The present study applied EWAS analytical method to UK biobank data in order to examine shared environmental factors existing in the AD and AMD. First, a multiple of environmental factors associating with emerging AD and AMD are listed from the existing literature. Second, using this environment-wide association approach, we identified 112 and 132 environmental factors that were associated with AD and AMD respectively. These factors cluster in the following domains: health condition, biological samples’ parameters, body index and attendance availability. Moreover, an examination of the co-environmental factors that influence AD and AMD reveals potential association of AD and AMD.
It remains a matter of debate whether AMD and AD are closely associated. A recent meta-analysis reported no significant association between AMD and incident dementia or AD30. On the contrary, another meta-analysis mainly enrolled case-controlled and cross-sectional studies documented the increased risk of AD in patients with AMD31. Based on GWAS data from 690,000 participants included in this study from nine multi-omics datasets, our unpublished results showed us that AD and AMD share common genetic factors such as APOC1 and APOE, which play a substantial role in the etiology. However, environmental exposures may also have a major impact on molecular and cellular systems for many diseases.
Referring to the constructed knowledge graph, most previous research has focused on testing the effect of one or two environmental influences that stemmed from one or two sources. Yet EWAS could provide a practical method to test a variety of exposures in human environment in an unbiased manner, similar to GWAS tests for genetic effects32.
After conducting EWAS analyses to explore shared environmental items between AD and AMD, we identified several common risk and protective factors. Factors including "no Started insulin," "Receive Attendance allowance and Disability living allowance," "Diabetes," "logMAR (mean arterial pressure)," "Started insulin within one year diagnosis of diabetes," "Cystatin C level," "Poor health condition," "Glucose," and "Glycated haemoglobin (HbA1c)" were found to be common risk factors for both diseases. On the other hand, "Haematocrit percentage," "Cholesterol level," "LDL level," "Haemoglobin concentration," "3mm asymmetry index for irregular astigmatism level (normal)," "3mm regularity index for irregular astigmatism level (normal)," "Good health condition," and "no Diabetes" were common protective factors for AD.
We found that the risk factors for AD and AMD include "no Started insulin," "Diabetes," "Started insulin within one year diagnosis of diabetes," "Glucose," and "Glycated haemoglobin (HbA1c)." These findings are consistent with previous research. A few of studies have highlighted the substantial similarities between AD and diabetes, including common metabolic alterations and genetic underpinnings. As such, AD has been referred to as "type 3 diabetes" (T3DM)33, 34, 35, 36, 37. Retinopathy is one of common and feared complications of diabetes, several studies have shown a relationship between glucose disturbance (GD) and AMD38. Additionally, a growing body of literature has identified diabetes as a risk factor for AMD 39, 40, 41.
Recent investigations have highlighted the correlation between variations in Cystatin C level and various disorders, such as AD and retinal inflammation, the latter being a prominent characteristic of AMD42, 43, 44. Ten years ago, several studies provided evidence that hypertension is associated with an increased risk of developing AD and AMD45, 46, 47. Epidemiological studies show a positive association between long-term exposure to nitrogen dioxide (NO2) air pollution and the risk of cognitive decline in older adults48, 49. Similar trends were also observed in AMD50, 51.
In addition, some protective factors have also been validated in the literatures. A recent prospective cohort study of 313,448 participants demonstrated a U-shaped association of hematocrit percentage and hemoglobin (HGB) concentration with dementia risk and concluded that HGB was causally associated with AD52. Plenty of epidemiological, genetic, and biochemical evidence suggests that cholesterol is a risk factor for AD 53, 54 and the receptor-binding domains of apolipoproteins comprise low-density lipoprotein (LDL) receptors that aid in the transport of amyloid peptides across the blood-brain barrier (BBB), thereby clearing them. Enhancing the expression of this receptor could be a promising therapeutic strategy for AD55, 56.
Our study identified 18 shared environmental factors associated with both AD and AMD, exhibiting a consistent trend across the comorbidity landscape. The majority of these factors have been previously validated. This finding may illustrate that there are complex shared environmental risk factors contributing to the pathogenesis of AD and AMD.
EWAS enable capturing the environome across levels, dimensions, and time in unprecedented depth and detail. This EWAS analysis helped us visualize the shared environmental factors between AMD and AD. However, our study has a few limitations that should be considered. Although assessment technologies have rapidly improved in recent years, capturing even one individual’s environome in its totality remains impossible to date.