A resistance gene in disguise for schizophrenia?

Paradox of schizophrenia genetics: is a paradigm shift occurring?

BACKGROUND

Genetic research of schizophrenia (SCZ) based on the nuclear genome model (NGM) has been one of the most active areas in psychiatry for the past two decades. Although this effort is ongoing, the current situation of the molecular genetics of SCZ seems disappointing or rather perplexing. Furthermore, a prominent discrepancy between persistence of the disease at a relatively high prevalence and a low reproductive fitness of patients creates a paradox. Heterozygote advantage works to sustain the frequency of a putative susceptibility gene in the mitochondrial genome model (MGM) but not in the NGM.

METHODS

We deduced a criterion that every nuclear susceptibility gene for SCZ should fulfill for the persistence of the disease under general assumptions of the multifactorial threshold model. SCZ-associated variants listed in the top 45 in the SZGene Database (the version of the 23rd December, 2011) were selected, and the distribution of the genes that could meet or do not meet the criterion was surveyed.

RESULTS

19 SCZ-associated variants that do not meet the criterion are located outside the regions where the SCZ-associated variants that could meet the criterion are located. Since a SCZ-associated variant that does not meet the criterion cannot be a susceptibility gene, but instead must be a protective gene, it should be linked to a susceptibility gene in the NGM, which is contrary to these results. On the other hand, every protective gene on any chromosome can be associated with SCZ in the MGM. Based on the MGM we propose a new hypothesis that assumes brain-specific antioxidant defenses in which trans-synaptic activations of dopamine- and N-methyl-d-aspartate-receptors are involved. Most of the ten predictions of this hypothesis seem to accord with the major epidemiological facts and the results of association studies to date.

CONCLUSION

The central paradox of SCZ genetics and the results of association studies to date argue against the NGM, and in its place the MGM is emerging as a viable option to account for genomic and pathophysiological research findings involving SCZ.

Schizophrenia: maternal inheritance and heteroplasmy of mtDNA mutations

Role of mitochondrial pathology in schizophrenia has not been fully clarified. We searched for distinctive variants in mtDNA extracted from the gray matter of postmortem brains and from peripheral blood samples. We screened mtDNA region containing 5 genes encoding subunits of cytochrome c oxidase and ATPases. Polymorphisms not already reported in databases are recorded as unregistered rare variants. Four unregistered, non-synonymous rare variants were detected in 4 schizophrenic samples. Seven registered non-synonymous variants were not previously detected in non-psychotic Japanese samples registered in the mtSNP database. These variants may contribute to disease pathophysiology. In one family, compound mutations showed co-segregation with schizophrenia. MtDNA mutations could confer a risk for schizophrenia in the Japanese population, although further analyses are needed.

Persistence criteria for susceptibility genes for schizophrenia: a discussion from an evolutionary viewpoint

BACKGROUND

The central paradox of schizophrenia genetics is that susceptibility genes are preserved in the human gene-pool against a strong negative selection pressure. Substantial evidence of epidemiology suggests that nuclear susceptibility genes, if present, should be sustained by mutation-selection balance without heterozygote advantage. Therefore, putative nuclear susceptibility genes for schizophrenia should meet special conditions for the persistence of the disease as well as the condition of bearing a positive association with the disease.

METHODOLOGY/PRINCIPAL FINDINGS

We deduced two criteria that every nuclear susceptibility gene for schizophrenia should fulfill for the persistence of the disease under general assumptions of the multifactorial threshold model. The first criterion demands an upper limit of the case-control difference of the allele frequencies, which is determined by the mutation rate at the locus, and the prevalence and the selection coefficient of the disease. The second criterion demands an upper limit of odds ratio for a given allele frequency in the unaffected population. When we examined the top 30 genes at SZGene and the recently reported common variants on chromosome 6p with the criteria using the epidemiological data in a large-sampled Finnish cohort study, it was suggested that most of these are unlikely to confer susceptibility to schizophrenia. The criteria predict that the common disease/common variant hypothesis is unlikely to fit schizophrenia and that nuclear susceptibility genes of moderate effects for schizophrenia, if present, are limited to 'rare variants', 'very common variants', or variants with exceptionally high mutation rates.

CONCLUSIONS/SIGNIFICANCE

If we assume the nuclear DNA model for schizophrenia, it should have many susceptibility genes of exceptionally high mutation rates; alternatively, it should have many disease-associated resistance genes of standard mutation rates on different chromosomes. On the other hand, the epidemiological data show that pathogenic genes, if located in the mitochondrial DNA, could persist through sex-related mechanisms.

Systematic association studies of mitochondrial DNA variations in schizophrenia: focus on the ND5 gene

Postmortem studies, as well as genetic association studies, have implicated mitochondrial dysfunction in schizophrenia (SZ). We conducted multistaged analysis to assess the involvement of mitochondrial DNA (mtDNA) variations in SZ. Initially, the entire mtDNA genome was sequenced in pools of DNA from SZ cases and controls (n = 180 in each group, set 1). Two polymorphisms localized to the NADH dehydrogenase subunit 5 (ND5) gene demonstrated suggestive case control allele frequency differences (mtDNA 13368 G/A, p = .019 and mtDNA 13708G/A, p = .043). Hence, the ND5 gene was sequenced in individual samples from the initial panel of cases and controls. Additional subjects from another independent set of cases and controls (set 2, cases, n = 244, controls n = 508) were also sequenced individually. No significant differences in allele frequencies for mtDNA 13368 G/A, and mtDNA 13708G/A were observed. However, we identified 216 other rare variants, 53 of which were reported earlier in association studies of other mitochondrial disorders. We compared the distribution of polymorphisms in both sets of cases and controls. No significant case-control differences were observed in the smaller, first set. In the second set, cases had more variants overall (p = 0.014), as well as synonymous variants (p = 0.02), but the difference for nonsynonymous variants was not significant (p = 0.19). Screening available first-degree relatives (n = 10) revealed 10 maternally inherited variations, suggesting that not all the variants are somatic mutations. Further investigations are warranted.