HAND1

In the third week of fetal development the rudimentary heart (bilaterally symmetrical cardiac tube) undergoes a characteristic dextral looping, forming an asymmetrical structure with bulges that represent the incipient ventricular and atrial chambers of the heart.[14] Arising from cells derived from the primary heart field in the cardiac crescent, HAND1 goes from being expressed on both sides of the heart tube to the ventral surface of the caudal heart segment and the aortic sac, then being restricted to the outer curvature of the left ventricle in the looped heart.[14][15][16] In conjunction with HAND2 (a fellow bHLH transcription factor), complementary and overlapping expression patterns are thought to play a role in interpreting asymmetrical signals in the developing heart which leads to the characteristic looping.[14][17] The two are implemented in cardiac development of embryos based on a crucial HAND gene dosage system. If HAND1 is over or under expressed then morphological abnormalities can form; most notable are cleft lips and palates. Expression was modeled with a knock-in of phosphorylation to turn on and off gene expression which induced the craniofacial abnormalities.[11]

HAND1 mutants also appear to develop a spectrum of cardiac abnormalities, as demonstrated in knock-out experimentation in the mouse model, where HAND1-null mice displayed defects in the ventral septum, malformation of the AV valve, hypoplastic ventricles, and outflow tract abnormalities.[17][18] In humans, evidence of a frameshift mutation in the bHLH domain of HAND1 has been correlated with hypoplastic left heart syndrome (a serious form of congenital heart disease where the left side of the heart is severely underdeveloped), aiding in the implication that HAND1 expression is a factor to patients with the disease.[12][19]

However, a lack of HAND1 in the distal regions of the Neural Crest has no effect on cranial feature formation.[11] Mutation of HAND1 has been shown to hinder the effect of GATA4, another vital cardiac transcription factor, and is associated with congenital heart disease.[13] The lack of HAND1 detection in the developing embryo leads to many of the structural defects that causes heart disease and facial deformities while the dosage of HAND1 relates to the severity of these maladies.[11]

In addition, HAND1 is uniquely expressed in trophoblasts and is essential for early trophoblast giant cell differentiation.[20] Trophoblast giant cells are necessary in order for placental development to proceed, participating in vital processes such as blastocyst implantation, remodeling of the maternal decidua, and secretion of hormones.[20] The importance of this relationship is demonstrated in HAND1-null mutant mice, which display significant abnormalities in trophoblast development, such as a reduced ectoplacental cone, thin parietal yolk sac, and reduced density of trophoblast giant cells.[21] These homozygous HAND1-null mutant embryos were arrested by E7.5 of gestation, though could be saved by contribution of wild-type cells to the trophoblast.[21]

Expressed in high levels in the extraembryonic membranes throughout development, HAND1 also plays a functional role in vascular development of the yolk sac.[22] Though not strictly required for vasculogenesis, data has shown that HAND1 contributes to the fine-tuning of the vasculogenic response in the yolk sac, recruiting smooth muscle cells to the endothelial network in order to refine the primitive endothelial plexus to a functional vascular system.[22][9] This relationship has been demonstrated in the HAND1-null mouse model, where embryos lacking the HAND1 gene had a yolk sac vasculature defect caused by lack of vasculature refinement leading to the accumulation of hematopoietic cells between the yolk sac and the amnion.[22]