7 May 2009. Drawing on a plethora of techniques to build its case—genotyping and genomic meta-analysis, structural and functional MRI, analysis of postmortem brain tissue, and electrophysiological experiments—an international team of researchers reports that KCNH2, a gene that codes for a potassium channel best known for its role in heart function, is a candidate risk gene for schizophrenia. Moreover, the group proposes that a KCNH2 isoform specific to the primate brain is associated with both schizophrenia and with a “schizophrenia-like shift” in brain anatomy and in physiological and cognitive function in subjects unaffected by the disease.
The new research, led by Daniel Weinberger at the NIMH, Bethesda, Maryland, and published in the May 3 online edition of Nature Medicine, is the latest in a series of genetic studies by the group that follow a multi-faceted, hypothesis-driven strategy to identify risk alleles (see, e.g., SRF related news story; SRF news story). In this case, the team took its lead from an earlier study that reported differential expression of many genes in the prefrontal cortex (PFC) of patients with schizophrenia (Prabakaran et al., 2004). Choosing 10 of those genes for further exploration, the authors genotyped 170 families with offspring that had been diagnosed with schizophrenia and found significant association with a region of chromosome 7q36.1 near the NOS3 (nitric oxide synthase-3) gene. The most strongly associated SNP lay in the nearby gene KCNH2. Additional genotyping in several samples and a pooled meta-analysis revealed that four SNPs in a small (~3 kb) region of the second intron of KCNH2 were strongly associated with schizophrenia.
KCNH2, also known as hERG (human ether-à-go-go related gene), is usually associated with myocardial function, where KCNH2 mutations contribute to long QT syndrome. But the gene is also highly expressed in the brain, particularly in the PFC and hippocampus (Guasti et al., 2005), where the distinctive slow activation, fast inactivation, and slow and voltage-dependent deactivation of the potassium channel coded by the gene are thought to contribute to oscillations in cortical and hippocampal networks (see, e.g., Bazhenov et al., 2004).
Even though none of the KCNH2 SNPs identified in the new study group achieved a statistically significant genomewide association with schizophrenia, the researchers decided to look further for possible structural or functional
brain phenotypes for the putative risk allele. The Weinberger group takes the position that “genes weakly associated with schizophrenia might show relatively robust effects on prefrontal cortex and hippocampal formation function in risk allele-carrying populations,” including healthy controls. Their related view that “the use of healthy controls for genetic association at the level of brain function avoids potential confounders related to chronic illness and medical treatment” (for a discussion, see Hariri and Weinberger, 2003) led the group to conduct structural and functional MRI comparisons of healthy non-carriers, heterozygous carriers, and homozygous carriers of KCNH2 risk alleles.
The researchers found that hippocampal volume decreased significantly in these healthy subjects with increasing “allelic load”; that is, homozygous carriers of KCNH2 risk alleles had significantly smaller hippocampi than heterozygous carriers or non-carriers. These structural correlations were reflected in cognitive and physiological fMRI measures: on tests of IQ and processing speed, carriers performed proportionally worse than non-carriers, but had significantly stronger BOLD (blood oxygenation level-dependent) signals in the hippocampus and prefrontal cortex during these tasks. The Weinberger team interprets these combined findings as an indication of overactive but far less efficient cognitive processing in carriers of KCNH2 risk alleles.
Finding a new isoform
To get at the possible mechanisms underlying these deficits, the team analyzed RNA in postmortem PFC samples from 10 individuals with schizophrenia. They identified a previously unknown KCNH2 isoform, KCNH2-3.1, in which the first 102 amino acids of the full-length transcript KCNH2-1A are replaced with six amino acids unique to KCNH2-3.1. Expression of the KCNH2-3.1 isoform was significantly greater than that of KCNH2-1A, and was also significantly associated with the same risk alleles tested in the group’s MRI experiments. However, expression of the isoform was most pronounced in those who had been diagnosed with schizophrenia.
Intriguingly, while expression of KCNH2-3.1 was roughly equal across brain regions, its expression was three orders of magnitude greater in brain tissue than heart tissue compared to the full-length KCNH2 transcript. Furthermore, KCNH2-3.1 homologs were undetectable in the mouse brains and other mammalian brains, but abundant in rhesus monkey brain. A comparison of human prenatal and adult brain tissue indicated that KCNH2-3.1 expression is significantly higher before birth, which suggests that it could influence the trajectory of neural development and susceptibility to schizophrenia.
To complete the hypothetical circle, the authors conducted electrophysiological experiments on cells, including cortical cells, transfected with KCNH2-1A and -3.1, and found that transfection with the shorter isoform abolished the signature “tail current” that characterizes the slow deactivation of normal KCNH2 channels.
Based on its “convergent experiments,” the Weinberger team argues that, while the sustained neuronal firing pattern associated with the KCNH2-3.1 isoform may have been a beneficial evolutionary adaptation for primate cognition, the overexpression of the isoform seen in its postmortem studies of brains from schizophrenic individuals may perhaps contribute to psychosis. In another evocative aside, they propose that the cardiac side effects of atypical antipsychotics that bind to KCNH2 might be minimized—and the antipsychotic therapeutic benefits increased—if new drugs could be targeted to tamp down the expression of the brain-enriched KCNH2-3.1 isoform.—Peter Farley.
Huffaker SJ, Chen J, Nicodemus KK, Sambataro F, Yang F, Mattay V, Lipska BK, Hyde
TM, Song J, Rujescu D, Giegling I, Mayilyan K, Proust MJ, Soghoyan A, Caforio G,
Callicott JH, Bertolino A, Meyer-Lindenberg A, Chang J, Ji Y, Egan MF, Goldberg
TE, Kleinman JE, Lu B, Weinberger DR. A primate-specific, brain isoform of KCNH2 affects cortical physiology, cognition, neuronal repolarization and risk of schizophrenia. Nat Med. 2009 May 3. Abstract