Chapter 71: Amyloidosis

DEFINITION

• Amyloidosis is a heterogeneous group of diseases characterized by tissue infiltration with misfolded protein precursors.

• The term amyloid is used to describe a substance with a homogeneous eosinophilic appearance by light microscopy, a green birefringence on polarizing light microscopy, and a characteristic β-pleated sheet appearance by x-ray diffraction.

• Terms such as primary, secondary, senile, dialysis-associated, and myeloma-associated have been abandoned in favor of the etiologically based, chemical terminology (Table 71–1) (eg, immunoglobulin light chain amyloidosis is termed AL amyloidosis).

EPIDEMIOLOGY

• The incidence of AL amyloidosis is rare, with an incidence of 8 per million persons per year and a median age at diagnosis of 67 years.

• Amyloid A (AA) amyloidosis is increasingly rare, occurring in less than 1% of persons with chronic inflammatory diseases in the United States and Europe.

• AA amyloidosis is more common in Turkey and the Middle East, where it occurs in association with familial Mediterranean fever.

• AA is the only type of amyloidosis that occurs in children.

• Amyloid β₂-microglobulin (Aβ₂M) amyloidosis usually manifests as deposits in the joint synovial and occurs in patients on long-term dialysis. The incidence is declining with changes in dialysis techniques.

• The inherited amyloidoses are rare in the United States, with an estimated incidence of less than approximately 1 per 100,000 persons.

• Amyloidogenic transthyretin (ATTR) amyloidosis is the most common form of familial amyloidosis and is associated with mutations of the gene encoding transthyretin (TTR).

ETIOLOGY AND PATHOGENESIS

• The exact mechanism of fibril formation is unknown and may be different among the various types of amyloid.

• Amyloid precursor proteins typically consist of long fibrils composed of relatively small precursor proteins with molecular weights between 4000 and 25,000 daltons.

• Each amyloid fibril protein has a precursor molecule in the serum.

  — The secondary structures of many of the proteins have substantial β-pleated sheet structure. The known exceptions include serum amyloid A (SAA) and cellular prion protein (PrP^c), which contain little or no β folding in the precursor protein despite extensive β-sheet in the deposited fibrils.

• Amyloid formation involves:

  — Stimulus-generated change in the serum concentration or primary structure of amyloid precursor proteins.

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