In the C. elegans early embryo, the chromatin in all blastomeres initially contain the euchromatic mark, di-methylation of histone H3 on lysine 4 (H3K4me2). However, upon their birth, the primordial germ cells Z2 and Z3 uniquely lose this mark. This appears to be a conserved characteristic of primordial germ cells as Drosophila pole cells also initially lack this modification. The loss of H3K4me2 has been proposed to be necessary for protecting the totipotency of the germline through chromatin-based transcriptional repression, but the mechanism of its removal is not understood. Recently, Shi et al. demonstrated that the mammalian amine oxidase LSD1 can specifically demethylate H3K4me2 (Shi et al. 2004). C. elegans has three
lsd1 homologs :
spr-5,
amx-1 and T08D10.2. In order to ask if these genes play a role in the chromatin remodeling of Z2 and Z3, we inactivated them individually and in combination.
RNAi or genetic mutation of any of these genes individually has no affect on H3K4me2 levels in Z2 and Z3. To test for redundant roles in the chromatin remodeling of Z2 and Z3, we generated a
spr-5;
amx-1 double mutant from two existing putative null mutations. Although the double mutants display multiple pleiotropic phenotypes, we observed no immediate effects on H3meK4 levels in Z2 and Z3. However, after multiple generations the double mutant exhibits a germline mortality phenotype, in which an increasing fraction of progeny in the population grow up sterile(black sterile phenotype). Strikingly, in these later generations, the double mutants exhibit retention of H3K4me2 in Z2 and Z3, further correlating repressive chromatin with maintenance of the germ cell lineage. In addition, the multiple generations that are required for this phenotype suggest that there can be stable inheritance of epigenetic defects through the germline and that the H3K4 demethylases may play a role in resetting H3K4 methylation at each generation.