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Prion Disease Lab Tests

Prion diseases, also known as transmissible spongiform encephalopathies, are neurodegenerative diseases with long incubation times. Approximately 1 case of sporadic CJD occurs per 1,000,000 people throughout the world. The mean age for the onset of disease is between 57 and 62 years, although rare cases have been reported in young adults and those over 80 years of age. Currently, there is no cure for prion diseases.

Five human prion diseases are currently recognized: Kuru, Creutzfeldt-Jakob disease (CJD), variant Creutzfeldt-Jakob disease (vCJD), Gerstmann-Sträussler-Scheinker syndrome (GSS), and fatal familial insomnia (FFI). Prion diseases are classified as sporadic, familial and acquired. Sporadic prion disease includes CJD, while familial includes familial CJD, fatal familial insomnia and GSS. Acquired prion disease includes iatrogenic CJD, vCJD and Kuru.

Sporadic prion diseases are defined as those that lack a mutation in the prion protein gene and a history of exposure to prion protein. CJD accounts for more than 90 percent of sporadic prion disease. Sporadic cases are believed to occur due to a change in another disease modifier that causes the prion protein to undergo an abnormal conformational change. Familial prion diseases have a mutation at codon 129 that encodes for prion protein with a greater tendency to undergo a conformational change and convert to PrPSc. Familial prion diseases account for approximately 10% of prion diseases.

Acquired prion diseases are caused by exposure to exogenous abnormal PrPSc by food ingestion and medical or surgical procedures. Kuru occurs in tribes in New Guinea that practice cannibalism. Variant CJD (vCJD) was caused in the United Kingdom by consumption of prion contaminated beef from cattle affected by bovine spongiform encephalopathy (BSE). Iatrogenic CJD (iCJD) has occurred following administration of growth hormone derived from cadaveric pituitary glands, dural graft transplants, dural mater in radiographic embolization procedures, corneal transplants, liver transplants, the use of contaminated neurosurgical instruments or stereotactic depth electrodes, and secondary infection with variant CJD through transfusion of infected blood products. Dural graft transplants and use of cadaveric pituitary hormones account for the vast majority of cases of iCJD. Transfusion has been associated with vCJD.

Human prion diseases have common neuropathologic features including neuronal loss, glial cell proliferation, absence of an inflammatory response, and spongiform degeneration. CSF usually contains no cells and has normal glucose. CSF protein may be elevated in about 40 percent of patients.

Prion disease is believed to be caused by the accumulation of an abnormal protein, called scrapie prion protein (PrPSc), which is believed to result from a change in the conformation of a normal cellular prion protein, PrPc. PrPSc is insoluble and precipitates in neurons, causing neurodegeneration. PrPSc is also believed to be transmissible.

Laboratory tests can assist in the diagnosis of prion disease. Detection of 14-3-3 protein in CSF is an adjunctive rather than absolute test for the diagnosis of prion disease. Sensitivity is 92% and specificity 80% in diagnosing sporadic CJD. A negative test does not exclude the diagnosis and a positive result can occur in non-prion diseases. False positive elevations in CSF 14-3-3 have been noted in patients with a variety of neurologic diseases including herpes simplex encephalitis, hypoxic encephalopathy, cerebral metastases, paraneoplastic disease, and metabolic encephalopathies. However, a positive test increases the probability of CJD when other clinical features are suggestive but not diagnostic. The National Prion Disease Pathology Surveillance Center (www.cjdsurveillance.com) based at Case Western Reserve University is the only laboratory in the United States that tests for the 14-3-3 protein.

Elevated tau protein level in CSF has superior sensitivity and specificity for CJD compared to 14-3-3 protein. Patients with CJD have elevated CSF tau levels, but not phosphorylated tau, resulting in an elevated ratio of total tau to phosphorylated tau. Use of this ratio significantly increases specificity for CJD. The National Prion Disease Pathology Surveillance Center measures both 14-3-3 and tau levels in CSF.

Real-time quaking-induced conversion (RT-QuIC), is an assay in which disease-associated prion protein (PrP) initiates a rapid conformational transition in recombinant PrP (recPrP), resulting in the formation of amyloid that can be monitored in real time. Sensitivity and specificity of second generation RT-QuIC have been reported to be 98% and 90%, respectively. The National Prion Disease Pathology Surveillance Center now reflexively performs RT-QuIC on all CSF samples that have 14-3-3 levels of 500 pg/mL or higher.

Specimen requirement is CSF obtained by lumbar puncture. The first 2 mL should be discarded. The next 2 to 5 mL should be collected and frozen as quickly as possible. Specimens should be stored at -80 C and shipped on dry ice.

Pathological studies of brain material to detect protease resistant PrPSc remain the gold standard for the diagnosis of prion diseases.

References

Skillbäck T, Rosén C, Asztely F, et al. Diagnostic performance of cerebrospinal fluid total tau and phosphorylated tau in Creutzfeldt-Jakob disease: results from the Swedish Mortality Registry. JAMA Neurol 2014; 71:476.

Pan T, Sethi J, Nelsen C, et al. Detection of misfolded prion protein in blood with conformationally sensitive peptides. Transfusion 2007; 47:1418.

McGuire LI, Peden AH, Orrú CD, et al. Real time quaking-induced conversion analysis of cerebrospinal fluid in sporadic Creutzfeldt-Jakob disease. Ann Neurol 2012; 72:278.

Atarashi R, Sano K, Satoh K, Nishida N. Real-time quaking-induced conversion: a highly sensitive assay for prion detection. Prion 2011; 5:150.

Orrú CD, Bongianni M, Tonoli G, et al. A test for Creutzfeldt-Jakob disease using nasal brushings. N Engl J Med 2014; 371:519.

Coulthart MB, Jansen GH, Cashman NR. Interpretation of cerebrospinal fluid protein tests in the diagnosis of sporadic Creutzfeldt-Jakob disease: an evidence-based approach. CMAJ 2014; 186:E333.

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