CPG island protects Rous sarcoma virus-derived vectors integrated into nonpermissive cells from DNA methylation and transcriptional suppression

CpG islands are important in the protection of adjacent housekeeping genes from de novo DNA methylation and for keeping them in a transcriptionally ac tive state. However, little is known about their capacity to protect hetero logous genes and assure position-independent transcription of adjacent tran sgenes or retroviral vectors. To tackle this question, we have used the mou se aprt CpG island to flank a Rous sarcoma virus (RSV)-derived reporter vec tor and followed the transcriptional activity of integrated vectors. RSV is an avian retrovirus which does not replicate in mammalian cells because of several blocks at all levels of the replication cycle. Here we show that o ur RSV-derived reporter proviruses linked to the mouse aprt gene CpG island remain undermethylated and keep their transcriptional activity after stabl e transfection into both avian and nonpermissive mammalian cells. This effe ct is most likely caused by the protection from de novo methylation provide d by the CpG island and not by enhancement of the promoter strength. Our re sults are consistent with previous finding of CpG islands in proximity to a ctive but not inactive proviruses and support further investigation of the protection of the gene transfer vectors from DNA methylation.