Abstract
Retroviral vectors are silenced in embryonic stem (ES) cells by epigenetic mechanisms whose kinetics are poorly understood. We show here that a 3'D4Z4 insulator directs retroviral expression with persistent but variable expression for up to 5 months. Combining an internal 3'D4Z4 with HS4 insulators in the long terminal repeats (LTRs) shows that these elements cooperate, and defines the first retroviral vector that fully escapes long-term silencing. Using FLP recombinase to induce deletion of 3'D4Z4 from the provirus in ES cell clones, we established retroviral silencing at many but not all integration sites. This finding shows that 3'D4Z4 does not target retrovirus integration into favorable epigenomic domains but rather protects the transgene from silencing. Chromatin analyses demonstrate that 3'D4Z4 blocks the spread of heterochromatin marks including DNA methylation and repressive histone modifications such as H3K9 methylation. In addition, our deletion system reveals three distinct kinetic classes of silencing (rapid, gradual or not silenced), in which multiple epigenetic pathways participate in silencing at different integration sites. We conclude that vectors with both 3'D4Z4 and HS4 insulator elements fully block silencing, and may have unprecedented utility for gene transfer applications that require long-term gene expression in pluripotent stem (PS) cells.