Toward the identification of peripheral epigenetic biomarkers of schizophrenia

Abstract

Schizophrenia (SZ) is a heritable, nonmendelian, neurodevelopmental disorder in which epigenetic dysregulation of the brain genome plays a fundamental role in mediating the clinical manifestations and course of the disease. The authors recently reported that two enzymes that belong to the dynamic DNA methylation/demethylation network-DNMT (DNA methyltransferase) and TET (ten-eleven translocase; 5-hydroxycytosine translocator)-are abnormally increased in corticolimbic structures of SZ postmortem brain, suggesting a causal relationship between clinical manifestations of SZ and changes in DNA methylation and in the expression of SZ candidate genes (e.g., brain-derived neurotrophic factor [BDNF], glucocorticoid receptor [GCR], glutamic acid decarboxylase 67 [GAD67], reelin). Because the clinical manifestations of SZ typically begin with a prodrome followed by a first episode in adolescence with subsequent deterioration, it is obvious that the natural history of this disease cannot be studied only in postmortem brain. Hence, the focus is currently shifting towards the feasibility of studying epigenetic molecular signatures of SZ in blood cells. Initial studies show a significant enrichment of epigenetic changes in lymphocytes in gene networks directly relevant to psychiatric disorders. Furthermore, the expression of DNA-methylating/demethylating enzymes and SZ candidate genes such as BDNF and GCR are altered in the same direction in both brain and blood lymphocytes. The coincidence of these changes in lymphocytes and brain supports the hypothesis that common environmental or genetic risk factors are operative in altering the epigenetic components involved in orchestrating transcription of specific genes in brain and peripheral tissues. The identification of DNA methylation signatures for SZ in peripheral blood cells of subjects with genetic and clinical high risk would clearly have potential for the diagnosis of SZ early in its course and would be invaluable for initiating early intervention and individualized treatment plans.

Keywords: DNA methylation; brain-derived nerve growth factor; glucocorticoid receptor; lymphocytes.

Fig 1

Schematic representation of putative DNA methylation/demethylation pathways. TET: ten-eleven translocation protein; DNMT: DNA-methyltransferase; AID/APOBEC: activation-induced deaminase/apolipoprotein B RNA editing catalytic component; TDG: thymine DNA glycosylase ; BER: Base excision repair; C: cytosine; 5mC: 5-methylcytosine; 5hmC: 5-hydroxymethylcytosine; 5hmU: 5-hydroxymethyluracil.

Fig 2

Plots of TET1 mRNA and 5-hydroxymethylcytosine (5-hmc) levels in the inferior cortico-parietal lobule of 11 Control (CTR) subjects and 19 Psychotic (PSY) patients (10 SZ and 9 BP). Figure obtained from the data originally reported by Dong et al. (2012). TET mRNA levels were measured with quantitative real time RT-PCR using beta-actin as internal standard. 5hmc levels on genomic DNA were measured by immune-dot-blot analyses. Reprinted with permission under the Creative Commons license. Fot the original report see doi:10.1038/npp.2011.221. The horizontal lines represent median values. (Dong et al 2012).

Fig 3

Increased 5-hydroxymethylcytosine (5hmC) levels at GAD67 and BDNF Promoters in Psychotic Patients (PSY). Figure obtained from the data originally reported by Gavin et al. (2012), and by Dong et al. (2012). Binding of 5hmC and 5mC to GAD67 and BDNF IXabcd promoters was measured by immunoprecipitation using specific 5hmC and 5mC antibodies with previously published procedures (Gavin, et al. 2012). We find significantly increased 5hmC in PSY compared with CTR at both regions of the GAD67 promoter examined and a region of the BDNF IXabcd gene around the putative transcription start site (-60 to +50), but not at a location within the BDNF IXd exon (+1185 to +1305). We also find increased 5mC at the -60 to +50 region of the BDNF IXabcd gene, but not at either of the GAD67 locations examined, nor at the +1185 to +1305 site within the BDNF IXd exon. * P<0.05 vs. CRT. Reprinted with permission under the Creative Commons license. For the original report see Dong et al., 2012; doi:10.1038/npp.2011.221 and Gavin et al., 2012 ; doi 10.1038/npp.2011.221.

Fig 4

DNMT1, TET1, MBD4, APOBEC3A, GCR, BDNF mRNA levels in lymphocytes of schizophrenia patients. mRNA levels were measured with quantitative real time RT-PCR using beta-actin as internal standard. The cohort is composed of 28 schizophrenia patients and 22 control subjects. * P<0.05 vs.control subjects. Potential confounding variables (age, gender, ethnicity, cigarette smoking, medications were not a significant factor for any of the mRNA measured. Modified from the original as reported in Auta et al. (2013); http//dx.doi.org/10.1016/j.schres.2013.07.030.

Fig 5

Pearson's correlation analysis between DNMT1 and BDNF IX mRNA levels in lymphocytes of 17 SZ patients. mRNA levels were measured with quantitative real time RT-PCR using beta-actin as internal standard. The original data are reported in Auta et al. (2013); http//dx.doi.org/10.1016/j.schres.2013.07.030.