Very little is known about the role of epigenetic processes in maintenance or deregulation of mental health. Understanding these epigenetic processes is of paramount importance in light of the fact that incidence of many developmental and psychiatric disorders of the brain, such as the Autism Spectrum Disorders (ASDs), are on the rise at an alarming rate and are now major public health concerns. Recently two closely related H2A.Z hypervariants (H2A.z1 and H2A.z2) have been reported that differ from each other by only three amino acids. Despite such subtle differences, preliminary data using microarrays suggest largely independent roles of these hypervariants in neuronal gene transcription. Among the approximately 1000 genes affected by H2A.z1 or H2A.z2 knockdown, less than 5% were affected by lack of either isoform, strongly suggesting largely non-overlapping functions of these isoforms in neuronal gene regulation. One such gene, strongly regulated by H2A.z isoforms, is homerl, which, when mutated, is known to be a risk factor for schizophrenia and ASDs. Preliminary data show that mRNA levels of the short inducible homer1 isoform, homer1a, decrease after H2A.z1 depletion but increase when H2A.z2 is similarly depleted. Interestingly, gene ontology analysis of the microarray data reveals that several genes sensitive to H2A.z1, but not H2A.z2, depletion are known ASD and schizophrenia candidate genes, indicative of a possible hypervariant-specific role of H2A.Z in the etiology of these brain disorders. Thus, this proposal is designed to study these hypervariants in the context of neuronal function and synaptic plasticity.
Three specific aims are proposed. First, the role of H2A.Z hypervariants will be studied in the rapid activity-induced induction of homer1a using RNA and chromatin immuno-precipitation techniques. Second, the DNA-binding sites of both H2A.Z isoforms across the entire genome and their roles in neuronal activity-dependent and -independent transcription will be studied by deep-sequencing of DNA and RNA from H2A.z1- or H2A.z2-depleted neurons. Third, the role of H2A.Z hypervariants in synaptic and non-synaptic neuronal function will be studied by electrophysiological and calcium imaging techniques.
Genes are expressed differentially in autistic brains despite similar genetic information as in normal counterparts, suggesting a role of epigenetic mechanisms in the disease etiology. This study is designed to study how very small differences in the sequence of two closely related histone proteins, a key component of epigenetic regulation of gene expression, may produce large differences in transcription and synaptic plasticitv in neurons.