Genomic organization and promoter analysis of a mouse homeobox gene, Hex

A homeobox gene, Hex, is mainly expressed in haematopoietic cells and hepatocytes. It is assumed to play a role in the early stage of differentiation of these cells. To understand the mechanisms involved in the regulation of the Hex gene expression in hepatocytes, we cloned and characterized the mouse Hex gene. The gene consists of four exons and three introns, and spans about 5.7 kb. All the exon-intron boundaries are consistent with the "GT-AG" rule. A single transcription start site was identified by primer extension and S1 mapping analyses. Although the 5'-flanking region is G/C rich (69%), it contains probable "TATA and CCAAT" boxes. Potential binding sequences for transcriptional regulatory proteins including Sp1 and AP-2 are also present in this region. Functional analysis of the Hex promoter was performed by transfecting MH1C1, HeLa, COS-7, and Caco-2 cells with Hex promoter region-luciferase constructs. We found three possible positive regulatory regions, comprising of nucleotides -199 and -172, -154 and -133, and -105 and -68, respectively, required for Hex gene expression in MH1C1 cells by analyses of a series of 5'-deletion constructs of the fusion genes. The activities of these constructs were extremely low in HeLa, COS-7, and Caco-2 cells suggesting that they possess cell-type specificity. Further analysis revealed two GC boxes, GC box1 and GC box2, at nucleotides -197 to -188 and -176 to -167, respectively, necessary for Hex gene expression. Thus, multiple regulatory elements contribute to the Hex gene expression in hepatocytes.