Neurodegenerative disorders are becoming increasingly common and an ever greater health care burden, as the average age in Western populations rises. The most common of these illnesses is Alzheimer's disease (AD), of which there are now about 5 million cases in the U.S. alone. Prion diseases (or prionoses) represent another neurodegenerative category, which currently is more rare than AD. However, the recent emergence of new variant Creutzfeldt-Jakob (nvCJD) has raised the possibility of a larger population at risk for this illness, as well as causing great concern regarding the safety of blood bank supplies (Jackson and Collinge, Mol Pathol. 2001; 54:393-9).
The term prionosis is used to describe any disease linked to conditions affecting the prion protein, also termed transmissible spongiform encephalopathies. The first prionosis to be described was scrapie, a disease of sheep recognized for over 250 years. The first identified human prionosis; kuru, is an illness of the Fore people living in the highlands of New Guinea (Gajdusek and Zigas, N. Eng. J. Med., 1957; 257:974-978; Gajdusek and Zigas, Am. J. Med., 1959; 26:442-469). Kuru is thought to be linked to ritualistic cannibalism. The most well-known of the human prionoses, Creutzfeldt-Jacob disease (CJD), initially described by in 1921, is found throughout the world with an incidence of about 1 per million. In addition to extensive cortical spongiosis (i.e., vacuolation of the brain parenchyma), gliosis (i.e., dense fibrous network of neuroglia) and neuronal loss, 10% of CJD cases are characterized by amyloid plaques (Prusiner et al., Prion Protein Biology, 1998; 93:337-348).
Other human prionoses include the autosomal dominantly inherited Gerstmann-Sträussler-Scheinker disease (GSS), described in a large kindred in 1936 (Gerstmann et al., Z Neurol., 1936; 154:736-762), and prion protein-congophilic angiopathy (PrP-CAA) (Ghetti et al., Proc. Natl. Acad. Sci. USA, 1996; 93:744-748). The neuropathological features of PrP-CAA as well as some kindreds of GSS (Ghetti et al., Mol. Neurobiol., 1994; 8:41-48) include neurofibrillary tangles (NET), which is an essential feature of AD. Congophilic angiopathy is also an essential feature of AD. Both these variants of prionoses further link the pathogenesis of AD and the prion related diseases.
Fatal familial insomnia (FFI) is a disorder presenting with intractable insomnia, dysautonomia, a variety of endocrine abnormalities and motor paralysis (Medori et al., N. Eng. J. Med., 1992; 326:444-449). Unlike other prionoses, spongiform change can be a minor feature or be absent altogether. AU patients with FFI have a missense mutation at codon 178 of the PrP gene where Asn is replaced by Asp, coupled with a Met at the polymorphic codon 129 (Goldfarb et al., Science, 1992; 258:806-808). The somewhat divergent clinical and neuropathological features of FFI, in comparison to other human prionoses, highlight the wide spectrum of disease associated with PrP dysfunction and suggests that there may be other human illnesses which have yet to be recognized as prionoses.
In cattle, there has been a recent epidemic of a new prionosis, bovine spongiform encephalopathy (BSE), that has led to more than 160,000 cattle deaths in the UK (Collinge, Hum. Mol. Genet, 1997; 6:1699-1705). This new disease is thought to be caused by meat and bone meal dietary supplements to cattle that were contaminated with scrapie infected sheep and other cattle with BSE. Some evidence suggests that BSE also has led to a new type of CJD, called new variant CJD (vCJD) (Collinge et al., Nature, 1996; 383:685-690). The first cases of vCJD were reported in 1995, when two cases of CJD were found in 2 British teenagers These cases had distinctive neuropathology that included so-called “florid” amyloid plaques which are reminiscent of kuru-associated PrP amyloid plaques (Collee and Bradley, Lancet, 1997; 349:636-641; Will et al., Lancet, 1997; 347:921-925). Since the original reports, there have been 14 cases with these distinctive features; all were in the UK except for one French case. The emergence of vCJD has raised the specter of an epidemic of prion related disease among the British population similar to that of BSE in cattle.
Prion disease is also found among wild animals. The disease, termed chronic wasting disease (CWD), attacks the brains of infected deer and elk, causing the animals to become emaciated, display abnormal behavior, lose bodily functions and die. The incidence of CWD in wild animals is of great concern. The disease was originally described in captive animals 35 years ago in Colorado. However, over the last five years, the disease has been found in wild herds in several surrounding states and Canada, and in early 2002, CWD was detected in wild deer in South Dakota, Wisconsin and New Mexico. The recent detection of CWD in the wild white-tailed deer herd in Wisconsin is of particular concern, since white-tailed deer appear more susceptible than muledeer and elk to CWD with a greater percentage of the herd becoming infected.
The most widely accepted hypothesis regarding the etiology of the prionoses is that the disease is caused by a protein or a “prion” (as in proteinous infectious particle) (Griffith, Nature, 1967; 215:1043-1044; Prusiner, Science, 1982; 216:136-144, Prusiner et al., Prion Protein Biology, 1998; 93:337-348). According to this hypothesis, a prion is a conformationally modified form, termed PrPSc, of a normal cellular protein, termed PrPC, which is a normal host protein found on the surface of many cells, particularly neurons. PrPC and PrPSc are thought to differ only in their conformation, with PrPC having a greater β-sheet content. When introduced into normal, healthy cells, PrPSc causes the conversion of PrPC into additional PrPSc molecules, initiating a self-perpetuating vicious cycle (Prusiner et al., Prion Protein Biology, 1998; 93:337-348). The etiology of prion diseases is thus the conversion of the normal prion protein, PrPC, into its infectious and pathogenic form, PrPSc (Prusiner et al., Prion Protein Biology, 1998; 93:337-348; Horwich and Weismann, Cell, 1997; 89:499-510).
The human PrP gene spans 20 kb and consists of a short, non-coding first exon, a 10-15 kb intron and a second exon that contains the entire 759 bp open reading frame encoding a 253 amino acid protein, and 1.64 kb of 3′ non-translated sequence (SEQ ID NO:1). The identification of the PrP gene, designated PRNP in humans, also allowed for the characterization of numerous mutations associated with familial prionoses (Prusiner et al., Prion Protein Biology, 1998; 93:337-348). Moreover, the PrP gene is highly conserved across mammalian species (see FIG. 1), and sequenced prion proteins include those of cow (SEQ ID NO:2); deer (SEQ ID NO:3), elk (SEQ ID NO:4), and muledeer (SEQ ID NO:5) (Cervenakova et al., Lancet, 1997; 350:219-90; Kaluz et al., Gene, 1997; 199:283-6); mouse (SEQ ID NO:6) and rat (SEQ ID NO:7); sheep (SEQ ID NO:8) and goat (SEQ ID NO:9); Syrian hamster (SEQ ID NO:10) and mink (SEQ ID NO:11); gorilla (SEQ ID NO:12) and chimpanzee (SEQ ID NO:13; Greater Kudu (SEQ ID NO:14); camel (SEQ ID NO:15); and pig (SEQ ID NO:16). Prion proteins in mammals are constitutively expressed in both neuronal and non-neuronal tissue (Kretzschmar et al., Am. J. Pathol., 1986; 122:1-5), and while the highest mRNA levels are found in neurons, in particular in the hippocampus, substantial amounts are also found in the heart and skeletal muscle.
Currently, there are no effective treatment or prevention methods for prion disease in humans or other animals, and only a limited number of approaches have been attempted. Experimental treatment approaches reported include the use of amphotericin B (Pocchiari et al., J. Gen. Virol., 1987; 68(Pt 1):219-223), Congo red (Caughey and Race, J. Neurochem., 1992; 59:768-771), sulphated polyanions (Ladogana et al., J. Gen. Virol., 1992; 73(Pt 3):661-665), anthracyclines (Tagliavini et al., Science, 1997; 276:1119-1122), β-sheet breaker peptides (Soto et al., Lancet, 2000; 355:192-197), porphyrin and phthalocyanine compounds (Priola et al., Science, 2000; 287:1503-1506). Some of these compounds delay the incubation time of animals infected with PrPSc but all have limitations in terms of toxicity and/or pharmacokinetics.
It has long been known that there is no specific immune response against prions, and the immune system appears to help rather than impair the propagation of prions (Aucouturier et al., Clin. Immunol, 2000; 96:79-85; Aucouturier et al., J. Clin. Invest., 2001; 108:703-708). However, therapeutic approaches based on the elicitation of an immune response against prion disease have been suggested (see, e.g., co-pending application PCT/US02/37634, filed Nov. 21, 2002, hereby incorporated by reference in its entirety, and by Wisniewski et al. (Curr. Neurosci Reports 2002; 2:400-4) and Wisniewski et al. (Biochem. Soc. Transact. 2002; 30:574-578)). Experimental studies in mouse models found that animals vaccinated intraperitoneally with recombinant mouse prion protein (rPrP) and complete Freund's adjuvant had a delay in the onset of prion disease, regardless of whether the vaccination was performed prior to or after peripheral prion exposure (Sigurdsson et al., Am J Pathol 2002; 161:13-17). In addition, anti-prion antibodies administered intraperitoneally post-inoculation of scrapie strain 139A increased the time from inoculation to onset of disease in experimental animals (Sigurdsson et al., Neurosci Lett, 2003; 336:185-7).
Active immunization has recently been tried in humans for another conformational disease; AD, however, significant toxicity resulted from the vaccine (Steinberg, New Scientist, 2002; 16:22; Munch et al., J. Neural Transm., 2002; 109:1081-1087; Schenk, Nat. Rev. Neurosci., 2002; 3:824-828). For example, in the human phase 2A clinical trial of the AD vaccine, 15 out of 360 patients worldwide developed symptoms of central nervous system inflammation, with symptoms apparently responding to immunosuppression in most patients (Steinberg, New Scientist, 2002; 16:22; Munch et al., J. Neural Transm., 2002; 109:1081-1087; Schenk, Nat. Rev. Neurosci., 2002; 3:824-828). One possible problem of the vaccine was that fibrillar Aβ1-42, an innately toxic peptide, was used. Also, the AD vaccine was administered subcutaneously with an adjuvant (saponin QS-21) that primarily stimulates cell mediated immunity (White et al., Adv. Exp. Med. Biol., 1991; 303:207-210), and the cerebral inflammation seen in the patients vaccinated appeared to be related to activation of CD8-positive cytotoxic T-cells within the central nervous system (Munch et al., J. Neural Transm., 2002; 109:1081-1087). These results emphasize the difficulties in designing safe and effective vaccines for AD and other conformational diseases.
Accordingly, there is a need for safe and efficient therapeutic and preventive methods for prion diseases. The invention addresses these and other needs in the art.