CHICAGO - Cutting-edge new research helps answer the puzzling question of why post-traumatic stress doesn't happen to everyone who endures horrible trauma.
In this case, the trauma was child abuse. The researchers found that survivors of child abuse were particularly likely to have symptoms of post-traumatic stress as adults if they also had specific variations in a stress-related gene.
Among adult survivors of severe child abuse, those with the specific gene variations scored more than twice as high (31) on a scale of post-traumatic stress, compared with those without the variations (13).
The worse the abuse, the stronger the risk in people with those gene variations.
The study of 900 adults is among the first to show that genes can be influenced by outside, nongenetic factors to trigger signs of post-traumatic stress disorder, or PTSD.
'The study is groundbreaking,' the largest of just two reports to show molecular evidence of a gene-environment influence on PTSD, said Karestan Koenen, a Harvard psychologist doing similar research. She wasn't involved in the new study.
'We have known for over a decade, from twin studies, that genetic factors play a role in vulnerability to developing PTSD, but have had little success in identifying specific genetic variants that increase risk of the disorder,' Koenen said.
The results suggest that there are critical periods in childhood when the brain is vulnerable 'to outside influences that can shape the developing stress-response system,' said Emory University researcher and study co-author Dr. Kerry Ressler.
The study appears in Wednesday's Journal of the American Medical Association.
Though preliminary, the study provides needed insight into a condition expected to hit rising numbers of veterans returning from Afghanistan and Iraq, said Dr. Thomas Insel, director of the National Institute of Mental Health. The agency paid for the study.
Insel said the results help explain why two people in the same jeep see a roadside bomb, and one simply experiences it as 'a bad day but goes back and is able to function,' while the other later develops paralyzing stress symptoms.
'This could be quite a wave that will hit us over the months and years ahead,' Insel said.
Study participants were mostly low-income black adults, aged 40 on average, who sought non-psychiatric health care at a public hospital in Atlanta. They were asked about experiences in childhood and as adults and gave saliva samples that underwent genetic testing.
The researchers focused on symptoms of PTSD rather than an actual diagnosis, and found that about 25 percent had stress symptoms severe enough to meet criteria for the disorder, Ressler said.
Childhood abuse and adult trauma each increased risks for PTSD symptoms in adulthood. But the most severe symptoms occurred in the 30 percent of child abuse survivors who had variations in the stress gene.
The researchers were not able to determine if the symptoms were reactions to the child abuse or to the more recent trauma - or both, said co-author Rebekah Bradley, also of Emory University.