Genetic Sex Differences in Early Human Neuronal Development : An Investigation in Embryo Tissue and Embryonic Stem Cells

Abstract: Sex differences in the human body affect many different organs and tissues, some of them have an effect on the human brain and its development. In the developing nervous system, sex differences can bias the number or functionality of neurons, glial cells or synapses. As a result, neural networks might develop with a sex-specific bias. A number of neurodevelopmental diseases, such as Tourette-Syndrome or Attention-Deficit/Hyperactivity Disorder, show sex differences in symptoms, onset and prevalence. It seems likely that sex differences in brain development contribute to differences in neurological disease susceptibility between males and females. In my work, I am investigating sex differences in gene expression during neuronal development in human embryo brain tissue, embryonic stem cells and neural stem cells. Of particular interest for sex differences are the genes of the sex chromosomes, since a large number of X-linked genes and even some Y-linked genes are implicated in neurodevelopment.In our first study, we found that Y chromosome genes are highly expressed in fetal brain tissues and 5 X/Y homologous genes have an increased gene dosage in male samples. We suggest 6 novel long non-coding RNAs that were expressed in previously unannotated regions of the Y chromosome in male fetal brain tissue. In our second study, we identified an increased rate of proliferation in male neural stem cells but similar neuronal differentiation trajectories in cells of both sexes. An increased expression of DCX and DLG4 suggests a faster differentiation of male neural stem cells, but sex differences disappeared after 14 days. Male cells overexpressed MASH1 and RELN, markers for Cajal-Retzius neurons, and the two demethylases KDM5D and UTY. Female cells overexpressed RMST a long non-coding RNA critical for neurogenesis. In the third study, sex-biased gene expression was investigated in human embryonic stem cells during 37 days of neuronal differentiation. Male and female cell lines showed sex-biased expression of genes involved in neurodevelopment, suggesting a sex difference in differentiation trajectory. We propose 13 sex-biased candidate genes that could strongly affect neuronal development. In addition, we confirmed the gene dosage compensation of X/Y homologs escaping XCI through the Y-homolog and identified a significant expression of the Y-homologs TXLNGY and UTY after 37 days of neuronal differentiation. We have also measured a significant increase of the Y-linked genes PCDH11Y, UTY and USP9Y during differentiation. The fourth study was an investigation of sex differences in H3 methylation and acetylation marks in embryonic stem cells. We found that H3K4me3, a transcription activation mark, was enriched at promotor sites of major pluripotency genes and related pathways, in female cell lines.In conclusion, we confirm the importance of Y chromosome genes for neuronal development and show that sex differences in gene expression exist during neuronal differentiation.