Posttraumatic stress disorder: From pathophysiology to pharmacology

Serotonin transporter (5-HTT) gene. Serotonin transporter is a monoamine transporter protein that terminates the neurotransmitter signal by transporting serotonin from the synaptic cleft back into the presynaptic neuron. It is encoded by the SLC6A4 gene, which resides on the long arm of chromosome 17(17q11.1-q12). It is a large gene with 31 kilo bases and 14 separate exons (transcribed regions).^33,34

This gene has several variants. The best-studied is a variation in the promoter region. A 44-bp insertion or deletion yields the “long” and “short” alleles, respectively. The proteins produced by the 2 alleles are identical, but the amount of expressed protein is different. The short allele (“S”) is associated with a nearly 50% reduction in 5-HTT expression in both homozygotes and heterozygotes.³⁵ A greater incidence of serotonin transporter promoter region (5-HTTLPR) S has been found in individuals with PTSD compared with those without PTSD,^36-38 and 5-HTTLPR S increases the risk of PTSD in individuals with low social support³⁹ or after very few traumatic events.⁴⁰ The short allele variant is also associated with depression in individuals who face adversity.^35,41

The overrepresentation of the short form of 5-HTTLPR in individuals who develop PTSD may represent a potential problem with current treatment paradigms, in which an antidepressant is the first-line treatment, because this allele is associated with reduced response to antidepressants.^42,43 More distressing is the possible association of this allele with increased suicide risk, particularly violent suicide⁴⁴ or repeated suicide attempts.⁴⁵

Furthermore, a functional MRI study of patients who were anxious revealed that in individuals with the short allele, administration of citalopram was associated with increased amygdala activity in response to negative stimuli, and reduced activity in response to positive stimuli.⁴⁶ This suggests that antidepressant treatment may actually worsen fear response in patients with PTSD. Clearly, additional research is needed to determine if having the SLC6A4 gene alters clinical outcomes in response to an antidepressant in a patient with PTSD. In the meantime, clinicians should use vigilance and a critical mindset when they administer antidepressants to a patient who has PTSD.

Catechol-o-methyltransferase (COMT) is one of the enzymes that degrades catecholamines such as dopamine, epinephrine, and norepinephrine (NE).⁴⁷ In humans, COMT protein is encoded by the COMT gene. This gene is associated with allelic variants; the best-studied of these is Val158Met. COMT Val158Met polymorphism (rs4860) has been linked to deficits in stress response and emotional resilience.^48,49 Val158Met is associated with a 40% reduction in enzyme activity and slower catalysis of catecholamines, resulting in increases in catecholamines levels in the brain, which may increase the risk of developing PTSD.⁵⁰ Individuals homozygous for this SNP (Met/Met) are highly susceptible to develop PTSD independently of the severity of the trauma they experienced.⁵¹ The Val158Met polymorphism may be associated with other abnormalities, such as cognitive problems with specific frontal cortical activity, and also with improved antidepressant response (valine homozygotes less responsive than methionine homozygotes).⁵² This gene is available on gene testing profiles.

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