Evolutionary expansion of a gene family may occur at both the DNA and RNA levels. The rat testis-specific Rtdpoz-T2 and -T1 (rT2 and rT1) retrogenes are members of the TD/POZ gene family which also includes the well-characterized SPOP gene. In this study, rT2/rT1 transcriptional activation in cancer cells is demonstrated; the cancer rT2/rT1 transcripts are structurally similar to the embryonic transcripts reported previously in frequent exonization of transposed elements. On database interrogation, we have identified an uncharacterized rT2/rT1-like SPOP paralog, designated as SPOP-like (SPOPL), in the human and rodent genomes. Ka/Ks analysis indicates that the SPOPL genes are under functional constraints implicating biological functions. Phylogenetic analyses further suggest that segmental duplication and retrotransposition events had occurred giving rise to new gene members or retrogenes in the human-rodent ancestors during the evolution of the TD/POZ gene family. Based on this and previous works, a model is proposed to map the routes of evolutionary expansion of the TD/POZ gene family. More importantly, different gene expression patterns of members of the family are depicted: intron-harboring members are ubiquitously expressed whereas retrogenes are expressed in tissue-specific and developmentally regulated manner, and are fortuitously re-activated in cancer cells involving exonization of transposed elements.
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