Synaptic markers in spinal fluid to tell behavior-related frontotemporal dementia apart from psychiatric disorders and Alzheimer's disease

Frontotemporal dementia (FTD) is a clinically, genetically, and pathologically heterogeneous disease and the second most common form of young-onset dementia after Alzheimer’s disease (AD) [1]. The most prevalent form is the behavioral variant of FTD (bvFTD), which is characterized by slowly progressive behavioral symptoms and impaired social cognition. In 30% of the bvFTD patients, the disease is caused by a pathogenic mutation (C9ORF72, MAPT, GRN), but the majority of cases are denoted sporadic bvFTD (70%) [2]. The lack of an early, disease-specific molecular biomarker in sporadic bvFTD together with its significant overlap in clinical symptoms with primary psychiatric disorders (PPD) and frontotemporal hypometabolic patterns on [¹⁸F]-2-deoxy-2-fluoro-D-glucose (FDG) positron emission tomography ([¹⁸F]FDG-PET) scans hampers diagnostic distinction. This leads to misdiagnosis in 50% of the cases with an average diagnostic delay of 6.4 years [3–5].

Evidence from post-mortem and clinical studies including pre-dementia stages show that loss of synaptic function is a predominant early feature in bvFTD and correlates with the level of cognitive impairment [6–14]. In accordance, a recent in vivo study using the [¹¹C]UCB-J PET tracer to detect synaptopathy showed widespread frontotemporal loss of synapses in symptomatic bvFTD patients, which is related to disease severity [14]. Also, lower brain synaptic densities on [¹⁸F]UCB-H PET were found in the temporal brain regions involved in social cognition, highlighting the clinical relevance of synaptopathy in the disease pathophysiology of FTD [11].

In agreement, using cerebrospinal fluid (CSF) biomarkers as indicators for synapse health, differential synaptic concentrations were found in genetic forms of FTD [12]. Previous studies on synaptic involvement in FTD predominately assessed genetic FTD cases, had a small sample size, or did not include PPD as a control group, whereas the latter is the most challenging to distinguish from bvFTD in clinical practice [10–12]. Identification of specific CSF synaptic markers in sporadic bvFTD might improve diagnostic accuracy and aid in a better understanding of FTD pathophysiology. In addition, specific CSF-synaptic panels can provide endpoints in future clinical trials of sporadic bvFTD as they might correlate and reflect cognitive and social functioning.

In an attempt to explore the synaptic pathology in FTD and AD, we recently performed a pilot study where we found that concentrations of CSF synaptic biomarkers synaptosomal-associated protein 25 (SNAP25) and neurogranin (Ng) were elevated in FTD compared with controls, while those of neuronal pentraxin 2 (NPTX2) were lower than in controls, suggesting these could be valuable biomarkers for FTD [15]. SNAP25 is a pre-synaptic vesicle protein involved in neurotransmission, while Ng in the post-synapse regulates calcium ion influxes, and NPTX2 present extracellularly in the synaptic cleft maintains synaptic plasticity [15]. Simultaneously, another recent study suggested that patients with primary psychiatric disorders had significantly lower expression of the post-synaptic protein glutamate receptor 4 (GluR4) in CSF compared with cognitively normal controls and therefore could be useful as biomarkers for PDD [16]. GluR4 is primarily involved in excitatory signal transmission [15]. The axonal protein neurofilament light chain (NfL) has emerged as a promising fluid biomarker to distinguish bvFTD from PPD. Several studies have reported the potential of NfL as a biomarker that correlates with brain atrophy, neurodegeneration, and cognition in dementias [17–20] and also with other neuronal damage, e.g., due to stroke, amyotrophic lateral sclerosis, and multiple sclerosis [3, 21–24].

In this study, we aimed to assess the diagnostic performance of CSF synaptic biomarkers in sporadic bvFTD versus PPD and their added value compared with NfL as well as their specificity towards bvFTD compared with AD and controls. Secondly, we assessed the association of CSF synaptic biomarkers with social cognition, cognitive performance, and disease severity in bvFTD.

In this study, we found differential concentrations of CSF synaptic markers between bvFTD, PPD, AD, and controls in which reduced NPTX2 concentrations were bvFTD specific, and increased concentrations of SNAP25 and Ng were AD specific. Adding NPTX2 to NfL in the biomarker panel to distinguish bvFTD and PPD patients provided added diagnostic value, although limited. Synaptic biomarker concentrations did not correlate with social cognition nor disease severity in FTD and showed weak correlations with cognitive performance. These results indicate that NPTX2, alongside NfL, may provide further insights into bvFTD pathophysiology, although it is relatively less suitable clinically as a diagnostic biomarker.

NfL is a reliable biomarker of neuroaxonal damage and is used for the diagnosis (although limited due to its non-disease specificity), prognosis, and monitoring of treatment response in several neurodegenerative conditions [38]. Several clinical reports have asserted the association of NfL with grey matter and hippocampal atrophy, neuronal impairment, and loss of cognition in neurodegenerative dementias [17–20]. Accumulating clinical evidence further suggests that NfL is a promising biomarker in clinical settings to differentiate patients with bvFTD from PPD [39]. However, high levels of NfL are not specific to bvFTD as this biomarker can be strongly elevated in several other neurodegenerative conditions as well [2, 40, 41]. Furthermore, clinical reports suggest that synaptic dysfunction precedes atrophy in patients with FTD [11, 14], which highlights the need for novel biomarkers that may aid in earlier and more accurate diagnosis of bvFTD over PPD.

Our findings of lower NPTX2 concentrations in sporadic FTD compared with PPD and controls are in line with our previous pilot study and with a recent study among patients with genetic forms of FTD, in which symptomatic mutation carriers showed lower concentrations of CSF NPTX2 compared with controls [13]. NPTX2 has also recently been identified as a promising biomarker for progression in genetic FTD [13]. NPTX2 is involved in the formation and stabilization of synapses, facilitating proper communication between neurons in the brain, and plays a crucial role in synaptic function and plasticity [42]. Studies have shown decreased synaptic density in the brains of FTD patients, particularly in brain regions affected by the disease including the salient network, inducing the characteristic impaired social cognition that is a hallmark feature of FTD [11]. Experimental evidence further suggests that downregulation of NPTX2 may lead to increased complement-mediated microglial activation, thereby causing abnormal elimination of synapses [43]. The downregulation of NPTX2 in both genetic and sporadic bvFTD may thus reflect a shared pathophysiology within the FTD disease heterogeneity and suggest that NPTX2 may play a crucial role in the pathogenesis of FTD by contributing to synaptic dysfunction [42]. Further investigation of NPTX2 and its mechanisms in FTD could provide valuable insights into the disease mechanisms and potentially lead to the development of novel therapeutic strategies targeting synaptic dysfunction.

While it has been reported elsewhere that CSF concentration of GluR4 is decreased in patients with mood disorder and schizophrenia compared with healthy controls [16], we did not find any diagnostic significance of this biomarker in this cohort. This might be due to our heterogeneous sample of PPD, including various subtypes such as mood disorders, personality disorders, autism spectrum disorder, anxiety disorder, functional disorder, schizophrenia, and other psychiatry, of which individual group levels did not reach statistically significant thresholds of lower GluR4.

Synaptic pathology is a shared mechanism across diseases, yet evidence presented here and elsewhere indicates that synaptic proteins participate differentially in various disease pathogeneses, underscoring the distinct impairments in synaptic functionality across diseases. For example, the AD-specific increase of CSF SNAP25 and Ng compared with bvFTD that we reported, corroborates previous findings [44, 45]. Ng is a postsynaptic protein that is important for maintaining synaptic plasticity and regulating calcium ion influxes, while SNAP25 is a presynaptic protein that plays a crucial role in synaptic vesicle fusion and neurotransmitter release, and both these proteins have been shown to play a key role in AD disease pathophysiology, although they may be less clinically relevant for FTD [10, 45–48]. While brain regions affected in AD, i.e., the hippocampus and cortex have a high expression of Ng, the anatomical distribution of SNAP25 is not well known, although it is expressed in the cortex [49–52]. Thus, a possible hypothesis for the increased concentrations of synaptic proteins SNAP25 and Ng in AD but not in bvFTD could be due to the topography of brain atrophy they reflect [45].

In our cohort, bvFTD patients performed worse on social cognition testing compared with PPD, but no association was found with CSF synaptic markers which might be due to the limited test scores available per diagnostic group, i.e., only 13 for bvFTD and 9 for PPD. A previous study, using [¹⁸F]UCBH-PET as a tracer for synaptic vesicle protein 2A (SV2A) which reflects synaptic density, showed a trend for synaptic loss in the temporal social brain in bvFTD, highlighting the clinical relevance of synaptopathy in disease pathophysiology of FTD [14]. Further studies assessing CSF synaptic markers might elucidate if SV2A is a superior synaptic marker in CSF correlating with social cognition in larger patient groups.

The association of the synaptic biomarkers with other cognitive functioning was absent or only moderate to weak in bvFTD and AD, while there were no correlations found in patients with PPD or with controls. Moreover, we did not detect any association of the synaptic proteins or NfL with FTLD-CDR disease severity scores [53, 54]. One plausible reason could be that the cohort included a homogenous sample of bvFTD patients with mild to moderate bvFTD disease severity (FTLD-CDR ≤ 16). The direction of correlation detected in the AD group between SNAP25 and attention was counterintuitive, and thus follow-up studies with greater statistical power are necessary to evaluate these findings.

The strengths of this study lie in including the assessment of concentrations of CSF synaptic proteins involved in several synaptic functions both upstream and downstream of the synapse, providing insight into pathophysiological mechanisms. Additionally, we included a well-phenotyped diverse PPD sample as a comparative group, which is the most important and clinically challenging to differentially diagnose from bvFTD, resembling clinical practice. There are also some limitations. For example, the odds ratios of the synaptic biomarkers for diagnostic distinction of bvFTD versus PPD and AD were modest and the clinical relevance of these biomarkers, particularly NPTX2, demands to be evaluated in larger cohorts. Since the PPD sample was heterogeneous, disease-specific synaptic concentrations in PPD could not be assessed and should be included in future studies. Furthermore, the sample sizes for cognitive test scores were limited, such as for social cognition (Ekman 60 faces test), disease severity (FTLD-CDR), and the domain visuospatial functioning.

We conclude that synaptic biomarker NPTX2 has additional, although limited diagnostic value to NfL in the differential diagnosis of bvFTD versus PPD. Our findings contribute insight into disease-specific mechanisms in bvFTD, by showing the bvFTD-specific decrease in concentrations of NPTX2. Furthermore, given that NfL likely reflects neuronal atrophy [17, 18], it is a clinically relevant biomarker at a progressed stage of the disease. Further investigation of NPTX2 and its mechanisms in bvFTD could provide valuable insights into the disease mechanisms for early diagnosis and prognosis, as well as potentially lead to the development of novel therapeutic strategies.