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Depressive symptoms and CSF Alzheimer's disease biomarkers in relation to clinical symptom onset of mild cognitive impairment

Abstract Introduction We sought to examine whether depressive symptoms and level of Alzheimer's disease (AD) pathology are independently or interactively associated with the risk of progression to mild cognitive impairment (MCI). Methods The study included a total of 216 participants from the Biomarkers for Older Controls at Risk for Alzheimer's Disease study, a cohort of individuals who were cognitively normal at baseline (mean age = 57) and followed for more than 20 years (mean = 12.7 years), who had baseline Hamilton Depression Scale (HAM‐D) scores and cerebrospinal fluid (CSF) amyloid beta (Aβ)1‐42 , t‐tau, and p‐tau measures available. Results Cox regression demonstrated that baseline HAM‐D and CSF AD biomarkers were both associated with time to onset of MCI. There was an interaction between HAM‐D scores and markers of AD pathology, in which depression was associated with time of onset in participants with low levels of AD pathology (hazard ratio = 0.64; 95% confidence interval = 0.43–0.95; P= .026). Discussion The effect of depressive symptoms on progression to clinical symptoms of MCI may be most evident among individuals with low levels of AD pathology.

mixed findings 7 or no association. 9,10 It also remains unclear whether depressive symptoms and AD pathology during the preclinical phase of AD interact with one another, or independently alter risk for subsequent cognitive decline, as few prospective studies have examined this issue. Gatchel et al. reported an association between increasing depressive symptoms and greater cognitive decline among older adults (mean age, 73.5 years) with high but not low amyloid, as measured by positron emission tomography (PET). 11 However, participants had a mean age of 73.5 at baseline, the impact of tau was not addressed, and the longitudinal follow-up was relatively limited (mean, 3.9 years).
By comparison, a retrospective neuropathological study by Wilson et al. among individuals with a mean baseline age of 76.6 years found that while both markers of AD pathology and depressive symptoms were independently associated with cognitive decline, neuropathologic markers were not related to levels of depression over time, and depressive symptoms did not alter the relationship between neuropathology and longitudinal cognitive trajectories. 12,13 However, ADrelated pathological processes occur up to decades before the onset of cognitive symptoms, 14 and these early changes may not be accounted for in these studies that included primarily older adults. Thus, a further understanding of early non-cognitive symptoms among middle-aged individuals is needed.
To our knowledge, no previous study has assessed the interaction among biomarkers of AD pathology, depressive symptoms at middle age, and the risk of progression from normal cognition to MCI. Our primary objective was to examine the combined effects of depression and AD pathology, as measured by cerebrospinal fluid (CSF) biomarkers of amyloid and tau, on risk of progression from normal cognition to onset of clinical symptoms of MCI. We also sought to examine the relationship between baseline depressive symptoms and the short-term rate of change in CSF AD biomarkers.

Study design and participant selection
Data for these analyses were derived from the Biomarkers for Older Controls at Risk for Alzheimer's Disease (BIOCARD) study, which was designed to recruit and follow a cohort of cognitively normal individuals to identify variables that could predict the subsequent development of mild to moderate symptoms of AD. By design, approximately 75% of the participants had a first-degree relative with dementia due to AD.
The BIOCARD study was initiated at the National Institutes of Health (NIH) in 1995. Recruitment procedures and baseline evaluations have previously been described in detail. 15

HIGHLIGHTS
-Low-severity depressive symptoms among cognitively normal, primarily middle-aged individuals were associated with an increased risk of progression to clinical symptom onset of mild cognitive impairment in individuals who "low Alzheimer's disease (AD) pathology" at baseline.
-Higher baseline Hamilton Depression Scale was associated with more rapid rate of decline in cerebrospinal fluid amyloid beta (Aβ)  and increase in the ratio of CSF ttau/Aβ 1-42 .
-Exploratory comparisons indicate that individuals with "low AD pathology" who progressed to mild cognitive impairment/dementia had higher rates of hypertension and higher white matter hyperintensity volumes relative to those who remained normal. -

Clinical and cognitive assessments
A comprehensive neuropsychological battery covering all major cognitive domains, including memory, executive function, language, visuospatial ability, attention, speed of processing, and psychomotor speed was completed annually (see Albert et al. 15  and dementia due to AD. 18 The clinical diagnosis was established without knowledge of CSF biomarker data. A diagnosis of "impaired not MCI" was made for individuals with contrasting information between the CDR 16 interview and the cognitive test scores. As in prior publications, these participants were included in the group of cognitively normal individuals (as they do not meet criteria for MCI) and excluded in sensitivity analyses.

Depression assessments
Depressive symptomatology was measured at baseline with the HAM-D. 19 The HAM-D is one of the most widely used scales for assessing depressive symptom severity. 20 It is a clinician-administered scale consisting of 21 items that measures somatic and affective symptoms of depression. Scores of 0 to 7 are generally considered to be normal. 21 Staff conducting the consensus diagnosis were blinded to HAM-D scores of the participants. In these analyses, HAM-D scores were treated as a continuous measure and also dichotomized by median split, creating two groups: HAM-D >1 and HAM-D 0-1. Previous analyses using method of categorization have found that subjects with HAM-D >1 were more likely to progress to MCI within 7 years than those with HAM-D 0-1. 2 Using all eligible baseline CSF measures, a dichotomous AD biomarker indicator variable, "high AD pathology," was created to reflect low Aβ 1-42 and high p-tau or low Aβ 1-42 and high t-tau, based on tertiles. In previous analyses using this method of classification, the comparable "high AD pathology" group has been associated with greater cognitive decline than groups with more normal levels of CSF AD biomarkers. 23 "Low" Aβ 1-42 was defined as Aβ 1-42 in the lower one third of the distribution, while "high" t-tau or p-tau was defined as the upper one third of the distribution. This results in a group similar to the "Stage 2″ hypothetical preclinical AD group, defined by abnormal biomarkers for both amyloid and tau, which has been proposed to be one of three successive stages for categorizing cognitively normal individuals along the spectrum of preclinical AD. 23 Individuals who did not meet criteria for "high AD pathology" are referred to as "low AD pathology." Note that the primary goal of these groupings was to identify "high" versus "low" pathology for statistical purposes, and not to categorize individuals as biomarker normal versus abnormal, which would require clinically validated cut-points.

Vascular risk factors
Baseline vascular risk factors were established by medical records or by self-report. A composite vascular risk score was calculated by summing five dichotomous vascular risk factors (each coded as 0 = absent or 1 = recent/remote), as previously published: 24 hypertension, hypercholesterolemia, diabetes, current smoking (in the past 30 days), and obesity (body mass index > 30 kg/m 2 ). slices) using a previously described automated method. 25

Statistical analysis
Group differences in demographics and baseline characteristics were compared to t-tests for continuous variables and chi-square tests for dichotomous variables. Cox regression models (ie, proportional hazard models) were used to determine whether the relationship between baseline HAM-D scores and time to clinical symptom onset was modified by the presence of AD pathology, as measured by the "high AD pathology" indicator variable. The models were designed to compare two groups, based on the diagnosis at their last visit: (1) participants who remained cognitively normal and (2) participants who were normal at baseline but were diagnosed with MCI or dementia at their last follow-up. The outcome variable was the time to MCI clinical symptom onset, for the participants who progressed to MCI or dementia.
All models were adjusted for left truncation because individuals were required to be symptom free at baseline, and the last date of diagnosis was used as the censoring time. The predictors were HAM-D score (continuous or dichotomous), the "high AD pathology" indicator, and the HAM-D × "high AD pathology" interaction term, which was of primary interest. The model was run with the "high AD pathology" indicator defined by CSF Aβ 1-42 and t-tau, and then with the "high AD pathology" indicator defined by CSF Aβ 1-42 and p-tau. Models were adjusted for age, education, and sex.
Linear mixed effects regression models were used to examine whether baseline HAM-D scores were associated with rate of change in the CSF biomarkers over time. Separate models were run for each of the CSF biomarkers and their ratios, including Aβ 1-42 , t-tau, p-tau, t-tau/Aβ 1-42 , and p-tau/Aβ 1-42 , which served as the dependent variables. The models were specified with a random intercept and slope.
All CSF variables were standardized before model fitting; the ratios of t-tau/Aβ 1-42 , and p-tau/Aβ 1-42 were log-transformed, to reduce skewness, before standardization. Predictors included baseline age, sex, baseline HAM-D score, time (in years), and the interaction of each predictor with time. In these models, we were primarily interested in the HAM-D score × time interaction, which tests whether the rate of change in a CSF biomarker over time differs as a function of baseline HAM-D scores.
To examine vascular factors as a possible mechanism for the association between depressive symptoms and risk of clinical symptom onset, we compared measures related to vascular risk in the subset of individuals with "low AD pathology." For these analyses, comparisons were made with t-tests or Kruskall-Wallis rank tests for continuous variables and chi-square tests or Fisher's exact tests for dichotomous variables, depending on the distribution of the variables.
All analyses were run in Stata (version 16.1). Significance was set at P < .05. with "low AD pathology" had higher mean baseline HAM-D scores and higher proportion of baseline HAM-D score >1 (all P ≤ .03) compared to those with "high AD pathology." Participants with "low AD pathology" were more likely to be diagnosed with MCI/dementia due to a non-AD or depressive etiology relative to those with "high AD pathology" (both P = .01) ( Table S3a). Among individuals who progressed to MCI/dementia with "low AD pathology," there were no differences in baseline CSF biomarkers between those with a HAM-D score of 0-1 (N = 11) versus HAM-D > 1 (N = 21, Tables S4a and S4b in supporting information). All three baseline CSF biomarkers were significantly different between groups with low versus high AD pathology (all P < .001; Table S6 in supporting information).

Relationship between HAM-D and CSF biomarkers of AD and time to onset of clinical symptoms of MCI
Our primary analyses examined the combined effects of depression and AD pathology, as measured by CSF biomarkers of amyloid and tau, on risk of progression from normal cognition to onset of clinical symptoms of MCI. Results of the Cox regression models, adjusted for age, sex, and education, are shown in Table 2. Kaplan-Meier plots for unadjusted models are shown in Figure 2 (AD pathology status defined by CSF Aβ 1-42 and p-tau) and Figure S1 in supporting information (AD pathology status defined by CSF Aβ 1-42 and t-tau). When coded dichotomously, baseline HAM-D scores > 1 were associated with an increased risk of clinical symptom onset of MCI associated with an increased risk of clinical symptom onset of MCI (both P < .05), whereas this association did not reach significance when HAM-D scores were coded continuously (both P < .11). "High AD pathology" was associated with an increased risk of clinical symptom onset of MCI in all models (all P < .001). There was also an interaction between baseline HAM-D scores and the "high AD pathology" indicator (all P < .03), indicating that higher baseline HAM-D scores were significantly associated with increased risk of clinical symptom onset among individuals with "low AD pathology," which was not the case for individuals who had "high AD pathology." The pattern of results was identical when apolipoprotein E (APOE)4 was included as an additional model covariate (Table S7 in supporting information).
In line with our primary findings, in a sensitivity analysis that only included individuals with "low AD pathology," HAM-D scores were associated with risk of progression, despite the smaller sample size ( Table 3). The pattern of results was similar when APOE4 was included as an additional covariate (Table S8 in      in the lower one third of the distribution, while "high" p-tau was defined as the upper one third of the distribution; Low AD pathology, group consisting of individuals who did not meet criteria for "high AD pathology." The pattern of results for "high AD pathology" defined by Aβ 1-42 in the lower one third of the distribution and t-tau in the upper one third of the distribution was similar, and can be found in Figure S1 in supporting information

TA B L E 3
Hazard ratios for baseline HAM-D scores in relation to time to onset of clinical symptoms of MCI or dementia in individuals with "low AD pathology" "Low AD pathology" defined by t-tau "Low AD pathology" defined by p-tau   with a similar trend for CSF p-tau/Aβ 1-42 (P = .057); however, these effects were not significant for the dichotomous baseline HAM-D models. Baseline HAM-D scores were not associated with rate of change in CSF t-tau or p-tau (Table 4).

Exploratory analysis of measures related to vascular risk
Findings from our primary analysis suggested a stronger relationship between depressive symptoms and progression to clinical symptoms of MCI in individuals with "low AD pathology" (Tables 2 and 3). To examine vascular risk as a potential non-AD mechanism for the association between depressive symptoms and progression to MCI/dementia, exploratory comparisons of baseline vascular risk in individuals with "low AD pathology" were conducted. Within this subgroup, individuals who progressed to MCI/dementia had higher rates of hypertension and higher WMH volumes relative to those who remained normal (both P < .003; Table S5 in

DISCUSSION
The primary finding in the present study was a significant interaction between baseline depressive symptoms and level of AD pathology, as measured by CSF, indicating that higher baseline HAM-D scores were significantly associated with increased risk of clinical symptom onset among individuals with "low AD pathology,", but not in "high AD pathology." A possible explanation is that, among individuals with higher levels of AD pathology, the effects of depression on the risk of progression to MCI are overshadowed by the well-established risks incurred by the presence of a high burden of AD pathology. 22,26 In individuals with low AD pathology, however, depressive symptoms may serve as an additional risk factor for cognitive symptom onset.
To our knowledge, this is the first study investigating the interac- to MCI is not mediated by AD pathology, there was no evidence that depressive symptoms were associated with higher baseline levels of AD pathology in the exploratory analysis of "low AD pathology" participants who progressed to MCI/dementia. Taken together with our previous finding that depressive symptoms are associated with increased risk of MCI symptom onset within 7 years, but not after 7 years, 2 it is possible that in individuals with low AD pathology, the relationship between depression and cognitive decline is mediated through mechanisms outside of amyloid and tau.
For example, in our exploratory analysis, we found that among individuals who progress to MCI/dementia, those with "low AD pathology" had higher prevalence of non-AD pathologies or other etiologies contributing to their clinical impairment, relative to individuals with "high AD pathology." Though depression itself can contribute to cognitive impairment, only 19% of the cohort with "low AD pathology" were diagnosed with MCI/dementia attributable to depression as a primary contributing etiology. With regards to vascular risk factors, we found that in individuals with "low AD pathology," those who do progress to MCI/dementia had higher prevalence of hypertension and higher mean WMH volume. This is consistent with previous evidence that depression may be a risk factor and/or prodrome for vascular dementia, 32 This study has several strengths, including the long duration of follow-up (mean = 12.7 years, maximum = 21.6 years). It includes a relatively large number of well-characterized, cognitively normal individuals who were primarily in middle age at baseline. These findings, however, should also be interpreted within the context of their limitations.
The participants were primarily well-educated, White, and had very low severity of depressive symptoms at baseline, which limits its gen- In summary, low-severity depressive symptoms among cognitively normal, primarily middle-aged individuals were associated with an increased risk of progression to clinical symptom onset of MCI in individuals with low AD pathology. These results suggest that the effect of depression on progression to MCI may be most evident among individuals with low levels of AD pathology, and that this association may occur through mechanisms outside of the amyloid and tau pathway.

ACKNOWLEDGMENTS
The BIOCARD Study consists of seven Cores with the following mem-