GAD1 alternative transcripts and DNA methylation in human prefrontal cortex and hippocampus in brain development, schizophrenia

Abstract

Genetic variations and adverse environmental events in utero or shortly after birth can lead to abnormal brain development and increased risk of schizophrenia. γ-Aminobutyric acid (GABA), the major inhibitory neurotransmitter in the mammalian brain, plays a vital role in normal brain development. GABA synthesis is controlled by enzymes derived from two glutamic acid decarboxylase (GAD) genes, GAD1 and GAD2, both of which produce transcript isoforms. While the full-length GAD1 transcript (GAD67) has been implicated in the neuropathology of schizophrenia, the transcript structure of GAD1 in the human brain has not been fully characterized. In this study, with the use of RNA sequencing and PCR technologies, we report the discovery of 10 novel transcripts of GAD1 in the human brain. Expression levels of four novel GAD1 transcripts (8A, 8B, I80 and I86) showed a lifespan trajectory expression pattern that is anticorrelated with the expression of the full-length GAD1 transcript. In addition, methylation levels of two CpG loci within the putative GAD1 promoter were significantly associated with the schizophrenia-risk SNP rs3749034 and with the expression of GAD25 in dorsolateral prefrontal cortex (DLPFC). Moreover, schizophrenia patients who had completed suicide and/or were positive for nicotine exposure had significantly higher full-length GAD1 expression in the DLPFC. Alternative splicing of GAD1 and epigenetic state appear to play roles in the developmental profile of GAD1 expression and may contribute to GABA dysfunction in the PFC and hippocampus of patients with schizophrenia.

Epigenetic Modifications in Schizophrenia and Related Disorders: Molecular Scars of Environmental Exposures and Source of Phenotypic Variability

“…While exposure to environmental adversities may readily contribute to epigenetically driven deficits in GABAergic gene transcription (Figure 2), several gene variants appear to do so as well (53,97,124,125). The latter effects are consistent with the considerable genetic influence on schizophrenia risk (2,5).…”

“…While certain epigenetic modifications in schizophrenia may have a genetic origin (12,13,53,97), converging evidence suggests that a substantial portion of epigenetic alterations may be acquired through environmental factors (98)(99)(100)(101). Hence, epigenetic modifications in schizophrenia may represent molecular scars of exposures to certain environmental adversities, especially when encountered during sensitive developmental periods (102)(103)(104)(105)(106).…”

Age-related Effects of Heroin on Gene Expression in the Hippocampus and Striatum of Cynomolgus Monkeys

“…Expression level of GAD1 is decreased in the hippocampus of patients with major depression and bipolar disorder and also in cocaine addicts [48,49]. Taken together, these findings imply that even a single injection of heroin negatively affects the GABAergic system, possibly promoting drug addictive behavior and psychiatric disorders.…”

DLPFC Transcriptome Defines Two Molecular Subtypes of Schizophrenia

“…For example, the extensive pathway analysis of the CMC RNAseq data by Fromer et al [2] might be profitably re-examined analyzing the Medical Examiner-based Pittsburgh cohort and the Hospital-based cohorts separately and taking into account schizophrenia subtype. Similarly, the recent study by Tao et al on the expression of alternate GAD1 transcripts in controls and schizophrenics included many subjects in the NIMH cohort [4]. As noted above, GAD1 is one of the genes differentially expressed in the DLPFC of type 2 but not type 1 schizophrenics.…”

“…Over a period of many years, and at great effort and expense, the Clinical Brain Disorders Branch of the NIMH intramural program assembled a large collection of frozen human brains from Medical Examiner patients and conducted detailed post-mortem psychiatric reviews to establish their diagnoses. The human tissue collection and processing protocols have been previously described [3,4]. Poly-A RNA was prepared from dorsolateral prefrontal cortex and hippocampus.…”