Against a backdrop of disappointing Alzheimer's disease clinical trial outcomes, two researchers are proposing a new approach for future study of the disease, according to a USC statement released Tuesday.
Caleb Finch of the USC Leonard Davis School of Gerontology and Alexander Kulminski of the Social Science Research Institute at Duke University have outlined a framework called the Alzheimer's Disease Exposome to address major gaps in understanding how environmental factors interact with genetic factors to increase or reduce risk for the disease.
The theoretical article appears Tuesday in Alzheimer's & Dementia: The Journal of the Alzheimer's Association.
"We propose a new approach to comprehensively assess the multiple brain-body interactions that contribute to Alzheimer's disease," Finch said. "The importance of environmental factors in gene-environment interactions is suggested by wide individual differences in cognitive loss, particularly among people who carry genes that increase the risk of Alzheimer's disease."
Increasing age is the most important known risk factor for Alzheimer's disease, but many other risk factors -- including environmental exposures -- are poorly understood. Previous studies of Swedish twins by USC's Margaret Gatz suggested that half of individual differences in Alzheimer's disease risk may be environmental.
The two classes of Alzheimer's disease genes are considered in the exposome model. One class called familial Alzheimer's genes are dominant, meaning someone who inherits those genes will ultimately develop Alzheimer's.
The other class include gene variants like apolipoprotein E4 (APOE4), where risk increases along with more copies of the genes. Among carriers of APOE4, a few people reach age 100 and older without ever developing the disease, demonstrating environmental risk is contributing to that variability.
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Cited in their article is a study that showed the onset of dementia was a decade earlier for carriers of a dominant Alzheimer's gene living in cities and who came from lower socioeconomic backgrounds or less education.
"Environmental factors, including exposure to air pollution and low socioeconomic status, shifted the onset curve by ten years," said Finch. "It's in the research literature but until now, no one has paid sufficient attention to it."
More examples come from Finch's recent research on the gene-environment interactions between parasitic infections and the Alzheimer's risk gene APOE4.
Among a pre-industrial tribe of Bolivian people called the Tsimane, the presence of the gene, along with chronic parasitic infections, seemed to lead to better, not worse, cognition. Finch says the startling results provide an example of how the exposome concept could lead to enhanced understanding of risks for dementia.
"Those are just some of the environmental interactions we have brought up in building a larger framework called the exposome, which lists the different types of environmental factors that are known to interact with Alzheimer's, but haven't been studied broadly for how they impact the Alzheimer's-linked genes," Finch said. "This is the story of the gene-environment interaction."
The exposome concept was first proposed by cancer epidemiologist Christopher Paul Wild in 2005 to draw attention to the need for more data on lifetime exposure to environmental carcinogens.
The exposome is now a mainstream model, eclipsing previous characterizations of environmental factors as affecting risk "one by one.
"Epidemiologists have been using the exposome to have a broader outlook on whatever they're studying, whether it's lead toxicity or head trauma," Finch said. "It's our point that a number of things need to be considered for interaction and many effects are additive."