Proteopathy

Proteopathy (Proteo- [pref. protein]; -pathy [suff. disease]; proteopathies pl.; proteopathic adj.). Proteopathy is the abnormal accumulation and toxicity of proteins in certain disease states.^[1] Also, selective hyperproteolytic diseases have been referred to this category, e.g. critical illness myopathies or tumor cachexia.^[2] The proteopathies comprise at least 30 diseases that affect a variety of organs and tissues, including Alzheimer's disease, Parkinson's disease, type 2 diabetes, amyloidosis, and a wide range of other disorders (see Table).^[3][4]

The proteopathies are also called protein conformational diseases,^[5] because a change in the 3-dimensional folding (conformation) of a protein increases the tendency of the protein to misfold and polymerize into aggregates that are resistant to clearance, and can become pathogenic. Because of the common structure of the polypeptide backbone, all proteins have the potential to misfold under some conditions.^[6]

Only certain proteins are linked to proteopathy, possibly due to instability or other structural features of the monomeric protein that increase the probability of misconformation, which in nearly all instances involves an increase in beta-sheet secondary structure. Potential risk factors for proteopathic diseases augment the tendency of vulnerable proteins to self-assemble. They include destabilizing changes in the primary amino acid sequence of the protein, post-translational modifications (such as hyperphosphorylation), changes in temperature or pH, an increase in production of a protein, or a decrease in its clearance. Advancing age frequently is a risk factor.

In some proteopathies, abnormal assembly can be templated on an exogenous protein, typically a misfolded form of the same protein. In this way, the disease state can be induced in a susceptible host by the introduction of diseased tissue extract from an afflicted donor. The best known form of such infectious (or transmissible) proteopathy is prion disease, which can be transmitted by exposure of a host organism to purified prion protein in a disease-causing conformation.^[7][8] There is now evidence that other proteopathies are inducible by a similar mechanism, including AA amyloidosis, apolipoprotein AII amyloidosis, and Aβ amyloidosis.^[9][10] In all of these instances, an aberrant form of the protein itself appears to be the pathogenic agent.

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