Differential Impairment of Cerebrospinal Fluid Synaptic Biomarkers in the Genetic Forms of Frontotemporal Dementia
Authors: Sogorb-Esteve Aitana , Nilsson Johanna , Swift Imogen J , Heller Carolin , Russell Lucy L , Peakman Georgia , Convery Rhian S , Swieten John C van , Seelaar Harro , Borroni Barbara , Galimberti Daniela , Sánchez-Valle Raquel , Laforce Robert , Moreno Fermin , Synofzik Matthis , Graff Caroline , Masellis Mario , Tartaglia Maria Carmela , Rowe James B , Vanderberghe Rik , Finger Elisabeth , Tagliavini Fabrizio , Santana Isabel , Butler Chris R , Ducharmer Simon , Gerhard Alexander , Danek Adrian , Levin Johannes , Otto Markus , Sorbi Sandro , Ber Isabelle Le , Paquier Florence , Gobom Johan , Brinkmalm Ann , Blennow Kaj , Zetterberg Henrik , Rohrer Jonathan D 
Published: January 2022

Abstract
Background: Approximately a third of frontotemporal dementia (FTD) is genetic with mutations in three genes accounting for most of the inheritance: C9orf72, GRN and MAPT. Impaired synaptic health is a common mechanism in all three genetic variants and so developing fluid biomarkers of this process could be useful as a readout of cellular dysfunction within therapeutic trials.Methods: A total of 193 cerebrospinal fluid (CSF) samples from the GENetic FTD Initiative including 77 presymptomatic (31 C9orf72, 23 GRN, 23 MAPT) and 55 symptomatic (26 C9orf72, 17 GRN, 12 MAPT) mutation carriers as well as 61 mutation-negative controls were measured using a microflow LC PRM-MS set-up targeting 15 synaptic proteins: AP-2 complex subunit beta, complexin-2, beta-synuclein, gamma-synuclein, 14-3-3 proteins (eta, epsilon, zeta/delta), neurogranin, Rab GDP dissociation inhibitor alpha (Rab GDI alpha), syntaxin-1B, syntaxin-7, phosphatidylethanolamine-binding protein 1 (PEBP-1), neuronal pentraxin receptor (NPTXR), neuronal pentraxin 1 (NPTX1), and neuronal pentraxin 2 (NPTX2). Mutation carrier groups were compared to each other and to controls using a bootstrapped linear regression model, adjusting for age and sex. Results: CSF levels of eight proteins were increased only in symptomatic MAPT mutation carriers (compared with controls) and not in symptomatic C9orf72 or GRN mutation carriers: beta-synuclein, gamma-synuclein, 14-3-3-eta, neurogranin, Rab GDI alpha, syntaxin-1B, syntaxin-7, and PEBP-1, with three other proteins increased in MAPT mutation carriers compared with the other genetic groups (AP-2 complex subunit beta, complexin-2, and 14-3-3 proteins zeta/delta). In contrast, CSF NPTX1 and NPTX2 levels were affected in all three genetic groups (decreased compared with controls), with NPTXR concentrations being affected in C9orf72 and GRN mutation carriers only (decreased compared with controls). No changes were seen in the CSF levels of these proteins in presymptomatic mutation carriers. Concentrations of the neuronal pentraxins were correlated with brain volumes in the presymptomatic period for C9orf72 and GRN groups, suggesting that they become abnormal in proximity to symptom onset.Conclusions: Differential synaptic impairment is seen in the genetic forms of FTD, with abnormalities in multiple measures in those with MAPT mutations, but only changes in neuronal pentraxins within the GRN and C9orf72 mutation groups. Such markers may be useful in future trials as measures of synaptic dysfunction, but further work is needed to understand how these markers change throughout the course of the disease.