Helix-loop-helix (E2-5, HEB, TAL1 and Id1) protein interaction with the TCRαδ enhancers

In order to dissect the correlation between aberrant TAL1 basic-helix-loop-helix (b-HLH) expression and the exclusive development of T cell acute lymphoblastic leukemias (T-ALL) of the TCR alpha beta lineage, we have assessed the ability of class A b-HLH proteins to regulate the TCR alpha and delta enhancers. We demonstrate that E47S binds to TCR alpha but not to TCR delta E-boxes in vitro. Despite this, neither E2-5 nor HEB transactivate the TCRa enhancer in NIH 3T3, nor did Id1 modify endogenously driven TCR alpha [alpha E1-4] activity in a TCR alpha beta cell line. We also demonstrate that TAL1 inhibits both binding of E47S to alpha E3 and alpha E4 and endogenous transactivation of the TCR alpha enhancer. Comparison of the activity of the minimal [alpha E1-2] fragment, which contains no E-boxes, with the accessory [alpha E3-4] fragment, which contains two, suggested some contribution from the latter to TCR alpha enhancer activity in HPB-ALL, TCR [alpha E1-2] activity was partially (40%) inhibited by TAL1 but not at all by Id1, In contrast, [aE3-4] activity was almost completely inhibited by TAL1 (80%) and slightly reduced by Id1 (15%). These data demonstrate that class A b-HLH regulation of the TCR alpha enhancer E-boxes differs from their B lymphoid Ig mu counterparts and suggest a novel mechanism of transcriptional inhibition by TAL1, which may be, at least partly, independent of E-box-mediated activation, as we currently recognize it. They also clearly demonstrate that the restriction of TAL1 deregulation to T-ALL of the TCR alpha beta lineage is not due to induction of TCR alpha enhancer activity by the TAL1 protein.