We here confirm CSF VILIP-1 as a diagnostic marker for AD in a large cohort, and now show that CSF VILIP-1 and VILIP-1/Aβ42 are strong prognostic measures for early AD. To our knowledge, this is the first study to show that CSF VILIP-1 and VILIP-1/Aβ42 predict future cognitive impairment in cognitively normal individuals. Furthermore, we provide new data from a large cohort of well characterized individuals that CSF VILIP-1 negatively correlates with whole brain and regional brain volumes in AD, and positively correlates with amyloid load in preclinical AD.
We have previously demonstrated that CSF VILIP-1 is increased in a small cohort of individuals with AD33
. We here validate these findings in a much larger cohort, including individuals in the earliest symptomatic stage of AD (CDR 0.5 or MCI). Our results suggest that CSF VILIP-1 and VILIP-1/Aβ42 offer diagnostic sensitivity for AD that is comparable to that of CSF tau, p-tau181, or Aβ42, and tau/Aβ42 or p-tau181/Aβ42, respectively. Notably, since our CDR 0.5 cohort includes individuals who may elsewhere be classified as MCI or pre-MCI, these findings reveal the promise of CSF VILIP-1 and VILIP-1/Aβ42 as diagnostic biomarkers for the earliest symptomatic stage of AD. Moreover, our study is the first to investigate the diagnostic utility of plasma VILIP-1 levels in AD. In agreement with the CSF findings, our plasma results suggest that significant, albeit smaller, differences in VILIP-1 levels between AD and non-demented controls can be detected peripherally.
Additionally, by including individuals with other neurodegenerative disorders (e.g. FTLD, PSP, and LBD), it appears that CSF VILIP-1 may confer some diagnostic specificity for AD. It is possible that elevated CSF VILIP-1 levels in the setting of chronic neurodegeneration may be influenced by relatively disease-specific mechanisms or alterations in signaling pathways 34–37
. It will be of interest in the future to validate these findings in multiple larger cohorts, including more individuals with non-AD dementias, and to investigate the mechanisms by which VILIP-1 levels may be preferentially altered in AD.
While significant differences in mean CSF VILIP-1 levels exist between AD and CNC, some overlap between the two groups was observed. Overlap is likely due in part to the inclusion of cognitively normal individuals with preclinical AD as well as some individuals with AD who may have alternative diagnoses. Consistent with previous reports14, 30
, our CNC cohort (60 years or older) includes individuals with amyloid binding on PET-PIB and some with elevated tau. Most of these individuals have CSF VILIP-1 levels that are comparable to those in AD (). Moreover, our AD cohort predominantly consists of individuals with very mild dementia (CDR 0.5), potentially decreasing the difference in CSF levels between the AD and CNC cohorts. Very similar overlap is observed with other CSF biomarkers such as tau, Aβ42, and tau/Aβ42 when used to compare AD and CNC based only on clinical diagnoses, not taking into account underlying AD pathology on PET-PIB (). Importantly, our results suggest that VILIP-1/Aβ42, tau/Aβ42, and p-tau181/Aβ42 are better indicators of PIB-positivity than of clinical diagnoses ().
VILIP-1 is abundantly expressed in neurons, but not other cell types, in the brain (Supplementary Figure 4
, and has demonstrated utility as a marker of neuronal injury in brain injury models 18
. Our findings that CSF VILIP-1 negatively correlates with whole brain and regional brain volumes, at least as well as tau and p-tau181, in even the earliest symptomatic stages of AD, and with amyloid load in cognitively normal individuals, support the potential utility of CSF VILIP-1 as a biomarker surrogate for neurodegeneration in AD. The correlation between CSF VILIP-1 and CSF tau or p-tau181 levels likely reflects close associations of these proteins in AD (Supplementary Figure 4
) 35, 38
and, perhaps, the ability of these markers to measure neurodegeneration since VILIP-1 is not a component of neurofibrillary tangles. In contrast, CSF Aβ42 levels appear to decrease years prior to the onset of cognitive impairment and remain relatively stable with further disease progression 28, 31, 39, 40
. Similarly to tau and p-tau18131
, the correlation between CSF VILIP-1 and amyloid load in preclinical AD suggests that increasing amyloid accumulation in these early stages 5, 6
is associated with greater neuronal injury. On the other hand, cortical amyloid deposition is likely close to reaching its maximal extent by the time individuals with AD become cognitively impaired 41
, and does not appear to change considerably with further disease progression 42
There is accumulating evidence that progressive neuronal and synaptic loss is the best surrogate for cognitive decline and disease progression in AD 41
. Neuronal counts in the entorhinal and hippocampal regions are comparable in preclinical AD and “healthy aging”, but decrease substantially as the earliest signs of cognitive impairment appear16
. We here demonstrate that CSF VILIP-1, alone or in combination with Aβ42 (VILIP-1/Aβ42), offers predictive value for future cognitive impairment in a large cohort of cognitively normal elderly. Together with CSF tau and Aβ42 levels, these data support the notion that increased tau levels (reflective of NFT, and perhaps neurodegeneration) and VILIP-1 levels (generally reflective of neuronal injury) occur following decreases in Aβ42 and increasing amyloid load over this short follow-up period, and herald the onset of symptomatic cognitive impairment. Furthermore, our findings suggest that CSF VILIP-1 complements the prognostic utility of CSF tau, p-tau181, and Aβ42 (collectively) in predicting future cognitive impairment over this follow-up period.
In the cohort studied herein, VILIP-1/Aβ42 appears to predict future cognitive impairment at least as well as tau/Aβ42 and p-tau181/Aβ42, the current “gold standard” prognostic biomarkers for AD ( and ). These results further suggest that while the initial deposition of amyloid plaques and neurofibrillary tangles probably begins a decade or longer prior to the onset of clinical symptoms 5, 14, 15
, it is only after a threshold of neuronal injury has been reached in vulnerable brain regions that the clinical manifestations of cognitive impairment and later dementia appear 16
. Therefore, it is likely that CSF markers of neuronal injury together with markers of amyloid may predict future cognitive impairment over short follow-up periods with comparable ability to that of CSF markers of tau together with amyloid.
Together, these findings highlight the potential use of CSF VILIP-1 and VILIP-1/Aβ42 in guiding trial design, treatment decisions, outcome assessments, and response to therapies in clinical trials of disease-modifying therapies. CSF VILIP-1 may have potential value as a secondary or tertiary outcome measure in such trials, and complement diagnostic and prognostic information provided by CSF tau, p-tau, and Aβ42. It will be important to validate these findings in larger cohorts of well characterized individuals with similar or longer durations of follow-up.