diseases are not common, the major affected animals are limited to
humans, sheep, and cattle, so it is not surprising that few animals
have orthologs to the human prion protein.
Amino acid sequence alignment of human prion protein and six orthologs. From HomoloGene
|House Mouse||NP 035200.1|
|Norway Rat||NP 036763.1|
|Chicken||NP 990796.1|Figure 2.
A cladogram proposing a phylogenetic tree for the evolution of the prion-encoding gene. From ClustalW
sequence alignment shows that there is high level of conservation
between the prion proteins of the various species. These six are
a limited selection of the orthologs found, but representative as all
were vertebrates, mostly mammals with the occasional bird and
amphibian. Orthologs were looked for in Arabidopsis
, yeast, C. elegans
, E. coli
, and Drosophila
, but none were found.
Centers for Disease Control and Prevention argues that there is
evidence that many of the human and non-human prion diseases are
descended from each other. On their website the CDC writes that
bovine spongiform encephalopathy (BSE or mad cow) could have resulted
from feeding cattle scrapie-infected, a sheep prion disease, sheep
products. There is also evidence that the human prion disease
variant Creutzfeldt-Jakob disease (vCJD) resulted from exposure to
BSE-contaminated food. The fact that one species's prions can
also cause disease in another species speaks to the high conservation.