With the increasing proportion of elderly individuals, improving understanding of the biological mechanisms involved in healthy aging is of utmost importance. Retinal vascular complexity obtained from retinal fundus photographs, and evaluated as fractal dimension (ie, Df), a measure of the geometric pattern of the retinal vasculature, has recently emerged as a valuable indicator of diseases and aging processes. The purpose of our study was to elucidate possible mechanisms underlying those relationships. By integrating retinal Dfwith genomic and plasma proteome biomarkers, we uncovered novel pathways involved in determining Df and its links with cardiometabolic diseases and longevity. First, we performed a genome wide association study (GWAS) of retinal vascular Df by meta-analysing signals from 74,434 participants from three large epidemiological cohorts, the Canadian Longitudinal Study of Aging (CLSA), the Genetics of Diabetes Audit and Research in Tayside Scotland (GoDARTS), and the UK Biobank (UKBB). We replicated four known loci associated with Df, and identified one novel locus within or near DAAM1, as well as found seven suggestive associations within or near CSNK1A1L, MAST4, HIVEP1, LOC283038, FNDC3A, SP100, and SUGP1 genes. GWAS summary statistics confirmed a negative correlation between Df and cardiovascular disease, stroke, and inflammation, but a positive correlation with longevity. Next, employing Mendelian randomization which combined genetic determinants of Df, and cis-protein Quantitative Trait Loci from 1,159 circulating proteins from the Prospective Urban and Rural Epidemiological (PURE) cohort (n=12,066), we identified eight causal mediators for Df, namely IgG Fc receptorIIb, BST1, LILRB2, IL16, MMP12, PON2, ALPP, and PDL2. Notably, MMP12 and IgG Fc receptorIIb have been suggested to connect Df to cardiometabolic outcomes (i.e., coronary artery disease, stroke, peripheral artery disease, and type 2 diabetes) and longevity, respectively. Pathway enrichment analyses highlighted the role of IgG Fc receptorIIb in immune and inflammatory responses to aging processes, referred to as inflammaging. This multi-pronged approach unveils IgG Fc recIIb and MMP12 as key mediators in immune and inflammation pathways, linking lower Df to a higher risk of cardiometabolic diseases and a shorter lifespan. Therefore, retinal Df may be a convenient way to estimate inflammaging, such that lower retinal Df, may indicate a higher inflammaging status. Targeting MMP12 and IgG Fc ReceptorIIb may promote healthy aging and mitigating the occurrence of cardiometabolic diseases in the elderly population.

The authors have declared no competing interest.

This research has been conducted using the UK Biobank Resource under project 788. This research was funded in part by the Medical Research Council grant (MR/N013166/1) to A.V.V. M.O.B. gratefully acknowledges funding from: Foundation Leducq Transatlantic Network of Excellence (17 CVD 03); EPSRC grant no. EP/X025705/1; British Heart Foundation and The Alan Turing Institute Cardiovascular Data Science Award (C-10180357); Diabetes UK (20/0006221); Fight for Sight (5137/5138); the SCONe projects funded by Chief Scientist Office, Edinburgh & Lothians Health Foundation, Sight Scotland, the Royal College of Surgeons of Edinburgh, the RS Macdonald Charitable Trust, and Fight For Sight; the Neurii initiative which is a partnership among Eisai Co., Ltd, Gates Ventures, LifeArc and HDR UK. This research has been conducted using the CLSA dataset 1906019_McMaster_GPare_Baseline (Baseline Comprehensive (COM) version 4.2), under Application Number 1906019. The CLSA is led by Drs. Parminder Raina, Christina Wolfson and Susan Kirkland. M.P. was supported by the E.J. Moran Campbell Internal Career Research Award from McMaster University, and the Early Career Research Award from Hamilton Health Sciences (HHS). Retinal images analyses of the CLSA were supported by a New Investigator Fund from HHS (NIF-18453 to M.P.), and the Canadian Institutes of Health Research (CIHR) (ACD-170312 and PJT-178302 to M.P.). The PURE study is an investigator-initiated study funded by the Canadian Institutes of Health Research (CIHR), Heart and Stroke Foundation of Ontario, the Ontario Ministry of Health and Long-Term Care, and through unrestricted grants from industry including AstraZeneca, Sanofi-Aventis, Boehringer Ingelheim, Servier, and GalaxoSmithKline. The biomarker project was led by PURE investigators at the Population Health Research Institute (Hamilton, Canada) in collaboration with Bayer scientists. Bayer directly compensated the Population Health Research Institute for measurement of the biomarker panels, and scientific, methodological, and statistical work. Genetic analyses were supported by CIHR (G-18-0022359) and the Heart and Stroke Foundation of Canada (399497) in the form of funding to GP.

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

UK Biobank study was approved by the National Research Ethics Committee, reference 11/NW/0382 For PURE data, the research ethics committees at each study location, including Hamilton Health Sciences, approved the project (Hamilton, ON, Canada). Written informed consent was obtained from each study subject.

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All data produced in the present study will be available upon publication