Background
Schizophrenia is a mental disease that occurs in the total population with the high frequency of about 1% and particularly in or after adolescence. Vulnerability to this disease is determined by several gene polymorphisms and environmental factors. At MIT in 2003, Tonegawa et al. reported that PPP3CC, the gene coding the calcineurin catalyzer subunit γ, is involved in the schizophrenia of Caucasian- and African-Americans. Calcineurin is the dephosphorylation enzyme that is expressed most abundantly in the neural system. It is a dimer of the catalyzer and regulatory subunits and plays an important role in memory and neuron death. Recently, we examined whether the calcineurin pathway also acts as a genetic vulnerability factor in schizophrenia cases among the Japanese.
Present Results
![Fig. 1: Upper panel: linkage disequilibrium structure (x, y axes) and association signals with schizophrenia [z axis: -log (P value)] in 8p21.3 region, Middle panel: arrangement of local genes, Bottom panel: genomic structure of EGR3.](files/research0101-big.jpg)
We obtained cooperation from schizophrenia family lines and analyzed 84 single-nucleotide polymorphisms (SNP) selected from 14 calcineurin-associated genes. As a result, we found that among Japanese people the PPP3CC gene is also involved in this disease. Most interestingly, we also found other genes that presented a statistical significance; these were EGR2, EGR3 and EGR4. The genes composing the EGR (Early Growth Response) family are numbered from 1 to 4, and each of them codes a transcription factor.
As the EGR3 gene is located in close proximity to the PPP3CC gene in the p-arm of chromosome 8, we next examined if the effects of these two genes are independent of each other. For this purpose, we selected 49 SNPs from the 564 kb genomic region that contain both genes and conducted a detailed genetic analysis of them. We found that the two genes are not interlocked by being inherited from parent to child, but that they contribute independently to the onset of schizophrenia (Fig. 1). We also prepared another set of samples and re-sequenced (re-checked the base sequence) the genome in the EGR3 genetic region (about 10 kb) and found that the new SNP [adenine (A) or guanine (G) depending on the case] in intron 1 is involved in an onset risk. As a result of functional analysis, it was also indicated that intron 1 has an enhancer activity (which promotes transcription of the EGR3 gene), while base sequence A that is found often in schizophrenia cases also has a weak enhancer activity. A gene expression analysis using postmortem brains also showed that the expression of EGR3 is reduced in the schizophrenia group and the expressions of EGR1 and EGR2 are also reduced in the disease group (a stable result was not obtained for EGR4 because its expression is a trace amount in the prefrontal region).
Future Research Topics
The present research indicated: 1) the possibility that factors composing the calcineuron signal transmission system form the foundation for the onset of schizophrenia is universal, and does not depend on the ethnic group, and 2) the possibility that the EGR family gene group is a new schizophrenia-associated group. Although pharmacological research in the past has indicated that the dysfunctioning of neurotransmitters such as dopamine and glutamates may be one of the causes of schizophrenia, we believe that the current research suggests that the calcineuron pathway is located at the confluence of the cascades of the two neurotransmitters (Fig. 2). Different scientists have suggested various possible causes of schizophrenia but we believe that the present research indicates the potential that fruitful discussions may ensue if this issue is viewed from the wider viewpoint of the intracellular metabolic cascade. From this standpoint, topics that might fruitfully be studied in the future are those regarding the identification of the target genes of the transcription control of the EGR genes, the study of their relationships with schizophrenia and the study of the clinical effects of drugs that act on the calcineurin pathway.
