August 24, 2013. Cortical interneuron dysfunction has been implicated in schizophrenia, particularly in the cognitive deficits of the disorder, and a key suspect is transcriptional dysregulation of GAD1. A study published July 17, 2013, in the Journal of Neuroscience, led by Schahram Akbarian of the University of Massachusetts Medical School in Worcester, takes a fresh look at the GAD1 promoter from a three-dimensional perspective and finds a 200 kb chromosomal loop formation that is decreased in postmortem prefrontal cortex tissue from subjects with the illness.
Glutamic acid decarboxylase 1 (GAD1), the gene encoding the GABA synthesizing enzyme GAD67, has received much attention in schizophrenia. The widely replicated finding of lower levels of GAD67 mRNA and protein in the illness is thought to reflect disturbances in inhibitory interneuron functioning that underlie the cognitive deficits of schizophrenia (see SRF related news story). Studies have also demonstrated that single nucleotide polymorphisms in GAD1 confer risk for schizophrenia and are associated with working memory performance (see SRF related news story). Although many hypotheses to explain these findings have been put forth, Akbarian and others have found altered chromatin structures at the GAD1 promoter, suggesting that transcriptional dysregulation may play a role in the interneuron alterations of the illness (see SRF related news story).
In the current study, first author Rahul Bharadwaj and colleagues explored the 3D architecture of the 200 kb that surrounds the GAD1 transcription start site (TSS) at human chromosome 2q31. Using a technique called chromosome conformation capture, they found evidence for a promoter/enhancer loop that is thought to allow conserved DNA elements 50 kb upstream of GAD1 to be positioned close to the TSS, thereby enhancing GAD1 expression. This loop was specific for GABA interneuron chromatin and conserved in rodents, suggesting that "the 'genome in 3D' is fully open to study in human brain and preclinical models alike," said Akbarian's team.
Activity-dependent expression of GAD1 is critical for maintaining the balance between excitation and inhibition in the cortex that may be disrupted in schizophrenia (Lau and Murthy, 2012), so Bharadwaj and colleagues examined whether the loop formation was also similarly regulated. Administration of the GABAA receptor antagonist picrotoxin (which increases neuronal activity) to mouse hippocampal cultures upregulated the loop formation and GAD1 expression, consistent with activity-dependent regulation.
When the researchers looked at the GAD1 chromosomal loop in prefrontal cortex tissue from 10 subjects with schizophrenia and seven healthy controls, they found a decrease in both the loop and GAD1 expression in those with the illness. These findings need to be replicated in future studies in larger cohorts and to determine the cause of the reduced loop formation in schizophrenia. The authors suggest that genetic factors, developmental defects, altered NMDA signaling, or an adaptive response to cortical dysfunction are all possible candidates. "In either case, a prolonged decrease of GAD1 is likely to be detrimental for higher cognitive function and the inhibitory networks in the cortex…," they conclude.—Allison A. Curley.
Bharadwaj R, Jiang Y, Mao W, Jakovcevski M, Dincer A, Krueger W, Garbett K, Whittle C, Tushir JS, Liu J, Sequeira A, Vawter MP, Gardner PD, Casaccia P, Rasmussen T, Bunney WE, Mirnics K, Futai K, Akbarian S. Conserved Chromosome 2q31 Conformations Are Associated with Transcriptional Regulation of GAD1 GABA Synthesis Enzyme and Altered in Prefrontal Cortex of Subjects with Schizophrenia. J Neurosci. 2013 Jul 17; 33(29):11839-11851. Abstract