Isolation and identification of homeobox genes from the human placenta including a novel member of the Distal-less family, DLX4

We have carried out a DNA binding site screen of a 32-week human placental cDNA library using a consensus homeodomain binding site as a probe. This study represents the first library screen carried out to isolate homeobox genes from the human placenta. We have shown that three homeobox genes known to be expressed in the embryo, HB24, GAX and MSX2 are also expressed in the placenta. We have also identified a novel homeobox gene, DLX4, that shows 85% sequence identity with the homeodomain encoded by the Drosophila Distal-less (Dll) gene. DLX4 therefore represents a new member of the Distal-less family of homeobox genes. This is the first evidence that members of the Distal-less family of homeobox genes are expressed in the placenta. Using fluorescence in situ hybridisation (FISH), DLX4 has been assigned to human chromosome 17q21-q22. This places DLX4 in the same region of chromosome 17 as another member of the Distal-less family, DLX3, and the HOX-B homeobox gene cluster. Members of the Distal-less family (DLX1 and DLX2; DLX5 and DLX6) are found as closely linked pairs on human chromosomes. We predict that DLX3 and DLX4 are closely linked and have arisen through gene duplication and divergence from a common ancestral precursor.