Researchers look at two genes involved in our ability to remember things as we age. The notorious genetic marker of Alzheimer’s disease and other forms of dementia, ApoE4, may not be a lone wolf.

Researchers from USC and The University of Manchester have found that another gene, TOMM40, complicates the picture. Although ApoE4 plays a greater role in some types of aging–related memory ability, the researchers believe that TOMM40 may pose an even greater risk for other types.

TOMM40 and APOE genes are neighbors, adjacent to each other on chromosome 19, and they are sometimes used as proxies for one another in genetic studies.

USC researchers believe their new findings raise a significant research question: Has TOMM40 been misunderstood as a sidekick to ApoE4 when it is really a mastermind, particularly when ApoE3 is present?

“Typically, ApoE4 has been considered the strongest known genetic risk factor for cognitive decline, memory decline, Alzheimer’s disease or dementia–related onset,” said T. Em Arpawong, the study’s lead author and a postdoctoral fellow in the USC Dornsife College of Letters, Arts and Sciences’ Department of Psychology.

“Although prior studies have found some variants of this other gene TOMM40 may heighten the risk for Alzheimer’s disease, our study found that a TOMM40 variant was actually more influential than ApoE4 on the decline in immediate memory — the ability to hold onto new information,” Arpawong explained.

Studies have shown that the influence of genes associated with memory and cognitive decline intensifies with age. That is why the scientists chose to examine immediate and delayed verbal test results over time in conjunction with genetic markers.

“An example of immediate recall is someone tells you a series of directions to get somewhere and you’re able to repeat them back,” said Carol A. Prescott, the paper’s senior author and professor of psychology at USC Dornsife and professor of gerontology at the USC Davis School of Gerontology. “Delayed recall is being able to remember those directions a few minutes later, as you’re on your way.”

The study was published in the journal PLOS ONE on Aug. 11.

The team of researchers used data from two surveys: the U.S. Health and Retirement Study and the English Longitudinal Study of Ageing. Both data sets are nationally representative samples and include results of verbal memory testing and genetic testing.

The research team used verbal test results from the U.S. Health and Retirement Survey, collected from 1996 to 2012, which interviewed participants via phone every two years. The researchers utilized the verbal memory test scores of 20,650 participants, aged 50 and older who were tested repeatedly to study how their memory changed over time.

To test immediate recall, an interviewer read a list of 10 nouns and then asked the participant to repeat the words back immediately. For delayed recall, the interviewer waited five minutes and then asked the participant to recall the list. Test scores ranged from 0 to 10.

The average score for immediate recall was 5.7 words out of 10, and the delayed recall scoring average was 4.5 words out of 10. A large gap between the two sets of scores can signal the development of Alzheimer’s or some other form of dementia.

“There is usually a drop–off in scores between the immediate and the delayed recall tests,” Prescott said. “In evaluating memory decline, it is important to look at both types of memory and the difference between them. You would be more worried about a person who has scores of 10 and 5 than a person with scores of 6 and 4.”

The first person is worrisome because five minutes after reciting the 10 words perfectly, he or she can recall only half of them, Prescott said. The other person wasn’t perfect on the immediate recall test, but five minutes later, was able to remember