LOSS OF TAL-1 PROTEIN-ACTIVITY INDUCES PREMATURE APOPTOSIS OF JURKAT LEUKEMIC T-CELLS UPON MEDIUM DEPLETION

Transcriptional activation of the tal-1 gene occurs in similar to 30% of patients with T cell Acute Lymphoblastic Leukemia and is therefore likely to be involved in human T cell leukemogenesis. However, the TAL -1 protein functional properties involved in this process have not bee n assessed so far. We have derived a clonal subline of the Jurkat T ce ll line which produced solely a mutant truncated form of TAL-1 protein . Sequencing of genomic DNA and cDNAs showed that the only transcribed tal-1 allele of this mutant subline harbored a G nucleotide insertion at codon 270. The resulting frameshift modifies TAL-1 residues 272-27 8 and creates a stop at codon 279. Although the deletion of the 53 car boxy-terminal residues of the TAL-1 protein did not directly affect th e TAL-1 basic helix-loop-helix domain (residues 185-243), it had drast ic effects on TAL-1 functional properties, since the mutant subline ex hibited a dramatic decrease of protein binding activity to the TAL-1 D NA consensus sequence. Growth curves indicated that the mutant subline exhibited premature apoptosis upon medium depletion or serum reductio n when compared with the parental cells. However, no difference betwee n Jurkat and the mutant subline was observed in etoposide- or Fas/APO- 1-triggered apoptosis. Stable expression of the mutant TAL-1 protein i n Jurkat cells resulted in a phenotype that was similar to that of the mutant Jurkat subline, indicating that the TAL-1 mutant protein behav ed like a dominant negative mutant and that the premature apoptosis of the mutant subline upon medium depletion was the consequence of the l oss of TAL-1 protein activity.