Elsevier

Schizophrenia Research

Volume 185, July 2017, Pages 9-16
Schizophrenia Research

Exome array analysis suggests an increased variant burden in families with schizophrenia

https://doi.org/10.1016/j.schres.2016.12.007Get rights and content

Abstract

The exome array assays rare-but-recurrent, likely deleterious, exonic variants and represents an intermediary between single nucleotide polymorphism (SNP) arrays and sequencing for genetic association studies. Multiplex families with multiple affected individuals may be enriched for disease-associated variants of this class compared to unrelated populations. We present an exome array study of schizophrenia in 99 multiplex families (n = 341, including 118 cases) from the Western Australian Family Study of Schizophrenia (WAFSS).

Compared to 55,726 individuals from the DIAGRAM sample not selected for schizophrenia, overall allele frequency of exome variants was higher in the WAFSS (P < 2.2E-16). This was pronounced in variants nominally associated (P < 0.05) with schizophrenia.

Genes harbouring variants present only in WAFSS cases were enriched (FDR-corrected P = 0.05) for membership of the ‘extracellular matrix (ECM) - receptor interaction’ biological pathway, adding to evidence that processes affecting the composition or turnover of ECM may contribute to neuropsychiatric disease.

We did not find individual variants significantly associated with schizophrenia, although like previous studies, power to detect associations of small effect size was low. Cases did not exhibit a higher burden of variants compared to their unaffected relatives and the finding of previous exome chip studies of unrelated samples that ‘schizophrenia gene-sets’ were enriched for case-only variants was not replicated in the WAFSS.

The higher frequency of moderately rare, exonic variants in these multiplex families compared to a population-based sample may account for some of their genetic liability to schizophrenia, and adds to evidence for a role of exome array variants from previous studies of unrelated samples.

Introduction

Schizophrenia (lifetime risk ~ 1%) is a potentially disabling disorder with a multifactorial aetiology comprising a major genetic contribution (broad-sense heritability estimates range from 60 to 80% (Lichtenstein et al., 2009, Sullivan et al., 2003) and a host of environmental factors, ranging from neurodevelopmental insults to psychosocial adversity (Matheson et al., 2013). The genetic underpinnings of schizophrenia are likely to be complex, and involve genetic variation across the frequency spectrum - from rare, highly penetrant variants such as copy number variants (CNVs) to common single nucleotide polymorphisms (SNPs) with individually small effect sizes.

The largest genome wide association study (GWAS) to date, conducted by the Psychiatric Genetic Consortium, identified 108 loci associated with schizophrenia at genome-wide significance (Ripke et al., 2014). Studies from the same consortium have shown that schizophrenia is highly polygenic (Bulik-Sullivan et al., 2015b), and composite risk profile scores account for ~ 7% of variation on the liability scale.

Sequencing studies of schizophrenia have so far been underpowered for the detection of very rare variants showing study-wide significant association. Exome sequencing of unaffected parent-singleton affected offspring trios has identified a higher burden of de novo variants in sporadic cases relative to expected rates, or rates in unaffected relatives (Fromer et al., 2014, Girard et al., 2011, Xu et al., 2011). Furthermore, case-control studies have revealed an increased burden of rare, likely pathogenic variants in cases versus controls (Kenny et al., 2014, Need et al., 2012, Purcell et al., 2014). Sequencing studies in multiplex families have focussed on rare, highly penetrant causative variants segregating with schizophrenia and reported protein-altering variants in 3 genes associated with function of the NMDA receptor – GRM5, PPEF2 and LRP1B (Timms et al., 2013); in the SHANK2 post-synaptic density scaffolding protein and the SMARCA1 transcriptional regulator (Homann et al., 2016) and in UNC13B, (Egawa et al., 2016), which encodes a protein involved in the priming step of synaptic vesicle exocytosis.

An intermediate step between GWAS and sequencing studies is to investigate so-called ‘goldilocks’ variants (Price et al., 2010) – variants common enough to be detected in population samples of a reasonable size and rare enough to be under purifying selection (minor allele frequency, MAF > 0.5% & ≤ 2%). Simulations for complex traits suggest that this class of variant may be quite frequent, with at least one variant present in up to one-eighth of genes affecting the trait (Price et al., 2010). The exome array was designed to target likely deleterious, recurrent variants of this type, and includes 97–98% of the nonsynonymous variants detected in an average genome through exome sequencing.

Two case-control studies of schizophrenia have been published recently using the exome chip, both of which implicated these variants in schizophrenia. Loohuis et al. (2015) described an increased burden of variants with MAF  0.01% in cases versus controls, and enrichment of rare variants in neurodevelopmental genes. Richards et al. (2016) reported that the rare (MAF < 0.1%) variant association signal was enriched in genes previously implicated in both common and rare variant studies of schizophrenia.

We hypothesised that families with multiple affected individuals may be enriched for disease-associated variants of this class compared to unrelated populations. Here, we report the results of an exome chip study in a sample comprising 99 pedigrees from the Western Australian Family Study of Schizophrenia (WAFSS), all of which included a proband with a diagnosis of schizophrenia and at least one first degree relative with schizophrenia. We assessed the association of variants with schizophrenia both individually and collectively, to ascertain whether exome chip variant burden is enriched in cases compared to their unaffected relatives. We looked for enrichment of variants in the schizophrenia-related gene-sets reported by Richards et al. (2016) and also conducted agnostic pathway analyses. Finally, we investigated whether rare variant burden is increased in this multiplex family dataset as a whole, compared to a large unselected sample of similar ethnicity.

Section snippets

Materials and methods

A full description of methods is provided in Supplementary Methods. Unless otherwise specified, all statistical analyses were performed in R version 3.2.0.

Association with individual variants

Quantile-Quantile plots (Supplementary Figs. 9 and 10) indicate that there was a slight deflation of the test statistics for association with schizophrenia in all 56,398 variants compared to the null distribution, which disappears when only common (MAF > 5%) variants are included. None of the variants were individually associated with schizophrenia beyond Bonferroni threshold (P < 8.8E-07). The top ten most significant associations are shown in Supplementary Table 9. Most of these ten variants were

Discussion

This study set out to investigate for the first time whether there is evidence that rare-but-recurrent, likely-deleterious variants assayed on the exome array play a role in schizophrenia in multiplex families which we hypothesised might be enriched for rare, schizophrenia-associated variants.

Minor allele frequencies of rare variants were on average higher in the WAFSS compared to the DIAGRAM, suggesting that these multiplex families may harbour a greater variant burden compared to a

Conflict of interest

All other authors declare that they have no conflicts of interest.

Contributors

N.S.M., E.K.M., P.M, A.J., J.C.B., J.B., V.A.M. were involved in the design of the study.

N.S.M. carried out all analysis with input from S.V.W., S.M.A., M.D., M.L.C., B. M and J.P.R.

N.S.M. wrote the manuscript which was subsequently revised by all authors.

All authors contributed to and have approved the final manuscript.

Funding

This work was supported by National Health and Medical Research Council Australia (APP1064582 to AJ, EKM, NSM, PEM, JCB, VAM 2014–2016), and by The Society for Mental Health Research (SMHR) Postdoctoral Fellowship [to NSM, 2016] and the Medical Research Foundation, Royal Perth Hospital (Small Project Grant to NSM 2015). Support from the Cooperative Research Centre for Mental Health Australia is also gratefully acknowledged.

Acknowledgements

We would like to thank all of the participants in the Western Australian Family Study of Schizophrenia.

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