Cardiac expression of the ventricle-specific homeobox gene Irx4 is modulated by Nkx2-5 and dHand

We report the isolation and characterization of the cDNAs encoded by the mu rine and human homeobox genes, Irx4 (Iroquois homeobox gene 4). Mouse and h uman Irx4 proteins are highly conserved (83%) and their 63-aa homeodomain i s more than 93% identical to that of the Drosophila Iroquois patterning gen es. Human IRX4 maps to chromosome 5p15.3, which is syntenic to murine chrom osome 13. Irx4 transcripts are present in the developing central nervous sy stem, skin, and vibrissae, but are predominantly expressed in the cardiac v entricles. In mice at embryonic day (E) 7.5, Irx4 transcripts are found in the chorion and at low levels in a discrete anterior domain of the cardiac primordia. During the formation of the linear heart tube and its subsequent looping (E8.0-8.5), Irx4 expression is restricted to the ventricular segme nt and is absent from both the posterior (eventual atrial) and the anterior (eventual outflow tract) segments of the heart. Throughout all subsequent stages in which the chambers of the heart become morphologically distinct ( E8.5-11) and into adulthood, cardiac Irx4 expression is found exclusively i n the ventricular myocardium. Irx4 gene expression was also assessed in emb ryos with aberrant cardiac development: mice lacking RXR alpha or MEF2c hav e normal Irx4 expression, but mice lacking the homeobox transcription facto r Nkx2-5 (Csx) have markedly reduced levels of Irx4 transcripts. dHand-null embryos initiate Irx4 expression, but cannot maintain normal levels. These data indicate that the homeobox gene Irx4 is likely to be an important med iator of ventricular differentiation during cardiac development, which is d ownstream of Nkx2-5 and dHand. (C) 2000 Academic Press.