Amyloidosis: A Pathology Perspective

Definition

Physical Nature of Amyloid

• Continuous, nonbranching fibrils, approximately 8-10 nm in diameter
• Each fibril made up of stacks of protofilaments arranged in a beta-pleated sheet (cross-β) conformation

Classification and Pathogenesis

1. AL Amyloidosis (Amyloid Light Chain)

• Protein source: Monoclonal immunoglobulin light chains (usually lambda > kappa), produced by a clonal plasma cell population
• Clinical setting: Multiple myeloma, other plasma cell dyscrasias, sometimes without overt malignancy (primary amyloidosis)
• Most common systemic form overall
• Preferentially involves: heart, GI tract, respiratory tract, peripheral nerves, skin, tongue

2. AA Amyloidosis (Amyloid-Associated)

• Protein source: Serum amyloid A (SAA), an acute-phase protein synthesized by the liver; proteolytically cleaved to form AA fibrils
• Clinical setting: Chronic inflammatory states - rheumatoid arthritis, inflammatory bowel disease, familial Mediterranean fever, chronic infections (TB, osteomyelitis, bronchiectasis), certain cancers
• Preferentially involves: kidneys, liver, spleen, lymph nodes, adrenals, thyroid

3. ATTR Amyloidosis (Transthyretin)

• Protein source: Transthyretin (TTR), a plasma protein that transports thyroxine and retinol
  • ATTRwt (wild-type): Normal TTR aggregates; occurs in males >70 years; cardiac predominance - "senile cardiac amyloidosis"; increasingly prevalent with aging population
  • ATTRv (variant/hereditary): Mutant TTR; familial; deposits in heart, peripheral nerves, and other tissues

Other Forms

1. Normal proteins that have an inherent tendency to misfold when produced in excess or when degradation is impaired (SAA in AA; wild-type TTR in ATTRwt)
2. Variant (mutant) proteins that are structurally prone to misfolding (mutant TTR in ATTRv)

Histopathology and Staining

Light Microscopy (H&E)

Congo Red Stain (Diagnostic Gold Standard)

• Under ordinary light: Pink-red deposits
• Under polarized light: Characteristic apple-green birefringence
• This reaction is shared by all forms of amyloid and is conferred by the cross-beta pleated fibril configuration

Other Stains

• Crystal violet / methyl violet: Metachromatic staining (purple-red color)
• Thioflavin T/S: Fluorescent staining - bright yellow-green fluorescence under UV (very sensitive, used for frozen sections)
• PAS: Weakly positive, diastase-resistant
• Electron microscopy: Amorphous nonoriented thin fibrils

Specific Typing

• Immunohistochemistry (IHC): Specific antibodies against AA, AL (kappa/lambda light chains), ATTR - required for subtyping
• Mass spectrometry (laser capture + LC-MS/MS): Emerging gold standard for typing, especially when IHC fails

Morphology by Organ

Gross Appearance

Kidney (most common + most serious)

• Early: Subtle mesangial matrix thickening + uneven widening of glomerular basement membranes
• Progressive: Capillary narrowing and distortion of the glomerular vascular tuft
• Advanced: Capillary lumens obliterated; glomerulus flooded by confluent masses of amyloid - "obsolescent glomerulus"
• Interstitium, arteries, and arterioles also involved
• Grossly: normal size or shrunken (from ischemia due to vascular amyloid deposition)

Spleen

• Sago spleen: Deposits limited to splenic follicles; tapioca-like granules on gross inspection
• Lardaceous spleen: Deposits in sinus walls and red pulp connective tissue; large maplike areas; marked splenomegaly (up to 800 g)

Liver

• May be inapparent to massively enlarged
• Deposits begin in space of Disse (between sinusoidal endothelium and hepatocytes)
• Progress to compress hepatic parenchyma
• Portal tracts and hepatic vessels also affected in advanced disease

Heart

• May be enlarged and firm, or no gross change
• Subendocardial focal deposits initially, then between myocardial fibers
• Expansion causes pressure atrophy of myofibers
• Subendocardial deposits damage the conduction system - arrhythmias, heart block
• Produces restrictive cardiomyopathy pattern

Tongue and GI Tract

• Macroglossia: Nodular deposits causing tongue enlargement; characteristic of AL amyloid
• Stomach/intestine: malabsorption, diarrhea
• Vascular fragility: spontaneous or traumatic bleeding; factor X inactivation by AL amyloid causes coagulopathy

Nervous System

• Peripheral and autonomic neuropathies (hereditary ATTR amyloidosis)
• Carpal tunnel syndrome (bilateral) - classic feature of β2-microglobulin amyloidosis (hemodialysis patients)
• Alzheimer disease: cerebral plaques (senile plaques) + cerebrovascular amyloid angiopathy

Adrenals, Thyroid, Pituitary

• Involved in generalized secondary (AA) amyloidosis

Clinical Features

Diagnosis

• Biopsy is required for histologic confirmation
• Common biopsy sites:
  • Abdominal fat pad aspiration - simple, minimally invasive, ~80% sensitivity in AL/AA
  • Rectal biopsy - submucosa sampled
  • Kidney - when renal manifestations predominate
  • Bone marrow - often positive in AL
  • Organ-specific biopsy if above negative but suspicion remains
• SAP scintigraphy (radiolabeled serum amyloid P scan) - whole-body imaging of amyloid burden (not widely available)
• Serum/urine immunofixation + free light chain assay - screening for AL
• Genetic testing - for ATTR variant mutations

Summary Table: Key Amyloid Types