Genes VII

Prion diseases occur in sheep and man, and, more recently, in cows. The basic phenotype is an ataxia Xa neurodegenerative disorder that is manifested by an inability to remain upright. The name of the disease in sheep, scrapie, reflects the phenotype: the sheep rub against walls in order to stay upright. Scrapie can be perpetuated by inoculating sheep with tissue extracts from infected animals. The disease kuru was found in New Guinea, where it appeared to be perpetuated by cannibalism, in particular the eating of brains. Related diseases in Western populations with a pattern of genetic transmission include Gerstmann-Straussler syndrome; and the related Creutzfeldt-Jakob disease (CJD) occurs sporadically. Most recently, a disease resembling CJD appears to have been transmitted by consumption of meat from cows suffering from "mad cow" disease (Hsiao et al., 1989).

When tissue from scrapie-infected sheep is inoculated into mice, the disease occurs in a period ranging from 75 V150 days. The active component is a protease-resistant protein. The protein is coded by a gene that is normally expressed in brain. The form of the protein in normal brain, called PrP^C, is sensitive to proteases. Its conversion to the resistant form, called Prp^Sc, is associated with occurrence of the disease. The infectious preparation has no detectable nucleic acid, is more sensitive to UV irradiation at wave lengths that damage protein than nucleic acid, but has a low infectivity (1 infectious unit / 10⁵ PrP^Sc proteins). This corresponds to an epigenetic inheritance in which there is no change in genetic information, because normal and diseased cells have the same PrP gene sequence, but the PrP^Sc form of the protein is the infectious agent, whereas PrP^C is harmless (McKinley et al., 1983; Oesch et al., 1985; Basler et al., 1986; for review see Prusiner, 1982).

The basis for the difference between the PrP^Sc and Prp^C forms is not known. Both proteins are glycosylated and linked to the membrane by a GPI-linkage. No changes in these modifications have been found. The PrP^Sc form has a high content of β sheets, which is absent from the PrP^C form.

The assay for infectivity in mice allows the dependence on protein sequence to be tested. Figure 19.58 illustrates the results of some critical experiments. The leftmost column shows the normal situation in which PrP^Sc protein extracted from an infected mouse will induce disease (and ultimately kill) when it is injected into a recipient mouse.

If the PrP gene is "knocked out", a mouse becomes resistant to infection. This experiment demonstrates two things. First, the endogenous protein is necessary for an infection, presumably because it provides the raw material that is converted into the infectious agent. Second, the cause of disease is not the removal of the PrP^C form of the protein, because a mouse with no PrP^C survives normally: the disease is caused by a gain-of-function in PrP^Sc (Bueler et al., 1993).

The existence of species barriers allows hybrid proteins to be constructed to delineate the features required for infectivity. The original preparations of scrapie were perpetuated in several types of animal, but these cannot always be transferred readily. For example, mice are resistant to infection from prions of hamsters. This means that hamster-PrP^Sc cannot convert mouse-PrP^C to PrP^Sc. However, the situation changes if the mouse PrP gene is replaced by a hamster PrP gene. (This can be done by introducing the hamster PrP gene into the PrP knockout mouse.) A mouse with a hamster PrP gene is sensitive to infection by hamster PrP^Sc. This suggests that the conversion of cellular PrP^C protein into the Sc state requires that the Sc and C proteins have matched sequences.

There are different "strains" of PrP^Sc, which are distinguished by characteristic incubation periods upon inoculation into mice. This implies that the protein is not restricted solely to alternative states of PrP^C and PrP^Sc, but that there may be multiple Sc states. These differences must depend on some self-propagating property of the protein other than its sequence. If conformation is the feature that distinguishes PrP^Sc from PrP^C, then there must be multiple conformations, each of which has a self-templating property when it converts PrP^C (Scott et al., 1993; for review see Prusiner and Scott, 1997).

The probability of conversion from PrP^C to PrP^Sc is affected by the sequence of PrP. Gerstmann-Straussler syndrome in man is caused by a single amino acid change in PrP. This is inherited as a dominant trait. If the same change is made in the mouse PrP gene, mice develop the disease. This suggests that the mutant protein has an increased probability of spontaneous conversion into the Sc state. Similarly, the sequence of the PrP gene determines the susceptibility of sheep to develop the disease spontaneously; the combination of amino acids at three positions (codons 136, 154, and 171) determines susceptibility.

The prion offers an extreme case of epigenetic inheritance, in which the infectious agent is a protein that can adopt multiple conformations, each of which has a self-templating property. This property is likely to involve the state of aggregation of the protein.