Spinal muscular atrophy (SMA) is a heterogeneous group of neuromuscular disorders caused by degeneration of lower motor neurons. Although functional loss of SMN1 is associated with autosomal-recessive childhood SMA, the genetic cause for most families affected by dominantly inherited SMA is unknown. Here, we identified pathogenic variants in bicaudal D homolog 2 (Drosophila) (BICD2) in three families afflicted with autosomal-dominant SMA. Affected individuals displayed congenital slowly progressive muscle weakness mainly of the lower limbs and congenital contractures. In a large Dutch family, linkage analysis identified a 9q22.3 locus in which exome sequencing uncovered c.320C>T (p.Ser107Leu) in BICD2. Sequencing of 23 additional families affected by dominant SMA led to the identification of pathogenic variants in one family from Canada (c.2108C>T [p.Thr703Met]) and one from the Netherlands (c.563A>C [p.Asn188Thr]). BICD2 is a golgin and motor-adaptor protein involved in Golgi dynamics and vesicular and mRNA transport. Transient transfection of HeLa cells with all three mutant BICD2 cDNAs caused massive Golgi fragmentation. This observation was even more prominent in primary fibroblasts from an individual harboring c.2108C>T (p.Thr703Met) (affecting the C-terminal coiled-coil domain) and slightly less evident in individuals with c.563A>C (p.Asn188Thr) (affecting the N-terminal coiled-coil domain). Furthermore, BICD2 levels were reduced in affected individuals and trapped within the fragmented Golgi. Previous studies have shown that Drosophila mutant BicD causes reduced larvae locomotion by impaired clathrin-mediated synaptic endocytosis in neuromuscular junctions. These data emphasize the relevance of BICD2 in synaptic-vesicle recycling and support the conclusion that BICD2 mutations cause congenital slowly progressive dominant SMA.

Heterozygous variants in BICD cargo adapter 2 (BICD2) cause autosomal dominant spinal muscular atrophy, lower extremity-predominant 2 (SMALED2). The disease is usually characterized by a benign or slowly progressive, congenital or early onset muscle weakness and atrophy that mainly affects the lower extremities, although some affected individuals show involvement of the arms and the shoulder girdle. Here we report unusual extremes of BICD2-related diseases: A severe form of congenital muscular atrophy with arthrogryposis multiplex, respiratory insufficiency and lethality within four months. This was caused by three BICD2 variants, (c.581A>G, p.(Gln194Arg)), (c.1626C>G, p.(Cys542Trp)) and (c.2080C>T, p.(Arg694Cys)), two of which were proven to be de novo. Affected individuals showed reduced fetal movement, weak muscle tone and sparse or no spontaneous activity after birth. Despite assisted ventilation, the condition led to early death. At the other extreme, we identified an asymptomatic woman with a known BICD2 variant (c.2108C>T, p.(Thr703Met)). Radiological examination showed fatty degeneration of selected thigh and calf muscles without clinical consequences. Instead, her son carrying the same variant is affected by a mild childhood onset disease with myopathic and neurogenic features. Mechanisms leading to variable expressivity and onset of BICD2-related disease may include alterations in molecular interactions of BICD2 and suggest the presence of genetic modifiers that may act in a protective fashion to ameliorate or abrogate disease. Our data define an additional severe disease type caused by BICD2 and emphasize a possibly variable etiology of BICD2-opathies with regard to primary muscle and neuronal involvement.

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Our findings extend the phenotypic spectrum of BICD2-associated disorders by features of a chronic myopathy and show a pathomechanism of BICD2 defects in skeletal muscle.