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In mammals, genomic imprinting governed by DNA methyltransferase DNMT3A and its cofactor DNMT3L is essential for functional gametes. Oocyte-specific methylation imprints are established during oocyte growth concomitant with DNMT3A/DNMT3L expression, although the mechanisms of oocyte-specific imprinting are not fully understood. To determine whether the presence of DNMT3A/DNMT3L in oocytes is sufficient for acquisition of methylation imprints, we produced transgenic mice to induce DNMT3A/DNMT3L expression prematurely in oogenesis and analyzed DNA methylation imprints. The results showed that 2- to 4-fold greater expression of DNMT3A/DNMT3L was achieved in non-growing (ng) oocytes versus fully grown oocytes derived from wild-type mice, but the analyzed imprint domains were not methylated. Thus, the presence of DNMT3A/DNMT3L in ng oocytes is insufficient for methylation imprints, and imprinted regions are resistant to DNMT3A/DNMT3L in ng oocytes. In contrast, excess DNMT3A/DNMT3L accelerated imprint acquisition at Igf2r, Lit1, Zac1 and Impact but not Snrpn and Mest in growing oocytes. Therefore, DNMT3A/DNMT3L quantity is an important factor for imprint acquisition. Transcription at imprinted domains is proposed to be involved in de novo methylation; however, transcription at Lit1, Snrpn and Impact was observed in ng oocytes. Thus, transcription cannot induce DNMT3A catalysis at imprinted regions even if DNMT3A/DNMT3L is present. However, the accelerated methylation imprints in oocytes, with the exception of Igf2r, were erased during embryogenesis. In conclusion, a sufficient amount of DNMT3A/DNMT3L and a shift from the resistant to permissive state are essential to establish oocyte-specific methylation imprints and that maintenance of the acquired DNA methylation imprints is essential for functional imprinting.

human molecular genetics online/human molecular genetics

Forced expression of DNA methyltransferases during oocyte growth accelerates the establishment of methylation imprints but not functional genomic imprinting