Growing oocyte-specific transcription-dependent de novo DNA methylation at the imprinted Zrsr1-DMR

Zrsr1 is a paternally expressed imprinted gene located in the first intron of Commd1, and the Zrsr1 promoter resides in a differentially methylated region (DMR) that is maternally methylated in the oocyte. However, a mechanism for the establishment of the methylation has remained obscure. Commd1 is transcribed in the opposite direction to Zrsr1 with predominant maternal expression, especially in the adult brain.

We found Commed1 transcribed through the DMR in the growing oocyte. Zrsr1-DMR methylation was abolished by the prevention of Commd1 transcription. Furthermore, methylation did not occur at the artificially unmethylated maternal Zrsr1-DMR during embryonic development when transcription through the DMR was restored in the zygote. Loss of methylation at the maternal Zrsr1-DMR resulted in biallelic Zrsr1 expression and reduced the extent of the predominant maternal expression of Commd1.

These results indicate that the establishment of methylation at Zrsr1-DMR occurs in a transcription-dependent and oocyte-specific manner and caused Zrsr1 imprinting by repressing maternal expression. The predominant maternal expression of Commd1 is likely caused by transcriptional interference by paternal Zrsr1 expression.