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21 February 2007. A report published in the February 20 issue of PNAS suggests a genetic association between schizophrenia and a gene encoding the protein early growth response 3 (EGR3), which plays a key role in neuronal development. EGR3 is one of a family of transcription factors that regulate gene activity, and it is under the control of calcineurin, a protein that has previously been linked to schizophrenia. The finding, from the labs of Takeo Yoshikawa at the RIKEN Brain Science Institute, Saitama, Japan, and Susumu Tonegawa at MIT, strengthens the link between schizophrenia and calcineurin signaling.
Calcineurin, otherwise known as protein phosphatase 2B, is a key regulatory molecule, helping to modulate numerous signaling pathways by removing phosphate groups that have been added to proteins by various kinases. The addition of phosphate, or phosphorylation, is one of the primary mechanisms by which cellular processes are turned on and off, and phosphatases have evolved to reset those switches to ensure that cellular signaling does not get out of control. In neurons, calcineurin regulates phosphorylations elicited by both glutamatergic and dopaminergic signaling (see, for example, Nishi et al., 2002), making it particularly interesting to those studying schizophrenia since there is considerable evidence to suggest that both forms of neurotransmission may be dysfunctional in the disease.
The Tonegawa lab previously reported that genetic variations in the gene for the γ subunit of calcineurin associate with schizophrenia in U.S. families (see Gerber et al., 2003) and that knocking out the gene in the mouse forebrain causes abnormal behavior reminiscent of that seen in schizophrenia, including impaired prepulse inhibition and altered social activity (see Miyakawa et al., 2003). The latest evidence linking the phosphatase to schizophrenia comes from a study of calcineurin-related genes. First author Kazuo Yamada and colleagues analyzed single nucleotide polymorphisms (SNPs) in 124 Japanese schizophrenia families, focusing on 14 genes that have been implicated in calcineurin signaling, including calcineurin itself. The researchers confirmed the previously described association of the γ subunit of calcineurin, also called PPP3CC, with schizophrenia in American families. They also found an association with EGR3 and two of its family members, EGR2 and EGR4.
Because the EGR3 and PPP3CC genes lie very close together on chromosome eight, the researchers wondered if the two genes confer risk separately. To test this, they looked to see which variants of EGR3 are present in families that do not have the schizophrenia-linked variant of PPP3CC. That they found an association with two EGR3 SNPs in these families indicates that the two genes may independently contribute to the risk for schizophrenia.
To confirm the association with EGR3 the researchers turned to a wider case-control sample set. They found that two of four haplotypes, or groups of SNPs, are linked to the disease. In schizophrenia patients one of these haplotypes turned up more often than would be expected by chance, while the other was less frequent. Although statistically significant, the associations were not dramatic. The authors suggest that both calcineurin and EGR3 “may independently elicit a modestly increased risk for schizophrenia.”
Exactly how these genetic polymorphisms translate into disease risk is unclear. Yamada and colleagues analyzed postmortem brain samples to see if production of the calcineurin γ subunit or the EGR proteins is different from control samples. While they found no difference between levels of PPP3CC in schizophrenia and normal dorsolateral prefrontal cortex (see also Kozlovsky et al., 2006), they did find that all three EGRs were downregulated in the schizophrenia samples. In this regard it is noteworthy that the EGR3 SNP that seemed to confer the strongest risk is one that falls in a highly conserved intron. Because these non-coding sequences are not normally conserved, the authors wondered if this particular intron, and the SNP, may have some regulatory function. To test this, they measured the activity of the gene in cultured cells and found that when the SNP was present, production of the protein flagged. The findings are consistent with the idea that the EGR3 SNP somehow reduces production of the transcription factor in the brain.
While there is much more work to do to tease out any potential role of EGR3 in the pathology of schizophrenia, the authors make one other interesting observation: EGR3 can be regulated by the activity of neuregulin (see Jacobson et al., 2004), also implicated in schizophrenia (see SRF related news story), providing another potential genetic link between EGR3 and this complex disease.—Tom Fagan.
Reference:
Yamada K, Gerber DJ, Iwayama Y, Ohnishi T, Ohba H, Toyota T, Aruga J, Minabe Y, Tonegawa S, Yoshikawa T. Genetic analysis of the calcineurin pathway identifies members of the EGR gene family, specifically EGR3, as potential susceptibility candidates in schizophrenia. PNAS 2007 Feb 20;104:2815-2820.
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