Cell cycle checkpoints are implemented to safeguard the genome, avoiding the accumulation of genetic errors1,2. Checkpoint loss results in genomic instability and contributes to the evolution of cancer. Among G1-, S-, G2- and M-phase checkpoints, genetic studies indicate the role of an intact S-phase checkpoint in maintaining genome integrity3,4. Although the basic framework of the S-phase checkpoint in multicellular organisms has been outlined, the mechanistic details remain to be elucidated. Human chromosome-11 band-q23 translocations disrupting the MLL gene lead to poor prognostic leukaemias5,6,7,8,9. Here we assign MLL as a novel effector in the mammalian S-phase checkpoint network and identify checkpoint dysfunction as an underlying mechanism of MLL leukaemias. MLL is phosphorylated at serine 516 by ATR in response to genotoxic stress in the S phase, which disrupts its interaction with, and hence its degradation by, the SCFSkp2 E3 ligase, leading to its accumulation. Stabilized MLL protein accumulates on chromatin, methylates histone H3 lysine 4 at late replication origins and inhibits the loading of CDC45 to delay DNA replication. Cells deficient in MLL showed radioresistant DNA synthesis and chromatid-type genomic abnormalities, indicative of S-phase checkpoint dysfunction. Reconstitution of Mll−/− (Mll also known as Mll1) mouse embryonic fibroblasts with wild-type but not S516A or ΔSET mutant MLL rescues the S-phase checkpoint defects. Moreover, murine myeloid progenitor cells carrying an Mll–CBP knock-in allele that mimics human t(11;16) leukaemia show a severe radioresistant DNA synthesis phenotype. MLL fusions function as dominant negative mutants that abrogate the ATR-mediated phosphorylation/stabilization of wild-type MLL on damage to DNA, and thus compromise the S-phase checkpoint. Together, our results identify MLL as a key constituent of the mammalian DNA damage response pathway and show that deregulation of the S-phase checkpoint incurred by MLL translocations probably contributes to the pathogenesis of human MLL leukaemias.
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We thank J. Y. Wang and Z. You for discussions during the inception and the completion of this study, respectively. H.L. is supported by the Scholar award of the American Society of Hematology. The Mll+/ex7(stop)CBP mice were provided by S. Armstrong and the late S. Korsmeyer. This study is supported by CA119008, the Scholar award of the American Society of Hematology, the Scholar award of the American Cancer Society, to J.J.-D.H., and CA129537/CA123232, to T.K.P.
This file contains a Supplementary Discussion, Supplementary Figures 1-13 with legends and additional references.
About this article
Pharmacology & Therapeutics (2018)