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Title: Diagnostic utility of RT-QuIC for the diagnosis of Sporadic Creutzfeldt-Jakob Disease
Author: Davidson, Louise Ruth Robertson
ISNI:       0000 0004 7969 2029
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2019
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Sporadic Creutzfeldt-Jakob Disease (sCJD) remains the commonest type of prion disease with an incidence of approximately 1 per million and accounting for approximately 80% of cases. The exact infectious mechanism is unknown. However, the most widely accepted theory is the 'protein-only hypothesis' initially suggested by Prusiner and later developed over the years. It has been proposed that prion diseases result from the post translational change of a normally expressed protein (PrPC) into a disease associated form (PrPSc). PrPSc is partially protease resistant and reserves the ability to self-propagate by inducing further PrPC to undergo conformational change thus producing more PrPSc, a so called 'seeding' effect. PrPSc subsequently aggregates throughout the brain, producing the neuro-pathological hall mark of prion diseases which includes spongiform change, neuronal loss and astrocytic gliosis. Despite its rarity, the clinical presentation is well described and classically follows a characteristic course of rapid onset dementia with associated neurological decline that often includes cerebellar ataxia, myoclonus, eventually leading to a state of akinetic mutism and death within a period of 4-6 months. Atypical presentations are less common but are well documented in the literature. Currently, there is not a disease specific ante-mortem test available for the diagnosis of sCJD with post mortem remaining the definitive means of diagnosis. Current diagnostic criteria rely on clinical presentation in association with the MRI, EEG and CSF 14-3-3 protein. However, none of these investigations are specific for sCJD. Atypical presentations can be diagnostically challenging and there can be a delay in diagnosis which can be distressing for relatives. This identified a need for a disease-specific, reliable diagnostic test that can provide an earlier and more accurate diagnosis. A recently developed assay called real time quaking induced conversion (RT-QuIC) exploits the seeded conversion of normal prion protein to the abnormal form and therefore detects disease-associated prion protein in the cerebrospinal fluid. Based on recent evidence, it has RT-QuIC has been reported to be highly sensitive and specific for diagnosing sporadic CJD and has the potential to identify cases that conventional diagnostic techniques such as electroencephalogram and MRI may miss, potentially contributing to an earlier and more accurate diagnosis. This study aimed to provide a prospective analysis of the utility of RTQuIC in routine clinical practice. Aims of Thesis The aims of this work were: - To prospectively assess the diagnostic utility of RT-QuIC in routine clinical practice - To assess if certain clinical factors affect the RT-QuIC result, for example, codon 129 genotype, age at onset, symptoms at onset and duration of disease. - To determine the value of RT-QuIC in cases where there is diagnostic uncertainty using current diagnostic techniques and establish whether it can provide an earlier diagnosis - To review the operational parameters (for example CSF volume, timing of lumbar puncture) of RT-QuIC throughout the course of the study and contribute to the optimisation and development of this new diagnostic test. Methods 162 suspected cases of sCJD were referred to the National CJD Research and Surveillance Unit (NCJDRSU) within the 18 month study period. 146 suspected cases underwent a lumbar puncture and 115 of the clinically suspected cases had 14-3-3 and RT-QuIC analysis performed. All cases were examined by the author where possible and the family were interviewed using a standardised questionnaire. All cases were classified according to the current EuroCJD criteria pre and post review. MRI and EEG were reviewed where possible. Genetic testing and codon 129 were analysed following obtained consent. Brain tissue following post mortem examination was also reviewed where possible. Results 44 cases were classified as pathologically confirmed sCJD. Of these, 35 (79.9%) were RTQuIC positive. 36 (81.8%) were 14-3-3 positive, 14 (31.8%) were EEG positive and 24 (54.5%) were MRI positive. 45 cases were classified as 'probable' according to WHO criteria, 38 (84.4%) were RT-QuIC positive, 40 (88.8%) were 14-3-3 positive, 14 (31.1%) were EEG positive and 27 (60.1%) were MRI positive. There were 5 cases classified as 'possible' sCJD of which 3(60%) were RT-QuIC, 0 were 14-3-3, MRI or EEG positive. 7 cases were classified as 'unknown' cases of sCJD of which 2(28.6%) were positive for both RT-QuIC and 14-3-3. 3(42.8%) had a positive MRI and 0 cases had a positive EEG. Overall, RT-QuIC has a sensitivity of 80% and specificity of 100% with a positive predictive value of 100%. In comparison, 14-3-3 has a sensitivity of 82%, specificity of 82% and PPV of 95%. EEG has a sensitivity of 32%, specificity of 91% and PPV of 93%. MRI has a sensitivity of 55%, specificity of 100% and PPV of 100%. During the initial data period, 15μl of CSF was used to perform the RT-QuIC test although over the course of the study, it was found that 30μl gave better discrimination between positive and negative RT-QuIC results. However, some CSF samples from cases of suspected sporadic CJD had a negative RT-QuIC response at 30μl but a postive RT-QuIC response was obtained using 15μl. Therefore, CSF samples were tested at both 15μl and 30μl. Discussion In concordance with current literature, RT-QuIC is similar in sensitivity to 14-3-3 but more specific and has a greater positive predictive value. It is considerably more sensitive than EEG and more specific. It is more sensitive than MRI but has equal specificity and positive predictive value. The 3 cases classified as 'possible' and RT-QuIC positive were highly suspected to be cases but died without a post-mortem. The 2 cases classified as 'unknown' and RT-QuIC positive were both clinically suspected to be cases but also died without a postmortem. However, since this study was started, based on further European and International research on RT-QuIC, the European diagnostic criteria for the diagnosis of sCJD has changed as of January 2017 to include RT-QuIC. Therefore, based on the new criteria, all of these cases would now be considered likely cases of sCJD. Overall, this is a highly sensitive, specific and reliable test for the diagnosis of sCJD, especially in cases where the diagnosis is difficult and when conventional tests may fail. This study supports the current literature.
Supervisor: Knight, Richard ; Will, Robert Sponsor: Not available
Qualification Name: Thesis (M.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Sporadic Creutzfeldt-Jakob disease ; sCJD ; prion ; quaking induced conversion ; RT-QuIC ; lumbar puncture ; diagnostic criteria