RAPAMYCIN RESISTANCE IN ATAXIA-TELANGIECTASIA

The gene mutated in the human genetic disorder ataxia-telangiectasia ( A-T) has been described recently (Savitsky et al., 1995a) and the comp lete coding sequence of this gene, ATM, has been reported (Savitsky et al., 1995b). The derived amino acid sequence demonstrates significant homologies to several proteins containing a phosphatidylinositol 3-ki nase (PI3-kinase) domain, including the yeast TOR proteins and the hum an protein FRAP. Since the TOR and FRAP proteins are targets for the i mmunosuppressive drug rapamycin, we have investigated the effects of t his compound on A-T cells. We report here that 3 A-T cell lines are mo re resistant than control cells to rapamycin's growth inhibiting effec ts but were more sensitive to the PI3-kinase inhibitor wortmannin. As expected rapamycin (1 nM) inhibited the rate of exit of control cells from G(1) phase but failed to perturb the progression of A-T cells. Th is difference in cell cycle progress after rapamycin treatment is refl ected in ribosomal S6 protein kinase (p70(S6K)) by both a downward mob ility shift on SDS-PAGE and inhibition of activity. Furthermore, the G , phase cyclin-dependent kinase, cyclin E-cdk2, was rapidly inhibited in control cells post-treatment, whereas in A-T cells it took consider ably longer to observe inhibition. There was no evidence that a GST-FK BP12 fusion protein specifically precipitated the ATM protein in the p resence of rapamycin in either cell type. These results demonstrate th at the ATM protein is not a direct target for rapamycin but its functi onal loss renders cells more resistant to this compound.