Amyloid is an abnormal insoluble extracellular protein that deposits in the different tissues and causes organic dysfunction and a wide variety of clinical syndromes. In systemic amyloidosis, amyloid gradually accumulate and amyloid deposition is widespread in the viscera, blood vessel walls, and in the different connective tissues. Primary (AL) amyloidosis) is the most common type of amyloidosis. It results from aggregation and deposition of monoclonal immunoglobulin (Ig) light chains that usually produced by plasma cell clones. Secondary amyloidosis is associated with chronic inflammation (such as tuberculosis or rheumatoid arthritis). Hereditary (or familial) amyloidosis are autosomal dominant diseases that inherited variant proteins cause the production and deposition of amyloid fibrils. Some neurodegenerative disorders such as Parkinson’s disease, Alzheimer, and Huntington’s disease may occur in localised amyloidosis. On microscopic pathology, typical green birefringence under polarized light after Congo red staining (appears in red under normal light).

• Amyloid is an abnormal insoluble extracellular protein that deposits in the different tissues and causes organic dysfunction and a wide variety of clinical syndromes.^[1]
• These abnormal amyloids are derived from misfolding and aggregation of normally soluble proteins.
• Amyloid deposition can disrupt tissue structure of involved organ and consequently leads to organ failure.^[2]

• In systemic amyloidosis, amyloid gradually accumulates and amyloid deposition is widespread in the viscera, blood vessel walls, and different connective tissues.^[3][4]

• Primary (AL) amyloidosis) is the most common type of amyloidosis.^[5]
• It results from aggregation and deposition of monoclonal immunoglobulin (Ig) light chains that are usually produced by plasma cell clones.
• Change in the secondary structure or tertiary structure of a monoclonal light chain results in abnormal folding of the light chain that abnormally form amyloid fibrils.
• This type of amyloidosis most frequently involve the kidney (nephrotic syndrome) and the heart.
• In primary (AL) amyloidosis survival rate depends on:
  • Type of organ involvement (amyloid heart disease is the main prognostic factor)
  • The severity of different organ involvement
  • Hematological response to treatment
• The median survival of patients with AL amyloidosis is approximately 3.8 years.

• Secondary amyloidosis occurs as a reaction to an existing illness.
• Secondary amyloidosis is associated with chronic inflammation (such as tuberculosis or rheumatoid arthritis).^[6]
• Secondary or reactive amyloidosis (AA) comprises approximately 45% of the systemic amyloidoses.
• Pathogenesis of secondary amyloidosis is multifactorial, including:
  • Primary structure of the precursor protein
  • Acute phase response
  • Nonfibril proteins (amyloid P component, apo E, GAGs, proteoglycans and basement membrane proteins)
  • Receptors
  • Lipid metabolism
  • Proteases

• Hereditary amyloidosis is an autosomal dominant disorder.^[7]
• It can have a heterogeneous nature of presentation and can be complicated by significant disability and mortality.^[8]
• Inherited variant proteins cause the production and deposition of amyloid fibrils.^[6]
• Hereditary amyloidosis is due to amyloidogenic mutations and the subsequent deposition of amyloids which include:
  • Transthyretin (TTR) (most common inherited mutation)
  • Fibrinogen
  • Apolipoprotein A1
  • Apolipoprotein A2
  • Lysozyme
  • Gelsolin genes

• In this type of amyloidosis, amyloid deposition occurs only in the organ of origin or tissue of precursor protein.
• Neurodegenerative disorders, such as Parkinson’s disease, Alzheimer’s disease, and Huntington’s disease, may occur in localized amyloidosis.

• Localized amyloidoses can develop due to the deposition of intracellular and/or extracellular amyloid.
  • Huntington’s disease: intracellular protein deposition
  • Parkinson’s disease: intracellular protein deposition
  • Alzheimer’s disease: intracellular (Tau protein fibrils) and extracellular (amyloid β fibrils) deposition

• Hereditary amyloidosis involves mutations in the following genes:^[9]
  • LYZ gene
  • Fibrinogen A alpha polypeptide gene
  • FGA gene
  • APOA1 gene
  • Lysozyme gene
  • B2M gene

• Primary localized cutaneous amyloidosis is inherited in autosomal dominant manner.^[10]

• Primary localized cutaneous amyloidosis involves mutations in the following genes:^[11]
  • OSMR gene

Conditions associated with amyloidosis include:^[12]

• MEN2A

On gross pathology, the organs affected by amyloidosis can be characterized by the following features:

• Porcelain like or waxy appearance
• Enlargement

On microscopic histopathological analysis, amyloidosis is characterized by:^[6][16]

• Green birefringence under polarized light after Congo red staining (appears red under normal light)
• Linear non-branching fibrils (indefinite length with an approximately same diameter)
• Distinct X-ray diffraction pattern consistent with Pauling’s model of a cross-beta fibril