Graduate School of Biomedical Sciences, Program in Neuroscience
Schizophrenia; Epigenesis, Genetic; Prefrontal Cortex; Glutamate Decarboxylase; Academic Dissertations
Schizophrenia is a neurodevelopmental disorder affecting 1% of the general population. Dysfunction of the prefrontal cortex (PFC) is associated with the etiology of schizophrenia. Moreover, a substantial deficit of GAD1 mRNA in schizophrenic PFC has been reported by different groups. However, the underlying molecular mechanisms are still unclear. Interestingly, epigenetic factors such as histone modifications and DNA methylation could be involved in the pathogenesis of schizophrenia during the maturation of the PFC. In my work, I identified potential epigenetic changes in schizophrenic PFC and developmental changes of epigenetic marks in normal human PFC. Furthermore, mouse and neuronal precursor cell models were used to confirm and investigate the molecular mechanisms of the epigenetic changes in human PFC.
My initial work examined whether chromatin immunoprecipitation can be applied to human postmortem brain. I used micrococcal nuclease (MNase)-digested chromatin instead of cross-linked and sonicated chromatin for further immunoprecipitation with specific anti-methyl histone antibodies. Surprisingly, the integrity of mono-nucleosomes was still maintained at least 30 hrs after death. Moreover, differences of histone methylation at different genomic loci were detectable and were preserved within a wide range of autolysis times and tissue pH values. Interestingly, MNase-treated chromatin is more efficient for subsequent immunoprecipitation than crosslinked and sonicated chromatin.
During the second part of my dissertation work, I profiled histone methylation at GABAergic gene loci during human prefrontal development. Moreover, a microarray analysis was used to screen which histone methyltransferase (HMT) could be involved in histone methylation during human prefrontal development. Mixed-lineage leukemia 1 (MLL1), an HMT for methylation at histone H3 lysine 4 (H3K4), appears to be the best candidate after interpreting microarray results. Indeed, decreased methylation of histone H3 lysine 4 at a subset of GABAergic gene loci occurred in Mll1 mutant mice. Interestingly, clozapine, but not haloperidol, increased levels of trimethyl H3K4 (H3K4me3) and Mll1 occupancy at the GAD1 promoter. I profiled histone methylation and gene expression for GAD1 in schizophrenics and their matched controls. Interestingly, there are deficits of GAD1 mRNA levels and GAD1 H3K4me3 in female schizophrenics. Furthermore, I was also interested in whether the changes of GAD1 chromatin structure could contribute to cortical pathology in schizophrenics with GAD1 SNPs. Remarkably, homozygous risk alleles for schizophrenia at the 5’ end of the GAD1 gene are associated with a deficit of GAD1 mRNA levels together with decreased GAD1 H3K4me3 and increased GAD1 H3K27me3 in schizophrenics.
Finally, I shifted focus on whether DNA methylation at the GAD1 promoter could contribute to a deficit of GAD1 mRNA in schizophrenia. However, no reproducible techniques are available for extracting genomic DNA specifically from GABAergic neurons in human brain. Therefore, I used an alternative approach that is based on immunoprecipitation of mononucleosomes with anti-methyl-histone antibodies differentiating between sites of active and silenced gene expression. The methylation frequencies of CpG dinucleotides at the GAD1 proximal promoter and intron 2 were determined from two chromatin fractions (H3K4me3 and H3K27me3) separately. Consistently, the proximal promoter region of GAD1 is more resistant to methylation in comparison to intron 2 of GAD1 in either open or repressive chromatin fractions. Interestingly, overall higher levels of DNA methylation were seen in repressive chromatin than in open chromatin. Surprisingly, schizophrenic subjects showed a significant decrease of DNA methylation at the GAD1 proximal promoter from repressive chromatin.
Taken together, my work has advanced our knowledge of epigenetic mechanisms in human prefrontal development and the potential link to the etiology of schizophrenia. It could eventually provide a new approach for the treatment of schizophrenia, especially in the regulation of methylation at histone H3 lysine 4.
Huang, H. Epigenetic Determinants of Altered Gene Expression in Schizophrenia: a Dissertation. (2008). University of Massachusetts Medical School. GSBS Dissertations and Theses. Paper 365. http://escholarship.umassmed.edu/gsbs_diss/365
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