Cell fate is determined by gene expression, which in turn is determined by chromatin modifications that regulate access to DNA. Gametes are a highly specialized cell type, and as such, have a distinctive chromatin landscape. This necessitates that after fertilization, an epigenetic reprogramming event must occur to erase the gamete fate and allow the single-celled zygote to achieve totipotency. In this process, some chromatin modifications are erased by maternally-provided factors, while others are maintained to be propagated throughout its development. The failure to reprogram the chromatin landscape at fertilization is debilitating for development and may be a factor in human disease. A critical aspect of reprogramming is the addition and removal of methylation marks at histone protein tails, which regulate access to DNA (and therefore gene expression). In the nematode C. elegans, we have recently demonstrated that the histone 3 lysine 4 (H3K4) demethylase SPR-5 is required to remove H3K4 methylation (commonly considered a mark of active transcription), while the histone 3 lysine 9 (H3K9) methyltransferase MET-2 is subsequently required to add H3K9 methylation (considered a repressive mark). The progeny of mutants lacking both enzymes aberrantly accumulate H3K4 di-methylation at sperm genes, which correlates with the increased expression of these genes in somatic cells. These double mutant progeny have a severe developmental delay, along with defects in intestinal morphology, oogenesis, and vulva formation. In mice, the maternal loss of the SPR-5 homolog, LSD1/KDM1A, or the MET-2 homolog, SETDB1, leads to embryonic arrest by the 2-cell stage, demonstrating that histone modifiers are indispensable for vertebrate development. Furthermore, a recent clinical study showed that human patients with disruptions in LSD1 function exhibit developmental delay and craniofacial abnormalities. Together, these findings suggest a new disease paradigm where the inappropriate inheritance of histone methylation leads to developmental defects. Although chromatin reprogramming is essential for the proper regulation of development, we do not understand how the inappropriate propagation of histone methylation compromises normal development.
The aims proposed here will begin to decipher this mechanism by AIM 1) characterizing the failure to distinguish between a germline or somatic fate, AIM 2) determining the mechanism that allows the misexpression of germline genes in somatic tissues, and AIM 3) examining how the inappropriate expression of germline genes in specific tissues leads to physiological defects. Human studies have indicated that the inappropriate inheritance of histone methylation between generations might be a novel mechanism of disease. Our work will provide basic insight into this new mechanism to generate a solid foundation for later translational efforts. !
Data from mouse and humans suggest a new disease paradigm where the inappropriate inheritance of histone methylation, caused by defects in chromatin reprogramming between generations, leads to developmental abnormalities. We have developed a C. elegans model that can be used to elucidate the mechanisms underlying this new disease paradigm. The proposed studies will use our C. elegans model to determine how germline genes become ectopically expressed in somatic tissues and how this ectopic expression leads to phenotypic defects.
