Monoclonal Gammopathy of Undetermined Significance

Etiology

No specific cause of non-IgM MGUS has been identified. However, the disease may be associated with some non-malignant disorders such as connective tissue disorders, peripheral neuropathies, dermatological diseases such as acquired C1 esterase inhibitor deficiency (angioedema), endocrine diseases, and liver infections such as hepatitis C virus infection and HIV liver disease.[2][4][5] Population-based studies from northern Europe and the United States show increased risk of MGUS among first-degree relatives of those with MGUS or myeloma, supporting a role for germline susceptibility genes, shared environmental influences, or an interaction between both.[6][7]

Epidemiology

Monoclonal gammopathy of undetermined significance (MGUS) is found in approximately 2% to 3% of adults over age 50 and in 5% of adults older than the age of 70. MGUS is more common in men than in women (1.5:1) and 2 to 3-fold more common in African Americans compared to Caucasians. Non-IgM MGUS represents up to 85% of MGUS cases; IgM MGUS represents up to 15% of MGUS cases.[2][4]

Pathophysiology

The clonal plasma cells producing non-IgM MGUS reside in the bone marrow (BM). These cells harbor somatic hypermutation of the variable regions and are class-switched. The majority of non-IgM MGUS are IgG, followed by IgA, bi-clonal, and IgD.[1][5]

History and Physical

The majority of non-IgM MGUS patients are asymptomatic. According to the 2014 International Myeloma Working Group updated criteria, the diagnosis of non-IgM MGUS is based on three criteria:

1. Serum monoclonal protein less than 30 g/L (3 g/dL)
2. Less than 10% of clonal plasma cells in the bone marrow
3. The absence of CRAB symptoms (hypercalcemia, renal insufficiency, anemia and, bone lesions suspicious for PCM)

Non-IgM MGUS is usually diagnosed as an incidental finding on protein electrophoresis performed as part of an evaluation for disorders or disease presentation of peripheral neuropathy, vasculitis, hemolytic anemia, skin rashes, hypercalcemia, elevated erythrocyte sedimentation rate. Non-IgM MGUS is generally not considered a neoplastic process, with an annual risk of progression of 1%. Non-IgM MGUS may progress to PCM, solitary plasmacytoma, or amyloidosis. Two important factors influence non-IgM MGUS progression. The first is the size and type of M protein, and the second is the level of immunoglobulin (less than 5 g/L and greater than 25 g/L).[8][1][5]

Evaluation

Non-IgM MGUS needs a thorough set of investigations to exclude this disease entity from the other plasma cell neoplasms. Investigations should include complete blood count (CBC), bone marrow aspirate/biopsy (BMA/BMB), immunohistochemical analysis (IHC), serum calcium and creatinine, serum protein electrophoresis and immunofixation, urine protein electrophoresis and immunofixation, serum-free light chains (FLC) assay, quantification of immunoglobulins, immunophenotyping utilizing flow cytometry and cross-sectional imaging studies.[8][1]

CBC/peripheral smear is usually normal. However, some cases may show rouleaux formation. BMB usually shows 3% to 5% mature plasma cells (less than 10%) evenly scattered or in occasional small clusters. CD138 is useful in highlighting these plasma cells in the BMB. IHC of the plasma cells for kappa and lambda will demonstrate monoclonal restriction. MGUS is characterized by the presence of a monoclonal (M) protein which is produced by clonal plasma cells and detected by serum and urine protein electrophoresis, and immunofixation of the serum and urine. Immunophenotyping shows monoclonal plasma cells that are CD38+ (bright) cells with an aberrant CD56 population (may also be negative). Molecular testing for non-IgM MGUS has shown that this disease entity usually shows a normal karyotype because of the relatively small number of plasma cells. Some patients show chromosomal alterations of PCM that include: t(11;14), t(4;14), t(14;16), deletions of 13q and hyperdiploid. No clinical correlation for these genetic alterations has been found in non-IgM MGUS.[9][1]

The 2019 International myeloma working group (IMWG), consensus recommends the following approach for cross-sectional imaging in MGUS:[10]

Whole-body imaging is recommended only in high-risk MGUS. Since IgM-MGUS usually progresses to Waldenstrom macroglobulinemia and not multiple myeloma, routine bone imaging is not recommended.

In suspected high-risk non-IgM MGUS, a whole-body CT to rule out multiple myeloma is recommended. CT scan has superior sensitivity compared with a skeletal survey for the detection of osteolytic lesions in patients with multiple myeloma.

If whole-body CT is not available, conventional skeletal survey or whole-body MRI are alternatives.

In patients with equivocal findings on whole-body CT (or conventional skeletal survey) in whom there is a concern for myeloma development, a whole-body MRI is recommended (or MRI of the spine and pelvis if whole-body MRI is not available).

If whole-body CT is positive, a PET/CT should be done. A follow-up bone imaging is not recommended unless there are signs of progression to symptomatic disease (e.g., pain or increase in serological parameters).

In patients with MGUS with positive imaging findings for focal and osteolytic lesions, other malignancies should be ruled out as well. If needed, a biopsy of such a lesion should be performed.

Treatment / Management

Treatment is usually not recommended for patients with MGUS. As there is a risk of progression to lymphoproliferative malignancy long-term follow-up is advised.

If a patient has an IgM M-protein, bone marrow biopsy and computed tomography (CT) scanning of the abdomen may be helpful in detecting Waldenström macroglobulinemia or other lymphoproliferative disorders.

MGUS-associated neuropathies are usually not treated, except in the case of a disabling IgM monoclonal gammopathy or IgG/IgA MGUS associated with chronic inflammatory demyelinating neuropathy (CIDP). About 80% of patients with IgG/IgA MGUS CIDP respond to one of the typical CIDP treatments and some patients stabilize without therapy. MGUS patients with associated osteopenia or osteoporosis may benefit from treatment with intravenous bisphosphonates.

The intensity of follow-up in patients with MGUS is guided by risk stratification. Initial follow-up at six months is recommended, with subsequent visits scheduled according to the level of risk. If clinical trials of preventive strategies are available, patients at high risk for progression should be encouraged to participate.

Differential Diagnosis

It is crucial to distinguish non-IgM MGUS from other advanced plasma cell neoplasms since these diseases will require a different management approach. The differential diagnosis of non-IgM MGUS include:

• Plasma cell myeloma (smoldering or symptomatic): Any patient with a non-IgM serum monoclonal protein greater than or equal to 30 g/L or with greater than or equal to 10% clonal plasma cells in the bone marrow should not be diagnosed as non-IgM MGUS. Many providers consider smoldering PCM as a transition stage between non-IgM MGUS and symptomatic PCM. Smoldering PCM is distinguished from MGUS based on the size of the M protein and the percentage of clonal plasma cells in the bone marrow. Smoldering PCM is distinguished from symptomatic PCM by the presence of CRAB symptoms.[5]
• Waldenström macroglobulinemia (smoldering or symptomatic): WM is a distinct clinicopathologic disorder that shows lymphoplasmacytic lymphoma (LPL) in the bone marrow and an IgM monoclonal gammopathy. Symptoms include blood hyperviscosity, lymphadenopathy, or splenomegaly. The WHO (2017) diagnostic criteria of WM include:[5]
  1. IgM monoclonal gammopathy
  2. Clonal plasma cells less than 10%
  3. Absence of end-organ damage (CRAB symptoms)
  4. Absence of symptoms or signs of amyloidosis

• Monoclonal gammopathy of renal significance: This disease entity is diagnosed when the patient has diagnostic criteria for MGUS as well as renal insufficiency and monoclonal immunoglobulin deposits in the kidney by immunofluorescence.[11]
• Light chain smoldering multiple myeloma (idiopathic Bence Jones proteinuria): The diagnostic criteria of LC-SMM include:
  1. Monoclonal light chains in the urine (Bence Jones proteinuria)
  2. No immunoglobulin heavy chain expression in the serum or urine
  3. No symptoms of PCM, WM, or amyloid light chain amyloidosis

• Primary (amyloid light chain) amyloidosis and light chain deposition disease: This entity of plasma cell neoplasms is associated with the pathologic deposition of monoclonal light chains in tissue. The diagnostic criteria include the presence of amyloid in tissue and evidence of plasma cell neoplasm.
• B cell lymphoproliferative disorder

Prognosis

Non-IgM MGUS is considered a preneoplastic condition with an annual risk of progression of approximately 1%. The risk of progression is increased when M protein greater than or equal to 15 g/L and with an abnormal free light chain ratio. Non-IgM MGUS does not require treatment. However, the management of patients with non-IgM MGUS requires an understanding of the risk of progression of the disease. Generally, close follow-up is recommended for these patients. Most providers believe that all non-IgM MGUS patients should undergo a clinical examination and laboratory evaluation for disease progression annually.[8] The patients with MGUS who are at risk for progressing to advanced disease can be risk-stratified based on the following criteria:

1. Serum monoclonal protein level greater than or equal to 15 g/L (1.5 g/dL)
2. Non-IgG MGUS (i.e., IgA, IgM, and IgD MGUS)
3. Abnormal serum free light chain ratio

Patients with three risk factors are categorized as high-risk MGUS; patients with two risk factors are categorized as high-intermediate risk MGUS. Patients with one risk factor are categorized as low-intermediate risk MGUS. Patients with no risk factors are categorized as low-risk MGUS. Despite close observation, some non-IgM MGUS patients may show progression to PCM abruptly.[9][5][12][13][14]

Complications

Monoclonal gammopathy of undetermined significance (MGUS) can progress to develop myeloproliferative disorders or other serious conditions such as:

• Multiple myeloma
• Light chain amyloidosis
• Waldenstrom macroglobulinemia
• Lymphoma
• Osteoporosis
• Venous thromboembolism