Soma-To-Germline Transition and Sexually Dimorphic Transcription in a Chromatin-Linked Neurodevelopmental Disorder

Abstract

Many male-biased neurodevelopmental disorders (NDDs), including intellectual disability (ID) and autism spectrum disorders (ASDs), are due to or associated with mutations in chromatin regulators. However, it is unclear why chromatin regulation is so essential for neurodevelopment and if this male-bias is due to male-specific functions. While this relationship may be partially explained by their influence upon neuron-specific genes, loss of some chromatin regulators can also lead to the misexpression of tissue-specific genes with unknown consequences. My dissertation work aims to explore the impact of sex and cellular identity in chromatin-linked NDDs by investigating sexually dimorphic transcription and germline gene misexpression in mice lacking lysine demethylase 5c (KDM5C). Loss of KDM5C in male and female mice and humans results in mild to severe intellectual disability and aberrant aggression, yet it is unclear why KDM5C is required for neurodevelopment as it is ubiquitously expressed. KDM5C is thought to suppress gene expression by erasing histone 3 lysine 4 di and tri-methylation (H3K4me2/3), marks enriched at active gene promoters. mRNA sequencing (mRNA-seq) of the male Kdm5c-knockout (-KO) brain surprisingly revealed widespread misexpression of germline genes that have no known function in somatic tissues like the brain. It is currently unclear how this ectopic expression of germline genes arises, if they lead to functional consequences, if this phenotype is sex-specific, and whether this apparent soma-to-germline transition contributes to Kdm5c mutant neuronal impairments.

To elucidate the shared and sex-specific consequences of Kdm5c loss upon neurodevelopment, we performed mRNA-seq in the Kdm5c mutant male and female postnatal day 6 (P6) hippocampus and cortex. We found many sex-specific gene expression changes, including genes relevant to neurodevelopment that may influence behavioral phenotypes. We additionally found many genes dysregulated in both Kdm5c mutant male and female brains, including testis and ovary-enriched germline genes. These data indicate male and female somatic cells lacking Kdm5c fail to decommission germline identity during early development. We then assessed germline gene misexpression in male and female Kdm5c mutant epiblast-like cells (EpiLCs), an in vitro model of when germline genes are typically decommissioned. Unlike in the mature brain, Kdm5c mutant EpiLCs expressed master regulators of germline identity, including Dazl, and their misexpression was exacerbated by the presence of retinoic acid. These data, in conjunction with previous work, suggest germline gene misexpression in the mature Kdm5c mutant brain is due to failure to suppress master regulators early in life. In support of this, we found additional heterozygous loss of Dazl (Kdm5c-KO;Dazl-HET) ameliorated the transcriptional dysregulation observed in the Kdm5c-KO brain, including rescue of non-germline genes. We additionally found early evidence that germline gene misexpression leads to germline-like phenotypes in Kdm5c-KO somatic cells, such as the dysregulation of genomic imprinting and transposable element expression. These preliminary phenotypes offer promising avenues by which the somatic acquisition of germline identity contributes to Kdm5c-KO neurodevelopmental impairments. Ultimately, this work provides insight into the molecular demarcation of somatic and germline identity and offers a novel route by which chromatin regulators are essential for proper neurodevelopment in both sexes.