The DST gene in neurobiology.

DST is a gene whose alternative splicing yields epithelial, neuronal, and muscular isoforms. The autosomal recessive Dstdt (dystonia musculorum) spontaneous mouse mutation causes degeneration of spinocerebellar tracts as well as peripheral sensory nerves, dorsal root ganglia, and cranial nerve ganglia. In addition to Dstdt mutants, axonopathy and neurofilament accumulation in perikarya are features of two other murine lines with spontaneous Dst mutations, targeted Dst knockout mice, DstTg4 transgenic mice carrying two deleted Dst exons, DstGt mice with trapped actin-binding domain-containing isoforms, and conditional Schwann cell-specific Dst knockout mice. As a result of nerve damage, Dstdt mutants display dystonia and ataxia, as seen in several genetically modified models and their motor coordination deficits have been quantified along with the spontaneous Dst nonsense mutant, the conditional Schwann cell-specific Dst knockout, the conditional DstGt mutant, and the Dst-b isoform specific Dst mutant. Recent findings in humans have associated DST mutations of the Dst-b isoform with hereditary sensory and autonomic neuropathies type 6 (HSAN-VI). These data should further encourage the development of genetic techniques to treat or prevent ataxic and dystonic symptoms.

A novel variant in the dystonin gene causing hereditary sensory autonomic neuropathy type VI in a male infant: Case report and literature review.

The DST gene is located on chromosome 6p and encodes for a large protein. Alternative splicing of this protein produces the neuronal (a1-a3), muscular (b1-b3), and epithelial (e) isoforms. Hereditary sensory and autonomic neuropathy (HSAN) type VI is a rare autosomal recessive disorder due to mutations affecting the a2 isoform. We present a case of HSAN-VI in a male neonate born to consanguineous parents. Genome sequencing revealed a novel homozygous variant (DST_c.1118C > T; p.Pro373Leu) inherited from both parents. This case further expands the phenotype and genotype of this rare syndrome.

Dystonin loss-of-function leads to impaired autophagosome-endolysosome pathway dynamics.

The neuronal dystonin protein (DST-a) is a large cytoskeletal linker important for integrating the various components of the cytoskeleton. Recessive Dst mutations lead to a sensory neuropathy in mice, known as dystonia musculorum (Dstdt). The disease is characterized by ataxia, autonomic disturbances, and ultimately, death, which are associated with massive degeneration of the sensory neurons in the dorsal root ganglion (DRG). Recent investigation of Dstdt sensory neurons revealed an accumulation of autophagosomes and a disruption in autophagic flux, which was believed to be due to insufficient availability of motor protein. Motor protein levels and the endolysosomal pathway were assessed in pre-symptomatic (postnatal day 5; P5) and symptomatic (P15) stage wild-type and Dstdt DRGs. Levels of mRNA encoding molecular motors were reduced, although no significant reduction in the protein level was detected. An increase in lysosomal marker LAMP1 in medium-large size Dstdt-27J sensory neurons was observed, along with an accumulation of electron-light single-membraned vesicles in Dstdt-27J DRG tissue at the late stages of disease. These vesicles are likely to have been autolysosomes, and their presence in only late-stage Dstdt-27J sensory neurons is suggestive of a pathological defect in autophagy. Further investigation is necessary to confirm vesicle identity, and to determine the role of Dst-a in normal autophagic flux.

Disruption of dystonin in Schwann cells results in late-onset neuropathy and sensory ataxia.

Dystonin (Dst) is a causative gene for Dystonia musculorum (dt) mice, which is an inherited disorder exhibiting dystonia-like movement and ataxia with sensory degeneration. Dst is expressed in a variety of tissues, including the central nervous system and the peripheral nervous system (PNS), muscles, and skin. However, the Dst-expressing cell type(s) for dt phenotypes have not been well characterized. To address the questions whether the disruption of Dst in Schwann cells induces movement disorders and how much impact does it have on dt phenotypes, we generated Dst conditional knockout (cKO) mice using P0-Cre transgenic mice and Dst gene trap mice. First, we assessed the P0-Cre transgene-dependent Cre recombination using tdTomato reporter mice and then confirmed the preferential tdTomato expression in Schwann cells. In the Dst cKO mice, Dst mRNA expression was significantly decreased in Schwann cells, but it was intact in most of the sensory neurons in the dorsal root ganglion. Next, we analyzed the phenotype of Dst cKO mice. They exhibited a normal motor phenotype during juvenile periods, and thereafter, started exhibiting an ataxia. Behavioral tests and electrophysiological analyses demonstrated impaired motor abilities and slowed motor nerve conduction velocity in Dst cKO mice, but these mice did not manifest dystonic movements. Electron microscopic observation of the PNS of Dst cKO mice revealed significant numbers of hypomyelinated axons and numerous infiltrating macrophages engulfing myelin debris. These results indicate that Dst is important for normal PNS myelin organization and Dst disruption in Schwann cells induces late-onset neuropathy and sensory ataxia. MAIN POINTS: Dystonin (Dst) disruption in Schwann cells results in late-onset neuropathy and sensory ataxia. Dst in Schwann cells is important for normal myelin organization in the peripheral nervous system.

Novel Compound Heterozygous DST Variants Causing Hereditary Sensory and Autonomic Neuropathies VI in Twins of a Chinese Family.

Background: Hereditary sensory and autonomic neuropathies (HSANs) are a rare and severe group of sensory axonal neuropathies. HSANs have been classified into eight groups based on mode of inheritance, clinical features, and the involved genes. HSAN-VI, perhaps the most notable type, is an autosomal recessive disease, which manifests as the severely impaired pain sensitivity, autonomic disturbances, distal myopathy, spontaneous or surgical amputations, and sometimes early death. Mutations in DST have been identified as the cause of HSAN-VI. DST encodes dystonin, a member of the plakin protein family that is involved in cytoskeletal filament networks. Dystonin has seven major isoforms in nerve, muscle, and epithelium. Material and Methods: The present study investigated a Chinese family with HSAN and explored potential pathogenic variants using whole-exome sequencing (WES). Variants were screened and filtered through bioinformatics analysis and prediction of variant pathogenicity. Co-segregation analysis was subsequently conducted. Results: We identified compound heterozygous variants of DST (c.3304G>A, p.V1102I and c.13796G>A, p.R4599H) in two patients. Conclusion: We reported on a Chinese family with HSAN-VI family and detected the disease-causing variants. Our description expands the spectrum of known DST variants and contributes to the clinical diagnosis of HSAN-VI.

Functional and Genetic Analysis of Neuronal Isoforms of BPAG1.

The neuronal isoforms of bullous pemphigoid antigen 1 (BPAG1, and also known as dystonin) are a group of large cytoskeletal linker proteins predominantly expressed in sensory neurons. The major neuronal isoforms consist of the spectraplakins (BPAG1/dystonin-a1, -a2, -a3), which have an N-terminus actin-binding domain and a C-terminus microtubule-binding domain. These proteins have crucial roles in cytoskeletal organization and stability, organelle integrity, and intracellular transport. BPAG1 loss-of-function in mice results in a lethal movement disorder known as dystonia musculorum (dt), which is likely caused by rapid sensory neuron degeneration. A human disease termed hereditary and sensory autonomic neuropathy type VI was also identified to be associated with mutations in the BPAG1 gene (DST). This chapter provides an overview of the type of experiments used for analysis of the different isoforms of BPAG1.

Disruption in the autophagic process underlies the sensory neuropathy in dystonia musculorum mice.

A homozygous mutation in the DST (dystonin) gene causes a newly identified lethal form of hereditary sensory and autonomic neuropathy in humans (HSAN-VI). DST loss of function similarly leads to sensory neuron degeneration and severe ataxia in dystonia musculorum (Dst(dt)) mice. DST is involved in maintaining cytoskeletal integrity and intracellular transport. As autophagy is highly reliant upon stable microtubules and motor proteins, we assessed the influence of DST loss of function on autophagy using the Dst(dt-Tg4) mouse model. Electron microscopy (EM) revealed an accumulation of autophagosomes in sensory neurons from these mice. Furthermore, we demonstrated that the autophagic flux was impaired. Levels of LC3-II, a marker of autophagosomes, were elevated. Consequently, Dst(dt-Tg4) sensory neurons displayed impaired protein turnover of autophagosome substrate SQTSM1/p62 and of polyubiquitinated proteins. Interestingly, in a previously described Dst(dt-Tg4) mouse model that is partially rescued by neuronal specific expression of the DST-A2 isoform, autophagosomes, autolysosomes, and damaged organelles were reduced when compared to Dst(dt-Tg4) mutant mice. LC3-II, SQTSM1, polyubiquitinated proteins and autophagic flux were also restored to wild-type levels in the rescued mice. Finally, a significant decrease in DNAIC1 (dynein, axonemal, intermediate chain 1; the mouse ortholog of human DNAI1), a member of the DMC (dynein/dynactin motor complex), was noted in Dst(dt-Tg4) dorsal root ganglia and sensory neurons. Thus, DST-A2 loss of function perturbs late stages of autophagy, and dysfunctional autophagy at least partially underlies Dst(dt) pathogenesis. We therefore conclude that the DST-A2 isoform normally facilitates autophagy within sensory neurons to maintain cellular homeostasis.