A US team looked at the DNA from 200 members of 12 families who survived the 1988 Armenian earthquake.
It found those who carried two gene variants which affect the production of serotonin – which affects mood and behaviour – were more likely to display symptoms of PTSD.
The research is published in Journal of Affective Disorders.
PTSD can arise after any kind of experience that causes trauma, whether that be in war, after a natural disaster or because of child abuse or sexual assault.
Symptoms can include flashbacks, feeling emotionally numb or hyper-alert to danger, and avoiding situations that act as reminders of the original trauma.
It is estimated that up to 3% of the general population is likely to be affected by PTSD at some point.
The Armenian earthquake, which had a magnitude of 7.1, occurred on 7 December 1988. It hit the northern part of Armenia, which was then part of the Soviet Union. It killed at least 25,000 people.
All those who took part in the study had experienced the earthquake, 90% saw dead bodies and 92% saw people who had been seriously injured.
They also undertook a recognised assessment to evaluate what, if any, PTSD symptoms they had experienced.
Continue reading the main story
A diagnostic tool based upon TPH1 and TPH2 could enable military leaders to identify soldiers who are at higher risk of developing PTSD”
Dr Armen Goenjian,
When the University of California, Los Angeles (UCLA) researchers analysed the Armenian families’ DNA, they saw that those who had experienced more PTSD symptoms were more likely to have two specific gene variants – TPH1 and TPH2.
Dr Armen Goenjian, a research professor of psychiatry who led the work, said: "We suspect that the gene variants produce less serotonin, predisposing these family members to PTSD after exposure to violence or disaster.
"Our next step will be to try and replicate the findings in a larger, more heterogeneous population."
But he said that, if larger studies did confirm the finding, they could eventually lead to new ways to screen people at risk of PTSD, and target specific medicines for preventing and treating the disorder.
Dr Goenjian added: "A diagnostic tool based upon TPH1 and TPH2 could enable military leaders to identify soldiers who are at higher risk of developing PTSD, and reassign their combat duties accordingly.
"Our findings may also help scientists uncover alternative treatments for the disorder, such as gene therapy or new drugs that regulate the chemicals responsible for PTSD symptoms."
Psychologist Dr Jennifer Wild, who is an expert in PTSD, said the finding was significant, but it would be "premature" to give it too much emphasis.
She added: "The link between the gene variant and PTSD is strong, which is promising in terms of guiding future research and possibly leading to a genetic screening test.
"But the gene variant discovered in the study only accounted for a small proportion of PTSD symptoms in sufferers. Psychological factors, such as past history of trauma or a trait called neuroticism, tend to predict PTSD with much greater precision and they are less costly to assess."
And she said that the fact people were asked to think back to their experiences – rather than studied while they were experiencing them – was also problematic.
The commonly used antidepressants SSRIs (selective serotonin re-uptake inhibitors) improve mood by prolonging the effect of serotonin in the brain by slowing its absorption by brain cells.
But Dr Wild said the recommended treatment for PTSD was psychological therapy, and that SSRIs were not recommended in the UK for PTSD treatment.