A genetic culprit of Alzheimer’s disease may also contribute to faltering mental health in your golden years, according to research published Tuesday in Biological Psychiatry.

The study authors delved into data taken from a long-running Swedish population study of older adults without depression and dementia, the broad term for any number of brain disorders that gradually reduces the sufferer’s mental functioning and typically strikes the elderly. Alzheimer’s disease represents the single largest cause of dementia, accounting for 60 to 80 percent of cases.

They looked at a sample of 839 people (ages 70 to 92), and tracked down their health status in 2001, 2005, and 2009. In addition to extensive tests of their physical and mental health, the subjects were also tested for the presence of the ε4 version of the apolipoprotein gene (APOE ε4). Located in the 19th chromosome, having one or two copies of the APOE ε4 gene is known to increase the risk of late-onset Alzheimer's as well as speed along its progression (other variants of APOE decrease that risk or are simply neutral).

The study authors then attempted to determine the relationship, if any, between having the APOE ε4 gene and later depression. They also chose to exclude subjects who were diagnosed with dementia in-between 2001 and 2009, since the degenerative condition is known to be a risk factor for depression.

"In our study, the presence of the APOE ε4 predicted future depression, even after excluding individuals who later developed dementia," said study author Dr. Silke Kern of the University of Gothenburg in a statement. "It was also related to dementia. APOE ε4 might be a marker for identifying older persons at risk to develop depression or dementia, which could be important for prevention and early detection of these common disorders." More specifically, older adults without dementia but with the APOE ε4 gene were 75 percent more likely to report any form of depression (major or minor) than those without the gene, and twice as likely to report minor depression.

Meanwhile, APOE ε4 is believed to be carried by around 20 percent of the general population, though this figure varies among different ethnic populations, and it is carried by 40 to 60 percent of Alzheimer’s sufferers. APOE ε4 is also known to be strongly tied to atherosclerosis, cardiovascular disease and even the more rapid progression of multiple sclerosis.

Though the study does not definitively prove a cause-and-effect relationship between APOE ε4 and depression, it does potentially point the way to new avenues of research.

"Late-life depression is an under-appreciated source of distress and disability in older people," commented Dr. John Krystal, Editor of Biological Psychiatry. "The current study suggests a new link to the biology of Alzheimer's disease, even among people who do not show signs of memory impairment."

Source: Skoog I, Waern M, Duberstein P, et al. A 9-Year Prospective Population-Based Study on the Association Between the APOE*E4 Allele and Late-Life Depression in Sweden. Biological Psychiatry. 2015.