Bortezomib-induced “BRCAness” sensitizes multiple myeloma cells to PARP inhibitors

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 ...

DNA repair pathways in human multiple myeloma

“…130,131 The decrease in FANCD2 and H2Ax ubiquination by PI is explained by the accumulation of poly-ubiquitinated proteins and, consequently, the decrease of the pool of available nuclear ubiquitin. 132 Bortezomib treatment may prevent DNA resection through inhibiting proteasomal degradation of proteins involved in chromatin relaxation, thus preventing recruitment of RPA onto ssDNA. 130,131 This effect of Bortezomib on blocking DNA repair could explain why a phase II study has shown that the combination of Bortezomib together with highdose melphalan before autologous stem cell transplantation could increase by 3-fold the complete remission rate (35% vs. 11%).…”

The future of autologous stem cell transplantation in myeloma

“…13, 46,47 Targeted therapy has already demonstrated some efficacy, but may merely select for resistant subclones, unless a given mutation drives the disease as recently has been described for KRAS/NRAS activating mutations. 48 High-risk myeloma even with modern genetic analysis will likely remain a formidable opponent in years to come.…”

Evidence for a role of the histone deacetylase SIRT6 in DNA damage response of multiple myeloma cells

“…9,10 In MM cells, direct evidence of homozygous loss or mutations in BRCA1/2 or other DDR genes is lacking, but increased DNA repair activity has been reported. 11,12 Thus, identification of adaptive pathways for coping with genomic instability in MM may similarly provide the framework for new therapeutic strategies. …”