No abstract

Chromosomal instability is a defining feature of clonal myeloma plasma cells that results in the perpetual accumulation of genomic aberrations. In addition to its role in protein homeostasis, the ubiquitinproteasome system is also involved in the regulation of DNA damage-repair proteins. In the present study, we show that proteasome inhibition induces a "BRCAness" state in myeloma cells (MM), with depletion of their nuclear pool of ubiquitin and abrogation of H2AX polyubiquitylation, an essential step for the recruit- IntroductionGenomic integrity is continuously challenged by both exogenous and endogenous stressors. 1 To counteract DNA damage, cells have evolved repair mechanisms specific for many types of lesions. [2][3][4][5][6] Single-strand DNA breaks (SSBs) are repaired through the nucleotide excision repair or the base excision repair machinery, which require the activation of poly-ADP-ribose polymerase (PARP). PARP1, and to a lesser extent PARP2, bind DNA SSBs and catalyze the synthesis and addition of large chains of poly-ADP-ribose (PAR) polymers on target proteins, including the histones H1 and H2B and PARP1 itself. These polymers serve to recruit variable proteins needed to activate DNA-damage repair (DDR). [7][8][9] If persistent or left unrepaired, SSBs encountered by replication forks lead to the formation of potentially lethal double-strand DNA breaks (DSBs). These genomic DSBs encountered in the S/G 2 phases are predominantly repaired by the homologous recombination (HR) pathway, in which the MRN (MRE11-RAD50-NBS1) complex senses the DSBs and initiates a dynamic protein recruitment to DNA-repair foci. 10,11 MRN first recruits the ATM kinase to the vicinity of the lesions, with resulting ATM-mediated phosphorylation of the histone variant H2AX that leads to the accumulation of the MDC1 protein and its binding partners. These include the MRN complex and RNF8 and RNF168, 2 ubiquitin ligases that initiate histone H2AX Lys63 mono-and polyubiquitylation at sites of DNA damage. This histone ubiquitylation allows for a second wave of protein accumulation, including factors such as 53BP1 and the BRCA1 A complex that are critically important for DSB repair and for the maintenance of genomic integrity. [12][13][14] Deregulation of the DDR machinery fuels the genomic instability needed to drive cancer-cell development and clonal evolution. Recognition of these deregulated DDR pathways has led to the discovery of novel therapeutics that result in synthetic lethality in transformed cells. Recent studies have demonstrated the efficacy of targeting PARP1 in tumors with impaired HR resulting from the homozygous loss of the BRCA1 or BRCA2 genes. [15][16][17] Furthermore, genetic screens have identified a host of HR-related genes (including RAD51, ATR, and PCNA) that upon deletion or silencing render cells hypersensitive to PARP inhibitors. 18 Therefore, tumor cells with any HR deficiency or "BRCAness" are likely to be particularly sensitive to PARP inhibitors because they are unable to cope effectively with the ...

Poly-ADP ribose polymerase (PARP) inhibitors have shown promise in the treatment of human malignancies characterized by deficiencies in the DNA damage repair proteins BRCA1 and BRCA2 and preclinical studies have demonstrated the potential effectiveness of PARP inhibitors in targeting ataxia-telangiectasia mutated (ATM)-deficient tumours. Here, we show that mantle cell lymphoma (MCL) cells deficient in both ATM and p53 are more sensitive to the PARP inhibitor olaparib than cells lacking ATM function alone. In ATM-deficient MCL cells, olaparib induced DNA-PK-dependent phosphorylation and stabilization of p53 as well as expression of p53-responsive cell cycle checkpoint regulators, and inhibition of DNA-PK reduced the toxicity of olaparib in ATM-deficient MCL cells. Thus, both DNA-PK and p53 regulate the response of ATM-deficient MCL cells to olaparib. In addition, small molecule inhibition of both ATM and PARP was cytotoxic in normal human fibroblasts with disruption of p53, implying that the combination of ATM and PARP inhibitors may have utility in targeting p53-deficient malignancies.

Integrin-␤7 (ITGB7) mRNA is detected in multiple myeloma (MM) cells and its presence is correlated with