Propensity to Develop PTSD May Reflect Genetic Factors

Why, in the face of trauma, do some people develop posttraumatic stress disorder while others do not?

Certainly an individual’s gender and mental health status play a role—that is, females and persons with preexisting anxiety or depression seem to be especially vulnerable. But genes also appear to play a substantial role, a study of Vietnam War veterans divulged a few years ago, and a newly published study bolsters the case.

The study was conducted by Murray Stein, M.D., a professor of psychiatry at the University of California at San Diego, in conjunction with colleagues in San Diego and at the University of British Columbia. The results were reported in the October American Journal of Psychiatry.

Stein and his colleagues recruited 406 pairs of twins, both identical and fraternal, to take part in their investigation so that they could tease out genetic and environmental influences on trauma exposure and on posttraumatic stress disorder symptoms. The subjects consisted of 222 identical twin pairs and 184 fraternal twin pairs. All subjects were asked to fill out questionnaires at home.

Subjects had to indicate whether they had ever been exposed to various types of traumas, such as being robbed; being held captive; being sexually assaulted; being in a car accident, a tornado, or a flood; experiencing a sudden family death; having their life threatened in some other way; and so forth, and how often they had experienced such traumas.

The 612 subjects who recorded exposure to one or more traumas, that is, 75 percent of all subjects, also had to answer questions indicating how they had reacted to the trauma that had disturbed them most. These questions were adapted from the Posttraumatic Diagnosis Scale, a clinically validated measure of posttraumatic stress disorder symptoms.

First the researchers attempted to determine whether there was greater similarity in the number of nonassaultive traumas experienced by identical twins compared with fraternal twins. Greater concordance among the identical twins would have implied a genetic influence. However, they did not find greater similarity among the identical twins. So the risk of being exposed to nonassaultive traumas like car accidents or natural disasters is essentially due to environmental factors, not genes, they concluded.

Second, the investigators attempted to determine whether there was greater concordance in the number of assaultive traumas experienced by identical twins than in the number experienced by fraternal twins. Once again, greater similarity among the identical twins would have suggested a genetic influence. And indeed, they did find greater concordance among the identical twins than among the fraternal ones. So the risk of being exposed to assaultive traumas like robbery or rape is inherited to some degree, they concluded. And while this finding on the surface appears counter-intuitive, the researchers conceded in their study report, they believe that a genetic risk for assaultive trauma might consist of the following: A person inherits certain personality traits—say, a propensity to anger and irritability—and the anger and irritability might then make it more likely that he gets into fights and increases his risk for assaultive trauma.

Third, they ascertained whether there was greater similarity in the number of posttraumatic stress disorder symptoms experienced by identical twins who had had trauma exposure than in the number of such symptoms experienced by fraternal twins. Once again, greater concordance among the identical twins would have signaled an inherited component. And indeed, the researchers did find greater similarity among the identical twins than the fraternal twins. Thus, susceptibility to posttraumatic stress disorder symptoms is moderately inherited, they concluded.

Finally came what was probably the most interesting aspect of the study. The researchers attempted to determine whether genetic susceptibility to trauma exposure and genetic susceptibility to posttraumatic stress disorder symptoms overlapped in their subjects.

There was substantial overlapping, they found. Thus, “genetic influences on posttraumatic stress disorder are probably mediated through a causal pathway that includes genes that simultaneously influence personality (i.e., exposure proneness) and posttraumatic stress disorder symptoms following exposure,” they pointed out in their study report.

But how might such a dual genetic liability for posttraumatic stress disorder work? One possibility, Stein and his team speculated in their report, is that personality traits that prompt people to put themselves in danger of assaultive trauma—say, traits that lead to anger, irritability, and getting into fights—may also cause them to become emotionally aroused as a result of experiencing such trauma and thereby increase the risk for posttraumatic stress disorder.

The study was funded by grants from the Medical Research Council of Canada and the National Institute of Mental Health.

The study, “Genetic and Environmental Influences on Trauma Exposure and Posttraumatic Stress Disorder Symptoms: A Twin Study,” is posted on the Web at http://ajp.psychiatryonline.org/cgi/content/full/159/10/1675. ▪

Am J Psychiatry 2002 159 1675