Background
Frontotemporal dementia (FTD) is a common cause of early-onset dementia, presenting with behavioural change (behavioural variant FTD [bvFTD]) or language impairment (primary progressive aphasia [PPA]). Around one-third of cases are familial, associated most commonly with mutations in progranulin (
GRN), microtubule-associated protein tau (
MAPT) or chromosome 9 open reading frame 72 (
C9orf72) [
1]. Pathologically, the majority of individuals have frontotemporal lobar degeneration (FTLD) with inclusions containing tau or transactive response DNA binding protein 43 (TDP-43), although some, particularly those with the logopenic variant of PPA (lvPPA), have Alzheimer’s disease (AD) pathology [
2]. Reliable biomarkers that differentiate the pathological changes underlying sporadic FTD in vivo or that predict disease onset, severity or progression in sporadic and familial FTD are currently lacking. There is growing evidence that neuroinflammation and microglial dysfunction play a role in FTD, particularly in familial FTD secondary to
GRN mutations [
3,
4]. Inflammatory markers are variably altered in blood or cerebrospinal fluid (CSF) of patients with neurodegenerative disease, including across the clinical, genetic and pathological spectrum of FTD, and they could be useful as disease biomarkers in future clinical trials.
The protein triggering receptor expressed on myeloid cells 2 (TREM2) is an innate immune receptor expressed on microglia and on myeloid cells outside the brain [
5,
6]. TREM2 is upregulated on activated microglia and involved in microglial phagocytosis [
7‐
11], survival [
12] and chemotaxis and response to neuronal injury [
13]. Homozygous
TREM2 mutations lead to a rare syndrome called Nasu-Hakola disease [
14], which is associated with an early-onset FTD-like dementia, and homozygous
TREM2 variants are associated with FTD-like syndromes without bony involvement [
15‐
18]. TREM2 undergoes cleavage of its ectodomain to release a soluble TREM2 (sTREM2) fragment into the extracellular space [
9], which is measurable in CSF and blood. Although raised CSF sTREM2 levels were initially described in neuroinflammatory conditions such as multiple sclerosis [
19,
20], establishing the relationship between sTREM2 and other markers of disease has recently become of great interest in neurodegenerative disorders.
Most studies of CSF sTREM2 levels in dementia have focused on AD, but non-stratified patient cohorts have produced conflicting results, including increased [
21,
22], reduced [
9] or similar [
23] levels in patients with AD compared with healthy controls. However, CSF sTREM2 levels may change according to disease stage in AD, with raised levels in mild cognitive impairment and early sporadic [
24,
25] or pre-symptomatic familial [
26] disease, but lower levels (similar to controls) in established disease [
26]. This suggests that sTREM2 levels could be useful in tracking the disease course in AD or for determining proximity to disease onset in pre-symptomatic familial cases, and this may also be the case for other neurodegenerative diseases, such as FTD. CSF sTREM2 levels correlate with CSF levels of total tau (T-tau), a marker of neuronal injury, in AD cohorts, but generally not with β-amyloid 1–42 (Aβ42) levels [
21,
22,
24‐
26]. This suggests that CSF sTREM2 may be a useful marker of microglial activation in response to neuronal injury, regardless of amyloid pathology, and hence worth exploring in FTD.
Previous studies of CSF sTREM2 levels in FTD have included small numbers of patients with undefined clinical subtypes and have found widely differing results, including lower [
9], higher [
22] or similar [
21] levels in patients with FTD compared with healthy controls. It remains unclear whether CSF sTREM2 levels are altered in FTD or whether they differ between the various clinical subtypes of FTD. To our knowledge, no studies have compared CSF sTREM2 levels across groups of individuals with familial FTD to determine whether levels differ between the genetic subtypes of FTD. In addition, individuals may develop clinical syndromes consistent with FTD (bvFTD or PPA) due to underlying AD, rather than FTLD, pathology. It is unclear whether CSF sTREM2 levels differ in patients with similar clinical syndromes but contrasting pathologies.
Given the heterogeneous clinical, genetic and pathological nature of FTD and the urgent need for disease biomarkers, in this study we aimed to examine how CSF sTREM2 levels vary within a well-phenotyped cohort of symptomatic individuals with different clinical and genetic subtypes of FTD. We also aimed to clarify how CSF sTREM2 levels differ between individuals with clinical FTD syndromes due to AD versus FTLD pathology (as determined by their CSF neurodegenerative biomarker profile), as well as to establish whether sTREM2 levels correlate with levels of other CSF biomarkers previously explored in AD: T-tau, tau phosphorylated at position threonine 181 (P-tau) and Aβ42.
Discussion
This study shows that CSF sTREM2 levels do not differ overall between individuals with FTD and cognitively normal controls or between the various clinical subtypes of FTD. However, CSF sTREM2 levels are higher in those with familial FTD due to GRN mutations, albeit within a small cohort, and in individuals with a clinical syndrome consistent with FTD but CSF biomarkers consistent with underlying AD pathology. In addition, CSF sTREM2 levels are positively associated with levels of CSF T-tau in individuals with FTD and also with P-tau in individuals with likely AD pathology, and they are influenced by both age and disease duration.
Using a well-phenotyped cohort, we were able to compare CSF sTREM2 levels between individuals with FTD and controls, and across a variety of more distinct, clinically defined FTD syndromes, which, to our knowledge, has not been described previously. Other studies have found significantly lower [
9] or higher [
22] CSF sTREM2 levels in FTD than in controls. However, these studies assessed a much smaller number of FTD cases and without clearly defined clinical subgroups. There was significant variability in sTREM2 levels within our clinical subgroups, and large intergroup variability has been noted in studies of AD, where the substantial overlap between levels in each group limited the utility of sTREM2 to differentiate between AD and controls, despite a higher level in the AD group overall [
21,
26]. CSF sTREM2 is also raised in a number of different neuroinflammatory [
19,
20] and other neurodegenerative [
21] diseases, limiting its diagnostic specificity for FTD. However, given the negative association with disease duration in our study, there may be a differential profile of CSF sTREM2 according to disease stage in FTD, as has been identified in the continuum from mild cognitive impairment to AD [
24,
25], or according to disease intensity, as has been identified for serum and CSF neurofilament light chain levels in FTD [
30,
31], which would be clinically useful.
We included a number of individuals with familial FTD in our study, enabling an exploratory analysis of differences in CSF sTREM2 between individuals with the three most common mutations linked to FTD (
GRN,
MAPT and
C9orf72) and compared with cognitively normal controls. This adds to previous studies of raised inflammatory CSF markers in familial FTD, particularly in
GRN mutation carriers [
32‐
34]. Although we were able to include only a small number of individuals in each mutation group, we found that individuals with FTD due to
GRN mutations had higher CSF sTREM2 levels than those with
MAPT or
C9orf72 mutations, and compared with controls. This may be due to a link between
GRN and
TREM2, both of which are expressed by microglia, and thought to regulate microglial function and immune pathways in general.
GRN-knockout mice have upregulated
TREM2 gene expression [
3,
35] and excessive synaptic pruning mediated by aberrantly activated microglia [
3]. sTREM2 promotes release of inflammatory cytokines and enhanced microglial activation and survival in mice [
36]. Mouse models of homozygous
GRN mutations and patients with heterozygous
GRN mutations display excessive microglial activation on post-mortem brain tissue analysis [
3,
37‐
40] and dysregulated levels of other inflammatory markers [
32‐
34,
41]. Increased microglial activation in the context of
GRN haploinsufficiency could lead to enhanced
TREM2 expression by microglia, increasing release of sTREM2 into the CSF and promoting survival of dysfunctional microglia or exacerbating neuronal damage through excessive phagocytosis.
Although downregulating
TREM2 expression or reducing CSF sTREM2 levels could be a therapeutic target for individuals with FTD secondary to
GRN mutations, animal models of multiple sclerosis deteriorated with TREM2 inhibition [
42]. In addition, the homozygous
TREM2 mutations that cause the frontal lobe dementia associated with Nasu-Hakola disease impair TREM2 function, locking microglia in a homeostatic, rather than phagocytic, state [
10] and produce very low CSF sTREM2 levels [
9]. This suggests that TREM2 may be protective in some circumstances. There is clearly a fine balance between TREM2 activity and suppression. However, CSF sTREM2 levels could rise in pre-symptomatic
GRN mutation carriers before symptom onset and act as a useful marker of disease proximity, as in familial AD [
26].
Because several studies have shown higher CSF sTREM2 levels in amnestic AD than in controls [
21,
22,
24], we hypothesised that individuals with a clinical diagnosis of an FTD syndrome (i.e., bvFTD or PPA) but underlying AD pathology would have higher sTREM2 levels than controls. We were also keen to establish whether sTREM2 levels differ by underlying pathology (AD versus FTLD) in those with FTD. This is of particular use in the FTD field because certain patients, particularly those with lvPPA, may have underlying AD pathology and differing relationships between CSF sTREM2 and other disease biomarkers compared with individuals with FTLD, which could prove an issue for future clinical trials. By stratifying individuals with FTD by their CSF neurodegenerative biomarker profile (into AD biomarker-positive and AD biomarker-negative groups), we were able compare CSF sTREM2 levels (and relationships with other biomarkers) between biochemically defined, rather than clinically defined, syndromes. The significantly higher CSF sTREM2 levels in individuals with FTD but AD-like CSF (AD biomarker-positive group) than in those with non-AD-like CSF (AD biomarker-negative group), who most likely have FTLD, in our study suggests that individuals with significant neuronal injury due to AD (combined tau and amyloid pathology) may have more microglial activation and sTREM2 release into the CSF than those with FTLD. This is supported by our finding that CSF sTREM2 levels did not significantly differ between individuals with FTD and non-AD-like CSF and controls. There was a trend towards higher CSF sTREM2 levels in those with FTD and AD-like CSF than controls, although this did not reach significance, perhaps due to small group size. Interestingly, the difference in CSF sTREM2 levels between individuals with likely underlying AD versus FTLD became more pronounced with increasing disease duration. This most likely occurred because CSF sTREM2 levels were negatively associated with disease duration in those with FTLD, and there was a trend towards a positive association with disease duration in those with AD. In individuals with FTLD, microglial activation could decrease more over time, leading to a gradual decline in CSF sTREM2 levels. Alternatively, this separation may have occurred because several individuals with FTD and non-AD-like CSF had much longer disease durations (perhaps due to a less intense disease process) and much lower sTREM2 levels than the majority of those with AD-like CSF. Longitudinal CSF data from both groups are required to examine this further.
We went on to explore the relationship between levels of CSF sTREM2 and validated neurodegenerative biomarkers that reflect neuronal injury (T-tau), hyperphosphorylated tau (P-tau) and amyloid pathology (Aβ42). In individuals with FTD but likely AD pathology, higher CSF sTREM2 levels were associated with higher CSF T-tau and P-tau levels. This is consistent with associations between CSF sTREM2 and T-tau and/or P-tau levels in amnestic AD [
21,
24,
25], as well as in our lvPPA subgroup, the majority of whom had AD-like CSF. We found that CSF sTREM2 levels were associated with T-tau (but not P-tau) levels in those with FTD due to likely FTLD, consistent with the theory that sTREM2 levels may also rise in the context of neuronal injury without concurrent hyperphosphorylated tau or amyloid pathology [
22,
24‐
26]. In our control group, higher CSF sTREM2 levels were associated with higher CSF T-tau levels, but not with P-tau or Aβ42. Other studies have found associations with levels of T-tau, P-tau, or all three markers (T-tau, P-tau and Aβ42) in healthy controls [
23‐
26]. This may reflect differential effects of microglial activation in response to mild age-related neuronal injury between individual cohorts, the variability of CSF T-tau and P-tau levels in healthy ageing [
43,
44], or the variety of control group age distributions across studies. Surprisingly, we found a small but significant positive association between CSF sTREM2 and Aβ42 levels in FTD overall and in those with FTD and likely FTLD (rather than AD) pathology. This association with Aβ42 was observed in the control groups of three other studies [
23‐
25], one of which speculated that the positive correlation between sTREM2 and Aβ42 levels was due to very early pre-symptomatic AD in some control individuals, because CSF Aβ42 may transiently increase due to reduced clearance before it decreases [
23]. The positive association in our overall FTD group likely reflects that some individuals in this group were in a different stage of the AD pathology continuum than others, particularly because it included individuals with lvPPA. It remains unclear why there was also a positive association with Aβ42 in individuals with likely FTLD, although there may well be reduced clearance of amyloid in the context of extensive other pathology. However, overall in FTD there appears to be more of an association between CSF sTREM2 and T-tau and P-tau levels, than with Aβ42 levels, suggestive of a stronger link between sTREM2 and neuronal injury, than amyloid pathology itself. This is consistent with previous studies in AD [
21‐
26].
We also examined relationships between sTREM2 levels and relevant clinical parameters such as age, disease duration and sex, which may affect TREM2 expression. The positive association between CSF sTREM2 levels and age in FTD is consistent with studies of AD [
22‐
25]. Increased microglial activity associated with ageing leads to increased microglial TREM2 expression in healthy individuals [
45] and in AD [
46], and this could also be the case in FTD. We did not find a significant association with age in controls, as has been found previously [
23,
25], perhaps because our control group was generally younger (to match to the FTD group) and smaller than those in other studies. Other studies’ control groups had a broader age range and so may have included some older, cognitively normal individuals with early AD pathology (and higher sTREM2 levels), leading to an apparently positive association between sTREM2 levels and age. We would suggest that future studies of CSF sTREM2 levels in neurodegenerative disease cohorts explore associations with age and consider adjusting group analyses for age at CSF collection.
Because previous research has shown that CSF sTREM2 levels vary by disease stage in mild cognitive impairment and AD [
24,
25], we examined the relationship between CSF sTREM2 levels and disease duration in FTD. CSF sTREM2 levels were negatively associated with disease duration in FTD overall and particularly in bvFTD, where the widest range of disease durations was present. In the early stages of FTD, CSF sTREM2 levels may be high (due to florid microglial activation in response to incipient neurodegeneration), whereas later on in disease, levels may decrease, perhaps as compensatory microglial overactivation is overwhelmed by neurodegeneration. It has been postulated that this may occur in AD, and CSF sTREM2 levels could therefore act as a marker of disease progression [
24]. An alternative explanation for our findings is that CSF sTREM2 levels may just be lower in individuals with FTD who have less aggressive disease and hence longer disease durations. CSF sTREM2 levels could therefore be a biomarker of rate of neuronal injury and disease intensity in FTD, in keeping with the positive association with CSF T-tau levels in individuals with non-AD-like CSF, who most likely have FTLD. Although the lack of longitudinal data in our study precludes a conclusion that levels of CSF sTREM2 rise and then fall over the disease course in FTD, our findings emphasise the importance of future studies assessing associations between CSF biomarker levels and disease duration.
Although CSF sTREM2 levels did not differ by sex within our whole cohort or in our dementia or control groups, there was a difference in the svPPA subgroup, with higher CSF sTREM2 levels in males. The majority of studies focusing on AD have not found an association between CSF sTREM2 levels and sex [
21,
23‐
25], although significantly higher levels [
22] or a trend towards higher levels [
26] have been observed in males. Although it remains unclear whether sex affects sTREM2, we adjusted all analyses for sex.
A limitation of our study is that some of the FTD clinical and genetic subgroups were rather small, which may have limited our power to detect significant differences between groups. However, this is inherent to a disease such as FTD, where rarer subtypes exist, and it is difficult to avoid when analysing biomarker levels across a broad clinical and pathological spectrum of disease and when employing CSF collection and biomarker analysis at one centre in order to minimise inter-centre variation. Other studies with multi-centre CSF sources have shown significant variability in sTREM2 levels between centres [
25], which we were keen to avoid. Our dementia group contained individuals with a diagnosis of an FTD syndrome, including those typically associated with FTLD-TDP or FTLD-tau (bvFTD, svPPA and nfvPPA) and those associated with AD pathology (lvPPA), which in theory could have differentially affected sTREM2 levels within the group as a whole. However, we were able to dissect out any differences in sTREM2 linked to differing pathologies through stratification of all patients with dementia by their CSF biomarker profile. Although most cases of dementia were not pathologically confirmed, all met recent diagnostic criteria for bvFTD [
27] or PPA [
28], and our CSF ratio cut-off was intentionally stringent to minimise misclassification of cases into the wrong pathology subgroup. Our dementia group combined individuals with a wide range of disease durations, which we showed was independently associated with sTREM2 levels. However, we adjusted analyses for disease duration wherever possible to account for this. We did not include any individuals with mild cognitive impairment, because this is typically a ‘pre-AD’ rather than a ‘pre-FTD’ state, nor did we analyse longitudinal CSF samples or samples from pre-symptomatic mutation carriers at risk of familial FTD. This means we cannot definitively conclude whether CSF sTREM2 levels change over the disease course, and therefore reflect disease proximity, intensity or progression in FTD, or how sTREM2 relates to changes in other CSF biomarkers such as T-tau over time.