Journal Article

Phosphorylation of MLL by ATR is required for execution of mammalian S-phase checkpoint

Cheng, Emily H.-Y., Liu, Han, Kumar, Rakesh, Hsieh, James J.-D., Brown, Eric J., Pandita, Tej K., Takeda, Shugaku, Westergard, Todd D.

2010

Overview

Linking leukaemia and DNA repair The S-phase checkpoint in mitosis, activated by DNA damage, maintains genome stability by allowing the cell time to repair the damage before it progresses through the cell cycle. Liu et al . now report that the MLL (mixed lineage leukaemia) gene, frequently translocated in leukaemia, is part of the S-phase checkpoint. When DNA is damaged, MLL is phosphorylated by the checkpoint kinase ATR, causing it to accumulate on chromatin and to methylate histone H3 at lysine residue 4. This histone modification blocks activation of late replication origins. MLL translocations disrupt this pathway and promote genomic instability. Cell cycle checkpoints, such as the S-phase checkpoint, delay cell division to give the cell time to repair any damaged DNA. Here it is shown that the MLL gene — frequently disrupted in leukaemia — is part of the S-phase checkpoint. When DNA is damaged, MLL is phosphorylated by the ATR protein, causing MLL to accumulate on chromatin and methylate histone H3 on lysine 4. This delays DNA replication. MLL translocations, such as those that occur in leukaemia, disrupt this pathway and cause genomic instability. Cell cycle checkpoints are implemented to safeguard the genome, avoiding the accumulation of genetic errors 1 , 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 integrity 3 , 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 leukaemias 5 , 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 SCF Skp2 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.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 631/337/1427/2566 / 631/45/612/1229 / 631/80/641/2187 / 692/699/67/1990/283 / Alleles / Amino acids / Animals / Ataxia Telangiectasia Mutated Proteins / Biological and medical sciences / Cell cycle / Cell Cycle Proteins - metabolism / Cell cycle, cell proliferation / Cell Line / Cell physiology / Chromatin - metabolism / Deoxyribonucleic acid / Deregulation / DNA / DNA Damage / DNA Replication - physiology / Fundamental and applied biological sciences. Psychology / Gene expression / Genes, Dominant - genetics / Genomic Instability - physiology / Genotype & phenotype / Histone-Lysine N-Methyltransferase / Histones - chemistry / Histones - metabolism / Human genome / Humanities and Social Sciences / Humans / Kinases / letter / Leukemia / Leukemia - genetics / Lysine - metabolism / Mammals / Methylation / Mice / Molecular and cellular biology / multidisciplinary / Myeloid Progenitor Cells - metabolism / Myeloid-Lymphoid Leukemia Protein - chemistry / Myeloid-Lymphoid Leukemia Protein - deficiency / Myeloid-Lymphoid Leukemia Protein - genetics / Myeloid-Lymphoid Leukemia Protein - metabolism / Observations / Phosphorylation / Phosphoserine - metabolism / Physiological aspects / Properties / Protein Binding / Protein Serine-Threonine Kinases - metabolism / S Phase - physiology / S-Phase Kinase-Associated Proteins - metabolism / Science / Science (multidisciplinary) / Signal Transduction / Studies / Translocation, Genetic - genetics