The role of DNMT3A in neuronal function and its potential for reversing behavioural deficits in TBRS

Tatton-Brown-Rahman syndrome (TBRS) is a neurodevelopmental disorder resulting from de novo mutations in DNA methyltransferase 3A (DNMT3A). It is characterised by overgrowth, intellectual disability and behavioural deficits such as autism spectrum disorders and anxiety disorders, as well as a multitude of several different pathologies. DNMT3A is well documented to be involved in neurodevelopment, behaviour, learning and memory, but the ways in which TBRS manifests in terms of neuronal structure, functional connectivity and molecular function are largely unexplored. TBRS currently has no treatment, with current approaches focusing on improving quality of life for affected individuals. I investigated the ways in which TBRS alters brain morphology, functional connectivity, molecular dynamics and behaviour using a heterozygous knockout mouse model of Dnmt3a. I first began by characterizing the behavioural deficits in the model and discovered a sex-specific anxiety-like behaviour phenotype for female mice, but no cognitive deficits in either sex. I then assessed differences in gross brain morphology in two developmental time points and found that despite some alterations being present in adolescence, gross brain morphology returns to physiological levels in adulthood. I then investigated fine brain morphology and found that female, but not male TBRS mice showcase longer, more complex dendrites in the dorsal CA1, as well as increased spine density. I then addressed differences in functional connectivity in the dorsal CA1 and discovered increased glutamatergic activity in female, but not male TBRS mice. This finding was further assessed to not be derived from increased glutamatergic receptor subunit expression in the dorsal CA1 and was found to affect neuronal activation in response to a novel environment in female TBRS mice. Even though, glutamatergic receptor subunit expression was unaltered in ventral CA1 and the dentate gyrus, the ventral CA1 also showed evidence of increased neuronal activation in response to a novel environment. Additionally, I assessed neuronal morphology, glutamatergic receptor subunit expression and neuronal activation in the amygdala but found no differences in mice of either sex. My results serve to bridge the gap of how lifelong heterozygosity can affect neuronal morphology, functional connectivity and molecular dynamics in the context of TBRS and report a novel sex-specific behavioural effect. Taken together my results suggest a mechanism in which alterations in dorsal CA1 neuronal structure lead to increased glutamatergic transmission resulting in increased neuronal activation, which could underlie the anxiety-like behaviour observed. Finally, I addressed if reintroduction of DNMT3A2 in the dorsal hippocampus of adult animals is enough to reverse the behavioural deficits but found this is not the case. My results inform that intervention in the form of gene-reintroduction requires either both isoforms of Dnmt3a or an earlier intervention. Taken together, the results of this study serve to inform future studies investigating the manifestation of TBRS due to Dnmt3a heterozygosity by uncovering previously unknown neuronal structural, functional, molecular and behavioural manifestations of the disorder and helps guide future research by proposing different areas of interest and therapeutic avenues that will aid in fully elucidating TBRS pathology and subsequent therapeutic interventions.