The unique functions of Runx1 in skeletal muscle maintenance and regeneration are facilitated by an ETS interaction domain

The conserved Runt-related (RUNX) transcription factor family are master regulators of developmental and regenerative processes. Runx1 and Runx2 are expressed in satellite cells (SC) and in skeletal myotubes. Conditional deletion of Runx1 in adult SC negatively impacted self-renewal and impaired skeletal muscle maintenance even though Runx2 expression persisted. Runx1 deletion in C2C12 cells that retain Runx2 expression identified unique molecular functions of Runx1 that cannot be compensated by Runx2. The reduced myoblast fusion in vitro caused by Runx1 loss was due in part to ectopic expression of Mef2c, a target repressed by Runx1 . Structure-function analysis demonstrated that the Ets-interacting MID/EID region of Runx1, absent from Runx2, is critical to Runx1 myoblast function and for Etv4 binding. Analysis of ChIP-seq datasets from Runx1 (T-cells, muscle) versus Runx2 (preosteoblasts) dependent tissues identified a composite Ets:Runx motif enriched in Runx1 -dependent tissues. The Ets:Runx composite motif was enriched in peaks open exclusively in ATAC-seq datasets from WT cells compared to ATAC peaks unique to Runx1KO cells. Thus, engagement of a set of targets by the RUNX1/ETS complex define the non-redundant functions of Runx1 .