Multiple Myeloma (MM) is an incurable hematological malignancy developing as a result of clonal proliferation of plasma cells originating from post–germinal-center B cells (1). Age-adjusted rates of new cases and deaths, covering the period from 2011–2015, show that new myeloma cases incidence was 6.6 per 100,000 people per year, while the number of deaths was 3.3 per 100,000 people per year (2). MM accounts for ~10% of all hematologic malignancies (3), and mostly affects elderly people, with 69 years being the median age at diagnosis (2).

One of the distinguished characteristics of MM is the clonal progression of the disease, which is reflected through precursor stages, named monoclonal gammopathy of undetermined significance (MGUS), and smoldering multiple myeloma (SMM) (4–6). Genomic events occurring across the course of the disease can be divided into primary and secondary genomic events, which altogether form a unique genomic landscape of the disease. Primary events are further divided into hyperdiploid (HRD) and non-HRD subtypes, which are mutually exclusive. Primary HRD events are usually trisomies of odd-numbered chromosomes 3, 5, 7, 9, 11, 15, 19, and/or 21 (7). Primary non-HRD events include translocations of the immunoglobulin (Ig) heavy chains (IGH), with five most frequently occurring translocations being t(11;14) (15%), t(4;14) (12%), t(14;16) (3%), t(14;20) (2%), and t(6;14) (1%), as well as del13q, which is the most frequent deletion in MM (59%) (8). All these primary events are usually detected at the MGUS stage of the disease development. Most frequent copy number gains and losses found in MM are del13q (45%), 1q+ (40%), del14q (39%), del16q (35%), del6q (33%), del1p (30%), and del8p (25%), all representing secondary mutation events, that can be also seen as driver events (9, 10).

TP53 gene is located at the chromosome 17p13.1, coding for p53 tumor suppressor protein, which is also known as guardian of the genome. The whole p53 signaling network is turned off under the normal physiological conditions, when p53 itself is expressed at low levels. In response to cellular stress, such as hypoxia, DNA damage, heat shock, p53 pathway becomes activated by means of protein stabilization through posttranslational mechanisms, which include phosphorylation and acetylation. The resulting accumulation of p53 protein in the nucleus triggers activation of various downstream pathways, which work in cooperation to keep the genomic integrity and homeostasis of the cell. This is achieved through several tumor suppressive mechanisms, including cell cycle arrest, apoptosis, and angiogenesis inhibition (11, 12).

In general, mutations in TP53 have been found in 50% of all human cancers (13). In MM, the frequency of TP53 alterations—by means of mutations and deletions—are more frequent in late stages of the disease and are associated with treatment resistance (7, 14). In this review we focus on the role of deregulated p53 in the progression of MM and the latest the therapeutic approaches designed to target specifically this tumor suppressor.

Ten years after the initial discovery of p53 in 1979 as a host cell protein bound to T antigen in SV40-transformed mouse cells (15), it was finally established that this 53 kDa protein performs its role as a tumor suppressor in cell culture, contrary to the popular belief that it functions as an oncogene (16). Immediately after this discovery inactivating TP53 mutations were confirmed to be a common event in the colorectal cancer tumorigenesis (17), and mutations occurring within this tumor suppressor were detected as a distinctive feature of Li-Fraumeni syndrome (18).

The p53 protein is structurally organized into several domains that are crucial for maintaining several of its functions. Two N-terminal transactivation domains, TAD1, and TAD2 respectively, are followed by a conserved proline-rich domain, which plays a role in DNA repair in response to γ-radiation (19). Positioned centrally, DNA binding domain is responsible for the site-specific DNA-binding function of p53 and represents a hot spot for the most of tumor-derived missense mutations (20). Within the C-terminal domain are located sequences necessary for nuclear localization and non-specific DNA binding, as well as an oligomerization domain mediating the formation of homo- and hetero-tetramers (21) (Figure 1).

Since p53 is a transcription factor that can sense cellular stress and gets activated in response to the DNA damage and oncogene activation, during homeostasis its levels are kept low through interaction with E3 ubiquitin-protein ligase MDM2. MDM2 works on p53 inhibition in two ways, by interacting with its transactivation domain and by targeting p53 for proteasomal degradation, conferring a very short half-life of p53 in the range from 5 to 30 min (22, 23). Moreover, MDM2 expression is regulated by p53, meaning that low levels of tumor suppressor are maintained via negative feedback loop in normal physiological conditions. Depending on the nature of the cellular stress, mechanisms of activation of p53 can be different—DNA damage results in inhibition of MDM2-mediated degradation of p53 (24), whereas oncogenic signaling activates the ARF tumor suppressor, which also prevents MDM2 from degrading p53 (25).

During the response to oncogenic stress, ATM/ATR, CHK1, and CHK2 kinases phosphorylate p53, disrupting its binding to MDM2. At the same time, CBP and KAT5 acetyl-transferases induces acetylation of the tumor suppressor, specifically at lysine 120 (K120) within the DNA-binding domain (24, 26, 27).

Once activated, p53 induces either cell-cycle arrest or apoptosis depending on the cellular context, by transactivation of its downstream target genes, such as CDKN1A (coding for p21 protein), resulting in cell cycle arrest and senescence, and BAX, PUMA, and NOXA, triggering apoptosis (23). Recently it was reported that p53 is also involved in metabolism regulation by decreasing expression of the cystine/glutamate antiporter XCT2 (transcribed from SLC7A11 gene) and upregulating expression of glutaminase 2 (GLS2 gene) (28, 29).

Although the clinical outcome of patients with MM was significantly improved with the introduction of novel therapeutic strategies such as new-generation proteasome inhibitors, immunomodulatory drugs, anti-CD38 antibodies, and more recently CAR-T cells, MM remains incurable.

Patients with a 17p13 deletion demonstrated significantly lower response rate to lenalidomide treatment compared to patients not bearing this abnormality (60). Moreover, combinational treatment with lenalidomide, Adriamycin, and dexamethasone showed similar results in relapsed patients with 17p13 deletion (61). These results emphasize the need for novel therapeutic approaches that would overcome the 17p13 deletion related resistance to conventional treatment.

Two main approaches in reestablishing normal function of p53 in MM and in cancer in general are based inhibiting the interaction of the protein with its negative regulators (such as MDM2 and MDM4), and on restoring the function of protein product of the mutated TP53 gene (Figure 2).

Nutlin was detected as the first potential inhibitor of p53-MDM2 interaction, as it was found that prevents binding of p53 to MDM2, which results in stabilization, accumulation, and activation of p53 signaling in cancer cells. However, nutlin works only in cells with preserved p53 signaling pathway and wt p53 (62). Nutlin-3 was showed to have significant activity in MM in experiments performed in vitro and ex vivo, as well as in synergy with drugs such as melphalan, etoposide, and bortezomib (63, 64), however the drug induces resistance and clonal selection.

Another compound called RITA (Reactivation of p53 and Induction of Tumor Cell Apoptosis) binds directly to the N-terminal domain of p53, which reduces its affinity for MDM2 as a consequence of conformational changes in the structure of p53 protein (65), although later study questioned this mechanism of action (66). It appears that RITA sensitivity correlates with induction of DNA damage as resistant cells show increased DNA cross-link repair. Inhibition of FancD2 restores RITA sensitivity (67). In MM cells, which acquired resistance to other therapeutic approaches, RITA induced cell cycle arrest and apoptosis (68). However, when the efficacies of nutlin-3a and RITA were compared in a panel of HMCLs with different TP53 statuses, it was found that, unlike nutlin-3a which exhibited toxicity only in HMCLs with wt-TP53, RITA killed 25% of HMCLs independently of the TP53 status, and showed efficacy independently of the presence or absence of 17p deletion in primary patient samples, implying that RITA could be a therapeutic approach of choice in patients with TP53 abnormalities, who showed resistance to current therapies (69).

When overexpressed in MM cells, MDM2 maintains low levels of p53, thus inhibiting its tumor suppressive functions (70, 71). Recently, a compound CMLD010509, which is a synthetic analog of the rocaglate, was found to inhibit the oncogenic translation program in MM cells and in mouse models of MM, targeting MDM2 oncoprotein among other targets (72). This can be explained by the fact that rocaglates are inhibitors of translation initiation factors and that MDM2 has a short half-life, suggesting that these proteins are preferentially affected in case of low translational flux.

Another approach to interact with MDM2 is to inhibit the ubiquitin-specific protease 7 (USP7), which is as a deubiquitinase removing ubiquitin specifically from MDM2. The inhibition of UPS7 results in the degradation of MDM2 and leads to re-activation of p53 (73). The small molecule FT671 is a potent USP7 inhibitor with a potent effect in vitro and in vivo in various cancers.

In was demonstrated that TP53-deficient cells are highly sensitive to the inhibition of Ataxia-Telangiectasia Rad3-related (ATR), one of the kinases responsible for DNA damage response (DDR) (74). Likewise, MM cells lines bearing TP53 mutation showed better response to ATR inhibition compared to TP53 wild-type MM cell lines, implying that, in the subset of MM clones with increased replicative stress, and DNA damage, inhibition of ATR could be potentially exploited as a synthetic lethal therapeutic approach (75).

PRIMA-1 (p53 reactivation and induction of massive apoptosis) has been introduced as therapeutic agent with aim of restoring the original function of mutant p53, through obtaining the wt conformation of the protein (76). The more effective derivative, PRIMA-1^MET, except restoring mutant p53, is also acting in a TP53-independent manner by inducing reactive oxygen species (ROS) in cancer cells, with a good efficacy in MM (77–81). In fact, the accumulation of mutant-p53 protein suppresses the expression of SLC7A11, a component of the cystine/glutamate antiporter, through binding to the master antioxidant transcription factor NRF2. This diminishes glutathione synthesis, rendering mutant-p53 tumors susceptible to oxidative damage (82).

Other therapeutic approaches have been recently investigated (83, 84). A novel compound PK11007, belonging to the class of selective thiol alkylators 2-sulfonylpyrimidines, was found to alkylate surface-exposed cysteines 182 and 277 of p53, which results in stabilization of its DNA binding domain in vitro. This compound is particularly effective in cancer cell lines with null or mutant p53 background (83). An innovative approach consists of functional screening of phage display libraries for peptides, which carry the potential for reactivation of the mutant p53 (84).

TP53 alterations are of adverse prognosis in MM as in cancer in general. Recent publications of next generation sequencing on large cohort of patients with MM have enabled to better define the incidence of TP53 alteration across the course of the disease. In MM, TP53 is deregulated through different molecular mechanism, such as deletion17p, TP53 mutations, MDM2 overexpression, methylation of TP53 promoter, and deregulation of specific miRNAs. The knowledge of these molecular mechanisms of TP53 pathway alteration has helped the development of therapeutic strategies to target this pathway. Different approaches are currently tested: blocking the interaction between MDM2 and p53 with small molecules such as Nutlin or RITA, targeting MDM2 by inhibiting translation initiation with rocaglate or by blocking its deubiquitination by USP7 inhibitors, and restoring the function of altered p53 proteins. All these therapeutic strategies are currently being tested and need to be further validated for clinical use. Considering that TP53 alterations increase during progression of MM and induce drug resistance, it will be necessary to interact with TP53 pathway to be able to cure MM, or at least develop strategies that do not select TP53 subclones.

KJ, GE, JD, AW, ZV, MF, PC, TF, BQ, and SM wrote the manuscript, generated the figures, and revised the review.