Transgenes encompassing dual-promoter CpG islands from the human TBP and HNRPA2B1 loci are resistant to heterochromatin-mediated silencing.

The genetic elements that are responsible for establishing a transcriptionally competent, open chromatin structure at a region of the genome that consists only of ubiquitously expressed, housekeeping genes are currently unknown. We demonstrate for the first time through functional analysis in stably transfected tissue culture cells that transgenes containing methylation-free CpG islands spanning the dual divergently transcribed promoters from the human TATA binding protein (TBP)-proteasome component-B1 (PSMB1) and heterogeneous nuclear ribonucleoprotein A2/B1 (HNRPA2B1)-heterochromatin protein 1Hs-gamma (chromobox homolog 3, CBX3) gene loci are sufficient to prevent transcriptional silencing and a variegated expression pattern when integrated within centromeric heterochromatin. In addition, only transgene constructs extending over both the HNRPA2B1 and the CBX3 promoters, and not the HNRPA2B1 promoter alone, were able to confer high and stable long-term EGFP reporter gene expression. These observations suggest that methylation-free CpG islands associated with dual, divergently transcribed promoters possess an independent dominant chromatin opening function and may therefore be major determinants in establishing and maintaining a region of open chromatin at housekeeping gene loci.