Orally Infected monkey backs up BSE-CJD link
Between Cows and Monkeys ... analysis by Adriano Aguzzi
Family dementia history ... but normal prion gene in some v-CJD
Marmosets didn't acquire v-VJD from UK beef in monkey chow
Prion phylogeny revisited
Nando and Reuter Information Service
Infected monkey backs up BSE-CJD link
7 July 1996
LONDON (Jul 5, 1996 5:41 p.m. EDT) - French researchers said Friday they had discovered what they believe to be the first case of a non-laboratory monkey catching a form of mad cow disease.
The researchers, writing in the Lancet medical journal, said the discovery appears to back up theories that mad cow disease can be transmitted between species through the food chain. [Bons N et al (1996) "Spontaneous spongiform encephalopathy in a young adult rhesus monkey", The Lancet, 348, 6 July 1996, 55.]
The monkey, which died at Montpellier zoo in 1992, had been fed on meat products. The only monkeys previously reported to have contracted spongiform encephalopathy, the brain disease that affects cows as bovine spongiform encephalopathy (BSE or mad cow disease) and humans as Creutzfeldt-Jakob disease (CJD), were deliberately infected in laboratory experiments.
"As far as we are aware, this is the first reported case of spontaneously developed spongiform encephalopathy in a monkey," the research team from the University of Montpellier wrote.
"The feeding of this monkey with animal protein raises the possibility of cross-species transmission of the disease through contaminated foodstuff." Montpellier's zoo bought the rhesus monkey, born in 1982, from a British zoo in 1986. In 1991, the monkey developed the first symptoms of spongiform encephalopathy, lethargy and mood swings. It was fed on standard monkey food, including one containing meat products declared fit for human consumption.
NATURE 381, 734-735, 1996 (June 27)
Between Cows and Monkeys
From: Adriano Aguzzi
What would constitute definitive evidence that bovine spongiform encephalopathy (BSE) has spread to humans? This question - the importance of which cannot be overstated - is exercising scientists as much as politicians and public health officials and for several months it has been a front-page issue in the European press.
The latest storm was provoked by reports of a novel form of spongiform encephalopathy occurring in young people in Great Britain and, at least in one case, in France1,2. The salient feature (which was termed, perhaps somewhat conservatively, „variant of Creutzfeldt-Jakob disease" or vCJD) is the presence of abundant "florid plaques", large cortical deposits of pathological prion protein (PrPRES) decorated by vacuolated brain tissue in a daisy-like fashion (a in the figure).
Two weeks ago came news of a study with macaque monkeys, released at a press conference, which reinforces the suspicion of a link between BSE and vCJD, and adds pathogenetic evidence to epidemiological data. Elements of the study concerned, which was carried out by Lasmézas and colleagues, now appear in Scientific Correspondence on page 743 of this issue3.
The authors have found that intracerebral inoculation of three macaques with brain extracts from BSE-sick cattle produced a brain disease with plaques identical to those of vCJD patients (b in the figure), whereas no plaques were seen in two macaques inoculated with sporadic human CJD. The monkeys are reported to develop a syndrome consisting mainly of prominent cerebellar signs, but also more exotic signs such as "voracious appetite". The latter symptom was described by Gajdusek to occur in kuru-affected patients from Papua, and was ascribed to hypothalamic damage.
The BSE agent shows a characteristic behavior which has never been seen before in transmissible spongiform encephalopathies. For example, it is highly promiscuous in its choice of hosts. Unlike its counterpart in sheep, mice and hamsters, it appears to infect animals of other species easily, especially when transmitted orally. The molecular basis for these "strain-specific" traits is completely unknown.
Because kuru also exhibits abundant plaques, one might argue that the presumptive oral route of application is the main determinant of neuropathology in vCJD. The observations of Lasmézas et al. run counter to this argument and suggests that the neuropathological findings characteristic of vCJD in man represent a specific trait of the BSE strain of prions in primates. (Here I will use "prion" as a generic term to refer to the infectious agent involved in transmissible encephalopathies, irrespective of its actual physical nature. There is strong, but not yet clinching, evidence that the agent is an aberrant isoform of a normal cellular protein called PrPC. The agent is thought to recruit PrPC and transform it into further infectious protein.)
The results of Lasmezas et al., of course, leave many questions open, the first of which is whether the macaque system is a realistic model of the situation in humans? Although the inoculum used is large by molecular criteria, is equivalent to only 50-100 mg of infectious brain homogenate. It is unsettling that these amounts are well within the range of brain tissue present in commercial food products for human consumption until few years ago.
On the other hand, however spectacular (and worrying) these findings are, it should be stressed that transmission was achieved by direct intracerebral inoculation, and we can hope that the oral route of administration will be considerably less efficient. Although there is no available data on the relative efficiency of oral as opposed to intracerebral transmission of BSE to primates, we know that as little as 500 mg of brain extract administered orally produced encephalopathy in 1 out of 6 sheep3.
Next, how can we go about estimating the risk of contracting the disease after exposure to the BSE agent? One issue of utmost interest is the possibility of genetic susceptibility to infection. By extrapolation of the findings in transgenic and knockout mice, two factors may be expected to play a key role: first, the primary sequence of the PRNP gene which encodes the normal cellular prion protein (PrPC); and, second, the availability of PrPC for conversion into pathological PrPRES by the infectious agent (which may or may not be identical to PrPRES).
As far as the first issue goes, the polymorphism at codon 129 of the PRNP gene (which encodes either methionine or valine) is emerging as a crucial parameter4. All published British vCJD patients, as well as the recent case from France, were homozygous for 129Met, while a further suspected patient is homozygous for 129Val (J. Collinge, pers. comm). Therefore, 129Met/Val heterozygosity (prevalence: 51% in Caucasians) would appear to be a protective factor. Within the framework of the protein-only hypothesis5 this finding may indicate that heterozygous individuals produce two PrPC moieties which are subtly different from each other, and reduce the efficiency of PrPC-PrPRES conversion by reciprocal competition.
Regrettably, this important issue was not studied in detail: the codon 129 status in the three inoculated animals was not determined, and indeed it is unclear whether codon 129 is polymorphic in the macaque. A further potential risk factor may be represented by the amount of PrPC present in the brain. It has been repeatedly demonstrated that prion replication relies on the presence of PrPC (ref. 6), and that Prnp+/o heterozygous mice with reduced levels of PrPC develop a milder form of scrapie after challenge with infectious prions7,8. Moreover, PrPC is an essential mediator of pathogenesis since chronic exposure of Prnpo/o mice to large doses of prions does not induce any brain damage9. So it will be important to confirm that PrPC expression levels in macaque monkeys are similar to those of humans.
Another largely not understood issue relates to the mechanisms of neuro-invasiveness. How do prions, when administered via the gastrointestinal tract, travel across the body and colonize the nervous system? Several lines of evidence point to lymphatic organs as a possible link. As recently shown by Lasmézas and colleagues, the neuroinvasiveness of intraperitoneally administered prions is impaired in SCID mice (which suffer from major lymphatic defects), and can be reconstituted by transfer of spleen cells10.
While transgenic mouse models are proving helpful in dissecting this problem, they may fall short of reproducing all the peculiarities of the BSE agent. The reason why caution needs to be exerted is that, although BSE prions may replicate in the lymphoreticular system of most species (including sheep, hamster, and mouse), they show a surprising hesitance to infect spleen and lymph nodes of cattle.
One obvious question, therefore, which demands to be addressed with urgency, reads: Will BSE prions cause vCJD in macaques even if they are administered orally? If so, what is the minimal infectious dose? Such studies are demanding since they require exposure of a large number of primates to serial dilutions of the infectious agent over many years. Further experiments will have to address the molecular basis of the strain characteristics of the BSE and the human vCJD agents. A thorough understanding of the latter is likely to be a precondition to devising rational approaches to prevention and, perhaps, to treatment11.1. Will, R. G., Ironside, J. W., Zeidler, M., et al. Lancet 347, 921-925 (1996). 2. Chazot, G., Broussolle, E., Lapras, C. I., Blättler, T., Aguzzi, A. & Kopp, N. Lancet 347, 1181(1996). 3. Foster, J. D., Bruce, M., McConnell, I., Chree, A. & Fraser, H. Vet. Rec. 138, 546-548 (1996). 4. Palmer, M. S., Dryden, A. J., Hughes, J. T. & Collinge, J. Nature 352, 340-342 (1991). 5. Prusiner, S. B. Annu. Rev. Microbiol. 43, 345-374 (1989). 6. Bueler, H. R., Aguzzi, A., Sailer, A., et al. Cell 73, 1339-1347 (1993). 7. Bueler, H., Raeber, A., Sailer, A., Fischer, M., Aguzzi, A. & Weissmann, C. Molecular Medicine 1, 19-30 (1994). 8. Manson, J. C., Clarke, A. R., McBride, P. A., McConnell, I. & Hope, J. Neurodegeneration. 3, 331-340 (1994). 9. Brandner, S., Isenmann, S., Raeber, A., et al. Nature 379, 339-343 (1996). 10. Lasmezas, C., Deslys, J. P., Robain, O., et al. Nature this issue, (1996). 11. The author wishes to thank Drs. Charles Weissmann and John Collinge for helpful discussions.Figure Legend:
a: The brown immunohistochemical stain evidences the prion protein in the "florid plaques" characteristic of the new CJD variant in the brain of a 26-year old French patient (in a cortical biopsy taken several weeks before death).
b: Similar plaques are present in the brain of BSE-inoculated macaques.
Collinge et al report that in several of "the ten" cases of V-CJD reported on by CJDSU, there was a family history of dementia. However, they report that protein gene analysis has excluded these cases as being inherited forms of prion disease.
Collinge J et al (1996) "Prion protein gene analysis in new variant cases of Creutzfeldt-Jakob disease", The Lancet, 348, 6 July 1996, 56.
Ridley et al report on their breeding colony of marmosets for use in neuropsychological research, More than 100 born between 1980 and 1990 were fed for their entire lives, 5-10 years, on a diet including pellets containing 20% ruminant-derived protein (cf about 4% in cattle feed in the same period). None developed spongiform encephalopathy
The authors conclude "Our observation serves as a reminder that the oral route is probably an inefficient mode of infection for spongiform encephalopathy across the species barrier." [Marmosets, a new world monkey, also did not acquire BSE from intra-cranial inoculations -- webmaster]
Ridley R M et al (1996) "Failure to transmit bovine spongiform encephalopathy to marmosets with ruminant-derived meal", The Lancet, 348, 6 July 1996, 56.
Prion phylogeny revisited Nature 4 July 1996 ... Vol. 382, No. 6586 pages 32-33
by W Goldmann, N Hunter, R Somerville & J Hope
Institute for Animal Health, Edinburgh, UK