Disease-associated epigenetic changes in monozygotic twins discordant for schizophrenia and bipolar disorder

Emma L Dempster1^, Ruth Pidsley1^, Leonard C Schalkwyk1, Sheena Owens2, Anna Georgiades2, Fergus Kane2, Sridevi Kalidindi2, Marco Picchioni2,3, Eugenia Kravariti2, Timothea Toulopoulou2, Robin M Murray2, Jonathan Mill1*

^ these authors contributed equally to this work

Hum Mol Genet20(24): 4786-4796. PMID: 21908516 doi: 10.1093/hmg/ddr416

1 MRC Social, Genetic and Developmental Psychiatry Centre and

2 Psychosis Studies, Institute of Psychiatry, King's College London, De Crespigny Park, Denmark Hill, London. SE5 8AF. UK.

3 St Andrew's Academic Centre, Northampton, NN1 5BG. UK.

Studies of the major psychoses, schizophrenia and bipolar disorder, have traditionally focused on genetic and environmental risk factors, although more recent work has highlighted an additional role for epigenetic processes in mediating susceptibility. Since monozygotic (MZ) twins share a common DNA sequence, their study represents an ideal design for investigating the contribution of epigenetic factors to disease etiology. We performed a genome-wide analysis of DNA methylation on peripheral blood DNA samples obtained from a unique sample of MZ twin-pairs discordant for major psychosis. Numerous loci demonstrated disease-associated DNA methylation differences between twins discordant for schizophrenia and bipolar disorder individually, and together as a combined major psychosis group. Pathway analysis of our top loci highlighted a significant enrichment of epigenetic changes in biological networks and pathways directly relevant to psychiatric disorder and neurodevelopment. The top psychosis-associated differentially methylated region, significantly hypomethylated in affected twins, was located in the promoter of ST6GALNAC1 overlapping a previously reported rare genomic duplication observed in schizophrenia. The mean DNA methylation difference at this locus was 6%, but there was considerable heterogeneity between families with some twin-pairs showing a 20% difference in methylation. We subsequently assessed this region in an independent sample of postmortem brain tissue from affected individuals and controls, finding marked hypomethylation (>25%) in a subset of psychosis patients. Overall, our data provide further evidence to support a role for DNA methylation differences in mediating phenotypic differences between MZ twins and in the etiology of both schizophrenia and bipolar disorder.

SUPPLEMENTARY DATA / FIGURES

bipolar manhattan plot showing DNA methylation differences at individual CpG sites for each discordant MZ twin-pair

schizophrenia manhattan plot showing DNA methylation differences at individual CpG sites for each discordant MZ twin-pair

bipolar results [html] [excel] listing DNA methylation for each CpG included in our analysis

psychosis results [html] [excel] listing DNA methylation for each CpG included in our analysis

schizophrenia results [html] [excel] listing DNA methylation for each CpG included in our analysis