Investigation of the role of LAP1B in transcriptional regulation of muscle cells

The loss-of-function of the inner nuclear membrane protein LAP1B (lamina-associated polypeptide 1, isoform B) causes muscular dystrophy and cardiomyopathy in humans. The function of LAP1B in muscle is still unknown. The goal of this thesis is to contribute to the understanding of the molecular pathogenesis underlying muscular dystrophy caused by LAP1B, by determining transcriptional changes occurring throughout differentiation of muscle cells lacking LAP1B. For this purpose, primary fibroblasts isolated from healthy and one patient affected by LAP1B-related muscular dystrophy were immortalized and myoconverted with inducible MyoD gene transfer. Whereas control cells formed mature myotubes within eight days, mutant cells demonstrated very low fusion potential and failed to fully differentiate. Mutations causing knockdown of LAP1A/B expression were created in C2C12 mouse myoblasts and similar results were observed. By RNA sequencing, genes differentially expressed in control and mutant cells within the transcriptome and enriched pathways were identified. Muscle contraction, cell cycle, mitotic chromatid segregation and extracellular matrix organization were among the most significantly enriched pathways. It was shown by cell cycle assay that despite downregulation of p21 expression, mutant cells withdrew from the cell cycle. Finally, upregulation of p53 expression and increase in the number of micronuclei in mutant cells were related to cellular stress and DNA damage. These findings demonstrated that LAP1B is not involved in cell cycle exit but might suggest a role in DNA damage repair necessary for the induction of myogenin expression. Identification of previously unknown pathways for LAP1 will contribute to the discovery of novel targets for therapy.