IDENTIFICATION, CHARACTERIZATION, AND LOCALIZATION TO CHROMOSOME 17Q21-22 OF THE HUMAN TBX2 HOMOLOG, MEMBER OF A CONSERVED DEVELOPMENTAL GENE FAMILY

The T-box motif is present in a family of genes whose structural features and expression patterns support their involvement in developmental gene regulation. Previously, sequence comparisons among the T-box domains of ten vertebrate and invertebrate T-box (Tbx) genes established a phylogenetic tree with three major branches. The Tbx2-related branch includes mouse Mm-Tbx2 and Mm-Tbx3, Drosophila optomotor-blind (Dm-Omb), and Caenorhabditis elegans Ce-Tbx2 and Ce-Tbx7 genes. From the localization of Mm-Tbx2 to Chromosome (Chr) 11, we focused our search for the human homolog, Hs-TBX2, within a region of synteny conservation on Chr 17q. We used Dm-Omb polymerase chain reaction (PCR) primers to amplify a 137-basepair (bp) product from human genomic, Chr 17 monochromosome hybrid, and fetal kidney cDNA templates. The human PCR product showed 89% DNA sequence identity and 100% peptide sequence identity to the corresponding T-box segment of Mm-Tbx2. The putative Hs-TBX2 locus was isolated within a YAC contig that included three anonymous markers, D17S792, D17S794, and D17S948, located at Chr 17q21-22. Hybridization- and PCR-based screening of a 15-week fetal kidney cDNA library yielded several TBX2 clones. Sequence analysis of clone lambda cTBX2-1 confirmed homology to Mm-Tbx2-90% DNA sequence identity over 283 nt, and 96% peptide sequence identity over 94 amino acids, Similar analysis of Hs-TBX2 cosmid 154F11 confirmed the cDNA coding sequence and also identified a 1.7-kb intron located at the same relative position as in Mm-Tbx2. Phylogenetic analyses of the T-box domain sequences found in several vertebrate and invertebrate species further suggested that the putative human TBX2 and mouse Tbx2 are true homologs. Northern blot analysis identified two major TBX2 transcripts of 3.5 and 2.8 kb, with high levels of TBX2 expression in fetal kidney and lung, and in adult kidney, lung, ovary, prostate, spleen, and testis. Reduced expression levels were seen in heart, white blood cells, small intestine, and thymus. These results suggest that Hs-TBX2 could play important roles in both developmental and postnatal gene regulation.