Promyelocytic leukemia activates Chk2 by mediating Chk2 autophosphorylation

Chk2 is a kinase critical for DNA damage- induced apoptosis and is considered a tumor suppressor. Chk2 is essential for p53 transcriptional and apoptotic activities. Although mutations of p53 are present in more than half of all tumors, mutations of Chk2 in cancers are rare, suggesting that Chk2 may be inactivated by unknown alternative mechanisms. Here we elucidate one such alternative mechanism regulated by PML (promyelocytic leukemia) that is involved in acute promyelocytic leukemia (APL). Although p53 inactivating mutations are extremely rare in APL, t(15; 17) chromosomal translocation which fuses retinoic acid receptor (RAR alpha) to PML is almost always present in APL, while the other PML allele is intact. We demonstrate that PML interacts with Chk2 and activates Chk2 by mediating its autophosphorylation step, an essential step for Chk2 activity that occurs after phosphorylation by the upstream kinase ATM (ataxia telangiectasia- mutated). PML/ RAR alpha in APL suppresses Chk2 by dominantly inhibiting the autophosphorylation step, but inactivation of PML/ RAR alpha with alltrans retinoic acid (ATRA) restores Chk2 autophosphorylation and activity. Thus, by fusing PML with RAR alpha, the APL cells appear to have achieved functional suppression of Chk2 compromising the Chk2- p53 apoptotic pathway.