A superior visual memory is the best friend of artists and competitive card memorizers. But to people who've lived through traumatic events, it might be the enemy.
Researchers in Switzerland and Germany guessed that people with a better memory might be more susceptible to post-traumatic stress disorder, their minds clinging stubbornly to horrific events in the past. But studying the memories of people living with a mental illness is difficult, since the disorder itself might affect their memory. So when the researchers went on a hunt for genes that are linked to both memory and PTSD, they began in a healthy population.
A group of more than 700 Swiss young adults, free of any mental illness, participated in the first part of the study. They viewed several dozen pictures that were meant to elicit either a positive emotional response, a negative emotional response, or a neutral one. After being distracted for 10 minutes, they were given a surprise quiz on how many of the pictures they could recall.
The subjects's DNA underwent testing too. The researchers checked 2,005 individual spots in each person's genes called SNPs (pronounced "snips"). These are bits of DNA that vary across a population, such that some people might have a T nucleotide where others have a G, for example. All of the 2,005 SNPs the researchers checked had to do with certain multitasking molecules called protein kinases that seem to be involved in memory formation.
Out of the 2,005 gene variants in this haystack, one needle emerged: a bit of DNA that was significantly linked to subjects' performance on the memory test. There are two versions (or alleles) of the gene in question, which makes a molecule called PKC alpha. People with one of these alleles--an A rather than a G--remembered more of the pictures they'd seen. Although researchers were especially interested in their subjects' recall of emotionally negative pictures, the effect seemed to extend to positive and neutral ones as well.
Brain scans showed a difference inside the heads of these high-performing memorizers. Subjects with A alleles had more activity in parts of the prefrontal cortex while looking at the negative images. These same regions, the authors say, have been linked to emotional memory storage in other studies.
Now that the researchers had found a gene of interest, they could study it in some actual traumatized people. They turned to a group of 347 Rwandan refugees who fled their country during the civil war. After being interviewed thoroughly, 134 of the refugees were found to meet criteria for post-traumatic stress disorder. Rwandans who had the better-memory gene variant from the first part of the study were more likely to be in the PTSD group. They were also more likely to have the symptom of reliving a traumatic memory over and and over.
Among the healthy Swiss population, the better-memory A allele was more common than the worse-memory G allele. But among the Rwandan refugees, the opposite was true: The better-memory gene variant was the rare one. If it were more common, PTSD symptoms might have been even more frequent among the displaced Rwandans.
The genetics of mental illness are tricky to untangle, and what merits a diagnosis in one culture might be normal in another. Studies such as this one, though, could reveal who's most at risk for certain symptoms. And if scientists can figure out how exactly the genes in question are acting in the brain, we might see new drugs that can treat some of these symptoms--or prevent people's memories from turning against them in the first place.
de Quervain, D., Kolassa, I., Ackermann, S., Aerni, A., Boesiger, P., Demougin, P., Elbert, T., Ertl, V., Gschwind, L., Hadziselimovic, N., Hanser, E., Heck, A., Hieber, P., Huynh, K., Klarhofer, M., Luechinger, R., Rasch, B., Scheffler, K., Spalek, K., Stippich, C., Vogler, C., Vukojevic, V., Stetak, A., & Papassotiropoulos, A. (2012). PKC is genetically linked to memory capacity in healthy subjects and to risk for posttraumatic stress disorder in genocide survivors Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1200857109