Confirming the Disease
Currently available tests for prion diseases identify the abnormally-folded prion protein in brains of infected or dead animals through either antibody- or Western blot-based technologies. To date, no tests can identify the presence of the disease-causing proteins in easily accessible blood or tissues.
Disease can be confirmed only upon the appearance of outward symptoms and/or the availability of post-mortem brain and nervous symptom tissues which are examined immunohistochemically.
Within the last few years, cases of vCJD, or variant CJD, have been reported in Europe and Asia originating from transfusions. The appearance of these vCJD infections has been a driving force in the search for diagnostic tests that can find evidence of prion disease in accessible tissues and blood.
All these diseases involve changes in the conformation of prion precursor protein (PrPc), a protein normally expressed in mammalian cells, particularly in neurons, as a plasma membrane glycoprotein.
The inability to detect disease until animals are either sick or dead has seriously complicated control of infected herds. The inadvertent introduction of the bovine disease into the human food supply through an animal feed supplement fed to cows containing meat and bone meal from dead sheep prompted the British government to ban the use of animal-derived food supplements in 1988.
While this step contained the epidemic, the sick cows that sporadically appear in herds cause considerable alarm, as the disease is transmissible to humans through meat consumption.
Apparently absorbed through the intestinal tract upon ingestion of infected meat, abnormal prion particles migrate until they reach the brain to recruit normal prion particles into the disease-causing contorted forms that aggregate, accumulate, and cause brain destruction.
Mass slaughter of potentially infected animals causes considerable economic hardship; between 1996 and 2000, as the British government attempted to eradicate BSE-infected cattle, over five million cattle were killed with financial losses nearing $3 billion. Currently, a definitive diagnosis of BSE can be made only through post-mortem testing.
The unusual etiology of these diseases has fueled a long-standing scientific debate. Stanley B. Prusiner, M.D., of the University of California, San Francisco, first proposed that the causative agents were misfolded protein particles. This idea was considered highly unlikely based on what was understood about infectivity, that is, that a nucleic acid, either DNA or RNA, was required to ensure the replication of the infectious agent in a host.
Studies in Dr. Prusiners and other laboratories showed that the infectious agent in scrapie lacked any kind of nucleic acid, as would be expected if the infectious agent were bacterial or viral, and consisted only of protein. Brain extracts prepared from infected animals in which all nucleic acid had been destroyed could cause infection in healthy animals.
Further experimentation confirmed that a protease-resistant form of a normal brain protein could both cause and transmit this group of diseases. Ongoing research efforts are aimed at further verification and extension of the prion hypothesis and in developing rapid and reliable means of testing for evidence of the disease in animals and animal products.
The concept that proteins could be infectious received further support from studies that identified specific genetic mutations associated with disease development in families with inherited prion diseases. Characterizing the mutations that caused warped prions allowed the development of transgenic mice expressing both the normal prion protein and/or a mutated form.
Animals carrying the mutated gene developed scrapie and the homogenized brain tissue from such animals could transmit the disease to healthy animals. More recent studies have concentrated on developing formal proof that spongiform encephalopathies are caused solely by conformationally altered prion proteins and on developing methods for detection and diagnosis of these diseases.