Variants in MYH7 cause cardiomyopathies as well as myosin storage myopathy and Laing early-onset distal myopathy (MPD1). MPD1 is characterized by muscle weakness and atrophy usually beginning in the lower legs. Here, we generated iPSC lines from lymphoblastoid cells of three unrelated individuals heterozygous for the most common MPD1-causing variant; p.Lys1617del. iPSC lines showed typical morphology, expressed pluripotency markers, demonstrated trilineage differentiation potential, and had a normal karyotype. These lines represent the first iPSCs derived from MPD1 patients and complement existing MPD1 animal models. They can provide in vitro platforms to better understand and model MPD1 pathomechanisms and test therapies.

The myosin inhibitor mavacamten has transformed the management of obstructive hypertrophic cardiomyopathy (HCM) by targeting myosin ATPase activity to mitigate cardiac hypercontractility. This therapeutic mechanism has proven effective for patients with HCM independent of having a primary gene mutation in myosin. In this issue of the JCI, Buvoli et al. report that muscle hypercontractility is a mechanism of pathogenesis underlying muscle dysfunction in Laing distal myopathy, a disorder characterized by mutations altering the rod domain of β myosin heavy chain. The authors performed detailed physiological, molecular, and biomechanical analyses and demonstrated that myosin ATPase inhibition can correct a large extent of muscle abnormalities. The findings offer a therapeutic avenue for Laing distal myopathy and potentially other myopathies. This Commentary underscores the importance of reevaluating myosin activity's role across myopathies in general for the potential development of targeted myosin inhibitors to treat skeletal muscle disorders.

Proline substitutions within the coiled-coil rod region of the β-myosin gene (MYH7) are the predominant mutations causing Laing distal myopathy (MPD1), an autosomal dominant disorder characterized by progressive weakness of distal/proximal muscles. We report that the MDP1 mutation R1500P, studied in what we believe to be the first mouse model for the disease, adversely affected myosin motor activity despite being in the structural rod domain that directs thick filament assembly. Contractility experiments carried out on isolated mutant muscles, myofibrils, and myofibers identified muscle fatigue and weakness phenotypes, an increased rate of actin-myosin detachment, and a conformational shift of the myosin heads toward the more reactive disordered relaxed (DRX) state, causing hypercontractility and greater ATP consumption. Similarly, molecular analysis of muscle biopsies from patients with MPD1 revealed a significant increase in sarcomeric DRX content, as observed in a subset of myosin motor domain mutations causing hypertrophic cardiomyopathy. Finally, oral administration of MYK-581, a small molecule that decreases the population of heads in the DRX configuration, significantly improved the limited running capacity of the R1500P-transgenic mice and corrected the increased DRX state of the myofibrils from patients. These studies provide evidence of the molecular pathogenesis of proline rod mutations and lay the groundwork for the therapeutic advancement of myosin modulators.

Deglutition-induced atrial fibrillation is a rare clinical entity with a reported prevalence of 0.6%. Laing distal myopathy is a rare autosomal dominant muscular dystrophy that is the result of mutations within the slow skeletal muscle fibre myosin heavy chain gene (MYH7). Atrial fibrillation has not been previously reported in patients with Laing distal myopathy. We describe the first reported case of deglutition triggered atrial fibrillation in a female with a history of Laing distal myopathy. A 44-year-old female with a history of Laing distal myopathy diagnosed at age 32, began experiencing intermittent episodes of pre-syncope and palpitations which occurred after deglutition with food. An ambulatory 30-day patient triggered event monitor recorded episodes of atrial fibrillation with rapid ventricular response. Family history was significant for Laing distal myopathy, atrial fibrillation, as well as sudden cardiac death. Laboratory data, transthoracic echocardiogram, cardiac magnetic resonance imaging, and an exercise treadmill SPECT Imaging stress test were normal. An oesophagram revealed a mild oesophageal dysmotility with no other abnormalities. She was started on flecainide 50 mg p.o. every 8 h and verapamil 40 mg p.o. every 8 h with no further episodes of atrial fibrillation. Given the strong genetic component of this myopathy, one could postulate as to a possible genetic component in the development of atrial fibrillation in our patient. Although we cannot make definite correlation between deglutition-induced atrial fibrillation and Laing myopathy, it is important to report this unusual association which has not been described before.

Laing myopathy is characterized by broad clinical and pathological variability. They are limited in number and protocol of study. We aimed to delineate muscle imaging profiles and validate imaging analysis as an outcome measure. This was a cross-sectional and longitudinal cohort study. Data from clinical, functional and semi-quantitative muscle imaging (60 magnetic resonance imaging [MRI] and six computed tomography scans) were studied. Hierarchical analysis, graphic heatmap representation and correlation between imaging and clinical data using Bayesian statistics were carried out. The study cohort comprised 42 patients from 13 families harbouring five MYH7 mutations. The cohort had a wide range of ages, age at onset, disease duration, and myopathy extension and Gardner-Medwin and Walton (GMW) functional scores. Intramuscular fat was evident in all but two asymptomatic/pauci-symptomatic patients. Anterior leg compartment muscles were the only affected muscles in 12% of the patients. Widespread extension to the thigh, hip, paravertebral and calf muscles and, less frequently, the scapulohumeral muscles was commonly observed, depicting distinct patterns and rates of progression. Foot muscles were involved in 40% of patients, evolving in parallel to other regions with absence of a disto-proximal gradient. Whole cumulative imaging score, ranging from 0 to 2.9 out of 4, was associated with disease duration and with myopathy extension and GMW scales. Follow-up MRI studies in 24 patients showed significant score progression at a variable rate. We confirmed that the anterior leg compartment is systematically affected in Laing myopathy and may represent the only manifestation of this disorder. However, widespread muscle involvement in preferential but variable and not distance-dependent patterns was frequently observed. Imaging score analysis is useful to categorize patients and to follow disease progression over time.