Elucidation of mechanisms of germ cell reprogramming and establishment of the procedure to control epigenome.

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To improve understanding of the low efficiency of somatic cell nuclear transfer (SCNT), we performed global transcription analysis of mouse SCNT blastocysts. Many of the downregulated genes (21/39) identified in SCNT embryos were located on the X chromosome. When the downregulation of X-linked genes in SCNT embryos was corrected by using Xist-deficient donor cells, the birth rates increased 8- to 9-fold. Thus, SCNT may be significantly improved by correcting epigenetic errors specific for SCNT.In the next series of experiments, to understand the cause of consistent downregulation in the region of A7.2 and F3 on X chromosome (XqA7.2/F3) in SCNT blastocysts, we analyzed repressive modification, dimethylation of histone H3 lysine 9 (H3K9me2), with chromatin immunoprecipitation-on-chip data from cumulus cells. The results showed that H3K9me2 was broadly enriched in XqA7.2/F3 and formed tissue-specific blocks called as large organized chromatin K9 modification (LOCKs). These data suggested that downregulation of XqA7.2/F3 was caused by failure of reprogramming of LOCKs after nuclear transfer.