Novel diagnostic cerebrospinal fluid biomarkers for pathologic subtypes of frontotemporal dementia identified by proteomics.

INTRODUCTION: Reliable cerebrospinal fluid (CSF) biomarkers enabling identification of frontotemporal dementia (FTD) and its pathologic subtypes are lacking. METHODS: Unbiased high-resolution mass spectrometry-based proteomics was applied on CSF of FTD patients with TAR DNA-binding protein 43 (TDP-43, FTD-TDP, n = 12) or tau pathology (FTD-tau, n = 8), and individuals with subjective memory complaints (SMC, n = 10). Validation was performed by applying enzyme-linked immunosorbent assay (ELISA) or enzymatic assays, when available, in a larger cohort (FTLD-TDP, n = 21, FTLD-tau, n = 10, SMC, n = 23) and in Alzheimer's disease (n = 20), dementia with Lewy bodies (DLB, n = 20), and vascular dementia (VaD, n = 18). RESULTS: Of 1914 identified CSF proteins, 56 proteins were differentially regulated (fold change >1.2, P < .05) between the different patient groups: either between the two pathologic subtypes (10 proteins), or between at least one of these FTD subtypes and SMC (47 proteins). We confirmed the differential expression of YKL-40 by ELISA in a partly independent cohort. Furthermore, enzyme activity of catalase was decreased in FTD subtypes compared with SMC. Further validation in a larger cohort showed that the level of YKL-40 was twofold increased in both FTD pathologic subtypes compared with SMC and that the levels in FTLD-tau were higher compared to Alzheimer's dementia (AD), DLB, and VaD patients. Clinical validation furthermore showed that the catalase enzyme activity was decreased in the FTD subtypes compared to SMC, AD and DLB. DISCUSSION: We identified promising CSF biomarkers for both FTD differential diagnosis and pathologic subtyping. YKL-40 and catalase enzyme activity should be validated further in similar pathology defined patient cohorts for their use for FTD diagnosis or treatment development.

Stability of Progranulin Under Pre-Analytical Conditions in Serum and Cerebrospinal Fluid.

Progranulin (PGRN) levels in blood and cerebrospinal fluid (CSF) are increasingly studied as potential markers for neurodegenerative disorders. We aimed to 1) characterize two commercially available PGRN ELISAs on several assay validation parameters, 2) assess the stability of PGRN in serum and CSF under pre-analytical conditions, and 3) compare stability in the two assays. Intra- and inter-assay variation, inter-lot variation, linearity, lower limit of detection, and kit correlations were assessed for the Adipogen and R&D PGRN ELISA kits. Blood and serum samples were experimentally exposed to ≤9 freeze/thaw cycles, delayed processing for ≤24 h at room temperature and 4°C, and to temperature stability tests for ≤3 weeks at -20°C, 4°C, room temperature, and 37°C. Both commercial PGRN ELISA kits showed acceptable ranges for intra- and inter-assay variation, where the R&D kit performed more accurate than the Adipogen kit, especially for inter-assay variation (intra-assay serum: 6.7 and 8.3%, respectively; inter-assay serum: 9.2 and 21.0%; intra-assay CSF: 3.6 and 12.0%; inter-assay CSF: 16.0 and 44.5%). Absolute serum PGRN concentrations were 1.9-fold higher in Adipogen than R&D (p < 0.001) and strongly correlated between both kits (ρ= 0.86, p < 0.0001) and CSF PGRN levels were on the borderline of detection in both kits. PGRN was typically stable under all pre-analytical conditions addressed, although two weeks at 37°C resulted in decreased PGRN concentrations in CSF, only when using the Adipogen kit. These results support further examination of PGRN as a potential marker in neurodegenerative diseases, since PGRN is stable in serum and CSF and can be measured using ELISA kits from several providers.

Standard biobanking conditions prevent evaporation of body fluid samples.

Pre-analytical variation in biobanking procedures, e.g., long-term storage, could confound biomarker outcomes. We investigated evaporation in various body fluids at different storage temperatures and storage durations. Biobank sample tubes (Sarstedt 72.694.007) filled with water in different volumes (50, 100, 250, 500, 750, 1000, 1250, 1500µl) were stored at different temperatures (-80°C, -20°C, 4°C, room temperature (RT)) for 4.5years and weighed at regular intervals. Next, saliva, serum, plasma, and CSF were stored in different volumes (50, 250, 500, 1000µl) at different temperatures (-80°C, -20°C, 4°C, RT) for 2years. An extra set of CSF was stored in tubes with safe-lock cap (Eppendorf 0030 120.086) instead of a screw cap with o-ring. No evaporation of water stored in biobanking tubes at -80°C or -20°C occurred over 4.5years. Storage of saliva, serum, plasma, and CSF at -80°C or -20°C, monitored over 2years, protected these samples from evaporation too. At 4°C, evaporation was minor, approximately 1.5% (50µl) or 0% (1ml) yearly, where at RT it ranged from 38% (50µl) to 2% (1ml). Differences were observed neither between different body fluids, nor between tube caps. Our data provide support for long-term biobanking conform current biobanking guidelines, encouraging retrospective use of clinical cohorts.