Human prion diseases: molecular, cellular and population biology.

The past 20 years have witnessed a dramatic resurgence of interest in a hitherto obscure neurodegenerative disease, Creutzfeldt-Jakob disease (CJD). This was driven partly by the novelty of the prion hypothesis, which sought to provide an explanation for the pathogenesis of transmissible spongiform encephalopathies, involving a unique epigenetic mechanism, and partly by events in the UK, where an outbreak of a new prion disease in cattle (bovine spongiform encephalopathy or BSE) potentially exposed a large section of the UK population to prion infectivity through a dietary route. The numbers of cases of the resultant novel disease variant CJD (vCJD), have so far been limited and peaked in the UK in the year 2000 and have subsequently declined. However, the effects of BSE and vCJD have been far-reaching. The estimated prevalence of vCJD infection in the UK is substantially higher than the numbers of clinical cases would suggest, posing a difficult dilemma for those involved in blood transfusion, tissue transplantation and cellular therapies. The clinico-pathological phenotype of human prion diseases has come under close scrutiny and molecular classification systems have been developed to account for the different diseases and their phenotypic spectra. Moreover, enhanced human and animal surveillance and better diagnostic tools have identified new human and animal prion diseases. Lastly, as the prion hypothesis has gained widespread acceptance, the concepts involved have been applied to other areas, including extra-chromosomal inheritance in fungi, long-term potentiation in memory formation and the spread of molecular pathology in diverse conditions, such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. Studies at the molecular and cellular level have helped to provide a better understanding of human prion diseases, aided pathological diagnosis and helped inform public health decision-making.