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

Genetic spectrum of sarcoglycanopathies in a cohort of Russian patients

Sarcoglycanopathies encompass four distinct forms of limb-girdle muscular dystrophies (LGMD), denoted as LGMD R3-R6, arising from mutations within the SGCA, SGCB, SGCG, and SGCD genes. The global prevalence of sarcoglycanopathies is low, making it challenging to study these diseases. The principal objective of this study was to explore the spectrum of mutations in a cohort of Russian patients with sarcoglycanopathies and to ascertain the frequency of these conditions in the Russian Federation. We conducted a retrospective analysis of clinical and molecular genetic data from 49 Russian patients with sarcoglycan genes variants. The results indicated that variants in the SGCA gene were found in 71.4% of cases, with SGCB and SGCG genes each exhibiting variants in 12.2 % of patients. SGCD gene variants were detected in 4.1% of cases. Bi-allelic pathogenic and likely pathogenic variants were identified in 46 of the 49 cases of sarcoglycanopathies: LGMD R3 (n = 34), LGMD R4 (n = 4), LGMD R5 (n = 6), and LGMD R6 (n = 2). A total of 31 distinct variants were identified, comprising 25 previously reported and 6 novel variants. Two major variants, c.229C>T and c.271G>A, were detected within the SGCA, constituting 61.4% of all mutant alleles in Russian patients with LGMD R3. Both LGMD R6 cases were caused by the homozygous nonsense variant c.493C>T p.(Arg165Ter) in the SGCD gene. The incidence of sarcoglycanopathies in the Russian Federation was estimated to be at least 1 in 4,115,039, which is lower than the reported incidence in other populations.

Molecular diagnosis of Alpha-sarcoglycanopathies by NGS in seven Moroccan families and report of two novel variants

BACKGROUND

Limb-girdle muscular dystrophies constitute a heterogeneous group of neuromuscular diseases, both clinically and genetically. Limb-girdle muscular dystrophy by alpha-sarcoglycan deficiency or LGMD R3 α-sarcoglycan-related is a subtype of the autosomal recessive sarcoglycanopathies caused by variants in the alpha-sarcoglycan gene (SGCA) at 17q21.33. It appears in childhood by progressive weakness of pelvic and/or scapular girdle muscles and calf hypertrophy, with a wide range of clinical inter- and intra-familial clinical variability.

AIMS

Our report extends the molecular spectrum of SGCA gene with the identification of variant disease causing and will help for better management of patients and genetic counseling of families.

METHODS

In our study, seven unrelated families presented a clinical and paraclinical picture consistent with alpha-sarcoglycanopathy. A molecular study using Next-Generation Sequencing (NGS) was carried out on them.

RESULTS

Six different homozygous variants of the SGCA gene were identified in the patients analyzed, including four previously reported variants and two novel variants predicted to be deleterious by the prediction tools.

CONCLUSIONS

Our results expand the spectrum of variants in Moroccan patients with sarcoglycanopathy, specifically LGMDR3, most importantly as this form is not common in the Moroccan population.

Protein glycosylation in cardiovascular health and disease

Protein glycosylation, which involves the attachment of carbohydrates to proteins, is one of the most abundant protein co-translational and post-translational modifications. Advances in technology have substantially increased our knowledge of the biosynthetic pathways involved in protein glycosylation, as well as how changes in glycosylation can affect cell function. In addition, our understanding of the role of protein glycosylation in disease processes is growing, particularly in the context of immune system function, infectious diseases, neurodegeneration and cancer. Several decades ago, cell surface glycoproteins were found to have an important role in regulating ion transport across the cardiac sarcolemma. However, with very few exceptions, our understanding of how changes in protein glycosylation influence cardiovascular (patho)physiology remains remarkably limited. Therefore, in this Review, we aim to provide an overview of N-linked and O-linked protein glycosylation, including intracellular O-linked N-acetylglucosamine protein modification. We discuss our current understanding of how all forms of protein glycosylation contribute to normal cardiovascular function and their roles in cardiovascular disease. Finally, we highlight potential gaps in our knowledge about the effects of protein glycosylation on the heart and vascular system, highlighting areas for future research.

Exome sequencing revealed variants in SGCA and SIL1 genes underlying limb girdle muscular dystrophy and Marinesco-Sjögren syndrome patients

BACKGROUND

Inherited neuromuscular (NMD) and neurodegenerative diseases (NDD) belong to two distinct categories that disturb different components of the nervous system, leading to a variety of different symptoms and clinical manifestations. Both NMD and NDD are a heterogeneous group of genetic conditions. Genetic variations in the SGCA and SIL1 genes have been implicated in causing Limb Girdle Muscular Dystrophy (LGMD), a type of neuromuscular disorder, and Marinesco-Sjögren Syndrome (MSS) which is a neurodegenerative disorder.

METHODS

In the present study, we have investigated four patients presenting LGMD and five patients with MSS features. After collecting detailed clinical and family history, necessary laboratory investigations, including estimation of a skeletal muscle marker enzyme serum creatine kinase (CK), nerve conduction study (NCS), electromyography (EMG), echocardiography (Echo), Magnetic resonance imaging (MRI -brain), CT-brain and X-rays were performed. Whole exome followed by Sanger sequencing was employed to search for the disease-causing variants.

RESULTS

Physical examination in LGMD patients revealed poor muscle tone and facing difficulty in straightening up from the floor. Clinical history revealed frequent falls and strenuousness in climbing stairs. They started toe-walking in early childhood. Laboratory investigations confirmed elevated CK levels and abnormal NCS and EMG. The MSS patients showed abnormalities in gate and jerking movement, abnormal speech, and strabismus with cataract. MRI-brain showed cerebral atrophy in some MSS patients with elevated CK levels. Whole exome sequencing revealed a nonsense variant [c.C574T, p.(Arg192*)] in the SGCA gene and a frameshift [c.936dupG, p.(Leu313AlaFs*39)] in the SIL1 gene in LGMD and MSS patients, respectively.

CONCLUSION

Our study emphasizes the significance of integrating clinical and genetic analyses for precise diagnosis and tailored management strategies in inherited NMD and NDD disorders. To the best of our knowledge, this is the first study documenting SGCA and SIL1 recurrent variants in subcontinent populations with few rare clinical features. The recurrent mutations expanding the global understanding of the mutation's geographic and ethnic distribution and contributing valuable epidemiological data. The study will facilitate genetic counseling for families experiencing similar clinical features, both within Pakistani populations and in other regions.

Advanced therapeutic approaches in sarcoglycanopathies

Sarcoglycanopathies are rare autosomal recessive diseases belonging to the family of limb-girdle muscular dystrophies. They are caused by mutations in the genes coding for α-, β-, γ-, and δ-sarcoglycan. The mutations impair the assembly of a key structural complex, which normally protects the sarcolemma of striated muscle from contraction-derived stress. Although heterogeneous, sarcoglycanopathies are characterized by progressive muscle degeneration, increased serum creatine kinase levels, loss of ambulation often during adolescence, and variable cardio-respiratory impairment. Genetic defects can impair sarcoglycan synthesis or produce a protein that is defective in folding. There is currently no effective treatment available; however, both gene replacement strategy and small molecule-based approaches show great promise and have entered or are starting to enter clinical trials.

Is Cardiac Transplantation Still a Contraindication in Patients with Muscular Dystrophy-Related End-Stage Dilated Cardiomyopathy? A Systematic Review

Inherited muscular diseases (MDs) are genetic degenerative disorders typically caused by mutations in a single gene that affect striated muscle and result in progressive weakness and wasting in affected individuals. Cardiac muscle can also be involved with some variability that depends on the genetic basis of the MD (Muscular Dystrophy) phenotype. Heart involvement can manifest with two main clinical pictures: left ventricular systolic dysfunction with evolution towards dilated cardiomyopathy and refractory heart failure, or the presence of conduction system defects and serious life-threatening ventricular arrhythmias. The two pictures can coexist. In these cases, heart transplantation (HTx) is considered the most appropriate option in patients who are not responders to the optimized standard therapeutic protocols. However, cardiac transplant is still considered a relative contraindication in patients with inherited muscle disorders and end-stage cardiomyopathies. High operative risk related to muscle impairment and potential graft involvement secondary to the underlying myopathy have been the two main reasons implicated in the generalized reluctance to consider cardiac transplant as a viable option. We report an overview of cardiac involvement in MDs and its possible association with the underlying molecular defect, as well as a systematic review of HTx outcomes in patients with MD-related end-stage dilated cardiomyopathy, published so far in the literature.

Mutational spectrum and phenotypic variability of Duchenne muscular dystrophy and related disorders in a Bangladeshi population

Duchenne muscular dystrophy (DMD) is a severe rare neuromuscular disorder caused by mutations in the X-linked dystrophin gene. Several mutations have been identified, yet the full mutational spectrum, and their phenotypic consequences, will require genotyping across different populations. To this end, we undertook the first detailed genotype and phenotype characterization of DMD in the Bangladeshi population. We investigated the rare mutational and phenotypic spectrum of the DMD gene in 36 DMD-suspected Bangladeshi participants using an economically affordable diagnostic strategy involving initial screening for exonic deletions in the DMD gene via multiplex PCR, followed by testing PCR-negative patients for mutations using whole exome sequencing. The deletion mapping identified two critical DMD gene hotspot regions (near proximal and distal ends, spanning exons 8-17 and exons 45-53, respectively) that comprised 95% (21/22) of the deletions for this population cohort. From our exome analysis, we detected two novel pathogenic hemizygous mutations in exons 21 and 42 of the DMD gene, and novel pathogenic recessive and loss of function variants in four additional genes: SGCD, DYSF, COL6A3, and DOK7. Our phenotypic analysis showed that DMD suspected participants presented diverse phenotypes according to the location of the mutation and which gene was impacted. Our study provides ethnicity specific new insights into both clinical and genetic aspects of DMD.

SGCD Missense Variant in a Lagotto Romagnolo Dog with Autosomal Recessively Inherited Limb-Girdle Muscular Dystrophy

An 8-month-old female Lagotto Romagnolo dog was presented for a 1-month history of an initial severe reluctance to move, rapidly progressing to a marked stiff gait and progressive muscular weakness and evolving to tetraparesis, which persuaded the owner to request euthanasia. A primary muscle pathology was supported by necropsy and histopathological findings. Macroscopically, the muscles were moderately atrophic, except for the diaphragm and the neck muscles, which were markedly thickened. Histologically, all the skeletal muscles examined showed atrophy, hypertrophy, necrosis with calcification of the fibers, and mild fibrosis and inflammation. On immunohistochemistry, all three dystrophin domains and sarcoglycan proteins were absent. On Western blot analysis, no band was present for delta sarcoglycan. We sequenced the genome of the affected dog and compared the data to more than 900 control genomes of different dog breeds. Genetic analysis revealed a homozygous private protein-changing variant in the SGCD gene encoding delta- sarcoglycan in the affected dog. The variant was predicted to induce a SGCD:p.(Leu242Pro) change in the protein. In silico tools predicted the change to be deleterious. Other 770 Lagotto Romagnolo dogs were genotyped for the variant and all found to be homozygous wild type. Based on current knowledge of gene function in other mammalian species, including humans, hamsters, and dogs, we propose the SGCD missense variant as the causative variant of the observed form of muscular dystrophy in the index case. The absence of the variant allele in the Lagotto Romagnolo breeding population indicates a rare allele that has appeared recently.

Longitudinal Analysis of Respiratory Function of Different Types of Limb Girdle Muscular Dystrophies Reveals Independent Trajectories

Background and Objectives

The prevalence and progression of respiratory muscle dysfunction in patients with limb girdle muscular dystrophies (LGMDs) has been only partially described to date. Most reports include cross-sectional data on a limited number of patients making it difficult to gain a wider perspective on respiratory involvement throughout the course of the disease and to compare the most prevalent LGMD subtypes.

Methods

We reviewed the results of spirometry studies collected longitudinally in our cohort of patients in routine clinical visits from 2002 to 2020 along with additional clinical and genetic data. A linear mixed model was used to investigate the factors associated with the progression of respiratory dysfunction.

Results

We followed up 156 patients with 5 different forms of LGMDs for a median of 8 years (range 1-25 years). Of them, 53 patients had pathogenic variants in the Capn3 gene, 47 patients in the Dysf gene, 24 patients in the Fkrp gene, 19 in the Ano5 gene, and 13 in one of the sarcoglycan genes (SCG). At baseline, 58 patients (37.1%) had a forced vital capacity percentage predicted (FVCpp) below 80%, while 14 patients (8.9%) had peak cough flow (PCF) values below 270 L/min. As a subgroup, FKRP was the group with a higher number of patients having FVC <80% and/or PCF <270 L/min at initial assessment (66%). We observed a progressive decline in FVCpp and PCF measurements over time, being age, use of wheelchair, and LGMD subtype independent factors associated with this decline. Fkrp and sarcoglycan patients had a quicker decline in their FVC (Kaplan-Meier curve, F test, p < 0.001 and p = 0.02, respectively). Only 7 of the 58 patients with low FVCpp values reported symptoms of respiratory dysfunction, which are commonly reported by patients with FVCpp below 50%-60%. The number of patients ventilated increased from 2 to 8 during follow-up.

Discussion

Respiratory dysfunction is a frequent complication of patients with LGMDs that needs to be carefully studied and has direct implications in the care offered in daily clinics. Respiratory dysfunction is associated with disease progression because it is especially seen in patients who are full-time wheelchair users, being more frequent in patients with mutations in the Fkrp and sarcoglycan genes.

Comprehensive functional characterization of SGCB coding variants predicts pathogenicity in limb-girdle muscular dystrophy type R4/2E

Genetic testing is essential for patients with a suspected hereditary myopathy. More than 50% of patients clinically diagnosed with a myopathy carry a variant of unknown significance in a myopathy gene, often leaving them without a genetic diagnosis. Limb-girdle muscular dystrophy (LGMD) type R4/2E is caused by mutations in β-sarcoglycan (SGCB). Together, β-, α-, γ-, and δ-sarcoglycan form a 4-protein transmembrane complex (SGC) that localizes to the sarcolemma. Biallelic loss-of-function mutations in any subunit can lead to LGMD. To provide functional evidence for the pathogenicity of missense variants, we performed deep mutational scanning of SGCB and assessed SGC cell surface localization for all 6,340 possible amino acid changes. Variant functional scores were bimodally distributed and perfectly predicted pathogenicity of known variants. Variants with less severe functional scores more often appeared in patients with slower disease progression, implying a relationship between variant function and disease severity. Amino acid positions intolerant to variation mapped to points of predicted SGC interactions, validated in silico structural models, and enabled accurate prediction of pathogenic variants in other SGC genes. These results will be useful for clinical interpretation of SGCB variants and improving diagnosis of LGMD; we hope they enable wider use of potentially life-saving gene therapy.

A novel biallelic variant in the Popeye domain-containing protein 1 (POPDC1) underlies limb girdle muscle dystrophy type 25

POPDC1 also known as BVES, is a highly conserved transmembrane protein, important for striated muscle function and homeostasis. Pathogenic variants in the POPDC1 gene are associated with limb-girdle muscular dystrophy type 25 (LGMDR25). In the present study, we performed trio-whole exome sequencing (WES) followed by Sanger sequencing on a single family having LGMD clinical features. Protein modeling of all POPDC1 missense variants (POPDC1^Pro134Leu , POPDC1^Ile193Ser , and POPDC1^Ser201Phe ) associated with LGMDR25 were performed using Molecular Dynamics (MD) simulation. We identified a homozygous missense variant (c.401C>T; p.Pro134Leu) in the POPDC1 gene. Altered 3D structure, disruptive fluctuation, less compactness, and instability were observed in all the three variants of POPDC1 protein models. In comparison, POPDC1^Ser201Phe protein dynamics were more unstable than other variants. Functional study of newly identified variant would add key answers to underlying mechanisms of the disease.

Clinical exome sequencing identifies novel compound heterozygous mutations of the POMT2 gene in patients with limb-girdle muscular dystrophy

OBJECTIVE

Mutations in protein O-mannosyltransferase 2 (POMT2) (MIM#607439) have been identified in severe congenital muscular dystrophy such as Walker-Warburg syndrome (WWS) and milder limb-girdle muscular dystrophy type 2N (LGMD2N). The aim of this study is to investigate the genetic causes in patients with LGMD2N.

METHODS

Three patients diagnosed with mild limb-girdle muscular dystrophy were recruited. The genetically pathogenic variant was identified by clinical exome sequencing, and healthy controls were verified by Sanger sequencing.

RESULTS

Novel compound heterozygous mutations c.800A > G and c.1074_1075delinsAT of POMT2 were revealed in one affected individual by clinical exome sequencing. There was no report of these two variants and predicted to be highly damaging to the function of the POMT2.

CONCLUSION

The novel variants extend the spectrum of POMT2 mutations, which promotes the prognostic value of testing for POMT2 mutations in patients with LGMD2N.

Nintedanib Reduces Muscle Fibrosis and Improves Muscle Function of the Alpha-Sarcoglycan-Deficient Mice

Sarcoglycanopathies are a group of recessive limb-girdle muscular dystrophies, characterized by progressive muscle weakness. Sarcoglycan deficiency produces instability of the sarcolemma during muscle contraction, leading to continuous muscle fiber injury eventually producing fiber loss and replacement by fibro-adipose tissue. Therapeutic strategies aiming to reduce fibro-adipose expansion could be effective in muscular dystrophies. We report the positive effect of nintedanib in a murine model of alpha-sarcoglycanopathy. We treated 14 Sgca-/- mice, six weeks old, with nintedanib 50 mg/kg every 12 h for 10 weeks and compared muscle function and histology with 14 Sgca-/- mice treated with vehicle and six wild-type littermate mice. Muscle function was assessed using a treadmill and grip strength. A cardiac evaluation was performed by echocardiography and histological study. Structural analysis of the muscles, including a detailed study of the fibrotic and inflammatory processes, was performed using conventional staining and immunofluorescence. In addition, proteomics and transcriptomics studies were carried out. Nintedanib was well tolerated by the animals treated, although we observed weight loss. Sgca-/- mice treated with nintedanib covered a longer distance on the treadmill, compared with non-treated Sgca-/- mice, and showed higher strength in the grip test. Moreover, nintedanib improved the muscle architecture of treated mice, reducing the degenerative area and the fibrotic reaction that was associated with a reversion of the cytokine expression profile. Nintedanib improved muscle function and muscle architecture by reducing muscle fibrosis and degeneration and reverting the chronic inflammatory environment suggesting that it could be a useful therapy for patients with alpha-sarcoglycanopathy.

Homozygous missense variant in POPDC3 causes recessive limb girdle muscular dystrophy type 26

BACKGROUND

Limb-girdle muscular dystrophy (LGMD) is a heterogeneous group of diseases, which affects different muscles, predominantly skeletal muscles and cardiac muscles of the body. LGMD is classified into two main sub-types A and B, which are further sub-classified into eight dominant and thirty recessive sub-types. Three genes, mainly POPDC1, POPDC2 and POPDC3, encodes popeye domain-containing protein (POPDC), and the variants of POPDC1 and POPDC3 genes have been associated with LGMD.

METHODS

In this study, we performed whole-exome sequencing (WES) analysis on a single-family to investigate the hallmark features of LGMD. The results of WES were further confirmed by Sanger sequencing and 3D protein modeling was also performed.

RESULTS

WES data analysis and sanger sequencing revealed a homozygous missense variant (c.460A>G; p.Lys154Glu) at a highly conserved amino acid position in the POPDC3. Mutations in the POPDC3 gene have been previously associated with recessive limb-girdle muscular dystrophy type 26. 3D protein modeling further suggested that the identified variant might affect the POPDC3 structure and proper function.

DISCUSSION/CONCLUSIONS

This study confirms the role of POPDC3 in LGMD, and will facilitate in genetic counseling of the family to mitigate the risks of the carrier or affected in future pregnancies.

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.