Isoform-specific loss of dystonin causes hereditary motor and sensory neuropathy

Roles of dystonin isoforms in the maintenance of neural, muscle, and cutaneous tissues

Dystonin (DST), also known as bullous pemphigoid antigen 1 (BPAG1), encodes cytoskeletal linker proteins belonging to the plakin family. The DST gene produces several isoforms, including DST-a, DST-b, and DST-e, which are expressed in neural, muscle, and cutaneous tissues, respectively. Pathogenic DST mutations cause hereditary sensory and autonomic neuropathy type 6 (HSAN-VI) and epidermolysis bullosa simplex (EBS); therefore, it is important to elucidate the roles of DST isoforms in multiple organs. Recently, we have used several Dst mutant mouse strains, in which the expression of Dst isoforms is disrupted in distinct patterns, to gain new insight into how DST functions in multiple tissues. This review provides an overview of the roles played by tissue-specific DST isoforms in neural, muscle, and cutaneous tissues.

DST variants are responsible for neurogenic arthrogryposis multiplex congenita enlarging the spectrum of type VI hereditary sensory autonomic neuropathy

Arthrogryposis multiplex congenita (AMC) is a developmental condition characterized by multiple joint contractures resulting from reduced or absent fetal movements. Through whole-exome sequencing combined with arrayCGH from DNA of a fetus presenting with early onset AMC, we identified biallelic loss of function variants in Dystonin (DST): a stop gain variant (NM_001144769.5:c.12208G > T:p.(Glu4070Ter)) on the neuronal isoform and a 175 kb microdeletion including exons 25-96 of this isoform on the other allele [NC_000006.11:g.(56212278_56323554)_(56499398_56507586)del]. Transmission electron microscopy of the sciatic nerve revealed abnormal morphology of the peripheral nerve with severe hypomyelination associated with dramatic reduction of fiber density which highlights the critical role of DST in peripheral nerve axonogenesis during development in human. Variants in the neuronal isoforms of DST cause hereditary sensory and autonomic neuropathy which has been reported in several unrelated families with highly variable age of onset from fetal to adult onset. Our data enlarge the disease mechanisms of neurogenic AMC.

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.

Isoform-specific mutation in Dystonin-b gene causes late-onset protein aggregate myopathy and cardiomyopathy

Dystonin (DST), which encodes cytoskeletal linker proteins, expresses three tissue-selective isoforms: neural DST-a, muscular DST-b, and epithelial DST-e. DST mutations cause different disorders, including hereditary sensory and autonomic neuropathy 6 (HSAN-VI) and epidermolysis bullosa simplex; however, etiology of the muscle phenotype in DST-related diseases has been unclear. Because DST-b contains all of the DST-a-encoding exons, known HSAN-VI mutations could affect both DST-a and DST-b isoforms. To investigate the specific function of DST-b in striated muscles, we generated a Dst-b-specific mutant mouse model harboring a nonsense mutation. Dst-b mutant mice exhibited late-onset protein aggregate myopathy and cardiomyopathy without neuropathy. We observed desmin aggregation, focal myofibrillar dissolution, and mitochondrial accumulation in striated muscles, which are common characteristics of myofibrillar myopathy. We also found nuclear inclusions containing p62, ubiquitin, and SUMO proteins with nuclear envelope invaginations as a unique pathological hallmark in Dst-b mutation-induced cardiomyopathy. RNA-sequencing analysis revealed changes in expression of genes responsible for cardiovascular functions. In silico analysis identified DST-b alleles with nonsense mutations in populations worldwide, suggesting that some unidentified hereditary myopathy and cardiomyopathy are caused by DST-b mutations. Here, we demonstrate that the Dst-b isoform is essential for long-term maintenance of striated muscles.

Pathogenic DST sequence variants result in either epidermolysis bullosa simplex (EBS) or hereditary sensory and autonomic neuropathy type 6 (HSAN-VI)

DST encodes bullous pemphigoid antigen-1 (BPAG1), a protein with eight tissue-specific isoforms expressed in the skin, muscle, brain, and nerves. Accordingly, mutations in this gene are associated with epidermolysis bullosa simplex (EBS) and hereditary sensory and autonomic neuropathy type 6 (HSAN-VI). The genotypic spectrum is attested to by 19 distinct mutations but genotype-phenotype correlation for both disorders is not well established. In this study, we performed next-generation sequencing (NGS) on two families with different phenotypic presentations, one fetus (P1) with musculoskeletal and neurological malformations established by prenatal ultrasound and family history, and a 15-year-old female (P2) with skin blistering. P1 had a novel homozygous nonsense mutation, DST: NM_001144769, c.3805C>T, p.R1269* within a region of homozygosity (ROH). This mutation resides within the plakin domain of BPAG1 and ablates all isoforms of this protein, leading to novel extracutaneous phenotypes consistent with HSAN-VI in P1. P2 had a recurrent homozygous mutation DST: NM_001723.7, c.3370C>T, p.Gln1124* that presented with giant, trauma-induced skin blisters without extracutaneous involvement. This mutation is located within the coiled-coil domain present on the skin isoform of DST, BPGA1-e, associated with EBS. In summary, we report two families with pathogenic DST variants and expand the spectrum of DST genotype and phenotypes.

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.