The results of the present study replicated the association between the Met/Met genotype of COMT and an increased risk of OCD. This finding is in line with the studies of Niehaus et al. (2001), Karaiyorgou et al. (1997), Denys et al. (2006), Pooley et al. (2007), and Katerberg et al(4–6, 31, 32); however, in some studies, the association has been reported only in men(6, 32). The findings are inconsistent with the association studies of Ohara et al. (1998)(33), Meira-Lima et al. (2004)(11), Tukel et al. (2013)(34) and Sampaio et al. (2015)(35).
To illustrate the heterogeneous results of the studies, the effect of ethnicity could be considered. Studies have suggested that the effect of this polymorphism is stronger in some ethnicities than in others. The Met/Met genotype correlates with lower COMT gene activity, which results in a reduced amount of COMT enzyme and subsequently causes lower dopamine degradation, resulting in a hyperdopaminergic state that may be involved in the pathogenesis of OCD (36, 37). Consistent with the present results, Denys et al. (2006) suggested that patients with a low COMT activity genotype may have longer and more effective dopamine release that might be more vulnerable to the development of OC symptoms(5).
For the memory dimensions, audio and visual memory retrieval were more defective in patients with OCD than in the control group. Moreover, both the Immediate and General Memory indexes showed weaker performance in the OCD group, although significantly weaker Working Memory was observed only among females. The results of this study supported the hypotheses about poorer performance of patients with OCD in episodic memory than of healthy controls. Although the research literature on memory performance has mainly focused on nonverbal memory impairments (9, 10), our data present impairments in both verbal and nonverbal memory functions. According to the neuropsychological findings, if we assume that patients with OCD suffer from disturbances in organizational and semantic strategies used during encoding and learning, they are expected to exhibit similar problems in both verbal and nonverbal areas (19), an assumption that was confirmed by our findings. Patients with OCD tend to focus on the unrelated details of the objects; hence, the observed memory impairments in these patients can be explained by Savage et al. (1999), who suggested that when attention to details is overwhelming, it can prevent proper understanding of contextual information and regarding it as a whole, thereby resulting in retrieval deficits (38).
Previous studies on working memory revealed a discordant pattern. Martoni et al. suggested that working memory becomes more defective as task becomes more complex and difficult because of the increased need to keep unrelated information in mind and the use of memory management strategies. (35). Therefore, since the Working Memory task in this study was letter-number sequencing and spatial span, which are categorized as complex tasks, the observed result in females was expected; however, the dissimilar functions in males and females may suggest different mechanisms underlying episodic and working memory in both sexes. Another interesting hypothesis for weaker working memory in patients with OCD is that part of the patients' working memory is kept occupied by their obsessions and ruminations, which reduces the processing capacity available for the other tasks.
Regarding sex-related genotype analysis, the results showed an association of COMT genotypes with Working Memory only in females. The results of the descriptive analysis for Working Memory performance revealed that females with the Val/Val genotype had the weakest, those with the Met/Met genotype had moderate, and those with the Val/Met genotype had the best performance. These results are consistent with the inverted U shape theory for dopamine in OCD that states that COMT homozygotes exhibit the worst and the best and heterozygotes exhibit moderate performance in cognitive functions.
As mentioned earlier, there are numerous reports that describe the sexual dimorphism of COMT activity(26). This dimorphism has also been reported for cognitive functions(20, 39). To illustrate the sex-related effect of COMT, the role of estrogen is usually discussed. Xie et al. (1999) showed that there are two elements of the estrogen response in the COMT gene promoter and that estradiol at high physiological concentrations inhibits mRNA expression of COMT, leading to a relative decrease in activity and the expression of COMT(40). This inhibition by estrogens is consistent with the sex differences observed in COMT activity. Hippocampal and hepatic COMT enzyme levels were found to be lower in females than in males(9). Thus, women who inherit one or two alleles for the low-activity COMT isoform may have even lower levels of hippocampal enzyme activity than men with the same alleles. Given the important role suggested for the hippocampus in memory functions, this decrease in activity by affecting dopamine regulation might explain the different effects of COMT on the memory functions of men and women.
Therefore, it may be concluded that the rs4680 polymorphism is a susceptibility factor for OCD and female memory functioning. Met homozygosity could be a risk factor for OCD and predict better working memory functioning, while Val homozygosity is associated with worse episodic memory and working memory functioning in female patients with OCD.