FOXK1 is an Essential Transcriptional and Epigenetic Regulator of Cardiovascular Development

Background: Congenital heart disease (CHD) is the most common genetic birth defect and has considerable morbidity and mortality. Therefore, it is essential to define the mechanisms governing the specification and differentiation of mesodermal and cardiac progenitors to develop targeted therapies for CHD. FOXK1 is a forkhead/winged helix transcription factor known to regulate cell cycle kinetics, myogenic stem cell proliferation and tumorigenesis. During development, FOXK1 expression is restricted to mesodermal progenitors, somites and the heart. The role for FOXK1 in the developing heart is unknown and warrants investigation. In the present study, we describe a novel role for FOXK1 as an essential regulator of cardiovascular development. Approach and Results: We used a mouse embryoid body (EB) system to differentiate WT and Foxk1 null ESCs into mesodermal and cardiac progenitor cells. Flow cytometry analysis for FLK1+PDGFRa+ showed that in the absence of FOXK1, the cardiac lineage was significantly affected. Bulk RNAseq analysis of D3 and D5 EBs showed a significant induction of the cardiac molecular program in WT EBs compared to the Foxk1 null EBs. Since forkhead transcription factors have been shown to be important epigenetic regulators (i.e. pioneer factors), we performed ATACseq of D3 and D5 EBs, which showed that the chromatin landscape nearby known important regulators of cardiogenesis (Isl1, Gata4, Hand1, Hand2, etc.) was significantly relaxed in WT EBs compared to Foxk1 null EBs. Furthermore, we demonstrated that in the absence of Foxk1 , cardiac differentiation was markedly impaired by assaying for cTnT expression and cardiac contractility which was essentially absent in Foxk1 null EBs compared to WT EBs at D10 of differentiation. Additionally, using transcriptional, Co-IP and pulldown assays we demonstrated that FOXK1 is an important regulator of Notch and Wnt/β-catenin signaling pathways during cardiogenesis. These results were further supported as the Foxk1 null embryo was lethal at E9.5 and had perturbed heart development. Conclusions: These results identify FOXK1 as an essential transcriptional and epigenetic regulator of cardiovascular development using EB assays and gene disruption technologies.