Cognitive stimulation in the workplace, plasma proteins, and risk of dementia: three analyses of population cohort studies

Abstract Objectives To examine the association between cognitively stimulating work and subsequent risk of dementia and to identify protein pathways for this association. Design Multicohort study with three sets of analyses. Setting United Kingdom, Europe, and the United States. Participants Three associations were examined: cognitive stimulation and dementia risk in 107 896 participants from seven population based prospective cohort studies from the IPD-Work consortium (individual participant data meta-analysis in working populations); cognitive stimulation and proteins in a random sample of 2261 participants from one cohort study; and proteins and dementia risk in 13 656 participants from two cohort studies. Main outcome measures Cognitive stimulation was measured at baseline using standard questionnaire instruments on active versus passive jobs and at baseline and over time using a job exposure matrix indicator. 4953 proteins in plasma samples were scanned. Follow-up of incident dementia varied between 13.7 to 30.1 years depending on the cohort. People with dementia were identified through linked electronic health records and repeated clinical examinations. Results During 1.8 million person years at risk, 1143 people with dementia were recorded. The risk of dementia was found to be lower for participants with high compared with low cognitive stimulation at work (crude incidence of dementia per 10 000 person years 4.8 in the high stimulation group and 7.3 in the low stimulation group, age and sex adjusted hazard ratio 0.77, 95% confidence interval 0.65 to 0.92, heterogeneity in cohort specific estimates I2=0%, P=0.99). This association was robust to additional adjustment for education, risk factors for dementia in adulthood (smoking, heavy alcohol consumption, physical inactivity, job strain, obesity, hypertension, and prevalent diabetes at baseline), and cardiometabolic diseases (diabetes, coronary heart disease, stroke) before dementia diagnosis (fully adjusted hazard ratio 0.82, 95% confidence interval 0.68 to 0.98). The risk of dementia was also observed during the first 10 years of follow-up (hazard ratio 0.60, 95% confidence interval 0.37 to 0.95) and from year 10 onwards (0.79, 0.66 to 0.95) and replicated using a repeated job exposure matrix indicator of cognitive stimulation (hazard ratio per 1 standard deviation increase 0.77, 95% confidence interval 0.69 to 0.86). In analysis controlling for multiple testing, higher cognitive stimulation at work was associated with lower levels of proteins that inhibit central nervous system axonogenesis and synaptogenesis: slit homologue 2 (SLIT2, fully adjusted β −0.34, P<0.001), carbohydrate sulfotransferase 12 (CHSTC, fully adjusted β −0.33, P<0.001), and peptidyl-glycine α-amidating monooxygenase (AMD, fully adjusted β −0.32, P<0.001). These proteins were associated with increased dementia risk, with the fully adjusted hazard ratio per 1 SD being 1.16 (95% confidence interval 1.05 to 1.28) for SLIT2, 1.13 (1.00 to 1.27) for CHSTC, and 1.04 (0.97 to 1.13) for AMD. Conclusions The risk of dementia in old age was found to be lower in people with cognitively stimulating jobs than in those with non-stimulating jobs. The findings that cognitive stimulation is associated with lower levels of plasma proteins that potentially inhibit axonogenesis and synaptogenesis and increase the risk of dementia might provide clues to underlying biological mechanisms.


The IPD-Work consortium
Established at the Four Centres meeting in London in 2008, the Individual-Participant-Data meta-analysis in Working populations (IPD-Work) consortium is a collaborative research project of European cohort studies with the aim to estimate reliably the associations of work-related factors with chronic diseases, disability, and mortality. The original participating cohort studies were: -Whitehall II, UK; -GAZEL, France; -Belstress, Belgium; -Netherlands Working Conditions Survey (NWCS) and -Permanent Onderzoek Leefsituatie (POLS), Netherlands; -Intervention Project on Absence and Well-being (IPAW), -Copenhagen Psychosocial Questionnaire version (COPSOQ-I) and -Danish Work Environment Cohort Study (DWECS), Denmark; -Work, Lipids and Fibrinogen-Stockholm and -Work, Lipids and Fibrinogen-Norrland, Sweden (in combination WOLF); -Still working study, -Finnish Public Sector study (FPS), and -Health and Social Support (HeSSup) study, Finland.
Subsequently, the Helsinki Health Study, Finland (HHS, an occupational cohort study set up in collaboration with the Whitehall II study) has joined the consortium.
As described in the two background papers of the consortium, published research assessing work-related risk factors for chronic conditions had been inconsistent and subject to post hoc decision making, publication bias and reverse causation bias. 1 2 To strengthen evidence, the aim of the IPD-Work consortium is to use pre-defined and harmonized exposure definitions (to minimize selective reporting) and large pooled datasets (to allow confirmation of findings across subgroups and, in case of null finding, to show and publish absence of associations convincingly).
In agreement with these principles, measurements of job demands and job control were validated and harmonized across the IPD-Work cohort studies before extracting outcome data. 3 In addition to the present paper on dementia, the IPD-Work consortium has used these pre-defined measures in studies of diabetes, coronary heart disease, stroke, site-specific cancers, and respiratory diseases. [4][5][6][7][8][9][10] The work of the IPD-Work consortium has been supported by: the Wellcome Trust Collaborative Award in Science (221854/Z/20/Z), Medical Research Council (MR/S011676/1), and Economic and Social Science Research Council (ESRC, ES/J023299/1), UK; National Institute on Aging (NIH), US (R01AG062553); the European Union New OSH ERA ('New and Emerging Risks in Occupational Safety and Health') Research Programme; NordForsk, the Nordic Programme on Health and Welfare (75021); Academy of Finland (132944, 311492); Finnish Work Environment Fund (112253,115421,190424), and Helsinki Institute of Life Science (H970), Finland.

Description of participating study cohorts
Atherosclerosis Risk in Communities (ARIC) 11 is a prospective cohort study of men and women recruited from four communities: Forsyth County, North Carolina; Jackson, Mississippi; eight northern suburbs of Minneapolis, Minnesota; and Washington County, Maryland, US. A total of 15,792 participants received an extensive examination, including medical, social, and demographic data and were re-examined every three years with the first screen (baseline) occurring in 1987-89. Data on proteins, based on blood samples drawn in the 1993-95 examination, and dementia incidence were available for 11,395 participants, the analytic sample of the present study. The ARIC study has been approved by the Institutional Review Boards of all participating institutions. Unlike this study, all the remaining cohorts are part of the IPD-Work consortium. The study was approved by each site's institutional review board (at The Johns Hopkins University, Wake Forest University, University of Mississippi Medical Center, and University of Minnesota), and written informed consent was signed by all participants (and proxies, when required) Finnish Public Sector study (FPS) 12 is a prospective cohort study comprising the entire public sector personnel of 10 towns (municipalities) and 21 hospitals in the same geographical areas 65 . Participants, who were recruited from employers' records in 2000-2002, were individuals who had been employed in the study organisations for at least six months prior to data collection. 48 592 individuals (9 337 men and  39 255 women aged 17 to 65) responded to the questionnaire. Of these, 47,448 had data on cognitive  stimulation at work and were eligible for our meta-analyses and 47,455 had data on the job exposure  matrix index of cognitive stimulation. Ethical approval was obtained from the ethics committee of the  Finnish Institute of Occupational Health. GAZEL 13 is a prospective cohort study of 20 625 employees (15,011 men and 5,614 women) of France's national gas and electricity company, Electricité de France-Gaz de France (EDF-GDF). Since the study baseline in 1989, when the participants were aged 35-50 years, they have been posted an annual followup questionnaire to collect data on health, lifestyle, individual, familial, social, and occupational factors. Cognitive stimulation at work was measured in Gazel in 1997, which we treated as a baseline year for our analyses. 11 448 individuals participated that year and 11,362 of them had data on cognitive stimulation at work and were eligible for our meta-analysis. The GAZEL study received approval from the national commission overseeing ethical data collection in France (Commission Nationale Informatique et Liberté).
Health and Social Support (HeSSup) 14 is a prospective cohort study of a stratified random sample of the Finnish population in the following four age groups: 20-24, 30-34, 40-44, and 50-54 68 . The participants were identified from the Finnish population register and posted an invitation to participate, along with a baseline questionnaire, in 1998 (4). Job strain was measured in 1998 and of the 25 898 individuals who responded to the questionnaire, 15 534 were in employment and had data on cognitive stimulation at work and were thus eligible for our meta-analyses. The Turku University Central Hospital Ethics Committee approved the study.
Helsinki Health Study (HHS) 15 is a prospective cohort study comprising all employees of the City of Helsinki, who turned 40, 45, 50, 55, or 60 years in 2000-2002. We included in this study all participants who responded to the baseline survey (n=8960, response rate 67%) and provided an informed written consent to combine their survey responses on cognitive stimulation with retrospective and prospective register-based follow-up data on different diseases and mortality (n=6544). Ethical approvals for this study were obtained from the ethics committees of the health authorities of the City of Helsinki, and the Department of Public Health, University of Helsinki.
Still Working Study 16 is an ongoing prospective cohort study. In 1986, the employees (n = 12 173) at all Finnish centres of operation of Enso Gutzeit (a forestry products manufacturer) were invited to participate in a questionnaire survey on demographic, psychosocial and health-related factors. At baseline, 9 282 individuals responded, and of these 9 165 had data on cognitive stimulation at work and were eligible for our meta-analyses. The study was approved by the ethics committee of the Finnish Institute of Occupational Health.
The Whitehall II study 17 is a prospective cohort study set up to investigate socioeconomic determinants of health. At study baseline in 1985-1988, 10,308 civil service employees (6,895 men and 3,413 women) aged 35-55 and working in 20 civil service departments in London were invited to participate in the study. Data on cognitive stimulation at work, measured at phase 3 were available for 7475 men of the men and women who were eligible for our meta-analyses. The Whitehall II study protocol was approved by the University College London Medical School committee on the ethics of human research. Written informed consent was obtained at each data collection wave. WOLF (Work, Lipids, and Fibrinogen). 18 19 WOLF-Stockholm cohort includes participants aged 19-70 and working in companies in Stockholm county and the WOLF Norrland includes participants aged 19-65 working in companies in Jämtland and Västernorrland counties, a total of 10 368 participants with data on cognitive stimulation and dementia. At study baseline the participants underwent a clinical examination and completed a set of health questionnaires. For WOLF Stockholm, the baseline assessment was undertaken at 20 occupational health units between November 1992 and June 1995 and for WOLF Norrland at 13 occupational health service units in 1996-98. The Regional Research Ethics Board in Stockholm, and the ethics committee at Karolinska Institutet, Stockholm, Sweden approved the study. In analyses, data from these cohorts were pooled and WOLF treated as a single study.

Characteristics of participating cohort studies
Sample characteristics and the proportion of missing data for each variable are described in etables 1 to 7 by cohort.

Demand-control model of cognitive stimulation
The level of cognitive stimulation was defined based on the active-passive dimension, the lowest cognitive stimulation relating to passive jobs in which both job demands and job control are low and the highest cognitive stimulation to active jobs in which both job demands and job control are high (eFigure 2). Medium cognitive stimulation refers to jobs in which job demands are high, but job control low or job demands are low, but job control high.
Karasek describes the active-passive dimension as follows: "…incremental additions to competency are predicted to occur when the challenges of the situation are matched by the individual's skill or control in dealing with a challenge. When job demands and job decision latitude are simultaneously high, we define the job as "active" and hypothesize that it leads to development of new behaviour patterns both on and off the job (Diagonal B toward lower right). The model predicts that jobs at the opposite extreme (defined as "passive job") induce a decline in overall activity and a reduction in general problem-solving activity." 20 (P. 288) The other dimension in Karasek's model is job strain. This is typically operationalised as a dichotomous variable with high demands and low control referring to job strain and all other combinations of demands and control to no job strain. 20 21

eFigure 1. Definition of cognitive stimulation (A) and job strain (B) based on the Demand-Control Model
Measurement of cognitive stimulation: The assessment of cognitive stimulation at work was based on Karasek's model of two fundamental characteristics of work, job demands and job control (also labelled as job decision latitude). 20 In IPD-Work cohort studies, questions in the job demands and job control scales had Likert-type response formats. Mean response scores for job demand items and for job control items were computed for each participant. 4 We defined high job demands as having a job-demand score that was greater than the study-specific median score; similarly, we defined low job control as having a job control score that was lower than the study-specific median score. These categorisations are the original and most commonly used.
We used dichotomised demand and control measures to construct three categories of cognitive stimulation at work along the active-passive work dimension of Karasek's demand-control model 20 21 in which both high demands and high control indicate higher stimulation. Low cognitive stimulation at work was defined as 'low demands and low control', medium cognitive stimulation as 'high control and low demands' or 'high demands and low control' and high cognitive stimulation as 'high demands and high control'.
Although scale items slightly varied between studies, all versions were strongly related to the original gold standard scales (Pearson correlation coefficients between r = 0.76 and r = 0.98). 3

Measurement of additional covariates
Additional covariates in the FPS were: Social isolation (based on an affirmative response to questions on living alone), depression (based on hospital admission records with an ICD-10 code F32 or F33), traumatic brain injury (based on hospital admission records with an ICD-10 code S06), and atrial fibrillation (based on hospital admission records with an ICD-10 code I48). Social isolation was measured at baseline. We considered hospital admissions both at baseline and during follow-up until dementia diagnosis in dementia cases and the end of follow-up in non-cases. The distribution of these covariates is shown in etable 9. In the Whitehall-II random sample and ARIC studies, ethnicity was measured by self-report and was categorized into 'white' vs 'non-white'. The distribution of this covariate by cohort is shown in etable 10.

Measurement of plasma proteins
We measured plasma proteins using the SomaScan Assay, version 4. 22 The assay, its performance characteristics and the modified aptamer binding reagents have previously been described [23][24][25] and the annotated menu for all ~5,000 modified-aptamer binding reagents is available in our previous paper. 22 Briefly, the SomaScan Assay uses a 96-well plate including a mix of thousands of slow off-rate modified aptamers (SOMAmers). The SOMAmers are labelled with biotin and a 5′ fluorophore, photocleavable linker and immobilized on streptavidin-coated beads through biotin-streptavidin interaction. Participants' plasma samples were diluted and added to each well, which resulted in formation of cognate and nonspecific SOMAmer-protein complexes. After the unbound proteins were washed away, captured proteins were labelled with biotin. By using ultraviolet light photocleavage, the SOMAmer-protein complexes were released from the beads and incubated in a buffer containing unlabelled polyanionic competitor. This step separates the unspecific, fast off-rate SOMAmer-protein complexes from slow off-rate cognate and thus intended complexes, and can be viewed as a second element of specificity analogous to the effect of adding a second antibody in a conventional immunoassay.
Cognate SOMAmer-protein complexes were then recaptured on a second set of streptavidin-coated beads using biotin labelled proteins and additionally washed to further remove non-specific SOMAmerprotein interactions. In the next step, denaturing buffer released the SOMAmers from the proteins. The SOMAmers were then hybridized to complementary sequences on a DNA microarray chip and quantified by fluorescence for readout. In the SomaScan assay, fluorescence intensity for each SOMAmer is related to the relative availability of the three-dimensional shape-charge epitope on each protein (the binding site of the SOMAmer reagent) in the original sample. This reflects the concentration of each protein. Median intra-and inter-assay coefficients of variation are ~5% and assay sensitivity is comparable to that of typical immunoassays, with a median lower limit of detection in the femtomolar range. 23 Specificity of the modified aptamer reagents is high. 22