What is amyloidosis?
The word amyloid means “starch-like (see Figure 1).” In 1854 a German pathologist Rudolph Virchow used the term amyloid to describe the deposits found in AA (reactive/secondary) amyloidosis. Since then scientists have found that many proteins can form amyloid fibrils and deposit extracellularly and causes organ and tissue dysfunction. The characteristics of proteins that can form amyloid fibrils include positive staining with Congo red dye, apple green refringence under polarized light, and a β-pleated sheet conformation. Essentially the type of amyloid is recognized by the specific protein which forms the basis of amyloid tissue deposits in each of the diseases called amyloidosis.
The recommended nomenclature to describe the different forms of amyloidosis includes the letter “A” for amyloid followed by the abbreviation of the parent protein. For example ATTR amyloidosis refers to amyloidosis caused by fibril deposition of the protein transthyretin (TTR) extracellularly in organs and tissues. AL amyloidosis is defined as the deposition of immunoglobulin light chain fibrils. The parent protein that forms the amyloid fibril determines the clinical course. ATTR and AL are the most common forms of systemic amyloidosis.
Left: Amyloid fibrils isolated from the liver from a patient with AL amyloidosis. Right: Amyloid fibrils isolated from the heart in a patient with ATTR amyloidosis.