Axonopathy in the Central Nervous System Is the Hallmark of Mice with a Novel Intragenic Null Mutation of Dystonin

Abstract

Dystonia musculorum is a neurodegenerative disorder caused by a mutation in the dystonin gene. It has been described in mice and humans where it is called hereditary sensory autonomic neuropathy. Mutated mice show severe movement disorders and die at the age of 3-4 weeks. This study describes the discovery and molecular, clinical, as well as pathological characterization of a new spontaneously occurring mutation in the dystonin gene in C57BL/6N mice. The mutation represents a 40-kb intragenic deletion allele of the dystonin gene on chromosome 1 with exactly defined deletion borders. It was demonstrated by Western blot, mass spectrometry, and immunohistology that mice with a homozygous mutation were entirely devoid of the dystonin protein. Pathomorphological lesions were restricted to the brain stem and spinal cord and consisted of swollen, argyrophilic axons and dilated myelin sheaths in the white matter and, less frequently, total chromatolysis of neurons in the gray matter. Axonal damage was detected by amyloid precursor protein and nonphosphorylated neurofilament immunohistology. Axonopathy in the central nervous system (CNS) represents the hallmark of this disease. Mice with the dystonin mutation also showed suppurative inflammation in the respiratory tract, presumably due to brain stem lesion-associated food aspiration, whereas skeletal muscles showed no pathomorphological changes. This study describes a novel mutation in the dystonin gene in mice leading to axonopathy in the CNS. In further studies, this model may provide new insights into the pathogenesis of neurodegenerative diseases and may elucidate the complex interactions of dystonin with various other cellular proteins especially in the CNS. KEYWORDS axonopathy; dystonia musculorum; dystonin deficiency; genomic deletion; spontaneous mutation A spontaneously occurring mutant was described in the mouse, in which the gene dystonin was affected (Ledoux 2011). Dystonin (Dst) [human gene name, DST; former name, bullous pemphigoid antigen 1 (BPAG1)] is a large cytoskeletal linker protein and crucial for maintaining cellular structural integrity (Young and Kothary 2008). Recent research in this field concentrates on the role of dystonin in central nervous tissue and neurological diseases, but not, however, on its parallel expression in musculature and skin.Mice with a mutated dystonin gene develop a severe sensory neuropathy called dystonia musculorum (Duchen 1976). Characteristics are a progressive loss of coordination of the limbs (ataxia) and an early death (Kothary et al. 1988;Guo et al. 1995). There are only few reports about patients with mutations of the human dystonin gene (Giorda et al. 2004;Groves et al. 2010;Edvardson et al. 2012).Several dystonin isoforms are generated from one genomic locus of 400 kb. They are expressed in the central nervous system (CNS) (predominant neuronal isoform "a," 617 kDa and "n," 344 kDa), muscles (predominant muscle isoform "b," 834 kDa), and skin (predominant skin isoform "e," 302 kDa) ( Figure 1 Pool et al...

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

“…This finding is consistent with previous reports showing selective recombination in Schwann cells, as well as satellite cells, using independent P0-Cre transgenic mice line (Feltri et al, 1999). which is one of the histological hallmarks of neurodegeneration in dt mice (Campbell & Peterson, 1992;Dowling et al, 1997;Horie et al, 2014Horie et al, , 2016Seehusen et al, 2016;Sotelo & Guenet, 1988;Tseng et al, 2006;Tseng et al, 2008). Furthermore, studies have reported that axonal swelling and NF accumulation in peripheral nerves are the other neurodegenerative hallmarks of dt mice (Campbell & Peterson, 1992;Dalpé et al, 1998;Janota, 1972;Sotelo & Guenet, 1988); however, the Dst cKO mice did not manifest these F I G U R E 4 Late-onset demyelination in the Dst cKO mice.…”

“…Dystonic movement in dt mice may be caused due to complex defects in both the CNS and PNS. For instance, we and others have reported that neurons with abnormal NF accumulation were observed in both the PNS and CNS of dt mice, such as in the cerebral cortex, deep cerebellar nuclei, and brainstem nuclei, including reticular formation, vestibular nuclei, red nucleus, and deep layer of the superior colliculus, all of which are associated with motor coordination (Horie et al, 2016;Seehusen et al, 2016). The conditional Dst gene trap is a useful approach to identify the neuronal circuits or neuronal subtypes that are involved in the appearance of dystonic movement.…”

Characterization of gastrointestinal pathologies in the dystonia musculorum mouse model for hereditary sensory and autonomic neuropathy type VI

“…Indeed, we do see expression of Dst‐a/b , with Dst‐a/b3 expressed across all GIT levels, and Dst‐a/b1 and ‐a/b2 mainly being expressed in esophagus, ileum, and colon (Figure D). We also ruled out the potential involvement of the epithelial isoform (Dst‐e/BPAG1e) with relation to the GIT defects, as the presence of this isoform is confirmed in Dst dt‐27J skin (Figure E), indicating that this isoform is not disrupted in Dst dt‐27J mice . It should also be noted that BPAG1e is not expressed in the GIT (with exception of the esophagus) and is therefore not expected to be involved in this pathology …”

“…We also ruled out the potential involvement of the epithelial isoform (Dst-e/BPAG1e) with relation to the GIT defects, as the presence of this isoform is confirmed in Dst dt-27J skin ( Figure 1E), indicating that this isoform is not disrupted in Dst dt-27J mice. 30 It should also be noted that BPAG1e is not expressed in the GIT (with exception of the esophagus) and is therefore not expected to be involved in this pathology. 31,32 Histological analysis of P15 duodenum, jejunum, ileum, and colon by hematoxylin & eosin staining revealed no differences in villi length, villi width, crypt length, crypt width, smooth muscle thickness (normalized to intestinal wall thickness; Figure S1), or colon goblet cell number between Dst dt-27J and wild type mice (Figure 2A-G), with the exception of an increase in crypt width in…”

Diverse dystonin gene mutations cause distinct patterns of Dst isoform deficiency and phenotypic heterogeneity in D ystonia musculorum mice

“…The causative gene for dt mice is dystonin (Dst), also known as bullous pemphigoid antigen1 (Bpag1) (Brown et al, 1995;Guo et al, 1995). Because of the large size of the Dst locus (encoding over 100 exons) and obvious motor phenotypes, many untargeted Dst mutant mouse strains have been established, including many spontaneously occurring mutants [Dst dt (Duchen et al, 1964); Dst dt-alb (Messer and Strominger, 1980); dt Frk (Pool et al, 2005); Dst dt-23Rbrc (Horie et al, 2016); dt-MP (Seehusen et al, 2016)], a transgene insertion-induced mutant [Dst Tg4 (Kothary et al, 1988)] and chemically induced mutants [Dst dt-33J (MGI: 1889074); Dst dt-36J (MGI: 2681971)]. Several mutants were also intentionally generated, including a gene-targeting knockout [Dst tm1Efu (Guo et al, 1995)], a gene-trap mutant [Dst Gt(E182H05) (Horie et al, 2014)] and nuclease-mediated mutants [Dst em13Dcr (MGI: 6258920); Dst em14Dcr (MGI: 6258923)].…”