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The role of the histone variant H3.3 in epigenetic reprogramming of primordial germ cells

Final Report Summary - HISTONEGERMCELLS (The role of the histone variant H3.3 in epigenetic reprogramming of primordial germ cells)

Primordial germ cells (PGCs) are the precursors of the gametes, representing the enduring link between generations. During embryonic development of mammals, a small subset of cells is set aside to become PGCs, which subsequently undergoes extensive transcriptional and epigenetic changes to establish a unique cell state driving the development towards sperm and egg, respectively. When sperm fertilizes the egg, the resulting zygote becomes totipotent, the ability to form a whole organism. Hence, PGCs give rise to only one of two cell types, but at the same time set the molecular and epigenetic stage to establish totipotency in the zygote.

This project focussed on the fundamental questions in biology of how PGCs arise in the embryo and how they are programmed to establish their unique abilities. In recent years, it became evident that changes on the level of chromatin play a fundamental role in establishing the PGC fate. The basic unit of chromatin is the nucleosome, which consists of DNA wrapped around a defined set of histone proteins. This compact structure, which was long believed to be only important for DNA compaction, is target of numerous chemical modifications such as methylation and acetylation, which appear to be important for gene regulation.

The histone protein H3 can be replaced by the histone variant H3.3 which is mediated by the histone chaperone HIRA. Since HIRA appears to be particularly enriched in prospective PGCs, I focussed in this project on the role of H3.3 in establishing the PGC fate. Towards this goal, I performed a series of genetic and molecular studies, which showed that the incorporation of H3.3 into nucleosomes occurs mainly at regulatory DNA elements including enhancers, which are control units of gene activity. This results in the formation of ‘labile’ nucleosomes, which presumably allows the binding of transcription factors. Subsequently, transcription factors recruit co-factors to activate the transcription of a defined set of target genes. To gain further insight into the role of H3.3 in the context of PGC development, I mapped and characterised all enhancers in precursor cells and PGCs, revealing a specific set of enhancers, which are essential to specify PGCs and induce their programming towards totipotency. In precursor cells, these enhancers are inactive but exhibit a specific set of histone modifications, marking them for activation upon initiation of PGC development. Thus, our study reveals underlying mechanisms of H3.3 function at enhancers to induce and subsequently program the PGC fate.

This study addressed questions in the areas of mammalian development, germ cell biology and molecular control of gene activity. My study contributed to the understanding of the development and the properties of the germ cell lineage in mammals, which is of great importance considering the implications in disease (e.g. infertility) and relevance to the development of whole organisms. Moreover, this study has potentially general impact on stem cell research and on mechanisms to establish and maintain pluripotency.