulatory variants in mitochondrial disease diagnostics and pathogenesis.

Rigid spine syndrome is a rare childhood-onset myopathy characterized by slowly progressive or non-progressive scoliosis, neck and spine contractures, hypotonia and respiratory insufficiency. Biallelic variants in SELENON account for most cases of rigid spine syndrome, however, the underlying genetic cause in some patients remains unexplained. We used exome and genome sequencing to investigate the genetic basis of rigid spine syndrome in patients without a genetic diagnosis. In five patients from four unrelated families, we identified biallelic variants in HMGCS1 (3-hydroxy-3-methylglutaryl-coenzyme A synthase). These included six missense variants and one frameshift variant distributed throughout HMGCS1. All patients presented with spinal rigidity primarily affecting the cervical and dorso-lumbar regions, scoliosis and respiratory insufficiency. Creatine kinase levels were variably elevated. The clinical course worsened with intercurrent disease or certain drugs in some patients; one patient died from respiratory failure following infection. Muscle biopsies revealed irregularities in oxidative enzyme staining with occasional internal nuclei and rimmed vacuoles. HMGCS1 encodes a critical enzyme of the mevalonate pathway and has not yet been associated with disease. Notably, biallelic hypomorphic variants in downstream enzymes including HMGCR and GGPS1 are associated with muscular dystrophy resembling our cohort's presentation. Analyses of recombinant human HMGCS1 protein and four variants (p.S447P, p.Q29L, p.M70T, p.C268S) showed that all mutants maintained their dimerization state. Three of the four mutants exhibited reduced thermal stability, and two mutants showed subtle changes in enzymatic activity compared to the wildtype. Hmgcs1 mutant zebrafish displayed severe early defects, including immobility at 2 days and death by Day 3 post-fertilisation and were rescued by HMGCS1 mRNA. We demonstrate that the four variants tested (S447P, Q29L, M70T and C268S) have reduced function compared to wild-type HMGCS1 in zebrafish rescue assays. Additionally, we demonstrate the potential for mevalonic acid supplementation to reduce phenotypic severity in mutant zebrafish. Overall, our analyses suggest that these missense variants in HMGCS1 act through a hypomorphic mechanism. Here, we report an additional component of the mevalonate pathway associated with disease and suggest biallelic variants in HMGCS1 should be considered in patients presenting with an unresolved rigid spine myopathy phenotype. Additionally, we highlight mevalonoic acid supplementation as a potential treatment for patients with HMGCS1-related disease.

Background: Amyotrophic lateral sclerosis (ALS) is a severe neurodegenerative disorder that progressively affects motor neurons. Gain-of-function mutations in serine palmitoyltransferase (SPT) genes, notably SPTLC1 and SPTLC2, have been linked to juvenile ALS. Here, we describe two childhood-onset ALS cases with distinct SPTLC2 mutations, providing new insights into sphingolipid dysregulation and its role in ALS pathogenesis. Material and methods: Two Chinese patients with early-onset ALS, both carrying SPTLC2 mutations, were recruited from Beijing Children's Hospital. We conducted whole-exome sequencing (WES) to identify genetic variants, followed by Sanger sequencing for validation. Sphingolipid profiles were analyzed using ultra-high-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Clinical evaluations included neurological assessments, brain MRI and electromyography. Additionally, mutant cell lines were established to assess the functional effects of the specific mutations. Results: Patient 1, a 6-year-old male, exhibited a novel heterozygous de-novo SPTLC2 variant (c.197T > G, p.T66R). Patient 2, a 7-year-old female, had a recurrent heterozygous de-novo SPTLC2 variant (c.778G > A, p.E260K). Both patients showed elevated levels of specific sphingolipids compared to controls, with distinct profiles between the SPTLC2-ALS and SPTLC1-hereditary sensory and autonomic neuropathy type 1 (HSAN1) cases. The novel p.T66R mutation was predicted to alter protein interactions within the SPT complex, potentially impairing sphingolipid homeostasis. Functional studies further revealed that the p.T66R variant reduces the inhibitory regulation of SPT by ORMDL proteins, leading to unrestrained SPT activity and excess sphingolipid production. Conclusions: Our findings identify a novel SPTLC2 variant linked to childhood-onset ALS and reveal altered sphingolipid profiles associated with different genetic mutations. These results underscore the importance of sphingolipid metabolism in ALS and suggest potential avenues for targeted therapeutic interventions. Further research is needed to explore treatment options aimed at modulating sphingolipid levels and correcting genetic defects, as well as investigating potential biomarkers for early diagnosis.

Structural variants (SVs) of the nebulin gene (NEB), including intragenic duplications, deletions, and copy number variation of the triplicate region, are an established cause of recessively inherited nemaline myopathies and related neuromuscular disorders. Large deletions have been shown to cause dominantly inherited distal myopathies. Here we provide an overview of 35 families with muscle disorders caused by such SVs in NEB. Using custom Comparative Genomic Hybridization arrays, exome sequencing, short-read genome sequencing, custom Droplet Digital PCR, or Sanger sequencing, we identified pathogenic SVs in 35 families with NEB-related myopathies. In 23 families, recessive intragenic deletions and duplications or pathogenic gains of the triplicate region segregating with the disease in compound heterozygous form, together with a small variant in trans, were identified. In two families the SV was, however, homozygous. Eight of these families have not been described previously. In 12 families with a distal myopathy phenotype (of which 10 are previously unpublished), eight unique, large deletions encompassing 52–97 exons in either heterozygous (n = 10) or mosaic (n = 2) state were identified. In the families where inheritance was recessive, no correlation could be made between the types of variants and the severity of the disease. In contrast, all patients with large dominant deletions in NEB had milder, predominantly distal muscle weakness. For the first time, we establish a clear and statistically significant association between large NEB deletions and a form of distal myopathy. In addition, we provide the hitherto largest overview of the spectrum of SVs in NEB.

Biallelic variants in ARL6IP1 are associated with a rare, complicated form of progressive hereditary spastic paraplegia. Among the few cases reported thus far, two distinct phenotypic clusters with upper and lower motor neuron pathology and varying severities have emerged. Here, we describe a proband who presented with decreased fetal movements, intrauterine growth retardation, arthrogryposis multiplex congenita (AMC), dysmorphic features, weakness and hypotonia. Course was complicated by extubation failure and feeding problems at age 3 months. Muscle biopsy demonstrated neurogenic changes. Magnetic resonance imaging revealed thin corpus callosum and simplified gyri most notable at the insula with incomplete opercularization. The proband developed tongue fasciculations at 5 months, and passed away at 15 months of age. A homozygous deletion of exon 1-3 of ARL6IP1 was identified through exome sequencing. ARL6IP1-related phenotypes now include in utero involvement, neurogenic AMC, dysmorphic features, microcephaly and malformations of cortical development, in the absence of spastic paraplegia.

Pathogenic variants in HMGCR were recently linked to a limb-girdle muscular dystrophy (LGMD) phenotype. The protein product HMG CoA reductase (HMGCR) catalyzes a key component of the cholesterol synthesis pathway. The two other muscle diseases associated with HMGCR, statin-associated myopathy (SAM) and autoimmune anti-HMGCR myopathy, are not inherited in a Mendelian pattern. Statins inhibit HMGCR activity to generate their cholesterol-lowering effects and are known to cause multiple types of adverse effects on skeletal muscle, while the antibodies associated with anti-HMGCR myopathy specifically target this enzyme. The mechanism linking pathogenic variants in HMGCR with skeletal muscle dysfunction is unclear. We knocked down Hmgcr in mouse skeletal myoblasts, knocked down hmgcr in Drosophila, and expressed three pathogenic HMGCR variants (c.1327C>T, p.Arg443Trp; c.1522_1524delTCT, p.Ser508del; and c.1621G>A, p.Ala541Thr) in Hmgcr knockdown mouse myoblasts. Hmgcr deficiency was associated with decreased proliferation, increased apoptosis, and impaired myotube fusion. Transcriptome sequencing of Hmgcr knockdown versus control myoblasts revealed differential expression involving mitochondrial function, with corresponding differences in cellular oxygen consumption rates. Both ubiquitous and muscle-specific knockdown of hmgcr in Drosophila led to lethality. Overexpression of reference HMGCR cDNA rescued myotube fusion in knockdown cells, whereas overexpression of the pathogenic variants of HMGCR cDNA did not. These results suggest that the three HMGCR-related muscle diseases share disease mechanisms related to skeletal muscle development.

Gene therapy development presents multiple challenges, and early planning is vital in the successful implementation of such programs. The Platform Vector Gene Therapy (PaVe-GT) program is a National Institutes of Health (NIH) initiative developing adeno-associated virus (AAV) gene therapies for four low-prevalence rare diseases. Utilizing the platform-based approach, the program aims to incorporate efficiencies throughout the preclinical and clinical development processes followed by public dissemination of scientific and regulatory learnings. Early in development, the establishment of a Target Product Profile (TPP) by the research team is a critical step to guide product development and align preclinical studies to clinical objectives. Based on the specific needs of the investigational product as defined in the TPP, an overall regulatory strategy can then be outlined to meet the regulatory requirements for the first-in-human clinical trials. During the preclinical phase of development, sponsors may request meetings with the Food and Drug Administration (FDA) to gather feedback on the planned studies and regulatory strategy. To pave the way for PaVe-GT's first investigational AAV gene therapy lead candidate, AAV9-hPCCA, we sought early feedback from the FDA utilizing an INitial Targeted Engagement for Regulatory Advice on CBER/CDER ProducTs (INTERACT) meeting. Here, we elaborate on the value of establishing a TPP and the FDA INTERACT meeting by including our initial AAV9-hPCCA TPP, detailing our INTERACT meeting experience, providing all corresponding regulatory documentation, and highlighting lessons learned. The regulatory documents along with templates developed by our program can also be found on the PaVe-GT website (https://pave-gt.ncats.nih.gov/). This communication aims to provide stakeholders with resources that can be applied to drug development programs in establishing a viable regulatory path to clinical trial initiation.

Intracellular trafficking involves an intricate machinery of motor complexes, including the dynein complex, to shuttle cargo for autophagolysosomal degradation. Deficiency in dynein axonemal chains, as well as cytoplasmic light and intermediate chains, have been linked with ciliary dyskinesia and skeletal dysplasia. The cytoplasmic dynein 1 heavy chain protein (DYNC1H1) serves as a core complex for retrograde trafficking in neuronal axons. Dominant pathogenic variants in DYNC1H1 have been previously implicated in peripheral neuromuscular disorders (NMD) and neurodevelopmental disorders (NDD). As heavy-chain dynein is ubiquitously expressed, the apparent selectivity of heavy chain dyneinopathy for motor neuronal phenotypes remains currently unaccounted for. Here, we aimed to evaluate the full DYNC1H1-related clinical, molecular and imaging spectrum, including multisystem features and novel phenotypes presenting throughout life. We identified 47 cases from 43 families with pathogenic heterozygous variants in DYNC1H1 (aged 0-59 years) and collected phenotypic data via a comprehensive standardized survey and clinical follow-up appointments. Most patients presented with divergent and previously unrecognized neurological and multisystem features, leading to significant delays in genetic testing and establishing the correct diagnosis. Neurological phenotypes include novel autonomic features, previously rarely described behavioral disorders, movement disorders and periventricular lesions. Sensory neuropathy was identified in nine patients (median age of onset 10.6 years), of which five were only diagnosed after the second decade of life, and three had a progressive age-dependent sensory neuropathy. Novel multisystem features included primary immunodeficiency, bilateral sensorineural hearing loss, organ anomalies and skeletal manifestations, resembling the phenotypic spectrum of other dyneinopathies. We also identified an age-dependent biphasic disease course with developmental regression in the first decade and, following a period of stability, neurodegenerative progression after the second decade of life. Of note, we observed several cases in whom neurodegeneration appeared to be prompted by intercurrent systemic infections with double-stranded DNA viruses (Herpesviridae) or single-stranded RNA viruses (Ross River fever, SARS-CoV-2). Moreover, the disease course appeared to be exacerbated by viral infections regardless of age and/or severity of neurodevelopmental disorder manifestations, indicating a role of dynein in anti-viral immunity and neuronal health. In summary, our findings expand the clinical, imaging and molecular spectrum of pathogenic DYNC1H1 variants beyond motor neuropathy disorders and suggest a life-long continuum and age-related progression due to deficient intracellular trafficking. This study will facilitate early diagnosis and improve counselling and health surveillance of affected patients.

Background:Neurogenetic disorders caused by pathogenic variants in four genes encoding non-erythrocytic spectrins ( SPTAN1, SPTBN1, SPTBN2, SPTBN4) range from peripheral and central nervous system involvement to complex syndromic presentations. Heterozygous pathogenic variants in SPTAN1 are exemplary for this diversity with phenotypes spanning almost the entire spectrum. Methods:Through international collaboration we identified 14 families with genetically unsolved distal weakness and unreported heterozygous SPTAN1 loss-of-function variants including frameshift, nonsense and splice-acceptor variants. Clinical data, electrophysiology, muscle CT or MRI and muscle biopsy findings were collected and standardized. SPTAN1 protein, mRNA expression analysis and cDNA sequencing was performed on muscle tissue from two patients. Results:All 20 patients presented with early childhood onset distal weakness. The severity varied both within families and between different families. Foot abnormalities ranged from hammer toes and pes cavus to distal arthrogryposis. Electrophysiology showed mixed myogenic and neurogenic features. Muscle MRI or CT in 10 patients showed fatty infiltration of the distal lower limb anterior compartment and/or selective involvement of the extensor hallucis longus muscle. Muscle biopsy revealed myopathic changes with mild dystrophic and chronic neurogenic changes in 7 patients. Finally, we provide proof for nonsense mediated decay in tissues derived from two patients. Conclusions:We provide evidence for the association of SPTAN1 loss-of-function variants with childhood onset distal myopathy in 14 families. This finding extends the phenotypic spectrum of SPTAN1 loss-of-function variants ranging from intellectual disability to distal weakness with a predominant myogenic cause. KEY MESSAGES:SPTAN1 loss-of-function variants, including frameshift, nonsense and splice site variants cause a novel childhood onset distal weakness syndrome with primarily skeletal muscle involvement. Hereditary motor neuropathies and distal myopathic disorders present a well-known diagnostic challenge as they demonstrate substantial clinical and genetic overlap. The emergence of SPTAN1 loss-of-function variants serves as a noteworthy example, highlighting a growing convergence in the spectrum of genotypes linked to both hereditary motor neuropathies and distal myopathies.

Collagen VI-related dystrophies (COL6-RDs) manifest with a spectrum of clinical phenotypes, ranging from Ullrich congenital muscular dystrophy (UCMD), presenting with prominent congenital symptoms and characterised by progressive muscle weakness, joint contractures and respiratory insufficiency, to Bethlem muscular dystrophy, with milder symptoms typically recognised later and at times resembling a limb girdle muscular dystrophy, and intermediate phenotypes falling between UCMD and Bethlem muscular dystrophy. Despite clinical and immunohistochemical features highly suggestive of COL6-RD, some patients had remained without an identified causative variant in COL6A1, COL6A2 or COL6A3. With combined muscle RNA-sequencing and whole-genome sequencing we uncovered a recurrent, de novo deep intronic variant in intron 11 of COL6A1 (c.930+189C>T) that leads to a dominantly acting in-frame pseudoexon insertion. We subsequently identified and have characterised an international cohort of forty-four patients with this COL6A1 intron 11 causative variant, one of the most common recurrent causative variants in the collagen VI genes. Patients manifest a consistently severe phenotype characterised by a paucity of early symptoms followed by an accelerated progression to a severe form of UCMD, except for one patient with somatic mosaicism for this COL6A1 intron 11 variant who manifests a milder phenotype consistent with Bethlem muscular dystrophy. Characterisation of this individual provides a robust validation for the development of our pseudoexon skipping therapy. We have previously shown that splice-modulating antisense oligomers applied in vitro effectively decreased the abundance of the mutant pseudoexon-containing COL6A1 transcripts to levels comparable to the in vivo scenario of the somatic mosaicism shown here, indicating that this therapeutic approach carries significant translational promise for ameliorating the severe form of UCMD caused by this common recurrent COL6A1 causative variant to a Bethlem muscular dystrophy phenotype.