Identification of the potential risk factors for monoclonal gammopathy of undetermined significance of progression

Abstract

Backgrounds

Monoclonal gammopathy of undetermined significance (MGUS) is a premalignant plasma cell disorder. The etiology of MGUS progression remains unclear and is a current topic of investigation.

Methods

This review summarizes the essential features of MGUS and the potential risk factors for MGUS of progression.

Results

Many clinical studies have been conducted to identify the critical risk markers that play important roles in progression. Some clinical variables, such as immunophenotypic markers and cytogenetic changes, have been recognized as potential risk factors.

Conclusions

In this review, we discuss novel insights from recent studies of potential risk factors, and we propose future directions for clinical management and additional studies.

Keywords:

• Monoclonal gammopathy of undetermined significance
• Risk model
• Management

MGUS is a premalignant plasma cell (PC) disorder. The incidence rate is more than 3% of the general white population older than age 50. It is characterized by monoclonal immunoglobulin that is detectable in the patient's serum and/or urine as well as clonal PCs present in the bone marrow (BM).¹ According to one clinical investigation, individuals diagnosed with monoclonal gammopathy of undetermined significance (MGUS) have an ∼1% annual risk of progression to multiple myeloma (MM) or related malignancy.² Some reports state that almost all myeloma evolves from MGUS.³ Many clinical studies have been conducted to identify critical risk markers that play important roles in progression.⁴ There is a growing body of evidence showing that the risk of progression of MGUS into symptomatic MM can be predicted by using markers that evaluate the extent of tumor burden (e.g. M protein, marrow plasmacytosis, focal lesions by magnetic resonance imaging (MRI) or circulating PCs, and markers related to the level of clonal reflected by the balance between normal and clonal PCs (e.g. serum-free light or heavy light chains, immune paresis, and clonal excess by multiparameter flow cytometry (MFC)), or the possible mechanism responsible for the transition of MGUS to MM such as cytogenetic changes analyzed by gene expression profiling. However, the etiology of MGUS progression is a current topic of investigation which remains unclear. In this review, we discuss novel insights from recent studies on the possible risk factors, and we propose future directions for clinical management and additional studies.

Laboratory variables

According to the diagnostic criteria proposed by the International Myeloma Working Group (IMWG) in 2010, the diagnosis of MGUS must meet the following criteria: (i) serum monoclonal protein under 3 g/dl; (ii) clonal BM PCs under 10%; and (iii) the absence of end-organ damage, such as hypercalcemia, renal insufficiency, anemia, and bone lesions that maybe attributed to PC proliferative disorder.⁵ On the basis of different transition states and clinical characteristics, MGUS was divided into three distinct clinical subtypes: non-IgM (IgG, IgA, IgE, and IgD) MGUS, IgM MGUS, and light-chain MGUS.⁵ Most of the non-IgM MGUS will progress to smoldering MM (SMM) and, eventually, to MM. On the other hand, IgM MGUS is mostly predisposed to Waldenström's macroglobulinemia or non-Hodgkin's lymphomas, chronic lymphocytic leukemia, and primary amyloidosis, while rarely progressing to IgM MM. ‘Light chain MGUS’ represents the premalignant precursor of ‘light-chain MM,’ which accounts for almost 20% of all new MM cases .These subgroups have provided the rationale for researchers to investigate the nature of each MGUS subtype, and clinicians to identify the possible risk factors for the progression of MGUS to MM as laboratory variables.

There is a large-scale clinical investigation conducted by the Mayo Clinic indicate that M-protein subtype, serum M-protein concentration, and abnormal serum-free light-chain (FLC) ratio are the risk factors associated with the progression from MGUS to MM.⁵ After 20 years of follow-up, the results showed that variables such as non-IgG isotype, M-protein concentration over 1.5 g/dl, and an abnormal serum FLC ratio (normal reference 0.26–1.65) are the risk factors for progression from MGUS to MM. Among 1148 patients with MGUS, the absolute risk of progression is 5% for patients with zero risk factors, 21% for patients with one risk factor, 37% for patients with two risk factors, and 58% for patients with three risk factors. Thus, patients with M protein under 1.5 g/dl, with an IgG isotype, and with a normal FLC ratio are ‘low-risk MGUS’ patients. ‘Intermediate-/high-risk MGUS’ patients refer to patients with M proteins above 1.5 g/dl, with an IgA or IgM isotype, or with an abnormal FLC ratio (Table 1). Sigillary, the recent study conducted by the Mayo Clinic has found that the suppression of uninvolved immunoglobulins defined by heavy/light chain (HLC) pair suppression is a risk factor for progression of MGUS.⁶ Isotype-specific suppression of the uninvolved HLC (HLC pair suppression) was defined as a concentration of the polyclonal member of the HLC pair below the lower limit of the reference range concentration (e.g. in a MGUS patient with a monoclonal IgGκ protein, suppression of IgGλ below the lower limit of the reference range for IgGλ). They have examined 999 of the MGUS included in the former study and found that HLC pair suppression is a risk factor for the progression of MGUS. The effect of adding HLC pair suppression to their previous risk assessment model is meaningful, except for the lowest risk group. The inclusion of HLC pair suppression further divided the groups into lower and higher risk. Based on these data, the novel modified risk stratification model was developed using the four variables of M-spike concentration, FLC ratio, heavy-chain isotype, and HLC pair suppression. The model has five groups (zero, one, two, three, or four adverse risk factors), and the probability of progression to MM increases with the number of risk factors (Table 1).

Table 1. Mayo clinic for MGUS patients and the update^5,6

Molecular markers

In recent years, MFC immunophenotyping has demonstrated to be of great value in the differential diagnosis of B-cell lymphoproliferative disorders.⁷ Not only for baseline prognostication of patients with (non-IgM) MGUS and MM, but also useful in the diagnosis of Waldenstrom's macroglobulinemia (WM).^8,9 Immunophenotyping has also shown to provide accurate assessment of the expression of multiple markers and their fluorescence intensity in thousands of PC/tube, specific information being provided on individual PC. It allows (i) simultaneous analysis of multiple parameters on a single-cell basis, (ii) the study of high numbers of cells within a relatively short period of time, (iii) storage of information about individual cells for latter analyses, (iv) quantitative evaluation of antigen expression, and (v) combined detection of surface and intracellular antigens.¹⁰ Phenotypically aberrant PC (aPC) typically show under-expression of CD19, CD27, CD38, and CD45, over-expression of CD28, CD33, and CD56, and asynchronous expression of CD20, CD117, and surface immunoglobulins (sIg). Immunophenotypic panels containing simultaneous stainings for at least four markers in multicolor combinations are typically required for the identification and characterization of PC.

A Spanish study group has used MFC to identify aPC populations.⁸ Perez-Persona et al.⁸ studied 407 MGUS patients and identified the aPCs as CD38^dim and CD56⁺. These authors also observed a lack of CD19, and/or the absence of CD45 expression, which was obviously different from normal BM PCs expressing CD138^bright and CD38^bright. In addition, they established MGUS risk factors for progression, including anaPC/BMPC > 95% and DNA aneuploidy. Progression-free survival at 5 years for the MGUS patients with zero, one, and two risk factors was 2, 10, and 46% (P < 0.001), respectively⁸ (Table 2).

Table 2. Risk stratification model for MGUS using Spanish (PETHEMA) model⁸

Carulli et al.¹¹ used a six-color mode to identify abnormal plasma. They considered immunophenotypes such as CD19⁻, CD27⁻, over-expression of CD117, CD56, CD28, and asynchronous expression of CD20 as abnormal PCs. They also proposed that when abnormal PCs were ≥3.1% of the global PC population, MM could be predicted; when aPCs were ≤3%, MGUS could be predicted. Notably, the sensitivity and specificity achieved was 0.983 and 0.92.¹¹

Paiva et al.¹² investigated the potential utility of a MFC immunophenotyping computerized algorithm based on the simultaneous assessment of the tumor burden and the degree of clonality of the BM PC compartment to predict for the risk of malignant transformation of MGUS and SMM. They first differentiated the clonal PCs from normal PCs (NPC) which were identified on the basis of strong CD38 expression and intermediate side scatter signals. Clonal PCs were discriminated from NPCs by aberrant phenotypic expressions, typically consisting of simultaneous downregulation of CD19 and CD45, with or without the over-expression of CD56.¹³ The relative frequency of BM PCs plus the percentage of clonal and normal PCs within the whole BM PC compartment were defined as three variables and were shown by the automatic population separator (principal component 1 versus principal component 2) graphical representation of the Infinicyt software. The MFC immunophenotyping computerized algorithm could predict the transformation of MGUS.

Cytogenetic alterations

Translocations involving the immunoglobulin heavy-chain locus or hyperdiploidy are involved in the patients with the MGUS and may be stable for many years. Additional genetic events, which contribute towards disease progression leading to the invasive stages of the disease, have been identified continuously. There is significant interest in defining these driver mutations that push the malignant clone towards more invasive behavior, as they constitute good therapeutic targets. Combined with new methods, including metaphase karyotype, fluorescent in situ hybridization, comparative genomic hybridization, high-density single nucleotide polymorphism (SNP), and whole genome sequencing, various chromosomal or molecular heterogeneities in MGUS have been identified as potential risk factors for the progression to MM.

The main approach to identify these variables are to compare the genetics of samples taken from different individuals who have MGUS, SMM, or MM. Lopez-Corral et al.¹⁴ undertook a genomic comparison between the PCs from 20 cases of MGUS, 20 cases of SMM, and 34 cases of MM with a high-density 6.0 SNP array. They observed an increase in the incidence of copy number abnormalities (CNA) in MM in contrast to MGUS (P < 0.006). Insertions on chromosomes 1q, 3p, 6p, 9p, 11q, 19p, 19q, and 21q along with deletions on 1p, 16q, and 22q, were significantly less frequent in MGUS than in MM. The study not only identified new chromosomal regions involved in CNA but also cytogenetic risk factors for progression. Moreover, the gain or loss of chromosome 1q or 1p also seems to correlate with disease progression from myeloma precursor disease of MGUS and SMM. Landgren and Staudt¹⁵ reported that MYD88 L265P expression in IgM MGUS might be a precursor to WM when they determined that nine IgM MGUS patients, who, coupled with an IgM spike and MYD88 L265P expression in malignant cells, had both clonal B-cells and PCs in their BM. MYD88 L265P is a commonly recurring mutation in patients with WM and may be an etiology factor for WM. Xu et al.¹⁶ suggested that IgM MGUS with MYD88 L265P expression is a risk factor for progression to WM through the use of allele-specific polymerase chain reaction (PCR), while Varettoni et al.¹⁷ proposed that MYD88 L265P expression has no indicative value for the transformation between IgM MGUS and WM.

A way of accurately defining the genetic factors associated with disease progression is to study sequential samples from the same patient as they progress from one clinical stage to another. Walker et al.¹⁸ performed whole exome sequencing of DNA derived from a series of samples with MGUS (n = 4), MM (n = 22), and PCL (n = 2) and showed that the majority of the NS exonic changes were already present and that only a few additional changes were seen following transformation. They described acquired NS-SNV (non-synonymous single-nucleotide variant) in RUNX2 is seen following progression to MM, which could plausibly drive the expansion of the clone carrying it. Consistently, RUNX2 is expressed in myeloma cells and regulates osteopontin, a multifunctional bone matrix glycoprotein that is involved in angiogenesis, cell survival, and proliferation.^19,20 They described a translocation disrupting one allele of BRCA2. The other feature is several translocations into UNC5D, which is very complex and could be an interesting mechanism mediating disease progression.^21,22

Gene expression profiling

The gene expression profiling results are heterogeneous. It was reported by Zhan et al.,²³ who undertook the gene expression profiling of 22 healthy volunteers, patients with MGUS (n = 44), SMM (n = 12), or MM (n = 367) by microarrays and found that 52 genes were differentially expressed between the healthy volunteers and patients. Of the 52 genes, 41 exhibited a progressive increase in the expression levels along the transition from NPCs to MGUS to MM, and 1 gene had lower expression levels in MGUS compared with MM. The differential expression of the 52 genes in the 22 NPC and 24 MGUS patients was visualized through unsupervised hierarchical clustering. At the same time, another prospective study (SWOG S0120) conducted by Dhodapkar et al.²⁴ had found GEP data could predict the transformation of MGUS to MM. Madhav et al. have found the 70-gene model (GEP-70) risk score, which is defined by Shaughnessy based on a validated 70-gene model for high-risk MM, was useful to predict the risk of transformation. They have found that GEP-70 risk score > −0.26, and GEP proliferation index > −2.73 predicted an increased risk.²⁵ Moreover, Greenberg validated the association between 3p22.1 (rs1052501) and MGUS by TaqMan SNP genotyping assays from a clinic-based control, which included 378 people recruited as the control group and 388 MGUS cases participating as the experimental group. Rs1052501 maps to ULK4, encoding a kinase with serine/threonine activity, and may influence MGUS and induce progression.²⁶

And also, Chng et al.²⁷ analyzed the gene expression datasets generated from 22 MGUS and 101 MM patients through a gene-set enrichment analysis. They reported that MYC was activated in 67% cases of myeloma but not in MGUS. This result was confirmed by immunohistochemistry, using membrane CD138 and nuclear MYC double staining. They showed that MYC may serve as a potential marker of disease progression.²⁷ MicroRNA (miRNA) profiling has also yielded some interesting results, characterizing MGUS with MM patients.²⁸ Pichiorri et al.²⁸ used both miRNA microarrays and quantitative real-time PCR to profile miRNA expression in MM-derived cell lines (n = 49) and CD138⁺ BM PCs from subjects with MM (n = 16), MGUS (n = 6), and normal donors (n = 6). Compared with the healthy controls, the MGUS and MM patients have unregulated miR-21, miR-106b, miR-181a, and miR-181b. The up-regulation of miR-32 and the miR-17-92 cluster was observed in the MM patients but not in the MGUS patients, which might be a causal factor for the transmission from MGUS to MM.²⁸

Imaging techniques

Different techniques such as radiographic survey, multidetector computed tomography, whole-body MRI (WB-MRI), fluorodeoxyglucose positron emission tomography (FDG-PET) with or without computed tomography (CT), and 99mTc-methoxyisobutylisonitrile scintigraphy²⁹ have been widely used in diagnosing, staging, and therapeutical evaluation of patients with monoclonal PC diseases. Among the imaging techniques currently used in patients with monoclonal PC diseases, MRI has the highest sensitivity for detecting both diffuse and focal BM involvement, although FDG-PET in combination with low-dose CT are more sensitive than skeleton X-ray in screening and diagnosing monoclonal PC diseases.^30–32

Several comparative studies have revealed a higher sensitivity of MRI compared with conventional skeletal survey and CT for the detection of BM involvement,^31,33 demonstrating that MRI was useful for the prediction of the progression of MGUS to MM.^34–36

Vande et al. determined the frequency of abnormal BM magnetic resonance (MR) images in 37 patients with MGUS of the spinal and compared the outcomes in patients with normal and in those with abnormal MR studies. They found that either diffusion or focal BM abnormalities would predict the transformation to MM.³⁴

Hillengass et al. demonstrated the prognostic significance of the presence of focal lesions detected by WB-MRI in a well-documented retrospective group of individuals with MGUS. They have analyzed the findings of WB-MRI in 137 consecutive individuals with MGUS,^32,35,36 and had found that the presence and numbers of focal lesions were of independent prognostic significance for progression into a symptomatic disease requiring systemic treatment. The optimal cut point for the number of focal lesions as prognostic parameter was 0–1 versus >1 (P < 0.01). However, some researchers found that MRI has negative role in differentiating the transformation of MGUS. Bellaiche et al. found that 24 patients with MGUS underwent MRI of the thoracolumbar spine, all patients had normal scans, which has little effect on the differentiation of MGUS and MM.^37,38 Hence, a well-designed prospective clinical trial was needed for a clear demonstration of the exact role of imaging techniques.

Surveillance and follow-up

Surveillance of MGUS is based on clinical (general condition, bone pain, tumoral syndrome) and laboratory criteria (such as the isotype, the level of monoclonal component and the serum FLC ratio).^1–3 On the basis of the distinct risk categories, the IMWG has reached consensus on a follow-up plan. ‘Low-risk MGUS’ patients (an M protein under 1.5 g/dl, an IgG isotype, and a normal FLC ratio) should be followed with serum protein electrophoresis every 6 months, and if stable, they should be followed every 2–3 years or if symptoms arise. For ‘intermediate-/high-risk MGUS’ patients (an M protein above 1.5 g/dl, an IgA or IgM isotype, or an abnormal FLC ratio), BM aspiration and biopsy should be performed at baseline to rule out the underlying PC malignancy. The patient should be followed with serum protein electrophoresis and a complete blood count every 6 months, and then, annually for life.³⁹

However, a study showed that close follow-up of MGUS patients was rarely able to identify progression before serious complications occurred.^40,41 Among 80 MGUS patients with a regular follow-up, routine laboratory follow-up led to the diagnosis of MM in only 16%. Hence, the authors recommended that especially for the intermediate- and the high-risk MGUS patients, BM aspiration should be performed and flow cytometric analysis of the PC immunophenotype and ploidy and gene expression profiling should be routinely available.

Intervention strategies

The current standard of care for MGUS is follow-up without treatment until symptomatic disease develops. However, some researchers advocate changing the paradigm of delaying therapy. They hypothesize that a cause of incurable MM is a regimen of delayed therapy because most therapy is begun when the disease is cytogenetically and clinically more advanced. Hence, some researchers propose that the patients with high-risk MGUS should be involved in clinical trials. Many phase II studies have been conducted to determine whether MGUS patients will benefit from early intervention.

A randomized trial of curcumin versus placebo given as a preventive therapy in 19 patients with MGUS and 17 patients with SMM,⁴¹ showed lowering FLC levels by 25–50% in one-quarter of the patients and a decrease in the bone resorption marker urinary deoxypyridinoline.⁴² The follow-up study confirmed the results of the biological activity of curcumin which was published recently.⁴³ Although the sample size is quite small to lead to clinical practice, it highlights the primary data of intervention therapies for MGUS. Other atoxic medicines, including celoxicib, omega-3 fatty acids, and green tea extract, have been studied in well-designed clinical trials. It was predicted that the controversy as to early intervention will be resolved by identifying those patients who may benefit from early treatment and by allowing the development of intervention strategies based on rational science, thereby changing the landscape of current clinical management for MM precursor disease.

Conclusions and perspectives

MGUS is an asymptomatic PC disorder with a propensity to transform into MM and related malignancies. It is a consensus opinion that MGUS is a heterogenetic disease and presents a clinical challenge in MM precursor disease, which should be treated individually. In recent years, there have been improvements in the risk stratification models that have led to a better understanding of the biology of the disease and the probability of progression, including the molecular target and the examined techniques. However, consensus on the objective and robust factor to predict the transformation has not been achieved. Some novel imaging techniques which could detect and monitor more accurately the diffuse and focal patterns should be developed to truly estimate the tumor burden and define the transformation of MGUS to MM. Furthermore, robust and comparable multicenter manner FCM protocols to find the risk factor of transformation need to be established. To accurately define the genetic factors associated with disease progression, sequential samples from the same patient as they progress from one clinical stage to another should be studied in a well-designed clinical trial. To develop early intervention strategies designed to delay and prevent full-blown MM, large-scale, well-designed, double-blind, multicenter, and prospective clinical trials should be conducted.

Disclaimer statements

Contributors RL wrote the paper, JD revised the paper, and JH proposed the idea and revised the paper.

Funding This work was supported by grants from the National Natural Science Fund (Grant Nos. 30828017 and 81102284).

Conflict of interests None.

Ethics approval None.