Collagen VI glycine mutations: perturbed assembly and a spectrum of clinical severity

Optimized allele-specific silencing of the dominant-negative COL6A1 G293R substitution causing collagen VI-related dystrophy

Collagen VI-related dystrophies (COL6-RDs) are a group of severe, congenital-onset muscular dystrophies for which there is no effective causative treatment. Dominant-negative mutations are common in COL6A1, COL6A2, and COL6A3 genes, encoding the collagen α1, α2, and α3 (VI) chains. They act by incorporating into the hierarchical assembly of the three α (VI) chains and consequently produce a dysfunctional collagen VI extracellular matrix, while haploinsufficiency for any of the COL6 genes is not associated with disease. Hence, allele-specific transcript inactivation is a valid therapeutic strategy, although selectively targeting a pathogenic single nucleotide variant is challenging. Here, we develop a small interfering RNA (siRNA) that robustly, and in an allele-specific manner, silences a common glycine substitution (G293R) caused by a single nucleotide change in COL6A1 gene. By intentionally introducing an additional mismatch into the siRNA design, we achieved enhanced specificity toward the mutant allele. Treatment of patient-derived fibroblasts effectively reduced the levels of mutant transcripts while maintaining unaltered wild-type transcript levels, rescuing the secretion and assembly of collagen VI matrix by reducing the dominant-negative effect of mutant chains. Our findings establish a promising treatment approach for patients with the recurrent dominantly negative acting G293R glycine substitution.

Allele-specific CRISPR/Cas9 editing inactivates a single nucleotide variant associated with collagen VI muscular dystrophy

The application of allele-specific gene editing tools can expand the therapeutic options for dominant genetic conditions, either via gene correction or via allelic gene inactivation in situations where haploinsufficiency is tolerated. Here, we used allele-targeted CRISPR/Cas9 guide RNAs (gRNAs) to introduce inactivating frameshifting indels at a single nucleotide variant in the COL6A1 gene (c.868G>A; G290R), a variant that acts as dominant negative and that is associated with a severe form of congenital muscular dystrophy. We expressed spCas9 along with allele-targeted gRNAs, without providing a repair template, in primary fibroblasts derived from four patients and one control subject. Amplicon deep-sequencing for two gRNAs tested showed that single nucleotide deletions accounted for the majority of indels introduced. While activity of the two gRNAs was greater at the G290R allele, both gRNAs were also active at the wild-type allele. To enhance allele-selectivity, we introduced deliberate additional mismatches to one gRNA. One of these optimized gRNAs showed minimal activity at the WT allele, while generating productive edits and improving collagen VI matrix in cultured patient fibroblasts. This study strengthens the potential of gene editing to treat dominant-negative disorders, but also underscores the challenges in achieving allele selectivity with gRNAs.

Retrospective clinical and genetic analysis of COL6-RD patients with a long-term follow-up at a single French center

Collagen type VI-related dystrophies (COL6-RD) are rare diseases with a wide phenotypic spectrum ranging from severe Ullrich's congenital muscular dystrophy Ullrich congenital muscular dystrophy to much milder Bethlem myopathy Both dominant and recessive forms of COL6-RD are caused by pathogenic variants in three collagen VI genes (COL6A1, COL6A2 and COL6A3). The prognosis of these diseases is variable and difficult to predict during early disease stages, especially since the genotype-phenotype correlation is not always clear. For this reason, studies with long-term follow-up of patients with genetically confirmed COL6-RD are still needed. In this study, we present phenotypic and genetic data from 25 patients (22 families) diagnosed with COL6-RD and followed at a single French center, in both adult and pediatric neurology departments. We describe three novel pathogenic variants and identify COL6A2:c.1970-9G>A as the most frequent variant in our series (29%). We also observe an accelerated progression of the disease in a subgroup of patients. This large series of rare disease patients provides essential information on phenotypic variability of COL6-RD patients as well as on frequency of pathogenic COL6A gene variants in Southern France, thus contributing to the phenotypic and genetic description of Collagen type VI-related dystrophies.

Molecular and spatial landmarks of early mouse skin development

A wealth of specialized cell populations within the skin facilitates its hair-producing, protective, sensory, and thermoregulatory functions. How the vast cell-type diversity and tissue architecture develops is largely unexplored. Here, with single-cell transcriptomics, spatial cell-type assignment, and cell-lineage tracing, we deconstruct early embryonic mouse skin during the key transitions from seemingly uniform developmental precursor states to a multilayered, multilineage epithelium, and complex dermal identity. We identify the spatiotemporal emergence of hair-follicle-inducing, muscle-supportive, and fascia-forming fibroblasts. We also demonstrate the formation of the panniculus carnosus muscle (PCM), sprouting blood vessels without pericyte coverage, and the earliest residence of mast and dendritic immune cells in skin. Finally, we identify an unexpected epithelial heterogeneity within the early single-layered epidermis and a signaling-rich periderm layer. Overall, this cellular and molecular blueprint of early skin development-which can be explored at https://kasperlab.org/tools-establishes histological landmarks and highlights unprecedented dynamic interactions among skin cells.

COLLAGEN VI-RELATED MYOPATHIES: CLINICAL VARIABILITY, PHENOTYPE-GENOTYPE CORRELATION AND EXPLORATORY TRANSCRIPTOME STUDY

Collagen VI-related myopathies are a group of disorders that cause muscle weakness and joint contractures with significant variability in disease severity among patients. Here we report the clinical and genetic characteristics of 13 Chinese patients. Detailed histological, radiological and muscle transcriptomic evaluations were also conducted for selected representative patients. Across the cohort, fifteen putative disease causal variants were identified in three genes encoding collagen VI subunits, COL6A1 (n=6), COL6A2 (n=5), and COL6A3 (n=4). Most of these variants (12/15, 80%) were dominant negative and occurred at the triple helical domain. The rest (3/15, 20%) were located at the C-terminus. Two previously unreported variants, an in-frame mutation (COL6A1:c.1084_1092del) and a missense mutation (COL6A2:c.811G>C), were also noted. The transcriptome data from the muscle biopsies of two patients in the study with dominant negative mutations [COL6A2:c.811G>C and COL6A1:c.930+189C>T] supports the accepted aetiology of Collagen VI myopathy as dysfunction of the extracellular matrix. It also suggests there are perturbations to skeletal muscle differentiation and skeletal system development. It should be noted that although the phenotypes of patients can be mostly explained by the position and dominant-negative effect of the variants, exceptions and variability still exist and have to be reckoned with. This study provides valuable data explaining the varying severity of phenotypes among ethnically Chinese patients.

Childhood muscular dystrophies

Infancy- and childhood-onset muscular dystrophies are associated with a characteristic distribution and progression of motor dysfunction. The underlying causes of progressive childhood muscular dystrophies are heterogeneous involving diverse genetic pathways and genes that encode proteins of the plasma membrane, extracellular matrix, sarcomere, and nuclear membrane components. The prototypical clinicopathological features in an affected child may be adequate to fully distinguish it from other likely diagnoses based on four common features: (1) weakness and wasting of pelvic-femoral and scapular muscles with involvement of heart muscle; (2) elevation of serum muscle enzymes in particular serum creatine kinase; (3) necrosis and regeneration of myofibers; and (4) molecular neurogenetic assessment particularly utilizing next-generation sequencing of the genome of the likeliest candidates genes in an index case or family proband. A number of different animal models of therapeutic strategies have been developed for gene transfer therapy, but so far these techniques have not yet entered clinical practice. Treatment remains for the most part symptomatic with the goal of ameliorating locomotor and cardiorespiratory manifestations of the disease.

Ehlers-Danlos/myopathy overlap syndrome caused by a large de novo deletion in COL12A1

Autosomal dominant and recessive mutations in COL12A1 cause the Ehlers-Danlos/myopathy overlap syndrome. Here, we describe a boy with fetal hypokinesia, severe neonatal weakness, striking hyperlaxity, high arched palate, retrognathia, club feet, and pectus excavatum. His motor development was initially delayed but muscle strength improved with time while hyperlaxity remained very severe causing recurrent joint dislocations. Using trio exome sequencing and a copy number variation (CNV) analysis tool, we identified an in-frame de novo heterozygous deletion of the exons 45 to 54 in the COL12A1 gene. Collagen XII immunostaining on cultured skin fibroblasts demonstrated intracellular retention of collagen XII, supporting the pathogenicity of the deletion. The phenotype of our patient is slightly more severe than other cases with dominantly acting mutations, notably with the presence of fetal hypokinesia. This case highlights the importance of CNVs analysis in the COL12A1 gene in patients with a phenotype suggesting Ehlers-Danlos/myopathy overlap syndrome.

Collagen VI Muscle Disorders: Mutation Types, Pathogenic Mechanisms and Approaches to Therapy

Mutations in the genes encoding the major collagen VI isoform, COL6A1, COL6A2 and COL6A3, are responsible for the muscle disorders Bethlem myopathy and Ullrich congenital muscular dystrophy. These disorders form a disease spectrum from mild to severe. Dominant and recessive mutations are found along the entire spectrum and the clinical phenotype is strongly influenced by the way mutations impede collagen VI protein assembly. Most mutations are in the triple helical domain, towards the N-terminus and they compromise microfibril assembly. Some mutations are found outside the helix in the C- and N-terminal globular domains, but because these regions are highly polymorphic it is difficult to discriminate mutations from rare benign changes without detailed structural and functional studies. Collagen VI deficiency leads to mitochondrial dysfunction, deficient autophagy and increased apoptosis. Therapies that target these consequences have been tested in mouse models and some have shown modest efficacy in small human trials. Antisense therapies for a common mutation that introduces a pseudoexon show promise in cell culture but haven't yet been tested in an animal model. Future therapeutic approaches await new research into how collagen VI deficiency signals downstream consequences.

A novel variant in the COL6A1 gene causing Ullrich congenital muscular dystrophy in a consanguineous family: a case report

Background

Collagen VI-related dystrophies are a subtype of congenital muscular dystrophy caused by pathogenic variants in COL6A1, COL6A2 or COL6A3 genes affecting skeletal muscles and connective tissue. The clinical phenotype ranges from the milder Bethlem myopathy to the severe Ullrich congenital muscular dystrophy (UCMD). Herein, we report the first consanguineous Sri Lankan family with two children affected with UCMD due to a novel variant in the COL6A1 gene.

Case presentation

Two sisters, aged 10-years and 7-years, presented with progressive, bilateral proximal muscle weakness. Both probands had delayed motor milestones and demonstrated difficulty in standing from a squatting position, climbing stairs and raising arms above the shoulders. Cognitive, language and social development were age appropriate. Examination showed proximal muscle weakness of the upper and lower extremities and hyperlaxity of the wrist and fingers in both with some variability in clinical severity noted between the two siblings. Serum creatine kinase levels were elevated, and electromyography showed low polyphasic motor unit potentials in the 10-year-old and myopathic features with short duration motor unit potentials with no polyphasia in the 7-year-old. Whole exome sequencing (WES) was performed and a novel, homozygous missense, likely pathogenic variant in exon 25 of COL6A1 gene [NM_001848: c.1667G > T;NP_001839.2:p.Gly556Val] was identified in both probands. This variant was validated by Sanger sequencing in proband 1 as well as proband 2, and the parents and an unaffected sibling were found to be heterozygote carriers for the same variant.

Conclusions

The findings in this family add to the expanding number of COL6A1 variants identified and provides a better understanding of the genotype-phenotype correlations associated with UCMD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12883-021-02134-7.

Association of Initial Maximal Motor Ability With Long-term Functional Outcome in Patients With COL6-Related Dystrophies

OBJECTIVE

To accurately categorize the phenotypes of individuals with collagen VI-related dystrophies (COL6-RDs) during the first years of life to predict long-term motor function and pulmonary function, to provide phenotype-specific anticipatory care, and to improve clinical trial readiness.

METHODS

This retrospective, multicenter, international study analyzed the relationship of long-term motor and pulmonary function with the initial maximal motor ability achieved in individuals with COL6-RD.

RESULTS

We studied 119 patients with COL6-RD from Spain (n = 54) and the United States (n = 65). The early maximal motor milestones of ability to rise from the floor unassisted and ability to climb 4 steps without holding onto a railing demonstrated reliability in distinguishing between 3 COL6-RD phenotypic subgroups: (1) Ullrich congenital muscular dystrophy, (2) intermediate COL6-RD, and (3) Bethlem myopathy. Long-term motor function and pulmonary function are strongly correlated with the maximal motor ability achieved during the first years of life. Maximal motor capacity can predict other disease-relevant events such as the age at loss of ambulation and the need for the initiation of nocturnal noninvasive ventilation.

CONCLUSION

This work proposes a prospective phenotypic classification for COL6-RDs that will enable an accurate prediction of a patient's COL6-RD phenotype during the first years of life. The ability to establish a patient's COL6-RD phenotypic classification early will enable a more accurate prognosis of future motor and pulmonary function, thus improving anticipatory clinical care, and it will be instrumental in aiding the design of future clinical trials by allowing early stratification of trial cohorts.

Reservoir of Fibroblasts Promotes Recovery From Limb Ischemia

BACKGROUND

The angiogenic response to ischemia restores perfusion so as to preserve tissue. A role for mesenchymal-to-endothelial transition in the angiogenic response is controversial. This study is to determine if resident fibroblasts contribute to angiogenesis.

METHODS

We utilized the murine model of hindlimb ischemia, and in vivo Matrigel plug assay together with lineage tracing studies and single cell RNA-sequencing to examine the transcriptional and functional changes in fibroblasts in response to ischemia.

RESULTS

Lineage tracing using Fsp1-Cre: R26R-EYFP mice revealed the emergence within the ischemic hindlimb of a small subset of YFP⁺ CD144⁺ CD11b^- fibroblasts (E* cells) that expressed endothelial cell (EC) genes. Subcutaneous administration of Matrigel in Fsp1-Cre: R26R-EYFP mice generated a plug that became vascularized within 5 days. Isolation of YFP⁺ CD11b^- cells from the plug revealed a small subset of YFP⁺ CD144⁺ CD11b^- E* cells which expressed EC genes. Pharmacological or genetic suppression of innate immune signaling reduced vascularity of the Matrigel plug and abrogated the generation of these E* cells. These studies were repeated using human fibroblasts, with fluorescence-activated cell sorting analysis revealing that a small percentage of human fibroblasts that were induced to express EC markers in Matrigel plug assay. Pharmacological suppression or genetic knockout of inflammatory signaling abolished the generation of E* cells, impaired perfusion recovery and increased tissue injury after femoral artery ligation. To further characterize these E* cells, single cell RNA-sequencing studies were performed and revealed 8 discrete clusters of cells expressing characteristic fibroblast genes, of which 2 clusters (C5 and C8) also expressed some EC genes. Ischemia of the hindlimb induced expansion of clusters C5 and C8. The C8 cells did not express CD144, nor did they form networks in Matrigel, but did generate angiogenic cytokines. The C5 fibroblasts most resembled E* cells in their expression of CD144 and their ability to form EC-like networks in Matrigel.

CONCLUSIONS

Together, these studies indicate the presence of subsets of tissue fibroblasts which seem poised to contribute to the angiogenic response. The expansion of these subsets with ischemia is dependent on activation of innate immune signaling and contributes to recovery of perfusion and preservation of ischemic tissue.

Collagen VI-related myopathy: Expanding the clinical and genetic spectrum

INTRODUCTION

We aimed to analyze the clinical and genetic characteristics of collagen VI-related myopathy.

METHODS

We analyzed the clinical course and mutation spectrum in patients with collagen VI gene mutations among our congenital muscular dystrophy cohort.

RESULTS

Among 24 patients with mutations in collagen VI coding genes, 13 (54.2%) were categorized as Ullrich type, and 11 (45.8%) as non-Ullrich type. Congenital orthopedic problems were similarly observed in both types, yet multiple joint contractures were found only in the Ullrich type. Clinical courses and pathology findings varied between patients. Mutations in COL6A1, COL6A2, and COL6A3 were found in 15 (65%), 3 (13%), and 5 (22%) patients, respectively, without genotype-phenotype association. Five novel variants were detected.

DISCUSSION

We verified clinical heterogeneity of collagen VI-related myopathy, which emphasizes the importance of genetic testing. Genotype-phenotype association or early predictors for progression were not identified. Multiple joint contractures predict rapid deterioration. Muscle Nerve 58: 381-388, 2018.

A recurrent COL6A1 pseudoexon insertion causes muscular dystrophy and is effectively targeted by splice-correction therapies

The clinical application of advanced next-generation sequencing technologies is increasingly uncovering novel classes of mutations that may serve as potential targets for precision medicine therapeutics. Here, we show that a deep intronic splice defect in the COL6A1 gene, originally discovered by applying muscle RNA sequencing in patients with clinical findings of collagen VI-related dystrophy (COL6-RD), inserts an in-frame pseudoexon into COL6A1 mRNA, encodes a mutant collagen α1(VI) protein that exerts a dominant-negative effect on collagen VI matrix assembly, and provides a unique opportunity for splice-correction approaches aimed at restoring normal gene expression. Using splice-modulating antisense oligomers, we efficiently skipped the pseudoexon in patient-derived fibroblast cultures and restored a wild-type matrix. Similarly, we used CRISPR/Cas9 to precisely delete an intronic sequence containing the pseudoexon and efficiently abolish its inclusion while preserving wild-type splicing. Considering that this splice defect is emerging as one of the single most frequent mutations in COL6-RD, the design of specific and effective splice-correction therapies offers a promising path for clinical translation.

Genetic Disorders of the Extracellular Matrix

Mutations in the genes for extracellular matrix (ECM) components cause a wide range of genetic connective tissues disorders throughout the body. The elucidation of mutations and their correlation with pathology has been instrumental in understanding the roles of many ECM components. The pathological consequences of ECM protein mutations depend on its tissue distribution, tissue function, and on the nature of the mutation. The prevalent paradigm for the molecular pathology has been that there are two global mechanisms. First, mutations that reduce the production of ECM proteins impair matrix integrity largely due to quantitative ECM defects. Second, mutations altering protein structure may reduce protein secretion but also introduce dominant negative effects in ECM formation, structure and/or stability. Recent studies show that endoplasmic reticulum (ER) stress, caused by mutant misfolded ECM proteins, makes a significant contribution to the pathophysiology. This suggests that targeting ER‐stress may offer a new therapeutic strategy in a range of ECM disorders caused by protein misfolding mutations. Anat Rec, 2019. © 2019 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

Extracellular matrix-driven congenital muscular dystrophies

Skeletal muscle function relies on the myofibrillar apparatus inside myofibers as well as an intact extracellular matrix surrounding each myofiber. Muscle extracellular matrix (ECM) plays several roles including but not limited to force transmission, regulation of growth factors and inflammatory responses, and influencing muscle stem cell (i.e. satellite cell) proliferation and differentiation. In most myopathies, muscle ECM undergoes remodeling and fibrotic changes that may be maladaptive for normal muscle function and recovery. In addition, mutations in skeletal muscle ECM and basement proteins can cause muscle disease. In this review, we summarize the clinical features of two of the most common congenital muscular dystrophies, COL6-related dystrophies and LAMA2-related dystrophies, which are caused by mutations in muscle ECM and basement membrane proteins. The study of clinical features of these diseases has helped to inform basic research and understanding of the biology of muscle ECM. In return, basic studies of muscle ECM have provided the conceptual framework to develop therapeutic interventions for these and other similar disorders of muscle.

Collagen VI disorders: Insights on form and function in the extracellular matrix and beyond

Mutations in the three canonical collagen VI genes, COL6A1, COL6A2 and COL6A3, cause a spectrum of muscle disease from Bethlem myopathy at the mild end to the severe Ullrich congenital muscular dystrophy. Mutations can be either dominant or recessive and the resulting clinical severity is influenced by the way mutations impact the complex collagen VI assembly process. Most mutations are found towards the N-terminus of the triple helical collagenous domain and compromise extracellular microfibril assembly. Outside the triple helix collagen VI is highly polymorphic and discriminating mutations from rare benign changes remains a major diagnostic challenge. Collagen VI deficiency alters extracellular matrix structure and biomechanical properties and leads to increased apoptosis and oxidative stress, decreased autophagy, and impaired muscle regeneration. Therapies that target these downstream consequences have been tested in a collagen VI null mouse and also in small human trials where they show modest clinical efficacy. An important role for collagen VI in obesity, cancer and diabetes is emerging. A major barrier to developing effective therapies is the paucity of information about how collagen VI deficiency in the extracellular matrix signals the final downstream consequences - the receptors involved and the intracellular messengers await further characterization.

Somatic mosaicism represents an underestimated event underlying collagen 6-related disorders

BACKGROUND

Collagen VI-related disorders (COL6-RD) are a group of heterogenous muscular diseases due to mutations in the COL6A1, COL6A2 and COL6A3 genes, encoding for collagen VI, a critical component of the extracellular matrix. Ullrich congenital muscle disorder and Bethlem myopathy represent the ends of a clinical spectrum that includes intermediate phenotypes of variable severity. UCMD are caused by recessive loss of function mutations or de-novo dominant-negative mutations. The intermediate phenotype and BM are more commonly caused by dominantly acting mutations, and less commonly by recessive mutations. Recently parental mosaicism for dominant mutations in COL6 have been reported in four COL6-RD families and germinal mosaicism has been also identified in a family with recurrence of UCMD in two half-sibs.

METHODS AND RESULTS

Here we report three unrelated patients affected by a COL6-RD who carried de novo mosaic mutations in COL6A genes. These mutations, missed by Sanger sequencing, were identified by next generation sequencing.

CONCLUSIONS

This report highlights the importance of a complete diagnostic workup when clinical and histological finding are consistent with a COL6-RD and strengthen the impression that mosaicisms are underestimated events underlying COL6-RD.

Clinical, Pathologic, and Genetic Features of Collagen VI-Related Myopathy in Korea

Background and Purpose

Mutations in collagen VI-related genes (COL6A1, COL6A2, and COL6A3) cause Bethlem myopathy (BM) and Ullrich congenital muscular dystrophy (UCMD). These were previously believed to be separate disease entities, but they are now both classified as collagen VI-related myopathies, which cover a broad clinical spectrum. We aimed to analyze the clinical, pathologic, and genetic characteristics of patients with collagen VI-related myopathy in Korea.

Methods

We reviewed the clinical, pathologic, and genetic features in 22 patients with collagen VI-related myopathy from 13 families, as confirmed by genetic analysis of collagen VI-related genes.

Results

The mean ages of the 22 patients at first symptom presentation and diagnosis were 4.5 and 24.9 years, respectively. Four patients in 4 families showed the phenotype of intermediate collagen VI-related myopathies (IM), 16 patients in 7 families had the BM phenotype, and 2 patients in 2 families presented with the typical UCMD phenotype. Based on genetic analysis, five patients (five families) comprising four with IM and one with typical UCMD had missense mutations in the triple-helical domain of COL6A1, and ten patients (four families) with BM showed exon-14-skipping mutations. Additionally, we found two novel mutations: c.956A>G (p.K319R) in COL6A1 and c.6221G>T (p.G2074V) in COL6A3.

Conclusions

Missense mutations in the triple-helical domain of COL6A1 are the most common mutations related to collagen VI-related myopathy in Korea. Patients with these mutations have a tendency toward an earlier disease onset and more severe progression compared to patients with other mutations.

Skeletal muscle fibroblasts in health and disease

As the primary producer of extracellular matrix (ECM) proteins in skeletal muscle, fibroblasts play an important role providing structural support to muscle. Skeletal muscle ECM is vital for force transduction from muscle cells to tendons and bones to create movement. It is these ECM connections that allow the movement created in muscle to be transmitted to our skeleton. This review discusses how fibroblasts participate in maintaining this healthy ECM within skeletal muscle. Additionally, from a basic science perspective, we discuss current methods to identify and study skeletal muscle fibroblasts, as this is critical to bettering our understanding of these important cells. Finally, skeletal muscle fibrosis is discussed, which is a devastating clinical condition characterized by an overproduction of ECM within skeletal muscle. We discuss the role that fibroblasts and other cells play in muscle fibrosis as well as the implications of this work.

Paternal germline mosaicism in collagen VI related myopathies

BACKGROUND

Collagen VI-related disorders are a group of muscular diseases characterized by muscle wasting and weakness, joint contractures, distal laxity, serious respiratory dysfunction and cutaneous alterations, due to mutations in the COL6A1, COL6A2 and COL6A3 genes, encoding for collagen VI, a critical component of the extracellular matrix. The severe Ullrich congenital muscular dystrophy (UCMD) can be due to autosomal recessive mutations in one of the three genes with a related 25% recurrence risk. In the majority of UCMD cases nevertheless, the underlying mutation is thought to arise de novo and the recurrence risk is considered as low.

METHODS AND RESULTS

Here we report a family with recurrence of UCMD in two half-sibs. In both, the molecular analysis revealed heterozygosity for the c.896G > A missense mutation in COL6A1 exon 10 (Gly299Glu) and for the COL6A1 c.1823-8G > A variation within COL6A1 intron 29. The intronic variation was inherited from the father and RNA analysis in skin fibroblasts allowed to exclude its role in affecting COL6A1 transcript processing. The Gly299Glu mutation occurred apparently de novo in the two sibs.

CONCLUSION

The described mutational segregation strongly suggests the occurrence of paternal germline mosaicism. This is the first report of UCMD recurrence due to a germline mosaic COL6 gene mutation. Mosaicism deserves to be considered as possible inheritance pattern in genetic counseling and recurrence risk estimation in collagen VI-related diseases.

Aberrant mitochondria in a Bethlem myopathy patient with a homozygous amino acid substitution that destabilizes the collagen VI α2(VI) chain

Bethlem myopathy and Ullrich congenital muscular dystrophy (UCMD) sit at opposite ends of a clinical spectrum caused by mutations in the extracellular matrix protein collagen VI. Bethlem myopathy is relatively mild, and patients remain ambulant in adulthood while many UCMD patients lose ambulation by their teenage years and require respiratory interventions. Dominant and recessive mutations are found across the entire clinical spectrum; however, recessive Bethlem myopathy is rare, and our understanding of the molecular pathology is limited. We studied a patient with Bethlem myopathy. Electron microscopy of his muscle biopsy revealed abnormal mitochondria. We identified a homozygous COL6A2 p.D871N amino acid substitution in the C-terminal C2 A-domain. Mutant α2(VI) chains are unable to associate with α1(VI) and α3(VI) and are degraded by the proteasomal pathway. Some collagen VI is assembled, albeit more slowly than normal, and is secreted. These molecules contain the minor α2(VI) C2a splice form that has an alternative C terminus that does include the mutation. Collagen VI tetramers containing the α2(VI) C2a chain do not assemble efficiently into microfibrils and there is a severe collagen VI deficiency in the extracellular matrix. We expressed wild-type and mutant α2(VI) C2 domains in mammalian cells and showed that while wild-type C2 domains are efficiently secreted, the mutant p.D871N domain is retained in the cell. These studies shed new light on the protein domains important for intracellular and extracellular collagen VI assembly and emphasize the importance of molecular investigations for families with collagen VI disorders to ensure accurate diagnosis and genetic counseling.

Whole-genome sequencing and the clinician: a tale of two cities

BACKGROUND

Clinicians are faced with unprecedented opportunities to identify the genetic aetiologies of hitherto molecularly uncharacterised conditions via the use of high-throughput sequencing. Access to genomic technology and resultant data is no longer limited to clinicians, geneticists and bioinformaticians, however; ongoing commercialisation gives patients themselves ever greater access to sequencing services. We report an increasingly common medical scenario by describing two neuromuscular patients--a mother and adult son--whose consumer access to whole-genome sequencing affected their diagnostic journey.

RESULTS

Whole-genome sequencing initiated by the patients--to predict their risk of common diseases--revealed that they share several variants potentially relevant to neuromuscular diseases, which initially sidetracked diagnostic efforts. Since eventual clinical reassessment, including muscle imaging, pointed towards Bethlem myopathy, a collagen VI-related myopathy, we pursued Sanger sequencing of COL6A1, COL6A2 and COL6A3. This targeted approach revealed a heterozygous causative variant in COL6A3 (c.6365G>T (p.Gly2122Val)), shared by both individuals, that was not flagged by the interpretation of the whole-genome sequencing data.

CONCLUSIONS

This report highlights the essential interplay of clinical and genomic expertise in realising the potential of high-throughput sequencing. In an era when patients themselves may bring their own data to the table, definitively identifying clinically significant genomic variants will require close collaboration among clinicians, geneticists and bioinformaticians.

The collagenopathies: review of clinical phenotypes and molecular correlations

Genetic defects of collagen formation (the collagenopathies) affect almost every organ system and tissue in the body. They can be grouped by clinical phenotype, which usually correlates with the tissue distribution of the affected collagen subtype. Many of these conditions present in childhood; however, milder phenotypes presenting in adulthood are increasingly recognized. Many are difficult to differentiate clinically. Precise diagnosis by means of genetic testing assists in providing prognosis information, family counseling, and individualized treatment. This review provides an overview of the current range of clinical presentations associated with collagen defects, and the molecular mechanisms important to understanding how the results of genetic testing affect medical care.

Position of glycine substitutions in the triple helix of COL6A1, COL6A2, and COL6A3 is correlated with severity and mode of inheritance in collagen VI myopathies

Glycine substitutions in the conserved Gly-X-Y motif in the triple helical (TH) domain of collagen VI are the most commonly identified mutations in the collagen VI myopathies including Ullrich congenital muscular dystrophy, Bethlem myopathy, and intermediate (INT) phenotypes. We describe clinical and genetic characteristics of 97 individuals with glycine substitutions in the TH domain of COL6A1, COL6A2, or COL6A3 and add a review of 97 published cases, for a total of 194 cases. Clinical findings include severe, INT, and mild phenotypes even from patients with identical mutations. INT phenotypes were most common, accounting for almost half of patients, emphasizing the importance of INT phenotypes to the overall phenotypic spectrum. Glycine substitutions in the TH domain are heavily clustered in a short segment N-terminal to the 17th Gly-X-Y triplet, where they are acting as dominants. The most severe cases are clustered in an even smaller region including Gly-X-Y triplets 10-15, accounting for only 5% of the TH domain. Our findings suggest that clustering of glycine substitutions in the N-terminal region of collagen VI is not based on features of the primary sequence. We hypothesize that this region may represent a functional domain within the triple helix.

Novel collagen VI mutations identified in Chinese patients with Ullrich congenital muscular dystrophy

BACKGROUND

We determined the clinical and molecular genetic characteristics of 8 Chinese patients with Ullrich congenital muscular dystrophy (UCMD).

METHODS

Clinical data of probands were collected and muscle biopsies of patients were analyzed. Exons of COL6A1, COL6A2 and COL6A3 were analyzed by direct sequencing. Mutations in COL6A1, COL6A2 and COL6A3 were identified in 8 patients.

RESULTS

Among these mutations, 5 were novel [three in the triple helical domain (THD) and 2 in the second C-terminal (C2) domain]. We also identified five known missense or in-frame deletion mutations in THD and C domains. Immunohistochemical studies on muscle biopsies from patients showed reduced level of collagen VI at the muscle basement membrane and mis-localization of the protein in interstitial and perivascular regions.

CONCLUSIONS

The novel mutations we identified underscore the importance of THD and C2 domains in the assembly and function of collagen VI, thereby providing useful information for the genetic counseling of UCMD patients.

Diagnostic approach to the congenital muscular dystrophies

Congenital muscular dystrophies (CMDs) are early onset disorders of muscle with histological features suggesting a dystrophic process. The congenital muscular dystrophies as a group encompass great clinical and genetic heterogeneity so that achieving an accurate genetic diagnosis has become increasingly challenging, even in the age of next generation sequencing. In this document we review the diagnostic features, differential diagnostic considerations and available diagnostic tools for the various CMD subtypes and provide a systematic guide to the use of these resources for achieving an accurate molecular diagnosis. An International Committee on the Standard of Care for Congenital Muscular Dystrophies composed of experts on various aspects relevant to the CMDs performed a review of the available literature as well as of the unpublished expertise represented by the members of the committee and their contacts. This process was refined by two rounds of online surveys and followed by a three-day meeting at which the conclusions were presented and further refined. The combined consensus summarized in this document allows the physician to recognize the presence of a CMD in a child with weakness based on history, clinical examination, muscle biopsy results, and imaging. It will be helpful in suspecting a specific CMD subtype in order to prioritize testing to arrive at a final genetic diagnosis.

Bethlem myopathy: An autosomal dominant myopathy with flexion contractures, keloids, and follicular hyperkeratosis

Bethlem myopathy and Ullrich congenital muscular dystrophy form a spectrum of collagenopathies caused by genetic mutations encoding for any of the three subunits of collagen VI. Bethlem phenotype is relatively benign and is characterized by proximal dominant myopathy, keloids, contractures, distal hyperextensibility, and follicular hyperkeratosis. Three patients from a single family were diagnosed to have Bethlem myopathy based on European Neuromuscular Centre Bethlem Consortium criteria. Affected father and his both sons had slowly progressive proximal dominant weakness and recurrent falls from the first decade. Both children aged 18 and 20 years were ambulant at presentation. All had flexion contractures, keloids, and follicular hyperkeratosis without muscle hypertrophy. Creatinine kinase was mildly elevated and electromyography revealed myopathic features. Muscle imaging revealed severe involvement of glutei and vasti with “central shadow” in rectus femoris. Muscle biopsy in the father showed dystrophic changes with normal immmunostaining for collagen VI, sarcoglycans, and dysferlin.

Mutations in the collagen XII gene define a new form of extracellular matrix-related myopathy

Bethlem myopathy (BM) [MIM 158810] is a slowly progressive muscle disease characterized by contractures and proximal weakness, which can be caused by mutations in one of the collagen VI genes (COL6A1, COL6A2 and COL6A3). However, there may be additional causal genes to identify as in ∼50% of BM cases no mutations in the COL6 genes are identified. In a cohort of -24 patients with a BM-like phenotype, we first sequenced 12 candidate genes based on their function, including genes for known binding partners of collagen VI, and those enzymes involved in its correct post-translational modification, assembly and secretion. Proceeding to whole-exome sequencing (WES), we identified mutations in the COL12A1 gene, a member of the FACIT collagens (fibril-associated collagens with interrupted triple helices) in five individuals from two families. Both families showed dominant inheritance with a clinical phenotype resembling classical BM. Family 1 had a single-base substitution that led to the replacement of one glycine residue in the triple-helical domain, breaking the Gly-X-Y repeating pattern, and Family 2 had a missense mutation, which created a mutant protein with an unpaired cysteine residue. Abnormality at the protein level was confirmed in both families by the intracellular retention of collagen XII in patient dermal fibroblasts. The mutation in Family 2 leads to the up-regulation of genes associated with the unfolded protein response (UPR) pathway and swollen, dysmorphic rough-ER. We conclude that the spectrum of causative genes in extracellular matrix (ECM)-related myopathies be extended to include COL12A1.

Characterization of a rare case of Ullrich congenital muscular dystrophy due to truncating mutations within the COL6A1 gene C-terminal domain: a case report

Background

Mutations within the C-terminal region of the COL6A1 gene are only detected in Ullrich/Bethlem patients on extremely rare occasions.

Case presentation

Herein we report two Brazilian brothers with a classic Ullrich phenotype and compound heterozygous for two truncating mutations in COL6A1 gene, expected to result in the loss of the α1(VI) chain C2 subdomain. Despite the reduction in COL6A1 RNA level due to nonsense RNA decay, three truncated alpha1 (VI) chains were produced as protein variants encoded by different out-of-frame transcripts. Collagen VI matrix was severely decreased and intracellular protein retention evident.

Conclusion

The altered deposition of the fibronectin network highlighted abnormal interactions of the mutated collagen VI, lacking the α1(VI) C2 domain, within the extracellular matrix, focusing further studies on the possible role played by collagen VI in fibronectin deposition and organization.

Ullrich Congenital Muscular Dystrophy (UCMD): Clinical and Genetic Correlations

OBJECTIVE

Ullrich congenital muscular dystrophy (UCMD) corresponds to the severe end of the clinical spectrum of neuromuscular disorders caused by mutations in the genes encoding collagen VI (COL VI). We studied four unrelated families with six affected children that had typical UCMD with dominant and recessive inheritance.

MATERIALS & METHODS

Four unrelated Iranian families with six affected children with typical UCMD were analyzed for COLVI secretion in skin fibroblast culture and the secretion of COLVI in skin fibroblast culture using quantitative RT-PCR (Q-RT-PCR), and mutation identification was performed by sequencing of complementary DNA.

RESULTS

COL VI secretion was altered in all studied fibroblast cultures. Two affected sibs carried a homozygous nonsense mutation in exon 12 of COL6A2, while another patient had a large heterozygous deletion in exon 5-8 of COL6A2. The two other affected sibs had homozygote mutation in exon 24 of COL6A2, and the last one was homozygote in COL6A1.

CONCLUSION

In this study, we found out variability in clinical findings and genetic inheritance among UCMD patients, so that the patient with complete absence of COLVI was severely affected and had a large heterozygous deletion in COL6A2. In contrast, the patients with homozygous deletion had mild to moderate decrease in the secretion of COL VI and were mildly to moderately affected.

Muscle fiber atrophy and regeneration coexist in collagen VI-deficient human muscle: role of calpain-3 and nuclear factor-κB signaling

Ullrich congenital muscular dystrophy (UCMD) is a common form of muscular dystrophy associated with defects in collagen VI. It is characterized by loss of individual muscle fibers and muscle mass and proliferation of connective and adipose tissues. We sought to investigate the mechanisms by which collagen VI regulates muscle cell survival, size, and regeneration and, in particular, the potential role of the ubiquitin-proteasome and calpain-proteolytic systems. We studied muscle biopsies of UCMD (n = 6), other myopathy (n = 12), and control patients (n = 10) and found reduced expression of atrogin-1, MURF1, and calpain-3 mRNAs in UCMD cases. Downregulation of calpain-3 was associated with changes in the nuclear immunolocalization of nuclear factor-κB. We also observed increased expression versus controls of regeneration markers at the protein and RNA levels. Satellite cell numbers did not differ in collagen VI-deficient muscle versus normal nonregenerating muscle, indicating that collagen VI does not play a key role in the maintenance of the satellite cell pool. Our results indicate that alterations in calpain-3 and nuclear factor-κB signaling pathways may contribute to muscle mass loss in UCMD muscle, whereas atrogin-1 and MURF1 are not likely to play a major role.

Congenital muscular dystrophies: a brief review

Congenital muscular dystrophies (CMDs) are clinically and genetically heterogeneous neuromuscular disorders with onset at birth or in infancy in which the muscle biopsy is compatible with a dystrophic myopathy. In the past 10 years, knowledge of neuromuscular disorders has dramatically increased, particularly with the exponential boost of disclosing the genetic background of CMDs. This review will highlight the clinical description of the most important forms of CMD, paying particular attention to the main keys for diagnostic approach. The diagnosis of CMDs requires the concurrence of expertise in multiple specialties (neurology, morphology, genetics, neuroradiology) available in a few centers worldwide that have achieved sufficient experience with the different CMD subtypes. Currently, molecular diagnosis is of paramount importance not only for phenotype-genotype correlations, genetic and prenatal counseling, and prognosis and aspects of management, but also concerning the imminent availability of clinical trials and treatments.

ColVI myopathies: where do we stand, where do we go?

Collagen VI myopathies, caused by mutations in the genes encoding collagen type VI (ColVI), represent a clinical continuum with Ullrich congenital muscular dystrophy (UCMD) and Bethlem myopathy (BM) at each end of the spectrum, and less well-defined intermediate phenotypes in between. ColVI myopathies also share common features with other disorders associated with prominent muscle contractures, making differential diagnosis difficult. This group of disorders, under-recognized for a long time, has aroused much interest over the past decade, with important advances made in understanding its molecular pathogenesis. Indeed, numerous mutations have now been reported in the COL6A1, COL6A2 and COL6A3 genes, a large proportion of which are de novo and exert dominant-negative effects. Genotype-phenotype correlations have also started to emerge, which reflect the various pathogenic mechanisms at play in these disorders: dominant de novo exon splicing that enables the synthesis and secretion of mutant tetramers and homozygous nonsense mutations that lead to premature termination of translation and complete loss of function are associated with early-onset, severe phenotypes. In this review, we present the current state of diagnosis and research in the field of ColVI myopathies. The past decade has provided significant advances, with the identification of altered cellular functions in animal models of ColVI myopathies and in patient samples. In particular, mitochondrial dysfunction and a defect in the autophagic clearance system of skeletal muscle have recently been reported, thereby opening potential therapeutic avenues.

The collagen VI-related myopathies: muscle meets its matrix

The collagen VI-related myopathy known as Ullrich congenital muscular dystrophy is an early-onset disease that combines substantial muscle weakness with striking joint laxity and progressive contractures. Patients might learn to walk in early childhood; however, this ability is subsequently lost, concomitant with the development of frequent nocturnal respiratory failure. Patients with intermediate phenotypes of collagen VI-related myopathy display a lesser degree of weakness and a longer period of ambulation than do individuals with Ullrich congenital muscular dystrophy, and the spectrum of disease finally encompasses mild Bethlem myopathy, in which ambulation persists into adulthood. Dominant and recessive autosomal mutations in the three major collagen VI genes-COL6A1, COL6A2, and COL6A3-can underlie this entire clinical spectrum, and result in deficient or dysfunctional microfibrillar collagen VI in the extracellular matrix of muscle and other connective tissues, such as skin and tendons. The potential effects on muscle include progressive dystrophic changes, fibrosis and evidence for increased apoptosis, which potentially open avenues for pharmacological intervention. Optimized respiratory management, including noninvasive nocturnal ventilation together with careful orthopedic management, are the current mainstays of treatment and have already led to a considerable improvement in life expectancy for children with Ullrich congenital muscular dystrophy.

The collagen VI-related myopathies Ullrich congenital muscular dystrophy and Bethlem myopathy

Mutations in the genes COL6A1, COL6A2, and COL6A3, coding for three α chains of collagen type VI, underlie a spectrum of myopathies, ranging from the severe congenital muscular dystrophy-type Ullrich (UCMD) to the milder Bethlem myopathy (BM), with disease manifestations of intermediate severity in between. UCMD is characterized by early-onset weakness, associated with pronounced distal joint hyperlaxity and the early onset or early progression of more proximal contractures. In the most severe cases ambulation is not achieved, or it may be achieved only for a limited period of time. BM may be of early or later onset, but is milder in its manifestations, typically allowing for ambulation well into adulthood, whereas typical joint contractures are frequently prominent. A genetic spectrum is emerging, with BM being caused mostly by dominantly acting mutations, although rarely recessive inheritance of BM is also possible, whereas both dominantly as well as recessively acting mutations underlie UCMD.

Hedgehog signaling and laminin play unique and synergistic roles in muscle development

Hedgehog (Hh) signaling and laminin-111, a basement membrane protein, are required for early muscle development. Hh signaling specifies different populations of muscle fibers and laminin-111 is critical for early muscle morphogenesis. However, additional requirements for Hh signaling and laminin during later phases of muscle development are not known. Furthermore, interactions between Hh signaling and laminin in this context are unknown. We used laminin gamma1 mutant zebrafish and cyclopamine to block Hh signal transduction separately and in combination to investigate their functions and interactions. We found that both Hh signaling and laminin are required for normal myosin chain expression. In addition, Hh signaling and laminin act synergistically during fast-twitch fiber elongation: fast muscle cells do not elongate in embryos deficient for both Hh signaling and laminin. Finally, we present evidence that suggests that Hh signaling is indirectly required via slow fiber specification for recovery of fast fiber elongation in laminin gamma1 mutant embryos.

Consensus statement on standard of care for congenital muscular dystrophies

Congenital muscular dystrophies are a group of rare neuromuscular disorders with a wide spectrum of clinical phenotypes. Recent advances in understanding the molecular pathogenesis of congenital muscular dystrophy have enabled better diagnosis. However, medical care for patients with congenital muscular dystrophy remains very diverse. Advances in many areas of medical technology have not been adopted in clinical practice. The International Standard of Care Committee for Congenital Muscular Dystrophy was established to identify current care issues, review literature for evidence-based practice, and achieve consensus on care recommendations in 7 areas: diagnosis, neurology, pulmonology, orthopedics/rehabilitation, gastroenterology/ nutrition/speech/oral care, cardiology, and palliative care. To achieve consensus on the care recommendations, 2 separate online surveys were conducted to poll opinions from experts in the field and from congenital muscular dystrophy families. The final consensus was achieved in a 3-day workshop conducted in Brussels, Belgium, in November 2009. This consensus statement describes the care recommendations from this committee.

Early onset collagen VI myopathies: Genetic and clinical correlations

OBJECTIVE

Mutations in the genes encoding the extracellular matrix protein collagen VI (ColVI) cause a spectrum of disorders with variable inheritance including Ullrich congenital muscular dystrophy, Bethlem myopathy, and intermediate phenotypes. We extensively characterized, at the clinical, cellular, and molecular levels, 49 patients with onset in the first 2 years of life to investigate genotype-phenotype correlations.

METHODS

Patients were classified into 3 groups: early-severe (18%), moderate-progressive (53%), and mild (29%). ColVI secretion was analyzed in patient-derived skin fibroblasts. Chain-specific transcript levels were quantified by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR), and mutation identification was performed by sequencing of complementary DNA.

RESULTS

ColVI secretion was altered in all fibroblast cultures studied. We identified 56 mutations, mostly novel and private. Dominant de novo mutations were detected in 61% of the cases. Importantly, mutations causing premature termination codons (PTCs) or in-frame insertions strikingly destabilized the corresponding transcripts. Homozygous PTC-causing mutations in the triple helix domains led to the most severe phenotypes (ambulation never achieved), whereas dominant de novo in-frame exon skipping and glycine missense mutations were identified in patients of the moderate-progressive group (loss of ambulation).

INTERPRETATION

This work emphasizes that the diagnosis of early onset ColVI myopathies is arduous and time-consuming, and demonstrates that quantitative RT-PCR is a helpful tool for the identification of some mutation-bearing genes. Moreover, the clinical classification proposed allowed genotype-phenotype relationships to be explored, and may be useful in the design of future clinical trials.

Collagen VI microfibril formation is abolished by an {alpha}2(VI) von Willebrand factor type A domain mutation in a patient with Ullrich congenital muscular dystrophy

Collagen VI is an extracellular protein that most often contains the three genetically distinct polypeptide chains, α1(VI), α2(VI), and α3(VI), although three recently identified chains, α4(VI), α5(VI), and α6(VI), may replace α3(VI) in some situations. Each chain has a triple helix flanked by N- and C-terminal globular domains that share homology with the von Willebrand factor type A (VWA) domains. During biosynthesis, the three chains come together to form triple helical monomers, which then assemble into dimers and tetramers. Tetramers are secreted from the cell and align end-to-end to form microfibrils. The precise molecular mechanisms responsible for assembly are unclear. Mutations in the three collagen VI genes can disrupt collagen VI biosynthesis and matrix organization and are the cause of the inherited disorders Bethlem myopathy and Ullrich congenital muscular dystrophy. We have identified a Ullrich congenital muscular dystrophy patient with compound heterozygous mutations in α2(VI). The first mutation causes skipping of exon 24, and the mRNA is degraded by nonsense-mediated decay. The second mutation is a two-amino acid deletion in the C1 VWA domain. Recombinant C1 domains containing the deletion are insoluble and retained intracellularly, indicating that the mutation has detrimental effects on domain folding and structure. Despite this, mutant α2(VI) chains retain the ability to associate into monomers, dimers, and tetramers. However, we show that secreted mutant tetramers containing structurally abnormal C1 VWA domains are unable to associate further into microfibrils, directly demonstrating the critical importance of a correctly folded α2(VI) C1 domain in microfibril formation.

Cardiac findings in congenital muscular dystrophies

Cardiac involvement (CI) in congenital muscular dystrophies (CMDs) has been only rarely investigated so far. By means of a systematic literature search we reviewed the literature about CI in CMD and found that CI is apparently absent in Ullrich CMD or CMD with integrin deficiency and only mild in Bethlem CMD. CI in merosin deficiency includes dilated cardiomyopathy and systolic dysfunction. CI in dystroglycanopathies seems most prevalent among all CMDs and includes dilated cardiomyopathy, systolic dysfunction, and myocardial fibrosis in Fukuyama CMD. Among the nonspecified dystroglycanopathies, CI manifests as dilated cardiomyopathy, hypertrophic cardiomyopathy (CMP) or systolic dysfunction. With CMD type 1C, as well as with limb-girdle muscular dystrophy 2I, up to half of the patients develop dilated cardiomyopathy. In rigid-spine syndrome, predominantly the right heart is affected secondary to thoracic deformity. In patients who carry LMNA mutations, CI may manifest as dilated cardiomyopathy, hypertrophic cardiomyopathy, or fatal ventricular arrhythmias. Overall, CI in patients with CMD varies considerably between the different CMD types from absent or mild CI to severe cardiac disease, particularly in merosin deficiency, dystroglycanopathies, and laminopathies. Patients with CMD with CI require regular cardiologic surveillance so that severe, treatable cardiac disease is not overlooked.

Differentiating Emery-Dreifuss muscular dystrophy and collagen VI-related myopathies using a specific CT scanner pattern

Bethlem myopathy and Ullrich congenital muscular dystrophy are part of the heterogeneous group of collagen VI-related muscle disorders. They are caused by mutations in collagen VI (ColVI) genes (COL6A1, COL6A2, and COL6A3) while LMNA mutations cause autosomal dominant Emery-Dreifuss muscular dystrophy. A muscular dystrophy pattern and contractures are found in all three conditions, making differential diagnosis difficult especially in young patients when cardiomyopathy is absent. We retrospectively assessed upper and lower limb muscle CT scans in 14 Bethlem/Ullrich patients and 13 Emery-Dreifuss patients with identified mutations. CT was able to differentiate Emery-Dreifuss muscular dystrophy from ColVI-related myopathies in selected thigh muscles and to a lesser extent calves muscles: rectus femoris fatty infiltration was selectively present in Bethlem/Ullrich patients while posterior thigh muscles infiltration was more prominently found in Emery-Dreifuss patients. A more severe fatty infiltration particularly in the leg posterior compartment was found in the Emery-Dreifuss group.

Congenital muscular dystrophy. Part II: a review of pathogenesis and therapeutic perspectives

The congenital muscular dystrophies (CMDs) are a group of genetically and clinically heterogeneous hereditary myopathies with preferentially autosomal recessive inheritance, that are characterized by congenital hypotonia, delayed motor development and early onset of progressive muscle weakness associated with dystrophic pattern on muscle biopsy. The clinical course is broadly variable and can comprise the involvement of the brain and eyes. From 1994, a great development in the knowledge of the molecular basis has occurred and the classification of CMDs has to be continuously up dated. In the last number of this journal, we presented the main clinical and diagnostic data concerning the different subtypes of CMD. In this second part of the review, we analyse the main reports from the literature concerning the pathogenesis and the therapeutic perspectives of the most common subtypes of CMD: MDC1A with merosin deficiency, collagen VI related CMDs (Ullrich and Bethlem), CMDs with abnormal glycosylation of alpha-dystroglycan (Fukuyama CMD, Muscle-eye-brain disease, Walker Warburg syndrome, MDC1C, MDC1D), and rigid spine syndrome, another much rare subtype of CMDs not related with the dystrophin/glycoproteins/extracellular matrix complex.

Zebrafish models of collagen VI-related myopathies

Collagen VI is an integral part of the skeletal muscle extracellular matrix, providing mechanical stability and facilitating matrix-dependent cell signaling. Mutations in collagen VI result in either Ullrich congenital muscular dystrophy (UCMD) or Bethlem myopathy (BM), with UCMD being clinically more severe. Recent studies demonstrating increased apoptosis and abnormal mitochondrial function in Col6a1 knockout mice and in human myoblasts have provided the first mechanistic insights into the pathophysiology of these diseases. However, how loss of collagen VI causes mitochondrial dysfunction remains to be understood. Progress is hindered in part by the lack of an adequate animal model for UCMD, as knockout mice have a mild motor phenotype. To further the understanding of these disorders, we have generated zebrafish models of the collagen VI myopathies. Morpholinos designed to exon 9 of col6a1 produced a severe muscle disease reminiscent of UCMD, while ones to exon 13 produced a milder phenotype similar to BM. UCMD-like zebrafish have increased cell death and abnormal mitochondria, which can be attenuated by treatment with the proton pump modifier cyclosporin A (CsA). CsA improved the motor deficits in UCMD-like zebrafish, but failed to reverse the sarcolemmal membrane damage. In all, we have successfully generated the first vertebrate model matching the clinical severity of UCMD and demonstrated that CsA provides phenotypic improvement, thus corroborating data from knockout mice supporting the use of mitochondrial permeability transition pore modifiers as therapeutics in patients, and providing proof of principle for the utility of the zebrafish as a powerful preclinical model.

Recessive COL6A2 C-globular missense mutations in Ullrich congenital muscular dystrophy: role of the C2a splice variant

Ullrich congenital muscular dystrophy (UCMD) is a disabling and life-threatening disorder resulting from either recessive or dominant mutations in genes encoding collagen VI. Although the majority of the recessive UCMD cases have frameshift or nonsense mutations in COL6A1, COL6A2, or COL6A3, recessive structural mutations in the COL6A2 C-globular region are emerging also. However, the underlying molecular mechanisms have remained elusive. Here we identified a homozygous COL6A2 E624K mutation (C1 subdomain) and a homozygous COL6A2 R876S mutation (C2 subdomain) in two UCMD patients. The consequences of the mutations were investigated using fibroblasts from patients and cells stably transfected with the mutant constructs. In contrast to expectations based on the clinical severity of these two patients, secretion and assembly of collagen VI were moderately affected by the E624K mutation but severely impaired by the R876S substitution. The E624K substitution altered the electrostatic potential of the region surrounding the metal ion-dependent adhesion site, resulting in a collagen VI network containing thick fibrils and spots with densely packed microfibrils. The R876S mutation prevented the chain from assembling into triple-helical collagen VI molecules. The minute amount of collagen VI secreted by the R876S fibroblasts was solely composed of a faster migrating chain corresponding to the C2a splice variant with an alternative C2 subdomain. In transfected cells, the C2a splice variant was able to assemble into short microfibrils. Together, the results suggest that the C2a splice variant may functionally compensate for the loss of the normal COL6A2 chain when mutations occur in the C2 subdomain.

Dominant-negative effects of COL7A1 mutations can be rescued by controlled overexpression of normal collagen VII

Dominant-negative interference by glycine substitution mutations in the COL7A1 gene causes dominant dystrophic epidermolysis bullosa (DDEB), a skin fragility disorder with mechanically induced blistering. Although qualitative and quantitative alterations of the COL7A1 gene product, collagen VII, underlie DDEB, the lack of direct correlation between mutations and the clinical phenotype has rendered DDEB less amenable to therapeutic targeting. To delineate the molecular mechanisms of DDEB, we used recombinant expression of wild-type (WT) and mutant collagen VII, which contained a naturally occurring COL7A1 mutation, G1776R, G2006D, or G2015E, for characterization of the triple helical molecules. The mutants were co-expressed with WT in equal amounts and could form heterotrimeric hybrid triple helices, as demonstrated by affinity purification and mass spectrometry. The thermal stability of the mutant molecules was strongly decreased, as evident in their sensitivity to trypsin digestion. The helix-to-coil transition, T(m), of the mutant molecules was 31-34 degrees C, and of WT collagen VII 41 degrees C. Co-expression of WT with G1776R- or G2006D-collagen VII resulted in partial intracellular retention of the collagen, and mutant collagen VII had reduced ability to support cell adhesion. Intriguingly, controlled overexpression of WT collagen VII gradually improved the thermal stability of the collective of collagen VII molecules. Co-expression in a ratio of 90% WT:10% mutant increased the T(m) to 41 degrees C for G1776R-collagen VII and to 39 degrees C for G2006D- and G2015E-collagen VII. Therefore, increasing the expression of WT collagen VII in the skin of patients with DDEB can be considered a valid therapeutic approach.