The Value Of Tau Gene Analysis And Detection Of Tau Protein In Cerebrospinal Fluid And Serum For The Diagnosis Of Sporadic Creutzfeldt-Jakob Disease

Prion disease (CJD) is caused by a transmissible agentdesignated as proteinaceous infectious agent (prion). Priondiseases are infectious, inherited, or sporadic encephalopathieswith a poor prognosis and long latency. These disorders includescrapie of sheep and goats, bovine spongiform encephalopathy (BSE)of cattle, and Creutzfeldt-Jakob disease (CJD),Gerstmann-Straussler-Scheinker disease (GSS) and fatal familialinsomnia (FFI) of humans. CJD is a rare disease that occurs in threeforms: sporadic, genetic, and iatrogenic. More recently, a newvariant of CJD (nvCJD) has been reported. The most common form ofCJD is sporadic in nature ,and it is a rapidly progressive andultimately fatal disorder of the central nervous system belongingto the transmissible spongiform encephalopathies. With theappearance of bovine spongiform endephalopathy (BSE) in 1986 andin 1996 with the identification of an apparantly new variant of thehuman spongiform encephalopathy Creutzfeldt-Jakob disease (CJD),great concerns of a potential transmission of BSE to humans havebeen voiced.Diagnosis of CJD can only be definite at postmortem examinationor brain biopsy showing the pathological isoform of the prionprotein (PrPsc). Brain biopsy, however, is not accepted by patientsand may result in iatrogenic transimission. It is necessary to finda new way in the diagnosis of CJD.Up to now, the diagnosis has beenmade according to clinical and EEG criteria. The clinical diagnosisof CJD can be supported by biochemical analysis for NSE, S-100protein, or 14-3-3 protein. However, only 14-3-3 immunblot isincluded in the diagnostic criteria.Alzheimer's disease (AD) is the most common conditionmimicking CJD, sharing some clinical and neuropathologicalfeatures. Neuronal loss with collapse of the cytoarchitecture iscommon in both diseases. Tau protein is main constituent of theintracellar neurofibrillary tangles in AD. Amyloid plaques are seenin up to 10% of patients with CJD and occasionally true Aβdepositshave been reported.Furthermore, it is difficult to distinguish CJDin the early state of disease from AD.We analysed cerebrospinal fluid (CSF) samples and serum samplesof 53 patients for total tau protein and phosphorylated tau usingan enzyme linked immunoassay (ELISA). In the group of 13 patientswith CJD, total tau protein concentrations in CSF weresignificantly higher than in two control groups of patients withother diseases(median 8295 pg/ml, range 145-10934 pg/ml; P<0.05).One control group comprised 11 patients who were seen in the samestudy and were diagnosed as having other dementing diseases (tauconcentration: median 300 pg/ml, range 190-580 pg/ml). The secondcontrol group comprised 29 patients from our hospital with nodementing disease (tau concentration: median 160 pg/ml, range160-1200 pg/ml). Calculating the cut-off point for the groups ofCJD and OD, best results were obtained at a level of 500 pg/ml. Atthis cut-off point, sensitivity for diagnosis of CJD is 84.6% andspecificity 87.5%. The positive predictive value is 68.8% and thenegative predictive value is 94.5%, respectively.Theconcentrations of tau in CSF were increased in CJD patients withacute and subacute onset. There was a significant difference intotal tau protein concentrations in serum between the groups CJDand OD(p<0.05). Calculating the cut-off point for the groups of CJDand OD in serum, best results were obtained at a level of 250 pg/ml.At this cut-off point, sensitivity for diagnosis of CJD is 76.9%and specificity 63.6%. The positive predictive value is 71.4% andthe negative predictive value is 70%, respectively. Its sensitivityand specificity in serum is lower than in CSF. There was nodifference in phosphorylated tau protein concentrations in CSFbetween the 3 groups (p>0.05). Two distinct subgroups could beidentified among the 13 sCJD patients based in the detection ofphosphorylated tau at threonine 181. Phosphorylated tauconcentration was clinically correlated with a significantlyshorter disease duration (p<0.01) . Although the determination ofphosphorylated tau levels cannot be used as a diagnostic biomarker,it may prove useful for estimating the prognosis of an sCJD patient.The P-tau /total tau ratio was lowest in patients with CJD followedOD,and NDC(p<0.05). The P-tau /total tau ratio discriminatedpatients with CJD from other dementias Therefore, determination of total tau protein in the CSF is auseful marker for clinical diagnosis of CJD in the appropriateclinical setting. Certainly, the advantage of tau-protein ELISA isthat it is easy to use in routine laboratories. The prion protein (PrP), a neuronal membrane protein, plays akey role in the development of transmissible spongiformencephalopathies (TSEs). However, only 10% of human TSEs areassociated with mutations within the Prnp region encoding PrPprotein. A common polymorphism of the PrP gene (PRNP) at residue129 in its homozygous state has been associated to the developmentof acquired and sporadic TSEs. The polymorphism at codon 129 of theprion protein gene (PRNP129) is known to modulate the clinicalcharacteristics and neuropathological phenotype. However, thehomozygous state in codon 129 of PRNP is present in more than 50%of the normal population suggesting that other genes may beimplicated in the genetic risk to develop TSEs. Tau is a microtubule associated protein implicated in thepathogenesis of several neurological disorders. Two extendedhaplotypes, H1 and H2, that cover the entire codificant region ofthe tau gene (100kb length) have been described in a Caucasianpopulation. An overrepresentation of the most common haplotype, H1,was detected in several neurodegenerative disorders such asprogressive supranuclear paralysis, corticobasal degeneration andin Parkinson's disease. Tau aggregates are found in about 19% ofthe neuropathological studies of TSEs. Moreover, a significantincrease of tau CSF concentration has been described in a seriesof Creutzfeldt-Jakob disease (CJD) patients compared withcontrols. We therefore initated this work in order to determine whetherthe tau gene confers susceptibility to develop sporadic CJD.Thirteen patients with CJD and 10 healthy controls matched with thepatients according to age and gender were studied. Genomic DNA waspurified from peripheral blood by standard methods. Exons 9 of thetau gene were amplified by PCR using sense primer(5 '-CGAGTCCTGGCTTCACTCC-3 ') and anti-sense primer(5 '-CTTCCAGCCACAGCCATACC-3 '). Exons 9 were analyzed by directsequencing. Mutation screening in the study was restricted to theexons harboring known tau mutations, which include themicrothbule-binding domains of the tau protein. Thus, although taumutations are clearly not common in CJD, the possibility thatmutations outside this area may be involved in sCJD cannot beexcluded.