ORIGINAL ARTICLE Anxiety and Comorbid Measures Associated With PLXNA2 Naomi R. Wray, PhD; Michael R. James, PhD; Steven P. Mah, PhD; Matthew Nelson, PhD; Gavin Andrews, MB ChB, MD; Patrick F. Sullivan, MD, FRANZCP; Grant W. Montgomery, PhD; Andrew J. Birley, PhD; Andreas Braun, PhD, MD; Nicholas G. Martin, PhD Context: Reduction in adult neurogenesis has been pro- typed individuals from 990 families. Of these, 624 indi- posed as a mechanism for onset of depression. Semaphor- viduals with a diagnosis of anxiety or depression from ins and their coreceptors, plexins, have been implicated 443 families were used in the association analysis. in nervous system development and in adult neurogen- esis. A recent genomewide association study of schizo- Main Outcome Measures: All the participants com- phrenia identified a variant of the gene encoding plexin pleted the Composite International Diagnostic Inter- A2 (PLXNA2) to be most consistently associated across view, the 23-item Neuroticism scale of the revised Eysenck study samples. Common genetic liabilities have been re- Personality Questionnaire, and the 10-item Kessler Psy- ported between psychiatric and psychological measures, chological Distress Scale. Diagnoses of DSM-IV depres- but few examples exist of common genetic variants. sion and anxiety were determined from the Composite International Diagnostic Interview. Objective: To perform a genetic association study be- tween 6 single nucleotide polymorphisms from the Results: There was evidence of an allelic association be- PLXNA2 gene (rs3736963, rs2767565, rs752016, tween rs2478813 (and other single nucleotide polymor- rs1327175, rs2478813, and rs716461) and anxiety, de- phisms correlated with it) and anxiety, depression, neu- pression, neuroticism, and psychological distress. roticism, and psychological distress; the association with anxiety is significant after Bonferroni correction for mul- Design: Extreme discordant and concordant siblings. tiple testing (empirical PϽ.001). The mouse ortholog of PLXNA2 is located in a highly significant linkage region Setting: Australia. previously reported for anxiety in mice. Participants: Study participants were selected with re- Conclusion: PLXNA2 is a candidate for causal varia- spect to extreme neuroticism scores from a population tion in anxiety and in other psychiatric disorders through cohort of 18 742 twin individuals and their siblings. The its comorbidity with anxiety. participants and their parents (if blood or buccal samples were available) were genotyped, for a total of 2854 geno- Arch Gen Psychiatry. 2007;64:318-326 OR THE PAST 4 DECADES,1,2 specifically, that neurogenesis is a require- most theories of the etiology ment for the behavioral responses of anti- of major depression have cen- depressants to be affected.7 Therefore, genes tered on brain alterations in that mediate hippocampal neurogenesis are neurochemistry. Recently, a plausible candidates for having risk vari- Fnovel theory proposed that the waning and ants for depression. waxing of neurogenesis in the adult hip- Semaphorins are members of a large, pocampus is causal in the onset of and re- highly conserved family of molecular cues covery from episodes of clinical depres- that have been implicated in the develop- sion.3,4 Pivotal to this theory was the ment of the nervous system, the guidance discovery that neurogenesis is not re- of axonal projections, axonal fascicula- stricted to the developing brain but also oc- tion, dendritic guidance, and neuronal mi- curs in the mature adult brain.5 More re- gration.8,9 In addition to their widespread cently, it has been shown that lithium (a expression during neuronal development, commonly prescribed mood-stabilizing some semaphorins are persistently ex- Author Affiliations are listed at drug with antidepressant properties) en- pressed in the adult nervous system,8 are the end of this article. hances hippocampal neurogenesis6 and, involved in neuronal apoptosis,10 and have (REPRINTED) ARCH GEN PSYCHIATRY/ VOL 64, MAR 2007 WWW.ARCHGENPSYCHIATRY.COM 318 ©2007 American Medical Association. All rights reserved. Downloaded From: http://archpsyc.jamanetwork.com/ by a UQ Library User on 09/13/2015 been shown to induce pruning of hippocampal axon generalized anxiety disorder27 (in the study sample used branches in the mature brain.11 Plexins are members of in this article). In their review, Fanous and Kendler28 con- the semaphorin receptor family that act with neuropilins cluded that, although neuroticism, depression, and schizo- in mediating the effects of semaphorins.12 Neuropilins have phrenia may have shared specific genetic risk factors, more been found at synapses, suggesting that semaphorins also conclusive empirical evidence is needed. have a role as synaptic modulators.13 Plexin A2 (PLXNA2), Given (1) the comprehensive strategy used by Mah together with neuropilin 1 or 2, forms a functional recep- et al,16 which resulted in their proposal of PLXNA2 as a tor for class 3 semaphorins,14 one of which, semaphorin candidate gene for schizophrenia, (2) the possible shared 3A, has been found at elevated levels in patients with schizo- etiology of schizophrenia, depression, and anxiety dis- phrenia.15 The functional role of semaphorins and their orders, and (3) the function of plexins, which fits with receptors makes them plausible candidates for variants to the hypothesis that reduction in neurogenesis in the adult have a causal role in a broad range of psychiatric and psy- brain precipitates the onset of depression, we looked for chological disorders. associations between a set of 6 SNPs in PLXNA2 and mea- Recently, a genomewide association study of schizo- sures of neuroticism, psychological distress, depres- phrenia that analyzed more than 25 000 single nucleo- sion, and anxiety. In this study, we show evidence of as- tide polymorphisms (SNPs) from approximately 14 000 sociations between PLXNA2 SNPs and a spectrum of genes reported that, of 62 SNPs found to be associated psychiatric and psychological traits, particularly driven in the “discovery” case-control set, 1 (accession ID by an association with anxiety, which shows a high de- rs752016 at dbSNP [see www.ncbi.nlm.nih.gov/projects/ gree of comorbidity with the other phenotypic mea- SNP]) showed consistency of association across replica- sures. This study sample is unique because the same co- tion samples (odds ratio [OR], 1.49; P=.006 in individu- hort of people, selected as sibling pairs concordant or als of European descent). Fine mapping identified a region discordant for extreme neuroticism scores, completed de- spanning exons 5 to 26 (approximately 60 kilobase [kb]) tailed questionnaires generating multiple phenotypic mea- of the PLXNA2 gene. As yet, it is unknown whether the sures for each person, allowing us to follow up the ini- PLXNA2 variant associated with schizophrenia16 might tial association results to identify the phenotypic subtypes act during neuronal development, adult neurogenesis, or that drive the association in this sample. both. Currently, schizophrenia is viewed as a disorder of neurodevelopment and of synaptic activity regulated METHODS by neurotransmitters such as dopamine and seroto- 17 nin. However, the first study to implicate adult neuro- ASCERTAINMENT genesis in schizophrenia has just been published,18 in- viting the hypothesis that common variants may affect This study was approved by the Queensland Institute of Medi- adult neurogenesis in a spectrum of psychiatric disor- cal Research human research ethics committee. Australian twin ders (although the same study found no evidence of neu- families recruited from the Australian Twin Registry com- pleted self-report questionnaires between 1980 and 1995, in- ral stem cell proliferation, which is considered to be the 24 first stage of adult neurogenesis in patients with major cluding either the full 90-item EPQ revised (EPQ-R) with a 23-item neuroticism scale or a shortened questionnaire (EPQ-Rs) depression). 19 with a 12-item neuroticism scale. The EPQ-R or EPQ-Rs neu- Under the Kraepelin categorical disease model (and roticism scores were available for 18 742 Australian twin indi- in DSM-IV and International Statistical Classification of viduals and their siblings. Sibling pairs that were either con- Diseases, 10th Revision), schizophrenia and major de- cordant or discordant for extreme EPQ scores (1 sibling in the pression are considered separate conditions. However, top or bottom decile, the other sibling in the top or bottom quin- mounting evidence suggests that this clinically based dis- tile, and excluding monozygotic twin pairs) were recruited to tinction may not provide adequate guidance in attempt- complete a more detailed personality questionnaire. Using these ing to elucidate the etiologic basis of these disorders.20 criteria, multiple siblings were selected from some families. Blood There are few specific genetic risk factors that have been (or buccal) samples were obtained where possible from the se- associated conclusively with both schizophrenia and de- lected siblings and their parents. Full details of the recruit- ment procedure for this extreme discordant and concordant pression, the most widely studied being the neuropatho- study, including response rates and incidence of DSM-IV di- genic role of brain-derived neurotrophic factor (re- 21 agnoses of anxiety- and depression-related disorders and lon- viewed by Angelucci et al ). Neuroticism, a dimension gitudinal stability of neuroticism scores, are given by Kirk et