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 (TR alpha), is thought to contribute to avian erythroblastosis virus (AEV)-induced leukemic transform ation by constitutively repressing transcription of target genes. However, the binding of v-ErbA or any unliganded nuclear receptor to a chromatin-emb edded response element as well as the role of the N-CoR-SMRT-HDAC co-repres sor 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 fo otprinting shows that v-ErbA is constitutively bound to this HS2-VRE in tra nsformed, undifferentiated erythroblasts along with other transcription fac tors like GATA-1, Transfection assays show that the repressed HS2 region ca n be turned into a potent enhancer in v-ErbA-expressing cells by mutation o f the VRE, Differentiation of transformed cells alleviates v-ErbA binding c oncomitant with activation of CAII transcription, Co-expression of a gag-TR alpha fusion protein in AEV-transformed cells and addition of ligand derep resses CAII transcription. Treatment of transformed cells with the histone deacetylase inhibitor, trichostatin A, derepresses the endogenous, chromati n-embedded CAII gene, while a transfected HS2-enhancer construct remains re pressed. Taken together, our data suggest that v-ErbA prevents CAII activat ion by 'neutralizing' in cis the activity of erythroid transcription factor s.