Amyloidosis

Abstract

The clinical amyloidosis syndromes are a heterogeneous group of disorders characterised by abnormal extracellular accumulation of autologous protein material. Amyloid deposits are largely composed of insoluble protein fibrils which are intimately associated with sulphated glycosaminoglycan residues. Amyloid P component (AP) is a minor but almost universal constituent of the deposits and is derived from the normal circulating glycoprotein serum amyloid P component (SAP), a member of the pentraxin family of plasma proteins. The current classification of amyloidosis is based on the chemical nature of the fibril protein subunits. Systemic amyloidosis is well known as a relatively rare but important cause of serious morbidity, and vital organ involvement is usually fatal. In recent years it has become increasingly recognised that amyloid deposition in a variety of sites is a universal feature of ageing, and in particular that amyloid in the cerebral blood vessels and within the brain itself is an integral part of the pathology of Alzheimer's disease. Also recently, a new form of amyloid has emerged, confined to patients who have received long term haemodialysis for end stage renal failure, in whom β 2-microglobulin ( β 2M) is laid down as amyloid fibrils predominantly in periarticular and bony tissues. Considerable progress in knowledge of the composition, molecular structure and properties of many different constituents of amyloid has been made in the past 30 years. However, the precise mechanisms of amyloid fibril formation, deposition and persistence are not known and no generally effective therapy yet exists which can arrest amyloid deposition or promote its resolution. A major reason for our ignorance of the natural history of amyloidosis is that it is an exclusively histological diagnosis, at the time of which most patients with systemic disease have extensive amyloid deposits throughout many organs and a very poor prognosis. Some optimism has been generated from recent work suggesting that amyloid fibrils are not inherently non-biodegradable and that the use of radiolabelled SAP may permit non-invasive sensitive, scintigraphic imaging of amyloid deposits in vivo.