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."
The team further showed that PRODH deficiency increased the release of glutamate at synapses formed by CA3 and CA1 neurons in the part of the brain called the hippocampus. These synapses are routinely used as models of specific types of brain activity responsible for learning and memory. A synapse is the gap between an incoming nerve and its target cell across that gap. Signals pass from one cell on one side of the gap to another cell on the other side. The increased release of excitatory chemicals glutamate and proline due to PRODH deficiency inhibited the ability of the synapse to undergo a change called long-term potentiation (LTP)--a long-lasting strengthening in the connection between two nerve cells. LTP is an important step in forming memories, and disruption of this process interferes with the ability to store information.
Another study using the PRODH-deficiency model showed that the drug D-amphetamine causes exaggerated movement similar to that caused by amphetamine in humans with schizophrenia, according to the researchers. A PRODH deficiency caused lab models to have problems remembering how to respond to an audible tone in a way that was previously learned.
"These observations showed that lack of regulation of glutamate levels due to loss of PRODH function contributed to learning difficulties similar to those found in schizophrenia," Zakharenko said.
Moreover, the researchers showed that PRODH deficiency caused a reduction in the levels of three proteins that, in combination, are associated with dopamine function in the frontal cortex. Because these proteins cooperate with COMT to regulate the overall dopamine activity, the microdeletion 22q11 is likely to contribute to schizophrenia symptoms by eliminating PRODH. "This finding is further evidence that PRODH and COMT interact to control dopamine levels and further explains why the 22q11 microdeletion is associated with schizophrenia," Zakharenko said.
Finally, the investigators used the drug D-amphetamine to stimulate release of dopamine, while blocking COMT activity with a drug called tolcapone. Blocking COMT activity significantly increased the effect of D-amphetamine in PRODH-deficient models, proving that disruption of COMT disrupts the brain's ability to rein in dopamine activity.
"This genetic model offers a way to make additional predictions about how specific gene defects in addition to PRODH and COMT deletions contribute to the development of schizophrenia in patients with 22q11 microdeletions," said Zakharenko. "Further studies using this model will likely help to answer many more questions about this disease."
Other authors of the study include Joseph A. Gogos, Maria Karayiorgou, Marta Paterlini, Wen-Sung Lai, Jie Qin, Hui Zhang, Jun Mukai, David Sulzer, Paul Pavlidis and Steven A. Siegelbaum (Columbia University and Rockefeller University) and Koen G.C. Westphal and Berend Olivier (University of Utrecht).
This work was supported in part by the National Institutes of Health, the New York Academy of Sciences and by National Alliance for Research on Schizophrenia and Depression.
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