The expression of SpRunt during sea urchin embryogenesis

The runt box (runx) is a highly conserved DNA binding and protein-protein interaction domain that defines a family of heterodimeric transcription factors that regulate development in metazoans. The three mammalian runx genes are oncogenes with essential functions in normal development: Runx1 is required for hematopoiesis and is frequently mutated in human and murine leukemias; Runx2 is required for bone development and is associated with human cleidocranial dysplasia and murine leukemias; and Runx3 (the evolutionarily basal member of the mammalian family) regulates growth of the gut and functions as a tumor suppressor in the gastric epithelium (Westendorf and Hiebert, 1999; Li et al., 2002). The sea urchin Strongylocentrotus purpuratus contains a single runx gene, SpRunt. We present here the initial structural characterization of SpRunt, and its pattern of expression during embryogenesis. SpRunt contains two introns, the locations of which are identical to those of the second and third introns from promoter P2 of the mammalian runx genes. A approximately 6 kb transcript begins to accumulate during cleavage. At mesenchyme blastula stage, SpRunt transcripts are found throughout the embryo, but specifically enriched in the vegetal plate, skeletogenic mesenchyme, and part of the ectoderm. By late gastrula stage expression is localized to the endomesoderm and oral ectoderm. In the pluteus larva SpRunt transcripts remain confined to the endomesoderm and oral ectoderm, with highest levels of accumulation in the foregut and in the ciliary band. These data suggest that SpRunt expression is enhanced in proliferating cells.