Instability of retroviral DNA methylation in embryonic stem cells.

The epigenetic status of pluripotent stem cells has been demonstrated to be extremely unstable. In our current study, we have attempted to further investigate the epigenetic dynamics of the stem cell genome by monitoring the expression of the murine stem cell virus (MSCV) retroviral vector in embryonic stem (ES) cells. Although MSCV is progressively silenced by proviral DNA methylation in ES cells, a substantial number of MSCV-transduced ES cell clones do show variegated proviral expression. This expression profile is due in part to the transient and reversible properties of MSCV silencing. However, the spontaneous reactivation rates of the silenced proviruses differ significantly between these variegated clones, indicating that the reversibility of silencing is dependent on the proviral integration site. Our current data suggest that the fidelity of DNA methylation among the genomic sequences that flank the proviral integration sites may be the determinant of this reversibility of MSCV silencing. Given that the adjoining epigenome environment affects the epigenetic regulation of proviral DNA, the reversible MSCV silencing effect is thus likely to reflect a unique and interesting feature of ES cell epigenome regulation that has not previously been revealed.