SMCHD1 activates the expression of genes required for the expansion of human myoblasts (RNA-seq)

In human muscle, SMCHD1 mutations are associated with the onset of FSHD2, but the mechanism driving the disease onset remains unclear. A commonly accepted explanation is the loss of SMCHD1 binding to the D4Z4 locus activates the expression of DUX4 in FSHD2 muscle. In this study, we used human myoblasts having DUX4 non-permissive 4qB alleles as a model to study DUX4-independent functions of SMCHD1 on myoblast cell growth. Surprisingly, depletion of SMCHD1 in these cells resulted in a cell proliferation defect. Despite the absence of DUX4 target genes’ activation, these cells showed a repression of PAX7 target genes (a hallmark of FSHD) and similar changes in expression profile compared to FSHD myoblasts. Interestingly, downregulation of cell proliferation-related genes and dysregulation of fibroblasts-specific genes were observed in SMCHD1 knockdown myoblasts and FSHD2 myoblasts but not FSHD1 myoblasts. Additionally, we identified LAP2 as direct targets of SMCHD1. Depletion of LAP2 leads to cell proliferation defect similar to the effect after SMCHD1 knockdown. These data imply that DUX4 is not the only driver for the onset of FSHD, and SMCHD1 has DUX4-independent functions in muscle growth and development.