Hand2 loss-of-function in Hand1-expressing cells reveals distinct roles in epicardial and coronary vessel development.

RATIONALE The basic helix-loop-helix (bHLH) transcription factors Hand1 and Hand2 are essential for embryonic development. Given their requirement for cardiogenesis, it is imperative to determine their impact on cardiovascular function. OBJECTIVE To deduce the role of Hand2 within the epicardium. METHOD AND RESULTS We engineered a Hand1 allele expressing Cre recombinase. Cardiac Hand1 expression is largely limited to cells of the primary heart field, overlapping little with Hand2 expression. Hand1 is expressed within the septum transversum, and the Hand1 lineage marks the proepicardial organ and epicardium. To examine Hand factor functional overlap, we conditionally deleted Hand2 from Hand1-expressing cells. Hand2 mutants display defective epicardialization and fail to form coronary arteries, coincident with altered extracellular matrix deposition and Pdgfr expression. CONCLUSIONS These data demonstrate a hierarchal relationship whereby transient Hand1 septum transversum expression defines epicardial precursors that are subsequently dependent on Hand2 function.