Researchers at UCLA have identified mutations within three genes that they say may make some people more likely to develop symptoms of post-traumatic stress disorder (PTSD) and depression.

The researchers analyzed 200 adults from 12 multigenerational families who were exposed to the 1988 earthquake in Spitak, Armenia, most of whom saw dead bodies lying in the streets and people who were severely injured.

The participants underwent psychological screening and a genetic test 14 years after experiencing the earthquake. The researchers found that people with mutations in any of three genes responsible for secreting the happiness hormone serotonin had PTSD and depression symptoms.

Previous studies have suggested that PTSD is heritable among siblings who experience traumatic situations such as war. But this study suggests that the disorder is also heritable through multiple generations, according to Julia Bailey, assistant professor in the department of epidemiology at UCLA, and co-author of the study.

"We found that both PTSD and depression are heritable and that they share genes," said Bailey, who added that the findings are consistent with previous research suggesting a genetic connection between PTSD and depression.

Unlike previous studies, the participants in this study were not previously diagnosed with PTSD or depression, nor were they seeking any sort of treatment for their symptoms.

However, all of the participants were of the same ethnic background, so the findings may not apply to all people, the researchers wrote in their study published today in the Journal of Affective Disorders.

PTSD is currently diagnosed through physician observations and a series of behavioral based questions. But some scientists are hoping to understand why some who experience the same traumatic events are more resilient than others.

"For some we know, it's genetic makeup and sometimes it's how they were raised," said Armen Goenjian, research professor of department of psychiatry at UCLA, and lead author of the study.

Goenjian said his study is just one more step in finding better ways to diagnose and treat PTSD.

"Future studies I think will help us better define what we call PTSD based on biology rather than observations," said Goenjian.