Ante Mortem Cerebrospinal Fluid Levels of Calsyntenin-1 and Neurexin-2a Reflect Post-Mortem TDP-43 Pathology in a Pathological Cohort of Frontotemporal Lobar Degeneration

Background. Frontotemporal dementia (FTD) clinical diagnosis is challenged by the variable correspondence between the clinical syndrome and underlying neuropathological changes, the phenotypic overlap with Alzheimer's disease (AD) and the lack of pathophysiologic diagnostic biomarkers. As synapse degeneration is an early event in pathological frontotemporal lobar degeneration (FTLD), a surrogate marker of synapse loss could be used to monitor early pathologic changes. The aim of this study was to evaluate the relationship of antemortem cerebrospinal fluid (CSF) levels of 9 synaptic proteins with postmortem global tau and TDP-43 burden in a neuropathological FTLD cohort. Methods. We included patients with a neuropathological confirmation of FTLD-Tau (n=24, mean age-at-CSF 67 years+/-11), FTLD-TDP (n=25, 66 years+/-9) or AD (n=25, 73 years+/-6) as well as cognitively normal controls (n=35, 69 years+/-7) from the Penn FTD Center and ADRC. A subset of 39 FTLD patients presented without AD comorbidity (neurofibrillary tangle score of B0/B1 according to the NIA-AA classification) and 18 AD patients presented without TDP-43 comorbidity. We quantified the synaptic panel in antemortem CSF by targeted mass spectrometry using isotopically-labeled peptides as internal standards. We used a semi-quantitative measure of tau and TDP-43 inclusions to quantify pathological burden across 16 brain regions. Statistical methods included Spearman rank correlations, one-way analysis of covariance (controlling for sex and age-at-CSF), linear regression (controlling for age-at-death) and receiver-operating characteristic curves. Result. CSF calsyntenin-1 and neurexin-2a were correlated in all patient groups (rs=.55 to .88). In FTLD-TDP, we observed low antemortem CSF levels of calsyntenin-1 and neurexin-2a compared to AD (.72-fold, p=.001, .77-fold, p=.04, respectively) and controls (.80-fold, p=.02, .78-fold, p=.02, respectively), which were inversely associated with post-mortem global TDP-43 burden (regression r2=.36, p=.04 and r2=.35, p=.04, respectively). Neurexin-2a showed 75.0% (95% CI 60-88) accuracy to discriminate FTLD-TDP from controls. Calsyntenin-1 showed 75.1% (95% CI 62-87) accuracy to discriminate FTLD-TDP from AD. None of the synaptic proteins were altered in FTLD-Tau compared to controls (0.79 to 1.12-fold, p>.12) or associated with post-mortem global tau burden (r2=.12, p=.36). Comorbidity had a limited effect on these findings. Conclusion. CSF calsyntenin-1 and neurexin-2a have potential as objective measures of TDP-43-mediated degeneration in FTLD.