Cooperative Silencing of Chicken ρ- and β^H- Globin Gene Expression by Chicken β-Globin Regulatory Elements: Involvement of RNAi Mediated Repression.

The chicken β-globin locus represents a well characterized, model system where the relationship between chromatin structure, transcription and DNA replication can be studied. The locus contains several regulatory elements including an intergenic enhancer as well as upstream regulatory elements that may function either alone or in combination with the intergenic enhancer as an LCR. The availability of the recombination proficient chicken B cell line DT40 has allowed the introduction of mutations into the endogenous chicken β-globin locus and phenotypic analysis after microcell mediated chromosome transfer into human erythroleukemia (K562) cells. Using this system, we have introduced deletions in the chicken β-globin intergenic enhancer as well as 5′ HS 1,2, and 3. Expression of the embryonic ρ and fetal β^H chicken globin genes were repressed by the intergenic enhancer, 5′ HS1, or 5′HS2. No ρ or β^H globin gene expression was detected in K562 cells containing control chicken chromosomes, while ρ and β^H mRNA were activated when the intergenic enhancer, 5′ HS1, or 5′HS2 were deleted. Chromatin immunoprecipitation (ChIP) experiments that assayed RNA polmerase II (pol II), GATA-1 and NF-E2 p45/ p18 binding at regulatory elements and gene promoters in targeted cell lines supported this hypothesis and suggested a potential role for 5′HS3 in gene activation. However, targeted deletion of 5′ HS3, unlike the other chicken β-globin regulatory elements, showed no transcriptional phenotype. Our results demonstrate the intergenic enhancer, 5′HS1, and 5′ HS2 function through a common silencing mechanism involving pol II, GATA-1, and NF-E2/P18. The recent demonstration of the involvement of Pol II in the synthesis of miRNA’s prompted us to investigate the role of miRNA’s in gene silencing in this system. A small miRNA was identified at the intergenic enhancer region. ChIP assays showed the binding of two components of the RISC (Dicer and Ago2) at the chicken globin regulatory elements. These results are consistent with the involvement of RISC and miRNA’s in gene silencing in this system.