Disruption of the normal interaction between the genes PRODH and COMT contributes directly to major symptoms of schizophrenia by upsetting the balance of the brain chemicals glutamate and dopamine, according to a group of investigators that includes a scientist now at St. Jude Children's Research Hospital.
The investigators developed a model of schizophrenia that provides a way to study and understand how the loss of both PRODH and COMT gene activity contributes to the symptoms of schizophrenia.
The insights they gained into the disease with this model are important because the loss of the PRODH gene causes the imbalance in the levels of both glutamate and dopamine; and this imbalance contributes directly to the symptoms of schizophrenia, according to Stanislav Zakharenko, MD, PhD, an assistant member of the Department of Developmental Neurobiology at St. Jude.
The team investigated the roles of PRODH and COMT because these genes are located in the q11 region of human chromosome 22. Previous work by other scientists showed that a mutation in this region--the 22q11 microdeletion--is one of the major risk factors for developing schizophrenia.
The study's findings linked changes seen at the molecular level directly to symptoms of schizophrenia seen in humans, said Zakharenko, who is a co-author of a report on this work that appears in the November 15 issue of Nature Neuroscience. The work was completed by Zakharenko and his colleagues at Columbia University (New York), Rockefeller University (New York) and the University of Utrecht (the Netherlands). Zakharenko is continuing his work on the molecular causes of schizophrenia at St. Jude.
The key finding in the current study was that the models of PRODH deficiency had increased COMT activity in the frontal cortex of the brain. "This might reflect a response to the increased dopamine activity caused by PRODH deficiency," Zakharenko said. "And it shows that when PRODH is lost, the additional loss of COMT due to the 22q11 mutation may worsen the symptoms of schizophrenia by allowing dopamine levels to rise." The prefrontal cortex is the part of the brain involved in complex cognitive functioning (e.g., thinking and reasoning).
In the same issue of Nature Neuroscience, another group of investigators reports that their study of adolescents with the 22q11 deletion showed that low activity of COMT is a risk factor for loss of volume of the part of the brain called the prefrontal cortex; and that this same mutation also puts adolescents at risk for developing psychotic symptoms.
Using their model of schizophrenia, Zakharenko and collegues first discovered that the loss of PRODH function directly causes hyperactivity of nerves that use glutamate to signal other nerves in the brain. Next, they found that disruption of PRODH gene activity causes the upregulation of the COMT gene, which encodes for the enzyme that breaks down dopamine. Upregulation is the increase in the rate at which a gene is decoded so the protein it codes for can be manufactured by the cell.
Prior research had already shown that PRODH makes an enzyme that breaks down proline, an amino acid that mimics the action of glutamate on most nerves in the brain. When PRODH activity is low, proline levels are high, creating an excess of excitatory activity leading to overall hypersensitivity of nerve cells to stimulation that might contribute to some schizophrenia symptoms. "Our model of schizophrenia was particularly useful because it lacked only a part of PRODH gene, so the level of proline rose to approximately that seen in individuals with schizophrenia," Zakharenko said.
Although dopamine and glutamate systems were suspected to contribute separately to the development of schizophrenia, researchers had not found a clear connection between them, according to Zakharenko. However, the present study clearly shows this connection. Specifically, when PRODH activity is low, proline levels are high, and there is excess in dopamine activity, he said. The subsequent increase in COMT compensates for the increased release of this dopamine caused by PRODH deficiency. "This finding shows why loss of COMT activity is linked to symptoms of schizophrenia," Zakharenko said.
The study also showed why patients with schizophrenia who also have the 22q11 microdeletion are especially disadvantaged. "COMT upregulation appears to be a response that brings the level of dopamine signaling back to normal," Zakharenko said. "So patients with the 22q11 microdeletion are unable to compensate for their PRODH deficiency by upregulating COMT."