Regulation of Embryonic Cell Fate Decision By Histone Methylation

In C. elegans, two epigenetic enzymes, the H3K4me2 demethylase, SPR-5, and the H3K9 methyltransferase, MET-2, are maternally deposited into the oocyte and cooperate to reestablish the epigenetic ground state by modifying histone methylation. Progeny of worms lacking spr-5 and met-2 accumulate high levels of H3K4me2 in the subsequent generation. This inappropriate H3K4me2 is associated with a low level of embryonic lethality, as well as a severe developmental delay and sterility in the small number of animals that reach adulthood. In addition, the progeny of spr-5;met-2 mutants improperly express germline genes in somatic tissues. We hypothesized that this improper expression could result in cell lineage defects. To interrogate how reprogramming defects may affect early embryonic development, we are taking advantage of the invariant C. elegans embryonic lineage by performing automated lineage tracing experiments in spr-5;met-2 progeny. Thus far, we observe three stages of defects. First, in early embryogenesis, the shape of the embryos is rounder, with a diameter that is greater than N2, and the mutant cells divide faster than N2. Since these defects are prior to the onset of zygotic transcription, they may be due directly to the decondensed state of chromatin associated with increased H3K4me2, or indirectly due to altered maternal RNAs and proteins. Second, after the onset of zygotic transcription, we observe many changes in cell timing, along with defects in cell migration. These defects may be due to the inappropriate somatic transcription of germline genes that we observe in the progeny of spr-5;met-2 mutants. We are currently performing single-cell RNA sequencing in the progeny of spr-5;met-2 mutants to further explore these three stages of defects.