EXPRESSION OF THE MURINE HOXA4 GENE REQUIRES BOTH AUTOREGULATION AND A CONSERVED RETINOIC ACID RESPONSE ELEMENT

Analysis of the regulatory regions of the Hox genes has revealed a com plex array of positive and negative cis-acting elements that control t he spatial and temporal pattern of expression of these genes during em bryogenesis, In this study we show that normal expression of the murin e Hoxa4 gene during development requires both autoregulatory and retin oic acid-dependent modes of regulation, When introduced into a Hoxa4 n ull background, expression of a lacZ reporter gene driven by the Hoxa4 regulatory region (Hoxa4/lacZ) is either abolished or significantly r educed in all tissues at E10.5-E12.5. Thus, the observed autoregulatio n of the Drosophila Deformed gene is conserved in a mouse homolog in F ive, and is reflected in a widespread requirement for positive feedbac k to maintain Hoxa4 expression. We also identify three potential retin oic acid response elements in the Hoxa4 5' flanking region, one of whi ch is identical to a well-characterized element flanking the Hoxd4 gen e. Administration of retinoic acid to Hoxa4/lacZ transgenic embryos re sulted in stage-dependent ectopic expression of the reporter gene in t he neural tube and hindbrain, When administered to Hoxa4 null embryos, however, persistent ectopic expression was not observed, suggesting t hat autoregulation is required for maintenance of the retinoic acid-in duced expression, Finally, mutation of the consensus retinoic acid res ponse element eliminated the response of the reporter gene to exogenou s retinoic acid, and abolished all embryonic expression in untreated e mbryos, with the exception of the neural tube and prevertebrae, These data add to the evidence that Hox gene expression is regulated, in par t, by endogenous retinoids and autoregulatory loops.