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

Abstract

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.

Fig. 1.

Representative sequencing traces showing 2 common mitochondrial DNA (mtDNA) polymorphisms with significant case-control allele frequency differences in pooled DNA samples. Two polymorphisms were observed at ND5 in initial screening by pooled DNA sequencing. mt.13368A/G (A: case, B: control); mt.13708G/A (C: case, D: control). Affected base positions are marked by arrows.

Fig. 2.

Maternal inheritance of selected mitochondrial DNA (mtDNA) variants. A. DNA sequence traces displaying maternal inheritance of mt.13924C>T variation in and case and the affected sibling. The control trace shows wild-type mt.13924C. An arrow indicates the relevant base position. B. Maternal inheritance of heteroplasmic variation—mt.14131A/C. Proband showing maternal inheritance of mt.14131A/C heteroplasmy. The mt.14131C→A resulted in nonsynonymous variation (ND5: 599Leu → Met). The father is homoplasmic for wild-type mt.14131C. Position indicated by an arrow. For the mother, heteroplasmic mt.14131A/C is shown as “N.”