Prion

 

                                                        Prion

                                P.K.Ghatak,MD

Dr. Daniel Carleton Gajdusek in 1957, while studying a spongiform encephalitis called Kuru, in the local population of Papua New Guinea who were cannibals, proved that Kuru was an infectious disease when he successfully reproduced the disease by injection the brain extract of Kuru victims in chimpanzees. He theorized Kuru was due a slow growing unknown virus. It became evident that it was not a virus because Ultraviolet radiation did not kill the infective agent, on the other hand, was sterilized by a protein alerting chemical, bleach.

 In 1982,Stanley Prusiner of UC at SF called the infective agent "a proteinaceous particle and named it  Prion.

Prion is simply a protein molecule made up of 259 amino acids. All living animals and plants including yeast have prion in the cell membrane and in the cytoplasm. Neurons have the highest concentration of normal Prion molecules.

Chemical compounds like toxins, snake poison etc. are also compounds of proteins but Prions differ from toxins in that Prions multiply in huge numbers in the victims body, whereas, poison and toxins do not. The Medical community was naturally skeptical that an inert protein molecule could multiply in a victim with having no Nucleic acids or, DNA or, RNA.

Dr. Prusiner proved that the infectious Prion ( PrP sc) , once introduced in to the living cells, it induces change of configuration of normal similar protein molecules to adopt the shape and appearance of the PrPsc. The newly formed PrPsc, in turn make changes more protein molecules and the chain reaction follows.

Normal protein molecule is alpha helical in its molecular folding but abnormal Prion takes beta helical folding. Misfolded protein molecules prevent cells from carrying out normal cell functions, including elimination of metabolic wastes. Accumulated wastes cloak cells to death. Brain cells have high concentration of Prion molecules. As the misfolded cells die, they leave behind many small voids in the brain matters. Loss of vital functions of brain cells result in progressive dementia and movement disorder and other symptoms leading to premature end of life. The pathological process is called spongiform encephalopathy because of the resemblance with a sponge.

In normal individual, the nature has provided a gene called PRNP gene, that regulates rate of conversion of misfolded proteins. When mutation occurs in PRNP gene, the mutation is passed to the next generation by Autosomal Dominant fashion. Normal prion protein PRNP codon 129  also exists in polymorphic forms in association with variants Type 1 and Type 2 genes, arise out of coding errors for amino acid methionine and valine respectively. The combination of these mutations results in 6 subtypes. This explain the variable penetration and low frequency of disease in spite of dominant inheritance.

Daniel Carleton Gajdusek and Stanley Prusiner received Noble prize in medicine in 1976 and 1999 respectively.

These are the notation used when referring a particular Prion and a prion disease.

Normal- PrP.  Scrapi - PrPsc.  Creutzfeldt-Jackob disease - CJD.  Variant CJD - vCJD   Familial - f CJD , Spontaneous -sp CJD.

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Diseases caused by prions in humans are

CJD Creutzfeldt-Jacob Disease.

vCJD (variant CJD).

Kuru.

Gerstmann Straussler -Scheinker syndrome (GSSS),

Fatal Familial insomnia,

In mammals prion causes Scrapie in sheep,

Mad-cow-disease in cattle.

Chronic wasting disease in deer, elk, moose, reindeer and caribou.

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