2018 / 18 January

Modern classification of Amyloidosis


Amyloidosis is a group of the conditions characterized by extracellular and intracellular deposition of an insoluble protein called amyloid fibrils. Deposits of abnormal amyloid alter the normal function of tissues. The diagnosis requires tissue biopsy (needs to be congo-red positive) usually take from the rectum or abdominal fat pads.

What is Amyloid?
Around 90 percent of the deposits in Amyloidosis are amyloid fibrils that are formed by the aggregation of misfolded proteins. Amyloid is congo red positive and can’t be removed and damaged organs must be transplanted.
Historical classification systems for amyloidosis were clinically based on systemic or local. Modern classification systems are biochemically based.

Amyloid deposits visualized by Congo red stain, polarized light (apple green birefringence), and H&E stain (shows deposits in glomerular mesangial areas and tubular basement membranes).

What are two types of systemic amyloidosis?

  • Primary amyloidosis: when Ig light chain deposits systemically (in form of AL amyloid), it’s called primary amyloidosis. It’s associated with plasma cell dyscrasias (disease), when plasma cells make a ton of light chain, they will deposit.
  • Secondary amyloidosis: when SAA protein deposits systemically (in form of AA amyloid), it’s called secondary amyloidosis. SAA is acute phase reactant that is increased in chronic inflammatory states, malignancy, and Familial Mediterranean fever.
  • Dialysis-related amyloidosis: when β2-microglobulin deposits systemically. Seen in patients with ESRD and/or on long-term dialysis.

Amyloid is now classified chemically

Chemical classification of amyloid is the current classification standard. Investigation of a biopsy tissue sample allows chemical classification by detecting which type of protein is the characteristic of the amyloid involved. The amyloidosis nomenclature is based on the amyloid fibril proteins, followed by an abbreviation of the fibril protein precursor.

 

References:

  • Sipe JD, Benson MD, Buxbaum JN, Ikeda S, Merlini G, Saraiva MJ, Westermark P;  Nomenclature Committee of the International Society of Amyloidosis. Amyloid fibril protein nomenclature: 2012 recommendations from the Nomenclature Committee of the International Society of Amyloidosis. Amyloid. 2012 Dec;19(4):167-70. doi: 10.3109/13506129.2012.734345.
  • Sipe JD, Cohen AS. Review: history of the amyloid fibril. J Struct Biol. 2000 Jun;130(2-3):88-98.

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