The lifetime risk of developing schizophrenia or bipolar disorder in the UK is 0.7% – 0.85% and 0.3% – 1.5% respectively. Family, twin and adoption studies have shown that there is a large genetic component to both disorders. Multiply affected families are common, and there is good evidence that the susceptibility genes for schizophrenia can be transmitted by obligate carriers who have an abnormal EEG, abnormal eye tracking and volumetric deficits on MRI but have no formal mental illness 1. Replicated evidence from genetic linkage studies has confirmed that multiple chromosomal loci are involved in the heritability of both bipolar disorder and schizophrenia 23.

Evidence that a schizophrenia susceptibility locus maps to chromosome 13q has been reported by several groups 4567891011, although not consistently demonstrated 21213. However such variability in the outcome of linkage studies is to be expected given genetic heterogeneity.

Several studies have also found linkage between bipolar disorder (BPD) and markers on chromosome 13q. The distribution of these linked markers would suggest at least two distinct regions of linkage: 13q11-13q21 and 13q22-13q32. The distal 13q22-13q32 region is more robustly supported by the linkage data. A meta-analysis of linkage scans in bipolar disorder found the strongest evidence for a susceptibility locus on 13q at 79 cM 14. Shaw et al subsequently reported significant linkage with a lod score of 3.4 for markers mapping to this region 11. In addition, there have been a number of independent reports which support linkage to the 13q22-13q32 region 15161718.

Chumakov and colleagues initially investigated the 13q34 region in schizophrenia using 191 SNPs spanning 5 Mb in French Canadian and Russian samples 19. Six markers were found to be associated with schizophrenia in the French Canadian sample, two of which were also associated in the Russian sample. This region contains two overlapping genes, DAOA (G72) and G30, which are transcribed in opposing directions. The DAOA transcript was found to express a primate specific 153 amino acid protein that localises to the endoplasmic reticulum 19. However, more recently it has been shown that the LG72 transcript codes for a mitochondrial protein 20. No protein was identified for G30 in this study and it was postulated that because G30 is transcribed on the opposite strand it could act as a regulator for DAOA expression 19. Expression studies of DAOA in post-mortem brain have found evidence for over expression in schizophrenic brains compared to controls, but not for G30 21. A correlation between DAOA and G30 expression was also found supporting the notion that G30 acts as a regulator of DAOA 21.

DAOA was found to interact directly with the enzyme D-amino acid oxidase (DAO) leading to activation of DAO 19. However, this interaction could not be replicated by Kvajo et al 20. The DAO gene has been localised to chromosome 12q. Four SNPs within the gene were also found to be associated with schizophrenia in the French Canadian but not in the Russian sample 19. Logistic regression analysis demonstrated an interaction occurred between SNPs at the two loci 19. DAO is known to oxidise D-serine, which is a powerful activator of the NMDA-type glutamate receptor 22. It was found that levels of D-serine levels were higher in the serum of schizophrenic patients compared to normal controls 23. Elevated levels of D-serine were also found in cerebrospinal fluid of drug naïve patients with schizophrenia 24.

Hattori et al first reported association of variants at the DAOA/G30 locus with bipolar disorder 25. They examined 16 SNPs across a 157 kb region encompassing the DAOA/G30 locus in two family-based samples. Five SNPs showed individual association with BPD in the Clinical Neurogenetics (CNG) sample.

Subsequently a number of groups have attempted to replicate the associations between DAOA/G30 and schizophrenia, BPD, a psychosis phenotype, and panic disorder. A recent meta analysis of association findings for the DAOA/G30 locus published up until April 2007 found consistent evidence for association with markers at the locus with schizophrenia. 26. rs947267 (M18 SCZ in Asians) and rs778293 (M22 SCZ in Asians) were associated with schizophrenia in Asian populations. rs1421292 (M24) was associated with schizophrenia in European populations.

A number of studies were not included in the meta analysis or have been published since April 2007. Hall et al, who studied US and Afrikaans trios with schizophrenia, found association between rs2391191 (M15) and schizophrenia in the Afrikaans sample 27. Korostishevsky et al found suggestive evidence for association with schizophrenia at the DAOA locus in a Palestinian-Arab sample 28. Fallin et al used an Israeli sample of 337 BPD trios and 274 schizophrenia trios. They showed overlapping suggestive evidence of association with DAOA/G30 for both disorders. They also found evidence for association of DAO with BPD 29. Shin et al found association of rs947267 (M18) and rs778294 (M19) with schizophrenia in a Korean case control sample 30. Shinkai et al reported association of rs746187 (M7) and rs947267 (M18) with schizophrenia in their case control sample. They also found over transmission of alleles to affected offspring with markers rs746187 (M7), rs3918342 (M23) and rs1421292 (M24) in their family based sample 31. Corvin et al reported association with markers at both the DAO and DAOA loci in an Irish schizophrenia case control sample 32. Association was detected with the DAOA markers rs3916965 (M12 p = 0.005) and rs2391191 (M15 p = 0.01); and with the DAO markers rs2111902 (M4 p = 0.02) and rs391834 (M5 p = 0.003). A logistic regression based interaction analysis demonstrated evidence for epistatic interaction between the DAOA marker rs3916965 (M21) and the DAO marker rs3918346 (M5), p = 0.008. Of note these were not the same markers found to interact in the Chumakov (2002) study 19. Prata et al found no evidence for single marker association with bipolar disorder using markers at DAOA and DAO in a case control sample. They did however find some evidence for haplotype association at both loci 33.

A UK study looked at DAOA polymorphisms in both schizophrenia and bipolar samples 34. Association was detected in the bipolar sample but not in the schizophrenia sample 34. The authors went on to perform sub-analysis across traditional diagnostic boundaries. More significant associations were found when schizophrenia patients with a history of major mood disorder were added to the bipolar group. Shultze et al also attempted a phenotypic dissection of their bipolar sample. They concluded that the DAOA association was specifically linked to BPD patients with persecutory delusions 35. The authors also reported an independent haplotypic, but not allelic replication, in a Polish sample35.

However not all studies have supported the association of DAOA with schizophrenia 3637. Genome wide association studies of schizophrenia published to date 383940414243 do not further implicate DAOA or DAO. Two SNPs, rs1981272 (p = 0.0315) and rs9519697 (p = 0.0445), in the DAOA gene region were found to be associated in the WTCCC bipolar disorder genome wide association study 44. However there is sample overlap between this study and the studies by Williams et al 34 and Prata et al 33. No SNPs in the DOAO or DAO regions were found to be associated with bipolar disorder in a meta analysis of genome wide association data that included amongst others the UCL samples described here and the WTCCC samples 45.

In this investigation we test for allelic and haplotypic association between markers at the DAOA locus in our schizophrenia and BPD samples. Additionally we test for association of markers within the DAO locus with schizophrenia.

All the data collected in this study was found to be in Hardy Weinberg equilibrium and free from systematic errors.

Table 1 displays the allele frequencies and tests of association for the SNP markers genotyped at the DAOA locus schizophrenic patients, bipolar disorder patients and controls. The results for the DAO SNPs are found in table 2. Allelic association was detected with both diseases with marker rs3918342 (M23; schizophrenia χ² = 5.824 p = 0.016; bipolar disorder χ² = 4.293 p = 0.038; combined χ² = 7.044 p = 0.008). A trend toward association with schizophrenia was observed with rs3916967 (M14; χ² = 3.675 p = 0.055) and rs1421292 (M24; χ² = 3.499 p = 0.0615). For marker rs1421292 (M24) a nominally significant association was also detected in the combined analysis (χ² = 4.06 p = 0.044).

Pair-wise linkage disequilibrium (LD) was calculated between all pairs of markers at the DAOA locus using GENECOUNTING for the combined data from the schizophrenia, BPD and control samples (see Figure 1). Markers rs3916965 (M12) to rs778294 (M19) formed an LD block defined with a solid spine of LD with a D' >0.8. Markers rs778293 (M22), rs3918342 (M23) and rs1421292 (M24) also formed a separate LD block. Tests of haplotypic association with schizophrenia with markers rs778293 (M22), rs3918342 (M23) and rs1421292 (M24) were positive (global permutation p = 0.018). The haplotype most likely to increase the risk of schizophrenia comprised alleles G, T, A with estimated frequencies of 1.8% in controls and 3.6% in cases of schizophrenia, the results of this analysis are shown in table 3. Haplotype analysis of data from the BPD sample did not show evidence for association.

Association of rs3918342 (M23) using the following case definition was tested: schizophrenia with mood disorders, schizophrenia with mood disorders and BPD combined, BPD with persecutory delusions, and schizophrenia and BPD with persecutory delusions. Nominal association was found with the schizophrenia with mood disorders and BPD combined phenotype (χ² = 4.696 p = 0.030) and the schizophrenia and BPD with persecutory delusions phenotype (χ² = 3.968 p = 0.046).

We did not find statistically significant association between the three markers at the DAO locus and schizophrenia (Table 2). D' values for DAO are displayed in Figure 2.

Allelic association was detected between one DAOA marker (rs3918342, M23) and schizophrenia in our sample. The T allele at rs3918342 (M23) was associated with schizophrenia in our study. This is the same allele that was found to be associated in the study of Chumakov et al 19. However it should be stated that association between rs3918342 and schizophrenia was not found in the meta-analysis of DAOA studies 26. A trend towards association with schizophrenia was found with the marker rs1421292 (M24). The T allele at rs1421292 (M24) was more common in schizophrenic patients than controls. This is the same allele that was found to be associated in the study of Chumakov et al 19 and the meta-analysis26. These findings were supported by haplotypic association.

A modest association of the marker rs3918342 (M23) was also found with bipolar disorder. This provides some support for the involvement of DAOA in the aetiology of bipolar disorder. It should however, also be noted that rs3918342 (M23) is located approximately 50 kb from the 3' end of DAOA and that the LD structure shown in figure 1 suggests that this SNP may have little relevance to unknown functional variants located within the gene.

Three markers were genotyped at the DAO locus in our schizophrenia sample. These did not provide evidence for allelic or haplotypic association with the disease.

In light of the previous findings with DAOA and DAO using the same markers and with the level of LD between markers no correction for multiple testing has been applied to the data presented because the appropriate method for correcting these analyses is not clear.

Our findings point to a role for DAOA in both schizophrenia and bipolar disorder. However it is evident that this locus can account for only a small proportion of the genetic susceptibility to these disorders.

The authors declare that they have no competing interests.

NJB, SRD, AM, VP, KC carried out the sample preparation and molecular genetic studies. NJB, SRD & AM drafted the manuscript. ST, JL, DQ, JP participated in the design of the study. DC oversaw the statistical analysis. HMDG conceived of the study, and participated in its design and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.