Recently, the HAND subclass of bHLH transcription factors has been identified. This class contains two members, dHAND and eHAND, which are expressed in the embryonic heart, lateral mesoderm and neural crest-derived cells. Antisense oligonucleotides of both HAND genes arrest cardiac development of chick embryos between stage 11 and 12, when cardiac looping occurs, indicating that the HAND genes participate in the earliest events of cardiogenesis. To investigate these molecular events during mouse development, we initiated a study of the transcriptional control region of the dHAND gene. Using a 5' region of the dHAND promoter fused to a LacZ reporter, we generated mice that express LacZ within the developing heart.
LacZ expression is highest on the right side of the heart, consistent with in situ data.
Deletion mutants suggest that the cis -acting elements responsible for heart expression resided within a 900bp 5' enhancer. Analysis of this transgene demonstrates that enhancer elements present within the 900bp fragment drive transcription in heart, in a pattern consistent with expression of the endogenous dHAND gene. DNA sequence analysis reveals that this enhancer contains binding sites for a number of transcription factors expressed in these tissues. The right sided expression that dHAND exhibits within the developing heart taken together with the effects of HAND antisense oligonucleotides on cardiac morphogenesis is suggestive of dHAND having a role in cardiac looping. Ongoing studies focus on further characterization of the dHAND enhancer and identification of the specific trans-acting factors that control dHAND expression in the developing heart.