Haplotypes of the D-Amino Acid Oxidase Gene Are Significantly Associated with Schizophrenia and Its Neurocognitive Deficits
Haplotypes of the D-Amino Acid Oxidase Gene Are Significantly Associated with Schizophrenia and Its Neurocognitive Deficits
Yu-Li Liu 0 1 2
Sheng-Chang Wang 1 2
Hai-Gwo Hwu 0 2
Cathy Shen-Jang Fann 2
Ueng- Cheng Yang 2
Wei-Chih Yang 2
Pei-Chun Hsu 2 3
Chien-Ching Chang 2
Chun-Chiang Wen 0 2
Jyy-Jih Tsai-Wu 2
Tzung-Jeng Hwang 0 2
Ming H. Hsieh 0 2
Chen-Chung Liu 0 2
Yi- Ling Chien 0 2
Chiu-Ping Fang 1 2
Stephen V. Faraone 2 4
Ming T. Tsuang 2
Wei J. Chen 2
Chih-Min Liu 0 2
0 Department of Psychiatry, National Taiwan University Hospital and National Taiwan University College of Medicine , Taipei 10051, Taiwan , 3 Graduate Institute of Brain and Mind Sciences, College of Medicine, National Taiwan University , Taipei 10051, Taiwan , 4 Institute of Biomedical Sciences , Academia Sinica, Taipei 11529, Taiwan , 5 Institute of Bioinformatics, National Yang-Ming University , Taipei 112 , Taiwan
1 Center for Neuropsychiatric Research, National Health Research Institutes , Miaoli 35053 , Taiwan
2 Editor: Yong-Gang Yao, Kunming Institute of Zoology, Chinese Academy of Sciences , CHINA
3 Graduate Institute of Biomedical Electronics and Bioinformatics, National Taiwan University , Taipei 10051, Taiwan , 7 Department of Medical Research, National Taiwan University Hospital , Taipei 10051 , Taiwan
4 Medical Genetics Research Center and Department of Psychiatry and Neuroscience & Physiology, SUNY Upstate Medical University , Syracuse, NY 13210 , United States of America, 9 Harvard Institute of Psychiatric Epidemiology and Genetics, and Departments of Epidemiology and Psychiatry, Harvard University , Boston, Massachusetts, 02115 , United States of America, 10 Institute of Behavioral Genomics, University of California San Diego, San Diego, California 92093, United States of America, 11 Institute of Epidemiology and Preventive Medicine, College of Public Health, National Taiwan University , Taipei 10051 , Taiwan
D-amino acid oxidase (DAO) has been reported to be associated with schizophrenia. This study aimed to search for genetic variants associated with this gene. The genomic regions of all exons, highly conserved regions of introns, and promoters of this gene were sequenced. Potentially meaningful single-nucleotide polymorphisms (SNPs) obtained from direct sequencing were selected for genotyping in 600 controls and 912 patients with schizophrenia and in a replicated sample consisting of 388 patients with schizophrenia. Genetic associations were examined using single-locus and haplotype association analyses. In single-locus analyses, the frequency of the C allele of a novel SNP rs55944529 located at intron 8 was found to be significantly higher in the original large patient sample (p = 0.016). This allele was associated with a higher level of DAO mRNA expression in the Epstein-Barr virus-transformed lymphocytes. The haplotype distribution of a haplotype block composed of rs11114083-rs2070586-rs2070587-rs55944529 across intron 1 and intron 8 was significantly different between the patients and controls and the haplotype frequencies of AAGC were significantly higher in patients, in both the original (corrected p < 0.0001) and replicated samples (corrected p = 0.0003). The CGTC haplotype was specifically associated with the subgroup with deficits in sustained attention and executive function
Data Availability Statement: All relevant data are
available in the paper and its Supporting Information
and LCM], the National Health Research Institute,
Taiwan [grant number NHRI-EX-91,92,93-9113PP,
MD095PP14, MD096PP12, MD097PP02,
MD096PP12, MD097PP14, MD096SP01,
MD097SP01 to HGW, and PH098, 99-46 to SCW] ,
the National Institutes of Health, U.S.A. [grant number
IR01 MH 59624-01 to MTT], and National Taiwan
University [grant number 97HM00271~7 to HGW and
LCM]. The funders had no role in study design, data
collection and analysis, decision to publish, or
preparation of the manuscript.
and the AAGC haplotype was associated with the subgroup without such deficits. The DAO
gene was a susceptibility gene for schizophrenia and the genomic region between intron 1
and intron 8 may harbor functional genetic variants, which may influence the mRNA
expression of DAO and neurocognitive functions in schizophrenia.
D-amino acid oxidase (DAO) has been found to be involved in the signal transduction pathway
of the N-methyl-D-aspartic acid (NMDA) receptor [
] and has been hypothesized to be
implicated in the pathogenesis of schizophrenia. DAO [
] and DAO activator (DAOA)  have
been suggested as candidate genes for schizophrenia. The single nucleotide polymorphisms
(SNPs) found most consistently to have a significant association with schizophrenia included
DAO-M4 (rs2111902 at intron 1), DAO-M5 (rs3918346 at intron 3), and DAO-M6 (rs3741775
at intron 4) in French-Canadian, Russian [
], German [
] and Irish populations [
these results were not confirmed in a Chinese population [
], and in our previous study using a
Taiwanese sample [
The use of endophenotypes to refine the phenotypic characterization of schizophrenia has
been advocated [
]. There is substantial empirical evidence to support both sustained attention
impairments and executive dysfunction as endophenotypic markers for schizophrenia.
Sustained attention deficit measured by the Continuous Performance Test (CPT) [
] has been
shown to be present in patients with schizophrenia as well as subjects with schizotypal
personality disorder and in nonpsychotic relatives of schizophrenic patients [
] and their first degree relatives  often have impaired executive function, as
measured by the Wisconsin Card Sorting Test (WCST) [
]. Using performance on the CPT
and the WCST to define endophenotypes of both impaired sustained attention and executive
function in schizophrenia might help address the issue of heterogeneity in schizophrenia by
analysis of the association.
As the haplotype structure of the DAO genomic region is different in Caucasian and Asian
populations, the associated genomic region may be different in different ethnic samples.
Therefore, our aim was to re-sequence the genomic regions of this gene in our sample, and to study
the associations between the specific SNPs or haplotypes found and the subgroups of
schizophrenia as assessed by CPT and WCST.
Materials and Methods
Subjects for direct sequencing. Study subjects included the probands from the Study on
Etiological Factors of Schizophrenia, in which schizophrenic probands from both simplex (i.e.
without affected siblings) and multiplex families (i.e. at least two affected siblings) were
recruited from National Taiwan University Hospital (Taipei, Taiwan) and Ju-Shan Psychiatric
Hospital (Tao-Yuan County, Taiwan) from 2002 to 2005. The exclusion criteria were: severe
neurological abnormality, prominent substance abuse, mental retardation, and aboriginal
Taiwanese ancestry. Healthy persons with no history of psychiatric disorders and no family history
of schizophrenia in their first degree relatives were recruited as controls. A total of 50
independent multiplex patients, 50 simplex patients, and 50 normal controls were selected for the
direct sequencing study.
2 / 15
Subjects for genotyping. The patients were the affected siblings of two types of families:
the multiplex families and the simplex families. The multiplex families were recruited from two
research programs: the Multidimensional Psychopathology Study of Schizophrenia [
1993 to 2001; and the Taiwan Schizophrenia Linkage Study [
] from 1998 to 2002. The 93
families from the first research program were interviewed by research psychiatrists and
assessed using the Psychiatric Diagnostic Assessment . The 609 families from the second
research program were interviewed by well-trained research assistants using the Diagnostic
Interview for Genetic Studies [
]. For both studies, the final research diagnosis was
formulated by integrating the data from both interviews and clinical information from medical
records, based upon the criteria of the Diagnostic and Statistical Manual of Mental Disorders, 4th
edition (DSM-IV). To assure the independence of multiplex subjects, we randomly selected
only one affected sibling from each multiplex family. A total of 702 patients from these
multiplex families were included in this study. The simplex families were recruited from another
independent project [
]. They were interviewed by well-trained research assistants using the
Diagnostic Interview for Genetic Studies. A total of 210 simplex families were recruited.
Therefore, a total of 912 patients with schizophrenia were included in this genotyping study.
The normal controls were selected from a representative Taiwanese supernormal genomic
] with inclusion criteria of age over 60 and Short Portable Mental Status
Questionnaire score above 14. They were recruited by the Institute of Biomedical Science, Academia
Sinica. A total of 600 supernormal control individuals (313 males and 287 females) were
included in this study.
Subjects for replication. Sample 1: A total of 249 subjects were recruited from the
inpatient facilities and day-care units of three psychiatric hospitals located in northern Taiwan; the
Department of Health Taoyuan Mental Hospital, Bali Mental Hospital and Ju-Shan hospital.
The enrolled patients had fulfilled the diagnostic criteria for schizophrenia as outlined by
]. The diagnosis of schizophrenia was based on a review of all available medical
records and confirmed by two board-certified psychiatrists. Patients with a concurrent
diagnosis of a substance abuse disorder or with a clinically significant medical or neurological disease
were excluded. The study subjects were recruited between November 1, 2006 and December
Sample 2: A total of 139 subjects were recruited from the outpatient clinics of National
Taiwan University Hospital. The patients, aged 18 to 65 years, met the DSM-IV criteria for
schizophrenia. Subjects with mental retardation, schizoaffective disorders, bipolar affective disorder,
organic mental disorders, and substance-related disorders were excluded. All the patients
received blood withdrawn for DNA extraction and the examination of event-related potential
(ERP) for the measurement of mismatch negativity (MMN). Detailed recruiting information of
this sample has been published previously [
Subjects for the gene expression study. The subjects for the mRNA expression study
were from the same project as those in the direct sequencing study. A total of 92 patients and
34 controls were included. The mean age of controls was 36.5 (± 10.3) years and 41.7% were
male. The mean age of patients was 33.0 (± 9.1) years and 58.8% were male. There was no
significant difference in age and gender of patients and controls. The mean age at onset of the
patients was 20.2 (± 5.8) years and mean duration of illness was 12.8 (± 8.1). Most of them are
chronic patients on regular antipsychotics.
Ethical statements. All the above subject recruiting projects were approved by the
Institutional Review Boards of all the participating hospitals and institution, including National
Taiwan University Hospital, Ju-Shan Psychiatric Hospital, the Institute of Biomedical Science,
Academia Sinica, Department of Health Taoyuan Mental Hospital and Bali Mental Hospital.
Written informed consent was obtained from all participants in these projects. The capacity for
3 / 15
consent of these patients was assessed by their attending certified psychiatrists to rule out those
participants whose psychotic symptoms or mentality were so severe that impaired their
capacity for consent. All the psychiatric patients who were compulsory hospitalized did not allow
entering these projects. All informed consents were obtained from patients themselves.
Surrogate consent procedure was prohibited in these projects.
DAO genomic sequencing regions defined by bioinformatics
Exon, splicing variant, and isoform analyses. The exon regions of DAO were evaluated
not only for the 11 exons encoded in full-length cDNA, but also for transcripts and variants of
expressed sequence tags. The exon data were processed by the Distributed Annotation System
for the patterns of transcripts and variants located around the DAO genomic region.
The splicing variants and isoforms of DAO were processed by the Integrated Splicing
Variants database [
]. Two isoforms (DAO mRNA transcripts of T1 and T2) and three expressed
sequence tags variants were found in the DAO sequence region. The exons, splicing variants,
and isoforms were compared with the Ensembl databases and were found to be consistent with
Promoter and the highly conserved regions of the introns. The promoter region was
defined as the 2,000 base pairs before the start exon. The highly conserved regions were
predicted by MultiPipMaker (http://pipmaker.bx.psu.edu/pipmaker/) output results [
These conserved regions were arranged into clusters through percent identity plot (PIP)
among humans, chimpanzees, mice, and rats by MultiPipmaker. The identity rate was defined
by a similarity score divided by sequence length in a PCR product region. Regions were
considered highly conserved if the identity was higher than 50% and the gaps among these regions
were less than 10 bp.
The genomic positions of previously significant disease-associated haplotype regions [
and highly conserved regions were integrated. These regions were compared with exon regions
and the splicing junctions were defined. The sequencing regions of primers were designed by
Primer3 (http://frodo.wi.mit.edu/primer3/) [
] to avoid repeat sequencing in the same region.
The sequencing regions including the promoters, 11 exons (including the transcripts and
variants of expressed sequence tags, the highly conserved region and the previously significant
disease-associated haplotype region [
] are shown in Fig 1.
The genomic regions of DAO selected using the above bioinformatics procedures were
amplified by polymerase chain reaction (PCR). PCR products were purified with exonuclease I and
shrimp alkaline phosphatase (USB Corporation, Cleveland, OH, USA) and sequenced from
Fig 1. The sequenced regions of DAO in this study. DAO genetic sequencing regions (gray box) are displayed throughout the promoter (pro), and 11
exons (E) (black box). These sequenced regions include the transcripts (T) and variants (V) of expressed sequence tags, the highly conserved region (CR)
and the previously significant disease-associated haplotype region (HR). The primer pairs for all sequencing regions are presented in Table A in S2 File. The
SNPs of rs11114083, rs2070586, rs2070587, and rs55944529 genotyped in this study are indicated within the genetic region.
4 / 15
both ends. DNA sequencing reactions were performed with BigDye Terminator Cycle
Sequencing Version 3.1 (Applied Biosystems, Foster City, CA, USA) followed by analysis on an
ABI 3730xl DNA Analyzer (Applied Biosystems, CA, USA).
Sequence alignment and analyses. The resulting sequences were compared and aligned
using the Polyphred Sequence Alignment Editor (http://droog.gs.washington.edu/polyphred/).
The reference sequences were obtained from the National Center for Biotechnology
Information (NCBI) website (http://www.ncbi.nlm.nih.gov/).
The selection criteria for large scale genotyping of genetic variants identified from direct
sequencing were as follows: for less common variants (minor allele frequency in these subjects
between 0 to 10%), > 2% difference in minor allele frequency between normal controls and the
patients with schizophrenia; for common variants (minor allele frequency was between 10% to
50%), > 4% difference in minor allele frequency. All SNP genotyping was performed using the
matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF
]. Primers and probes flanking the SNPs were designed using SpectroDESIGNER
software (Sequenom, San Diego, CA, USA). A DNA fragment (100–300 bp) encompassing the
SNP site was amplified by PCR (GeneAmp 9700 thermocycler, Applied Biosystems, CA, USA)
according to the manufacturer’s instructions.
After removing the un-incorporated deoxynucleotide triphosphate (dNTP) and inactivating
the shrimp alkaline phosphatase from the PCR product, primer extension was performed by
adding the probe, Thermo Sequenase (Amersham Pharmacia, Piscataway, NJ, USA) and an
appropriate dideoxynucleotide triphosphate (ddNTP/dNTP) mixture, followed by 55 cycles of
denaturing at 94°C for 5 sec, annealing at 52°C for 5 sec, and extension at 72°C for 5 sec. The
other extension products were differentiated by mass through MALDI-TOF [
Real-time reverse transcriptase polymerase chain reaction (RT-PCR)
DAO expression was assessed using the lymphoblastoid cell lines transformed from
lymphocytes by the Epstein-Barr virus (EBV). These transformed lymphoblastoid cells were washed
once with 1x ice-cold phosphate buffered saline before total RNA extraction. Trizol Reagent
(Invitrogen Life Technologies, Grand Island, NY, USA) was used according to the
manufacturer’s guidelines to extract total RNA.
Real-time RT-PCR was performed for DAO and a housekeeping gene, TATA-box binding
protein (TBP), using pre-designed gene-specific TaqMan1 probes and primer sets (Hs01126151_m1
for DAO and Hs00920497_m1 for TBP) purchased from Applied Biosystems (Applied
Biosystems, CA, USA). Real-time RT-PCR amplification was conducted using RevertAid™ H minus first
strand cDNA synthesis kit (Fermentas, Waltham, MA) with random hexamer and real-time PCR
on an ABI StepOne Plus System (Applied Biosystems, CA, USA), according to the manufacturer’s
instructions. Gene expression was quantified relative to TBP expression using StepOne Software
(Applied Biosystems, CA, USA) and the relative quantification method. The relative expression
level of DAO compared with that of TBP was defined as CT = [CTDAOCTTBP], where CT was the
cycle threshold. The DAO mRNA/TBP mRNA ratio was calculated from 2 CT × K, in which K was
CPT. A CPT machine from Sunrise System, v. 2.20 (Pembroke, MA, USA), was used to
assess sustained attention. The procedure has satisfactory reliability, and has been described in
detail elsewhere [
]. Briefly, numbers from 0 to 9 were randomly presented for 50 msec each,
5 / 15
at a rate of one per second. Each subject undertook two CPT sessions: the undegraded 1–9 task
and the 25% degraded 1–9 task. Subjects were asked to respond whenever the number “9”
preceded by the number “1” appeared on the screen. A total of 331 trials, 34 (10%) of which were
target stimuli, were presented over 5 minutes for each session. One signal-detection index of
performance on the test, sensitivity (d´), was derived from the hit rate (probability of response
to target trials) and false-alarm rate (probability of response to non-target trials) [
Sensitivity is defined as an individual’s ability to discriminate target stimuli from non-target stimuli.
In this study, the z-score of d’ on CPT was used as the endophenotype indicator for
schizophrenia. The cut point of d’ was set as -2.5 from our previous family genetic study [
]. In this
study, 298 patients had a z score of d’ on undegraded CPT below -2.5 (deficit) and 456 patients
had a score above -2.5 (non-deficit); 374 patients had a z score of d’ on degraded CPT below
-2.5 (deficit) and 358 patients had a score above-2.5 (non-deficit).
WCST. A computerized version of the WCST [
] used in a previous study of a Taiwanese
population was used in this study. During the WCST, subjects were required to match response
cards to the four stimulus cards along one of three dimensions (color, form, or number) by
pressing one of the four number keys (1–4) on the computer keyboard. The testing in this
study continued until all 128 cards had been sorted. In this study, the indices of WCST for
association analyses were Perseverative Errors [
]. The indicator was found to be impaired in
schizophrenic probands [
] and in the first degree relatives of schizophrenic probands
. Based on the familial distributions of the z-score of Perseverative Errors, schizophrenic
patients with a z score of Perseverative Errors 1 were defined as having deficit on the WCST.
A total of 263 patients had a z score of Perseverative Errors above 1 (deficit) and 417 patients
had a score below 1 (non-deficit).
Measurement of mismatch negativity (MMN). The detail information about the
measurement of MMN has been published previously [
]. Briefly, audiometry testing was used to
exclude subjects who could not detect 40-dB sound pressure level tones at 500, 1000, and 6000
Hz presented to either ear. Subjects were seated in a comfortable recliner in a
sound-attenuating and electrically shielded booth. The auditory stimuli were generated by a Neuroscan STIM
system and were presented to subjects binaurally via foam insert earphones. The data was
recorded by a Neuroscan ACQUIRE system (NeuroScan, Inc., El Paso, TX). The EEG signals
were recorded with an electrode cap (Quik-Cap, NeuroScan, Inc., Charlotte, NC) from 32 scalp
locations (10–20 system). An auditory oddball paradigm of duration MMN of approximately
30-min duration was given. The duration of standard stimulation and deviant stimulation were
50 msec and 100 msec, respectively. Stimuli occurred in a pseudorandom order with
probability of occurrence 0.9 for standard tones and 0.1 for deviant tones. The MMN session was
continued until a minimum of 225 artifact-free deviant trials had been collected online.
All the electroencephalographic data were processed using Neuroscan Edit 4.3 software
(Compumedics USA, Charlotte, North Carolina). Semi-automated procedures using the Tool
Command Language (TCL) batch processing language began with EOG artifact reduction
through a built-in pattern-recognition algorithm. MMN waveforms were generated by
subtracting the standard ERP from the deviant ERP. MMN indices were measured as the mean
voltage from 135 to 205 milliseconds [
Hardy-Weinberg equilibrium was assessed by using the ALLELE procedure in SAS/GENETICS
release 8.2 [
]. Haploview software was used to construct haplotype blocks with strong linkage
disequilibrium for which the one-sided upper 95% confidence bound on D0 was >0.98 (that is,
consistent with no historical recombination) and the lower bound was above 0.7 according to
6 / 15
the criteria proposed by Gabriel et al. [
]. Haplotype frequencies were compared between the
case and control groups by using the chi-square test or Fisher’s exact test where appropriate.
The false discovery rate method [
] was used to correct for multiple comparisons. The relative
expression levels of DAO between normal controls and schizophrenics were compared using a
non-parametric Mann-Whitney U test for independent groups. A p-value less than 0.05 was
considered significant. The MMN indices of the Fz electrode among patients carrying different
number of the risk haplotype were compared using analysis of covariance (ANCOVA) after
controlling age as a covariate.
Association of single nucleotide polymorphism and schizophrenia
According to bioinformatics analyses (Fig 1), 15 amplicons were designed to search for the
genetic variants of the DAO gene (Table A in S2 File). We found 13 SNPs (4 on the promoter
region, 9 on the intronic region) through direct sequencing (Table B in S2 File). Five of the 14
SNPs fulfilled the criteria for further genotyping (Table 1). We found one novel SNP
(ss73689521) located at intron 8 and reported this to the NCBI. This SNP (ss73689521 or
rs55944529) showed a borderline significant association with schizophrenia (nominal
p = 0.016, corrected p = 0.077) in the original sample (912 patients and 600 controls). The risk
allele is the C allele and it affects the risk for schizophrenia with an odds ratio of 1.29 (95%
confidence interval between 1.05 and 1.58). This SNP did not show a significant association in the
replication sample due to its small sample size.
By analyzing the differences in the relative expression of mRNA among the genotypes of the
associated SNP rs55944529, we found a significantly higher level of DAO in the individuals
carrying the risk allele (C allele, genotypes of CC and CT) compared to those without the risk
allele (genotype of TT) (Z = -2.17, df = 125, p = 0.029) (Fig 2). However, there were no
significant differences in the relative mRNA expression of DAO between patients and controls.
DAO haplotype association with schizophrenia
A tetra-nucleotide (rs11114083- rs2070586-rs2070587-rs55944529) block was found in the
patient group (Fig A in S2 File) using Haploview. Table 2 shows the results of the haplotype
analysis of this haplotype block. We found highly significant differences in haplotype
distribution between patient and control groups both in the original sample and in the replicated
sample (global p < 0.0001 after multiple test correction). The haplotype frequencies of two
haplotypes of AAGC and CGTC were significantly higher in the patient group in the original
sample (corrected p < 0.0001 and p = 0.0089, respectively); however, the haplotype frequency
of CGTT was not significantly different between patients and controls. The haplotype
frequency of AAGC was also significantly higher in the patients in the replicated sample (n = 388)
and in combined analysis (n = 1300) compared with the original controls (corrected p = 0.0003
and p < 0.0001, respectively) (Table 2); however, the haplotype frequency of CGTC was not
significantly higher in the patients in the replicated sample, although significant in the
combined analysis (corrected p = 0.047). There were no significant differences in DAO mRNA
between these haplotypes (detailed data not shown).
Different haplotypes associated with different subgroups of schizophrenia
0 n eno ise 3 1 8 2
.1 .2 .2 .2
3 c G w - 0 0 0 0
p o -e p
d le ) G w - 0 0 0 0
irg le p lle ise .25 .11 .22 .78
o p 8 A w - 0 0 0 0
.3 .3 .3 .1 .
g R + M - 0 0 0 0 I)
n e d N
e p ) (
tesqu lt)rso teyno isep .96 .02 .44 .05 .260 .(013 itoanm
c n G w 0 0 0 0 0 r
re o fo
id c d In
foD lasm M C 0 0 0 0 0 laC
s a t n
SPN iirgn cFA itaen .049 .381 .370 .370 .133 itaoN t.e
ifve O M P 0 0 0 0 0 teh aR
tfeho bllleeA /AG /AC /AG /TG /TC t63na . ryvcoe
is io le is
laaysnn eenG itcaoonL ’5TRU tIron1n tIron1n tIron1n tIron8n iitrvssnoe illronaem lysaebFD
ita po is e 01
liliecsasscgnouoS itraonm rsehoooCmmaiitspono 1077819755 10781602138 1078180496 1078180727 10786515992 :rscsehoooeommm llllrrsseaeudenahe lllifrrycaeeequenon ltttrr-sccvoeedpuae lj..t./ranuopon01e5043105
. fo 58 40 58 58 45 o w m le 37
lae1b inPN IPND 0207 1111 0207 0207 5549 rhoCm lhoeT :FAM iltupm i:.1o10
T S S rs rs rs rs rs a b c d d
Fig 2. Differential DAO expression for genotypes of rs55944529. The transcriptional DAO expression is
measured by real-time RT-PCR in the EBV-transformed lymphoblasts of 126 subjects. The relative DAO
expressions for subjects with CC (n = 88) and CT (n = 26) genotypes, are significantly higher than that of TT
genotype (n = 12) (Z = 2.17, df = 125, p = 0.029, using Wilcoxon two-sample test). Data are presented as
mean ± SD.
schizophrenics with impairment in CPT and WCST. The corrected p values were significant in
three deficient subgroups (p = 0.002–0.005), compared to those of the non-deficient subgroups
(all were non-significant). We also observed that the AAGC haplotype was specifically
associated with the subgroups without impairments in CPT and WCST. The corrected p-values were
significant in three non-deficient subgroups (all p < 0.0001), compared to those of the deficient
subgroups (p = 0.045–0.22). We did not assess CPT and WCST for the patients in the
replicated sample, so that we cannot perform the analyses of the association between the haplotypes
and the neurocognitive subgroups.
Association between haplotypes and MMN
We found that the AAGC haplotype of the tetra-nucleotide block
(rs11114083rs2070586-rs2070587-rs55944529) was associated with the MMN index. Table 4 shows that
aThe rare haplotypes were not listed here.
bThe p value was calculated by Chi-square test and multiple test corrected by False Discovery Rate.
9 / 15
CPT: continuous performance test WCST: Wisconsin card sorting test PER: perseverative error
aDegree of freedom = 1
bThe p value was calculated by Chi-square test
cMultiple test corrected by False Discovery Rate
ddeficient subgroup was defined by z score of d’ < -2.5 on undegraded CPT or on degraded CPT, or by z score of perseverative error 1 on WCST
eNon-deficient subgroup was defined by z score of d’ -2.5 on undegraded CPT or on degraded CPT, or by z score of perseverative error < 1 on WCST.
the more AAGC haplotype the patients carry, the poorer MMN they have (the more negativity
means better MMN) after controlling for age.
By direct sequencing, we found a novel SNP rs55944529 located at intron 8 of the DAO gene.
The frequency of the C allele of this SNP was significantly higher in the original patient sample
than the controls, but not different in the replicated sample. The C allele of this SNP was
associated with a higher level of mRNA expression of DAO in the EBV-transformed lymphocytes.
The haplotype distribution of rs11114083- rs2070586-rs2070587-rs55944529 was significantly
different between schizophrenia and control groups, and the haplotype frequencies of AAGC
were significantly higher in the schizophrenia group, both in the original and replicated
samples. The haplotype of CGTC was specifically associated with the subgroup of schizophrenia
with deficits in sustained attention and executive function, while the haplotype of AAGC was
specifically associated with the subgroup of schizophrenia without deficits. The haplotype of
AAGC was associated with the poorer MMN index of ERP in schizophrenia patients.
*: p = 0.037 for the additive model, p = 0.016 for the dominant model using ANCOVA after controlling for
Number of AAGC haplotype
0 (N = 53)
1 (N = 59)
-0.31 ± 0.62
2 (N = 23)
-0.17 ± 0.54
10 / 15
The SNPs of DAO previously reported to be associated with schizophrenia are DAO-M4
(rs2111902 at intron 1), DAO-M5 (rs3918346 at intron 3), and DAO-M6 (rs3741775 at intron
4) in Caucasian populations [
]; however, this result was not replicated in Chinese [
Taiwanese populations [
]. In this study, we found a novel SNP rs55944529, located at intron
8. Although the novel SNP rs55944529 had a low significant level of association in the original
sample and not significant in the replicated sample, the haplotypes composed of this novel
SNP (rs55944529) and the other three SNPs in the public databank (rs11114083, rs2070586,
and rs2070587) did show a highly significant association both in the original and replicated
sample. This highlights the importance of exploring novel SNPs from a specific population
sample by direct sequencing, rather than by using SNPs only from the public databank. It also
suggests that the regions of DAO associated with schizophrenia might be different in different
The most recent report from the Psychiatric Genomics Consortium (PGC) didn’t reveal
association between DAO gene and schizophrenia (odds ratio = 0.979, p = 0.65 for the SNP
rs55944529 in CEU sample (Sweden 1–6)) [
]. The genetic analyses of major
histocompatibility region in Han Chinese in Taiwan and Caucasians revealed Taiwan’s Han Chinese differ
drastically in genotypic information compared with Caucasians but are relatively
homogeneous in itself [
]. The different population structure may explain the differences between the
results of the PGC and our study. The genome-wide association study (GWAS) in Japan, using
575 patients and 564 controls, didn’t reveal significant association evidence of DAO with
]. The GWAS study in China, using 498 patients and 2025 controls from the
Han Chinese population, revealed borderline significance (p = 0.06) for the SNP rs3918347
], which is 890 bp from our SNP rs55944529. They didn’t perform haplotype analyses in
their studies and it is still unclear whether the specific associated haplotype in our study is
replicated in these different samples. We suggest that the SNP rs55944529 is not the real associated
genetic variant, but the underlying genetic structure captured by the AAGC haplotype is
associated with schizophrenia.
The most consistent result of this study is the association evidence of AAGC haplotype with
schizophrenia. It has been reported that the MMN index is associated with NMDA receptor
]. The poorer MMN in schizophrenia patients is consistent with the evidence of
hypofunction of NMDA receptor function in schizophrenia [
]. The association between
the AAGC haplotype and poorer MMN implies that the haplotype AAGC might have negative
impact upon DAO activity (maybe elevation), next lower the function of NMDA receptor,
then influence MMN index, and at last elevate the risk of schizophrenia. However, we failed to
find the association of mRNA expression with this haplotype.
The genetic information captured by the haplotypes is much more than a single SNP. The
genetic mechanism underlying the haplotypes may be complex and multiple true associated
genetic variants may lie in the haplotype region. We found that the haplotype of CGTC was
specifically associated with the deficient subgroup, and the haplotype of AAGC with the
nondeficient subgroup. It implies there may be some SNPs in the haplotype region, influencing the
biological function of DAO, then having impact upon the neurocognitive function. Because the
rs55944529 is not the true functional genetic variant from our association results, we suggest
that the association of rs55944529 with the mRNA expression of DAO may be through the
association with the neighbor functional SNPs, which may have impact upon mRNA
The mechanism underlying the genetic association of DAO with schizophrenia remains
unclear. Several studies have reported increased DAO expression and activity in the
postmortem brain of schizophrenic patients [
]. Our novel finding reveals that the genotypes of a
SNP rs55944529 at intron 8 have effects on the mRNA expression of DAO. With regard to the
11 / 15
association between genotypes and other endophenotypes of schizophrenia, one study found that
an SNP in the 5’-UTR in DAO (rs4623951) was associated with sensorimotor gating, working
memory, and personality patterns in healthy males in a Greek population [
]. A functional
imaging study revealed that the DAO genotypes (rs3918346 at intron 3) were related to the
degree of deactivation in the left precuneus and greater activation in the right posterior cingulate
gyrus, reflecting a reallocation of cognitive resources [
]. One study found no association of
three DAO SNPs (rs3918346 at intron 3; rs3741775 at intron 4 and rs3918347 at intron 10) with
performance on a broad range of cognitive tasks, including CPT and WCST [
], while our
study found a tetra-nucleotide haplotype across intron 3 and intron 8 to be associated with
neurocognitive function in schizophrenia assessed by CPT and WCST. The various SNP coverages,
cognitive tests, and ethnicities across these studies may explain the diversity of these results.
There are some limitations to our study. First, we did not measure the plasma D-serine level
and DAO enzyme activity in the peripheral blood cells, which may relate to the DAO
functional pathway more directly than the level of DAO expression in the EBV-transformed
lymphocytes we measured in this study. Second, we did not have CPT and WCST data in the
replication sample for analysis. Third, though we found the risk AAGC haplotype was
associated with poorer MMN, we failed to find the association of this haplotype with the mRNA
expression of DAO.
In summary, the most consistent finding of our study was the evidence for an association
between the haplotypes of DAO and schizophrenia. Our findings suggest that the DAO gene is
a susceptibility gene for schizophrenia and the true functional genetic variants related to this
disorder may lie on the genomic region between intron 1 and intron 8.
S1 File. Genotyping raw data of the original sample.
S2 File. Table A, Table B, Fig A.
We gratefully acknowledge the sequencing work performed by the National Sequencing Core
Facility, National Yang-Ming University and the SNP genotyping work performed by the
National Center for Genome Medicine (NGCM), National Science Council, Taiwan. We would
like to thank Translational Resource Center for Genomic Medicine (TRC) of National Research
Program for Biopharmaceuticals (NRPB) for their service. We also thank Taiwan Han Chinese
Cell and Genome Bank of Academic Sinica for their support. Assistance was provided by the
Microarray and SNP Core Facility for Genomic Medicine in National Taiwan University
Hospital and by the Department of Medical Research, National Taiwan University Hospital.
Conceived and designed the experiments: YLL SCW HGH CML SVF MTT WJC. Performed
the experiments: CCW JJTW CPF. Analyzed the data: CSJF WCY CCC UCY PCH.
Contributed reagents/materials/analysis tools: SJF UCY JJTW. Wrote the paper: YLL SCW HGH
12 / 15
CML. Case recruitment and took part in the discussion of research project and manuscript
writing: TJH MHH CCL YLC.
13 / 15
14 / 15
1. Schell MJ . The N-methyl D-aspartate receptor glycine site and D-serine metabolism: an evolutionary perspective . Philos Trans R Soc Lond B Biol Sci . 2004 ; 359 ( 1446 ): 943 - 64 . Epub 2004/08/13. doi: 10 . 1098/rstb. 2003 . 1399 BJYG8C0DVBQP03R0 [pii]. PMID: 15306409; PubMed Central PMCID : PMC1693380 .
2. Wood LS , Pickering EH , Dechairo BM . Significant support for DAO as a schizophrenia susceptibility locus: examination of five genes putatively associated with schizophrenia . Biol Psychiatry . 2007 ; 61 ( 10 ): 1195 - 9 . PMID: 17055463 .
3. Verrall L , Burnet PW , Betts JF , Harrison PJ . The neurobiology of D-amino acid oxidase and its involvement in schizophrenia . Mol Psychiatry . 2010 ; 15 ( 2 ): 122 - 37 . Epub 2009/09/30. doi: 10 .1038/mp. 2009 . 99 PMID: 19786963; PubMed Central PMCID : PMC2811712 .
4. Chumakov I , Blumenfeld M , Guerassimenko O , Cavarec L , Palicio M , Abderrahim H , et al. Genetic and physiological data implicating the new human gene G72 and the gene for D-amino acid oxidase in schizophrenia . Proc Natl Acad Sci U S A . 2002 ; 99 ( 21 ): 13675 - 80 . PMID: 12364586 .
5. Schumacher J , Jamra RA , Freudenberg J , Becker T , Ohlraun S , Otte AC , et al. Examination of G72 and D-amino-acid oxidase as genetic risk factors for schizophrenia and bipolar affective disorder . Mol Psychiatry . 2004 ; 9 ( 2 ): 203 - 7 . PMID: 14966479 .
6. Corvin A , McGhee KA , Murphy K , Donohoe G , Nangle JM , Schwaiger S , et al. Evidence for association and epistasis at the DAOA/G30 and D-amino acid oxidase loci in an Irish schizophrenia sample . Am J Med Genet B Neuropsychiatr Genet . 2007 ; 144 ( 7 ): 949 - 53 . PMID: 17492767 .
7. Liu X , He G , Wang X , Chen Q , Qian X , Lin W , et al. Association of DAAO with schizophrenia in the Chinese population . Neurosci Lett . 2004 ; 369 ( 3 ): 228 - 33 . PMID: 15464270 .
8. Liu YL , Fann CS , Liu CM , Chang CC , Wu JY , Hung SI , et al. No association of G72 and D-amino acid oxidase genes with schizophrenia . Schizophr Res . 2006 ; 87 ( 1-3 ): 15 - 20 . PMID: 16842973 .
9. Gottesman II , Gould TD . The endophenotype concept in psychiatry: etymology and strategic intentions . Am J Psychiatry . 2003 ; 160 ( 4 ): 636 - 45 . Epub 2003/04/02. PMID: 12668349 .
10. Beck LH , Bransome ED Jr., Mirsky AF , Rosvold HE , Sarason I. A continuous performance test of brain damage . J Consult Psychol . 1956 ; 20 ( 5 ): 343 - 50 . Epub 1956/10/01. PMID: 13367264 .
11. Faraone SV , Seidman LJ , Kremen WS , Toomey R , Pepple JR , Tsuang MT . Neuropsychological functioning among the nonpsychotic relatives of schizophrenic patients: a 4-year follow-up study . J Abnorm Psychol . 1999 ; 108 ( 1 ): 176 - 81 . PMID: 10067004 .
12. Faraone SV , Kremen WS , Lyons MJ , Pepple JR , Seidman LJ , Tsuang MT . Diagnostic accuracy and linkage analysis: how useful are schizophrenia spectrum phenotypes? Am J Psychiatry . 1995 ; 152 ( 9 ): 1286 - 90 . PMID: 7653682 .
13. Cornblatt BA , Keilp JG . Impaired attention, genetics, and the pathophysiology of schizophrenia . Schizophr Bull . 1994 ; 20 ( 1 ): 31 - 46 . Epub 1994/01/01. PMID: 8197420 .
14. Chen WJ , Faraone SV . Sustained attention deficits as markers of genetic susceptibility to schizophrenia . Am J Med Genet . 2000 ; 97 ( 1 ): 52 - 7 . PMID: 10813804 .
15. Faraone SV , Seidman LJ , Kremen WS , Toomey R , Pepple JR , Tsuang MT . Neuropsychologic functioning among the nonpsychotic relatives of schizophrenic patients: the effect of genetic loading . Biol Psychiatry . 2000 ; 48 ( 2 ): 120 - 6 . PMID: 10903408 .
16. Chen WJ . Diagnostic Interview for Genetic Studies (DIGS) Mandarin version 2 . 01998 .
17. Chen WJ , Chang CH , Liu SK , Hwang TJ , Hwu HG . Sustained attention deficits in nonpsychotic relatives of schizophrenic patients: a recurrence risk ratio analysis . Biol Psychiatry . 2004 ; 55 ( 10 ): 995 - 1000 . PMID: 15121483 .
18. Goldberg TE , Weinberger DR , Berman KF , Pliskin NH , Podd MH . Further evidence for dementia of the prefrontal type in schizophrenia? A controlled study of teaching the Wisconsin Card Sorting Test . Arch Gen Psychiatry . 1987 ; 44 ( 11 ): 1008 - 14 . Epub 1987/11/01. PMID: 3675128 .
19. Koren D , Seidman LJ , Harrison RH , Lyons MJ , Kremen WS , Caplan B , et al. Factor structure of the Wisconsin Card Sorting Test: dimensions of deficit in schizophrenia . Neuropsychology . 1998 ; 12 ( 2 ): 289 - 302 . Epub 1998/04/29. PMID: 9556775 .
20. Wolf LE , Cornblatt BA , Roberts SA , Shapiro BM , Erlenmeyer-Kimling L. Wisconsin Card Sorting deficits in the offspring of schizophrenics in the New York High-Risk Project . Schizophr Res . 2002 ; 57 ( 2- 3 ): 173 . Epub 2002/09/12. S0920996401003012 [pii]. PMID: 12223248.
21. Robinson AL , Heaton RK , Lehman RA , Stilson DW . The utility of the Wisconsin Card Sorting Test in detecting and localizing frontal lobe lesions . J Consult Clin Psychol . 1980 ; 48 ( 5 ): 605 - 14 . Epub 1980/ 10/01. PMID: 7410659 .
22. Hwu HG , Chen CH , Hwang TJ , Liu CM , Cheng JJ , Lin SK , et al. Symptom patterns and subgrouping of schizophrenic patients: significance of negative symptoms assessed on admission . Schizophr Res . 2002 ; 56 ( 1-2 ): 105 - 19 . PMID: 12084425 .
23. Hwu HG , Faraone SV , Liu CM , Chen WJ , Liu SK , Shieh MH , et al. Taiwan schizophrenia linkage study: the field study . Am J Med Genet B Neuropsychiatr Genet . 2005 ; 134 ( 1 ): 30 - 6 . PMID: 15685625 .
24. Faraone SV , Hwu HG , Liu CM , Chen WJ , Tsuang MM , Liu SK , et al. Genome scan of Han Chinese schizophrenia families from Taiwan: confirmation of linkage to 10q22.3 . Am J Psychiatry . 2006 ; 163 ( 10 ): 1760 - 6 . PMID: 17012687 .
25. Hwu HG . Psychiatric diagnostic assessment . 2nd ed. ed: Publication Committee, College of Medicine, National Taiwan University Taipei.; 1999 .
26. Chen CH , Lee YR , Chung MY , Wei FC , Koong FJ , Shaw CK , et al. Systematic mutation analysis of the catechol O-methyltransferase gene as a candidate gene for schizophrenia . Am J Psychiatry . 1999 ; 156 ( 8 ): 1273 - 5 . PMID: 10450274 .
27. Chang SS , Liu CM , Lin SH , Hwu HG , Hwang TJ , Liu SK , et al. Impaired flush response to niacin skin patch among schizophrenia patients and their nonpsychotic relatives: the effect of genetic loading . Schizophr Bull . 2009 ; 35 ( 1 ): 213 - 21 . Epub 2008/01/22. doi: 10 .1093/schbul/sbm153 PMID: 18203758; PubMed Central PMCID : PMC2643969 .
28. Pan WH , Fann CS , Wu JY , Hung YT , Ho MS , Tai TH , et al. Han Chinese cell and genome bank in Taiwan: purpose, design and ethical considerations . Hum Hered . 2006 ; 61 ( 1 ): 27 - 30 . PMID: 16534213 .
29. American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) . Washington, DC.: American Psychiatric Publishing, Inc.; 1994 .
30. Lin YT , Liu CM , Chiu MJ , Liu CC , Chien YL , Hwang TJ , et al. Differentiation of schizophrenia patients from healthy subjects by mismatch negativity and neuropsychological tests . PloS one . 2012 ; 7 ( 4 ): e34454. doi: 10 .1371/journal.pone.0034454 PMID: 22496807; PubMed Central PMCID : PMC3320618 .
31. Fu CL , Yang UC . ISVdb: Integrated splicing variants database [Master thesis] . Institute of Biochemistry and Molecular Biology: National Yang-Ming University; 2004 .
32. Schwartz S , Zhang Z , Frazer KA , Smit A , Riemer C , Bouck J , et al. PipMaker-a web server for aligning two genomic DNA sequences . Genome Res . 2000 ; 10 ( 4 ): 577 - 86 . Epub 2000/04/26. PMID: 10779500; PubMed Central PMCID : PMC310868 .
33. Schwartz S , Elnitski L , Li M , Weirauch M , Riemer C , Smit A , et al. MultiPipMaker and supporting tools: Alignments and analysis of multiple genomic DNA sequences . Nucleic Acids Res . 2003 ; 31 ( 13 ): 3518 - 24 . Epub 2003/06/26. PMID: 12824357; PubMed Central PMCID : PMC168985 .
34. Rozen S , Skaletsky H. Primer3 on the WWW for general users and for biologist programmers . Methods Mol Biol . 2000 ; 132 : 365 - 86 . PMID: 10547847 .
35. Rodi CP , Darnhofer-Patel B , Stanssens P , Zabeau M , van den Boom D. A strategy for the rapid discovery of disease markers using the MassARRAY system . Biotechniques . 2002 ;Suppl: : 62 - 6 , 8 - 9 . Epub 2002/06/27. PMID: 12083400 .
36. Tost J , Gut IG . Genotyping single nucleotide polymorphisms by MALDI mass spectrometry in clinical applications . Clin Biochem . 2005 ; 38 ( 4 ): 335 - 50 . PMID: 15766735 .
37. Nuechterlein KH . Vigilance in schizophrenia and related disorders . Steinhauer SR , Gruzelier JH , Zubin J , editors. Amsterdam: Elsevier; 1991 . 397 -433 p.
38. Tien AY , Spevack TV , Jones DW , Pearlson GD , Schlaepfer TE , Strauss ME . Computerized Wisconsin Card Sorting Test: comparison with manual administration . Kaohsiung J Med Sci . 1996 ; 12 ( 8 ): 479 - 85 . Epub 1996/08/01. PMID: 8774117 .
39. Heaton RK , Chelune GI , Talley JL , Kay GG , Curtiss G . Wisconsin Card Sorting Test Manual: Revised and Expanded . Odessa, FL: Psychological Assessment Resources; 1993 .
40. Wynn JK , Sugar C , Horan WP , Kern R , Green MF . Mismatch negativity, social cognition, and functioning in schizophrenia patients . Biol Psychiatry . 2010 ; 67 ( 10 ): 940 - 7 . doi: 10 .1016/j.biopsych. 2009 . 11 . 024 PMID: 20074704; PubMed Central PMCID : PMC2862843 .
41. Light GA , Braff DL . Mismatch negativity deficits are associated with poor functioning in schizophrenia patients . Arch Gen Psychiatry . 2005 ; 62 ( 2 ): 127 - 36 . doi: 10 .1001/archpsyc.62.2.127 PMID: 15699289 .
42. Inc . SI. SAS/Genetics1 User's guide . Cary, North Carolina: SAS Institute Inc.; 2002 .
43. Gabriel SB , Schaffner SF , Nguyen H , Moore JM , Roy J , Blumenstiel B , et al. The structure of haplotype blocks in the human genome . Science . 2002 ; 296 ( 5576 ): 2225 - 9 . Epub 2002/05/25. doi: 10 .1126/ science.1069424 1069424 [pii]. PMID: 12029063.
44. Benjamini Y , Hochberg Y. Controlling the false discovery rate: a practical and powerful approach to multiple testing . J R Statist Soc B . 1995 ; 57 ( 1 ): 289 - 300 .
45. Schizophrenia Working Group of the Psychiatric Genomics C. Biological insights from 108 schizophrenia-associated genetic loci . Nature . 2014 ; 511 ( 7510 ): 421 - 7 . doi: 10 .1038/nature13595 PMID: 25056061; PubMed Central PMCID : PMC4112379 .
46. Yang HC , Lin CH , Hsu CL , Hung SI , Wu JY , Pan WH , et al. A comparison of major histocompatibility complex SNPs in Han Chinese residing in Taiwan and Caucasians . J Biomed Sci . 2006 ; 13 ( 4 ): 489 - 98 . doi: 10 .1007/s11373-006-9077-7 PMID: 16544196 .
47. Ikeda M , Aleksic B , Kinoshita Y , Okochi T , Kawashima K , Kushima I , et al. Genome-wide association study of schizophrenia in a Japanese population . Biol Psychiatry . 2011 ; 69 ( 5 ): 472 - 8 . doi: 10 .1016/j. biopsych. 2010 . 07 .010 PMID: 20832056 .
48. Wong EH , So HC , Li M , Wang Q , Butler AW , Paul B , et al. Common variants on Xq28 conferring risk of schizophrenia in Han Chinese . Schizophr Bull . 2014 ; 40 ( 4 ): 777 - 86 . doi: 10 .1093/schbul/sbt104 PMID: 24043878; PubMed Central PMCID : PMC4059435 .
49. Umbricht D , Koller R , Vollenweider FX , Schmid L . Mismatch negativity predicts psychotic experiences induced by NMDA receptor antagonist in healthy volunteers . Biol Psychiatry . 2002 ; 51 ( 5 ): 400 - 6 . PMID: 11904134 .
50. Umbricht D , Krljes S. Mismatch negativity in schizophrenia: a meta-analysis . Schizophr Res . 2005 ; 76 ( 1 ): 1 - 23 . doi: 10 .1016/j.schres. 2004 . 12 .002 PMID: 15927795 .
51. Coyle JT . NMDA receptor and schizophrenia: a brief history . Schizophr Bull . 2012 ; 38 ( 5 ): 920 - 6 . doi: 10 .1093/schbul/sbs076 PMID: 22987850; PubMed Central PMCID : PMC3446237 .
52. Burnet PW , Eastwood SL , Bristow GC , Godlewska BR , Sikka P , Walker M , et al. D-amino acid oxidase activity and expression are increased in schizophrenia . Mol Psychiatry . 2008 ; 13 ( 7 ): 658 - 60 . Epub 2008/06/19. doi: 10 .1038/mp. 2008 .47 PMID: 18560437; PubMed Central PMCID : PMC2629619 .
53. Madeira C , Freitas ME , Vargas-Lopes C , Wolosker H , Panizzutti R . Increased brain D-amino acid oxidase (DAAO) activity in schizophrenia . Schizophr Res . 2008 ; 101 ( 1-3 ): 76 - 83 . Epub 2008/04/02. doi: 10 .1016/j.schres. 2008 . 02 .002 PMID: 18378121 .
54. Bendikov I , Nadri C , Amar S , Panizzutti R , De Miranda J , Wolosker H , et al. A CSF and postmortem brain study of D-serine metabolic parameters in schizophrenia . Schizophr Res . 2007 ; 90 ( 1-3 ): 41 - 51 . Epub 2006/12/13. doi: 10 .1016/j.schres. 2006 . 10 .010 PMID: 17156977 .
55. Verrall L , Walker M , Rawlings N , Benzel I , Kew JN , Harrison PJ , et al. d-Amino acid oxidase and serine racemase in human brain: normal distribution and altered expression in schizophrenia . Eur J Neurosci . 2007 ; 26 ( 6 ): 1657 - 69 . Epub 2007/09/21. doi: 10 .1111/j.1460- 9568 . 2007 . 05769 . x PMID : 17880399 ; PubMed Central PMCID : PMC2121142 .
56. Kapoor R , Lim KS , Cheng A , Garrick T , Kapoor V . Preliminary evidence for a link between schizophrenia and NMDA-glycine site receptor ligand metabolic enzymes, d-amino acid oxidase (DAAO) and kynurenine aminotransferase-1 (KAT-1) . Brain Res . 2006 ; 1106 ( 1 ): 205 - 10 . Epub 2006/07/11. doi: 10 . 1016/j.brainres. 2006 . 05 .082 PMID: 16828464 .
57. Roussos P , Giakoumaki SG , Adamaki E , Anastasios G , Nikos RK , Bitsios P. The association of schizophrenia risk D-amino acid oxidase polymorphisms with sensorimotor gating, working memory and personality in healthy males . Neuropsychopharmacology . 2011 ; 36 ( 8 ): 1677 - 88 . Epub 2011/04/08. doi: 10 .1038/npp. 2011 .49 PMID: 21471957; PubMed Central PMCID : PMC3138651 .
58. Papagni SA , Mechelli A , Prata DP , Kambeitz J , Fu CH , Picchioni M , et al. Differential effects of DAAO on regional activation and functional connectivity in schizophrenia, bipolar disorder and controls . Neuroimage . 2011 ; 56 ( 4 ): 2283 - 91 . Epub 2011/03/23. doi: 10 .1016/j.neuroimage. 2011 . 03 .037 PMID: 21421061 .
59. Goldberg TE , Straub RE , Callicott JH , Hariri A , Mattay VS , Bigelow L , et al. The G72/G30 gene complex and cognitive abnormalities in schizophrenia . Neuropsychopharmacology . 2006 ; 31 ( 9 ): 2022 - 32 . Epub 2006/03/24. doi: 10 .1038/sj.npp.1301049 PMID: 16554747 .