Novel splice site mutation in the caveolin-3 gene leading to autosomal recessive limb girdle muscular dystrophy

The spectrum of rippling muscle disease

Rippling muscle disease (RMD) is a rare disorder of muscle hyperexcitability. It is characterized by rippling wave-like muscle contractions induced by mechanical stretch or voluntary contraction followed by sudden stretch, painful muscle stiffness, percussion-induced rapid muscle contraction (PIRC), and percussion-induced muscle mounding (PIMM). RMD can be hereditary (hRMD) or immune-mediated (iRMD). hRMD is caused by pathogenic variants in caveolin-3 (CAV3) or caveolae-associated protein 1/ polymerase I and transcript release factor (CAVIN1/PTRF). CAV3 pathogenic variants are autosomal dominant or less frequently recessive while CAVIN1/PTRF pathogenic variants are autosomal recessive. CAV3-RMD manifests with a wide spectrum of clinical phenotypes, ranging from asymptomatic creatine kinase elevation to severe muscle weakness. Overlapping phenotypes are common. Muscle caveolin-3 immunoreactivity is often absent or diffusely reduced in CAV3-RMD. CAVIN1/PTRF-RMD is characterized by congenital generalized lipodystrophy (CGL, type 4) and often accompanied by several extra-skeletal muscle manifestations. Muscle cavin-1/PTRF immunoreactivity is absent or reduced while caveolin-3 immunoreactivity is reduced, often in a patchy way, in CAVIN1/PTRF-RMD. iRMD is often accompanied by other autoimmune disorders, including myasthenia gravis. Anti-cavin-4 antibodies are the serological marker while the mosaic expression of caveolin-3 and cavin-4 is the pathological feature of iRMD. Most patients with iRMD respond to immunotherapy. Rippling, PIRC, and PIMM are usually electrically silent. Different pathogenic mechanisms have been postulated to explain the disease mechanisms. In this article, we review the spectrum of hRMD and iRMD, including clinical phenotypes, electrophysiological characteristics, myopathological findings, and pathogenesis.

Coexistence of a Heterozygous Caveolin-3 Deletion and a Novel Dystrophin Gene Mutation in a Duchenne Muscular Dystrophy Patient

Inherited muscular abnormalities are debilitating disorders that greatly diminish the quality of life in affected individuals. Mutations in proteins such as dystrophin and caveolin, which together with other proteins form structural connections between the cytoskeleton and the extracellular matrix, are frequently the culprit of muscular dystrophies. In this case report, we describe a patient with a novel pathogenic dystrophin mutation co-existing with a caveolin-3 deletion. While genetically composed of this unique combination, the patient phenotypically presented with a primary clinical manifestation of Duchenne muscular dystrophy (DMD) in contrast to other cases of dual mutations in dystrophin and dystrophin-associated proteins.

Muscle biopsy essential diagnostic advice for pathologists

Background

Muscle biopsies are important diagnostic procedures in neuromuscular practice. Recent advances in genetic analysis have profoundly modified Myopathology diagnosis.

Main body

The main goals of this review are: (1) to describe muscle biopsy techniques for non specialists; (2) to provide practical information for the team involved in the diagnosis of muscle diseases; (3) to report fundamental rules for muscle biopsy site choice and adequacy; (4) to highlight the importance of liquid nitrogen in diagnostic workup. Routine techniques include: (1) histochemical stains and reactions; (2) immunohistochemistry and immunofluorescence; (3) electron microscopy; (4) mitochondrial respiratory chain enzymatic studies; and (5) molecular studies. The diagnosis of muscle disease is a challenge, as it should integrate data from different techniques.

Conclusion

Formalin-fixed paraffin embedded muscle samples alone almost always lead to inconclusive or unspecific results. Liquid nitrogen frozen muscle sections are imperative for neuromuscular diagnosis. Muscle biopsy interpretation is possible in the context of detailed clinical, neurophysiological, and serum muscle enzymes data. Muscle imaging studies are strongly recommended in the diagnostic workup. Muscle biopsy is useful for the differential diagnosis of immune mediated myopathies, muscular dystrophies, congenital myopathies, and mitochondrial myopathies. Muscle biopsy may confirm the pathogenicity of new gene variants, guide cost-effective molecular studies, and provide phenotypic diagnosis in doubtful cases. For some patients with mitochondrial myopathies, a definite molecular diagnosis may be achieved only if performed in DNA extracted from muscle tissue due to organ specific mutation load.

The Experimentally Obtained Functional Impact Assessments of 5' Splice Site GT'GC Variants Differ Markedly from Those Predicted

Introduction:

5' splice site GT>GC or +2T>C variants have been frequently reported to cause human genetic disease and are routinely scored as pathogenic splicing mutations. However, we have recently demonstrated that such variants in human disease genes may not invariably be pathogenic. Moreover, we found that no splicing prediction tools appear to be capable of reliably distinguishing those +2T>C variants that generate wild-type transcripts from those that do not.

Methodology

Herein, we evaluated the performance of a novel deep learning-based tool, SpliceAI, in the context of three datasets of +2T>C variants, all of which had been characterized functionally in terms of their impact on pre-mRNA splicing. The first two datasets refer to our recently described “in vivo” dataset of 45 known disease-causing +2T>C variants and the “in vitro” dataset of 103 +2T>C substitutions subjected to full-length gene splicing assay. The third dataset comprised 12 BRCA1 +2T>C variants that were recently analyzed by saturation genome editing.

Results

Comparison of the SpliceAI-predicted and experimentally obtained functional impact assessments of these variants (and smaller datasets of +2T>A and +2T>G variants) revealed that although SpliceAI performed rather better than other prediction tools, it was still far from perfect. A key issue was that the impact of those +2T>C (and +2T>A) variants that generated wild-type transcripts represents a quantitative change that can vary from barely detectable to an almost full expression of wild-type transcripts, with wild-type transcripts often co-existing with aberrantly spliced transcripts.

Conclusion

Our findings highlight the challenges that we still face in attempting to accurately identify splice-altering variants.

First estimate of the scale of canonical 5' splice site GT>GC variants capable of generating wild-type transcripts

It has long been known that canonical 5' splice site (5'SS) GT>GC variants may be compatible with normal splicing. However, to date, the actual scale of canonical 5'SSs capable of generating wild-type transcripts in the case of GT>GC substitutions remains unknown. Herein, combining data derived from a meta-analysis of 45 human disease-causing 5'SS GT>GC variants and a cell culture-based full-length gene splicing assay of 103 5'SS GT>GC substitutions, we estimate that ~15-18% of canonical GT 5'SSs retain their capacity to generate between 1% and 84% normal transcripts when GT is substituted by GC. We further demonstrate that the canonical 5'SSs in which substitution of GT by GC-generated normal transcripts exhibit stronger complementarity to the 5' end of U1 snRNA than those sites whose substitutions of GT by GC did not lead to the generation of normal transcripts. We also observed a correlation between the generation of wild-type transcripts and a milder than expected clinical phenotype but found that none of the available splicing prediction tools were capable of reliably distinguishing 5'SS GT>GC variants that generated wild-type transcripts from those that did not. Our findings imply that 5'SS GT>GC variants in human disease genes may not invariably be pathogenic.

Untangling the complexity of limb-girdle muscular dystrophies

The limb-girdle muscular dystrophies (LGMDs) are a group of genetically heterogeneous, autosomal inherited muscular dystrophies with a childhood to adult onset, manifesting with hip- and shoulder-girdle muscle weakness. When the term LGMD was first conceptualized in 1954, it was thought to be a single entity. Currently, there are 8 autosomal dominant (LGMD1A-1H) and 26 autosomal recessive (LGMD2A-2Z) variants according to the Online Mendelian Inheritance in Man database. In addition, there are other genetically identified muscular dystrophies with an LGMD phenotype not yet classified as LGMD. This highlights the entanglement of LGMDs, which represents an area in continuous expansion. Herein we aim to simplify the complexity of LGMDs by subgrouping them on the basis of the underlying defective protein and impaired function. Muscle Nerve 58: 167-177, 2018.

The relative frequency of common neuromuscular diagnoses in a reference center

The diagnostic procedure in neuromuscular patients is complex. Knowledge of the relative frequency of neuromuscular diseases within the investigated population is important to allow the neurologist to perform the most appropriate diagnostic tests.

OBJECTIVE

To report the relative frequency of common neuromuscular diagnoses in a reference center.

METHODS

A 17-year chart review of patients with suspicion of myopathy.

RESULTS

Among 3,412 examinations, 1,603 (46.98%) yielded confirmatory results: 782 (48.78%) underwent molecular studies, and 821 (51.21%) had muscle biopsies. The most frequent diagnoses were: dystrophinopathy 460 (28.70%), mitochondriopathy 330 (20.59%), spinal muscular atrophy 158 (9.86%), limb girdle muscular dystrophy 157 (9.79%), Steinert myotonic dystrophy 138 (8.61%), facioscapulohumeral muscular dystrophy 99 (6.17%), and other diagnoses 261 (16.28%).

CONCLUSION

Using the presently-available diagnostic techniques in this service, a specific limb girdle muscular dystrophy subtype diagnosis was reached in 61% of the patients. A neuromuscular-appropriate diagnosis is important for genetic counseling, rehabilitation orientation, and early treatment of respiratory and cardiac complications.

Mutation in the caveolin-3 gene causes asymmetrical distal myopathy

Mutations in the gene encoding caveolin-3 (CAV3) can cause a broad spectrum of clinical phenotypes, including limb girdle muscular dystrophy, rippling muscle disease, distal myopathy (MD), idiopathic persistent elevation of serum creatine kinase and cardiomyopathy. MD is a relatively rare subtype of caveolinopathy. Here, we report a sporadic case of a middle-aged female Chinese patient with MD in which a CAV3 mutation was identical to that previously reported in cases of rippling muscle disease. T1-weighted enhanced skeletal muscle MRI of the lower limbs showed an abnormal signal in the distal and proximal muscles. A muscle biopsy revealed moderate dystrophic changes, and immunohistochemical staining showed reduced CAV-3 expression in the plasmalemma. Genetic analysis revealed a heterozygous c.136G > A (p.Ala46Thr) CAV3 mutation that appeared to be de novo because it was absent from the patient's parents. This study suggested that the CAV3 c.136G > A (p.Ala46Thr) mutation can cause MD as well as different phenotypes in different individuals, suggesting that additional unknown loci must affect the disease phenotypes.

Cardiomyopathy in neurological disorders

According to the American Heart Association, cardiomyopathies are classified as primary (solely or predominantly confined to heart muscle), secondary (those showing pathological myocardial involvement as part of a neuromuscular disorder) and those in which cardiomyopathy is the first/predominant manifestation of a neuromuscular disorder. Cardiomyopathies may be further classified as hypertrophic cardiomyopathy, dilated cardiomyopathy, restrictive cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy, or unclassified cardiomyopathy (noncompaction, Takotsubo-cardiomyopathy). This review focuses on secondary cardiomyopathies and those in which cardiomyopathy is the predominant manifestation of a myopathy. Any of them may cause neurological disease, and any of them may be a manifestation of a neurological disorder. Neurological disease most frequently caused by cardiomyopathies is ischemic stroke, followed by transitory ischemic attack, syncope, or vertigo. Neurological disease, which most frequently manifests with cardiomyopathies are the neuromuscular disorders. Most commonly associated with cardiomyopathies are muscular dystrophies, myofibrillar myopathies, congenital myopathies and metabolic myopathies. Management of neurological disease caused by cardiomyopathies is not at variance from the same neurological disorders due to other causes. Management of secondary cardiomyopathies is not different from that of cardiomyopathies due to other causes either. Patients with neuromuscular disorders require early cardiologic investigations and close follow-ups, patients with cardiomyopathies require neurological investigation and avoidance of muscle toxic medication if a neuromuscular disorder is diagnosed. Which patients with cardiomyopathy profit most from primary stroke prevention is unsolved and requires further investigations.

Alterations of excitation-contraction coupling and excitation coupled Ca(2+) entry in human myotubes carrying CAV3 mutations linked to rippling muscle

Rippling muscle disease is caused by mutations in the gene encoding caveolin-3 (CAV3), the muscle-specific isoform of the scaffolding protein caveolin, a protein involved in the formation of caveolae. In healthy muscle, caveolin-3 is responsible for the formation of caveolae, which are highly organized sarcolemmal clusters influencing early muscle differentiation, signalling and Ca²⁺ homeostasis. In the present study we examined Ca²⁺ homeostasis and excitation–contraction (E-C) coupling in cultured myotubes derived from two patients with Rippling muscle disease with severe reduction in caveolin-3 expression; one patient harboured the heterozygous c.84C>A mutation while the other patient harbored a homozygous splice-site mutation (c.102+ 2T>C) affecting the splice donor site of intron 1 of the CAV3 gene. Our results show that cells from control and rippling muscle disease patients had similar resting [Ca²⁺]_i and 4-chloro-m-cresol-induced Ca²⁺ release but reduced KCl-induced Ca²⁺ influx. Detailed analysis of the voltage-dependence of Ca²⁺ transients revealed a significant shift of Ca²⁺ release activation to higher depolarization levels in CAV3 mutated cells. High resolution immunofluorescence analysis by Total Internal Fluorescence microscopy supports the hypothesis that loss of caveolin-3 leads to microscopic disarrays in the colocalization of the voltage-sensing dihydropyridine receptor and the ryanodine receptor, thereby reducing the efficiency of excitation–contraction coupling. Hum Mutat 32:309–317, 2011. © 2011 Wiley-Liss, Inc.

Rippling muscle disease and cardiomyopathy associated with a mutation in the CAV3 gene

Caveolin-3, the myocyte-specific isoform of caveolins, is preferentially expressed in skeletal, cardiac and smooth muscles. Mutations in the CAV3 gene cause clinically heterogeneous neuromuscular disorders, including rippling muscle disease, or cardiopathies. The same mutation may lead to different phenotypes, but cardiac and muscle involvement rarely coexists suggesting that the molecular network acting with caveolin-3 in skeletal muscle and heart may differ. Here we describe an Italian family (a father and his two sons) with clinical and neurophysiological features of rippling muscle disease and heart involvement characterized by atrio-ventricular conduction defects and dilated cardiomyopathy. Muscle biopsy showed loss of caveolin-3 immunosignal. Molecular studies identified the p.A46V mutation in CAV3 previously reported in a German family with autosomal dominant rippling muscle disease and sudden death in few individuals. We suggest that cardiac dysfunction in myopathic patients with CAV3 mutations may be underestimated and recommend a more thorough evaluation for the presence of cardiomyopathy and potentially lethal arrhythmias.

Caveolinopathies: from the biology of caveolin-3 to human diseases

In muscle tissue the protein caveolin-3 forms caveolae--flask-shaped invaginations localized on the cytoplasmic surface of the sarcolemmal membrane. Caveolae have a key role in the maintenance of plasma membrane integrity and in the processes of vesicular trafficking and signal transduction. Mutations in the caveolin-3 gene lead to skeletal muscle pathology through multiple pathogenetic mechanisms. Indeed, caveolin-3 deficiency is associated to sarcolemmal membrane alterations, disorganization of skeletal muscle T-tubule network and disruption of distinct cell-signaling pathways. To date, there have been 30 caveolin-3 mutations identified in the human population. Caveolin-3 defects lead to four distinct skeletal muscle disease phenotypes: limb girdle muscular dystrophy, rippling muscle disease, distal myopathy, and hyperCKemia. In addition, one caveolin-3 mutant has been described in a case of hypertrophic cardiomyopathy. Many patients show an overlap of these symptoms and the same mutation can be linked to different clinical phenotypes. This variability can be related to additional genetic or environmental factors. This review will address caveolin-3 biological functions in muscle cells and will describe the muscle and heart disease phenotypes associated with caveolin-3 mutations.

Primary myopathies and the heart

Myopathies are frequently not confined to the skeletal muscles but also involve other organs or tissues. One of the most frequently affected organ in addition to the skeletal muscle is the heart (cardiac involvement, CI). CI manifests as impulse generation or conduction defects, focal or diffuse myocardial thickening, dilation of the cardiac cavities, relaxation abnormality, hypertrophic, dilated, restrictive cardiomyopathy, apical form of hypertrophic cardiomyopathy, noncompaction, Takotsubo phenomenon, secondary valve insufficiency, intra-cardiac thrombus formation, or heart failure with systolic or diastolic dysfunction. CI occurs in dystrophinopathies, Emery-Dreifuss muscular dystrophy, facioscapulohumeral muscular dystrophy, limb girdle muscular dystrophies, laminopathies, congenital muscular dystrophies, myotonic dystrophies, congenital myopathies, metabolic myopathies, desminopathies, myofibrillar myopathy, Barth syndrome, McLeod syndrome, Senger's syndrome, and Bethlem myopathy. Patients with myopathy should be cardiologically investigated as soon as their neurological diagnosis is established, since supportive cardiac therapy is available, which markedly influences prognosis and outcome of CI in these patients.

Caveolinopathy--new mutations and additional symptoms

Mutations in the caveolin-3 gene (CAV3) can lead to a broad spectrum of clinical phenotypes. Phenotypes that have so far been associated with primary caveolin-3 deficiency include limb girdle muscular dystrophy, rippling muscle disease, distal myopathy and hyperCKaemia. This is the first report describing the clinical, pathological and genetic features of patients with caveolinopathy from the UK. Ten patients (six families) were identified via the National Commissioning Group (NCG) service for patients with limb girdle muscle dystrophy in Newcastle. Myalgia was the most prominent symptom in our cohort of patients and for 50% it was the reason for referral. Muscle weakness was only found in 60% of the patients, whereas rippling muscle movement was present in 80%. One of the patients reported episodes of myoglobinuria and another one episodes of hypoglycaemia. Five different mutations were identified, two of which were novel and three that had previously been described. Caveolinopathy needs to be considered as a differential diagnosis in a range of clinical situations, including in patients who do not have any weakness. Indeed, rippling muscles are a more frequent symptom than weakness, and can be detected in childhood. Presentation with myalgia is common and management of it as well as of myoglobinuria and hypoglycaemia may have a major impact on the patients' quality of life.

Novel homozygous mutation of the caveolin-3 gene in rippling muscle disease with extraocular muscle paresis

We describe a 39-year-old Japanese man with rippling muscle disease who carried a novel homozygous mutation (Trp70 to a stop codon) in the caveolin-3 gene. The patient also had extraocular muscle paresis showing atrophy of the extraocular muscles on orbital MRI. The involvement of the extraocular muscles of patients with caveolinopathy is discussed.