In Human Beta-Globin Gene Locus, ERV-9 LTR Retrotransposon Interacts with and Activates Beta- but Not Gamma-Globin Gene

Retrotransposons including endogenous retroviruses and their solitary long terminal repeats (LTRs) comprise over 40% of the human genome. Many of them are located in intergenic regions far from genes. Whether these intergenic retrotransposons serve beneficial host functions was not known. In the human b-globin gene locus, an ERV-9 LTR retrtransposon is located near the 5’ end of the locus control region (LCR) at 40-70 kb upstream of the human fetal g- and adult b-globin genes. To address the functional significance of the intergenic ERV-9 LTR, we generated transgenic (Tg) mice carrying a 100 Kb BAC clone spanning the entire human b-globin gene locus from the ERV-9 LTR to b-globin gene and showed that the LTR retrotransposon serves long-range, beneficial host function (Pi et al., PNAS 2010): The ERV-9 LTR containing multiple CCAAT and GATA motifs competitively recruits high concentration of NF-Y and GATA-2 present in low abundance in adult erythroid cells to assemble an LTR/RNA polymerase II complex. Deletion of the ERV-9 LTR by Cre-loxP mediated in situ recombination in the BAC Tg mice suppresses transcription of b-globin gene but activates transcription of g-globin gene. The results indicate that the ERV-9 LTR activates transcription of b-globin gene in erythroid cells during development. Therefore, LTR deletion drastically suppressed b-globin gene and re-activated g-globin gene through a competitive mechanism of globin gene switching.

Alternatively, the primary effect of the ERV-9 LTR could be to suppress g-globin gene during development. Therefore, LTR deletion re-activated g-globin gene, which then suppressed transcription of b-globin gene. To differentiate between these two possibilities, we utilized Mx1-Cre mice to conditionally delete the ERV-9 LTR in the erythroid cells of the adult BAC transgene mice, in which g-globin gene was already silenced and b-globin gene fully activated. If the primary target of the ERV-9 LTR enhancer complex was g-globin gene, deletion of the ERV-9 LTR should not be able to activate the already silenced g-globin gene nor to suppress the fully active b-globin gene. However, the same effect on transcriptional suppression of b-globin gene and re-activation of g-globin gene was observed. These results indicate that the primary target of the ERV-9 LTR is the b-globin gene and not the g-globin gene. The molecular factors underlying the preferential interaction between the ERV-9 LTR and the b-globin gene are under investigation and will be presented