Loss of the Nuclear Envelope Protein LAP1B Disrupts Myogenic Differentiation of Patient-Derived Fibroblasts

Lamina-associated polypeptide 1 (LAP1) is a ubiquitously expressed inner nuclear membrane protein encoded by TOR1AIP1, and present as two isoforms in humans, LAP1B and LAP1C. While loss of both isoforms results in a multisystemic progeroid-like syndrome, specific loss of LAP1B causes muscular dystrophy and cardiomyopathy, suggesting that LAP1B has a critical role in striated muscle. To gain more insight into the molecular pathophysiology underlying muscular dystrophy caused by LAP1B, we established a patient-derived fibroblast line transdifferentiated into myogenic cells with inducible MyoD expression. Compared to controls, we observed strikingly reduced myogenic differentiation and fusion potentials. Similar defects were observed in C2C12 myoblasts carrying loss-of-function LAP1A/B mutations. Using RNA sequencing, we showed that despite MyoD overexpression and efficient cell cycle exit, transcriptional reprogramming of LAP1B-deficient cells into the myogenic lineage was impaired with delayed activation of myogenin and muscle-specific downstream genes. Gene set enrichment analysis suggested dysregulation of protein metabolism, extracellular matrix and chromosome organization. Finally, we found that LAP1B-deficient cells exhibit nuclear deformations such as increased micronuclei and altered morphometric parameters. This study reports the phenotypic and transcriptomic changes during myoconversion of patient-derived fibroblasts and provides a useful resource to gain new insights into the mechanisms implicated in LAP1B-associated nuclear envelopathies. Comparative gene expression profiling analysis of RNA-seq data for control and LGMD2Y patient-derived immortalized myoconverted fibroblasts.