Amyloidosis: Protein Misfolding and Deposition

• Amyloidosis is a group of diseases characterized by the deposition of extracellular fibrillary protein.
• These conditions can be due to abnormal amounts of normal protein accumulation or normal amounts of mutant protein accumulation.
• A common feature of all amyloidoses is the beta-pleated sheet structure.

Amyloid Deposition

• Misfolded proteins aggregate into insoluble fibrils, disrupting normal tissue function.
• Two main categories of amyloid deposition:
• Abnormal amounts of normal proteins accumulate
• Normal amounts of mutant proteins accumulate

Abnormal Amounts of Normal Proteins Accumulate

• The body produces a normal protein in excessive amounts, overwhelming degradation pathways and leading to fibril formation.
• Example: Chronic Inflammation and Serum Amyloid A (SAA)
• SAA is an acute-phase reactant protein produced by the liver in response to inflammation.
• Normally, SAA is degraded efficiently.
• In chronic inflammatory diseases, persistently high levels of SAA overwhelm clearance mechanisms.
• This leads to AA amyloidosis, where SAA misfolds and deposits in organs.

Normal Amounts of Mutant Proteins Accumulate

• The body produces a mutant protein that has an increased tendency to misfold, even at normal levels.
• Example: Mutant Transthyretin (TTR) in Familial Amyloidosis
• TTR is a normal transport protein for thyroid hormone and retinol.
• Mutations in the TTR gene cause it to misfold and aggregate into amyloid fibrils.
• This leads to hereditary ATTR amyloidosis, primarily affecting the heart and peripheral nerves.
• Wild-type TTR amyloidosis (senile systemic amyloidosis) can occur in elderly individuals without a mutation.

Systemic vs. Localized Amyloidosis

• Systemic amyloidosis involves the whole body, affecting various organs.
• Localized amyloidosis affects only one organ, such as the brain, thyroid, heart, or pancreas.

Diagnosis of Amyloidosis

• Diagnosed based on histological evidence of amyloid deposits, typically using biopsy and special staining techniques.

Biopsy of the Affected Organ

• A biopsy confirms the presence of amyloid fibrils in tissues.
• Localized amyloidosis requires a biopsy from the affected organ.
• Systemic amyloidosis often uses a fat pad biopsy as a minimally invasive option.

Fat Pad Biopsy

• A small sample of abdominal subcutaneous fat is taken.
• It is a preferred initial test for systemic amyloidosis, as it is less invasive than an organ biopsy.

Congo Red Staining

• Congo red stain is the standard histological stain used to confirm amyloid deposits.
• Under a normal light microscope, amyloid appears as pink-red extracellular deposits.
• Under polarized light, amyloid shows apple-green birefringence, a hallmark of amyloidosis.
• Liver biopsy findings in amyloidosis show amyloid deposits stain red with Congo red around hepatocytes, blood vessels, and bile ducts.
• This pattern suggests hepatic amyloidosis, which can cause hepatomegaly and liver dysfunction.

Protein Structure and Amyloid Formation

Secondary Structure: Beta-Pleated Sheets

• The secondary structure of a protein refers to its local folding pattern, held together by hydrogen bonds.
• Normal proteins typically form alpha-helices or beta-sheets.
• In amyloid proteins, misfolding causes the formation of beta-pleated sheets.
• Misfolded beta-pleated sheets accumulate and form a sheet-like structure, tightly packed in a non-normal manner.

Quaternary Structure: Fibrils (Cross-Linkage)

• Quaternary structure refers to the overall three-dimensional arrangement of multiple polypeptide chains or subunits.
• Amyloid proteins aggregate and cross-link to form fibrils.
• These amyloid fibrils are long, insoluble, and highly stable structures.
• They are composed of numerous misfolded protein molecules linked together.
• As fibrils form, they accumulate and deposit in tissues, disrupting organ function.

Gross Morphology

• Amyloid deposits have a distinctive waxy appearance when observed grossly.
• The waxy appearance is due to the dense, rigid nature of amyloid fibrils.
• In the kidneys, amyloid deposits may cause the renal cortex to look pale and waxy.
• In the liver, the surface may appear smooth and shiny.
• In the heart, it may cause a stiff, pale myocardium.
• Deposits have a firm, slightly granular feel similar to starch granules.

Mechanisms of Protein Misfolding

• Amyloidosis results from protein misfolding, leading to aggregation and deposition in tissues.
• Normal proteins are produced in excessive amounts, and a small fraction misfolds.
• Mutant proteins are produced in normal amounts but fold incorrectly.

Types of Amyloidosis

Primary Amyloidosis (AL Amyloidosis)

• Associated with plasma cell dyscrasias (e.g., multiple myeloma).
• Light chains are overproduced, misfold, and deposit in tissues.

Secondary Amyloidosis (AA Amyloidosis)

• Results from chronic inflammatory conditions.
• Serum amyloid A is an acute-phase reactant produced by the liver during inflammation.
• Commonly affects the kidneys, leading to nephrotic syndrome.
• Symptoms include proteinuria, hypoalbuminemia, edema, and hyperlipidemia.

Organ Involvement and Clinical Manifestations

• Kidneys can have nephrotic syndrome, proteinuria, hypoalbuminemia, and edema.
• Amyloid deposits damage the glomeruli.
• Heart can have restrictive cardiomyopathy and a stiff myocardium.
• Amyloid deposits accumulate in the ventricles, impairing the heart's ability to expand and contract.
• GI tract can have malabsorption and hepatomegaly.
• Amyloid deposits can accumulate in the GI tract, leading to digestive issues.
• Peripheral nerves can have neuropathy and autonomic dysfunction.
• Amyloid deposits in the peripheral nerves can cause numbness, tingling, weakness, and pain.

Localized Amyloidosis Examples

• Alzheimer’s disease → Aβ plaques in the brain, causing memory loss and neurodegeneration.
• Familial Amyloid Cardiomyopathy (ATTR Amyloidosis) → Mutant transthyretin (TTR) deposits in the heart, leading to restrictive cardiomyopathy.
• Senile Cardiac Amyloidosis → Wild-type TTR deposits in the heart, affecting elderly individuals.
• Type 2 Diabetes Mellitus → Islet amyloid polypeptide (IAPP) deposits in pancreatic beta cells, leading to dysfunction.

AL Amyloidosis (Primary Amyloidosis) – Kappa or Lambda Light Chains

• AL amyloidosis results from abnormal plasma cell proliferation, often seen in multiple myeloma or B-cell lymphomas.
• Plasma cells produce excess monoclonal immunoglobulin light chains (kappa or lambda) that misfold and form amyloid deposits.
• Commonly affects kidneys (nephrotic syndrome), the heart (restrictive cardiomyopathy), and nerves (peripheral neuropathy).

AA Amyloidosis (Secondary Amyloidosis) – Serum Amyloid A Protein

• AA amyloidosis occurs as a complication of chronic inflammatory conditions.
• Chronic inflammation leads to sustained elevations in SAA, which can misfold and deposit as amyloid fibrils in various organs.
• Common causes include rheumatoid arthritis, cystic fibrosis, cancer, and inflammatory bowel disease.
• Commonly affects kidneys (nephrotic syndrome), the spleen (splenomegaly), the liver (hepatomegaly), and the gastrointestinal system (malabsorption and diarrhea).

Senile Cerebral Amyloidosis (Alzheimer’s Disease) – Amyloid Beta (Aβ) Precursor Protein

• Senile cerebral amyloidosis is linked to Alzheimer’s disease, accumulating amyloid beta (Aβ) plaques in the brain.
• Aβ is derived from the amyloid precursor protein (APP), misfolding and aggregating into plaques.
• Amyloid plaques accumulating in the brain lead to memory loss, cognitive impairment, and behavioral changes.

Cardiac Amyloidosis – Atrial Natriuretic Peptide (ANP) and Aging

• Cardiac amyloidosis results in amyloid deposits accumulating in the heart, impairing its function.
• As individuals age, small amounts of ANP can misfold and form amyloid fibrils that deposit in the heart.
• Can cause restrictive cardiomyopathy and atrial fibrillation.

Endocrine Amyloidosis – Procalcitonin in Medullary Thyroid Carcinoma

• Endocrine amyloidosis is associated with the deposition of amyloid in endocrine organs.
• In medullary thyroid carcinoma, the thyroid gland accumulates procalcitonin.
• In MTC, C cells proliferate abnormally, leading to the overproduction of calcitonin.
• Can cause medullary thyroid carcinoma symptoms.