Characteristic findings of skeletal muscle MRI in caveolinopathies

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.

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.

Immunofluorescence-Based Analysis of Caveolin-3 in the Diagnostic Management of Neuromuscular Diseases

Immunohistochemistry- and/or immunofluorescence-based analysis of muscular proteins represents a standard procedure in the diagnostic management of patients suffering from neuromuscular diseases such as "Caveolinopathies" which are caused by mutations in the CAV3 gene encoding for caveolin-3. Human caveolin-3 is a 151 amino acid sized transmembrane protein localized within caveolae, predominantly expressed in cardiac and skeletal muscle cells and involved in a diversity of cellular functions crucial for muscle cell homeostasis. Loss of caveolin-3 protein abundance is indicative for the presence of pathogenic mutations within the corresponding gene and thus for the diagnosis of "Caveolinopathies." Moreover, description of abnormal immunoreactivity findings for the caveolin-3 protein is increasing in the context of other neuromuscular diseases suggesting that profound knowledge of abnormal caveolin-3-expression and/or distribution findings can be decisive also for the diagnosis of other neurological diseases as well as for a better understanding of the biology of the protein. Here, we summarize the current knowledge about the caveolin-3, report on a protocol for immunofluorescence-based analysis of the protein in the diagnostic workup of neuromuscular patients-also considering problems encountered-and confirm as well as summarize already published abnormal histological findings in muscular pathologies beyond "Caveolinopathies."

Coexistence of a CAV3 mutation and a DMD deletion in a family with complex muscular diseases

Whole-exome sequencing (WES) can comprehensively detect both pathogenic single nucleotide variants and copy number variants, enabling identification of a coexistence of two or more genetic etiologies. Here we report a family consisting of individuals with Becker muscular dystrophy and rippling muscle disease. The proband, a 12-year-old boy, was diagnosed with Becker muscular dystrophy with exon 45-55 DMD deletions at age 4. He had myalgia and muscle stiffness. Interestingly, percussion-induced muscle mounding (PIMM), which is a characteristic of rippling muscle disease, was also observed. The father also showed muscle stiffness, myalgia, fatigability, muscle rippling and PIMM. WES revealed a missense CAV3 mutation (NM_033337.2:c.80G>A) in the proband, the father, the oldest sister and the grandmother, who had an elevated serum creatine kinase (CK) level. The c.80G>A mutation was considered pathogenic according to ACMG guidelines. The second older sister, the mother and the paternal grandfather did not have the CAV3 mutation and had normal CK. Using two programs for copy number analysis with WES data, we successfully identified the DMD deletion in the proband, the older sister and the mother. We revealed the coexistence of the CAV3 mutation and the DMD deletion in a family with complex muscular diseases and confirmed the usefulness of WES for elucidating such etiology.