Regulatory analysis of the mouse Hoxb3 gene: Multiple elements work in concert to direct temporal and spatial patterns of expression

The expression pattern of the mouse Hoxb3 gene is exceptionally complex and dynamic compared with that of other members of the Herb cluster. There are multiple types of transcripts for Hoxb3 gene, and the anterior boundaries of its expression vary at different stages of development. Two enhancers na nking Hoxb3 on the 3' and 5' sides regulate Hoxb2 and Hoxb4, respectively, and these control regions define the two ends of a 28-kb interval in and ar ound the Hoxb3 locus. To assay the regulatory potential of DNA fragments in this interval we have used transgenic analysis with a lacZ reporter gene t o locate cis-elements for directing the dynamic patterns of Hoxb3 expressio n. Our detailed analysis has identified four new and widely spaced cis-acti ng regulatory regions that can together account for major aspects of the Ho xb3 expression pattern. Elements Ib, IIIa, and IVb control gene expression in neural and mesodermal tissues; element Va controls mesoderm-specific gen e expression. The most anterior neural expression domain of Hoxb3 is contro lled by an r5 enhancer (element Na); element IIIa directs reporter expressi on in the anterior spinal cord and hindbrain up to rb, and the region A enh ancer tin element I) mediates posterior neural expression. Hence, the regul ation of segmental expression of Hoxb3 in the hindbrain is different from t hat of Hoxa3, as two separate enhancer elements contribute to expression in r5 and r6,. The mesoderm-specific element (Va) directs reporter expression to prevertebra C1 at 12.5 dpc, which is the anterior limit of paraxial mes oderm expression for Hoxb3. When tested in combinations, these cis-elements appear to work as modules in an additive manner to recapitulate the major endogenous expression patterns of Hoxb3 during embryogenesis. Together our study shows that multiple control elements direct reporter gene expression in diverse tissue-, temporal-, and spatially restricted subset of the endog enous Hoxb3 expression domains and work in concert to control the neural an d mesodermal patterns of expression, (C) 2001 Academic Press.