Neurogenic arthrogryposis and the power of phenotyping

In this article we review the commonest cause of neurogenic arthrogryposis, termed Spinal Muscular Atrophy Lower Extremity Dominant (SMALED), due to variants in DYNC1H1 and BICD2. We discuss the characteristic clinical and radiological phenotype of this disorder and how this has facilitated the identification of the genetic cause of SMALED2. We also review the similarities and differences between the human SMALED phenotype and mouse models and how this has informed our understanding of the potential mechanisms governing motor neuron loss in these disorders.

Neuromuscular disease: 2021 update

This review highlights ten important advances in the neuromuscular disease field that were first reported in 2020. The overarching topics include (i) advances in understanding of fundamental neuromuscular biology; (ii) new / emerging diseases; (iii) advances in understanding of disease etiology and pathogenesis; (iv) diagnostic advances; and (v) therapeutic advances. Within this broad framework, the individual disease entities that are discussed in more detail include neuromuscular complications of COVID-19, supervillin-deficient myopathy, 19p13.3-linked distal myopathy, vasculitic neuropathy due to eosinophilic granulomatosis with polyangiitis, spinal muscular atrophy, idiopathic inflammatory myopathies, and transthyretin neuropathy/myopathy. In addition, the review highlights several other advances (such as the revised view of the myofibrillar architecture, new insights into molecular and cellular mechanisms of muscle regeneration, and development of new electron microscopy tools) that will likely have a significant impact on the overall neuromuscular disease field going forward.

SMALED2 with BICD2 gene mutations: Report of two cases and portrayal of a classical phenotype

The spinal muscular atrophies (SMA) affect lower motor neurons leading to important muscle atrophy and paralysis. Some cases of SMA affect mostly the lower limbs and are called autosomal dominant spinal muscular atrophy, lower extremity predominant (SMALED). So far, two genes have been identified to cause this phenotype, DYNC1H1 (SMALED1) and BICD2 (SMALED2). This pathology is rare, but patients exhibit classical features which should be recognised by physicians. We present two unrelated cases of SMALED2 with previously described c.320C>T BICD2 mutations. Our cases exhibit non-progressive weakness and atrophy of the lower limbs associated with contractures and unique muscle MRI findings suggestive of classical SMALED2. We also performed an extensive review of the literature to present the classical and atypical phenotypes of BICD2. Indeed, some features appear to be highly suggestive of the disease, including upper limb sparing, sparing of the adductors muscles on physical examination and MRI, congenital contractures and normal nerve conductions studies.

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.