| Hox genes encode a group of evolutionarily conserved transcription factors that play vital roles in establishing anterior-posterior pattern formation during vertebrate development. These genes are organized into four chromosomal clusters where genes at the 3' end of the cluster are expressed in anterior regions of the developing embryo and genes more 5' are expressed in posterior regions. Hox genes characteristically contain a homeobox region that encodes a 60-amino acid DNA binding domain. This domain allows Hox proteins to bind to the promoter regions of their target genes to regulate their transcription. Due to alternative splicing, the Hoxa-1 gene encodes two mRNAs that can direct the synthesis of two distinct proteins, one that contains the homeodomain (Hoxa1-993), and another truncated protein that lacks the DNA binding domain (Hoxal-399). The functions of this truncated transcript are not known. To determine the role of Hoxal-399 in cellular differentiation, I focused on the transcriptional activation of Hoxb1, a well-studied Hoxa1 target gene. Hoxa1-993, together with its cofactor, Pbx1, binds to the promoter region of Hoxb1 to transcriptionally activate Hoxb1. Data from reporter assays using F9 stem cells that ectopically express Hoxa1-399 (F9-399 cells) indicate that Hoxal-399 inhibits the transcriptional activation of the Hoxb1 promoter. Additionally, gel shift assays demonstrate that Hoxa1-399 inhibits the binding of the Hoxa1-993/Pbx1 protein complex on the Hoxb1 promoter sequence. Finally, GST-pull down experiments suggest that Hoxa1399 forms a trimer with Hoxa1-993 and Pbx1. However, F9-399 cells show no differences in RA-induced proliferation arrest or in Hoxb1, Pbx1, Hoxa5, Cyp26a1, GATA4, or Meis mRNA levels when compared to F9 wild type (Wt) cells. These data indicate that the Hoxa1-399 protein plays a negative role in the transcriptional activation of Hoxb1 at a specific region of the promoter. As Hoxa1 expression is altered in various disorders, understanding the function of truncated Hox proteins, particularly Hoxa1, may provide insights into the normal function of these proteins and thereby provide information as to how these proteins are altered in various disease states. |
