SLC6A4 and post-traumatic stress disorder: Another well-studied PTSD candidate is Catechol-O-methyltransferase (COMT), which encodes enzymes that degrade neurotransmitters such as dopamine and the serotonin transporter SERT. Increased COMT promoter methylation was associated with impaired fear inhibition in individuals with PTSD carrying the COMT met/met genotype (Meyer-Lindenberg and Weinberger, 2006).