AMYLOIDOSIS - University Examination Answer

DEFINITION

PROPERTIES OF AMYLOID

Physical Nature

• By electron microscopy: continuous, non-branching fibrils, diameter 8-10 nm
• Each fibril consists of stacked protofilaments in a beta (β)-pleated sheet conformation
• This β-pleated sheet conformation is responsible for all characteristic staining properties

Chemical Composition

• ~95% fibril proteins (amyloidogenic proteins)
• ~5% serum amyloid P (SAP) component + glycoproteins (heparan sulfate, dermatan sulfate proteoglycans)

Staining Characteristics (HIGH YIELD)

The apple-green birefringence under polarized light with Congo red is the GOLD STANDARD for diagnosis.

Fig: Amyloid fibril structure - four protofilaments wound around each other with regularly-spaced Congo red binding. (B) Apple-green birefringence under polarized light.

PATHOGENESIS

Fig: Mechanism - misfolded proteins oligomerize → aggregate into amyloid fibrils → extracellular deposition → tissue damage

• Normally, intracellular misfolded proteins are degraded in proteasomes
• Extracellular protein aggregates are cleared by macrophages
• In amyloidosis, both mechanisms fail → fibrillar protein accumulation outside cells

1. Normal proteins with inherent tendency to self-associate, particularly when overproduced (e.g., SAA in chronic inflammation, transthyretin wild-type)
2. Mutant/variant proteins structurally prone to misfolding (e.g., mutant transthyretin in familial amyloidosis)

• Abnormal tissue architecture from extracellular deposits
• Activation of ROS-producing signaling pathways
• Defective calcium homeostasis

CLASSIFICATION (Table - University Essential)

CLASSIFICATION BY DISTRIBUTION

• Involves multiple organs
• Subtypes: Primary (AL), Reactive/Secondary (AA), Heredofamilial (ATTR mutant), Dialysis-related (Aβ2M)

• Deposits limited to single organ/tissue
• Examples: Alzheimer's (brain), medullary thyroid carcinoma (calcitonin-derived), senile cardiac, cutaneous, endocrine tumors

MAJOR CLINICAL FORMS (Detailed)

1. AL Amyloidosis (Primary)

• Most common form - ~2000-3000 new cases/year in the USA
• Clonal plasma cell proliferation secreting abnormal Ig light chains
• λ chains 6x more likely to deposit as amyloid than κ chains
• 5-15% of multiple myeloma patients develop systemic amyloidosis
• Most cases: no overt B-cell neoplasm, but monoclonal Ig or free light chains (Bence Jones protein) detectable in blood/urine
• Organs affected: kidney (most common - nephrotic syndrome), heart, liver, spleen, peripheral nerves, tongue (macroglossia), GI tract

2. AA Amyloidosis (Reactive/Secondary)

• Composed of AA protein derived by proteolysis of serum amyloid-associated (SAA) protein (synthesized by liver)
• SAA synthesis stimulated by IL-6 and IL-1 (acute-phase response cytokines) during chronic inflammation
• Associated conditions:
  • Chronic infections: TB, osteomyelitis, bronchiectasis (now less common due to antibiotics)
  • Rheumatoid arthritis (most common current association) - occurs in ~3% of RA patients
  • Inflammatory bowel disease (Crohn's > UC)
  • Ankylosing spondylitis
  • Chronic IV/subcutaneous heroin use (skin infections)
  • Cancers: renal cell carcinoma, Hodgkin lymphoma
• Organs affected: kidney (proteinuria/nephrotic syndrome), liver, spleen, adrenals

3. Heredofamilial / Familial Amyloidosis

• Familial Mediterranean Fever (FMF): autosomal recessive; Armenian, Sephardic Jewish, Arabic populations; amyloid is AA type; mutation in MEFV gene (encodes pyrin/marenostrin); presents with recurrent inflammatory attacks (peritonitis, pleuritis, arthritis) + amyloidosis
• Familial Amyloid Polyneuropathy (FAP): autosomal dominant; mutant TTR (V30M most common, occurs in 30-45%); presents with peripheral neuropathy + cardiomyopathy
• Senile Systemic Amyloidosis: wild-type (normal) TTR deposits in heart in elderly patients (>70 years) - slowly progressive restrictive cardiomyopathy

4. Dialysis-Associated Amyloidosis

• β2-microglobulin, normally filtered by the kidney, accumulates in patients on long-term hemodialysis
• Deposits in joints, synovium, tendons, carpal tunnel (carpal tunnel syndrome)

MORPHOLOGY (Gross and Microscopic)

Gross Appearance

• Affected organs are enlarged and firm ("lardaceous" = waxy, bacon-like appearance)
• Classic test: Lugol's iodine → mahogany-brown; then H2SO4 → blue-violet color ("amyloid reaction")

Microscopic Appearance

• Deposits initially in mesangium, then capillary walls of glomeruli
• Later: tubular BM and vessels
• Results in nephrotic syndrome → chronic renal failure

• Sago spleen: deposits in splenic follicles (white pulp) → small, tapioca-like granules on cut surface
• Lardaceous spleen: deposits in splenic sinuses (red pulp) → large, irregular, waxy areas

• Deposits in space of Disse (between hepatocytes and sinusoidal endothelium)
• Progressive compression of hepatocytes → hepatomegaly (mild dysfunction; jaundice rare)

• Deposits in interstitium between myocardial fibers → restrictive cardiomyopathy
• Subendocardial deposits → "senile cardiac amyloidosis" pattern

• Macroglossia - classic feature of AL amyloidosis

DIAGNOSIS

CLINICAL FEATURES SUMMARY

• Renal: nephrotic-range proteinuria, progressive renal failure (commonest manifestation of systemic amyloidosis)
• Cardiac: restrictive cardiomyopathy, arrhythmias, heart failure
• Hepatic: hepatomegaly, rarely significant dysfunction
• GI: malabsorption, motility disorders, bleeding
• Neurologic: peripheral sensorimotor neuropathy, autonomic neuropathy (especially ATTR)
• Other: macroglossia (AL), periorbital purpura (AL - "raccoon eyes"), carpal tunnel syndrome

KEY EXAM POINTS (One-Liners)

1. Amyloid = misfolded proteins in β-pleated sheet conformation deposited extracellularly
2. Fibril diameter = 8-10 nm, non-branching
3. Congo red + polarized light = apple-green birefringence - pathognomonic
4. AL type = most common systemic form; associated with plasma cell dyscrasias
5. AA type = secondary to chronic inflammation; RA is the most common current association
6. SAA is the serum precursor of AA protein; synthesized in liver; induced by IL-6 and IL-1
7. Kidney is the most commonly affected organ in systemic amyloidosis
8. Sago spleen = follicular deposits; Lardaceous spleen = sinusoidal deposits
9. FMF (Familial Mediterranean fever) = most common heredofamilial form; autosomal recessive; AA type
10. TTR amyloidosis = familial (mutant TTR) or senile (wild-type TTR); cardiac > neurologic
11. Macroglossia = characteristic of AL amyloidosis (plasma cell disorders)
12. Normal protein quality control: proteasomes (intracellular) + macrophages (extracellular)