Genome-wide association analysis of age at onset in schizophrenia in a European-American sample

Whole-exome sequencing in a Chinese sample provides preliminary evidence for the link between rare/low-frequency immune-related variants and early-onset schizophrenia

Rare and low-frequency variants contribute to schizophrenia (SCZ), and may influence its age-at-onset (AAO). We examined the association of rare or low-frequency deleterious coding variants in Chinese patients with SCZ. We collected DNA samples in 197 patients with SCZ spectrum disorder and 82 healthy controls (HC), and performed exome sequencing. The AAO variable was ascertained in the majority of SCZ participants for identify the early-onset (EOS, AAO<=18) and adult-onset (AOS, AAO>18) subgroups. We examined the overall association of rare/low-frequency, damaging variants in SCZ versus HC, EOS versus HC, and AOS versus HC at the gene and gene-set levels using Sequence Kernel Association Test. The quantitative rare-variant association test of AAO was conducted. Resampling was used to obtain empirical p-values and to control for family-wise error rate (FWER). In binary-trait association tests, we identified 5 potential candidate risk genes and 10 gene ontology biological processes (GOBP) terms, among which PADI2 reached FWER-adjusted significance. In quantitative rare-variant association tests, we found marginally significant correlations of AAO with alterations in 4 candidate risk genes, and 5 GOBP pathways. Together, the biological and functional profiles of these genes and gene sets supported the involvement of perturbations of neural systems in SCZ, and altered immune functions in EOS.

Early-onset schizophrenia is associated with immune-related rare variants in a Chinese sample

Background

Rare variants are likely to contribute to schizophrenia (SCZ), given the large discrepancy between the heritability estimated from twin and GWAS studies. Furthermore, the nature of the rare-variant contribution to SCZ may vary with the "age-at-onset" (AAO), since early-onset has been suggested as being indicative of neurodevelopment deviance.

Objective

To examine the association of rare deleterious coding variants in early- and adult-onset SCZ in a Chinese sample.

Method

Exome sequencing was performed on DNA from 197 patients with SCZ spectrum disorder and 82 healthy controls (HC) of Chinese ancestry recruited in Hong Kong. We also gathered AAO information in the majority of SCZ samples. Patients were classified into early-onset (EOS, AAO<18) and adult-onset (AOS, AAO>18). We collapsed the rare variants to improve statistical power and examined the overall association of rare variants in SCZ versus HC, EOS versus HC, and AOS versus HC at the gene and gene-set levels by Sequence Kernel Association Test. The quantitative rare-variant association test of AAO was also conducted. We focused on variants which were predicted to have a medium or high impact on the protein-encoding process as defined by Ensembl. We applied a 100000-time permutation test to obtain empirical p-values, with significance threshold set at p < 1e -3 to control family-wise error rates. Moreover, we compared the burden of targeted rare variants in significant risk genes and gene sets in cases and controls.

Results

Based on several binary-trait association tests (i.e., SCZ vs HC, EOS vs HC and AOS vs HC), we identified 7 candidate risk genes and 20 gene ontology biological processes (GOBP) terms, which exhibited higher burdens in SCZ than in controls. Based on quantitative rare-variant association tests, we found that alterations in 5 candidate risk genes and 7 GOBP pathways were significantly correlated with AAO. Based on biological and functional profiles of the candidate risk genes and gene sets, our findings suggested that, in addition to the involvement of perturbations in neural systems in SCZ in general, altered immune responses may be specifically implicated in EOS.

Conclusion

Disrupted immune responses may exacerbate abnormal perturbations during neurodevelopment and trigger the early onset of SCZ. We provided evidence of rare variants increasing SCZ risk in the Chinese population.

The genetic basis of onset age in schizophrenia: evidence and models

Schizophrenia is a heritable neurocognitive disorder affecting about 1% of the population, and usually has an onset age at around 21-25 in males and 25-30 in females. Recent advances in genetics have helped to identify many common and rare variants for the liability to schizophrenia. Earlier evidence appeared to suggest that younger onset age is associated with higher genetic liability to schizophrenia. Clinical longitudinal research also found that early and very-early onset schizophrenia are associated with poor clinical, neurocognitive, and functional profiles. A recent study reported a heritability of 0.33 for schizophrenia onset age, but the genetic basis of this trait in schizophrenia remains elusive. In the pre-Genome-Wide Association Study (GWAS) era, genetic loci found to be associated with onset age were seldom replicated. In the post-Genome-Wide Association Study era, new conceptual frameworks are needed to clarify the role of onset age in genetic research in schizophrenia, and to identify its genetic basis. In this review, we first discussed the potential of onset age as a characterizing/subtyping feature for psychosis, and as an important phenotypic dimension of schizophrenia. Second, we reviewed the methods, samples, findings and limitations of previous genetic research on onset age in schizophrenia. Third, we discussed a potential conceptual framework for studying the genetic basis of onset age, as well as the concepts of susceptibility, modifier, and "mixed" genes. Fourth, we discussed the limitations of this review. Lastly, we discussed the potential clinical implications for genetic research of onset age of schizophrenia, and how future research can unveil the potential mechanisms for this trait.

Common genetic variants contribute to heritability of age at onset of schizophrenia

Schizophrenia (SCZ) is a complex disorder that typically arises in late adolescence or early adulthood. Age at onset (AAO) of SCZ is associated with long-term outcomes of the disease. We explored the genetic architecture of AAO with a genome-wide association study (GWAS), heritability, polygenic risk score (PRS), and copy number variant (CNV) analyses in 4 740 subjects of European ancestry. Although no genome-wide significant locus was identified, SNP-based heritability of AAO was estimated to be between 17 and 21%, indicating a moderate contribution of common variants. We also performed cross-trait PRS analyses with a set of mental disorders and identified a negative association between AAO and common variants for SCZ, childhood maltreatment and attention-deficit/hyperactivity disorder. We also investigated the role of copy number variants (CNVs) in AAO and found an association with the length and number of deletions (P-value = 0.03), whereas the presence of CNVs previously reported in SCZ was not associated with earlier onset. To our knowledge, this is the largest GWAS of AAO of SCZ to date in individuals from European ancestry, and the first study to determine the involvement of common variants in the heritability of AAO. Finally, we evidenced the role played by higher SCZ load in determining AAO but discarded the role of pathogenic CNVs. Altogether, these results shed light on the genetic architecture of AAO, which needs to be confirmed with larger studies.

Genome-wide methylation analysis of early-onset schizophrenia

OBJECTIVE

Schizophrenia (SCZ) is a debilitating disease with a complex genetic cause in which age at onset may reflect genetic vulnerability. Though there has been some association between genetic polymorphisms and age of onset, there has been little exploration of the role of epigenetic processes. We sought to explore the influence of DNA methylation, a key epigenetic mechanism, and its association with the age of onset of illness.

METHODS

One hundred thirty-eight participants aged 18-75 years and previously diagnosed with SCZ spectrum disorders by the Structured Clinical Interview for the Diagnostic and Statistical Manual of Mental Disorders (SCID DSM-5) were recruited. Venous blood was collected and genome-wide DNA methylation was quantified using the Illumina Infinium HumanMethylation450 BeadChip array. Individual CpG sites and regions of differential methylation were explored by the age of onset; covariates included age, sex, as well as white blood cell composition.

RESULTS

Binary grouping (early vs. late onset) revealed four intergenic CpG sites on chromosome 2 that were above the expected P-value threshold, with hypermethylation of the CpG site cg10392614 most strongly associated with early-onset SCZ. The four most strongly associated CpG sites, including cg 10392614, were intergenic. Continuous analysis revealed the top CpG site to be cg11723066 , which is linked to the JAM3 gene, with hypomethylation associated with earlier onset; however, results were below the expected P-value threshold.

CONCLUSION

Studies on DNA methylation in the first-episode psychosis population may help further our understanding of the role of epigenetics in the age of onset of SCZ.

NRN1 Gene as a Potential Marker of Early-Onset Schizophrenia: Evidence from Genetic and Neuroimaging Approaches

Included in the neurotrophins family, the Neuritin 1 gene (NRN1) has emerged as an attractive candidate gene for schizophrenia (SZ) since it has been associated with the risk for the disorder and general cognitive performance. In this work, we aimed to further investigate the association of NRN1 with SZ by exploring its role on age at onset and its brain activity correlates. First, we developed two genetic association analyses using a family-based sample (80 early-onset (EO) trios (offspring onset ≤ 18 years) and 71 adult-onset (AO) trios) and an independent case–control sample (120 healthy subjects (HS), 87 EO and 138 AO patients). Second, we explored the effect of NRN1 on brain activity during a working memory task (N-back task; 39 HS, 39 EO and 39 AO; matched by age, sex and estimated IQ). Different haplotypes encompassing the same three Single Nucleotide Polymorphisms(SNPs, rs3763180–rs10484320–rs4960155) were associated with EO in the two samples (GCT, TCC and GTT). Besides, the GTT haplotype was associated with worse N-back task performance in EO and was linked to an inefficient dorsolateral prefrontal cortex activity in subjects with EO compared to HS. Our results show convergent evidence on the NRN1 association with EO both from genetic and neuroimaging approaches, highlighting the role of neurotrophins in the pathophysiology of SZ.

Gene-based mediation analysis in epigenetic studies

Mediation analysis has been a useful tool for investigating the effect of mediators that lie in the path from the independent variable to the outcome. With the increasing dimensionality of mediators such as in (epi)genomics studies, high-dimensional mediation model is needed. In this work, we focus on epigenetic studies with the goal to identify important DNA methylations that act as mediators between an exposure disease outcome. Specifically, we focus on gene-based high-dimensional mediation analysis implemented with kernel principal component analysis to capture potential nonlinear mediation effect. We first review the current high-dimensional mediation models and then propose two gene-based analytical approaches: gene-based high-dimensional mediation analysis based on linearity assumption between mediators and outcome (gHMA-L) and gene-based high-dimensional mediation analysis based on nonlinearity assumption (gHMA-NL). Since the underlying true mediation relationship is unknown in practice, we further propose an omnibus test of gene-based high-dimensional mediation analysis (gHMA-O) by combing gHMA-L and gHMA-NL. Extensive simulation studies show that gHMA-L performs better under the model linear assumption and gHMA-NL does better under the model nonlinear assumption, while gHMA-O is a more powerful and robust method by combining the two. We apply the proposed methods to two datasets to investigate genes whose methylation levels act as important mediators in the relationship: (1) between alcohol consumption and epithelial ovarian cancer risk using data from the Mayo Clinic Ovarian Cancer Case-Control Study and (2) between childhood maltreatment and comorbid post-traumatic stress disorder and depression in adulthood using data from the Gray Trauma Project.

Why does age of onset predict clinical severity in schizophrenia? A multiplex extended pedigree study

Schizophrenia has substantial variation in symptom severity, course of illness, and overall functioning. Earlier age of onset (AOO) is consistently associated with negative outcomes and yet the causes of this association are still unknown. We used a multiplex, extended pedigree design (total N = 771; 636 relatives from 43 multigenerational families with at least 2 relatives diagnosed with schizophrenia and 135 matched controls) to examine among the schizophrenia relatives (N = 103) the relationship between AOO and negative and positive symptom severity, cognition, and community functioning. Most importantly, we assessed whether there are shared genetic effects between AOO and negative symptoms, positive symptoms, cognition, and community functioning. As expected, earlier AOO was significantly correlated with increased severity of negative and positive symptoms and poorer cognition and community functioning among schizophrenia patients. Notably, the genetic correlation between AOO of schizophrenia and negative symptoms was significant (R_g = -1.00, p = .007). Although the genetic correlations between AOO and positive symptoms, cognition, and community functioning were estimated at maximum and in the predicted direction, they were not statistically significant. AOO of schizophrenia itself was modestly heritable, although not significant and negative symptoms, positive symptoms, and cognition were all strongly and significantly heritable. In sum, we replicated prior findings indicating that earlier AOO is associated with increased symptom severity and extended the literature by detecting shared genetic effects between AOO and negative symptoms, suggestive of pleiotropy.

Maternal cerebellar gray matter volume is associated with daughters' psychotic experience

AIM

A substantial portion of children and adolescents show subthreshold psychotic symptoms called psychotic experience (PE). Because PE shares its biological and environmental risk factors with psychotic spectrum disorders, parental neuroanatomical variation could reflect a heritable biological underpinning of PE that may predict an offspring's PE.

METHODS

A total of 94 participants from 35 families without a diagnosis of major neuropsychiatric disorders were examined, including 14 mother-daughter, 17 mother-son, 12 father-daughter, and 16 father-son dyads. An offspring's PE was assessed with the Atypicality subscale of the Behavior Assessment System for Children - 2nd Edition, Self-Report of Personality form (BASCaty). We examined correlations between voxel-by-voxel parental gray matter volume and their offspring's BASCaty score.

RESULTS

Maternal cerebellar gray matter volume using voxel-based morphometry was positively correlated with their daughters' BASCaty scores. The findings were significant in a more robust approach using cerebellum-specific normalization known. We did not find significant correlation between paternal gray matter volume and BASCaty scores or between offspring gray matter volumes and their BASCaty scores.

CONCLUSION

Expanding upon parent-of-origin effects in psychosis, maternal neuroanatomical variation was associated with daughters' PE. The nature of this sex-specific intergenerational effect is unknown, but maternally transmitted genes may relate cerebellum development to PE pathogenesis.

Methionine sulfoxide reductase (Msr) dysfunction in human brain disease

Extensive research has shown that oxidative stress is strongly associated with aging, senescence and several diseases, including neurodegenerative and psychiatric disorders. Oxidative stress is caused by the overproduction of reactive oxygen species (ROS) that can be counteracted by both enzymatic and nonenzymatic antioxidants. One of these antioxidant mechanisms is the widely studied methionine sulfoxide reductase system (Msr). Methionine is one of the most easily oxidized amino acids and Msr can reverse this oxidation and restore protein function, with MsrA and MsrB reducing different stereoisomers. This article focuses on experimental and genetic research performed on Msr and its link to brain diseases. Studies on several model systems as well as genome-wide association studies are compiled to highlight the role of MSRA in schizophrenia, Alzheimer's disease, and Parkinson's disease. Genetic variation of MSRA may also contribute to the risk of psychosis, personality traits, and metabolic factors.

Unravelling the genetic basis of schizophrenia and bipolar disorder with GWAS: A systematic review

OBJECTIVES

To systematically review findings of GWAS in schizophrenia (SZ) and in bipolar disorder (BD); and to interpret findings, with a focus on identifying independent replications.

METHOD

PubMed search, selection and review of all independent GWAS in SZ or BD, published since March 2011, i.e. studies using non-overlapping samples within each article, between articles, and with those of the previous review (Li et al., 2012).

RESULTS

From the 22 GWAS included in this review, the genetic associations surviving standard GWAS-significance were for genetic markers in the regions of ACSL3/KCNE4, ADCY2, AMBRA1, ANK3, BRP44, DTL, FBLN1, HHAT, INTS7, LOC392301, LOC645434/NMBR, LOC729457, LRRFIP1, LSM1, MDM1, MHC, MIR2113/POU3F2, NDST3, NKAPL, ODZ4, PGBD1, RENBP, TRANK1, TSPAN18, TWIST2, UGT1A1/HJURP, WHSC1L1/FGFR1 and ZKSCAN4. All genes implicated across both reviews are discussed in terms of their function and implication in neuropsychiatry.

CONCLUSION

Taking all GWAS to date into account, AMBRA1, ANK3, ARNTL, CDH13, EFHD1 (albeit with different alleles), MHC, PLXNA2 and UGT1A1 have been implicated in either disorder in at least two reportedly non-overlapping samples. Additionally, evidence for a SZ/BD common genetic basis is most strongly supported by the implication of ANK3, NDST3, and PLXNA2.

A schizophrenia associated CMYA5 allele displays differential binding with desmin

CMYA5 is a candidate gene for schizophrenia because of the genetic association of variant rs10043986 (C > T) to this severe mental disorder. Studies of CMYA5 and its gene product, myospryn, in the brain and neuronal cells have not been previously reported. The SNP rs10043986 changes the 4,063rd amino acid from Pro to Leu, which is likely to alter protein function. To understand its potential role in the brain, we examined the neuronal expression of myospryn and its binding partner, desmin, an intermediate filament (IF) protein, and investigated how the two alleles of myospryn affect its binding to desmin. Myospryn and desmin are shown to be expressed in the brain and myospryn is shown to localize to the cytoplasm and nucleus of myoblast, neuroblastoma, and glioblastoma cell lines. Peripherin and vimentin, known brain IF proteins, have high protein similarity to desmin but were found not to interact with myospryn using yeast two-hybrid (Y2H). Using a quantitative Y2H assay and surface plasmon resonance, the T allele (Leu) of rs10043986 was found to have stronger binding to desmin than the C allele (Pro). Based on findings described in this report, we hypothesize that the interaction between myospryn to IF provides structural support and efficient rearrangement of the cytoskeleton network during early neuritogenesis.

Adhesion G Protein-Coupled Receptors as Drug Targets for Neurological Diseases

The family of adhesion G protein-coupled receptors (aGPCRs) consists of 33 members in humans. Although the majority are orphan receptors with unknown functions, many reports have demonstrated critical functions for some members of this family in organogenesis, neurodevelopment, myelination, angiogenesis, and cancer progression. Importantly, mutations in several aGPCRs have been linked to human diseases. The crystal structure of a shared protein domain, the GPCR Autoproteolysis INducing (GAIN) domain, has enabled the discovery of a common signaling mechanism - a tethered agonist - for this class of receptors. A series of recent reports has shed new light on their biological functions and disease relevance. This review focuses on these recent advances in our understanding of aGPCR biology in the nervous system and the untapped potential of aGPCRs as novel therapeutic targets for neurological disease.

Genetic Variants Within Molecular Targets of Antipsychotic Treatment: Effects on Treatment Response, Schizophrenia Risk, and Psychopathological Features

Schizophrenia (SCZ) is a common and severe mental disorder. Genetic factors likely play a role in its pathophysiology as well as in treatment response. In the present study, we investigated the effects of several single nucleotide polymorphisms (SNPs) within 9 genes involved with antipsychotic (AP) mechanisms of action. Two independent samples were recruited. The Korean sample included 176 subjects diagnosed with SCZ and 326 healthy controls, while the Italian sample included 83 subjects and 194 controls. AP response as measured by the positive and negative syndrome scale (PANSS) was the primary outcome, while the secondary outcome was the SCZ risk. Exploratory analyses were performed on (1) symptom clusters response (as measured by PANSS subscales); (2) age of onset; (3) family history; and (4) suicide history. Associations evidenced in the primary analyses did not survive to the FDR correction. Concerning SCZ risk, we partially confirmed the associations among COMT and MAPK1 genetic variants and SCZ. Finally, our exploratory analysis suggested that CHRNA7 and HTR2A genes may modulate both positive and negative symptoms responses, while PLA2G4A and SIGMAR1 may modulate respectively positive and negative symptoms responses. Moreover, GSK3B, HTR2A, PLA2G4A, and S100B variants may determine an anticipation of SCZ age of onset. Our results did not support a primary role for the genes investigated in AP response as a whole. However, our exploratory findings suggested that these genes may be involved in symptom clusters response.

Genetic loci associated with an earlier age at onset in multiplex schizophrenia

An earlier age at onset (AAO) has been associated with greater genetic loadings in schizophrenia. This study aimed to identify modifier loci associated with an earlier AAO of schizophrenia. A genome-wide association analysis (GWAS) was conducted in 94 schizophrenia probands with the earliest AAO and 91 with the latest AAO. Candidate single nucleotide polymorphisms (SNPs) were then genotyped in the co-affected siblings and unrelated probands. Multi-SNP genetic risk scores (GRS) composed of the candidate loci were used to distinguish patients with an early or late AAO. The 14-SNP GRS could distinguish the co-affected siblings (n = 90) of the earliest probands from those (n = 91) of the latest probands. When 132 patients with an earlier AAO and 158 patients with a later AAO were included, a significant trend in the 14-SNP GRS was detected among those unrelated probands from 4 family groups with the earliest, earlier, later, and latest AAO. The overall effect of the 14 SNPs on an AAO in schizophrenia was verified using co-affected siblings of the GWAS probands and trend effect across unrelated patients. Preliminary network analysis of these loci revealed the involvement of PARK2, a gene intensively reported in Parkinson’s disease and schizophrenia research.

A Rare Recurrent 4q25 Proximal Deletion Not Involving the PITX2 Gene: A Genomic Disorder Distinct from Axenfeld-Rieger Syndrome

Haploinsufficient microdeletions within chromosome 4q25 are often associated with Axenfeld-Rieger syndrome. A de novo 4q25 deletion, 675 kb proximal to PITX2, has previously been reported once in an adult patient. The patient did not have Axenfeld-Rieger anomaly, but instead had intellectual disability and a complex behavioral phenotype with withdrawn, stereotypic, and ritualistic behavior. Array comparative genome hybridization demonstrated a smaller, overlapping 4q25 deletion in a 2-year-old patient and his mother, both having significant phenotypic overlap with the initially reported patient. All 3 patients have distinct facial features (including mild hypotelorism and subtle mandibular asymmetry), developmental delay, and complex behavioral difficulties. A genotype-phenotype correlation narrows the shared phenotype to a common COL25A1 gene aberration and proposes that the concurrent EGF gene loss has a significant impact on the phenotypic severity. Overall, our patients provide data to support the existence of a novel 4q25 proximal deletion syndrome.

Recent genetic findings in schizophrenia and their therapeutic relevance

Over 100 loci are now associated with schizophrenia risk as identified by single nucleotide polymorphisms (SNPs) in genome-wide association studies. These findings mean that 'genes for schizophrenia' have unquestionably been found. However, many questions remain unanswered, including several which affect their therapeutic significance. The SNPs individually have minor effects, and even cumulatively explain only a modest fraction of the genetic predisposition. The remainder likely results from many more loci, from rare variants, and from gene-gene and gene-environment interactions. The risk SNPs are almost all non-coding, meaning that their biological significance is unclear; probably their effects are mediated via an influence on gene regulation, and emerging evidence suggests that some key molecular events occur during early brain development. The loci include novel genes of unknown function as well as genes and pathways previously implicated in the pathophysiology of schizophrenia, e.g. NMDA receptor signalling. Genes in the latter category have the clearer therapeutic potential, although even this will be a challenging process because of the many complexities concerning the genetic architecture and mediating mechanisms. This review summarises recent schizophrenia genetic findings and some key issues they raise, particularly with regard to their implications for identifying and validating novel drug targets.

Genetic modifiers and subtypes in schizophrenia: investigations of age at onset, severity, sex and family history

Schizophrenia is a genetically and clinically heterogeneous disorder. Genetic risk factors for the disorder may differ between the sexes or between multiply affected families compared to cases with no family history. Additionally, limited data support a genetic basis for variation in onset and severity, but specific loci have not been identified. We performed genome-wide association studies (GWAS) examining genetic influences on age at onset (AAO) and illness severity as well as specific risk by sex or family history status using up to 2762 cases and 3187 controls from the International Schizophrenia Consortium (ISC). Subjects with a family history of schizophrenia demonstrated a slightly lower average AAO that was not significant following multiple testing correction (p=.048), but no differences in illness severity were observed by family history status (p=.51). Consistent with prior reports, we observed earlier AAO (p=.005) and a more severe course of illness for men (p=.002). Family history positive analyses showed the greatest association with KIF5C (p=1.96×10(-8)), however, genetic risk burden overall does not differ by family history. Separate association analyses for males and females revealed no significant sex-specific associations. The top GWAS hit for AAO was near the olfactory receptor gene OR2K2 (p=1.52×10(-7)). Analyses of illness severity (episodic vs. continuous) implicated variation in ST18 (p=8.24×10(-7)). These results confirm recognized demographic relationships but do not support a simplified genetic architecture for schizophrenia subtypes based on these variables.

Sex differences in the genetic risk for schizophrenia: history of the evidence for sex-specific and sex-dependent effects

Although there is a long history to examinations of sex differences in the familial (and specifically, genetic) transmission of schizophrenia, there have been few investigators who have systematically and rigorously studied this issue. This is true even in light of population and clinical studies identifying significant sex differences in incidence, expression, neuroanatomic and functional brain abnormalities, and course of schizophrenia. This review highlights the history of work in this arena from studies of family transmission patterns, linkage and twin studies to the current molecular genetic strategies of large genome-wide association studies. Taken as a whole, the evidence supports the presence of genetic risks of which some are sex-specific (i.e., presence in one sex and not the other) or sex-dependent (i.e., quantitative differences in risk between the sexes). Thus, a concerted effort to systematically investigate these questions is warranted and, as we argue here, necessary in order to fully understand the etiology of schizophrenia.

Methylenetetrahydrofolate reductase (MTHFR) C677T polymorphism and age at onset of schizophrenia: no consistent evidence for an association in the Nordic population

Methylenetetrahydrofolate reductase (MTHFR) is an enzyme involved in metabolic pathways of importance for nucleotide synthesis and methylation of DNA, membranes, proteins and lipids. The MTHFR gene includes a common polymorphism (rs1801133 or C677T), which is associated with enzyme activity. The T-allele of the C677T polymorphism has been associated with earlier age at onset of schizophrenia in a Scandinavian population, although no association was found in replication attempts in other populations. Extending the study to five Nordic samples consisting of 2,198 patients with schizophrenia, including the original Scandinavian samples, there was no significant association between MTHFR C677T polymorphism and age at onset in schizophrenia. The present results do not suggest that the investigated MTHFR polymorphism has any significant influence on age at onset of schizophrenia in the Nordic population.

Molecular identification of hydroxylysine kinase and of ammoniophospholyases acting on 5-phosphohydroxy-L-lysine and phosphoethanolamine

The purpose of the present work was to identify the catalytic activity of AGXT2L1 and AGXT2L2, two closely related, putative pyridoxal-phosphate-dependent enzymes encoded by vertebrate genomes. The existence of bacterial homologues (40-50% identity with AGXT2L1 and AGXT2L2) forming bi- or tri-functional proteins with a putative kinase belonging to the family of aminoglycoside phosphotransferases suggested that AGXT2L1 and AGXT2L2 acted on phosphorylated and aminated compounds. Vertebrate genomes were found to encode a homologue (AGPHD1) of these putative bacterial kinases, which was therefore likely to phosphorylate an amino compound bearing a hydroxyl group. These and other considerations led us to hypothesize that AGPHD1 corresponded to 5-hydroxy-L-lysine kinase and that AGXT2L1 and AGXT2L2 catalyzed the pyridoxal-phosphate-dependent breakdown of phosphoethanolamine and 5-phosphohydroxy-L-lysine. The three recombinant human proteins were produced and purified to homogeneity. AGPHD1 was indeed found to catalyze the GTP-dependent phosphorylation of 5-hydroxy-L-lysine. The phosphorylation product made by this enzyme was metabolized by AGXT2L2, which converted it to ammonia, inorganic phosphate, and 2-aminoadipate semialdehyde. AGXT2L1 catalyzed a similar reaction on phosphoethanolamine, converting it to ammonia, inorganic phosphate, and acetaldehyde. AGPHD1 and AGXT2L2 are likely to be the mutated enzymes in 5-hydroxylysinuria and 5-phosphohydroxylysinuria, respectively. The high level of expression of AGXT2L1 in human brain, as well as data in the literature linking AGXT2L1 to schizophrenia and bipolar disorders, suggest that these diseases may involve a perturbation of brain phosphoethanolamine metabolism. AGXT2L1 and AGXT2L2, the first ammoniophospholyases to be identified, belong to a family of aminotransferases acting on ω-amines.