13 Dec 2008

Creutzfeldt-Jakob disease (CJD) is a fatal and transmissible neurodegenerative disease caused by a buildup of misfolded prion proteins in the brain. In the 1990s an outbreak of variant Creutzfeldt-Jakob disease (vCJD) among young people in England was tied to their consumption of contaminated meat from cows with the prion disease bovine spongiform encephalitis (BSE). The number of cases (~200) was only a small fraction of the millions of people who ate prion-laced meat, leading to the idea that there might be genetic determinants of susceptibility to the disease. One known variant of the PRNP gene (encoding the prion protein) has been identified as a clear risk factor. All of the people who got vCJD were homozygous for methionine at codon 129. However, so are one-third of the U.K. population, a situation that suggests other genetic risk factors are waiting to be discovered. This week, a first genomewide association study looking for additional risk factors for CJD has produced several new loci that may affect susceptibility or modulate the incubation period for the disease. The study, from John Collinge and colleagues at University College, London, raises the possibility that the cases of vCJD seen so far may be only the first wave of BSE hitting the most susceptible slice of the population, and that there may be more to come. The paper appeared online December 11 in Lancet Neurology.

In another report, a new mouse model of CJD offers the fullest recapitulation to date of the neurological signs of human disease. The mice could lead to a better understanding of the pathology of prions, and provide a vehicle to test treatments for the invariably fatal disease. That work, from Roberto Chiesa and colleagues at the Mario Nigra Institute for Pharmacological Research in Milan, Italy, appears in the November 26 issue of Neuron.

In the genetics study, first author Simon Mead and colleagues performed single nucleotide polymorphism (SNP) association analysis in several populations with CJD. In a first phase, they compared 119 British patients who had vCJD to 3,083 controls. This revealed two significant associations within the PRNP gene. As expected, the strongest association was with the 129Met allele. They also found a nearby SNP in the intergenic region downstream of the PRNP gene that independently increased risk.

For confirmation, the investigators looked at SNP associations in 506 additional patients with sporadic CJD, 28 with iatrogenic CJD, and 151 with kuru. In all cases, the codon 129 variant was most strongly associated, and the second PRNP SNP was replicated.

Two other SNP links cropped up, one being a nominal association with a SNP upstream of the retinoic acid receptor beta gene. This needs to be confirmed, but is interesting because retinoic acid has been shown to control the expression of the prion protein (Bate et al., 2004). The final SNP was located upstream of the STMN2 gene, which encodes SCG10, a regulator of microtubule stability in neuronal cells. This SNP was intriguing because it was associated with resistance to vCJD and kuru and with incubation time for the disease. It was not associated with risk for sporadic CJD, but with an earlier age at onset. The researchers made a tentative biological link between prion infection and the gene by showing that expression of the mouse SCG10 was dramatically downregulated in prion-infected mouse cells.

“Our data lend considerable support to the hypothesis that genetic susceptibility in addition to the PRNP codon 129 genotype has contributed significantly to the outbreak of vCJD to date. Whether these effects are on the incubation period rather than susceptibility, such that further waves of BSE-associated prion disease with longer incubation periods might occur in the years ahead and be associated with different genotypes at many risk loci, is unknown,” the authors write.

But what of the chances that many more cases will develop among the already infected? That is hard to gauge for a disease whose incubation period can range up to 50 years. As Hans Kretzschmar of the Ludwig-Maximilians University and Thomas Illig of the Helmholtz Zentrum, both in Munich, Germany, write in an accompanying commentary, “A second wave of CJD with a longer incubation time might hit these shores, but we do not know whether this will be a tidal wave or a trickle.”

Learning more about the genetic underpinnings and potential treatments for prion diseases depends on animal models, the subject of a recent report from the Milan group of Robert Chiesa. First authors Sara Dossena and Luca Imeri created a new mouse model of CJD by expressing the familial mutant D178N/V129 in the absence of any wild-type prion protein. The mice accumulate insoluble prion protein in brain tissue, and develop age-related neurological symptoms including movement problems and memory impairment, abnormal EEG readings, and sleep disturbances. The EEG abnormalities and sleep disruptions resemble those seen in a patient with the same mutation. The pathologies are dose-dependent, as they accelerate in mice expressing higher levels of the mutant protein. While existing mouse models of prion disease have motor abnormalities, the Tg(CJD) mouse is the first to show cognitive defects.

On a cell level, the pathology includes prion deposits in brain, inflammation, and loss of neurons. Cerebellar granule neurons show swelling of the endoplasmic reticulum, associated with altered trafficking of PrP and its retention in the ER. How such changes might lead to cell death is unknown, but ER disruption occurs in many neurodegenerative diseases.

“The fact that Tg(CJD) mice accumulate a misfolded form of mutant PrP in their brains and develop clinical features of CJ argues that essential aspects of pathogenesis are modeled in these animals,” the authors conclude. The animals should be useful for both investigating disease mechanisms and testing potential therapies of inherited prion disease, they write.—Pat McCaffrey

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References

Paper Citations

1. Bate C, Langeveld J, Williams A. Manipulation of PrPres production in scrapie-infected neuroblastoma cells. J Neurosci Methods. 2004 Sep 30;138(1-2):217-23. PubMed.

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