Titin, the largest human protein, is essential for sarcomere structure and function. The TTN gene, spanning 364 exons, undergoes extensive alternative splicing thus producing multiple isoforms. The M-band region, encoded by exons 359-364, plays a critical role in sarcomere integrity and mechanical stability. Exon 363 is of interest due to its involvement in titinopathies. Pathogenic truncating variants in this exon have been linked to recessive myopathies, including and mainly young-onset recessive distal titinopathy. A multicenter study was conducted on six patients from five unrelated families with confirmed recessive titinopathy and truncating variants in exon 363. Clinical evaluations were performed. Genetic testing and segregation analysis confirmed the phase of the variants. A novel truncating variant c.107578C>T, p.(Gln35860Ter) was identified in four unrelated patients of Eastern European ancestry, all carrying a second pathogenic variant in a canonical TTN exon. These patients exhibited juvenile/young-adult onset recessive distal titinopathy with progressive lower limb weakness, frequently asymmetric muscle involvement, and no cardiac or respiratory complications. A Belgian family presented with a congenital myopathy caused by a novel frameshift deletion c.107430delA, p.(Ser35811AlafsTer32) in exon 363, in compound heterozygosity with a truncating variant in exon 208. These patients showed a more severe phenotype. This study expands the spectrum of TTN-related myopathies, emphasizing exon 363's pathogenic significance. Truncating exon 363 variants contribute to young onset recessive distal and sometimes early onset titinopathy with contractures, and the phenotype severity is influenced by the second variant's location and exon usage.

To describe a 51-year-old woman with early-onset weakness and foot deformities carrying a biallelic truncating mutation in the VWA1 gene. The patient underwent laboratory tests, nerve conduction studies with electromyography, muscle magnetic resonance imaging, muscle biopsy, and genetic testing. Neurophysiological studies and biopsy revealed both myopathic and neuropathic features. Muscle magnetic resonance imaging showed a distinctive outside-in pattern of fatty replacement in the vastus lateralis, resembling that seen in type VI collagen-related myopathies. Whole-exome sequencing identified a homozygous pathogenic variant in VWA1, which encodes an extracellular matrix protein found in the basement membranes of nerves and muscles. Recently, truncating mutations in VWA1 have been associated with previously unsolved neuromyopathy cases. In one of these reports, as in the current case, the MRI pattern mimicked that of type VI collagen disorders. Biallelic VWA1 mutations may account for some genetically undiagnosed cases of neuropathy with myopathic features.

Distal myopathies comprise a clinically and genetically diverse group of muscle disorders characterized by initial involvement of the distal extremities. We describe siblings who developed progressive weakness in the ankle plantar flexors from adolescence to early adulthood. By their 50s, the lower legs exhibited severe fatty degeneration with pronounced involvement of the gastrocnemius and soleus. Genetic analysis identified a heterozygous p.L95F variant in KLHL9, previously associated with an early-onset autosomal dominant form of distal myopathy featuring tibialis anterior atrophy and sensory deficits. Though considering himself unaffected, the father harbored the same variant and exhibited an extremely mild phenotype. Muscle biopsy revealed chronic myopathic changes with normal expression of KLHL9. This may represent the second reported family with a KLHL9 variant, and is worth establishing KLHL9-linked distal myopathy. The combination of shared and distinct findings from the original family broadens the clinical phenotype and provides insight into the disease.

The J-domain proteins (JDPs), or HSP40s, are essential molecular co-chaperones that, in concert with HSP70, play a pivotal role in maintaining protein homeostasis, which is particularly critical in skeletal muscle. In recent years, pathogenic variants in several JDP-encoding genes have been identified as a cause of a growing group of inherited muscle diseases, termed JDP-related myopathies. This review provides a comprehensive overview of the current understanding of the molecular genetics, clinical phenotypes, muscle pathology, and pathomechanisms of myopathies caused by mutations in DNAJB6, DNAJB4, and DNAJB2. These disorders present with a wide spectrum of clinical features, including limb-girdle or distal weakness, and, in some cases, severe early-onset respiratory failure with axial rigidity. Pathologically, they are often characterized by rimmed vacuoles and sarcoplasmic protein inclusions. The underlying molecular mechanisms all involve disruption of the JDP-HSP70 chaperone system, but they are driven by distinct molecular events specific to each gene and mutation type. While loss-of-function is a primary mechanism for recessive forms of DNAJB4 and DNAJB2 myopathy, a toxic gain-of-function mediated by a dysregulated interaction with HSP70 is emerging as a central pathomechanism for dominant myopathies caused by DNAJB6 and DNAJB4 variants. A dominant-negative effect is proposed for dominant DNAJB2 neuromyopathy. This evolving mechanistic understanding is crucial as it informs the development of targeted therapeutic strategies, moving beyond supportive care. Potential future therapies include gene replacement for loss-of-function disorders, and for gain-of-function diseases, approaches including small molecule inhibitors of the JDP-HSP70 interaction or allele- and isoform-specific knockdown.

Myofibrillar myopathy (MFM) is a group of hereditary neuromuscular disorders with heterogenous manifestations in skeletal and cardiac muscles. Little is known about phenotype-genotype spectrum of MFM in Indian population. This study aims to characterize the clinico-genetic spectrum of 12 MFM ptients from India. A detailed description of the clinical, radiological and mutation spectrum of genetically confirmed MFM patients were done. The M:F ratio was 3:1. Median age of onset, presentation and illness duration were 20 (range: birth - 57 years), 31.5 (range: 6-59 years) and 9 (range: 1 - 28 years) years, respectively. Consanguinity was noted in n = 3 (25%) and motor developmental delay in n = 2 (16.7%). Clinical features noted include ptosis (n = 5, 41.7%) and ophthalmoparesis (n = 3, 25%), bifacial weakness (n = 3, 25%), flaccid dysarthria (n = 3, 41.7%), neck weakness (n = 5, 41.7%), limb-girdle weakness (n = 5, 41.7%), foot drop (n = 1, 8.3%), distal upper limb weakness (n = 2, 16.7%), proximo-distal weakness (n = 5, 41.7%), exertional dyspnoea (n = 4, 33.3%) and joint contractures (n = 8, 66.7%). Cardiac involvement (n = 4, 33.3%) including restrictive, dilated, hypertrophic cardiomyopathy. Median creatine kinase level was 884U/L (range: 347 - 3070 U/L). Muscle biopsy revealed reduced/absent sarcoplasmic desmin expression. Muscle MRI in three patients with predominant fatty infiltration in gluteus maximus and minimus, sartorius, gracilus and semitendinosus in DES; anterior and posterior compartments of distal legs in CRYAB; glutei, hamstrings, adductors of hip and legs with relative sparing of quadriceps, adductor magnus, medial gastrocnemius and peroneal muscles in TTN. Next generation sequencing (NGS) showed the most common gene involved is DES (n = 7, 58.3%) followed by other genes such as HSPB8 (n = 1), FLNC (n = 1), CRYAB (n = 1), LDB3 (n = 1) and TTN (n = 1). This is the first study on clinic-genetic features of MFM from India. The various novel phenotypes noted in our cohort include: CRYAB with late symptom onset without cardiac or bulbar involvement, LDB3 with early onset limb girdle syndrome, ptosis and FLNC with distal myopathy and cardiomyopathy and HSPB8 with limb girdle syndrome and ptosis, further expanding the phenotypic spectrum of MFM.