The T-Cell Oncogenic Protein HOX11 ActivatesAldh1Expression in NIH 3T3 Cells but Represses Its Expression in Mouse Spleen Development

<jats:p><jats:italic>Hox11</jats:italic>is a homeobox gene essential for spleen formation in mice, since atrophy of the anlage of a developing spleen occurs in early embryonic development in<jats:italic>Hox11</jats:italic>null mice. HOX11 is also expressed in a subset of T-cell acute leukemias after specific chromosomal translocations. Since the protein has a homeodomain and can activate transcription, it probably exerts at least some of its effects in vivo by regulation of target genes. Representational difference analysis has been used to isolate cDNA clones corresponding to mRNA species activated following stable expression of HOX11 in NIH 3T3 cells. The gene encoding the retinoic acid-synthesizing enzyme aldehyde dehydrogenase 1 (Aldh1), initially called Hdg-1, was found to be ectopically activated by HOX11 in this system. Study of<jats:italic>Aldh1</jats:italic>gene expression during spleen development showed that the presence of<jats:italic>Aldh1</jats:italic>mRNA inversely correlated with<jats:italic>Hox11. Hox11</jats:italic>null mouse embryos have elevated<jats:italic>Aldh1</jats:italic>mRNA in spleen primordia prior to atrophy, while<jats:italic>Aldh1</jats:italic>seems to be repressed by<jats:italic>Hox11</jats:italic>during organogenesis of the spleens of wild-type mice. This result suggests that expression of Aldh1 protein is negatively regulated by Hox11 and that abnormal expression of Aldh1 in<jats:italic>Hox11</jats:italic>null mice may cause loss of splenic precursor cells by aberrant retinoic acid metabolism.</jats:p>