Inhibition of cellular differentiation by the SCL/tal oncoprotein: transcriptional repression by an Id-like mechanism.

In cases of T-cell acute lymphoblastic leukemia (T-ALL), the basic helix-loop-helix (bHLH) oncogene SCL/tal undergoes frequent rearrangements activating ectopic expression. Despite the compelling epidemiological association of SCL/tal expression with T-ALL, no specific transforming function has been attributable to the protein product. However, investigators have recently demonstrated that forced overexpression of SCL/tal can block monocytic differentiation of M1 murine myeloid leukemia cells. Thus, inappropriate expression of wild-type SCL/tal protein may in part account for the maturation arrest phenotype observed in T-ALL cells. In this study, ectopic expression of the SCL/tal gene blocked the differentiation of C2C12 muscle precursor cells. Characterization of the mechanism of differentiation blockade showed that the SCL/tal protein repressed transcriptional activation by the myogenic bHLH factor MyoD. Protein interaction analysis showed that SCL/tal and MyoD compete for common partners (E bHLH proteins) but do not directly bind one other. A model is thus proposed in which ectopic SCL/tal protein, by its ability to titrate out E proteins, prevents the formation of bHLH complexes that drive cellular differentiation: the "Id-like" mechanism.