The term ‘paraprotein’ was introduced by Apitz in 1940 to describe the proteins in blood, urine or tissues that are produced by myeloma cells. However, the abnormally occurring protein in urine, later identified as the monoclonal light chain component of the immunoglobulin molecule was first described by William Maclntyre in 1846 and thereafter called ‘Bence-Jones protein’.
In a situation where there is proliferation of a single clone of B-cells, the antibody product will consist of a single heavy chain class and sub-class, light chain type and idiotype. Such antibody will represent a homogenous protein migrating as a compact band in electrophoretic separations. This is a paraprotein or monoclonal immunoglobulin.
Paraproteins are thus the earliest described tumour markers and remain an important diagnostic feature in multiple myeloma. However, it has now become clear that paraproteins are found in many individuals and are associated with a wide range of disorders, other than multiple myeloma. When hospital populations are surveyed, the most frequent association of paraproteinaemia is with the malignant conditions.
Surveys of symptomless populations have shown a low incidence of paraproteinaemia increasing with age and in the majority of cases this does not seem to signify early detection of malignancy, but rather the chance finding of an idiopathic benign condition. Current data would suggest a higher frequency than was recorded 20-30 years ago, this probably represents increased sensitivity of the detection methods rather than a true increased incidence. Significant studies at extreme ages are difficult to document, but frequencies of up to 10% in the tenth decade, and 20% in the twelfth decade, have been recorded.
Whether a paraprotein indicates a benign or malignant condition is important in patient management. In all cases of paraproteinaemia, and in suspected cases of a B-cell malignancy, both serum and urine analysis is essential. An important distinguishing feature is the fairly frequent finding, in the malignant conditions, of immunoglobulin fragments produced by tumour cells. These fragments, which may show as only subtle changes in the serum electrophoretic pattern, are usually of lower molecular weight than the complete immunoglobulin molecule and may be clearly visible in the urine, due to the concentration effect. The most frequently seen of these is the Bence-Jones protein (BJP).
- Myelomatosis (multiple myeloma) is defined by three major features:
- The production of paraproteins which can be detected in serum or urine.
- The presence of increased plasma cells within the bone marrow, usually 10% or more of the nucleated cell population.
- The destruction of bone, either in the form of osteolytic lesions or as extreme osteoporosis.
A confident diagnosis can be made when only two of these features are present.
In 20% of myeloma patients, BJP is the only tumour product and can be missed if only the serum is examined. BJP may also be implicated in deposition of amyloid and in the rare kappa chain disease, the idiotype of the BJP shows an affinity for kidney and other tissues without forming amyloid, so that diagnosis depends on immunochemical studies on biopsies.
Apparently localised plasmacytoma can present in a single bone, eg clavicle, femur or vertebra or in the soft tissues. In most patients a serum paraprotein or urinary Bence-Jones provides a tumour marker. In the absence of an immunoglobulin marker, serum b 2-microglobulin estimations may be useful.
This diagnosis should be a diagnosis reserved for those patients with IgM paraproteins who present with viscosity syndrome.
The underlying tumour is so indolent ( only slowly infiltrating the lymph nodes, spleen and bone marrow with lymphocytoid plasmacells, typically of uniform size) that the protein product is allowed to build up to a concentration above 30 g/L. It is this protein concentration, rather than symptoms from tumour invasion, which brings the usually elderly male patient to the doctor.
Heavy Chain Diseases
These are extremely rare in native Western populations – the most commonly seen in the UK is Alpha chain disease in patients from the Middle East. Clinically, it presents as a severe malabsorption syndrome with a stove-pipe-like involvement of the small intestine and an apparently high serum IgA concentration with almost absent IgG and IgM. A few cases have presented with a segmental involvement of the lung and similar serum findings.
Gamma chain disease
This was first described in association with oedema and redness of the uvula and soft palate. As more cases were discovered, it was apparent that less than 15% present in this way. The clinical picture may resemble that of malignant lymphoma. The disease has been described in association with a number of chronic conditions, eg rheumatoid arthritis, Sjogren’s syndrome, SLE, tuberculosis.
Mu chain disease
This occurs predominantly with Chronic Lymphatic Leukaemia and, unlike gamma and alpha chain, is often accompanied by BJP. The defect appears to be in the assembly of complete immunoglobulin, unlike the other heavy chain diseases where evidence indicates deletion of light chain synthesis.
The diagnosis of heavy chain disease remains the prerogative of the laboratory and the term should be reserved for those conditions in which the presence of free heavy chains can be clearly demonstrated.
Immunofixation can be used to detect free heavy chains. These may then be confirmed by the technique of immunoselection electrophoresis.
Other B-Cell Malignancies
Paraproteins may be seen with any B-cell malignancy, eg lymphoma, Chronic Lymphocytic Leukaemia and occasionally Kaposi’s Sarcoma. In most cases it is the effect of the tumour, rather than the paraprotein, that produces clinical symptoms.
This is the diagnosis made when all possible malignant conditions have been excluded and there has been no progression for at least five years, and preferably ten years, of follow-up. The term Monoclonal Gammopathy of Uncertain Significance (MGUS) is used to denote those cases where a monoclone has been identified but no other features of myelomatosis are present, the condition has not been monitored for a minimum five years and cannot, therefore, be defined as benign.
This is the most often seen against a background of immune suppression, particularly with a superimposed infection, and probably represents limited clonal expansion and/or immune imbalance. Some 80% of bone-marrow-transplant patients will show such bands during the reconstitution period.
Paraprotein Detection and Typing
Paraproteins are most conveniently demonstrated as narrow bands on electrophoresis. The method of choice for typing heavy and/or light chains of paraproteins is immunofixation.
Investigation and typing of paraprotein is necessary for:
- Diagnosis and prognosis in B-cell neoplasm.
- Assessing significance of the paraprotein.
- Monitoring of both benign and malignant paraproteinaemia.
Diagnosis and assessment of paraprotein
Quantitation and typing of a paraprotein, together with estimation of the normal immunoglobulins, can help to assess whether the underlying lesion is benign or malignant, and to establish the diagnosis. Unless the isotype of the paraprotein is known the concentration cannot be interpreted in terms of tumour bulk. The average serum concentrations (g/L) at the time of presentation of patients with myelomatosis are IgG 43; IgA 28; IgM 19; IgD 14; IgE 10. The IgM concentration in Waldenstrom’s macroglobulinaemia is usually greater than 30 g/L. In most cases of benign paraproteinaemia the paraprotein concentration is less than 10 g/L and in transient conditions, less than 5 g/L., although exceptions to both rules are recognised
Investigation of urine for BJP is important in diagnosis of BJ myeloma and for distinguishing benign from malignant conditions. Bence Jones Protein is very rarely seen with a benign condition.
See also b2microglobulin for prognostic significance in myeloma.
If a diagnosis of malignant immunocytoma is made, it is important to monitor the patient’s serum and urine every three months so that the patient’s response, or lack of response, to treatment can be assessed.
If the paraprotein disappears following treatment, measurement can be repeated 6 or 12 monthly. If the paraprotein does not disappear, this indicates residual disease particularly with solitary plasmacytoma.
Apparently benign paraproteinaemia should be monitored yearly if the patient is elderly, but three monthly if the patient is less than 50 years old. At this age, benign paraproteinaemia is rare and if due to myeloma, this can be swift to evolve.
5 mL serum in a screw capped glass bottle. The sample must be taken and separated at 37° C if the presence of cryoglobulins is suspected. A 25 mL sample of a 24 hour collection of urine in a screw capped universal container with 0.1% sodium azide should be submitted.
Random urine samples are of limited value for quantitative assessment of BJP although they can be used for screening purposes.
Centres offering this assay:
London St George’s Hospital PRU Diagnostic Service
Sheffield Northern General’s PRU Diagnostic Service