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

Abstract

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 TCRαβ lineage, we have assessed the ability of class A b-HLH proteins to regulate the TCRα and δ enhancers. We demonstrate that E47S binds to TCRα but not to TCRδ E-boxes in vitro. Despite this, neither E2-5 nor HEB transactivate the TCRα enhancer in NIH 3T3, nor did Id1 modify endogenously driven TCRα [αE1-4] activity in a TCRαβ cell line. We also demonstrate that TAL1 inhibits both binding of E47S to αE3 and αE4 and endogenous transactivation of the TCRα enhancer. Comparison of the activity of the minimal [αE1-2] fragment, which contains no E-boxes, with the accessory [αE3-4] fragment, which contains two, suggested some contribution from the latter to TCRα enhancer activity in HPB-ALL. TCR [αE1-2] activity was partially (40%) inhibited by TAL1 but not at all by Id1. In contrast, [αE3-4] 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 TCRα enhancer E-boxes differs from their B lymphoid Igμ 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 TCRαβ lineage is not due to induction of TCRα enhancer activity by the TAL1 protein.