Genetic landscape and novel disease mechanisms from a large LGMD cohort of 4656 patients

Current and Future Approaches to Classify VUSs in LGMD-Related Genes

Next-generation sequencing (NGS) has revealed large numbers of genetic variants in LGMD-related genes, with most of them classified as variants of uncertain significance (VUSs). VUSs are genetic changes with unknown pathological impact and present a major challenge in genetic test interpretation and disease diagnosis. Understanding the phenotypic consequences of VUSs can provide clinical guidance regarding LGMD risk and therapy. In this review, we provide a brief overview of the subtypes of LGMD, disease diagnosis, current classification systems for investigating VUSs, and a potential deep mutational scanning approach to classify VUSs in LGMD-related genes.

Sex differences in the involvement of skeletal and cardiac muscles in myopathic Ano5-/- mice

Limb-girdle muscular dystrophy R12 (LGMD-R12) is caused by recessive mutations in the Anoctamin-5 gene (ANO5, TMEM16E). Although ANO5 myopathy is not X-chromosome linked, we performed a meta-analysis of the research literature and found that three-quarters of patients with LGMD-R12 are males. Females are less likely to present with moderate to severe skeletal muscle and/or cardiac pathology. Because these sex differences could be explained in several ways, we compared males and females in a mouse model of LGMD-R12. This model recapitulates the sex differences in human LGMD-R12. Only male Ano5-/- mice had elevated serum creatine kinase after exercise and exhibited defective membrane repair after laser injury. In contrast, by these measures, female Ano5-/- mice were indistinguishable from wild type. Despite these differences, both male and female Ano5-/- mice exhibited exercise intolerance. Although exercise intolerance of male mice can be explained by skeletal muscle dysfunction, echocardiography revealed that Ano5-/- female mice had features of cardiomyopathy that may be responsible for their exercise intolerance. These findings heighten concerns that mutations of ANO5 in humans may be linked to cardiac disease.

First Identification of Rare Exonic and Deep Intronic Splice-Altering Variants in Patients With Beta-Sarcoglycanopathy

BACKGROUND

The precise genetic diagnosis of a sarcoglycanopathy or dystrophinopathy is sometimes extremely challenging, as pathogenic non-coding variants and/or complex structural variants do exist in DMD or sarcoglycan genes. This study aimed to determine the genetic diagnosis of three patients from two unrelated families with a suspected sarcoglycanopathy or dystrophinopathy based on their clinical, radiological, and pathological features, for whom routine genomic detection approaches failed to yield a definite genetic diagnosis.

METHODS

Muscle-derived reverse transcription-polymerase chain reaction analysis and/or TA cloning of DMD, SGCA, SGCB, SGCD, and SGCG mRNA were performed to identify aberrant transcripts. Genomic Sanger sequencing around the aberrant transcripts was performed to detect possible splice-altering variants. Bioinformatic and segregation studies of the detected genomic variants were performed in both families.

RESULTS

In patients F1-II1 and F1-II2, we identified two novel pathogenic compound heterozygous variants in SGCB. One is a deep intronic splice-altering variant (DISV), c.243 + 1558C > T in intron 2 causing the activation of an 87-base pair (bp) pseudoexon, and the other one is a non-canonical splicing site variant, c.243 + 6T > A leading to the partial intron inclusion of 10-bp sequence. A novel DISV, c.243 + 1576C > G causing a 106-bp pseudoexon activation, and a nonsense variant in SGCB were identified in compound heterozygous state in patient F2-II1. Unexpectedly, the predicted nonsense variant, c.334C > T in exon 3, created a new donor splice site in exon 3 that was stronger than the natural one, resulting in a 97-bp deletion of exon 3 (r.333_429del).

CONCLUSION

This is the first identification of rare exonic and DISVs in the SGCB gene.

Molecular Diagnosis of Muscular Dystrophy Patients in Western Indian Population: A Comprehensive Mutation Analysis Using Amplicon Sequencing

Muscular Dystrophies (MDs) are a group of inherited diseases and heterogeneous in nature. To date, 40 different genes have been reported for the occurrence and/or progression of MDs. This study was conducted to demonstrate the application of next-generation sequencing (NGS) in developing a time-saving and cost-effective diagnostic method to detect single nucleotide variants (SNVs) and copy number variants (CNVs) in a single test. A total of 123 cases clinically suspected of MD were enrolled in this study. Amplicon panel-based diagnosis was carried out for 102 (DMD/BMD) cases and the results were further screened using multiplex ligation-dependent probe amplification (MLPA). Whilst in the case of LGMD (N = 19) and UMD (N = 2), only NGS panel-based analysis was carried out. We identified the large deletions in 74.50% (76/102) of the cases screened with query DMD or BMD. Further, the large deletion in CAPN3 gene (N = 3) and known SNV mutations (N = 4) were identified in LGMD patients. Together, the total diagnosis rate for this amplicon panel was 70.73% (87/123) which demonstrated the utility of panel-based diagnosis for high throughput, affordable, and time-saving diagnostic strategy. Collectively, present study demonstrates that the panel based NGS sequencing could be superior over to MLPA.

Diagnostic delay in patients with FKRP-related muscular dystrophy

Diagnostic journey for people with FKRP mutations participating in a dystroglycanopathy natural history study (n = 68; NCT00313677) was analyzed. Earliest symptoms and age at muscular dystrophy diagnosis were abstracted from subject-reported medical history and record review. Initial signs/symptoms were classified as chronic motor dysfunction (e.g., delayed motor milestones, weakness, falling; n = 40, 59%), elevated transaminases (n = 7, 10%), or acute/intermittent symptoms (myoglobinuria, myalgia, febrile illness-associated acute weakness; n = 21, 31%). Median time from sign/symptom onset to diagnosis was 6.5 years and differed by symptom group: 7.5 years for motor group, 9 years for acute/intermittent group, and 4 years for elevated transaminases group. The sign/symptom category that most commonly resulted in a diagnosis was chronic motor dysfunction (n = 45). Of those without clear weakness as first symptom (n = 55), 36.4% were not diagnosed with MD until weakness became apparent. Median time to diagnosis was shortest for those with febrile illness-associated acute weakness (0.25 years). Median time from first sign/symptom to MD diagnosis has decreased incrementally from 18.8 years for those with onset in the 1970s to < 10 years for symptom onset occurring after 2000. Awareness of disease presentation variability will aid in earlier diagnosis, which is increasingly important with treatments in development.

Investigating Late-Onset Pompe Prevalence in Neuromuscular Medicine Academic Practices: The IPaNeMA Study

BACKGROUND AND OBJECTIVES

We investigated the prevalence of late-onset Pompe disease (LOPD) in patients presenting to 13 academic, tertiary neuromuscular practices in the United States and Canada.

METHODS

All successive patients presenting with proximal muscle weakness or isolated hyperCKemia and/or neck muscle weakness to these 13 centers were invited to participate in the study. Whole blood was tested for acid alpha-glucosidase (GAA) assay through the fluorometric method, and all cases with enzyme levels of ≤10 pmoL/punch/h were reflexed to molecular testing for mutations in the GAA gene. Clinical and demographic information was abstracted from their clinical visit and, along with study data, entered into a purpose-built REDCap database, and analyzed at the University of California, Irvine.

RESULTS

GAA enzyme assay results were available on 906 of the 921 participants who consented for the study. LOPD was confirmed in 9 participants (1% prevalence). Another 9 (1%) were determined to have pseudodeficiency of GAA, whereas 19 (1.9%) were found to be heterozygous for a pathogenic GAA mutation (carriers). Of the definite LOPD participants, 8 (89%) were Caucasian and were heterozygous for the common leaky (IVS1) splice site mutation in the GAA gene (c -32-13T>G), with a second mutation that was previously confirmed to be pathogenic.

DISCUSSION

The prevalence of LOPD in undiagnosed patients meeting the criteria of proximal muscle weakness, high creatine kinase, and/or neck weakness in academic, tertiary neuromuscular practices in the United States and Canada is estimated to be 1%, with an equal prevalence rate of pseudodeficiency alleles.

TRIAL REGISTRATION INFORMATION

Clinical trial registration number: NCT02838368.

A recurrent rare intronic variant in CAPN3 alters mRNA splicing and causes autosomal recessive limb-girdle muscular dystrophy-1 in three Pakistani pedigrees

Autosomal recessive limb-girdle muscular dystrophy-1 (LGMDR1) is an autosomal recessive disorder characterized by progressive weakness of the proximal limb and girdle muscles. Biallelic mutations in CAPN3 are reported frequently to cause LGMDR1. Here, we describe 11 individuals from three unrelated consanguineous families that present with typical features of LGMDR1 that include proximal muscle wasting, weakness of the upper and lower limbs, and elevated serum creatine kinase. Whole-exome sequencing identified a rare homozygous CAPN3 variant near the exon 2 splice donor site that segregates with disease in all three families. mRNA splicing studies showed partial retention of intronic sequence and subsequent introduction of a premature stop codon (NM_000070.3: c.379 + 3A>G; p.Asp128Glyfs*15). Furthermore, we observe reduced CAPN3 expression in primary dermal fibroblasts derived from an affected individual, suggesting instability and/or nonsense-mediated decay of mutation-bearing mRNA. Genome-wide homozygosity mapping and single-nucleotide polymorphism analysis identified a shared haplotype and supports a possible founder effect for the CAPN3 variant. Together, our data extend the mutational spectrum of LGMDR1 and have implications for improved diagnostics for individuals of Pakistani origin.

An integrated approach to the evaluation of patients with asymptomatic or minimally symptomatic hyperCKemia

Introduction/Aims

Currently, there are no straightforward guidelines for the clinical and diagnostic management of hyperCKemia, a frequent and nonspecific presentation in muscle diseases. Therefore, we aimed to describe our diagnostic workflow for evaluating patients with this condition.

Methods

We selected 83 asymptomatic or minimally symptomatic patients with persistent hyperCKemia for participation in this Italian multicenter study. Patients with facial involvement and distal or congenital myopathies were excluded, as were patients with suspected inflammatory myopathies or predominant respiratory or cardiac involvement. All patients underwent a neurological examination and nerve conduction and electromyography studies. The first step of the investigation included a screening for Pompe disease. We then evaluated the patients for myotonic dystrophy type II–related CCTG expansion and excluded patients with copy number variations in the DMD gene. Subsequently, the undiagnosed patients were investigated using a target gene panel that included 20 genes associated with isolated hyperCKemia.

Results

Using this approach, we established a definitive diagnosis in one third of the patients. The detection rate was higher in patients with severe hyperCKemia and abnormal electromyographic findings.

Discussion

We have described our diagnostic workflow for isolated hyperCKemia, which is based on electrodiagnostic data, biochemical screening, and first‐line genetic investigations, followed by successive targeted sequencing panels. Both clinical signs and electromyographic abnormalities are associated with increased diagnostic yields.

Anoctamin 5 Knockout Mouse Model Recapitulates LGMD2L Muscle Pathology and Offers Insight Into in vivo Functional Deficits

Mutations in the Anoctamin 5 (Ano5) gene that result in the lack of expression or function of ANO5 protein, cause Limb Girdle Muscular Dystrophy (LGMD) 2L/R12, and Miyoshi Muscular Dystrophy (MMD3). However, the dystrophic phenotype observed in patient muscles is not uniformly recapitulated by ANO5 knockout in animal models of LGMD2L. Here we describe the generation of a mouse model of LGMD2L generated by targeted out-of-frame deletion of the Ano5 gene. This model shows progressive muscle loss, increased muscle weakness, and persistent bouts of myofiber regeneration without chronic muscle inflammation, which recapitulates the mild to moderate skeletal muscle dystrophy reported in the LGMD2L patients. We show that these features of ANO5 deficient muscle are not associated with a change in the calcium-activated sarcolemmal chloride channel activity or compromised in vivo regenerative myogenesis. Use of this mouse model allows conducting in vivo investigations into the functional role of ANO5 in muscle health and for preclinical therapeutic development for LGMD2L.

FKRP mutations cause congenital muscular dystrophy 1C and limb-girdle muscular dystrophy 2I in Asian patients

Mutation in the fukutin-related protein (FKRP) gene causes alpha-dystroglycanopathies, a group of autosomal recessive disorders associated with defective glycosylated alpha-dystroglycan (α-DG). The disease phenotype shows a broad spectrum, from the most severe congenital form involving brain and eye anomalies to milder limb-girdle form. FKRP-related alpha-dystroglycanopathies are common in European countries. However, a limited number of patients have been reported in Asian countries. Here, we presented the clinical, pathological, and genetic findings of nine patients with FKRP mutations identified at a single muscle repository center in Japan. Three and six patients were diagnosed with congenital muscular dystrophy type 1C and limb-girdle muscular dystrophy 2I, respectively. None of our Asian patients showed the most severe form of alpha-dystroglycanopathy. While all patients showed a reduction in glycosylated α-DG levels, to variable degrees, these levels did not correlate to clinical severity. Fifteen distinct pathogenic mutations were identified in our cohort, including five novel mutations. Unlike in the populations belonging to European countries, no common mutation was found in our cohort.

Clinical and genetic spectrum of a large cohort of patients with δ-sarcoglycan muscular dystrophy

Sarcoglycanopathies include four subtypes of autosomal recessive limb-girdle muscular dystrophies (LGMDR3, LGMDR4, LGMDR5 and LGMDR6) that are caused, respectively, by mutations in the SGCA, SGCB, SGCG and SGCD genes. Delta-sarcoglycanopathy (LGMDR6) is the least frequent and is considered an ultra-rare disease. Our aim was to characterize the clinical and genetic spectrum of a large international cohort of LGMDR6 patients and to investigate whether or not genetic or protein expression data could predict diseasés severity. This is a retrospective study collecting demographic, genetic, clinical and histological data of patients with genetically confirmed LGMDR6 including protein expression data from muscle biopsies. We contacted 128 pediatric and adult neuromuscular units around the world that reviewed genetic data of patients with a clinical diagnosis of a neuromuscular disorder. We identified 30 patients with a confirmed diagnosis of LGMDR6 of which 23 patients were included in this study. Eighty seven percent of the patients had consanguineous parents. Ninety one percent of the patients were symptomatic at the time of the analysis. Proximal muscle weakness of the upper and lower limbs was the most common presenting symptom. Distal muscle weakness was observed early over the course of the disease in 56.5% of the patients. Cardiac involvement was reported in 5 patients (21.7%) and 4 patients (17.4%) required non-invasive ventilation. Sixty percent of patients were wheelchair-bound since early teens (median age of 12.0 years old). Patients with absent expression of the sarcoglycan complex on muscle biopsy had a significant earlier onset of symptoms and an earlier age of loss of ambulation compared to patients with residual protein expression. This study confirmed that delta-sarcoglycanopathy is an ultra-rare neuromuscular condition and described the clinical and molecular characteristics of the largest yet-reported collected cohort of patients. Our results showed that this is a very severe and quickly progressive disease characterized by generalized muscle weakness affecting predominantly proximal and distal muscles of the limbs. Similar to other forms of sarcoglycanopathies, the severity and rate of progressive weakness correlates inversely with the abundance of protein on muscle biopsy.

Molecular Diagnosis of Pompe Disease in the Genomic Era: Correlation with Acid Alpha-Glucosidase Activity in Dried Blood Spots

Measurement of alpha-glucosidase activity on dried blood spots has been the main method to screen for Pompe disease, but a paradigm shift has been observed in recent years with the incorporation of gene panels and exome sequencing in molecular diagnostic laboratories. An 89-gene panel has been available to Canadian physicians since 2017 and was analyzed in 2030 patients with a suspected muscle disease. Acid alpha-glucosidase activity was measured in parallel in dried blood spots from 1430 patients. Pompe disease was diagnosed in 14 patients, representing 0.69% of our cohort. In 7 other patients, low enzyme activities overlapping those of Pompe disease cases were attributable to the presence of pseudodeficiency alleles. Only two other patients had enzymatic activity in the Pompe disease range, and a single heterozygous pathogenic variant was identified. It is possible that a second variant could have been missed; we suggest that RNA analysis should be considered in such cases. With gene panel testing increasingly being performed as a first-tier analysis of patients with suspected muscle disorders, our study supports the relevance of performing reflex enzymatic activity assay in selected patients, such as those with a single GAA variant identified and those in whom the observed genotype is of uncertain clinical significance.

The evolving genetic landscape of congenital disorders of glycosylation

Congenital Disorders of Glycosylation (CDG) are an expanding and complex group of rare genetic disorders caused by defects in the glycosylation of proteins and lipids. The genetic spectrum of CDG is extremely broad with mutations in over 140 genes leading to a wide variety of symptoms ranging from mild to severe and life-threatening. There has been an expansion in the genetic complexity of CDG in recent years. More specifically several examples of alternate phenotypes in recessive forms of CDG and new types of CDG following an autosomal dominant inheritance pattern have been identified. In addition, novel genetic mechanisms such as expansion repeats have been reported and several already known disorders have been classified as CDG as their pathophysiology was better elucidated. Furthermore, we consider the future and outlook of CDG genetics, with a focus on exploration of the non-coding genome using whole genome sequencing, RNA-seq and multi-omics technology.

Phenotypic Spectrum of Myopathies with Recessive Anoctamin-5 Mutations

BACKGROUND

Biallelic variants in Anoctamin 5 (ANO5) gene are causative of limb-girdle muscular dystrophy (LGMD) R12 anoctamin5-related, non-dysferlin Miyoshi-like distal myopathy (MMD3), and asymptomatic hyperCKemia.

OBJECTIVE

To describe clinic, histologic, genetic and imaging features, of ANO5 mutated patients.

METHODS

Five patients, four from France (P1, P2, P3 and P4) and one from Mexico (P5), from four families were included. P1 and P2, belonging to group 1, had normal muscle strength; Group 2, P3, P4 and P5, presented with muscular weakness. Muscle strength was measured by manual muscle testing, Medical Research Council (MRC) grades 1/5 to 5/5. Laboratory exams included serum CK levels, nerve conduction studies (NCS)/needle electromyography (EMG), pulmonary function tests, EKG and cardiac ultrasound. ANO5 molecular screening was performed with different approaches.

RESULTS

Group 1 patients showed myalgias with hyperCKemia or isolated hyperCKemia. Group 2 patients presented with limb-girdle or proximo-distal muscular weakness. Serum CK levels ranged from 897 to 5000 UI/L. Muscle biopsy analysis in P4 and P5 showed subsarcolemmal mitochondrial aggregates. Electron microscopy confirmed mitochondrial proliferation and revealed discontinuity of the sarcolemmal membrane. Muscle MRI showed asymmetrical fibro-fatty substitution predominant in the lower limbs.P1 and P2 were compound heterozygous for c.191dupA (p.Asn64Lysfs*15) and c.1898 + G>A; P3 was homozygous for the c.692G>T. (p.Gly231Val); P4 harbored a novel biallelic homozygous exons 1-7 ANO5 gene deletion, and P5 was homozygous for a c.172 C > T (p.(Arg 58 Trp)) ANO5 pathogenic variant.

CONCLUSIONS

Our cohort confirms the wide clinical variability and enlarge the genetic spectrum of ANO5-related myopathies.

Causative variant profile of collagen VI-related dystrophy in Japan

BACKGROUND

Collagen VI-related dystrophy spans a clinical continuum from severe Ullrich congenital muscular dystrophy to milder Bethlem myopathy. This disease is caused by causative variants in COL6A1, COL6A2, or COL6A3. Most reported causative variants are de novo; therefore, to identify possible associated causative variants, comprehensive large cohort studies are required for different ethnicities.

METHODS

We retrospectively reviewed clinical information, muscle histology, and genetic analyses from 147 Japanese patients representing 130 families, whose samples were sent for diagnosis to the National Center of Neurology and Psychiatry between July 1979 and January 2020. Genetic analyses were conducted by gene-based resequencing, targeted panel resequencing, and whole exome sequencing, in combination with cDNA analysis.

RESULTS

Of a total of 130 families with 1-5 members with collagen VI-related dystrophy, 120 had mono-allelic and 10 had bi-allelic variants in COL6A1, COL6A2, or COL6A3. Among them, 60 variants were in COL6A1, 57 in COL6A2, and 23 in COL6A3, including 37 novel variants. Mono-allelic variants were classified into four groups: missense (69, 58%), splicing (40, 33%), small in-frame deletion (7, 6%), and large genomic deletion (4, 3%). Variants in the triple helical domains accounted for 88% (105/120) of all mono-allelic variants.

CONCLUSIONS

We report the causative variant profile of a large set of Japanese cases of collagen VI-related dystrophy. This dataset can be used as a reference to support genetic diagnosis and variant-specific treatment.

Guidelines for genetic testing of muscle and neuromuscular junction disorders

Despite recent advances in the understanding of inherited muscle and neuromuscular junction diseases, as well as the advent of a wide range of genetic tests, patients continue to face delays in diagnosis of sometimes treatable disorders. These guidelines outline an approach to genetic testing in such disorders. Initially, a patient's phenotype is evaluated to identify myopathies requiring directed testing, including myotonic dystrophies, facioscapulohumeral muscular dystrophy, oculopharyngeal muscular dystrophy, mitochondrial myopathies, dystrophinopathies, and oculopharyngodistal myopathy. Initial investigation in the remaining patients is generally a comprehensive gene panel by next-generation sequencing. Broad panels have a higher diagnostic yield and can be cost-effective. Due to extensive phenotypic overlap and treatment implications, genes responsible for congenital myasthenic syndromes should be included when evaluating myopathy patients. For patients whose initial genetic testing is negative or inconclusive, phenotypic re-evaluation is warranted, along with consideration of genes and variants not included initially, as well as their acquired mimickers.

A Journey with LGMD: From Protein Abnormalities to Patient Impact

The limb-girdle muscular dystrophies (LGMD) are a collection of genetic diseases united in their phenotypical expression of pelvic and shoulder area weakness and wasting. More than 30 subtypes have been identified, five dominant and 26 recessive. The increase in the characterization of new genotypes in the family of LGMDs further adds to the heterogeneity of the disease. Meanwhile, better understanding of the phenotype led to the reconsideration of the disease definition, which resulted in eight old subtypes to be no longer recognized officially as LGMD and five new diseases to be added to the LGMD family. The unique variabilities of LGMD stem from genetic mutations, which then lead to protein and ultimately muscle dysfunction. Herein, we review the LGMD pathway, starting with the genetic mutations that encode proteins involved in muscle maintenance and repair, and including the genotype–phenotype relationship of the disease, the epidemiology, disease progression, burden of illness, and emerging treatments.

Mutation spectrum of hereditary myopathies in Turkish patients and novel variants

Hereditary myopathies are a heterogeneous disorder known to be associated with more than 100 genes. Although hereditary myopathy subgroups can be partially described with traditional methods such as muscle biopsy, next-generation sequencing (NGS) is essential to reveal the disease's underlying genetic etiology and molecular mechanisms. In this study, we performed clinical exome sequencing or whole-exome sequencing (CES/WES) in 20 unrelated Turkish patients. Thirteen pathogenic or likely pathogenic variants, including five novel variantswere detected in the 16 known hereditary myopathy genes. We achieved a high rate of diagnosis (65%) compared to previous studies. The most common condition noticed was limb-girdle muscular dystrophy (LGMD), which should not be ignored in patients diagnosed with myopathy. CES or WES provides a certain molecular diagnosis from a broad perspective to demonstrate underlying genetic causes in heterogeneous disorders. Therefore, exome sequencing offers a higher and more complete diagnosis than the gene panel.

A form of muscular dystrophy associated with pathogenic variants in JAG2

JAG2 encodes the Notch ligand Jagged2. The conserved Notch signaling pathway contributes to the development and homeostasis of multiple tissues, including skeletal muscle. We studied an international cohort of 23 individuals with genetically unsolved muscular dystrophy from 13 unrelated families. Whole-exome sequencing identified rare homozygous or compound heterozygous JAG2 variants in all 13 families. The identified bi-allelic variants include 10 missense variants that disrupt highly conserved amino acids, a nonsense variant, two frameshift variants, an in-frame deletion, and a microdeletion encompassing JAG2. Onset of muscle weakness occurred from infancy to young adulthood. Serum creatine kinase (CK) levels were normal or mildly elevated. Muscle histology was primarily dystrophic. MRI of the lower extremities revealed a distinct, slightly asymmetric pattern of muscle involvement with cores of preserved and affected muscles in quadriceps and tibialis anterior, in some cases resembling patterns seen in POGLUT1-associated muscular dystrophy. Transcriptome analysis of muscle tissue from two participants suggested misregulation of genes involved in myogenesis, including PAX7. In complementary studies, Jag2 downregulation in murine myoblasts led to downregulation of multiple components of the Notch pathway, including Megf10. Investigations in Drosophila suggested an interaction between Serrate and Drpr, the fly orthologs of JAG1/JAG2 and MEGF10, respectively. In silico analysis predicted that many Jagged2 missense variants are associated with structural changes and protein misfolding. In summary, we describe a muscular dystrophy associated with pathogenic variants in JAG2 and evidence suggests a disease mechanism related to Notch pathway dysfunction.

Compound heterozygous CAPN3 variants identified in a family with limb-girdle muscular dystrophy recessive 1

Limb-girdle muscular dystrophy recessive 1 (LGMDR1), a rare subtype of muscular dystrophy, is characterized by progressive muscle weakness and degeneration with a predominant presentation on the shoulder, pelvic and proximal limb muscles. Variants in calcium-activated neutral proteinase 3 (CAPN3), which encodes an enzyme, calpain 3, are considered the major cause of LGMDR1. The present study was conducted to identify the variants responsible for clinical symptoms in a Chinese patient with limb-girdle muscular dystrophies (LGMDs) and explore its genotype-phenotype associations. A series of clinical examinations were conducted, including blood tests and magnetic resonance imaging scans of the lower legs, electromyography and muscle biopsy on the proband diagnosed with muscular dystrophies. Genomic DNA was extracted from the peripheral blood of a three-person family with LGMDs and pathogenic variants detected by whole-exome sequencing (WES) were verified by Sanger sequencing. The WES of this patient revealed compound heterozygous variants in CAPN3, c.2120A>G/p.(Asp707Gly) in exon 20 and c.2201_2202delAT/p.(Tyr734*) in exon 21, which were inherited from his parents and absent from 200 control individuals of similar ethnic origin, indicating that these variants are the pathogenic triggers of the LGMDR1 phenotype. Notably, these CAPN3 sequence variants were related to LGMDR1 pathogenesis in this three-person family. The newly discovered c.2201_2202delAT/p.(Tyr734*) expands the current CAPN3 variant spectrum, improving the understanding of the conditions required to develop molecular diagnostic tools and for genetic counseling, particularly for families with a history of autosomal recessive LGMDs.

TRIM32 and Malin in Neurological and Neuromuscular Rare Diseases

Tripartite motif (TRIM) proteins are RING E3 ubiquitin ligases defined by a shared domain structure. Several of them are implicated in rare genetic diseases, and mutations in TRIM32 and TRIM-like malin are associated with Limb-Girdle Muscular Dystrophy R8 and Lafora disease, respectively. These two proteins are evolutionary related, share a common ancestor, and both display NHL repeats at their C-terminus. Here, we revmniew the function of these two related E3 ubiquitin ligases discussing their intrinsic and possible common pathophysiological pathways.

Late onset Pompe Disease in India - Beyond the Caucasian phenotype

We evaluated the clinical histories, motor and pulmonary functions, cardiac phenotypes and GAA genotypes of an Indian cohort of twenty patients with late onset Pompe disease (LOPD) in this multi-centre study. A mean age at onset of symptoms and diagnosis of 9.9 ± 9.7 years and 15.8 ± 12.1 years respectively was identified. All patients had lower extremity limb-girdle muscle weakness. Seven required ventilatory support and seven used mobility assists. Of the four who used both assists, two received ventilatory support prior to wheelchair use. Cardiac involvement was seen in eight patients with various combinations of left ventricular hypertrophy, tricuspid regurgitation, cardiomyopathy, dilated ventricles with biventricular dysfunction and aortic regurgitation. Amongst 20 biochemically diagnosed patients (low residual GAA enzyme activity) GAA genotypes of 19 patients identified homozygous variants in eight and compound heterozygous in 11: 27 missense, 3 nonsense, 2 initiator codon, 3 splice site and one deletion. Nine variants in 7 patients were novel. The leaky Caucasian, splice site LOPD variant, c.-32-13T>G mutation was absent. This first study from India provides an insight into a more severe LOPD phenotype with earlier disease onset at 9.9 years compared to 33.3 years in Caucasian patients, and cardiac involvement more than previously reported. The need for improvement in awareness and diagnosis of LOPD in India is highlighted.

ANO5 ensures trafficking of annexins in wounded myofibers

Mutations in ANO5 (TMEM16E) cause limb-girdle muscular dystrophy R12. Defective plasma membrane repair is a likely mechanism. Using myofibers from Ano5 knockout mice, we show that trafficking of several annexin proteins, which together form a cap at the site of injury, is altered upon loss of ANO5. Annexin A2 accumulates at the wound to nearly twice the level observed in WT fibers, while annexin A6 accumulation is substantially inhibited in the absence of ANO5. Appearance of annexins A1 and A5 at the cap is likewise diminished in the Ano5 knockout. These changes are correlated with an alteration in annexin repair cap fine structure and shedding of annexin-positive vesicles. We conclude that loss of annexin coordination during repair is disrupted in Ano5 knockout mice and underlies the defective repair phenotype. Although ANO5 is a phospholipid scramblase, abnormal repair is rescued by overexpression of a scramblase-defective ANO5 mutant, suggesting a novel, scramblase-independent role of ANO5 in repair.

Null variants in DYSF result in earlier symptom onset

We investigated the clinical, laboratory, and genetic spectra in Korean patients with dysferlinopathy to clarify its genotype-phenotype correlation. We retrospectively reviewed 101 patients from 96 unrelated families with pathogenic variants of DYSF. The most common initial phenotype was Miyoshi myopathy in 50 patients. Median ages at examination and symptom onset were 23 [interquartile range (IQR): 18-30] and 36 years [IQR: 27-48], respectively. We observed 38 variants, including nine novel variants. Four variants (c.2494C > T, c.1284 + 2 T > C, c.663 + 1G > C, and c.2997G > T) in DYSF accounted for 62% of total allele frequencies of pathogenic variants. To analyze the genotype-phenotype correlation, we compared the clinical phenotype between patients with null/null (N/N; n = 55) and null/missense variants (N/M; n = 35). The N/N group had an earlier symptom onset age (median: 20 years [IQR: 17-25]) than the N/M group (median: 29 years [IQR: 23-35], p < .001). Total manual muscle testing scores in lower extremities were lower in the N/N group (median: 80 [IQR: 56-92]) than in the N/M group (median: 89 [IQR: 78-98], p = .013). Our study is the first to report that null variants in DYSF result in an earlier symptom onset than missense variants.

Genetic variations and clinical spectrum of dystroglycanopathy in a large cohort of Chinese patients

Dystroglycanopathy is a group of muscular dystrophies with deficient glycosylation of alpha-dystroglycan (α-DG). We recruited patients from 36 tertiary academic hospitals in China. In total, 143 patients with genetically diagnosed dystroglycanopathy were enrolled. Of these, limb girdle muscular dystrophy was the most common initial diagnosis (83 patients) and Walker-Warburg syndrome was the least common (1 patient). In 143 patients, mutations in FKRP gene were the most prevalent (62 patients), followed by POMT2, POMT1 (16), POMGNT1, ISPD (14), FKTN, GMPPB, B3GALNT2, DPM3, and DAG1. Several frequent mutations were identified in FKRP, POMT1, POMGNT1, ISPD, and FKTN genes. Many of these were founder mutations. Patients with FKRP mutations tended to have milder phenotypes, while those with mutations in POMGNT1 genes had more severe phenotypes. Mental retardation was a clinical feature associated with mutations of POMT1 gene. Detailed clinical data of 83 patients followed up in Peking University First Hospital were further analyzed. Our clinical and genetic analysis of a large cohort of Chinese patients with dystroglycanopathy expanded the genotype variation and clinical spectrum of congenital muscular dystrophies.

[Calpainopathies: state of the art and therapeutic perspectives]

Calpainopathies are inherited limb-girdle muscular dystrophies, most often following an autosomal recessive (AR) transmission. Autosomal dominant (AD) forms with less severe presentation are increasingly reported. Calpainopathies with autosomal recessive (AR) mutations of the calpain3 gene (CAPN3) are associated with limb girdle muscular dystrophy type R1 (LGMD-R1, OMIM 253600) also referred to as LGMD-2A according to the old nomenclature. LGMD-R1 is the commonest form of all LGMDs, with an estimated prevalence of 10 to 70 cases per million inhabitants, that is a cohort of between 670 and 4,200 patients in France theoritically. Patients present a symmetrical proximal axial myopathy manifesting itself between the first and second decade. The clinical course is variable. The level of Creatine- Kinase (CK) is usually high and there is no cardiac involvement. From a therapeutic perspective, the autosomal recessive form of calpainopathy is quite suitable to gene replacement strategies; the viability of recombinant AAV-mediated calpain 3 transfer has been demonstrated in animal models and clinical trials are expected in the coming years. Meanwhile, natural history studies are needed to prepare for future clinical trials.

Mutational Spectrum of CAPN3 with Genotype-Phenotype Correlations in Limb Girdle Muscular Dystrophy Type 2A/R1 (LGMD2A/LGMDR1) Patients in India

BACKGROUND

Limb girdle muscular dystrophy recessive type 1 (LGMDR1, Previously LGMD2A) is characterized by inactivating mutations in CAPN3. Despite the significant burden of muscular dystrophy in India, and particularly of LGMDR1, its genetic characterization and possible phenotypic manifestations are yet unidentified.

MATERIAL AND METHODS

We performed bidirectional CAPN3 sequencing in 95 LGMDR1 patient samples characterized by calpain-3 protein analysis, and these findings were correlated with clinical, biochemical and histopathological features.

RESULTS

We identified 84 (88.4%) cases of LGMDR1 harboring 103 CAPN3 mutations (71 novel and 32 known). At least two mutant alleles were identified in 79 (94.2%) of patients. Notably, 76% exonic variations were enriched in nine CAPN3 exons and overall, 41 variations (40%) correspond to only eight exonic and intronic mutations. Patients with two nonsense/out of frame/splice-site mutations showed significant loss of calpain-3 protein as compared to those with two missense/inframe mutations (P = 0.04). We observed a slow progression of disease and less severity in our patients compared to European population. Rarely, presenting clinical features were atypical, and mimicked other muscle diseases like FSHMD, distal myopathy and metabolic myopathies.

CONCLUSION

This is first systematic study to characterize the genetic framework of LGMDR1 in the Indian population. Preliminary calpain-3 immunoblot screening serves well to direct genetic testing. Our findings prioritized nine CAPN3 exons for LGMDR1 diagnosis in our population; therefore, a targeted-sequencing panel of nine exons could serve well for genetic diagnosis, carrier testing, counseling and clinical trial feasibility study in LGMDR1 patients in India.

Fukutin-Related Protein: From Pathology to Treatments

Fukutin-related protein (FKRP) is a glycosyltransferase involved in the functional glycosylation of α-dystroglycan (DG), a key component in the link between the cytoskeleton and the extracellular matrix (ECM). Mutations in FKRP lead to dystroglycanopathies with broad severity, including limb-girdle and congenital muscular dystrophy. Studies over the past 5 years have elucidated the function of FKRP, which has expanded the number of therapeutic opportunities for patients carrying FKRP mutations. These include small molecules, gene delivery, and cell therapy. Here we summarize recent findings on the function of FKRP and describe available models for studying diseases and testing therapeutics. Lastly, we highlight preclinical studies that hold potential for the treatment of FKRP-associated dystroglycanopathies.

A Revisited Diagnosis of Collagen VI Related Muscular Dystrophy in a Patient with a Novel COL6A2 Variant and 21q22.3 Deletion

The genetic etiology of collagen VI related muscular dystrophies is heterogenous. Genomic deletions in one allele involving COL6A2 or both COL6A1 and COL6A2 unmasking a pathogenic variant in the second nondeleted allele have been described in the etiology. We aimed to report the clinical and molecular findings of a 13-year-old boy with ring chromosome 21 who presented to our clinic with easy fatigability, muscle weakness, and waddling gait. Phenotypic delineation along with chromosomal microarray analysis and DNA sequencing were performed. Affymetrix CytoScan Optima array platform and DNA sequencing revealed a 2,202 kb de novo deletion at 21q22.3, including COL6A1 and COL6A2, and a novel heterozygous variant at position c.2875G > A;p.(Glu959Lys) in COL6A2, respectively. Before his admission to our center, the patient was evaluated for hypotonia elsewhere when he was 15 months old. He was diagnosed with ring chromosome 21 on peripheral blood karyotype analysis; however, no further assessment was performed at that time. He had normal growth with mild dysmorphic facial features, distal laxity, gastrocnemius hypertrophy, proximal muscle weakness, increased lordotic posture with mild flexion contractures at the knees, and gait disturbance. Although the phenotype does not fit into classical Ullrich congenital muscular dystrophies, muscle magnetic resonance imaging (MRI) revealed a complementary pattern consistent with collagen VI related myopathies. Genetic testing confirmed the clinical diagnosis as well. This patient yet represents another example of the effect of large genomic deletions leading to recessive disorders through unmasking a pathogenic variant in the second nondeleted allele.

Genetic cause of heterogeneous inherited myopathies in a cohort of Greek patients

Inherited muscle disorders are caused by pathogenic changes in numerous genes. Herein, we aimed to investigate the etiology of muscle disease in 24 consecutive Greek patients with myopathy suspected to be genetic in origin, based on clinical presentation and laboratory and electrophysiological findings and absence of known acquired causes of myopathy. Of these, 16 patients (8 females, median 24 years-old, range 7 to 67 years-old) were diagnosed by Whole Exome Sequencing as suffering from a specific type of inherited muscle disorder. Specifically, we have identified causative variants in 6 limb-girdle muscular dystrophy genes (6 patients; ANO5, CAPN3, DYSF, ISPD, LAMA2, SGCA), 3 metabolic myopathy genes (4 patients; CPT2, ETFDH, GAA), 1 congenital myotonia gene (1 patient; CLCN1), 1 mitochondrial myopathy gene (1 patient; MT-TE) and 3 other myopathy-associated genes (4 patients; CAV3, LMNA, MYOT). In 6 additional family members affected by myopathy, we reached genetic diagnosis following identification of a causative variant in an index patient. In our patients, genetic diagnosis ended a lengthy diagnostic process and, in the case of Multiple acyl-CoA dehydrogenase deficiency and Pompe's disease, it enabled specific treatment to be initiated. These results further expand the genotypic and phenotypic spectrum of inherited myopathies.

Panorama of the distal myopathies

Distal myopathies are genetic primary muscle disorders with a prominent weakness at onset in hands and/or feet. The age of onset (from early childhood to adulthood), the distribution of muscle weakness (upper versus lower limbs) and the histological findings (ranging from nonspecific myopathic changes to myofibrillar disarrays and rimmed vacuoles) are extremely variable. However, despite being characterized by a wide clinical and genetic heterogeneity, the distal myopathies are a category of muscular dystrophies: genetic diseases with progressive loss of muscle fibers. Myopathic congenital arthrogryposis is also a form of distal myopathy usually caused by focal amyoplasia. Massive parallel sequencing has further expanded the long list of genes associated with a distal myopathy, and contributed identifying as distal myopathy-causative rare variants in genes more often related with other skeletal or cardiac muscle diseases. Currently, almost 20 genes (ACTN2, CAV3, CRYAB, DNAJB6, DNM2, FLNC, HNRNPA1, HSPB8, KHLH9, LDB3, MATR3, MB, MYOT, PLIN4, TIA1, VCP, NOTCH2NLC, LRP12, GIPS1) have been associated with an autosomal dominant form of distal myopathy. Pathogenic changes in four genes (ADSSL, ANO5, DYSF, GNE) cause an autosomal recessive form; and disease-causing variants in five genes (DES, MYH7, NEB, RYR1 and TTN) result either in a dominant or in a recessive distal myopathy. Finally, a digenic mechanism, underlying a Welander-like form of distal myopathy, has been recently elucidated. Rare pathogenic mutations in SQSTM1, previously identified with a bone disease (Paget disease), unexpectedly cause a distal myopathy when combined with a common polymorphism in TIA1. The present review aims at describing the genetic basis of distal myopathy and at summarizing the clinical features of the different forms described so far.

Clinical exome sequencing in the diagnosis of pediatric neuromuscular disease

BACKGROUND

The diagnosis of uncommon pediatric neuromuscular disease (NMD) is challenging due to genetic and phenotypic heterogeneity, yet is important to guide treatment, prognosis, and recurrence risk. Patients with diagnostically challenging presentations typically undergo extensive testing with variable molecular diagnostic yield. Given the advancement in next generation sequencing (NGS), we investigated the value of clinical whole exome sequencing (ES) in uncommon pediatric NMD.

METHODS

A retrospective cohort study of 106 pediatric NMD patients with a combination of ES, chromosomal microarray (CMA), and candidate gene testing was completed at a large tertiary referral center.

RESULTS

A molecular diagnosis was achieved in 37/79 (46%) patients with ES, 4/44 (9%) patients with CMA, and 15/74 (20%) patients with candidate gene testing. In 2/79 (3%) patients, a dual molecular diagnosis explaining the neuromuscular disease process was identified. A total of 42 patients (53%) who received ES remained without a molecular diagnosis at the conclusion of the study.

CONCLUSIONS

Due to NGS, molecular diagnostic yield of rare neurological diseases is at an all-time high. We show that ES has a higher diagnostic rate compared to other genetic tests in a complex pediatric neuromuscular disease cohort and should be considered early in the diagnostic journey for select NMD patients with challenging presentations in which a clinical diagnosis is not evident.

Clinical and Genomic Evaluation of 207 Genetic Myopathies in the Indian Subcontinent

Objective: Inherited myopathies comprise more than 200 different individually rare disease-subtypes, but when combined together they have a high prevalence of 1 in 6,000 individuals across the world. Our goal was to determine for the first time the clinical- and gene-variant spectrum of genetic myopathies in a substantial cohort study of the Indian subcontinent.

Methods: In this cohort study, we performed the first large clinical exome sequencing (ES) study with phenotype correlation on 207 clinically well-characterized inherited myopathy-suspected patients from the Indian subcontinent with diverse ethnicities.

Results: Clinical-correlation driven definitive molecular diagnosis was established in 49% (101 cases; 95% CI, 42–56%) of patients with the major contributing pathogenicity in either of three genes, GNE (28%; GNE-myopathy), DYSF (25%; Dysferlinopathy), and CAPN3 (19%; Calpainopathy). We identified 65 variant alleles comprising 37 unique variants in these three major genes. Seventy-eight percent of the DYSF patients were homozygous for the detected pathogenic variant, suggesting the need for carrier-testing for autosomal-recessive disorders like Dysferlinopathy that are common in India. We describe the observed clinical spectrum of myopathies including uncommon and rare subtypes in India: Sarcoglycanopathies (SGCA/B/D/G), Collagenopathy (COL6A1/2/3), Anoctaminopathy (ANO5), telethoninopathy (TCAP), Pompe-disease (GAA), Myoadenylate-deaminase-deficiency-myopathy (AMPD1), myotilinopathy (MYOT), laminopathy (LMNA), HSP40-proteinopathy (DNAJB6), Emery-Dreifuss-muscular-dystrophy (EMD), Filaminopathy (FLNC), TRIM32-proteinopathy (TRIM32), POMT1-proteinopathy (POMT1), and Merosin-deficiency-congenital-muscular-dystrophy-type-1 (LAMA2). Thirteen patients harbored pathogenic variants in >1 gene and had unusual clinical features suggesting a possible role of synergistic-heterozygosity/digenic-contribution to disease presentation and progression.

Conclusions: Application of clinically correlated ES to myopathy diagnosis has improved our understanding of the clinical and genetic spectrum of different subtypes and their overlaps in Indian patients. This, in turn, will enhance the global gene-variant-disease databases by including data from developing countries/continents for more efficient clinically driven molecular diagnostics.

Heterozygous CAPN3 missense variants causing autosomal-dominant calpainopathy in seven unrelated families

AIMS

Recessive variants in CAPN3 gene are the cause of the commonest form of autosomal recessive limb girdle muscle dystrophy. However, two distinct in-frame deletions in CAPN3 (NM_000070.3:c.643_663del21 and c.598_621del15) and more recently, Gly445Arg and Arg572Pro substitutions have been linked to autosomal dominant (AD) forms of calpainopathy. We report 21 affected individuals from seven unrelated families presenting with an autosomal dominant form of muscular dystrophy associated with five different heterozygous missense variants in CAPN.

METHODS

We have used massively parallel gene sequencing (MPS) to determine the genetic basis of a dominant form of limb girdle muscular dystrophy in affected individuals from seven unrelated families.

RESULTS

The c.700G> A, [p.(Gly234Arg)], c.1327T> C [p.(Ser443Pro], c.1333G> A [p.(Gly445Arg)], c.1661A> C [p.(Tyr554Ser)] and c.1706T> C [p.(Phe569Ser)] CAPN3 variants were identified. Affected individuals presented in young adulthood with progressive proximal and axial weakness, waddling walking and scapular winging or with isolated hyperCKaemia. Muscle imaging showed fatty replacement of paraspinal muscles, variable degrees of involvement of the gluteal muscles, and the posterior compartment of the thigh and minor changes at the mid-leg level. Muscle biopsies revealed mild myopathic changes. Western blot analysis revealed a clear reduction in calpain 3 in skeletal muscle relative to controls. Protein modelling of these variants on the predicted structure of calpain 3 revealed that all variants are located in proximity to the calmodulin-binding site and are predicted to interfere with proteolytic activation.

CONCLUSIONS

We expand the genotypic spectrum of CAPN3-associated muscular dystrophy due to autosomal dominant missense variants.

Cardiomyopathy in limb girdle muscular dystrophy R9, FKRP related

Introduction

Reported frequencies of cardiomyopathy in limb girdle muscular dystrophy R9 (LGMDR9) vary. We describe the frequency and age at onset of cardiomyopathy in an LDMDR9 cohort.

Methods

Echocardiograms from 56 subjects (157 echocardiograms) with LGMDR9 were retrospectively reviewed. The cumulative probability of having an abnormal echocardiogram as a function of age was assessed by survival analysis for interval‐censored data by genotype. Correlations between cardiac and clinical function were evaluated.

Results

Twenty‐five (45%) participants had cardiomyopathy. The median age at first abnormal echocardiogram for subjects homozygous for the c.826C>A variant was 54.2 y compared to 18.1 y for all other fukutin‐related protein (FKRP) genotypes (P < .0001). There was a weak correlation between ejection fraction and 10‐Meter Walk Test speed (r = 0.25), but no correlation with forced vital capacity (r = 0.08).

Discussion

Cardiomyopathy is prevalent among those with LGMDR9 and occurs later in subjects homozygous for the c.826C>A mutation. These data will help to guide surveillance and management.

The ties that bind: functional clusters in limb-girdle muscular dystrophy

The limb-girdle muscular dystrophies (LGMDs) are a genetically pleiomorphic class of inherited muscle diseases that are known to share phenotypic features. Selected LGMD genetic subtypes have been studied extensively in affected humans and various animal models. In some cases, these investigations have led to human clinical trials of potential disease-modifying therapies, including gene replacement strategies for individual subtypes using adeno-associated virus (AAV) vectors. The cellular localizations of most proteins associated with LGMD have been determined. However, the functions of these proteins are less uniformly characterized, thus limiting our knowledge of potential common disease mechanisms across subtype boundaries. Correspondingly, broad therapeutic strategies that could each target multiple LGMD subtypes remain less developed. We believe that three major "functional clusters" of subcellular activities relevant to LGMD merit further investigation. The best known of these is the glycosylation modifications associated with the dystroglycan complex. The other two, mechanical signaling and mitochondrial dysfunction, have been studied less systematically but are just as promising with respect to the identification of significant mechanistic subgroups of LGMD. A deeper understanding of these disease pathways could yield a new generation of precision therapies that would each be expected to treat a broader range of LGMD patients than a single subtype, thus expanding the scope of the molecular medicines that may be developed for this complex array of muscular dystrophies.

Limb Girdle Muscular Dystrophies

The limb girdle muscular dystrophies (LGMDs) are genetic muscle diseases with primary skeletal muscle involvement in persons with the ability to walk independently at some point in the disease course. They usually have increased creatine kinase levels along with patterns of fatty and fibrous deposition on muscle imaging and/or dystrophic features on muscle biopsy. Distinctive clinical features provide valuable diagnostic clues to the diagnosis and sometimes treatment of these disorders. The advent of gene and cell-based therapies; gene replacement, editing, and modulation; along with stem cell and small molecule therapies may significantly ameliorate clinical severity in the LGMDs.

Deep phenotyping of facioscapulohumeral muscular dystrophy type 2 by magnetic resonance imaging

BACKGROUND AND PURPOSE

The aim was to define the radiological picture of facioscapulohumeral muscular dystrophy 2 (FSHD2) in comparison with FSHD1 and to explore correlations between imaging and clinical/molecular data.

METHODS

Upper girdle and/or lower limb muscle magnetic resonance imaging scans of 34 molecularly confirmed FSHD2 patients from nine European neuromuscular centres were analysed. T1-weighted and short-tau inversion recovery (STIR) sequences were used to evaluate the global pattern and to assess the extent of fatty replacement and muscle oedema.

RESULTS

The most frequently affected muscles were obliquus and transversus abdominis, semimembranosus, soleus and gluteus minimus in the lower limbs; trapezius, serratus anterior, latissimus dorsi and pectoralis major in the upper girdle. Iliopsoas, popliteus, obturator internus and tibialis posterior in the lower limbs and subscapularis, spinati, sternocleidomastoid and levator scapulae in the upper girdle were the most spared. Asymmetry and STIR hyperintensities were consistent features. The pattern of muscle involvement was similar to that of FSHD1, and the combined involvement of trapezius, abdominal and hamstring muscles, together with complete sparing of iliopsoas and subscapularis, was detected in 91% of patients. Peculiar differences were identified in a rostro-caudal gradient, a predominant involvement of lower limb muscles compared to the upper girdle, and in the higher percentage of STIR hyperintensities in FSHD2.

CONCLUSION

This multicentre study defines the pattern of muscle involvement in FSHD2, providing useful information for diagnostics and clinical trial design. Both similarities and differences between FSHD1 and FSHD2 were detected, which is also relevant to better understand the pathogenic mechanisms underlying the FSHD-related disease spectrum.

Genetically confirmed limb-girdle muscular dystrophy type 2B with DYSF mutation using gene panel sequencing: A case report

RATIONALE

The limb-girdle muscular dystrophies (LGMDs) are a heterogeneous group of disorders characterized by progressive proximal muscle weakness and have more than 30 different subtypes linked to specific gene loci, which manifest as highly overlapping and heterogeneous phenotypes.

PATIENT CONCERNS

A 59-year-old male presented for evaluation of progressive muscle weakness since his late twenties. When he was 38 years old, he had muscle weakness in the upper extremities and had a waddling gait, hyper lordosis of lower back, and anterior pelvic tilt. His gait disturbance and muscle weakness slowly progressed. When he was 55 years old, he could not walk at all and had to use a wheelchair for ambulation.

DIAGNOSIS

Next-generation sequencing using a custom target capture-based gene panel including specific genes responsible for muscular dystrophy was performed. As a result, the proband was genetically diagnosed as LGMD type 2B, carrying 2 compound heterozygous mutations (NM_003494.3:c.1663C>T, p.Arg555Trp; rs377735262 and NM_003494.3:c.2997G>T, p.Trp999Cys; rs28937581) of the DYSF gene.

INTERVENTIONS

Physical and occupational therapy were prescribed properly for the first time Bracing and assistive devices were adapted specifically to the patient's deficiencies to preserve mobility and function and prevent contractures.

OUTCOMES

The patient with LGMD has periodic assessments of physical and occupational therapy for the prevention and management of comorbidities. However, in the 3 years after the gene panel sequencing diagnoses, his weakness was slowly progress and the patient still could not walk.

LESSONS

Gene panel sequencing allows for the correct recognition of different LGMD subtypes, improving timely treatment, management, and enrolment of molecularly diagnosed individuals in clinical trials.

Novel CAPN3 variant associated with an autosomal dominant calpainopathy

AIMS

The most common autosomal recessive limb girdle muscular dystrophy is associated with the CAPN3 gene. The exclusively recessive inheritance of this disorder has been recently challenged by the description of the recurrent variants, c.643_663del21 [p.(Ser215_Gly221del)] and c.598_612del15 [p.(Phe200_Leu204del)], associated with autosomal dominant inheritance. Our objective was to confirm the existence of autosomal dominant calpainopathies.

METHODS

Through our activity as one of the reference centres for genetic diagnosis of calpainopathies in France and the resulting collaborations through the French National Network for Rare Neuromuscular Diseases (FILNEMUS), we identified four families harbouring the same CAPN3 heterozygous variant with supposedly autosomal dominant inheritance.

RESULTS

We identified a novel dominantly inherited CAPN3 variant, c.1333G>A [p.(Gly445Arg)] in 14 affected patients from four unrelated families. The complementary phenotypic, functional and genetic findings correlate with an autosomal dominant inheritance in these families, emphasizing the existence of this novel transmission mode for calpainopathies. The mild phenotype associated with these autosomal dominant cases widens the phenotypic spectrum of calpainopathies and should therefore be considered in clinical practice.

CONCLUSIONS

We confirm the existence of autosomal dominant calpainopathies as an entity beyond the cases related to the in-frame deletions c.643_663del21 and c.598_612del15, with the identification of a novel dominantly inherited and well-documented CAPN3 missense variant, c.1333G>A [p.(Gly445Arg)]. In addition to the consequences for genetic counselling, the confirmation of an autosomal dominant transmission mode for calpainopathies underlines the importance of re-assessing other myopathies for which the inheritance is considered as strictly autosomal recessive.

Genetic Counseling and NGS Screening for Recessive LGMD2A Families

Genetic counseling applied to limb–girdle muscular dystrophies (LGMDs) can be very challenging due to their clinical and genetic heterogeneity and the availability of different molecular assays. Genetic counseling should therefore be addressed to select the most suitable approach to increase the diagnostic rate and provide an accurate estimation of recurrence risk. This is particularly true for families with a positive history for recessive LGMD, in which the presence of a known pathogenetic mutation segregating within the family may not be enough to exclude the risk of having affected children without exploring the genetic background of phenotypically unaffected partners. In this work, we presented a family with a positive history for LGMD2A (OMIM #253600, also known as calpainopathy) characterized by compound heterozygosity for two CAPN3 mutations. The genetic specialist suggested the segregation analysis of both mutations within the family as a first-level analysis. Sequentially, next-generation sequencing (NGS) analysis was performed in the partners of healthy carriers to provide an accurate recurrence/reproductive risk estimation considering the genetic background of the couple. Finally, this work highlighted the importance of providing a genetic counseling/testing service even in unaffected individuals with a carrier partner. This approach can support genetic counselors in estimating the reproductive/recurrence risk and eventually, suggesting prenatal testing, early diagnosis or other medical surveillance strategies.

Pathogenic variants in COL6A3 cause Ullrich-like congenital muscular dystrophy in young Labrador Retriever dogs

The collagen VI-related muscular dystrophies in people include a broad spectrum of diseases ranging from the severe Ullrich congenital muscular dystrophy to the mild Bethlem myopathy. Clinical features are attributable to both muscle and connective tissue and include progressive muscle weakness and respiratory failure, hyperlaxity of distal joints, and progressive contracture of large joints. Here we describe two different COL6A3 pathogenic variants in Labrador Retriever dogs that result in autosomal recessive or autosomal dominant congenital myopathies with hyperlaxity of distal joints and joint contracture, similar to the condition in people.

The Limb-Girdle Muscular Dystrophies

PURPOSE OF REVIEW

As a group, the limb-girdle muscular dystrophies (LGMDs) are the fourth most prevalent genetic muscle disease, yet they are still not well known or understood. This article defines and describes LGMDs, delineates a diagnostic strategy, and discusses treatment of the LGMDs.

RECENT FINDINGS

In 2018, the definition of the LGMDs was further refined, and a new nomenclature was proposed. Diagnosis of the LGMDs was long guided by the distinctive clinical characteristics of each particular subtype but now integrates use of genetics-with next-generation sequencing panels, exomes, and full genome analysis-early in the diagnostic assessment. Appreciation of the phenotypic diversity of each LGMD subtype continues to expand. This emphasizes the need for precision genetic diagnostics to better understand each subtype and formulate appropriate management for individual patients. Of significant relevance, the explosion of research into therapeutic options accentuates the need for accurate diagnosis, comprehensive disease characterization, and description of the natural histories of the LGMDs to move the field forward and to mitigate disease impact on patients with LGMD.

SUMMARY

The LGMDs are genetic muscle diseases that superficially appear similar to one another but have important differences in rates of progression and concomitant comorbidities. Definitive diagnoses are crucial to guide management and treatment now and in the future. As targeted treatments emerge, it will be important for clinicians to understand the nomenclature, diagnosis, clinical manifestations, and treatments of the LGMDs.