Correction: We originally wrote that the mRNA assays were carried out in postmortem tissue from patients with schizophrenia, when they were actually carried out in individuals without psychiatric or neurologic disorders.
23 January 2006. Several studies have found that the protein CNP, a marker of myelinating oligodendrocytes, is reduced in schizophrenia, adding support to hypotheses about white matter abnormalities in the disorder. Researchers at Cardiff University in Wales now report in the January issue of the Archives of General Psychiatry that the lower expressing allele of a CNP polymorphism is associated with increased risk of schizophrenia.
Converging evidence from imaging, microarray, genetic, and other research suggests that abnormalities in the myelin that speeds conduction through nerve fibers, and more broadly, the oligodendrocytes that give rise to myelin may contribute to the disorder (for a review, see Stewart and Davis, 2004). Additional, circumstantial evidence includes the observation that psychosis is a manifestation of several white matter disorders (see Walterfang et al., 2005). The CNP (2',3'-cyclic nucleotide 3'-phosphodiesterase) protein, which has long been used as a marker to identify mature, myelinating oligodendrocytes (though its function is still unclear), is an especially interesting subject for study: Both the mRNA (Hakak et al., 2001; Tkachev et al., 2003; Aston et al., 2004) and protein product (Flynn et al., 2003) of the CNP gene are down-regulated in schizophrenia, and in a genome-wide scan in families with multiple cases of schizophrenia, the Cardiff research group, led by Mike Owen, reported several years ago that schizophrenia is linked to chromosome 17, in the same region where the CNP gene is located (Williams et al., 2003).
In their new study, first author Tim Peirce and colleagues set out to test the hypothesis that differences in CNP expression could affect schizophrenia risk. Their strategy was to first find SNPs in the CNP gene that lead to differential expression, then test if these are linked to schizophrenia. Although they found several new variants of the CNP gene sequence in DNA from 14 unrelated people with schizophrenia, the SNP that was most useful for study, because it showed the highest minor allele frequency, was the previously identified SNP rs2070106. This variant is exonic, though it does not encode an amino acid change.
The researchers then applied an allele-specific mRNA expression assay, which, though it does not reveal absolute values of mRNA in tissue, does reveal relative differences in expression between alleles. In a sample of postmortem brain tissue from 25 individuals without psychiatric or neurologic disorders who were heterozygous for the minor (A) and major (G) alleles, the minor allele was expressed at lower levels in all cases (24 percent reduction, on average). Given the previous findings of low CNP in schizophrenia, this led naturally to the hypothesis that allele A of rs2070106 would be associated with schizophrenia.
Pooled genotyping of the marker revealed a trend for association. Subsequent individual genotyping in the entire case-control study cohort of 708 subjects with schizophrenia and 711 controls indicated a significant association of the lower expressing (A) rs2070106 allele and schizophrenia (P = .04; OR, 1.2, 95 percent CI 1.0-4.0).
In order to search for any other markers displaying an association with schizophrenia, or to find any markers with possible functional implications for the gene that might be contributing to the expression differences, the researchers examined whether there was any functional locus in linkage disequilibrium (LD) with the rs2070106 variant, but neither of two different approaches to LD mapping (pooling of genetic markers or a haplotype-based analysis) revealed any evidence of association.
Finally, Peirce and colleagues carried their explorations to a family with multiple cases of schizophrenia that the research group had previously found to be linked to chromosome 17q (Williams et al., 2003), the region where the CNP gene is located. They found that all six affected family members carried two copies of the minor, lower-expression A allele at rs2070106. "Conditional on the genotypes and transmission patterns of the parental chromosomes, this observation can be expected to occur as an incidental finding around three times in 100 and provides fully independent support for CNP as a susceptibility gene for schizophrenia. However, we would also stress that, with a rate in the general population of around 10 percent, homozygosity for the putative susceptibility allele in CNP is not a highly penetrant recessive genotype and cannot alone account for our linkage finding in this family," the authors write.
If these results are replicated, the hypothesis of abnormal myelination in the disorder is given a boost, and CNP, in particular, becomes a strong candidate for a schizophrenia susceptibility gene. However, the authors also write, "That altered CNP expression can be observed and replicated in small samples of cases and controls suggests a schizophrenia-related influence on CNP expression that is probably too large and homogeneous to attribute entirely to genetic variation at the CNP locus itself. If altered CNP expression is relevant to pathogenesis, it is probably as a final common pathway resulting from multiple trans-acting genetic and environmental risk factors."
Whatever the influences are that converge to reduce expression of CNP, a broader question that remains to be addressed is how changes in the biological activity of CNP might contribute to schizophrenia. Although the protein has long been used as a marker of actively myelinating oligodendrocytes, its function is still unclear. It is detected mostly in the soma and interacts with mitochondrial and cytoskeletal proteins (see, e.g., Bifulco et al., 2002). Peirce and colleagues also point to a study in which CNP knockout mice show gross abnormalities associated with schizophrenia, such as decreased brain size, enlarged ventricles, and corpus callosum atrophy (Lappe-Siefke et al., 2003).—Hakon Heimer.
Peirce TR, Bray NJ, Williams NM, Norton N, Moskvina V, Preece A, Haroutunian V, Buxbaum JD, Owen MJ, O'donovan MC. Convergent evidence for 2',3'-cyclic nucleotide 3'-phosphodiesterase as a possible susceptibility gene for schizophrenia. Arch Gen Psychiatry. 2006 Jan;63(1):18-24. Abstract