Annotation, nomenclature and evolution of four novel homeobox genes expressed in the human germ line.

The homeobox genes comprise a large gene superfamily characterised by a conserved DNA motif encoding the homeodomain. Most homeodomain proteins function as transcription factors, and many have important roles in embryonic development and cell differentiation. Here we describe, annotate and name four novel homeobox genes in the human genome: ARGFX, DPRX, TPRX1 and DUXA. Each has generated multiple retrotransposed (processed) pseudogenes; these are reliable indicators of germ-line expression because only in germ-line cells can retrotransposition result in inheritance to the next generation. The retrotransposed sequences were exploited here as a novel means to deduce exon-intron boundaries. All four novel genes show accelerated rates of protein sequence evolution. This fast rate of sequence change may be connected with roles in human reproductive biology. Deducing the evolutionary origins of these genes is not straightforward, but we propose that TPRX1, DPRX and DUXA are highly divergent derivatives of the CRX gene, itself a member of the Otx homeobox gene family.