Hereditary lower motor neuron diseases (LMND) other than 5q‐spinal muscular atrophy (5q‐SMA) can be classified according to affected muscle groups. Proximal and distal forms of non‐5q‐SMA represent a clinically and genetically heterogeneous spectrum characterized by significant overlaps with axonal forms of Charcot‐Marie‐Tooth (CMT) disease. A consensus for the best approach to molecular diagnosis needs to be reached, especially in the light of continuous novel gene discovery and falling costs of next generation sequencing (NGS). We performed exome sequencing (ES) in 41 families presenting with non‐5q‐SMA or axonal CMT, 25 of which had undergone a previous negative neuromuscular disease (NMD) gene panel analysis. The total diagnostic yield of ES was 41%. Diagnostic success in the cohort with a previous NMD‐panel analysis was significantly extended by ES, primarily due to novel gene associated‐phenotypes and uncharacteristic phenotypic presentations. We recommend early ES for individuals with hereditary LMND presenting uncharacteristic or significantly overlapping features. As mitochondrial dysfunction was the underlying pathomechanism in 47% of the solved individuals, we highlight the sensitivity of the anterior horn cell and peripheral nerve to mitochondrial imbalance as well as the necessity to screen for mitochondrial disorders in individuals presenting predominant lower motor neuron symptoms.

Primary intraosseous vascular malformation (VMPI, #606893) is an ultra-rare disorder caused by biallelic pathogenic variants in ELMO2 . To date, only six families with pathogenic ELMO2 variants causing a VMPI phenotype have been described. VMPI is characterized by vascular malformations that compress the facial bones, often leading to life-threatening complications, such as massive bleeding and intracranial herniation. In VMPI, vascular malformations are progressive and there is no causal therapy available. We report on four unreported individuals with classical VMPI harbouring biallelic truncating variants in ELMO2 , including a novel homozygous 25 bp duplication c.579_603dup; p.(Leu202Profs*47), detected by whole-exome sequencing. We present extensive clinical follow-up data, including a close monitoring of an individual from prenatal diagnosis onwards. Using computed tomography or magnetic resonance imaging angiography, we described the radiological characteristics of vascular malformations with fast-flow properties in the affected individuals. Additionally, we conducted a comprehensive histopathological evaluation of samples from one individual. This analysis revealed not only the similar morphological features described previously but also some atypical findings, such as increased de novo bone formation. Furthermore, we report for the first time the use of propranolol and sirolimus in VMPI. While we noted a reduction of bleeding episodes in one individual, no significant clinical improvement was observed overall in the other individuals treated with sirolimus. Moreover, sirolimus led to severe infectious complications with abscess formation in two individuals. Conversely, propranolol was relatively well tolerated, although it did not result in any notable clinical outcomes. During follow-up, one individual died due to severe bleeding.

Background: The nuclear-encoded enzyme polymerase gamma (Pol-γ) is crucial in the replication of the mitochondrial genome (mtDNA), which in turn is vital for mitochondria and hence numerous metabolic processes and energy production in eukaryotic cells. Variants in the POLG gene, which encodes the catalytic subunit of Pol-γ, can significantly impair Pol-γ enzyme function. Pol-γ-associated disorders are referred to as POLG-spectrum disorders (POLG-SDs) and are mainly autosomal-recessively inherited. Clinical manifestations include muscle weakness and fatigue, and severe forms of the disease can lead to premature death in infancy, childhood, and early adulthood, often associated with seizures, liver failure, or intractable epilepsy. Here, we analyzed fibroblasts from a compound heterozygous patient with the established pathogenic variant c.2419C>T; p.(Arg807Cys) and a previously undescribed variant c.678G>C; p.(Gln226His) with a clinical manifestation compatible with POLG-SDs, sensory ataxic neuropathy, and infantile muscular atrophy. We conducted a battery of functional studies for Pol-γ and mitochondrial dysfunction on the patient’s fibroblasts, to test whether the novel variant c.678G>C; p.(Gln226His) may be causative in human disease. Aims/Methods: We analyzed skin-derived fibroblasts in comparison to a first-degree relative (the mother of the patient), an asymptomatic carrier harboring only the established c.2419C>T; p.(Arg807Cys) mutation. Assessments of mitochondrial function included measurements of mtDNA content, mRNA levels of mitochondrial genes, mitochondrial mass, and mitochondrial morphology. Case Presentation and Results: A 13-year-old male presented with symptoms starting at three years of age, including muscle weakness and atrophy in the lower extremities and facial muscles, which later extended to the upper limbs, voice, and back muscles, without further progression. The patient also reported fatigue and muscle pain after physical activity, with no sensory deficits. Extensive diagnostic tests such as electromyography, nerve conduction studies, muscle biopsy, and MRI were unremarkable. Exome sequencing revealed that he carried the compound heterozygous variants in POLG c.678G>C; p.(Gln226His) and c.2419C>T; p.(Arg807Cys), but no other potential genetic pathogenic causes. In comparison to a first-degree relative (his mother) who only carried the c.2419C>T; p.(Arg807Cys) pathogenic mutation, in vitro analyses revealed a significant reduction in mtDNA content (~50%) and mRNA levels of mtDNA-encoded proteins. Mitochondrial mass was reduced by approximately 20%, and mitochondrial interconnectivity within cells was impaired, as determined by fluorescence microscopy and mitochondrial staining. Conclusions: Our findings suggest that the c.678G>C; p.(Gln226His) variant, in conjunction with the c.2419C>T; p.(Arg807Cys) mutation, may compromise mtDNA replication and mitochondrial function and could result in clinically significant mitochondriopathy. As this study is based on one patient compared to a first-degree relative (but with an identical mitochondrial genome), the pathogenicity of c.678G>C; p.(Gln226His) of POLG should be confirmed in future studies, in particular, in conjunction with other POLG-variants.