Amyloidosis - Pathology (15 Marks)

Definition

Physical Nature of Amyloid

• By electron microscopy: continuous, non-branching fibrils, ~8-10 nm in diameter
• Each fibril consists of stacked protofilaments arranged in a beta-pleated sheet conformation
• This beta-pleated sheet structure is responsible for the characteristic Congo red staining
• All amyloid types share this identical physical structure regardless of their protein composition

Chemical Nature of Amyloid

• ~95% fibril proteins (the disease-specific protein)
• ~5% serum amyloid P-component (SAP) and glycoproteins

• Aβ (beta-amyloid): from APP cleavage - Alzheimer disease
• Aβ2m: β2-microglobulin - long-term hemodialysis (carpal tunnel syndrome)
• ACal: calcitonin precursor - medullary carcinoma of thyroid
• AIAPP: islet amyloid peptide - Type 2 diabetes mellitus
• Prion proteins: a minority of CJD/prion disease cases

Pathogenesis

1. Normal protein overproduced - e.g., in chronic inflammation, SAA is overproduced as an acute-phase reactant. The liver synthesizes excess SAA; proteolysis yields AA protein which aggregates.
2. Variant (mutant) protein - inherently prone to misfolding regardless of quantity (e.g., variant transthyretin in familial amyloidosis).

Classification

I. Systemic (Generalized) Amyloidosis

• Most common systemic form
• Associated with multiple myeloma and other B-cell/plasma cell neoplasms
• Bence-Jones proteins (free κ or λ light chains) are amyloidogenic
• λ chains more amyloidogenic than κ chains
• Not all myeloma patients develop amyloidosis - specific amino acid sequence determines amyloidogenic potential

• Complicates chronic inflammatory conditions:
  • Rheumatoid arthritis, ankylosing spondylitis, IBD (Crohn disease)
  • Chronic infections: tuberculosis, osteomyelitis, bronchiectasis
  • Familial Mediterranean fever (hereditary AA amyloid)
• SAA is produced by the liver as an acute-phase reactant - chronically elevated levels lead to deposition

• Wild-type transthyretin aggregates in the heart, predominantly in males >70 years
• Increasingly prevalent with aging populations

II. Hereditary (Familial) Amyloidosis

• Familial amyloidotic neuropathies: variant TTR mutations - peripheral nerve + cardiac involvement
• Familial Mediterranean fever: AA amyloid, autosomal recessive
• ATTRv: variant transthyretin

III. Localized Amyloidosis

• Deposits confined to a single organ/tissue
• Senile cerebral (Alzheimer disease) - Aβ
• Endocrine organs: islets of Langerhans (Type 2 DM), medullary carcinoma thyroid
• Isolated atrial amyloidosis (ANF)

Morphology (Gross and Microscopic)

Gross Appearance

• Affected organs enlarge, firm, waxy, pale
• Liver/spleen: cut surface has "lardaceous" (bacon-like) waxy appearance
• "Sago spleen": early deposits around follicles giving grey, translucent granules resembling sago grains
• "Lardaceous spleen": diffuse deposits replacing red pulp

Microscopic Appearance

• Amyloid appears as amorphous, eosinophilic, hyaline, extracellular substance
• Progressively obliterates the parenchymal architecture

• Under ordinary light: pink-red deposits
• Under polarized light: characteristic apple-green birefringence - this is pathognomonic

• Crystal violet / methyl violet: metachromasia (purple-red color)
• Thioflavin T and S: fluorescence under UV light (yellow-green)
• PAS positive, diastase resistant

Organ-Specific Morphology

• Amyloid deposits first in mesangium and glomerular capillary walls
• Progressive glomerular obliteration
• Vascular wall deposits
• Results in nephrotic syndrome → renal failure

• Sago spleen (periarterial/follicular deposits) vs. lardaceous spleen (diffuse red pulp)

• Deposits in space of Disse (between sinusoids and hepatocytes)
• Hepatomegaly; hepatic function remarkably well preserved early on

• Subendocardial deposits and within myocardium
• Restrictive cardiomyopathy pattern
• Conduction disturbances and arrhythmias (serious, potentially fatal)
• "Rubber-like" rigid heart on gross exam

• Macroglossia (tongue involvement in AL amyloidosis)
• Respiratory tract - larynx to bronchioles
• Peripheral + autonomic nerves in familial forms
• Joints (hemodialysis-related Aβ2m - carpal tunnel syndrome)

Clinical Features

Diagnosis

1. Tissue biopsy + Congo red staining with polarized microscopy - gold standard
   • Sites: kidney (if renal features present), rectal mucosa, gingival tissue, abdominal fat aspirate
   • Abdominal fat aspirate: high specificity but low sensitivity for systemic amyloidosis
2. Serum/urine protein electrophoresis and immunoelectrophoresis - for AL type (detect M protein)
3. Bone marrow biopsy - often shows increased plasma cells in AL type
4. Imaging - echocardiography for cardiac amyloidosis (granular sparkling appearance)
5. Potassium permanganate pretreatment (useful to distinguish types):
   • AA amyloid loses Congo red affinity after permanganate treatment
   • AL amyloid retains Congo red affinity
6. Immunohistochemistry on biopsy - to definitively type the amyloid protein

Prognosis and Treatment

• Prognosis depends on the organs involved and degree of functional compromise
• Renal failure and cardiac arrhythmias are the most common causes of death
• AL amyloidosis: treat the underlying plasma cell dyscrasia (chemotherapy, stem cell transplant)
• AA amyloidosis: treat the underlying chronic inflammatory disease; colchicine useful in FMF
• ATTR amyloidosis: tafamidis (TTR stabilizer), small interfering RNA agents (patisiran, vutrisiran)
• No treatment reverses established amyloid deposits effectively