The 4-kilodalton (39 to 43 amino acids) amyloid beta protein (beta AP), which is deposited as amyloid in the brains of patients with Alzheimer's diseases, is derived from a large protein, the amyloid beta protein precursor (beta APP). Human mononuclear leukemic (K562) cells expressing a beta AP-bearing, carboxyl-terminal beta APP derivative released significant amounts of a soluble 4-kilodalton beta APP derivative essentially identical to the beta AP deposited in Alzheimer's disease. Human neuroblastoma (M17) cells transfected with constructs expressing full-length beta APP and M17 cells expressing only endogenous beta APP also released soluble 4-kilodalton beta AP, and a similar, if not identical, fragment was readily detected in cerebrospinal fluid from individuals with Alzheimer's disease and normal individuals. Thus cells normally produce and release soluble 4-kilodalton beta AP that is essentially identical to the 4-kilodalton beta AP deposited as insoluble amyloid fibrils in Alzheimer's disease.

The prion protein (PrP) plays an essential role in the pathogenesis of a group of sporadic, genetically determined and infectious fatal degenerative diseases, referred to as "prion diseases", affecting the central nervous system of humans and other mammals. The cellular PrP is encoded by a single copy gene, highly conserved across mammalian species. In prion diseases, PrP undergoes conformational changes involving a shift from alpha-helix to beta-sheet structure. This conversion is important for PrP amyloidogenesis, which occurs to the highest degree in the genetically determined Gerstmann-Sträussler-Scheinker disease (GSS) and prion protein cerebral amyloid angiopathy (PrP-CAA), while it is less frequently seen in other prion diseases. GSS and PrP-CAA are associated with point mutations of the prion protein gene (PRNP); these conditions show a broad spectrum of clinical presentation, the main signs being ataxia, spastic paraparesis, extrapyramidal signs and dementia. In GSS, parenchymal amyloid may be associated with spongiform changes or neurofibrillary lesions; in PrP-CAA, vascular amyloid is associated with neurofibrillary lesions. A major component of the amyloid fibrils in the two diseases is a 7 kDa peptide, spanning residues 81-150 of PrP.

Curvilinear fibrils with the tinctorial properties of amyloid were isolated from a patient with bone and joint involvement complicating chronic dialysis for renal disease. Subunit fractions of 24,000 and 12,000 mol wt were identified after gel filtration under dissociating conditions, the latter containing a significant amount of a dimer of the former. This was confirmed by Edman degradation of each fraction, which yielded the amino terminal sequence of normal human beta-2 microglobulin (B2M) to residues 20 and 30, respectively. The size of the subunit protein (12,000 mol wt) and the amino acid composition make it likely that intact B2M is a major constituent of the fibrils. B2M is thus another example of a low molecular weight serum protein, with a prominent beta-pleated sheet structure, that may adopt the fibrillar configuration of amyloid in certain pathologic states.

Clinical and neuropathological findings are reported in 63 patients with hereditary cerebral haemorrhage with amyloid angiopathy. Patients hadf mostly recurrent strokes, and at least 80% of these were haemorrhages. Almost a third ofthe patients died within a year of their first and only recorded haemorrhage, half of them within two weeks. This angiopathy was restricted to the cerebral and cerebeliar cortex and its covering leptomeninges. As the most important consequence, haemorrhagic infarcts and haemorrhages occurred in the subcortical white matter-that is, the region most vulnerable to impaired cortical circulation. Further development of these subcortical lesions gives rise to the fatal haemorrhages seen at necropsy. In so far as dementia occurs this is likely to result from multiple microinfarcts or haemorrhages. In most cases preamyloid lesions or diffuse plaques and early plaques were seen. No other type ofplaque or neurofibrillary degeneration was found. The plaques occur in conjunction with the angiopathy, but may not occur even when the angiopathy is severe. In one patient plaques were totally absent. Angiopathy and plaques may be the result of the same mutation, the expression of which is governed by tissue factors or phenotypic differences between individual subjects. (7 Neurol Neurosurg Psychiatry 1995;59:699-705)