Functions of the histone linker protein H1.4 in neuronal development and intellectual disability

Abstract

In the following document, I will describe three distinct projects which together provide information to better understand the genetic basis of autism spectrum disorder (ASD) as well as the genetic and molecular basis of Rahman syndrome. Specifically, the majority of my thesis focuses on how the gene H1-4 can contribute to intellectual disability (ID). First, in Chapter 2, I will provide an early project in my graduate career, which provides an illustration of the need to better understand the relationship between epigenetics and ASD. My work studying the DNA demethylase, TET1, highlights the effects that epigenetic dysregulation can have on ASD-like behaviors. Utilizing a novel Tet1 knockout mouse, we have shown that lack of TET1 results in reduction of the Oxytocin receptor (Oxtr) gene transcript. These Tet1 knockout animals display abnormal splicing of the Oxtr transcript, increased aggressive behaviors, decreased maternal care, and altered firing in neurons. This work sheds light on how two ASD implicated genes (TET1 and OXTR) interact to generate abnormal gene products and functions that relate to ASD-like behaviors. Second, in Chapters 3 and 4, I will present evidence to better understand how frameshift mutations in the histone linker, H1-4, contribute to pathogenicity of Rahman syndrome (RMNS), a syndromic form of ID. My work here utilized a combination of multiple approaches including review of clinical data, use of RMNS patient derived cells, and ex vivo neuronal cellular cultures to determine the effects of frameshift H1.4 (H1.4fsRMNS) on cellular transcription and function. I investigated transcriptional dysregulation in RMNS patient derived cells, as well as the effect of exogenous expression of H1.4fsRMNS on neuronal structure and function. My work in these chapters provide the first evidence for aberrant function of H1.4fsRMNS in post mitotic neurons of the brain. These data also help to better understand the role this mutant protein plays in creating an ID related phenotype. Of specific interest to this thesis, literature searches for these experiments highlight a gap in the literature with regards to what is known about the function of wild-type (WT) H1.4 in the brain. Finally, in Chapter 5, I characterize the expression of H1.4 as well as H1.2 and .3 throughout murine brain development. I show that these “replication-dependent” histones are actually expressed and present in mature post-mitotic neurons. I utilize a combination of transcriptional investigation, microscopy, top-down proteomics, and endogenously tagged H1f4 mouse model to systemically track expression of H1.4 throughout development, in multiple brain regions, and within a single, specific cell type (the CGN). My characterization of H1s during murine brain development together affirm that there is little known about distinct expression and function of individual somatic replication-dependent H1 proteins. Specifically, in a spatiotemporal specific manner in the brain. My work is the first to systemically interrogate the expression and binding of H1.4 in vivo in mature neurons. Further, I show that expression and post-translational modification of individual H1 subtypes is developmentally dependent and may have an effect on gene expression. My thesis sheds light on the molecular function of H1.4 within the brain. My work is useful for not only laying the foundation of how frameshift mutant H1.4fsRMNS leads to RMNS. But also, in understanding the function of the WT endogenous H1.4 protein. Together, this data lays the foundation to better understand the relationship between H1.4 function and brain development. Better understanding the expression of both the WT and frameshift mutant H1.4fsRMNS, will be useful in providing a basis for future studies and potential gene therapy strategies for RMNS.