Downregulated kynurenine 3-monooxygenase gene expression and enzyme activity in schizophrenia and genetic association with schizophrenia endophenotypes

Abstract

Context: Kynurenic acid, a metabolite of the kynurenine pathway of tryptophan degradation, is an antagonist at N-methyl-d-aspartate and α7 nicotinic acetylcholine receptors and modulates glutamate, dopamine, and acetylcholine signaling. Cortical kynurenic acid concentrations are elevated in the brain and cerebrospinal fluid of schizophrenia patients. The proximal cause may be an impairment of kynurenine 3-monooxygenase (KMO), a rate-limiting enzyme at the branching point of the kynurenine pathway.

Objectives: To examine KMO messenger RNA expression and KMO enzyme activity in postmortem tissue from the frontal eye field (FEF; Brodmann area 6) obtained from schizophrenia individuals compared with healthy control individuals and to explore the relationship between KMO single-nucleotide polymorphisms and schizophrenia oculomotor endophenotypes.

Design: Case-control postmortem and clinical study.

Setting: Maryland Brain Collection, outpatient clinics.

Participants: Postmortem specimens from schizophrenia patients (n = 32) and control donors (n = 32) and a clinical sample of schizophrenia patients (n = 248) and healthy controls (n = 228).

Main outcome measures: Comparison of quantitative KMO messenger RNA expression and KMO enzyme activity in postmortem FEF tissue between schizophrenia patients and controls and association of KMO single-nucleotide polymorphisms with messenger RNA expression in postmortem FEF and schizophrenia and oculomotor endophenotypes (ie, smooth pursuit eye movements and oculomotor delayed response).

Results: In postmortem tissue, we found a significant and correlated reduction in KMO gene expression and KMO enzyme activity in the FEF in schizophrenia patients. In the clinical sample, KMO rs2275163 was not associated with a diagnosis of schizophrenia but showed modest effects on predictive pursuit and visuospatial working memory endophenotypes.

Conclusion: Our results provide converging lines of evidence implicating reduced KMO activity in the etiopathophysiology of schizophrenia and related neurocognitive deficits.

Figure 1

The kynurenine pathway of tryptophan degradation. A, Metabolism is initiated by the oxidative ring opening of tryptophan by indoleamine 2,3-dioxygenase and tryptophan 2,3-dioxygenase. In the brain, the pivotal metabolite kynurenine is enzymatically converted to 3-hydroxykynurenine and kynurenic acid in microglial cells and astrocytes, respectively. B, In schizophrenia, a persistent reduction of microglial kynurenine 3-monooxygenase activity would result in increased kynurenic acid formation in, and release from, astrocytes. This could cause increased inhibition of neuronal α7 nicotinic receptors (α7nAChRs) and N-methyl-d-aspartate receptors (NMDARs) (modified from Wonodi and Schwarcz). NAD⁺ indicates nicotinamide adenine dinucleotide.

Figure 2

KMO messenger RNA (mRNA) expression and kynurenine 3-monooxygenase (KMO) enzyme activity. A, Mean KMO mRNA expression and KMO enzyme activity in frontal eye field (FEF) tissues obtained post mortem from schizophrenia (SZ) patients and healthy control individuals. Both measures are significantly reduced in the FEF of SZ patients: *P<.05, †P=.005 (analysis of variance). Error bars indicate SD; RQ, relative quantitation. B, Scatterplot of Pearson correlation (r= 0.66) between KMO gene expression (RQ values, normalized to GAPDH) and KMO enzyme activity in FEF tissue from 30 healthy controls and 30 SZ patients.

Figure 3

Mean effects of kynurenine 3-monooxygenase (KMO) rs2275163 genotype groups on KMO gene expression (ie, relative quantitation values normalized to GAPDH) in postmortem frontal eye field (FEF) samples. KMO gene expression is compared between carriers of the minor allele (TT and CT) (black bars) and carriers that are homozygous for the major allele (CC) (gray bars) in control and schizophrenia FEF tissue specimens: *P<.05, nominal (analysis of variance). Error bars indicate SD. TT and CT: control specimens: n=18, schizophrenia specimens: n=17; CC: control specimens: n=14, schizophrenia specimens: n=15.

Figure 4

Results of single-nucleotide polymorphism (SNP) genotyping. A, Effect of kynurenine 3-monooxygenase (KMO) SNP rs2275163 CC genotype on predictive pursuit function in the combined clinical sample of schizophrenia patients and healthy control individuals. Participants with the CC genotype had significantly worse predictive pursuit function compared with participants with CT or TT genotypes: *P<.003 (analysis of variance post hoc test). B, Effect of KMO SNP rs2275163 CC genotype on visuospatial working memory in the combined clinical sample of schizophrenia patients and healthy control participants. Participants with the CC genotype made significantly more visuospatial (spatial) working memory errors compared with participants with the CT or TT genotypes: *P<.009 (analysis of variance post hoc test).