5‐HTTLPR and DISC1 risk genotypes for elevated PTSD symptoms in US military veterans
Authors: Keith A. Young, Sandra B. Morissette, Robert Jamroz, Eric C. Meyer, Matthew S. Stanford, Li Wan, Nathan A. Kimbrel
Journal: World Psychiatry (2017)

The serotonin transporter linked polymorphic region (5-HTTLPR) genotype is a candidate vulnerability factor for post-traumatic stress disorder (PTSD). Recently, Telch et al1 reported in this journal that the 5HTTLPR-S’ variant was associated with elevated levels of PTSD symptoms in military personnel deployed to Iraq, supporting similar findings in deployed military personnel by our group2. Variants in several other genes have been associated with greater severity of PTSD symptoms in civilian populations. In a further study, we screened a panel of nine candidate genetic variants with potential to influence the development of PTSD in veterans deployed in support of the wars in Afghanistan and Iraq. Study subjects were male US military veterans registered to receive health care at the Central Texas Veterans Health Care System. The local institutional review board approved and monitored the study. Subjects who did not have a chart diagnosis of schizophrenia or bipolar disorder and who provided informed consent were enrolled in the study independent of their mental health status. Recruitment took place in a variety of venues, including the main lobby of the hospital and mental health waiting rooms. Participants (N=102) completed the Impact of Events Scale Revised (IES) and donated saliva for genetic research. The IES has been increasingly used as a self-report instrument to assess current PTSD symptom severity. It yields a total symptom severity score and three psychologically and biologically relevant sub-factor scores: avoidance, intrusion and hyperarousal. Saliva samples were collected with Oragene DNA collection kits (DNA Genotek, Ottawa, Canada), and genomic DNA was extracted and purified. 5-HTTLPR genotyping was accomplished with a triplex/double digestion PCR protocol2. Alleles were re-classified using rs25531 into the triallelic classification. A Taqman SNP panel for COMT (rs4680), FKBP5 (rs1360780), DTNBP1 (rs9370822), PACAP receptor ADCYAP1R1 (rs2267735), 5-HT1A (rs6295), DISC1 S704C (rs821616), BDNF val66met (rs6265) and NTRK1 (rs6336) was performed using ABI TaqMan SNP Genotyping Assays (Applied Biosystems, Inc., Foster City, CA) on ABI PRISM 7900 HT Sequence Detection System (Applied Biosystems). Distributions did not vary from Hardy-Weinberg equilibrium (all p values >0.05). Stepwise regression modeling identified two genetic variants that significantly contributed to variance in PTSD symptoms: 5-HTTLPR and DISC1 S704C. Risk genotypes selected by the regression (model cumulative R2=0.30) were 5-HTTLPR-S’ carriers (vs. 5-HTTLPR-L'L’; p<0.031) and DISC1 TT (vs. A carriers; p<0.034). The next closest gene to significance in the model was NTRK1 (p<0.11). When the stepwise procedure was repeated after removing 5-HTTLPR and DISC1, no other variant significantly contributed to PTSD symptom load. The two genetic variants continued to be significant factors in the ANCOVA analysis, which included age and education as covariates. The model also remained significant in a follow-up analysis in which participants who identified as African-American (N=15) were excluded. Carriers of the 5HTTLPR-S’ allele had increased PTSD symptoms compared to individuals homozygous for the L’ allele (IES mean score: L'L’=47.3 ± 5.3, S=59.8 ± 4.1). For DISC1, individuals homozygous for the T allele had increased PTSD symptoms compared to A carriers (A=45.3 ± 2.8, TT=61.9 ± 7.2). In ANCOVA analysis of symptom sub-factors, 5-HTTLPR and DISC1 selectively influenced intrusion and hypervigilance symptoms, but did not affect avoidance symptoms. PTSD symptom severity (total IES scores) increased by an average of 40% with each risk genotype (none=38.4, one= 54.5, two=65.6). These data support prior observations of 5-HTTLPR effects on PTSD symptoms in military veterans1, 2. Although 5-HTTLPR has been identified as a potential contributor to PTSD susceptibility in civilian-based populations, its effect may be less robust in those populations, due to lower overall level of trauma exposure3. The effects of 5-HTTLPR on PTSD in military veterans after deployment to a war zone may be more robust because of a universal and constant exposure to threat, military training, and/or separation from family and home social support. In addition to 5-HTTLPR, genetic variation in DISC1, a gene associated with susceptibility to multiple mental disorders, was found to contribute to PTSD symptom severity. Possessing both DISC1 and 5-HTTLPR risk genotypes resulted in a 1.7-fold increase in PTSD symptoms. Although this is the first report of DISC1 S704C TT allele as a risk factor for PTSD, the finding is not surprising, considering that this allele has been identified as a risk factor for major depression4. DISC1 variants interfere with a protein complex important for organelle transport and in tethering of mitochondria, interfering with dendritic development and reducing densities of dendritic spines in the frontal cortex, paralleling our recent report of spine density reductions in the frontal cortex in PTSD5-7. This study was powered to screen for candidate genes with relatively large effect sizes on PTSD symptoms in combat veterans, which may be different from sets of genes affecting PTSD in civilian populations. Study of the serotonin system in PTSD is motivated in large part by the therapeutic utility of serotonin uptake inhibitors to treat symptoms of PTSD. Our data provide additional impetus for continued study of this system in PTSD pharmacotherapy. In addition, antipsychotics such as risperidone have been shown to reverse DISC1-related behavioral deficits and pathophysiology in animal models, suggesting the possibility that such agents could be re-examined for use as alternative pharmacotherapies for PTSD8, 9. Keith A. Young1-3, Sandra B. Morissette4, Robert Jamroz1, Eric C. Meyer1-3, Matthew S. Stanford5, Li Wan3, Nathan A. Kimbrel6-8 1Central Texas Veterans Health Care System, Temple, TX, USA; 2Department of Veterans Affairs VISN 17 Center of Excellence for Research on Returning War Veterans, Waco, TX, USA; 3Department of Psychiatry and Behavioral Science, Texas A&M Health Science Center, Temple, TX, USA; 4University of Texas at San Antonio, San Antonio, TX, USA; 5Hope and Healing Center & Institute, Houston, TX, USA; 6Durham Veterans Affairs Medical Center, Durham, NC, USA; 7VA Mid-Atlantic Mental Illness Research, Education, and Clinical Center, Durham, NC, USA; 8Duke University Medical Center, Durham, NC, USA This work was supported with resources and use of facilities at the Central Texas Veterans Health Care System, Temple, TX. The views expressed here are those of the authors and do not necessarily reflect the position or policy of the Department of Veterans Affairs (VA) or the US government. N.A. Kimbrel was supported by a Career Development Award (IK2 CX000525) from the Clinical Science Research and Development Service of the VA Office of Research and Development. S.B. Morissette was supported by a Merit Award (RX000304-04A1) from the VA Rehabilitation, Research and Development section.