CpG islands are important in the protection of adjacent housekeeping genes from de novo DNA methylation and for keeping them in a transcriptionally active state. However, little is known about their capacity to protect heterologous genes and assure position-independent transcription of adjacent transgenes or retroviral vectors. To tackle this question, we have used the mouse aprt CpG island to flank a Rous sarcoma virus (RSV)-derived reporter vector 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 our RSV-derived reporter proviruses linked to the mouse aprt gene CpG island remain undermethylated and keep their transcriptional activity after stable transfection into both avian and nonpermissive mammalian cells. This effect is most likely caused by the protection from de novo methylation provided by the CpG island and not by enhancement of the promoter strength. Our results are consistent with previous finding of CpG islands in proximity to active but not inactive proviruses and support further investigation of the protection of the gene transfer vectors from DNA methylation.

Institute of Molecular Genetics Academy of Sciences of the Czech Republic Flemingovo nám 2 CZ 16637 Prague 6 Czech Republic

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Transcriptional provirus silencing as a crosstalk of de novo DNA methylation and epigenomic features at the integration site

Proviruses selected for high and stable expression of transduced genes accumulate in broadly transcribed genome areas

The core element of a CpG island protects avian sarcoma and leukosis virus-derived vectors from transcriptional silencing