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Paraproteins - Short Notes for Pathology MD Exam
1. Definition
Paraproteins (also called M proteins, M components, or myeloma proteins) are electrophoretically and immunologically homogeneous (monoclonal) immunoglobulins or immunoglobulin fragments secreted by a clonal population of plasma cells. The term "M" originally stood for "myeloma" but now denotes "monoclonal."
- Synonyms: monoclonal protein, M protein, M component, paraprotein
- Represent a clonal product - all molecules are identical in class, subclass, and light chain type
- Distinguished from polyclonal hypergammaglobulinemia (seen in infections, autoimmune diseases) where a broad diffuse band is present on electrophoresis
- Goldman-Cecil Medicine, 22e; Robbins & Kumar Basic Pathology
2. Structure of Immunoglobulins (Quick Recap)
| Component | Normal Role | Paraprotein Relevance |
|---|---|---|
| Heavy chains (IgG, IgA, IgM, IgD, IgE) | Determine Ig class | Determines M protein type |
| Light chains (kappa κ, lambda λ) | Associate with heavy chains | Free light chains = Bence Jones protein |
| Free light chains | Minor amount in serum | Markedly elevated in myeloma/AL amyloidosis |
- Complete M proteins: MW ~160 kDa - confined to plasma/extracellular fluid
- Free light chains: MW ~25 kDa - pass through glomerular slit diaphragms into urine
3. Classification of Plasma Cell Disorders Producing Paraproteins
I. Premalignant Monoclonal Gammopathies
- MGUS (IgM type, non-IgM type, light chain MGUS)
- Biclonal/triclonal gammopathies of undetermined significance
- Smoldering multiple myeloma
II. Malignant Monoclonal Gammopathies
- Multiple myeloma (IgG, IgA, IgD, IgE, light chain only)
- Plasma cell leukemia
- POEMS syndrome
- Solitary plasmacytoma
- Waldenström macroglobulinemia (IgM)
III. Heavy Chain Diseases (HCDs)
- γ-HCD (Franklin disease), α-HCD (most common HCD), μ-HCD
IV. Cryoglobulinemia (Types I, II, III)
V. Immunoglobulin Light Chain (AL) Amyloidosis
- Goldman-Cecil Medicine, Table 173-1
4. MGUS - Monoclonal Gammopathy of Undetermined Significance
Diagnostic Criteria (Mayo Clinic)
| Feature | Threshold |
|---|---|
| Serum M protein | < 3 g/dL |
| Clonal plasma cells in bone marrow | < 10% |
| Absence of | Myeloma-defining events, amyloidosis, macroglobulinemia |
Epidemiology
- Prevalence: ~1% in >50 years, >5% in >70 years
- Higher in males; ~2x higher in Black Americans vs. Whites
- Risk of progression: 1% per year (fixed, unrelenting)
Pathogenesis
- ~40% IgH translocations (chr 14q32) involving: 11q13 (CCND1/cyclin D1), 4p16.3 (FGFR-3/MMSET), 6p21 (CCND3), 16q23 (c-maf), 20q11 (mafB)
- ~40% hyperdiploid (trisomies of odd-numbered chromosomes)
- On flow cytometry: normal plasma cells coexist alongside clonal cells (distinguishes from myeloma)
Risk Stratification (Mayo Model)
| Risk Group | Features | 20-yr progression risk |
|---|---|---|
| Low | M protein <1.5 g/dL, IgG subtype, normal FLC ratio (0.26-1.65) | 5% |
| Low-intermediate | 1 abnormal factor | 21% |
| High-intermediate | 2 abnormal factors | 37% |
| High | All 3 factors abnormal | 58% |
- Goldman-Cecil Medicine, Table 173-3
5. Multiple Myeloma
M Protein Distribution
| Type | Frequency |
|---|---|
| IgG | ~60% |
| IgA | ~20-25% |
| Light chain only (κ or λ) | ~15-20% |
| IgD, IgE, IgM | Rare |
Pathogenesis
- Chromosomal translocations fusing IgH locus (chr 14) to oncogenes (cyclin D1, cyclin D3)
- IL-6 (from marrow stromal fibroblasts/macrophages) is the primary growth factor for myeloma cells
- Late stage: MYC translocations - mark aggressive disease
Effects on Organs (for exam)
Skeleton:
- RANKL upregulation by myeloma → osteoclast activation + osteoblast inhibition
- Net result: bone resorption → "punched-out" lytic lesions, pathologic fractures, hypercalcemia
- Sites: vertebra, ribs, skull, pelvis, femur, clavicle, scapula
Immune system:
- Paradox: elevated total Ig (M protein) but depressed functional antibody production
- High risk of bacterial infections (hypogammaglobulinemia)
Kidney ("Myeloma kidney"):
- Bence Jones protein casts in distal tubules and collecting ducts - surrounded by multinucleate giant cells (macrophage-derived)
- Light chain deposition as AL amyloid or linear deposits in glomeruli/interstitium
- Hypercalcemia → dehydration + renal stones
- Bacterial pyelonephritis from hypogammaglobulinemia
Morphology
- Lytic lesions: 1-4 cm punched-out defects
- Marrow: plasma cells >30% cellularity
- Myeloma cells: may have prominent nucleoli, cytoplasmic inclusions (Russell bodies) containing Ig
- End stage: leukemic spread
6. Bence Jones Proteins
- Free immunoglobulin light chains (κ or λ) in urine
- MW ~25 kDa - filtered by glomerulus
- Classic thermal property: precipitate at 40-60°C, redissolve at 100°C (historic test, now replaced by immunofixation)
- Detected by: urine immunofixation electrophoresis (UIFE)
- Highly nephrotoxic - direct tubular epithelial toxicity
- Present in: multiple myeloma, AL amyloidosis, light chain MGUS
7. Waldenström Macroglobulinemia
- Syndrome of IgM paraprotein producing hyperviscosity
- Usually associated with lymphoplasmacytic lymphoma (LPL)
- IgM: pentameric, large - even small amounts cause marked hyperviscosity
- Marrow involvement is defining; LPL is the 1st type
- 2nd type: IgA/IgG/no paraprotein, or LPL without marrow involvement
8. Detection of Paraproteins - Lab Methods
Serum Protein Electrophoresis (SPEP)
- Separates proteins into albumin, α1, α2, β, γ bands
- Paraprotein appears as a narrow, sharp "M-spike" in the β or γ region
- Polyclonal Ig: broad diffuse hypergammaglobulinemia
Immunofixation Electrophoresis (IFE) - Gold Standard
- Confirms M protein and identifies class (heavy chain) and type (light chain)
- More sensitive than SPEP - detects low-level M proteins
- SIFE = serum IFE; UIFE = urine IFE
Serum Free Light Chain (FLC) Assay
- Measures free κ and λ (not bound to heavy chains)
- Normal κ/λ ratio: 0.26-1.65
- Abnormal ratio = clonal excess of one light chain type
- Most sensitive for detecting light chain only disease and AL amyloidosis
- Used in risk stratification of MGUS
Key Point for Lab Interference
- High paraprotein concentrations cause factitious results on chemistry analyzers:
- Factitious hyperbilirubinemia, hyponatremia, hypouricemia, hypoalbuminemia, hypercalcemia
- Turbidity from paraprotein can cause factitious hypercalcemia
- Increased viscosity interferes with automated pipetting
- Can prolong PFA-100 closure time and disrupt platelet aggregation studies
- Platelet dysfunction: especially common with IgA and IgM paraproteins (mask surface receptors)
- Tietz Textbook of Laboratory Medicine, 7th ed.; Quick Compendium of Clinical Pathology, 5th ed.
9. Paraprotein-Associated Organ Damage (Exam High-Yield)
| System | Manifestation | Mechanism |
|---|---|---|
| Kidney | Cast nephropathy (myeloma kidney) | Bence Jones protein casts + tubular toxicity |
| Kidney | AL amyloidosis | Light chain deposition as fibrils |
| Kidney | Light chain deposition disease | Non-amyloid linear deposits |
| Nerves | MGUS-associated neuropathy | Unclear; anti-MAG antibodies in IgM MGUS |
| Nerves | AL amyloid neuropathy | Amyloid infiltration |
| Nerves | CIDP-like | IgM/IgG paraprotein-associated CIDP |
| Bone | Lytic lesions | RANKL/osteoclast activation |
| Skin | Necrobiotic xanthogranuloma | IgG paraprotein deposition |
| Skin | EED (Erythema elevatum diutinum) | IgA paraprotein association |
| Blood | Hyperviscosity syndrome | IgM > IgA > IgG (size/polymerization) |
| Blood | Cryoglobulinemia | Ig precipitate at low temp |
| Blood | Platelet dysfunction | Masking of surface receptors |
| Blood | Acquired vWD | Adsorption of high-MW vWF multimers |
10. Cryoglobulins
| Type | Composition | Disease Association |
|---|---|---|
| Type I | Monoclonal Ig only (IgM or IgG) | Myeloma, Waldenström - hyperviscosity |
| Type II | Monoclonal IgM (RF) + polyclonal IgG | HCV (most common), autoimmune |
| Type III | Polyclonal IgM + polyclonal IgG | Autoimmune, chronic infections |
- Types II and III = mixed cryoglobulinemia (contain RF activity)
- Symptomatic triad: purpura, arthralgia, weakness (Meltzer triad)
- Vasculitis of small vessels (leukocytoclastic vasculitis on skin biopsy)
11. AL Amyloidosis
- Most common form of systemic amyloidosis in developed world
- Monoclonal light chains (usually λ > κ) fold abnormally and deposit as β-pleated sheet fibrils
-
90% have monoclonal Ig detectable by IFE (serum or urine) or FLC assay
- Congo red stain: apple-green birefringence under polarized light
- Organs affected: heart, kidney, liver, nerves, tongue (macroglossia)
- 20% have overt multiple myeloma; remainder have small clonal plasma cell population
12. POEMS Syndrome (Mnemonic)
| Letter | Feature |
|---|---|
| P | Polyneuropathy (progressive, sensorimotor) |
| O | Organomegaly (hepato/splenomegaly, lymphadenopathy) |
| E | Endocrinopathy (multiple) |
| M | M protein (usually IgA or IgG, λ light chain) |
| S | Skin changes (hyperpigmentation, hemangiomata, hypertrichosis) |
- Sclerotic bone lesions (not lytic as in myeloma)
- VEGF is markedly elevated - pathogenic
Summary Table - Key Differentials
| Feature | MGUS | Smoldering Myeloma | Multiple Myeloma |
|---|---|---|---|
| M protein | <3 g/dL | ≥3 g/dL | Any level |
| Marrow plasma cells | <10% | 10-60% | >10% (often >30%) |
| Symptoms | None | None | CRAB criteria |
| Lytic lesions | Absent | Absent | Present |
| Anemia/renal/Ca | Absent | Absent | Present (CRAB) |
| Progression risk | 1%/yr | ~10%/yr | N/A (already malignant) |
CRAB = hyperCalcemia, Renal failure, Anemia, Bone lesions
Sources: Robbins & Kumar Basic Pathology; Goldman-Cecil Medicine 22e; Tietz Textbook of Laboratory Medicine 7e; Quick Compendium of Clinical Pathology 5e; Adams & Victor Principles of Neurology 12e
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