Experimental Research Center for Infectious Diseases Laboratory of Mouse Model

Histone H3 lysine 9 (H3K9) methylation is a repressive epigenetic mark for heterochromatin formation and transcriptional silencing. Genetic loss of a H3K9 lysine methylatransferase (KMTase), G9a leads drastic reduction of H3K9me2 and exhibits embryonic lethality at mid-gestation in mice, indicating that G9a and/or G9a-mediated H3K9 methylation is essential for mouse embryogenesis. However, how the loss of G9a affects mouse embryonic development is still unclear. Since G9a suppresses tissue specific genes in ES cells via H3K9 methylation, we hypothesize that G9a may control developmentally regulated genes in mouse embryogenesis. To gain insights into the transcriptional control of G9a in vivo, we isolated RNA from embryo proper and trophoblast cells of wild-type and G9a knockout (G9a-KO) litters. We then introduced them into microarray analysis to examine the differences of expression profiles between them. Subsequently, it was revealed that specific Hox family genes (reproductive homeobox: Rhox) were ectopically reactivated in G9a-KO construct. Rhox genes, which are localized on X-chromosome, are novel Hox-family genes characterized recently (Cell, 120 p369, 2005). It is shown that Rhox genes are expressed predominantly in reproductive tissues (testis and ovary) and placenta. Rhox genes display temporal and quantitative colinear pattern of expression, however, the molecular basis which contribute the establishment and maintenance of these unique expression remains to be solved. Interestingly, it was revealed that transcriptional suppression of some Rhox genes were achieved by DNA methylation. Furthermore, the DNA methylation-dependent silencing is occurred in a lineage-specific manner (specifically occuerd in embryo proper-lineage) (Genes Dev., 20, p3382, 2006). Regarding this notion, we speculate that both H3K9 methylation and DNA methylation machineries may cooperatively regulate colinear expression of Rhox genes. We are now going to set up chromatin immunoprecipitation analysis on Rhox-loci using embryo proper and trophoblasts of wild-type and G9a-KO mice.