Phenotypic and pathologic evaluation of the myd mouse. A candidate model for facioscapulohumeral dystrophy

Congenital hearing impairment associated with peripheral cochlear nerve dysmyelination in glycosylation-deficient muscular dystrophy

Hearing loss (HL) is one of the most common sensory impairments and etiologically and genetically heterogeneous disorders in humans. Muscular dystrophies (MDs) are neuromuscular disorders characterized by progressive degeneration of skeletal muscle accompanied by non-muscular symptoms. Aberrant glycosylation of α-dystroglycan causes at least eighteen subtypes of MD, now categorized as MD-dystroglycanopathy (MD-DG), with a wide spectrum of non-muscular symptoms. Despite a growing number of MD-DG subtypes and increasing evidence regarding their molecular pathogeneses, no comprehensive study has investigated sensorineural HL (SNHL) in MD-DG. Here, we found that two mouse models of MD-DG, Large^myd/myd and POMGnT1-KO mice, exhibited congenital, non-progressive, and mild-to-moderate SNHL in auditory brainstem response (ABR) accompanied by extended latency of wave I. Profoundly abnormal myelination was found at the peripheral segment of the cochlear nerve, which is rich in the glycosylated α-dystroglycan–laminin complex and demarcated by “the glial dome.” In addition, patients with Fukuyama congenital MD, a type of MD-DG, also had latent SNHL with extended latency of wave I in ABR. Collectively, these findings indicate that hearing impairment associated with impaired Schwann cell-mediated myelination at the peripheral segment of the cochlear nerve is a notable symptom of MD-DG.

Hearing loss (HL) is one of the most common sensory impairments and heterogeneous disorders in humans. Up to 60% of HL cases are caused by genetic factors, and approximately 30% of genetic HL cases are syndromic. Although 400–700 genetic syndromes are associated with sensorineural HL (SNHL), caused due to problems in the nerve pathways from the cochlea to the brain, only about 45 genes are known to be associated with syndromic HL. Muscular dystrophies (MDs) are neuromuscular disorders characterized by progressive degeneration of skeletal muscle accompanied by non-muscular symptoms. MD-dystroglycanopathy (MD-DG), caused by aberrant glycosylation of α-dystroglycan, is an MD subtype with a wide spectrum of non-muscular symptoms. Despite a growing number of MD-DG subtypes (at least 18), no comprehensive study has investigated SNHL in MD-DG. Here, we found that hearing impairment was associated with abnormal myelination of the peripheral segment of the cochlear nerve caused by impaired dystrophin–dystroglycan complex in two mouse models (type 3 and 6) of MD-DG and in patients (type 4) with MD-DG. This is the first comprehensive study investigating SNHL in MD-DG. Our findings may provide new insights into understanding the pathogenic characteristics and mechanisms underlying inherited syndromic hearing impairment.

Immunobiology of Inherited Muscular Dystrophies

The immune response to acute muscle damage is important for normal repair. However, in chronic diseases such as many muscular dystrophies, the immune response can amplify pathology and play a major role in determining disease severity. Muscular dystrophies are inheritable diseases that vary tremendously in severity, but share the progressive loss of muscle mass and function that can be debilitating and lethal. Mutations in diverse genes cause muscular dystrophy, including genes that encode proteins that maintain membrane strength, participate in membrane repair, or are components of the extracellular matrix or the nuclear envelope. In this article, we explore the hypothesis that an important feature of many muscular dystrophies is an immune response adapted to acute, infrequent muscle damage that is misapplied in the context of chronic injury. We discuss the involvement of the immune system in the most common muscular dystrophy, Duchenne muscular dystrophy, and show that the immune system influences muscle death and fibrosis as disease progresses. We then present information on immune cell function in other muscular dystrophies and show that for many muscular dystrophies, release of cytosolic proteins into the extracellular space may provide an initial signal, leading to an immune response that is typically dominated by macrophages, neutrophils, helper T-lymphocytes, and cytotoxic T-lymphocytes. Although those features are similar in many muscular dystrophies, each muscular dystrophy shows distinguishing features in the magnitude and type of inflammatory response. These differences indicate that there are disease-specific immunomodulatory molecules that determine response to muscle cell damage caused by diverse genetic mutations. © 2018 American Physiological Society. Compr Physiol 8:1313-1356, 2018.

Integrin and dystroglycan compensate each other to mediate laminin-dependent basement membrane assembly and epiblast polarization

During early embryogenesis, endodermal γ1-laminin expression is required for basement membrane (BM) assembly, promoting conversion of non-polar pluripotent cells into polarized epiblast. The influence of laminin-111 (Lm111) and its integrin and dystroglycan (DG) receptors on epiblast in embryoid bodies (EBs), a model for differentiation of the embryonic plate, was further investigated. Lm111 added to the medium of EBs initiated conversion of inner nonpolar cell to the polarized epiblast epithelium with an exterior-to-central basal-to-apical orientation. Microinjection of Lm111 into EB interiors resulted in an interior BM with complete inversion of cell polarity. Lm111 assembled a BM on integrin-β1 null EBs with induction of polarization at reduced efficiency. β-Integrin compensation was not detected in these nulls with integrin adaptor proteins failing to assemble. A dimer of laminin LG domains 4-5 (LZE3) engineered to strongly bind to α-dystroglycan almost completely inhibited laminin accumulation on integrin β1-null EBs, reducing BM and ablating cell polarization. When Lm111 was incubated with integrin-β1/dystroglycan double-knockout EBs, laminin failed to accumulate on the EBs, the EBs did not differentiate, and the EBs underwent apoptosis. Collectively the findings support the hypotheses that the locus of laminin cell surface assembly can determine the axis of epithelial polarity. This requires integrin- and/or dystroglycan-dependent binding to laminin LG domains with the highest efficiency achieved when both receptors are present. Finally, EBs that cannot assemble a matrix undergo apoptosis.

Facioscapulohumeral dystrophy: the path to consensus on pathophysiology

Although the pathophysiology of facioscapulohumeral dystrophy (FSHD) has been controversial over the last decades, progress in recent years has led to a model that incorporates these decades of findings and is gaining general acceptance in the FSHD research community. Here we review how the contributions from many labs over many years led to an understanding of a fundamentally new mechanism of human disease. FSHD is caused by inefficient repeat-mediated epigenetic repression of the D4Z4 macrosatellite repeat array on chromosome 4, resulting in the variegated expression of the DUX4 retrogene, encoding a double-homeobox transcription factor, in skeletal muscle. Normally expressed in the testis and epigenetically repressed in somatic tissues, DUX4 expression in skeletal muscle induces expression of many germline, stem cell, and other genes that might account for the pathophysiology of FSHD. Although some disagreements regarding the details of mechanisms remain in the field, the coalescing agreement on a central model of pathophysiology represents a pivot-point in FSHD research, transitioning the field from discovery-oriented studies to translational studies aimed at developing therapies based on a sound model of disease pathophysiology.

Congenital muscular dystrophy type 1D (MDC1D) due to a large intragenic insertion/deletion, involving intron 10 of the LARGE gene

Mutation of the LARGE gene is the rarest of the six known genetic causes of α-dystroglycanopathy. We report further a family with MDC1D due to a complex genomic rearrangement that was not apparent on standard sequencing of LARGE. Two sisters in a consanguineous family had moderate mental retardation and cerebellar malformations, together with dystrophic changes and markedly reduced α-dystroglycan glycosylation staining on muscle biopsy. There was homozygous linkage to the LARGE locus but sequencing of LARGE coding regions was normal. Analysis of LARGE cDNA showed an abnormal sequence inserted between exons 10 and 11, in most of the transcripts, predicted to introduce a premature stop codon. The abnormal sequence mapped to a spliced EST (DA935254) of unknown function, normally located at 100 kb centromeric of LARGE on chromosome 22q12.3. Quantitative PCR analysis of the EST and adjacent regions showed twice the normal copy number in patients' genomic DNA samples, consistent with a large intra-chromosomal duplication inserted into intron 10 of LARGE in a homozygous state. This insertion was associated with deletion of a central region of intron 10, but the exact break points of the deletion/duplication were not found, suggesting that an even more complex rearrangement may have occurred. The exact function of LARGE, a golgi protein, remains uncertain. POMT and POMGnT enzyme activities were normal in patients' lymphoblast cells, suggesting that defects in LARGE do not affect the initiation of O-mannosyl glycans.

Intragenic deletion in the LARGE gene causes Walker-Warburg syndrome

Intragenic homozygous deletions in the Large gene are associated with a severe neuromuscular phenotype in the myodystrophy (myd) mouse. These mutations result in a virtual lack of glycosylation of alpha-dystroglycan. Compound heterozygous LARGE mutations have been reported in a single human patient, manifesting with mild congenital muscular dystrophy (CMD) and severe mental retardation. These mutations are likely to retain some residual LARGE glycosyltransferase activity as indicated by residual alpha-dystroglycan glycosylation in patient cells. We hypothesized that more severe LARGE mutations are associated with a more severe CMD phenotype in humans. Here we report a 63-kb intragenic LARGE deletion in a family with Walker-Warburg syndrome (WWS), which is characterized by CMD, and severe structural brain and eye malformations. This finding demonstrates that LARGE gene mutations can give rise to a wide clinical spectrum, similar as for other genes that have a role in the post-translational modification of the alpha-dystroglycan protein.

The role of defective glycosylation in congenital muscular dystrophy

The dystrophin glycoprotein complex (DGC) is an assembly of proteins spanning the sarcolemma of skeletal muscle cells. Defects in the DGC appear to play critical roles in several muscular dystrophies due to disruption of basement membrane organization. O -mannosyl oligosaccharides on alpha-dystroglycan, a major extracellular component of the DGC, are essential for normal binding of alpha-dystroglycan to ligands (such as laminin) in the extracellular matrix and subsequent signal transmission to actin in the cytoskeleton of the muscle cell. Muscle-Eye-Brain disease (MEB) and Walker-Warburg Syndrome (WWS) have mutations in genes encoding glycosyltransferases needed for O -mannosyl oligosaccharide synthesis. Myodystrophic myd mice and humans with Fukuyama Congenital Muscular Dystrophy (FCMD), congenital muscular dystrophy due to defective fukutin-related protein (FKRP) and MDC1D have mutations in putative glycosyltransferases. These human congenital muscular dystrophies and the myd mouse are associated with defective glycosylation of alpha-dystroglycan. It is expected other congenital muscular dystrophies will prove to have mutations in genes involved in glycosylation.

The sarcolemma in the largemyd mouse

In the Large(myd) mouse, dystroglycan is incompletely glycosylated and thus cannot bind its extracellular ligands, causing a muscular dystrophy that is usually lethal in early adulthood. We show that the Large(myd) mutation alters the composition and organization of the sarcolemma of fast-twitch skeletal muscle fibers in young adult mice. Costameres at the sarcolemma of the tibialis anterior muscle of Large(myd) mice contain reduced levels of several membrane cytoskeletal proteins, including dystrophin and beta-spectrin. In the quadriceps, longitudinally oriented costameric structures tend to become thickened and branched. More strikingly, proteins of the dystrophin complex present between costameres in controls are absent from Large(myd) muscles. We propose that the absence of the dystrophin complex from these regions destabilizes the sarcolemma of the Large(myd) mouse and thereby contributes to the severity of its muscular dystrophy. Thus, the positioning of sarcolemmal proteins may have a profound effect on the health of skeletal muscle.

Hearing loss in the laminin-deficient dy mouse model of congenital muscular dystrophy

Sensorineural hearing loss is found in many inherited forms of muscular dystrophy. We investigated the dy mouse model, which has congenital muscular dystrophy due to a defect in laminin alpha 2, for evidence of cochlear dysfunction. Auditory brainstem response (ABR) audiometry to pure tones was used to evaluate 3-month-old homozygous dy/dy and age-matched C57 control mice. The average ABR thresholds to tone-burst stimuli for four frequencies (4, 8, 16, and 32 kHz) were determined and statistically compared by ANOVA. The dy/dy mice demonstrated elevated auditory thresholds ranging from 25 to 27 dB at each frequency tested (p<0.0001). Anatomic evaluations of the ears revealed pathology ranging from extensive connective tissue infiltration within the inner ear to possible minor defects in the cells of the organ of Corti. These anatomic and physiologic observations suggest that the extracellular matrix protein laminin plays a crucial role in normal cochlear function. Furthermore, the dy congenital muscular dystrophy mouse offers a novel model for evaluation of sensorineural hearing loss associated with muscular dystrophy.

Mutations in the fukutin-related protein gene (FKRP) cause a form of congenital muscular dystrophy with secondary laminin alpha2 deficiency and abnormal glycosylation of alpha-dystroglycan

The congenital muscular dystrophies (CMD) are a heterogeneous group of autosomal recessive disorders presenting in infancy with muscle weakness, contractures, and dystrophic changes on skeletal-muscle biopsy. Structural brain defects, with or without mental retardation, are additional features of several CMD syndromes. Approximately 40% of patients with CMD have a primary deficiency (MDC1A) of the laminin alpha2 chain of merosin (laminin-2) due to mutations in the LAMA2 gene. In addition, a secondary deficiency of laminin alpha2 is apparent in some CMD syndromes, including MDC1B, which is mapped to chromosome 1q42, and both muscle-eye-brain disease (MEB) and Fukuyama CMD (FCMD), two forms with severe brain involvement. The FCMD gene encodes a protein of unknown function, fukutin, though sequence analysis predicts it to be a phosphoryl-ligand transferase. Here we identify the gene for a new member of the fukutin protein family (fukutin related protein [FKRP]), mapping to human chromosome 19q13.3. We report the genomic organization of the FKRP gene and its pattern of tissue expression. Mutations in the FKRP gene have been identified in seven families with CMD characterized by disease onset in the first weeks of life and a severe phenotype with inability to walk, muscle hypertrophy, marked elevation of serum creatine kinase, and normal brain structure and function. Affected individuals had a secondary deficiency of laminin alpha2 expression. In addition, they had both a marked decrease in immunostaining of muscle alpha-dystroglycan and a reduction in its molecular weight on western blot analysis. We suggest these abnormalities of alpha-dystroglycan are caused by its defective glycosylation and are integral to the pathology seen in MDC1C.

Mutant glycosyltransferase and altered glycosylation of alpha-dystroglycan in the myodystrophy mouse

Spontaneous and engineered mouse mutants have facilitated our understanding of the pathogenesis of muscular dystrophy and they provide models for the development of therapeutic approaches. The mouse myodystrophy (myd) mutation produces an autosomal recessive, neuromuscular phenotype. Homozygotes have an abnormal gait, show abnormal posturing when suspended by the tail and are smaller than littermate controls. Serum creatine kinase is elevated and muscle histology is typical of a progressive myopathy with focal areas of acute necrosis and clusters of regenerating fibers. Additional aspects of the phenotype include sensorineural deafness, reduced lifespan and decreased reproductive fitness. The myd mutation maps to mouse chromosome 8 at approximately 33 centimorgans (cM) (refs. 2, 4-7). Here we show that the gene mutated in myd encodes a glycosyltransferase, Large. The human homolog of this gene (LARGE) maps to chromosome 22q. In myd, an intragenic deletion of exons 4-7 causes a frameshift in the resultant mRNA and a premature termination codon before the first of the two catalytic domains. On immunoblots, a monoclonal antibody to alpha-dystroglycan (a component of the dystrophin-associated glycoprotein complex) shows reduced binding in myd, which we attribute to altered glycosylation of this protein. We speculate that abnormal post-translational modification of alpha-dystroglycan may contribute to the myd phenotype.

Normal cochlear function in mdx and mdx(Cv3) Duchenne muscular dystrophy mouse models

OBJECTIVES/HYPOTHESIS

Sensorineural hearing loss has been found in association with inherited muscular dystrophies in humans and in mouse models. An increased brainstem auditory evoked response threshold has been previously reported in the dystrophin-deficient mdx mouse model for Duchenne muscular dystrophy, suggesting that full-length dystrophin (Dp427) is involved in hearing. The objective of the present study was to confirm cochlear dysfunction with this gene defect and determine whether the shorter carboxyl terminus isoforms of dystrophin are also critical in maintaining normal hearing.

STUDY DESIGN

Case controlled. Animal model.

METHODS

Auditory brainstem response (ABR) audiometry to pure tones was used to evaluate cochlear function. Fourteen mdx, 4 mdx(Cv3), and 13 age-matched control (C57BL/6J and C57BL/10ScSn) male mice were tested at 5 weeks and 11 weeks of age. The ABR thresholds to tone-burst stimuli at 4, 8, 16, and 32 kHz were obtained for each ear and statistically compared (ANOVA) for potential group differences.

RESULTS

Both mdx and mdx(Cv3) mice demonstrated normal ABR thresholds when compared with controls.

CONCLUSIONS

Both mdx and mdx(Cv3) mouse models have normal hearing by ABR. The authors' data suggest that dystrophin and its carboxyl terminus isoforms do not play a critical role in hearing in the mouse. This was unexpected, as previous studies using the brainstem auditory evoked response method suggested that the mdx mouse has an increased threshold for hearing.

Facioscapulohumeral dystrophy: a distinct regional myopathy with a novel molecular pathogenesis. FSH Consortium

Facioscapulohumeral muscular dystrophy (FSHD) is one of the most common inherited diseases of muscle. Until recently, FSHD had received little attention because of its relatively benign course and the perception that it represented a syndrome rather than a distinct myopathy. Research interest into this disease was reignited with the demonstration of linkage of FSHD to chromosome 4q35 in 1990. Clinical and molecular genetic research in FSHD has since helped define it as a distinct clinical entity, outlined its natural history, and defined the primary molecular defect associated with the condition. FSHD is now known to be associated with large deletions of variable size on chromosome 4q35. These deletions, however, do not appear to disrupt a transcribed gene but are thought to interfere with the expression of a gene or genes located proximal to the deletions. These observations complicate the search for the FSHD gene but also imply the presence of a potentially novel molecular pathogenesis.

Sensorineural hearing loss in the mdx mouse: a model of Duchenne muscular dystrophy

Sensorineural hearing loss has been identified in several types of muscular dystrophy, but few studies have investigated any relationship between Duchenne muscular dystrophy and hearing. An animal model of Duchenne muscular dystrophy, the mdx mouse, exhibits the same genetic defect as humans. We performed brainstem auditory evoked responses on mdx and control mice in order to assess sensorineural hearing loss. The amplitude and latency of wave I for each animal were measured at increasing sound pressure levels. A significant increase in threshold and a decrease in wave I amplitude were found in the mdx mice. These results indicate that significant sensorineural hearing loss is associated with muscular dystrophy in the mdx mouse. Possible cellular mechanisms contributing to the hearing deficit are presented.