A study on schizophrenia has implicated machinery that maintains the flow of potassium in cells and revealed a potential molecular target for new treatments. Expression of a previously unknown form of a key such potassium channel was found to be 2.5 fold higher than normal in the brain memory hub of people with the chronic mental illness and linked to a hotspot of genetic variation.
An extensive series of experiments suggest that selectively inhibiting this suspect form could help correct disorganized brain activity in schizophrenia without risk of cardiac side effects associated with some existing antipsychotic medications. Scientists at the National Institutes of Health and European colleagues report on threads of converging evidence in the May, 2009 issue of the journal Nature Medicine.
"The end game in linking genes with complex disorders like schizophrenia requires that we not only demonstrate statistical association, but also show how a gene version acts biologically to confer risk," explained Daniel Weinberger, M.D., director of National Institute of Mental Health's (NIMH) Genes Cognition and Psychosis Program, who led the research. "We found schizophrenia-like effects in brain circuitry and mental processing in perfectly healthy people who carry the risk-associated version of this potassium channel gene, even though they don't show any psychotic behavior."
Evidence suggests that schizophrenia stems from complex interactions between multiple genes and environmental factors. Several candidate genes have recently been statistically linked to the illness in large genome-wide association studies.
"Our study goes further, spanning discovery of a new gene variant, confirmation of its association with the illness, and multi-level probes into how it works in human post mortem brain tissue, the living human brain, and neurons," added Weinberger.
By regulating the flow of potassium ions into the cell, potassium channels control when neurons fire electrically discharge and release a chemical messenger that signals neighboring neurons in a circuit. This flow is regulated, in part, by activity of the chemical messenger dopamine, the main target of antipsychotic medications used to treat schizophrenia.
One type of potassium channel, called KCNH2, attracted the researchers' interest for its potential role in sustaining the type of neuronal firing that supports the higher mental functions disturbed in schizophrenia. Spurred by hints from postmortem studies of genetic variation linked to schizophrenia in the genomic neighborhood of KCNH2, the researchers analyzed the gene's association with the illness in 5 independent samples comprising hundreds of families. This pinpointed 4 variations associated with schizophrenia within a small region of the KCNH2 gene.
"Yet this statistical association didn't imply a mechanism," said Weinberger. "It didn't explain how KCNH2 might increase risk for schizophrenia. So we went back to the post-mortem brain tissue in search of an answer."
It was only then that the researchers discovered a previously unknown version of KCNH2, called Isoform 3.1, that soared to levels 2.5 times higher-than-normal in the hippocampus (memory hub) of people who had schizophrenia especially those with the risk-associated variations. Isoform 3.1 was also higher-than-normal in healthy individuals who carried the risk-associated variations. This signaled the existence of a risk-associated version of the KCNH2 gene.
Healthy controls carrying the risk gene version also:
In addition, Isoform 3.1:
Even though it is normally important for our higher order executive functioning, such over expression of Isoform 3.1 in schizophrenia could result in "abnormally increased neuronal excitability, runaway circuit activity and inefficient information processing," suggested Stephen Huffaker, Ph.D., the article's lead author, now a medical student at Harvard. The researchers propose that a treatment designed to inhibit just Isoform 3.1, might spare any heart-related side effects while improving the disorganized neural firing characteristic of the brain in schizophrenia.
|Contact: Jules Asher|
NIH/National Institute of Mental Health