Novel insights into SMALED2: BICD2 mutations increase microtubule stability and cause defects in axonal and NMJ development.

Bicaudal D2 (BICD2) encodes a highly conserved motor adaptor protein that regulates the dynein-dynactin complex in different cellular processes. Heterozygous mutations in BICD2 cause autosomal dominant lower extremity-predominant spinal muscular atrophy-2 (SMALED2). Although, various BICD2 mutations have been shown to alter interactions with different binding partners or the integrity of the Golgi apparatus, the specific pathological effects of BICD2 mutations underlying SMALED2 remain elusive. Here, we show that the fibroblasts derived from individuals with SMALED2 exhibit stable microtubules. Importantly, this effect was observed regardless of where the BICD2 mutation is located, which unifies the most likely cellular mechanism affecting microtubules. Significantly, overexpression of SMALED2-causing BICD2 mutations in the disease-relevant cell type, motor neurons, also results in an increased microtubule stability which is accompanied by axonal aberrations such as collateral branching and overgrowth. To study the pathological consequences of BICD2 mutations in vivo, and to address the controversial debate whether two of these mutations are neuron or muscle specific, we generated the first Drosophila model of SMALED2. Strikingly, neuron-specific expression of BICD2 mutants resulted in reduced neuromuscular junction size in larvae and impaired locomotion of adult flies. In contrast, expressing BICD2 mutations in muscles had no obvious effect on motor function, supporting a primarily neurological etiology of the disease. Thus, our findings contribute to the better understanding of SMALED2 pathology by providing evidence for a common pathomechanism of BICD2 mutations that increase microtubule stability in motor neurons leading to increased axonal branching and to impaired neuromuscular junction development.

Phenotypic extremes of BICD2-opathies: from lethal, congenital muscular atrophy with arthrogryposis to asymptomatic with subclinical features.

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.

Expanding the phenotype of BICD2 mutations toward skeletal muscle involvement.

OBJECTIVE: To expand the spectrum of bicaudal D, Drosophila, homologue 2 (BICD2) gene-related diseases, which so far includes autosomal dominant spinal muscular atrophy with lower extremity predominance 2 and hereditary spastic paraplegia due to mutations in the BICD2 gene. METHODS: We analyzed 2 independent German families with clinical, genetic, and muscle MRI studies. In both index patients, muscle histopathologic studies were performed. Transfection studies were carried out to analyze the functional consequences of the disease-causing mutations. RESULTS: We identified the mutations p.Ser107Leu and p.Thr703Met in the BICD2 gene in the 2 families, respectively. In contrast to other patients carrying the same mutations, our patients present features of a myopathy with slow progression. Immunofluorescence studies and immunoelectron microscopy showed striking impairment of Golgi integrity, vesicle pathology, and abnormal BICD2 accumulation either within the nuclei (p.Ser107Leu) or in the perinuclear region (p.Thr703Met). Transfection studies confirmed BICD2 aggregation in different subcellular locations. CONCLUSIONS: 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.

Dominant spinal muscular atrophy is caused by mutations in BICD2, an important golgin protein.

Spinal muscular atrophies (SMAs) are characterized by degeneration of spinal motor neurons and muscle weakness. Autosomal recessive SMA is the most common form and is caused by homozygous deletions/mutations of the SMN1 gene. However, families with dominant inherited SMA have been reported, for most of them the causal gene remains unknown. Recently, we and others have identified heterozygous mutations in BICD2 as causative for autosomal dominant SMA, lower extremity-predominant, 2 (SMALED2) and hereditary spastic paraplegia (HSP). BICD2 encodes the Bicaudal D2 protein, which is considered to be a golgin, due to its coiled-coil (CC) structure and interaction with the small GTPase RAB6A located at the Golgi apparatus. Golgins are resident proteins in the Golgi apparatus and form a matrix that helps to maintain the structure of this organelle. Golgins are also involved in the regulation of vesicle transport. In vitro overexpression experiments and studies of fibroblast cell lines derived from patients, showed fragmentation of the Golgi apparatus. In the current review, we will discuss possible causes for this disruption, and the consequences at cellular level, with a view to better understand the pathomechanism of this disease.

Neuronal-specific deficiency of the splicing factor Tra2b causes apoptosis in neurogenic areas of the developing mouse brain.

Alternative splicing (AS) increases the informational content of the genome and is more prevalent in the brain than in any other tissue. The splicing factor Tra2b (Sfrs10) can modulate splicing inclusion of exons by specifically detecting GAA-rich binding motifs and its absence causes early embryonic lethality in mice. TRA2B has been shown to be involved in splicing processes of Nasp (nuclear autoantigenic sperm protein), MAPT (microtubule associated protein tau) and SMN (survival motor neuron), and is therefore implicated in spermatogenesis and neurological diseases like Alzheimer's disease, dementia, Parkinson's disease and spinal muscular atrophy. Here we generated a neuronal-specific Tra2b knock-out mouse that lacks Tra2b expression in neuronal and glial precursor cells by using the Nestin-Cre. Neuronal-specific Tra2b knock-out mice die immediately after birth and show severe abnormalities in cortical development, which are caused by massive apoptotic events in the ventricular layers of the cortex, demonstrating a pivotal role of Tra2b for the developing central nervous system. Using whole brain RNA on exon arrays we identified differentially expressed alternative exons of Tubulinδ1 and Shugoshin-like2 as in vivo targets of Tra2b. Most interestingly, we found increased expression of the cyclin dependent kinase inhibitor 1a (p21) which we could functionally link to neuronal precursor cells in the affected brain regions. We provide further evidence that the absence of Tra2b causes p21 upregulation and ultimately cell death in NSC34 neuronal-like cells. These findings demonstrate that Tra2b regulates splicing events essential for maintaining neuronal viability during development. Apoptotic events triggered via p21 might not be restricted to the developing brain but could possibly be generalized to the whole organism and explain early embryonic lethality in Tra2b-depleted mice.

Exome sequencing identifies Laing distal myopathy MYH7 mutation in a Roma family previously diagnosed with distal neuronopathy.

We describe a Hungarian Roma family, originally investigated for autosomal dominant distal muscular atrophy. The mother started toe walking at 3 years and lost ambulation at age 27. Her three daughters presented with early steppage gait and showed variable progression. Muscle biopsies were nonspecific showing myogenic lesions in the mother and lesions resembling neurogenic atrophy in the two siblings. To identify the causative abnormality whole exome sequencing was performed in two affected girls and their unaffected father, unexpectedly revealing the MYH7 mutation c.4849_4851delAAG (p.K1617del) in both girls, reported to be causative for Laing distal myopathy. Sanger sequencing confirmed the mutation in the affected mother and third affected daughter. In line with variable severity in Laing distal myopathy our patients presented a more severe phenotype. Our case is the first demonstration of Laing distal myopathy in the Roma and the successful use of whole exome sequencing in obtaining a definitive diagnosis in ambiguous cases.

Mutations in BICD2, which encodes a golgin and important motor adaptor, cause congenital autosomal-dominant spinal muscular atrophy.

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.