Induced frameshifting mechanism of replication for an information-carrying scrapie prion

Octa-repeat domain of the mammalian prion protein mRNA forms stable A-helical hairpin structure rather than G-quadruplexes

Misfolding and aggregation of prion protein (PrP) causes neurodegenerative diseases like Creutzfeldt-Jakob disease (CJD) and scrapie. Besides the consensus that spontaneous conversion of normal cellular PrPC into misfolded and aggregating PrPSc is the central event in prion disease, an alternative hypothesis suggests the generation of pathological PrPSc by rare translational frameshifting events in the octa-repeat domain of the PrP mRNA. Ribosomal frameshifting most commonly relies on a slippery site and an adjacent stable RNA structure to stall translating ribosome. Hence, it is crucial to unravel the secondary structure of the octa-repeat domain of PrP mRNA. Each of the five octa-repeats contains a motif (GGCGGUGGUGGCUGGG) which alone in vitro forms a G-quadruplex. Since the propensity of mRNA to form secondary structure depends on the sequence context, we set to determine the structure of the complete octa-repeat region. We assessed the structure of full-length octa-repeat domain of PrP mRNA using dynamic light scattering (DLS), small angle X-ray scattering (SAXS), circular dichroism (CD) spectroscopy and selective 2'-hydroxyl acylation analysis by primer extension (SHAPE). Our data show that the PrP octa-repeat mRNA forms stable A-helical hairpins with no evidence of G-quadruplex structure even in the presence of G-quadruplex stabilizing agents.

Frameshifted prion proteins as pathological agents: quantitative considerations

A quantitatively consistent explanation for the titres of infectivity found in a variety of prion-containing preparations is provided on the basis that the ætiological agents of transmissible spongiform encephalopathy comprise a very small population fraction of prion protein (PrP) variants, which contain frameshifted elements in their N-terminal octapeptide-repeat regions. A mechanism for the replication of frameshifted prions is described and calculations are performed to obtain estimates of the concentration of these PrP variants in normal and infected brain, as well as their enrichment in products of protein misfolding cyclic amplification. These calculations resolve the lack of proper quantitative correlation between measures of infectivity and the presence of conformationally-altered, protease-resistant variants of PrP. Experiments, which could confirm or eventually exclude the role of frameshifted variants in the ætiology of prion disease, are suggested.

Prion diseases and the frame-shifting hypothesis

A group of neurological diseases, which includes scrapie, bovine spongiform encephalopathy and Creutzfeldt-Jakob disease, is caused by prion agents which appear to be comprised solely of an abnormal variant of a cellular protein, PrP. Infectious agents which contain no genetic material pose a conundrum for molecular biologists. Theoretical analysis of the gene encoding PrP has suggested a possible mechanism of replication for this proteinaceous infectious agent. The putative mechanism involves frame-shifting during translation of the PrP gene. The normal and abnormal forms of PrP are predicted to have different amino acid sequences.

A critical review of the nature of the spongiform encephalopathy agent: protein theory versus virus theory

All spongiform encephalopathies (SEs) result in brain disorders brought about by a slow virus. Since the origin of bovine SE (BSE), the infectious nature of the disease has been firmly established. Tubulofilamentous particles/scrapie termed nemavirus (NVP) and scrapie-associated fibrils (SAF) are ultrastructural markers, whereas protease-resistant protein (PrP(sc)) is a protein marker. The PrP molecules aggregate to form SAF. Each NVP consists of three layers: an outer protein coat, an intermediate ssDNA layer, and inner PrP/SAF. Therefore, ssDNA and PrP/SAF are physically associated with each other. The existence of at least 20 stable strains of SEs implies that a nucleic acid molecule serves as the information molecule. Animals inoculated with PrP(sc) do not develop the clinical disease, however, ssDNA purified from scrapie-hamster brains by alkaline gel electrophoresis mixed with binding proteins before inoculation developed the clinical disease. It appears that an "accessory protein" coded by the ssDNA of the NVP interacts with normal PrP(c) molecules, resulting in their conversion to PrP(sc)/SAF. The pathogenesis process in the infected animal, with increasing incubation periods, reveals that larger amounts of normal PrP molecules are modified to form SAF. This interferes with the normal supply of PrP to cell membranes, which become disrupted and eventually fragment, resulting in the vacuoles typical of those found in the SEs. Critical review of scientific literature has demonstrated that the agent contains a DNA genome.

Mechanisms of prion diseases described as long-lasting disorders of the blood-brain barrier system

Prison diseases can be described as disorders wherein circulating prion molecules of external origin intervene with the normal synthesis of similar molecules in the central nervous system. The hypothesis is that the endogenous prion-like molecules denote the blood-brain barrier discontinuity. In the case of barrier discontinuity, small numbers of hypothetical signal molecules enter circulation and attach to the receptor sites in the neighbouring blood vessels. The specific receptors of the cells in the blood vessels stimulated by endogenous prion-like molecules might initiate the repairing processes of the blood-brain barrier. In prion diseases, prion molecules from external sources are similar to endogenous prion-like molecules or to the hypothetical signal molecules described here. Large numbers of prion molecules enter circulation and initiate repairing processes in large brain areas causing the tissue damage. This damage leads to new barrier discontinuities that would provoke pathological process even when the exogenous prion molecules are no longer present in the circulation.

Polymorphism analysis of the prion gene in BSE-affected and unaffected cattle

Polymerase chain reaction (PCR) primers designed to amplify the octapeptide repeat region of the bovine prion gene were used to test the association of genotypes with bovine spongiform encephalitis (BSE) in 56 BSE-affected and 177 unaffected animals. Three alleles (A,B,C) were detected as single-strand conformation polymorphisms (SSCPs) and two alleles (1,2--representing six or five copies of the octapeptide repeat respectively) were detected as amplified double-strand fragment length polymorphisms (AMFLPs). Observed genotypes of SSCPs and AMFLPs were analysed by chi-square. The SSCP genotypes of nuclear family members of animals with BSE and BSE-affected animals were different (P < 0.001, P < 0.01) from unrelated animals of the same breed without BSE. No genotypic differences were found between the BSE-affected animals and their relatives (P > 0.469). No AMFLP genotypic differences were detected between BSE-affected animals, their relatives, unrelated animals of the same breed or animals of different breeds (P > 0.05). These data suggest that BSE-affected animals and their relatives are more likely to have the AA SSCP genotype than unrelated animals of the same breed or animals of different breeds.

Transmissible familial Creutzfeldt-Jakob disease associated with five, seven, and eight extra octapeptide coding repeats in the PRNP gene

The PRNP gene, encoding the amyloid precursor protein that is centrally involved in Creutzfeldt-Jakob disease (CJD), has an unstable region of five variant tandem octapeptide coding repeats between codons 51 and 91. We screened a total of 535 individuals for the presence of extra repeats in this region, including patients with sporadic and familial forms of spongiform encephalopathy, members of their families, other neurological and non-neurological patients, and normal controls. We identified three CJD families (in each of which the proband's disease was neuropathologically confirmed and experimentally transmitted to primates) that were heterozygous for alleles with 10, 12, or 13 repeats, some of which had "wobble" nucleotide substitutions. We also found one individual with 9 repeats and no nucleotide substitutions who had no evidence of neurological disease. These observations, together with data on published British patients with 11 and 14 repeats, strongly suggest that the occurrence of 10 or more octapeptide repeats in the encoded amyloid precursor protein predisposes to CJD.

A 'unified theory' of prion propagation

There is now very persuasive evidence that the transmissible agent for spongiform encephalopathies such as scrapie, consists of a modified form of the normal host protein PrPc, devoid of any nucleic acid. On the other hand, because there are many different strains of scrapie agent with distinct phenotypes which can be propagated in animals homozygous for the PrPc gene, it has been suggested that a nucleic acid must be a component of the agent. Can the two views be reconciled?