Deseret News Archives,
Thursday, August 3, 1995
At first, doctors thought the 41-year-old high school music director had multiple sclerosis because of his episodes of numbness, weakness and blurred vision But treatment did not help, and within a year he got worse: He needed a cane to walk, and he had plunged into depression. He was forgetful at school and had trouble concentrating, and he asked where his grandmother was even though she had been dead for 15 years. He also had trouble sleeping at night. And in the end, that was the key.
Genetic testing showed that the man had an extraordinarily rare, inherited disease that causes what one scientist calls ``industrial-strength'' insomnia for months or even a year before the patient dies. The music director died 15 months after his first symptoms appeared. Although he had not known of any prior cases in his family, researchers found he was distantly related to a previously reported family with the disease.
Now scientists report that by injecting brain tissue from the man into mice, they were able to transmit the disease to the animals. That confirms the idea that the disease, called fatal familial insomnia, belongs to a small group of neurological disorders caused by an abnormal protein called a prion.
Fewer than 10 families in the world are known to have the lethal inherited insomnia, said Dr. Paul Brown of the National Institute of Neurological Disorders and Stroke in Bethesda, Md. A defect in a certain gene produces the disease after age 30.
In contrast to the case of the music director, insomnia is usually a prominent feature early in the disease, said Dr. Raymond Roos of the University of Chicago Medical Center, who with Brown and others reported the mouse experiment in Thursday's issue of the journal Nature.
``It's a heavy-duty industrial-strength insomnia,'' Brown said. ``It's a lot worse than having trouble sleeping.'' Because the insomnia makes them doze during the day, people with the disease often have trouble concentrating on the job and may get into traffic accidents, Roos said. The disease also brings episodes of fever, racing pulses, impotence, blood pressure fluctuations and unexplained sweating. Other problems can include unsteady gait, jerking in the arms or legs, depression and loss of mental powers.
Roos said he suspects patients might be able to live longer if their insomnia can be treated, noting that animal studies suggest that prolonged sleeplessness itself can be lethal. Scientists had prior indications that the inherited fatal insomnia was among the half-dozen human brain diseases caused by the prion protein. Such diseases can be inherited or can be transmitted by an unusual contamination, such as by tissue transplants.
The best known of these conditions is Creutzfeldt-Jakob disease, which strikes about one in 1 million people in Western countries each year. It killed the choreographer George Balanchine, and some cases have been caused by injections of human growth hormone derived from cadaver pituitary glands. Doctors now use synthetic growth hormone instead.
Transmissible spongiform encephalopathies (TSEs) are neurodegenerative diseases which affect many mammalian species including man, sheep and cattle. The human forms of the disease have been classified into three categories: familial, acquired or iatrogenic, and sporadic. Familial forms are transmitted in an autosomal-dominant fashion, while acquired cases are attributed to transplantation of tissues or administration of products derived from individuals suffering from (unrecognized) TSE. Sporadic TSEs are evenly distributed worldwide, and no exogenous or endogenous causes have been identified. TSEs are readily transmissible to members of the same species and in many instances cross-species transmission has been achieved, albeit mostly with low efficiency.
The nature of the agent responsible for TSEs, the so-called prion, has been the subject of impassioned controversies. The prion is able to survive conventional and even extraordinary desinfection procedures and appears to be devoid of nucleic acids (1). The hypothesis that it is a modified, pathogenic form of a normal host protein and that it multiplies by converting the normal form into a likeness of itself has gained wide acceptance since it was first proposed by Griffith (2) and cast into precise molecular terms by Prusiner (3). The normal host protein, PrPC, which occurs predominantly at the outer cell surface of neurons and glia, and its modified form, PrPSc, are both encoded by the same singular gene (4,5) and are believed to be conformational isomers (6).
The so-called "protein only" hypothesis (7) is supported by much biochemical and genetic evidence and its popularity surged when it became apparent that all known hereditary forms of human prion diseases are tightly linked to one of a variety of mutations within the open reading frame of PRNP, the gene encoding PrP (8,9) (Fig. 1), and when it was shown that knockout mice lacking PrP are resistant to mouse-adapted scrapie and do not propagate infectivity (10,11).
The best-known form of human disease, Creutzfeldt-Jakob disease (CJD), mostly starts with rapidly progressing dementia and usually leads to death in less than one year. Gerstmann-Sträussler-Scheinker syndrome (GSS) is a hereditary condition with slightly different histopathological features, but with progressive cerebellar signs, followed by mental deterioration and slower progression to death. Both diseases usually appear in the fourth to sixth decade of life. Intraspecies transmissibility of prion diseases in man is well-documented by the erstwhile occurrence of kuru, a CJD-like disease which was transmitted by ritual cannibalism and reached epidemic proportions in New Guinea (12), as well as by iatrogenic transmission of CJD to young individuals by administration of pituitary hormones derived from cadaveric glands, prior to the availability of recombinant hormones produced in E. coli.
The broad spectrum of clinical and histopathological presentations in both hereditary and sporadic prion diseases gave rise to speculations that other "idiopathic" neurological disorders might be caused by prions. As a consequence, screening for transmissibility and/or for PRNP mutations in obscure neurodegenerative conditions became a respectable pastime for a number of neuropathologists and led to the characterization of fatal familial insomnia (FFI) as a prion disease, first by virtue of its linkage to mutations in the PRNP gene (13) and now, as reported recently in Nature and Lancet (14,15), by demonstration of its transmissibility to mice.
FFI was first described by a research consortium including the neurology research groups of Bologna and of Cleveland in a large North Italian kindred, with as many as 29 affected individuals among 288 family members over a period spanning six generations (16). The salient clinical trait is the loss of the ability to sleep at ages varying between 40 and 60, along with dream-like episodes.
This devastating disease, which is invariably fatal within 6-18 months, progressively affects the autonomic and motor systems. Positron emission tomography and histopathological examinations revealed hypometabolism and selective neuronal loss, in many subjects confined to the anterior ventral and mediodorsal nuclei of the thalamus (9).
The classical hallmarks of other prion diseases, such as spongy degeneration of the cerebral cortex and deposition of PrP plaques, are not typical of FFI (16). The subsequent identification of at least sixteen additional kindreds in Italy, France, and the United States (P. Gambetti, pers. comm.) suggests that FFI may be no rarer a disease than the more widely known GSS syndrome, which occurs about once per 10 to the 8 individuals per year.
The first breakthrough in the elucidation of the molecular pathogenesis of this disease came about with the unequivocal demonstration that all cases of FFI co-segregate with an Asp178®Asn178 mutation within the PRNP reading frame (13). Curiously, the very same Asn178 mutation had also been described in some families presenting with classical CJD (17).
What then determines the remarkable differences in the clinical and histopathological features of FFI and CJD?
An answer was suggested by Gambetti and colleagues (18). Codon 129 of the PRNP gene is polymorphic and its distribution in the normal Caucasian population is 37% for 129Met/Met, 51% for 129Met/Val, and 12% for 129Val/Val. The presence of one or the other amino acid in itself does not predispose for prion diseases, however most cases of sporadic and iatrogenic CJD are associated with homozygosity for one or the other amino acid (19).
Amazingly, the combination of Val129 and Asn178 on one PRNP allele is associated with familial CJD, but that of Met129 and Asn178 with FFI (18). Moreover, the presence of a Met129 codon in the normal allele of FFI patients is associated with a more severe clinical course of the disease (9). As mentioned above, transmission of prion disease between different species is hampered by the species barrier; this has been attributed to differences in the sequence of the incoming PrpSc and that of the recipient's PrPC which would impede the presumed interactions required for conversion (10).
In the case of transmission from hamster to mouse, the barrier was overcome by introduction of multiple hamster PrP transgenes into the mouse (10,20); similarly, transmission of CJD and GSS to mice (21) occurs more readily when the mice carry human PrP transgenes, particularly in the absence of functional mouse PrP genes (22). Because FFI, while associated with PRNP mutations, does not show histopathology typical for TSE's, it was of particular interest to ascertain whether or not this disease was transmissible. Using different mouse strains as recipients, this was recently shown to be so for one case of FFI by the groups of Tateishi and Gajdusek (14) and for two additional cases by Collinge and his colleagues (15) . This promotes FFI to full membership of the growing family of human prion diseases.
Some features of these reports merit further comment. The case transmitted by Tateishi, although phenotypically indistinguishable from FFI, was somewhat atypical in that the PRNP coding sequence had a deletion on the same allele carrying the FFI mutation (23). While Tateishi et al. transmitted FFI to wild-type mice, Collinge et al. utilized wild-type mice transgenic for human PRNP. In both reports, 12-20 months after inoculation mice developed rapidly progressive clinical symptoms leading to death within few days and typical TSE histopathology, incidentally without showing evidence of sleeplesness.
In neither paper, however, was the titer of prions in the brains of the inoculated mice determined by further transmission studies. Although Tateishi and his colleagues found heavy deposits of pathological PrP in the brain of all FFI-inoculated mice (and, provokingly, show a picture of prominent thalamic PrP deposition!), Collinge et al. (15) were unable to demonstrate protease-resistant PrP in their terminally sick FFI-inoculated mice; the sensitivity of the assay would have permitted the detection of PrPSc at a level 300 times lower that that found in mice inoculated with CJD agent. However, since the titer of infectious agent may have been low, no conclusions can be drawn regarding the relationship of infectious agent to protease-resistant PrP.
We believe that the next interesting, albeit time-consuming steps will be to confirm that the inoculated mice succumbed to prion disease by characterizing the infectious agent in their brain as a heat-resistant entity whose propagation depends on the presence of PrP in the host and to determine the infectious titers and PrPSc levels in subsequent transmissions to mice of different PrP transgenotypes, i.e. human or murine.
While genetic and biochemical evidence strongly supports the role of the PrP gene product as constituent of the infectious agent, it is still unclear whether the prion is identical with the protease-resistant entity designated as PrPSc or with some other form of PrP. The biochemical characterization of the infectious agent is rendered difficult by the fact that the ratio of infectious units to PrP molecules even in highly purified preparations is 1:105 (24). The fact that this ratio may vary considerably in various tissues and as a function of time after inoculation (25), as well as recent findings (D. Riesner, personal communication) showing that PrP resistant to protease but devoid of infectivity can be generated in vitro, suggest that not all forms of protease-resistant PrP are infectious. Moreover, the findings that mice may succumb to what is believed to be prion disease in the absence of detectable levels of PrPSc (15), the great variability in the extent of PrP deposition in sporadic CJD (26) and in atypical dementias with PRNP mutations (27), as well as the lack of scrapie pathology in Prn-p knockout mice chronically exposed to a continuous source of PrPSc (28) raise the question whether the presence of PrPSc in diseased brain is an epiphenomenon rather than a cause of neuronal degeneration.
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December 1994 -- Stanley B. Prusiner, M.D., a professor of neurology and biochemistry at the University of California, San Francisco, received this year's Albert Lasker Basic Medical Research Award for his discovery of a new class of disease-causing agents called prions (pronounced pree-ons). The Lasker Award is the most prestigious medical research prize awarded in the United States.
Dr. Prusiner first received funding from Alzheimer's Diseases Research (ADR) for his pioneering research on the prion in 1986. Since then, he has received nearly $850,000 from ADR to support his investigations on the prion theory.
Much of our knowledge about neurodegeneration, the destruction of nerve cells in the brain and elsewhere in the body, has come from studies of neurodegenerative diseases in animals. Scrapie, a disease affecting sheep, posed a remarkable challenge to scientists because despite clear evidence that the disease is infectious ( meaning it can be transmitted by transplantation or ingestion of affected tissue), the infectious agent was not a typical bacteria or virus. Two diseases that affect humans, Creutzfeldt-Jakob Disease (CJD) and syndrome (GSS), cause neurodegeneration similar to that seen in scrapie, and are also infectious under certain conditions.
Over the past decade, evidence for a unique infectious agent as the cause of scrapie, CJD, and GSS has accumulated. Studies of these diseases led to the hypothesis that this type of neurodegenerative disease is caused by an agent which differs from other infectious agents such as bacteria, viruses and parasites. In 1982, Dr. Prusiner introduced, amid controversy in the scientific community, the term "prion" to distinguish the protein particles that cause scrapie, CJD, and GSS from viruses.
ADR first supported Dr. Prusiner's studies of the structures and properties of prions, and studies that led to the purification and identification of the prion protein in scrapie- infected brains. With continued funding from ADR, Dr. Prusiner has gone on to study the two human diseases, CJD and GSS. It is known that the prion protein is present in normal cells and that the gene that codes for the production of the prion protein is part of a normal human chromosome. In patients with CJD and GSS, the gene for the prion protein contains a mutation that apparently alters the function of the protein, resulting in neurodegeneration, dementia, and finally death.
Through his prion work, Dr. Prusiner has furthered understanding of the role protein shape plays in directing biological processes. There are many similarities between the loss of brain function in prion diseases and in Alzheimer's disease, and it is likely that an understanding of how prion diseases begin and develop will add to our understanding of what happens to the brain in Alzheimer's disease.
The exact incidence of the syndrome is unknown but is estimated to be between one and ten per hundred million. Patients initially suffer from ataxia or dementia and deteriorate until they die, in one to ten years. Protease-resistant prion protein (PrP) and PrP-immunoreactive amyloid plaques with characteristic morphology accumulate in the brains of these patients. Current diagnostic criteria for Gerstmann-Straussler syndrome incorporate clinical and neuropathological features, as animal transmission studies can be unreliable.
Prion protein is implicated in the pathogenesis and transmission of the condition and in scrapie, an equivalent animal disease. It was discovered by enriching scrapie-infected hamster brain fractions for infectivity. Because there is compelling evidence that the scrapie isoform of PrP is a necessary component of the infectious particle, it seemed possible that the PrP gene on the short arm of human chromosome 20 in Gerstmann-Straussler syndrome might be abnormal.
We show here that PrP codon 102 is linked to the putative gene for the syndrome in two pedigrees, providing the best evidence to date that this familial condition is inherited despite also being infectious, and that substitution of leucine for proline at PrP codon 102 may lead to the development of Gerstmann-Straussler syndrome.