Biallelic variants in NDUFS6, encoding an accessory subunit of mitochondrial complex I, were initially associated with lethal neonatal mitochondrial encephalopathy and Leigh syndrome. Recent studies have demonstrated that NDUFS6 variants can also cause childhood- or adolescent-onset axonal neuropathy and Charcot-Marie-Tooth (CMT)-like phenotypes, indicating marked clinical heterogeneity. Here, we report a patient with a novel homozygous truncating NDUFS6 variant presenting with a neuropathy-predominant phenotype accompanied by epilepsy, in the absence of neonatal metabolic decompensation. The patient presented with childhood-onset progressive gait abnormality, pes cavus deformity, distal weakness requiring Achilles tendon-release surgery, pyramidal signs, urinary incontinence, and focal epileptiform EEG findings. Brain MRI showed bilateral lenticular nucleus abnormalities. Whole-exome sequencing identified a novel homozygous NDUFS6 nonsense variant (c.130C>T, p.Gln44*). While neuropathy has previously been reported primarily in association with the recurrent splice-site variant c.309+5G>A, our findings demonstrate that truncating NDUFS6 mutations can also underlie a neuropathy-predominant phenotype. Together with previously published cases, our findings support a phenotypic heterogeneity ranging from lethal encephalopathy to neuropathy and reinforce the role of NDUFS6 as a disease-causing gene for inherited peripheral neuropathy. These data support inclusion of NDUFS6 among established neuropathy and Charcot-Marie-Tooth genes.

The dystonin gene (DST) encodes three major isoforms, DST-a, DST-b and DST-e. Biallelic pathogenic variants in DST have previously been associated with two allelic monogenic disorders: hereditary sensory and autonomic neuropathy type VI (caused by a loss of DST-a) and epidermolysis bullosa simplex 3 (caused by a loss of DST-e). We investigated patients diagnosed with congenital myopathy using exome or genome sequencing. In 19 affected individuals from 14 unrelated families, we identified nine different variants in biallelic state located in exons 40-41, specific to DST-b. Affected individuals presented with severe neonatal myopathy characterized by arthrogryposis, hypotonia and dilated cardiomyopathy. Postnatal CPAP ventilation was required in nine patients, and seven died within the first three years of life. Survivors showed an improvement of symptoms, with the oldest three patients, now over 25 years old, exhibiting normal cognition and being ambulatory. RNA analyses demonstrated that transcripts encoding DST-b are predominantly expressed in skeletal muscle, heart tissue and cultured fibroblasts, but not in brain, matching the phenotypic spectrum. Patient-derived fibroblasts exhibited reduced DST mRNA expression. Proteomic analysis confirmed a reduction of DST protein levels due to an absence of the DST-b isoform. Muscle biopsies from four patients aged 1 month to 3 years revealed mild, non-specific myopathic changes. Ultrastructural analysis in three individuals showed mild and focal myofibrillar disruption and non-specific undulating nuclear membranes, with these changes observed in two cases each. Additionally, we identified two homozygous variants affecting both DST-a and DST-b isoforms in four patients from two unrelated families; all presented with severe arthrogryposis and died intrauterine or shortly after birth. Genotype-phenotype correlation in these patients and previously published cases with respective variants resulted in the definition of a DST-associated lethal congenital contracture syndrome. Our findings demonstrate that biallelic variants exclusively affecting DST-b cause an autosomal recessive congenital myopathy. Variants that also impact DST-a besides DST-b result in a more severe, lethal congenital contracture syndrome. The location of the variant within DST allows for phenotype prediction. We propose redefining DST as a disease-associated gene linked to four distinct allelic disease phenotypes.

This study aims to comprehensively delineate the phenotypic spectrum of ACTL6B-related disorders, previously associated with both autosomal recessive and autosomal dominant neurodevelopmental disorders. Molecularly, the role of the nucleolar protein ACTL6B in contributing to the disease has remained unclear. We identified 105 affected individuals, including 39 previously reported cases, and systematically analyzed detailed clinical and genetic data for all individuals. Additionally, we conducted knockdown experiments in neuronal cells to investigate the role of ACTL6B in ribosome biogenesis. Biallelic variants in ACTL6B are associated with severe-to-profound global developmental delay/intellectual disability, infantile intractable seizures, absent speech, autistic features, dystonia, and increased lethality. De novo monoallelic variants result in moderate-to-severe global developmental delay/intellectual disability, absent speech, and autistic features, whereas seizures and dystonia were less frequently observed. Dysmorphic facial features and brain abnormalities, including hypoplastic corpus callosum, and parenchymal volume loss/atrophy, are common findings in both groups. We reveal that in the nucleolus, ACTL6B plays a crucial role in ribosome biogenesis, particularly in pre-rRNA processing. This study provides a comprehensive characterization of the clinical spectrum of both autosomal recessive and dominant forms of ACTL6B-associated disorders. It offers a comparative analysis of their respective phenotypes provides a plausible molecular explanation and suggests their inclusion within the expanding category of "ribosomopathies."

Background/Objectives: PPP2R1A encodes the scaffold subunit Aα of protein phosphatase 2A (PP2A). Pathogenic variants cause Houge-Janssens syndrome 2, a rare neurodevelopmental disorder characterized by developmental delay, intellectual disability, epilepsy, and brain malformations. We systematically reviewed published cases to define the clinical spectrum, characterize the mutational landscape, and explore genotype-phenotype correlations. Methods: We conducted systematic searches of PubMed, Embase, and Web of Science from inception to March 2025, supplemented by GeneReviews and OMIM references. Studies reporting PPP2R1A variants with clinical data were included. Data extraction followed PRISMA guidelines, encompassing study characteristics, genetic findings, and phenotypic features. Results: We identified 16 studies representing 60 patients with PPP2R1A-related disorders. Twenty-six distinct pathogenic variants were identified; these were predominantly de novo heterozygous missense changes clustering within HEAT repeats 5-7. Recurrent hotspots included p.Arg182Trp (n = 12) and p.Arg183Gln (n = 5). Developmental delay and intellectual disability were universally present in all patients for whom data were available (100%, 58/58). Epilepsy occurred in 50.9% (29/57), and structural brain abnormalities in 83.1% (49/59), with corpus callosum abnormalities (40.7%, 24/59) and ventriculomegaly (32.2%, 19/59) being most frequent. Microcephaly was reported in 17.2% (10/58) and macrocephaly in 25.9% (15/58), while dysmorphic features were present in 53.4% (31/58). The phenotypic spectrum ranged from severe neonatal presentations with high mortality to milder neurodevelopmental courses, with prenatal manifestations including ventriculomegaly, corpus callosum abnormalities, and rare cardiac defects. Clear genotype-phenotype correlations emerged, with HEAT5 variants (p.Arg182Trp, p.Arg183Gln) associated with severe phenotypes and increased mortality, while p.Arg258His variants demonstrated comparatively milder courses. Conclusions: PPP2R1A-related disorders encompass a broad clinical spectrum ranging from lethal neonatal disease to survivable forms with variable neurodevelopmental outcomes. Prenatal features including ventriculomegaly and corpus callosum abnormalities enable early genetic diagnosis, informing reproductive counseling. Recognition of recurrent hotspot variants and their phenotype associations facilitates diagnosis, prognosis, and genetic counseling. These findings provide evidence-based guidance for clinical management and highlight the importance of variant-specific prognostication in this emerging neurodevelopmental disorder.

Prader-Willi syndrome (PWS) is a multigenic disorder caused by the loss of 7 contiguous paternally expressed genes. Mouse models with inactivation of all PWS genes are lethal. KO mouse models for each candidate gene have been generated, but they lack the functional interactions between PWS genes. Here, we revealed an interplay between Necdin and Magel2 PWS genes and generated a mouse model (named Del Ndn-Magel2 mice) with a deletion including both genes. A subset of Del Ndn-Magel2 mice showed neonatal lethality. Behaviorally, surviving mutant mice exhibited sensory delays during infancy and alterations in social exploration at adulthood. Del Ndn-Magel2 mice had a lower body weight before weaning, persisting after weaning in males only, with reduced fat mass and improved glucose tolerance as well as altered puberty. Adult mutant mice displayed increased ventilation and a persistent increase in apneas following a hypercapnic challenge. Transcriptomics analyses revealed a dysregulation of key circadian genes and alterations of genes associated with axonal function similar to patients with PWS. At neuroanatomical levels, Del Ndn-Magel2 mice had an impaired maturation of oxytocin neurons and a disrupted development of melanocortin circuits. Together, these data indicate that the Del Ndn-Magel2 mouse is a pertinent and genetically relevant model of PWS.