Cerebrospinal fluid total prion protein: a potential in vivo marker of cerebral prion pathology.

Severalhumandegenerativediseasesappear as a result of the misfolding and aggregation of proteins.1 The prototype central nervous system proteinopathy is Creutzfeldt-Jakob disease (CJD), in which neuronal prion protein (PrP) with high α-helical content switches into a stable structure rich in β-pleated sheets in a selfcatalyzingprocess that eventually,afteraprolongedbuild-upphase, turnsneurotoxic, causing a plethora of neurological and psychiatric symptoms.2 Althoughhumanpriondiseases are rare, affecting around 1 to 2 people permillion individuals each year,3 they are frequent differentialdiagnoseswhenpatientspresentwithclinical signs of rapidly progressive central nervous system disease. Reliable tools for diagnosis are neededbecause priondiseases are transmissible, which became clear in the 1930swhen healthy sheep were inoculated with brain tissue from sheep affected by scrapie (a sheep prion disease) and fell ill following an incubation period of a few years.4 The transmissibility was further emphasizedduring the 1980s epidemic of bovine spongiform encephalopathy, a prion disease of cattle in the United Kingdom,5 which eventually also made it clear that bovine spongiform encephalopathy could be transmitted to humans in the form of variant CJD, typically affecting younger individuals showingprionaggregation inperipheral tissuesaswell as in the central nervous system.6-8 Clinical, epidemiological, and experimental research has demonstrated that prions are transmissible by multiple routes, may have long incubation periods, and are resistant to conditions that would inactivatemost previously recognized pathogens, such as formalin treatment and heat denaturation. The first fluid biomarker for CJD was discovered using a classicproteomicsapproach inwhichcerebrospinal fluid (CSF) proteins were separated on 2-dimensional gels that were stained with silver, followed by mass spectrometric identification of proteins with differential expression in CJD vs control samples.9 IncreasedCSF levels of theneuronal 14-3-3 protein were established as a distinctive feature of CJD and developed into a diagnostic test that made it into clinical diagnostic criteria.10Presently, thediagnosisofhumanpriondiseases relies on identifying typical patterns of clinical features (dementia, cerebellar or visual dysfunction, pyramidal or extrapyramidal dysfunction, and akinetic mutism) combined with supportive investigation findings (elevatedCSF levels of 14-3-3 protein, periodic sharp wave complexes on electroencephalogram, or typical patterns of signal change on magnetic resonance imaging of the brain) to reach a probable diagnosis. However, large-scale studies have shown that this results inadiagnostic specificityofonly71%for sporadicCJD.10 Adefinitediagnosisof sporadicCJDcanonlybereachedbyconfirming the presence of typical neuropathological changes (spongiosis, astrogliosis, and deposition ofmisfolded PrP in a characteristic pattern) in brain tissue obtained either at autopsy or biopsy. Another potential biomarker for CJD is CSF tau, an intraaxonal protein selectively expressed within the central nervoussystem.Cerebrospinal fluid levelsof total tau (T-tau,measuredusing assays that donot discriminate betweendifferent tau isoforms) are believed to correlate with the rate of axonal degeneration across several neurological diseases, including CJD.11 In contrast, CSF levels of tau proteins phosphorylated at specific amino acid residues (P-tau) are increased inAlzheimer disease (AD) but generally not in other progressive neurological diseases. Predictive values of T-tau and the T-tau to P-tau ratio suggest that they are of value especially in conjunction with other diagnostic methods to differentiate CJD fromAD.12-14However,as is thecase for the14-3-3protein,other conditionswith severe neuronal injurymay cause increase in T-tau. Thus, the T-tau to P-tau ratio lacks in diagnostic specificity against non-AD disorders. In recent years, a number of novel molecular techniques for thedetectionand/oramplificationofdisease-associatedPrP species have been developed. These techniques have the potential to be specific for the basic pathogenic process in prion diseases. There are approaches based on the ProteinMisfolding Cyclic Amplification techniquewhere the templatedmisfolding of PrP is seededby a test sample containingmisfolded PrP and the accumulated misfolded protein is then detected havingshownpromise.15Quaking-InducedConversion,amodified form of Protein Misfolding Cyclic Amplification combined with an immunoprecipitation step, has been shown to detect extremely small amounts of brain-derived diseaseassociatedPrPwhenCJDbrainhomogenate is diluted intohuman plasma, according to work published by Orru et al.16 As high as 1014-folddilutions (estimated to containonly a fewattograms of abnormal PrP per mL) can be differentiated from dilutions of nonprion disease brain homogenate16 and the assay can be used to detect CJD-causing misfolded PrP in CSF samples with high analytical sensitivity and specificity and gooddiagnostic accuracy.17However, these techniquesaredifficult to standardize and are currently being performed only in a few specialized laboratories around the world. In an attempt to develop a screening test for variant CJD, a solidstate binding matrix to capture and concentrate diseaseRelated article page 267 Opinion