DNA repair of myeloma plasma cells correlates with clinical outcome: the effect of the nonhomologous end-joining inhibitor SCR7

Abstract

Key Points• Responders to melphalan therapy are characterized by slower rates of NER and DSB/R mechanisms and higher apoptotic rates.• The DSB/R inhibitor SCR7 enhances cytotoxicity of melphalan against myeloma plasma cells.DNA repair activity of malignant cells seems to influence therapeutic outcome and patients' survival. Herein, we investigated the mechanistic basis for the link between DNA repair efficiency and response to antimyeloma therapy. Nucleotide excision repair (NER), interstrand cross-links repair (ICL/R), double-strand breaks repair (DSB/R), and chromatin structure were evaluated in multiple myeloma (MM) cell lines (melphalan-sensitive RPMI8226; melphalan-resistant LR5) and bone marrow plasma cells (BMPCs) from MM patients who responded (n 5 17) or did not respond (n 5 9) to subsequent melphalan therapy. The effect of DSB/R inhibition was also evaluated. Responders' BMPCs showed slower rates of NER and DSB/R (P < .0022), similar rates of ICL/R, and more condensed chromatin structure compared with nonresponders. Moreover, apoptosis rates of BMPCs were inversely correlated with individual DNA repair efficiency and were higher in responders' cells compared with those of nonresponders (P 5 .0011). Similarly, RPMI8226 cells showed slower rates of NER and DSB/R, comparable rates of ICL/R, more condensed chromatin structure, and higher sensitivity than LR5 cells. Interestingly, cotreatment of BMPCs or cell lines with DSB/R inhibitors significantly reduced the rates of DSB/R and increased melphalan sensitivity of the cells, with the nonhomologous end-joining inhibitor SCR7 showing the strongest effect. Together, responders' BMPCs are characterized by lower efficiencies of NER and DSB/R mechanisms, resulting in higher accumulation of the extremely cytotoxic ICLs and DSBs lesions, which in turn triggers the induction of the apoptotic pathway. Moreover, the enhancement of melphalan cytotoxicity by DSB/R inhibition offers a promising strategy toward improvement of existing antimyeloma regimens. (Blood. 2016;128(9):1214-1225

Autocyclized and oxidized forms ofSCR7 induce cancer cell death by inhibiting nonhomologousDNAend joining in a LigaseIVdependent manner

“…Importantly, SCR7-pyrazine also inhibited the joining of different 5 0 -5 0 noncompatible ends, which requires processing of DNA breaks prior to ligation, and is dependent on classical NHEJ proteins [29,31]. This was also consistent with the observed inhibition of NHEJ by SCR7 [17][18][19][46][47][48]. Besides, similar to SCR7-cyclized, SCR7-pyrazine also showed specific inhibition of joining catalyzed by Ligase IV and their effect on Ligase III, Ligase I, and T4 DNA ligase-mediated joining was minimal in vitro.…”

XRCC1‐mediated DNA repair is associated with progression‐free survival of multiple myeloma patients after autologous stem cell transplant

“…This is consistent with previous findings that rs25487 primarily impacts the DNA repair function of XRCC1 protein, not the mRNA and protein expression of XRCC1 . One laboratory has published PBMCs in MM patients metabolize melphalan similarly to BMPCs . Hence, it is likely that myeloma cells from patients carrying rs25487 AA/AG have impaired XRCC1‐mediated DNA repair activity and are more susceptible to melphalan‐induced DNA damage.…”

“…Like other DNA‐alkylating agents, melphalan induces DNA damage through DNA alkylation and DNA interstrand crosslinks (ICLs) in cells (Figure ). It is assumed that the ability of myeloma cells to excise melphalan monoadducts and ICLs is central to the sensitivity to melphalan therapy as myeloma cells with effective DNA repair capacity are more likely to survive after HDM treatment . It is known that the nucleotide excision repair pathway (NER) and the base excision repair pathway (BER) play pivotal roles in the repair of melphalan‐induced monoadducts, whereas the single‐ and double‐strand break repair pathways (SSBR and DSBR) are involved in the repair of melphalan‐induced interstrand crosslinks (ICLs) .…”

Polymorphism in ANRIL is associated with relapse in patients with multiple myeloma after autologous stem cell transplant

“…In addition, with regard to melphalan sensitivity, PBMC response to melphalan strongly correlates with bone marrow plasma cells (BMPCs) . Recently, it has been shown that responders to high dose melphalan‐based autologous stem cell transplantation have slower rates of excision repair and double‐strand breaks repair rates compared to non‐responders . Taken together, these findings suggest that transcriptional and post‐transcriptional regulation of p53 contributes to a patient's overall response to melphalan.…”

“…The p14ARF‐MDM2‐p53 axis is one of the primary pathways regulating p53 (Figure ) . The MDM2 oncoprotein specifically binds to the N‐terminal transactivation domain of p53 and represses its ability to activate transcription, thus down‐regulating p53‐mediated apoptosis .…”