T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignant disease of thymocytes that mainly affects children and has very poor prognosis with high rates of relapse. A prominent feature observed in 60% of T-ALL childhood patients is the ectopic expression of a key hematopoietic transcription factor TAL1/SCL. Although several enhancers has been identified to play an important role in normal hematopoietic differentiation, the histone modification patterns and chromatin organization over the whole TAL1 locus reveled that none of them is active in T-ALL cell lines such as Jurkat and Rex cells. It remains currently unknown how TAL1 is activated in the majority of T-ALL patients lacking the TAL1 locus rearrangements. To understand the molecular mechanism underlying regulation of the TAL1 oncogene in leukemic T-cells, we employed circularized chromosome conformation capture (4C) methodology to identify new regulatory elements that activate TAL1 specifically in T-ALL leukemia. Using the TAL1 promoter 1a as the bait, we discovered that the TAL1 promoter 1a interacts with the TIL16 element (TAL1 interacting locus in chromosome 16) that is located at ∼15 Kb downstream of T-cell specific CD2BP2 gene in T-ALL cell line Jurkat, but not in erythroid progenitor K562 cells. The CD2BP2 protein is a cellular adapter protein that was originally identified as a binding partner of the T cell adhesion protein CD2 in the context of T cell signaling. The TIL16 element contains the bind sites for several transcription factors that are important for hematopoiesis such as C-Maf, Pax5, HoxA7 and USF2. The inter-chromosomal interaction between the TIL16 and the TAL1 promoter 1a was further confirmed by chromosome conformation capture (3C) assay in three TAL1 over-expressing T-ALL cell lines, Jurkat, REX and Molt4, but not in K562 cells. Recent genome wide study has correlates H3K4 mono- or dimethyl marks with distal enhancers while trimethyl H3K4 is enriched in promoters of active genes. To further test if the TIL16 acts as T-cell specific enhancer for TAL1 activation in T-ALL cells, we carried out ChIP-seq and ChIP analysis in CD4 T cells, Jurkat, and K562 cells. We found that the TIL16 element is specifically marked by H3K4me1 in Jurkat and CD4+ T-cells but not in K562 cells. The enrichment of H3K4me1 is correlated with the binding of c-Maf, a T-cell specific transcription factor. To further test whether the TIL16 element contributes to transcription activity, a DNA fragments containing the TIL16 element were cloneed into SV40 minimal promoter driven luciferase reporter and introduced into K562 and several T-ALL cell lines. Compared to the pGL3-SV40 vector that showed only minimal luciferase activity, the 1 Kb TIL element specifically activated transcription of the luciferase reporter in T-ALL cells, but not in erythroid progenitor K562 cells suggesting that the TIL16 element functions as a T-cell specific TAL1 enhancer. Thus, our data revealed a novel epigenetic mechanism by which the TAL1 oncogene is ectopically activated in T-cell leukemia.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.