Leukemic transformation by the v-ErbA oncoprotein entails constitutive binding to and repression of an erythroid enhancer in vivo

v-ErbA, a mutated thyroid hormone receptor alpha (TRalpha), is thought to contribute to avian erythroblastosis virus (AEV)-induced leukemic transformation by constitutively repressing transcription of target genes. However, the binding of v-ErbA or any unliganded nuclear receptor to a chromatin-embedded response element as well as the role of the N-CoR-SMRT-HDAC co-repressor complex in mediating repression remain hypothetical. Here we identify a v-ErbA-response element, VRE, in an intronic DNase I hypersensitive site (HS2) of the chicken erythroid carbonic anhydrase II (CAII) gene. In vivo footprinting shows that v-ErbA is constitutively bound to this HS2-VRE in transformed, undifferentiated erythroblasts along with other transcription factors like GATA-1. Transfection assays show that the repressed HS2 region can be turned into a potent enhancer in v-ErbA-expressing cells by mutation of the VRE. Differentiation of transformed cells alleviates v-ErbA binding concomitant with activation of CAII transcription. Co-expression of a gag-TRalpha fusion protein in AEV-transformed cells and addition of ligand derepresses CAII transcription. Treatment of transformed cells with the histone deacetylase inhibitor, trichostatin A, derepresses the endogenous, chromatin-embedded CAII gene, while a transfected HS2-enhancer construct remains repressed. Taken together, our data suggest that v-ErbA prevents CAII activation by 'neutralizing' in cis the activity of erythroid transcription factors