Effect of COMT Val<sup>108/158</sup> Met genotype on frontal lobe function and risk for schizophrenia

M Egan

T Goldberg

B Kolachana

J Callicott

^{Clinical Brain Disorders Branch, Building 10, Center Drive, National Institute of Mental Health, Bethesda, MD 20892;}^{Laboratory of Neurogenetics, 12501 Washington Avenue, Park 5, 451 National Institute of Alcohol Abuse and Alcoholism, Rockville, MD 20852; and}^{Department of Psychiatry, Virginia Commonwealth University, Richmond, VA 23298}

^{To whom reprint requests should be addressed. E-mail:}vog.hin.hmin.artni@mnage.

Communicated by P. S. Goldman-Rakic, Yale University School of Medicine, New Haven, CT

Received 2000 Aug 7; Accepted 2001 Mar 20.

Abstract

Abnormalities of prefrontal cortical function are prominent features of schizophrenia and have been associated with genetic risk, suggesting that susceptibility genes for schizophrenia may impact on the molecular mechanisms of prefrontal function. A potential susceptibility mechanism involves regulation of prefrontal dopamine, which modulates the response of prefrontal neurons during working memory. We examined the relationship of a common functional polymorphism (Val^{Met) in the catechol-}O-methyltransferase (COMT) gene, which accounts for a 4-fold variation in enzyme activity and dopamine catabolism, with both prefrontally mediated cognition and prefrontal cortical physiology. In 175 patients with schizophrenia, 219 unaffected siblings, and 55 controls, COMT genotype was related in allele dosage fashion to performance on the Wisconsin Card Sorting Test of executive cognition and explained 4% of variance (P = 0.001) in frequency of perseverative errors. Consistent with other evidence that dopamine enhances prefrontal neuronal function, the load of the low-activity Met allele predicted enhanced cognitive performance. We then examined the effect of COMT genotype on prefrontal physiology during a working memory task in three separate subgroups (n = 11–16) assayed with functional MRI. Met allele load consistently predicted a more efficient physiological response in prefrontal cortex. Finally, in a family-based association analysis of 104 trios, we found a significant increase in transmission of the Val allele to the schizophrenic offspring. These data suggest that the COMT Val allele, because it increases prefrontal dopamine catabolism, impairs prefrontal cognition and physiology, and by this mechanism slightly increases risk for schizophrenia.

Abstract

Schizophrenia is a complex genetic disorder characterized by chronic psychosis, cognitive impairment, and functional disability. Linkage studies have implicated several possible susceptibility loci, including regions on chromosomes 1q, 6p, 8p, 13q, and 22q (1–3). Attempts to replicate these findings have met with limited success, perhaps due to the weak effects of susceptibility loci and limited power of linkage (4, 5). Of genes mapped to 22q11, a common functional polymorphism of catechol-O-methyltransferase (COMT), a methylation enzyme that metabolizes released dopamine (6), has been a popular candidate because of the long hypothesized role of dopamine in schizophrenia (7). Although two family-based association studies using the transmission disequilibrium test (TDT) have provided evidence for a role of COMT in schizophrenia (8–10), several small case-control association studies of COMT alleles have been negative, and it has been unclear how either protein variation would increase risk for schizophrenia (11, 12).

One approach that may improve power to find genes for complex disorders is to target biological traits found in ill subjects and their unaffected relatives, so-called intermediate phenotypes, rather than clinical diagnosis (13, 14). Such traits may be more directly related to the biological effects of susceptibility genes. Abnormal function of the prefrontal cortex, a cardinal aspect of schizophrenia, also may represent an intermediate phenotype related to genetic risk for schizophrenia (15, 16). Stable deficits in cognitive functions referable to the dorsolateral prefrontal cortex and cortical physiological abnormalities during performance of such tasks have been consistently reported in studies of schizophrenia (17–22). Recent evidence indicates that healthy siblings of patients, including monozygotic cotwins, show similar cognitive and physiological abnormalities (14–16, 22, 24).^¶

Prefrontal deficits also are appealing phenotypes for genetic studies because the molecular mechanisms underlying such deficits have been sufficiently clarified to permit an hypothesis-driven test of candidate functional polymorphisms (25, 26). Electrophysiological studies in primates (27, 28) and rodents (29), and neuroimaging studies in humans (30, 31), have shown that dopamine plays an important role in modulating the activity of prefrontal circuitry during performance of working memory tasks. Although there are many proteins involved in the biological actions of dopamine, COMT, because it metabolizes released dopamine, may be an important factor during such prefrontally mediated tasks. Despite COMT's widespread distribution in nondopaminergic neurons and glia, pharmacological studies have shown that catabolic flux of synaptic dopamine through the COMT pathway is characteristic of the prefrontal cortex in contrast to the striatum (32). Studies of COMT knockout mice, similarly, have demonstrated that dopamine levels are increased only in prefrontal cortex (33) and, remarkably, that memory performance is enhanced.^‖ This regionally selective effect of COMT may be because, in contrast to striatum, in prefrontal cortex dopamine transporters are expressed in low abundance and not within synapses (34, 35). As a consequence, released synaptic dopamine appears to be inactivated by diffusion, receptor internalization, and COMT degradation. These findings strongly support the notion that variation in COMT activity may have neurobiological effects specific to the prefrontal cortex.

The COMT gene contains an evolutionarily recent G to A missense mutation that translates into a substitution of Met for Val at codon 108/158 (Val^{Met) (GenBank accession no.}{"type":"entrez-nucleotide","attrs":{"text":"Z26491","term_id":"403303","term_text":"Z26491"}}Z26491). The enzyme containing Met is unstable at 37°C and has 1/4 of the activity of the enzyme containing Val (36). The alleles are codominant, as heterozygous individuals have enzyme activity that is midway between homozygote individuals (6). Thus, genetically determined variations in COMT activity might affect prefrontal cortical activity, especially during executive and working memory tasks. We hypothesized that the high-activity Val allele, because it leads to increased dopamine catabolism, would be associated with relatively compromised prefrontal function, and, by virtue of this effect, would increase risk for schizophrenia.

To test these hypotheses, we studied prefrontal executive cognition and physiology in control subjects, patients with schizophrenia, and their unaffected siblings. To measure executive cognition and working memory, we used the Wisconsin Card Sorting Test (WCST). Deficits in WCST performance are enduring and core features of schizophrenia and predict long term-disability, independent of other cognitive deficits (17, 21); healthy siblings of patients with schizophrenia also perform abnormally on it (24, 37). Functional neuroimaging studies have found that the WCST activates the dorsolateral prefrontal cortex (17, 38) and that dopamimetic drugs improve performance on this task in patients with schizophrenia and enhance the signal to noise of the prefrontal physiological response (30, 31). We hypothesized, therefore, that COMT genotype would affect WCST performance and that Val/Val individuals would have the poorest performance.

To assay prefrontal physiology, we used functional MRI (fMRI) while subjects performed the N-back task. This task has been shown to activate dorsolateral prefrontal cortex as well as a distributed cortical working memory network (20, 39). In studies of patients with schizophrenia who perform relatively well on the N-back and similar tasks, fMRI activation of dorsolateral prefrontal cortex is “inefficient,” i.e., there is excessive activity for a given level of performance (19, 20). Similar fMRI results have been described in their unaffected siblings,^¶ suggesting that inefficient prefrontal information processing is related to genetic risk for schizophrenia. Using the N-back fMRI paradigm, Mattay et al. recently reported analogous inefficiency in hypodopaminergic patients with Parkinson's disease.^** In contrast, the efficiency of the N-back fMRI response in dorsolateral prefrontal cortex is enhanced by the dopamimetic drug, amphetamine, in healthy individuals whose performance remains stable (40). Thus, deviations of prefrontal physiology can be appreciated with this in vivo fMRI assay even if there is compensation at the level of performance accuracy, and changes in cortical dopaminergic function impact on physiological efficiency during this task. We hypothesized, therefore, that COMT genotype would affect the efficiency of the prefrontal fMRI response during this task and predicted an allele dosage relationship with activation, with Val/Val individuals being least efficient.

Means ± SD.

Means ± SD. Within each group (patients or controls), there is no significant difference between genotype for any variable.

Frequency is ± SE.

Acknowledgments

We thank the following for their assistance: Lew Bigelow, Mary Weirick, Venkatta Mattay, Tonya Gscheidle, Ashley Bone, Tom Weickert, Andreas Myer-Lindenberg, Alan Barnett, and the patients and their families whose participation made this project possible. This project was supported by funding from the National Institute of Mental Health and the Stanley Foundation (to D.R.W.).

Acknowledgments

Abbreviations

COMT	catechol-O-methyltransferase
fMRI	functional MRI
WCST	Wisconsin Card Sorting Test
TDT	transmission disequilibrium test
WRAT	Wide Range Achievement Test

Abbreviations

Footnotes

^{Callicott, J., Egan, M., Mattay, V., Bertolino, A., Jones, K., Goldberg, T. & Weinberger, D. (1998)}NeuroImage7, S895 (abstr.).

^{Kneavel, M., Gogos, J., Karayiorgou, K. & Luine, V., Society for Neuroscience 30th Annual Meeting, November 5–10, 2000, New Orleans, 571.20 (abstr.).}

^{Mattay, V. S., Tessitore, A., Callicott, J. H., Bertolino, A., Duyn, J., Frank, J. A., Goldberg, T., Chase, T., Hyde, T. & Weinberger, D. R., Society for Neuroscience 30th Annual Meeting, November 5–10, 2000, New Orleans, 746 (abstr.).}

Footnotes

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