Damage to DNA occurs due to external [ultraviolet (UV) and ionizing radiation, genotoxic drugs] or internal factors (oxidative stress, telomere erosion, replication fork collapse) [
13]. To overcome these alterations in the chemical structure of DNA, cells have developed a complex system of pathways, called DDR network that recognize and resolve the damage, thus protecting the integrity of the genome [
14]. DDR is triggered following the detection of a DNA lesion. Next, a signal transduction cascade is activated and results in the stimulation of sophisticated mechanisms for genome protection, including DNA repair pathways, cell cycle checkpoints and apoptosis. On the other hand, deregulated DDR may result in mutagenesis and genomic instability [
15]. Given that DDR regulates the cellular decision to remove the DNA damage or to activate apoptosis, it is involved in the onset and progression of several diseases, including cancer, as well as in the response to therapeutic interventions.
2.1. The DDR Network in the Onset and Progression of MM
Previous studies have shown that the DNA repair mechanisms are altered in MM, thus allowing the onset and progression of the disease as well as inducing therapy resistance (
Figure 1). In fact, deregulation in the Base Excision Repair (BER) pathway plays an important role in MM progression. For example, Liao and colleagues reported that two major BER-related apurinic/apyrimidinic nucleases (APEX1 and APEX2) crosstalk with p73, a transcriptional regulator of RAD51, and results in its transcriptional upregulation, thus increasing the efficiency of Homologous Recombination (HR) and driving genomic instability in MM [
16]. Moreover, a study of polymorphisms of BER-associated genes correlates alterations in APEX1 with a reduction in MM patient’s overall survival [
17]. In patients’ samples, both APEX1 and APEX2 gene expressions are increased during myelomagenesis [
18]. Also, researchers found that high expressions of certain BER genes, such as MPG (N-methylpurine DNA glycosylase) and PARP3 [Poly (ADP-ribose) polymerase 3], are linked to improved overall survival in MM patients who received autologous stem cell transplantation. On the other hand, increased expressions of PARP1 and POLD2 (DNA polymerase delta subunit 2) are associated with worse outcomes in MM, suggesting that targeting BER pathway might improve treatment effectiveness [
19,
20,
21].
Moreover, the gene expression patterns in normal plasma cells and newly diagnosed MM samples reveal that upregulation of the Nucleotide Excision Repair (NER) protein ERCC3 (excision repair cross-complementation group 3) is linked to poorer survival. Additionally, researchers have identified 34 NER-related genes with differential expression in MM plasma cells, along with 23 genes with copy-number variations [
22]. Interestingly, polymorphisms of NER have been shown to impact treatment outcomes in MM patients undergoing autologous bone marrow transplantation [
23].
It is known that defective Mismatch Repair (MMR) mechanism results in increased mutation rates, particularly in microsatellite DNA regions. This defect, known as microsatellite instability, was observed in many MM patients and becomes more common as the disease progresses and during relapse [
24]. Alterations in MMR genes (hMSH2, hMLH1 and hPMS1) have been identified in malignant disorders of B-cell and were associated with aggressive behavior [
25,
26]. Defective MMR is also implicated in drug resistance [
27,
28].
The homologous recombination repair (HR/R) mechanism removes DSBs that are formed following therapeutic treatment with several anti-myeloma drugs, such as topoisomerase inhibitors and DNA crosslinking agents. Previous studies have shown elevated expression of HR/R-associated genes, namely RAD50 and RAD51, and increased HR/R activity in MM cell lines and primary MM cells compared with normal plasma cells [
29,
30]. Since HR/R plays an important role in the recovery of the stalled replication fork and the repair of interstrand cross-links (ICLs), it is of particular importance in drug resistance of the fraction of proliferating MM cells. Indeed, previous studies have shown that following treatment of MM patients with high-dose melphalan (HDM) and autologous stem cell transplantation (ASCT) higher expressions of BRCA1, PRKDC (DNA-PK) and PARP1 genes are linked to poorer outcomes [
31]. Moreover, genetic variations in PARP, RAD51, MUTYH, OGG1, PCNA, TPMT and XPC are associated with disease progression [
32].
Studies in mice have highlighted the crucial roles of core proteins involved in Non-Homologous End Joining (NHEJ) repair mechanism in preserving genomic stability [
33]. In some MM cell lines, such as RPMI-8226, NHEJ activity appears to be compromised, while remains functional in others, including U266 and OPM2 [
34]. A study on MM patients treated with thalidomide also revealed that those with specific gene polymorphisms in ERCC1, ERCC5 or XRCC5 (KU80) had higher response rates, with longer overall survival being associated with polymorphisms in ERCC1 and XRCC5. Polymorphisms or abnormal expression of genes such as XRCC4, XRCC6 (KU70), DCLRE1C/Artemis and DNA ligase IV (LIG4) have also been linked to MM risk [
30,
35,
36]. Indeed, increased levels of DCLRE1C/Artemis, DNA–PKcs and XRCC4 proteins were observed in MM cells, while elevated expressions of XRCC5 and DCLRE1C/Artemis genes were linked to poorer prognosis in MM patients [
37]. Previous reports have also shown that NSD2 (Nuclear Receptor Binding SET Domain Protein 2), a factor with many biological functions, including DNA repair, plays a crucial role in MM relapse and treatment resistance [
38]. In line with these data, loss of NSD2 function reduces the expression of DNA repair genes like RAD51, TP53BP1 and XRCC4 and enhances DNA damage accumulation. On the other hand, overexpression of NSD2 increases DNA repair efficiency, which may contribute to drug resistance, particularly in t(4;14) MM cases [
39]. Alternative NHEJ (alt-NHEJ) is a DNA repair pathway, vital for genomic instability and the survival of MM cells. Higher gene expression of LIG3 (component of alt-NHEJ; also involved in NER and BER) in MM patients is linked to shorter survival, especially in advanced disease stages. LIG3 protein levels are elevated in bortezomib-resistant compared to bortezomib-sensitive MM cells; knocking down LIG3 increases DNA damage and inhibits MM cell growth both
in vitro and
in vivo [
40].
Fanconi anemia (FA) is a rare chromosomal instability syndrome, which has been linked to pathogenic variations in at least 22 genes that make up the FA pathway. Interestingly, FA patients’ cells are very sensitive to ICL-inducing drugs, suggesting that FA pathway is implicated in the repair of ICLs [
41]. In line with these data, melphalan-resistant myeloma cells express high levels of FANCF (FA Complementation Group F) and RAD51C; depletion of FANCF helps overcome resistance [
42].
Gene expression analyses of MM patients treated with HDM and ASCT have revealed the prognostic significance of genes involved in several DNA repair pathways, including NHEJ, HR/R, FA, NER, MMR and BER [
43]. Among 84 examined genes, 22 were found to have prognostic value for both event-free and overall survival. These genes included five related to NHEJ [three with negative (NSD2, RIF1, XRCC5/KU80) and two with positive prognostic value (PNKP and POLL)], six to HR/R [five with negative (EXO1, BLM, RPA3, RAD51, MRE11) and one with positive prognostic value (ATM)], three related to FA [all with negative prognostic value (RMI1, FANCI and FANCA)], eight to NER [six with negative (PCNA, RPA3, LIG3, POLD3, ERCC4, POLD1) and two with positive prognostic value (ERCC1, ERCC5)], two involved in MMR [both with negative prognostic value (EXO1 and MSH2)] and one in BER with negative prognostic value (LIG3).
2.2. The DDR Network in the Outcome of Anti-Myeloma Therapy
Extensive observations suggest that the DDR network is implicated in the outcome of genotoxic therapy. Indeed, studies have shown that
in vitro resistance to the nitrogen mustard melphalan [
44], is linked to increased efficiency of DNA repair mechanisms, including ICL repair [
45] and FA/BRCA pathway [
42]. In order to elucidate the role of DDR in the outcome of melphalan-treated patients, previous studies reported the formation and repair of DNA damage in peripheral blood mononuclear cells (PBMCs) and bone marrow plasma cells (BMPCs) following
in vivo (therapeutic) or
ex vivo melphalan treatment [
46,
47,
48,
49]. The authors reported that ΜΜ patients, responders to melphalan therapy, are characterized by lower DNA repair capacity and higher accumulation of melphalan-induced DNA damage than non-responders, suggesting that quantification of drug-induced DNA damage formation/repair may help in the selection of patients who may profit from melphalan therapy. Interestingly, they reported that DSB repair (DSB/R) inhibitors, such as the NHEJ inhibitor SCR7, significantly enhanced the cytotoxicity of melphalan against malignant plasma cells, suggesting a promising strategy for the treatment of MM [
48].
DSB/R inhibitors are not the only DDR modifiers used in MM therapy. Indeed, previous studies have shown that the combined treatment with inhibitors of ATM (KU-55933) and ATR (VE-821) seriously reduced survival of MM cell lines that exhibited high levels of endogenous DNA damage [
50]. Also, PIM-2, a serine/threonine kinase that interacts with DDR and plays a critical role in promoting cell survival and preventing apoptosis, is commonly found upregulated in MM [
51,
52]. Another study has shown that LT-171-861, a synthetic new PIM-2 inhibitor, induced DNA damage by inhibiting HR/R pathway, activated apoptosis in MM cells and suppressed tumor growth in MM xenograft models [
53]. Moreover, the PARP inhibitor olaparib could amplify the anticancer effect of LT-171-861 by reducing the growth of tumors in MM xenografted nude mice.
Panobinostat is an HDACi, which blocks cell cycle progression, induces apoptosome formation and down-regulates the anti-apoptotic Bcl-2 gene [
54]. Panobinostat showed synergistic anti-MM effects when combined with genotoxic drugs
in vitro [
55]. Indeed, in a recent report the authors studied the biological effects of the
ex vivo co-treatment of panobinostat and melphalan in BMPCs and PBMCs from MM patients [
56]. They found that this combination treatment reduced the efficiency of critical DNA repair mechanisms (NER and DSB/R), augmented the accumulation of cytotoxic DSB lesions and induced apoptosis in BMPCs, but not in PBMCs from the same patients or healthy controls. These data suggest that, compared with melphalan alone, the combined treatment of melphalan and panobinostat showed increased anti-myeloma efficacy and lower side effects.