Prions - The Disease Causing Agent

BSE is not caused by a bacterium or virus (it contains no nucleic acid). The disease appears to be caused by an unconventional infectious agent, originally believed to be a "slow virus," a "self-replicating protein.

Recently the causitive agent was found to be an abnormal conformation form (PrP^sc) of a normally occurring protien (PrP). PrP is the abbreviation used to identify the normal "prion" protein. This agent is extremely resistant to heat and to normal sterilization processes. PrP^sc does not evoke a detectable immune response or inflammatory reaction in host animals.

 

PrP	PrPsc
structure high in alpha-helix	structure high in beta-structure
protease susceptible	core residues are protease resistant
monomeric species	forms multimeric aggregates

click on image to view a 56k cartoon of possible structural changes in PrP

The prion protein, PrP, is a normal cell protein present on nerve cell membranes. The gene for PrP is present in most mammals. However, PrP's normal function is unclear; mice without the protein appear to be fine.

PrP's amino acid sequences are highly conserved across species (>85% identity between human, mouse, sheep and cattle). This similarity permits PrP from one species to interact with PrP from another species (a requirement for infectivity).

click on image to view a 33k version for comparison of PrP sequences between species

The sequence of amino acids in PrP and PrP^sc is identical within a species. This means that PrP^sc is formed from PrP after synthesis from the gene. The change is in conformation or shape. PrP changes conformation, from predominantly helix to predominantly pleated sheet, creating PrP^sc.

The altered PrP^sc protein is catalytic or "self-propagating". One PrP^sc causes other PrP molecule to change into PrP^sc. The PrP^sc protein core is very resistant to endogenous protease that would destroy the normal protein. Because the PrP^sc can't be broken down, it builds up, aggregates, then precipitates forming plaques and causing spongiform damage in the brain.

click on image to view a 72k image depicting the method of conversion

For the same reason that cells can not destroy PrP^sc , prions are very heat resistant.

• Some loss of infectivity occurs at temperatures above 100°C
• But 30 to 60 minutes at more than 130°C is needed for inactivation, although autoclaving at 134°C (Marsh, 1991) to 138°C for 60 minutes does not result in complete inactivation
• Prions remain infective after sterilizing levels of radiation, formalin exposure, extremes of pH, non-polar organic solvents, burying for years, passing through 0.1 µm filters (2.2 µm filters remove bacteria) and cremation at 343°C (Marsh, 1991 ) or 360°C
• Prions infectivity is destroyed by 1M NaOH @ 55°C or chlorine bleach @ 20,000 ppm (household bleach is 50,000 ppm)
• Exposure for two hours to a sodium dichloroisocyanurate solution (16,000 ppm of available chlorine) did not completely deactivate infectiveness
• Homogenates of BSE-infected bovine brain and rodent brain infected with scrapie-agent and exposed to 2M sodium hydroxide for two hours did not inactivate the prion agents. High-infectivity hamster-adapted scrapie is extremely resistant to heat, chemicals, and processing

What causes the original change in conformation of normal PrP to disease causing PrP^sc? While the spontaneous conversion of PrP to PrP^sc is unfavorable (corresponding to a low incidence of sporadic CJD), mutations in the PrP gene can make the spontaneous conversion more likely. There are at least 20 known mutations in the PrP gene sequence resulting in "spontaneous" PrP^sc formation. Each mutation results in a slightly different conformation, and slightly different pathology. This results in different prion "strains." Regardless of the mutation the "spontaneous" PrP^sc can change non-mutated PrP into PrP^sc.