Structure and allele-specific methylation state of mouse Copg2

Abstract

Copg2 located on mouse chromosome 6 is known to be imprinted partially in mouse brain. To understand the genomic structure of mouse Copg2, genomic clones of Copg2 were screened from mouse 129 genomic DNA library. DNA sequence revealed that Copg2 contains 24 exons and encompasses ＞40 kb of genomic DNA. Primer extension analysis showed that transcription starts at a site 61 bp upstream from the translation start codon. Copg2 exists as a single copy in the mouse genome and shows ubiquitous expression with the highest level in testis. A CpG island spans from the promoter to the region surrounding exon 1 and part of intron 1. Methylation analysis on the CpG island was performed by genomic Southern hybridization and bisulfite sequencing. F1 interspecific mice (C57BL/6J females×M. spretus males) were used to determine imprinting status and the methylation status at the CpG island. Interestingly, Copg2 was not imprinted in the F1 hybrid in spite of the existence of differentially methylated region (DMR) at the CpG island. Therefore, paternal-specific methylation of the mouse Copg2 is not accompanied by allele-specific expression of Copg2. These results suggest that COPG2 shows species- and strain-dependent relaxation of imprinting and raises a possibility of finding additional imprinted gene around this locus. A noble antisense transcript found in the promoter region of Copg2 showed mono-allelic expression. Although its size, initiation site, and function remain to be elucidated, the expression of the antisense transcript may be closely related to the differentially methylation pattern in the CpG island of Copg2.