270P Single nucleus RNA sequencing reveals unique myonuclei populations in late-onset myopathy

Genetic myopathies represent a large, heterogeneous group of diseases. Typical outcomes include progressive weakness over time, painful joints, and even deteriorating heart and respiratory function. There is tremendous variation in which muscles are affected, disease severity, and curiously, the age of onset. Mutations in a muscle specific protein would be expected to cause immediate consequences to muscular function. However, some patients remain unaffected until after normal muscle development has completed, only experiencing weakness starting in their 20s or later in life. The mechanisms behind the delay in onset of these diseases remain obscure. Single-cell technologies are a powerful method to obtain quantitative, cell-specific transcriptional information. This is a promising approach to studying skeletal muscle disease because it provides a high-resolution look at the many cell types regulating muscle homeostasis and repair. Our goal was to use single nucleus RNA sequencing to find transcriptional similarities between late-onset myopathy patients. To this end, we developed a novel nuclei isolation approach from frozen human skeletal muscle biopsies. We isolated nuclei from the muscle of 10 patients with a diverse range of myopathies and 4 age and sex matched controls. After processing, this yielded over 100,000 nuclei with quality control metrics in line with literature values. We identified 12 cell types, and interestingly, a unique population of differentiating myonuclei derived almost entirely from myopathic patients. These myonuclei express markers of senescence, aging, and impaired differentiation potential. Very few control nuclei were present in this population, implying a cell state that is specific to disease. Overall, our findings suggest that myogenic progenitors in late-onset myopathic muscle may be aging prematurely.