HELIX-LOOP-HELIX (E2-5, HEB, TAL1 AND ID1) PROTEIN-INTERACTION WITH THE TCR-ALPHA-DELTA 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, w e 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 a lpha 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 modif y endogenously driven TCR alpha [alpha E1-4] activity in a TCR alpha b eta cell line. We also demonstrate that TAL1 inhibits both binding of E47S to alpha E3 and alpha E4 and endogenous transactivation of the TC R 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 l atter to TCR alpha enhancer activity in HPB-ALL, TCR [alpha E1-2] acti vity was partially (40%) inhibited by TAL1 but not at all by Id1, In c ontrast, [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 the ir B lymphoid Ig mu counterparts and suggest a novel mechanism of tran scriptional inhibition by TAL1, which may be, at least partly, indepen dent 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 alp ha enhancer activity by the TAL1 protein.