DNA Methylation and Protein Markers of Chronic Inflammation and Their Associations With Brain and Cognitive Aging

Abstract

Background and objectives: To investigate chronic inflammation in relation to cognitive aging by comparison of an epigenetic and serum biomarker of C-reactive protein and their associations with neuroimaging and cognitive outcomes.

Methods: At baseline, participants (n = 521) were cognitively normal, around 73 years of age (mean 72.4, SD 0.716), and had inflammation, vascular risk (cardiovascular disease history, hypertension, diabetes, smoking, alcohol consumption, body mass index), and neuroimaging (structural and diffusion MRI) data available. Baseline inflammatory status was quantified by a traditional measure of peripheral inflammation-serum C-reactive protein (CRP)-and an epigenetic measure (DNA methylation [DNAm] signature of CRP). Linear models were used to examine the inflammation-brain health associations; mediation analyses were performed to interrogate the relationship between chronic inflammation, brain structure, and cognitive functioning.

Results: We demonstrate that DNAm CRP shows significantly (on average 6.4-fold) stronger associations with brain health outcomes than serum CRP. DNAm CRP is associated with total brain volume (β = -0.197, 95% confidence interval [CI] -0.28 to -0.12, p _FDR = 8.42 × 10^-6), gray matter volume (β = -0.200, 95% CI -0.28 to -0.12, p _FDR = 1.66 × 10^-5), and white matter volume (β = -0.150, 95% CI -0.23 to -0.07, p _FDR = 0.001) and regional brain atrophy. We also find that DNAm CRP has an inverse association with global and domain-specific (speed, visuospatial, and memory) cognitive functioning and that brain structure partially mediates this CRP-cognitive association (up to 29.7%), dependent on lifestyle and health factors.

Discussion: These results support the hypothesis that chronic inflammation may contribute to neurodegenerative brain changes that underlie differences in cognitive ability in later life and highlight the potential of DNAm proxies for indexing chronic inflammatory status.

Classification of evidence: This study provides Class II evidence that a DNAm signature of CRP levels is more strongly associated with brain health outcomes than serum CRP levels.

Figure 1. Chronic Inflammation Increases With Age and May Contribute to Variance in Cognitive Ability

(A) Schematic demonstrating lifespan curves for chronic inflammation. Inflammatory load tends to increase with age; lifestyle, genetics, and health conditions can all influence susceptibility to chronic inflammation and account for variance in inflammation levels between individuals.^, (B) Chronic inflammation can be measured by inflammatory proteins taken from a blood sample, such as serum levels and DNA methylation (DNAm) proxies of C-reactive protein (CRP). (C) Lifespan curves for cognitive ability outlining how there is considerable interindividual heterogeneity in rate and timing of cognitive decline, with some people on more accelerated cognitive aging trajectories than others. (D) Trajectories of LBC1936 participants respective of inflammation scores over age, as outlined in Stevenson et al, illustrating comparative stability of DNAm inflammation marker compared to serum CRP.

Figure 2. Flowchart Depicting the Step-by-Step Selection Process of the Lothian Birth Cohort 1936 (LBC1936) Participants Included in the Final Sample for Data Analyses

CRP = C-reactive protein; CT = cortical thickness; CV = cortical volume; gFA = general fractional anisotropy; GM = gray matter; gMD = general mean diffusivity; hsCRP = high-sensitivity C-reactive protein; MMSE = Mini-Mental State Examination; NAWM = normal appearing white matter; QC = quality control; TB = total brain volume; WMH = white matter hyperintensities.

Figure 3. DNAm CRP Shows Stronger and More Widespread Associations With Global and Regional Brain Structure Than Serum CRP

Regional cortical volume regressed against serum C-reactive protein (CRP) (A, D) and DNA methylation (DNAm) CRP (B, E) (n = 521). Colors denote the magnitude (T-maps; top) and significance (Q values; bottom) of the negative associations between inflammation and brain cortical volume. (C) Percentage attenuation for the significant associations between DNAm-CRP and cortical volume when also controlling for serum CRP. Conjunction plot (F) shows the spatial extent of independent contributions and overlap (red) in cortical loci that exhibit false discovery rate (FDR)–corrected unique associations with simultaneously modeled serum (pink) and epigenetic (blue) inflammation measures; results are corrected for sex, age, and intracranial volume.

Figure 4. DNA Methylation C-Reactive Protein is Associated With White Matter Microstructure in Specific White Matter Tracts

(A) Standardized regression coefficients for associations between white matter tract–averaged fractional anisotropy (FA; left) and mean diffusivity (MD; right). Bars show standardized coefficients and standard errors. Asterisks indicate where associations are significantly larger for DNA methylation than for serum (*p < 0.05, **p < 0.01). (B) Illustration of the respective white matter tracts measured using probabilistic neighborhood tractography in one LBC1936 study participant.

Figure 5. Brain Structure Partly Mediates the Association of DNAm CRP With Cognitive Ability

Top panel (A, B) displays single mediator models, bottom panel (C, D) displays multiple mediator models. (A) Model 1 structural equation model path diagrams showing that in model 1, the association between DNA methylation (DNAm) C-reactive protein (CRP) and general cognitive ability (path c) was significantly and partially mediated by normal-appearing white matter volume (path ab = −0.033, p = 0.001), attenuating the c path by 21% (path c′) and (C) 29.7% by multiple MRI variables (ab = −0.047, p = 0.002). (B) Single mediator models indirect effect size and standard error bars. (D) Multiple mediator models indirect effect size and standard error bars. Light bars show model 1 (includes covariates age and sex), dark bars show model 2, which contains additional health covariates (age + sex + body mass index [BMI] + hypertension + smoking status + alcohol use + cardiovascular disease [CVD] history + diabetes); *p < 0.05; **p < 0.01. g_f = general cognitive ability; gFA = general fractional anisotropy; GM = gray matter; gMD = general mean diffusivity; NAWM = normal-appearing white matter; WMH = white matter hyperintensity.

Figure 6. Mechanisms of Neurodegeneration via Increased Systemic Chronic Inflammation

(A) Chronic inflammation is pertinent to brain aging in that inflammatory mediators in the periphery can damage the blood–brain barrier (BBB), permitting entry into the brain where they go on to disrupt neurons and glia and perpetuate a chronic inflammatory state. This directly contributes to various neurodegenerative pathways (illustrated) that lead to brain cell death. (B) Suggested mechanisms by which the causes of inflammaging (immunosenescence, lifestyle, clinical health) and related consequences may drive brain health (structural and cognitive) outcomes. (C) Study model: chronic inflammation is a key driver of cognitive decline through its effects on brain structure. Left shows generic directed acyclic graph for mediation analysis (left panel) and for the study example (right panel). A = exposure; BDNF = brain-derived neurotrophic factor; C = confounder; CRP = C-reactive protein; IGF-1 = insulin-like growth factor 1; IL1 β = interleukin-1β; IL6 = interleukin-6; M = mediator; PGE2 = prostaglandin E2; ROS = reactive oxygen species; SASP = senescence associated secretory phenotype; TNF-α = tumor necrosis factor-α; Y = outcome. Created with BioRender.com.