miRNAs and Multiple Myeloma: Focus on the Pathogenesis,Prognosis,and Drug Resistance

Abstract

Multiple myeloma (MM) produces clonal plasma cells and aberrant monoclonal antibody accumulation in patients’ bone marrow (BM). Around 1% of all cancers and 13% of hematological malignancies are caused by MM, making it one of the most common types of cancer. Diagnostic and therapeutic methods for managing MM are currently undergoing extensive research. MicroRNAs (miRNAs) are short noncoding RNAs that reduce or inhibit the translation of their target mRNA after transcription. Because miRNAs play an influential role in how myeloma develops, resources, and becomes resistant to drugs, miRNA signatures may be used to diagnose, do prognosis, and treat the myeloma response. Consequently, researchers have investigated the levels of miRNA in plasma cells from MM patients and developed tools to test whether they directly impacted tumor growth. This review discusses the latest discoveries in miRNA science and their role in the development of MM. We also emphasize the potential applications of miRNAs to diagnose, prognosticate, and treat MM in the future.

Introduction

An abnormal accumulation of monoclonal antibodies and clonal plasma cells are the hallmarks of multiple myeloma (MM), a clonal B-cell malignancy.^1,2 Worldwide, it is among the most common types of hematological cancer. ³ MM is responsible for around 1% of all cancers and 13% of hematological malignancies.^4,5 MM, a neoplastic plasma cell disease of the bone marrow (BM), is characterized by anemia, myelosuppression, bone degeneration, kidney function problems, and other symptoms. ⁶ Nearly always, monoclonal gammopathy of unclear significance (MGUS), an asymptomatic premalignant stage, comes before MM.^7–11 MGUS and MM are related conditions. However, unlike myeloma, M-protein levels in MGUS remain stable, and individuals may not experience the clinical symptoms associated with MM. The risk of developing MM from MGUS is estimated at 1% per year. ¹² African Americans have a higher frequency of MGUS, which may contribute to a rise in the incidence of MM in this community.^13–15 There is no known cause of MGUS nor a good indicator of when MGUS will turn into myeloma.^12,16 Smoldering MM (SMM), a disease stage between MGUS and MM, develops in certain people. SMM patients are asymptomatic regarding end-organ damage caused by myeloma, with an M protein level of more than 30 g/L and greater than 10% clonal PC in the bone marrow. Ten percent of SMM patients acquire MM during the first five years of diagnosis; after that, the risk of progression falls. ¹⁷

The exact causes and risk factors contributing to the development of MM remain to be fully elucidated. Nevertheless, existing studies have identified several significant factors that play key roles in the pathogenesis of this disease. For instance, obesity, positive family history, and radiation have been proposed as key risk factors. In addition, the role of genetic mutations should not be ignored.^18–20 About 90% of individuals diagnosed with MM are estimated to exhibit genetic abnormalities within their plasma cells. Remarkably, these genetic alterations diverge as the disease advances into a malignant state. Some of these defects and cytogenetic variations have been reported to play a role in the progression of MGUS/SMM to MM. ²¹ In this regard, deletion mutations such as del (13), del (17p), and del (1p) can be examples. Plus, there are some other mutations like t(14;20) (q32;q11), t(4;14) (p16;q32), t(11;14) (q13;q32), isolated monosomy 13, monosomy 14, and gain(1q) mutations. ²² Significantly, these genetic alterations have a noteworthy influence on the survival rate of patients and their overall prognosis. Moreover, they can result in MM cells becoming resistant to current chemotherapy drugs. ²²

Proteasome inhibitors, immunomodulators, and monoclonal antibodies are effective therapeutics with tolerable side effects that have improved MM treatment options and patient survival. ²³ Nevertheless, MM is still incurable, and despite receiving those, as mentioned earlier, new agent-based therapy, practically all MM patients still, have relapses. ³

miRNAs and Cancer

What Are miRNAs

Endogenous noncoding RNAs called microRNAs (miRNAs) are short, 19–22-nucleotide RNAs. Recent studies have shown that miRNAs regulate gene expression primarily at the transcriptional level by recruiting the RNA-induced silencing complex to target mRNAs via their 3′ untranslated regions, suppressing or encouraging DNA translation.^24–26 Single miRNA influences several hundred mRNAs. In addition, over half of the genes that code for proteins in cells are regulated by miRNAs, according to recently published research.^27,28 There are two categories of miRNAs, tumor suppressor and oncogenic miRNAs, which highly depend on the environment in which they are expressed. ²⁹ Specifically, the first group is characterized by miRNAs that are downregulated in cancer cells and typically inhibit cancer progression by suppressing proto-oncogene expression. In contrast, the second is characterized by miRNAs that are upregulated in malignant cells and inhibit tumor suppressor genes, promoting their development. ³⁰

miRNAs’ Role in MM Research

miRNA signatures could serve as a biomarker for myeloma diagnosis, prognosis, and treatment response. This function is due to miRNAs playing an influential role in how MM develops, recurs, and becomes drug resistant.^31,32 Several cancers, such as MM, have been linked to deregulated miRNAs as tumor suppressors or oncogenic regulators that modulate progression and onset. Many processes regulate miRNA expression in MM, including epigenetics (acetylation and methylation, for example).^33–35

Numerous miRNA expression anomalies connected to developing and treating malignancies, including MM, have recently been discovered. ³⁶ Al Masriet et al provided the first proof of miRNA's role in MM's pathogenesis. Compared to healthy cells, MM cell lines and patient samples expressed significantly lower levels of miR-125b, miR-133a, miR-1, miR-124a, miR-15, and miR-16. ³⁷ For instance, miR-15 and miR-16 expression may be lost or reduced in MM patients with completely deleted or one copy of the 13q14 region in their genome, which is associated with carcinogenesis and the onset of MM.^31,32 Additionally, it has been discovered that miR-15a/16 blocks the expression of cyclin D1, cyclin D2, and CDC25A, targets the apoptosis inhibitor BCL-2, and inhibits the AKT3 and NF-kB pathways, which are regarded to be critical drivers of MM proliferation. ³⁸ Additionally, it was shown that miR-15a and miR-16-1 expression was negatively linked with BCL-2 and cyclin E expression and considerably lower in MM plasma cells compared with noncancerous and MGUS plasma cells. ³⁹

It has been noted that MM plasma cells express more miR-21 than normal control cells do. Several studies have revealed that this miRNA has an oncogenic potential: in MM cells, ectopic miR-21 expression reduces apoptosis without an existing IL-6 stimulation, while miR-21 suppression inhibits MM cell proliferation. IL-6 increases miR-21 expression via STAT3 signaling.^40–43 miR-92a, a well-known hypoxia-regulated miRNA, has been connected to the C-Jun pathway, which has been demonstrated to have a role in the progression of MM. ⁴⁴ Furthermore, laboratory studies have shown that miR-29b induces apoptosis in cholangiocarcinoma cells and decreases the expression of antiapoptotic protein MCL1. These results have been observed across various MM cell lines, providing consistent evidence. ⁴⁵

A growing body of data suggests that MM formation and progression depend critically on communication between MM cells and components of the bone marrow microenvironment (BMM). A change in miRNA expression of MM-BMSCs (bone marrow stromal cells) or exosomes leads to changes in the expression of miRNA-related targets such as adhesion molecules, signaling molecules, DNA methyltransferase, and cytokines.^46–52 According to Shen et al, miR-202 negatively modulates BAFF, thereby preventing MM cells from surviving, growing, and adhering to BMM. BMSCs up-expressing miR-202 made MM cells more susceptible to bortezomib. ⁴⁷ In 2005, Masri A. and colleagues initially found that malignant cells in both clinical plasma samples of affected patients and human myeloma cell lines (HMCL) displayed substantial changes in miRNA expression patterns compared to those of unaffected individuals. ³⁷ Gutierrez et al looked into the relationships between miRNA and their associated target genes and discovered that the downregulation of many miRNAs caused the overexpression of cyclin D2 (CCND2) in MM. These scientists showed that the miRNA expression profile in MM was first connected to genetic anomalies. ⁵³ When compared to information from BM samples of MM patients, Yyusnita et al found that upregulation of miR-449, miR-181a, and miR-181b and downregulation of miR-16 and let-7c showed similar expression patterns in peripheral blood. ⁵⁴ Nonetheless, multiple miRNAs, including miR-21, miR-181a/b, and miR-106b-25 cluster, have been identified as oncogenes and are suggested to be involved in the transformation of MGUS plasma cells into the malignant state of MM.^40,55 Further evidence that this miRNA cluster was linked to carcinogenesis and a poor prognosis in MM came from higher miR-17-92 cluster-expressed individuals who experienced shorter progression-free survival (PFS) than patients with low-level expression. ⁵⁶

An exosome containing miR-135b produced from hypoxia-resistant MM (HR-MM) cells may directly suppress the endothelial production of factor-inhibiting hypoxia-inducible factor 1 (FIH-1), according to a recent study. ⁴⁸

The Pathogenesis of Multiple Myeloma and miRNA Role

Pathogenesis of MM

Differentiated B cells convert to plasma cells, which secrete immunoglobulins and play a vital part in humoral immunity. Errors throughout physiological B-cell development may result in primary genetic events, including hyperdiploidy and translocations. Symptomatic MM occurs when a second wave is added to the first event (eg, oncogenic pathway mutation, tumor suppressor loss, epigenetic alterations, and BM microenvironment alterations).^57–59

Most translocations involve immunoglobulin H (IgH), immunoglobulin lambda (IgL), and the IgK locus.^20,60,61 Cyclin D (CCND) dysregulation and IgH-NSD2 are the most common consequences of IgH translocations. ⁶² NSD2 is essential for transformation. ⁶³ It can play a role in the change in proliferation rate, cellular adhesion, and tumorigenicity. ⁶⁴ CCND2 (12;14) expression enhances MM cell adherence to BMSCs and integrin B7. ⁶⁵ When the original clone fully differentiates into a long-lived plasma cell, late oncogenic processes are expected to occur in the BM. ⁶⁶

Crosstalking cells and BMM are well recognized as crucial stages in the pathogenesis of MM. ⁶⁷ The microenvironment comprises cellular (eg, bone BMSC, endothelial cells, mesenchymal stem cells (MSCs), and immune cells) and soluble factors. The studies show a significant difference between BMM composition in MM patients and healthy individuals. ²¹

Examples of cytokines that mediate cell proliferation, survival, migration, and medication resistance development in the BMM of MM patients include IL6, CXCL12, IGF1, and VEGFA. Besides the above factors, sick cells can secrete cytokines like TNF-α, TGF-β, and VEGF, which result in a positive feedback loop.^68,69 This condition can cause a variety of diseases called plasma cell dyscrasias. MM is the most common plasma cell disorder and accounts for almost 10% of hematologic malignancies.^57–59 MM malignant cells reproduce in the BM and produce monoclonal immunoglobulins, chemokines, and cytokines, followed by varied clinical symptoms.^57,70

miRNAs and Tumor Suppressor Genes

miRNAs, as small noncoding RNAs, modulate gene expression, protein synthesis, and cellular signaling patterns by affecting messenger RNA (mRNA).^30,71–73 Studies estimate that about 3-4% of our genes encode miRNA. ⁷⁴ As we write this article, more than 38 000 miRNAs have been discovered. ⁷⁵ It is shown that one miRNA species may have an impact on several targets. In contrast, several miRNAs can cooperate to target a specific mRNA and provide the according expression. ⁷⁴

Previous studies showed that miRNAs can play pivotal roles in the pathogenesis and prevention of different human pathologies. Intentional modification of specific miRNAs is a unique method for developing novel therapeutic procedures, especially in cancers.^73,76 Some miRNAs may act as oncogenes (oncomiRNAs and metastamiRNAs) or tumor suppressors. Many miRNAs are located in the genome's cancer-related sites, which witness their crucial part in cancer pathogenesis.^30,73 As a result, miRNAs may be used as diagnostic, prognostic, and therapeutic biomarkers for cancer.

As we discussed, tumor suppressor miRNAs show oncosuppressor characteristics by targeting mRNAs involved in coding oncoproteins. Oncosuppressor miRNAs are often downregulated in cancerous cells. ⁷³ If tumoral activity is detected in cells, these miRNAs can start pathways to cell cycle delay, apoptosis induction, and growth suppression. ⁷⁷

They work with some known genome guardians (p53, p73, and p63), tumor, and metastasis suppressors to protect the cell cycle from going out of control. More precisely, genome guardians increase the production of miRNAs that target tumor genes (eg, let-7, miR-15/16a, miR-26, miR-29, miR-30, miR-34, miR-145, and miR-146a), tumor suppressor genes (eg, CDKN1a/b/c, CDKN2a/b/c/d, PTEN, and RBs), and metastasis suppressors (eg, CycG2, Raf kinase inhibitory protein, and DEC2) following the detection of a defect in the cell cycle to inhibit tumor growth, angiogenesis, invasion, metastasis, and proliferation of cancer stem cells. ⁷⁷

miRNAs and Oncogenes

Cancer is defined by uncontrolled cell proliferation or survival. However, protective mechanisms have been developed to ensure cell division, differentiation, and death adhere to a well-balanced sequence. Cells may switch on or off many regulators to put things in order. ⁷⁸

miRNAs can also play roles as oncomiRNAs and are involved in the onset and progression of human cancers. There has been evidence that some miRNAs are associated with metastatic features of tumors. These miRNAs can act as metastasis promoters or tumor suppressor inhibitors.^73,79,80

miRNAs’ Role in the Pathogenesis of MM

As we explained before, myelomagenesis consists of multiple steps. MM pathogenesis is initiated by a premalignant disorder called MGUS. Patients often show no symptoms at this stage, and the chance of malignancy is 1% per year. ⁸¹ The next step that typically follows MGUS is SMM, which results in an accumulation of 10-60% plasma cells in the BM. The risk of MM is about 10% per year in these patients Although there is considerable information about these phases, no factor has yet been identified as a promising biomarker to predict malignant transformation.^82,83

Nowadays, we know that miRNA deregulation is related to tumorigenesis and disease development as well as plasma cell differentiation and myelomagenesis. ⁸³ miRNAs, in conjunction with other factors, work as a network to cause or prevent cancers. This fact makes them a valuable subject to investigate for cancer therapies. Despite many advances in the treatment of MM (eg, monoclonal antibodies, immunomodulators, and proteasome inhibitors) and improvements in raising the life quality and outcomes of the disease, MM remains untreatable and nearly all of the patients will experience a relapse even while receiving novel agent-based therapies, because of its noticeable heterogenic nature. ³⁶

Efforts have been made during the last 2-3 decades to find the most effective way to approach MM with methods based on miRNA therapy. The miRNA expression pattern in MM patients shows significant differences compared with healthy subjects.^36,84 miRNAs are significant regulators of BMM, immune system, methylation, drug resistance, and genomic instability, which are some factors involved in the pathogenesis of MM (Table 1). ³⁶

Table 1.

Summary of the MicroRNAs Widely Validated and Universal for Their Role in Multiple Myeloma.

miRNAs	Targets	Expression	Function	Ref.
miR-15a miR-16	Cyclin D1, D2, CDC25A, Cyclin E, BCL-2	Downregulated	MM proliferation	38,39
miR-29b	MCL1	Downregulated	Inducing apoptosis	45
miR-202	BAFF	Downregulated	MM cell lines survival	47
miR-135b	FIH-1	Overexpressed	Angiogenesis	85
miR-181a miR-181b, miR-106b	PCAF	Downregulated	Control P53 activity	40
miR-15a miR-16a	AKT kinase, MAPK, ribosomal protein s6, MAP3K, IP3	Downregulated	Oncosuppressor	38,82
miR-4268	TLR4	downregulated	Myocardial injury in MM	86
miR-221 miR-222	ATG12, P27, mTOR	overexpressed	Resistance to dexamethasone	87
miR-155	CD47	overexpressed	Resistance to dexamethasone	88
miR-615-3p	PRKCD	downregulated	Sensitivity to bortezomib	85
miR-21	PTEN, BTG2, Rho-B	overexpressed	Induces resistance to MM cell apoptosis triggered by dexamethasone, doxorubicin or bortezomib	89

AKT kinase, protein kinase B; ATG12, autophagy-related 12; BAFF, B cell activating factor; BCL-2, B-cell lymphoma 2; BTG2, BTG antiproliferation factor 2; CDC25A, cell division cycle 25A; CD47, cluster of differentiation 47; FIH-1, factor inhibiting HIF-1; IP3, inositol triphosphate; MAPK, mitogen-activated protein kinases; MCL1, myeloid cell leukemia sequence 1; miR, microRNA; mTOR, mammalian target of rapamycin; PCAF, P300/CBP-associated factor; PRKCD, protein kinase C delta; PTEN, phosphatase and tensin homolog; Rho-B, Ras homolog gene family, member B; TLR4, Toll-like receptor 4.

Nowadays, we know that not only are cells involved in cancer progression but the microenvironment around cells also has undeniable roles in oncogenesis, progression, metastasis, and recurrence of cancer. ⁹⁰ The microenvironment and cells interact bidirectionally in both standard and tumor growth conditions. This communication undeniably impacts disease development and prognosis.^90,91

Previous studies examined miRNA expression profiles in plasma cells of healthy subjects, MGUS patients, and MM patients. They demonstrated an elevation of miR181-a, miR181-b, miR106b-25 cluster (esp. miR-93.25 and miR-106b), and miR-21 in MGUS. Moreover, the increasing of miR-21, miR-18a, and miR106b-25 clusters in MM, both compared with healthy subjects (Figure 1), suggesting that this pattern of miRNA expressions plays an essential role in transitions of the abnormal plasma cells to MGUS and MM.^40,83

Figure 1.

Role of miRs in the pathogenesis of MM. The number of plasma cells with different levels of miRs increases in multiple myeloma. The levels of miR-106b-25, miR-181a, miR-181b, miR-21, miR-Let7b, and miR-Let7a increase, while the levels of miR-15a and miR-16 decrease. Because of increasing the level of PCAF, MYC, AKTK, IP3, and MAPK and decreasing the level of BIM and P53, cells’ apoptosis decreases. miR, micro-RNA; PCAF, P300/CBP-associated factor; AKTK, AKT kinases; IP3, inositol trisphosphate 3; MAPK, mitogen-activated protein kinase; BIM, B cell lymphoma-2-like 11; P53, tumor protein 53.

In vitro, experiments showed overexpression of miRNA-181a and miRNA-181b, probably leading plasma cells to MM by controlling the cell's life cycle. Another suggestive mechanism is the alteration of the p53 pathway. According to studies, p53 is significantly deregulated in MM. P300/CBP-associated factor (PCAF), which is favorably involved in p53 regulation through an acetylation mechanism. It targets three miRNAs, including miRNA-181a, miRNA-181b, and miRNA-106b. ⁴⁰

Another mechanism is suppressing tumor suppressants (esp. PTEN, BIM, and P21) which induce apoptosis and inhibit cell survival in cancerous cells. The miRNA-21 and miRNA-106b-25 clusters may be involved in suppression. ⁹² MicroRNA (miRNA) expression in myeloma development was the subject of another investigation. They compared miRNA expression levels in MGUS, SMM, and MM patients with healthy donors and found that some deregulations cause plasma cell dyscrasia, eg, decreased expression of some miRNA groups (let-7i and miRNA-15a, -16, and -106b) and (let-7a, let-7b, let-7i, and miRNA-15a, -15b, -16, -106b, and -20a) was associated with MGUS and SMM. Interestingly, they found some miRNAs (miR-21, miR-223, and miR-361) that were lowered in MM but not in MGUS or SMM, which suggests later events in the evolution of the disease. ⁸²

We also know that the let-7a and let-7b miRNAs regulate the expression of the MYC oncogene. ⁸² The expression of MYC, a transcription factor with various effects on cellular activities (eg, cell cycle, apoptosis, hematopoiesis, and DNA damage response), is regulated at several levels due to its multifunctionality. ⁹³ As a primary regulator of MM, MYC plays a role in the transformation of MGUS into MM. ⁹⁴ Another study evaluated the effect of miRNAs on some known oncosuppressors. AKT kinase, the MAP kinases (MAPK), the ribosomal protein S6, and MAP3K-inositol trisphosphate (IP3) are oncogenes suppressed by miR-15a and miR-16. Several studies show that MGUS, SMM, and MM patients have considerably lower levels of these miRNAs in their blood than the general population.^38,82 Furthermore, evidence shows global suppression of miRNAs during myeloma genesis due to DNA hypermethylation in the miRNA regions. ⁹⁵ This evidence confirms the epigenetic regulatory role of miRNAs as a driver of myelomagenesis.

The Involvement of Organs in MM and the Role of miRNAs

Common Organs Involved in MM

The most commonly affected organs in MM are the bones, kidneys, and blood. However, other organs, such as the liver, heart, peripheral nerves, soft tissues, and intestines, can also be involved.^96,97

The findings demonstrate a relationship between miRNA expression levels and clinical characteristics of myeloma. ⁹⁸ In tissues and organs, light chains of M protein and polysaccharide aggregation damage the functions of the corresponding organs, resulting in hypercalcemia, bone abnormalities, cardiovascular complications, anemia, and renal failure. ⁹⁹

The Role of miRNAs in Organ Failure

Bone and Bone Marrow Disease

The main consequence of MM progression is osteolytic bone damage. ¹⁰⁰ The interface between MM cells and the BMM is essential to cell survival, proliferation, differentiation, apoptosis, and treatment resistance. Extracellular matrix cells, known as osteoclasts, can increase MM cell viability and prevent cell death. ¹⁰¹ miRNAs are crucial regulatory molecules that control the transcription of genes related to bone homeostasis by inhibiting transcription or cleaving mRNA. ¹⁰² Many miRNAs impede osteogenesis by directly or indirectly targeting the osteogenic components. ¹⁰³

Many studies have demonstrated how dysregulated miRNAs affect osteoblast differentiation. For instance, miR-21 stimulates MSCs to differentiate into osteoblasts by controlling the PI3K/β-catenin pathway. ¹⁰⁴ It has been observed that MM-BMSCs express higher levels of several miRNAs relative to their normal cells. These miRNAs include miR-223, miR-16, miR-519d, and miR-485-5p. ¹⁰⁵ Also, studies have revealed that a variety of miRNAs, including miR-133, miR-141, and miR-34a, regulate important transcription elements and markers of osteogenesis involved in osteoblast signaling to either promote or inhibit the differentiation of osteoblasts (Figure 2).^106–110 Based on miR29b's pro-OBL and anti-MM functions, Rossi et al found that this miRNA gradually declines throughout osteoclast (OCL) development. ¹¹¹

Figure 2.

The role of miRs in MM involves vital organs. Some miRNAs, such as miR-223, miR-16, miR-519d, and miR-485-5p, were found to be much higher in MM-BMSCs than in regular BMSCs. In addition, studies have revealed that a variety of miRNAs, including miR-133, miR-141, and miR-34a, regulate key transcription elements and osteogenic markers in the osteoblast signaling cascade to either promote or inhibit the differentiation of osteoblasts. Moreover, the study demonstrated a significant reduction in the exosomal levels of let-7c-5p, let-7d-5p, miRNA-140-3p, miRNA-185-5p, and miRNA-425-5p in MM patients. Moreover, the study demonstrated a significant reduction in the exosomal levels of let-7c-5p, let-7d-5p, miRNA-140-3p, miRNA-185-5p, and miRNA-425-5p in MM patients. BMSCs, bone marrow stromal cells; miR, microRNA; MM, multiple myeloma.

Fan et al found that miR-221-5p expression levels in myeloma bone disease (MBD)-MSCs were considerably lower than in normal MSCs. Additionally, they discovered that miR-221-5p knockdown may encourage MBD-MSCs to differentiate into osteoblasts. This process was demonstrated by the elevated levels of expression of ALP, OPN, and OC mRNAs; these are typical indicators of osteoblast development. Also, osteogenic differentiation markers’ expression was suppressed by miR-221-5p overexpression. These findings indicated that miR-221-5p is a negative regulator in MBD-MSCs’ osteogenic development. ¹¹²

Heart Failure

Anemia or cardiac amyloidosis can lead to severe cardiomyopathy and heart failure resulting from MM comorbidities. ¹¹³ Heart involvement usually has no apparent clinical signs at first. If the problem progresses, restrictive cardiomyopathy, heart failure, and irregular cardiac rhythms will all emerge. ¹¹⁴ The upregulation of circ-G042080, which sponges miRNA-4268 and blocks its function on TLR4, may contribute to myocardial injury. Among the findings of this pathway, decreased miR-4268 is related to decreased H9C2 cell numbers and increased autophagia. ⁸⁶

Renal Failure

MM itself or other diseases could have a multifaceted role in renal failure. ¹¹⁵ According to the IMWG criteria in 2003, having a blood creatinine concentration that is 40% higher than the usual upper limit is clinically indicative of urinary insufficiency in patients with MM (approximately >2 mg/dL). ¹¹⁶ Compared to healthy adults in both primary and MM cell lines, miR-29c expression is low. Renal failure and β2-MG have an antagonistic relationship with miR-29c expression. ¹⁰¹ Exosomes of MM patients contained significantly lower amounts of let-7c-5p, let-7d-5p, miRNA-140-3p, miRNA-185-5p, and miRNA-425-5p than those of healthy people (Figure 2). Exosomal miRNA concentrations were linked to clinical feature-related indicators, including IL-6, β-CTx, 2-microglobulin, and creatinine in serum. ⁹⁸

MM Prognosis and the Role of miRNAs

Various criteria have been used to determine prognosis aspects, with the Revised International Staging System (R-ISS) method being the most often utilized prognostic variable in recently diagnosed MM. The relationship between MM and miRNAs has been considered. As human gene regulators, miRNAs are involved in various biological processes. ¹¹⁷ Some studies have tried to explain the prognostic role of miRNAs in hematological malignancies, including MM.^118–120 We summarize the widely validated and universally relevant miRNAs in multiple myeloma progression in Table 1.

Che et al pointed out that increased expression of miR-27 was associated with poor prognosis in MM patients. It may be because miR-27 is an oncogene in MM since it negatively regulates SPRY2 in this process. ¹²¹ According to a study conducted by Han et al, there are five specific miRNAs (miR-4484, miR-663b, miR-3687, miR-670-5p, and miR-4417) that have a strong correlation with the overall survival or progression-free survival of patients with MM. ¹²² Furthermore, a study by Pula et al investigated the potential of selected miRNAs (hsa-miR-409-3p and hsa-miR-328-3p) in the serum of MM patients treated with bortezomib-based regimens to predict early mortality. The study found that the expression of selected miRNAs in the serum of MM patients could predict early mortality. ¹²³ Another study by Rossi et al found that event-free survival is better in MM patients with low miRNA-455 expression. ¹²⁴ Chen et al illustrated those patients with low expression of miRNA-153, -296, -490, -500, -642, and -744 have superior event-free survival (EFS). Although, high expression of miRNA-17, -20, -92, -92a, -373, -410, -548d, -554, -886, -5p, and -888 led to poor prognosis. ³⁶

The findings would have been more helpful if they had used miRNA as prognostic biomarkers in clinical trials. One source of weakness in this is the low volume of research in this area likely influenced the findings that cannot be trusted. Overall, miRNAs play an essential role in MM prognosis and may be potential biomarkers for predicting early mortality and survival. However, more research is needed to fully understand the role of miRNAs in MM prognosis and develop effective miRNA-based therapies.

miRNAs as Biomarkers

miRNAs are present in cells and various body fluids, including peripheral blood, urine, and tumor specimens. Recent research has shown that miRNAs can be potential biomarkers for various diseases, including MM.

miRNAs in Peripheral Blood

Peripheral blood is a readily accessible source of miRNAs that can be used for diagnostic purposes. Unlike invasive tissue biopsies, which may not always be feasible, peripheral blood samples can be easily obtained from patients. Several studies have shown that miRNAs are present in peripheral blood and can be biomarkers for various diseases.^125,126 For example, in a study by Xiang et al, researchers investigated the expression levels of miRNAs in the peripheral blood of patients with MM. They found that specific miRNAs were differentially expressed in MM patients compared to healthy controls, suggesting their potential as diagnostic biomarkers. ¹²⁵

miRNAs in Urine and Tumor Specimens

While peripheral blood is a valuable source of miRNAs, other body fluids, such as urine and tumor specimens, also contain miRNAs that can serve as biomarkers. Urine is an easily accessible and noninvasive sample that can be collected repeatedly, making it suitable for monitoring disease progression or treatment response. Condrat et al identified specific miRNAs significantly dysregulated in bladder cancer patients compared to healthy individuals, suggesting their potential as noninvasive biomarkers for early detection and monitoring of bladder cancer. ¹²⁷ Tumor specimens, such as biopsies or surgical resections, are another valuable source of miRNAs. By analyzing miRNA expression patterns in tumor specimens, researchers can gain insights into the molecular characteristics of tumors and identify potential therapeutic targets. Handa et al found that specific miRNAs (miR-15a, miR-16, miR-21, and miR-221) were dysregulated in MM patients and functioned as oncogenes, promoting cancer progression. ¹²⁸ These findings suggest that miRNAs in tumor specimens can serve as diagnostic biomarkers and provide valuable insights into the underlying mechanisms of tumorigenesis.

MM Chemoresistance and miRNA

Chemotherapy and Chemoresistance

Therapeutic agents used in the management of MM have changed in the last decade. Today, the most common chemotherapy drugs are immunomodulators, including pomalidomide, thalidomide, and lenalidomide. They are also proteasome inhibitors, including ixazomib, bortezomib, carfilzomib, monoclonal antibodies, and histone deacetylase inhibitors.^57,129–131

Chemotherapy regimens vary according to the patient's condition. The standard regimen for young patients is a bortezomib, lenalidomide, and dexamethasone (VRd) combination. ¹³² In elderly patients who are transplant ineligible, the combination of bortezomib, melphalan, prednisone, and daratumumab (Dara-VMP) has better efficacy. ¹³³ Resistance to chemotherapy agents is problematic and reduces the drug's therapeutic effect. MM drug resistance debilitates overall survival leading to patient death. ¹³⁴

The Potential Role of miRNAs

miRNAs have a varied range of potential roles in MM. They are potential players in the treatment of MM, and there are two strategies for this purpose: the replacement approach and the miRNA inhibition approach. ¹³⁵ miRNAs may also cause improved tumor markers for MM. ⁹⁸ They can be diagnostic biomarkers for this type of cancer and classify it. miRNAs can contribute to evaluating the prognosis of this disease.^{118,136–138}

miRNA-Induced Chemoresistance

Resistance to chemotherapeutic agents is a high-risk condition, and factors that affect this problem are essential. miRNAs can affect this issue. For instance, miR-221/222 overexpression reduces autophagy by affecting autophagy-related gene 12 (ATG12) and p27kip (p27)-mammalian target of rapamycin (mTOR) pathway, which results in resistance to Dexamethasone. ⁸⁷ Another miRNA contributing to drug resistance is miR-129-5p. The related mechanism is circ_0007841 which increases resistance to bortezomib by sponging miR-129-5p. ¹³⁹ Also, miR-155 plays a role in resistance to dexamethasone by targeting the CD47/TNFAIP8 axis. ⁸⁸ Examples of miRNAs that contribute to chemoresistance include but are not limited to the ones mentioned before. The variety of role players in this mechanism is quite broad.

miRNA-Induced Chemosensitivity

miRNAs affect chemosensitivity in different ways and by different miRNAs. For example, miRNA-221/222 levels have an inverse correlation with dexamethasone sensitivity. ⁸⁷ Also, circITCH increases sensitivity to bortezomib by affecting the miR-615-3p/PRKCD axis. ⁸⁵ Another study suggests that overexpression of miRNA-1252-5p alleviates heparinase expression, and increased numbers of this miRNA cause more sensitivity to bortezomib (Bz) in MM cells. ¹⁴⁰ Another study demonstrates that OTX015 causes lower expression of miR-205, leading to more sensitivity in MM cells to lenalidomide. ¹⁴¹ Other related miRNAs in chemosensitivity are miR-16-5p, miR-15a-5p, miR-20a-5p, and miR-17-5p, which affect sensitivity to Bz, thalidomide, and lenalidomide. ¹⁴²

Conclusion

In this paper, we discuss miRNAs’ effect on managing MM's pathogenesis, prognosis, and drug resistance. As miRNAs play critical roles in MM's initiation, progression, recurrence, and medication resistance, miRNA profiles may be employed as biomarkers for these aspects of the disease. miRNA has been linked to the etiology of several malignancies, including MM, where they have been hypothesized to act as tumor suppressors or oncogenic regulators. In writing this evaluation, we aimed to help medical professionals diagnose and treat MM and associated organ involvement more effectively.