CpG dinucleotides are known to play a crucial role in regulatory domains, affecting gene expression in their natural context. Here, we demonstrate that intragenic CpG frequency and distribution impacts transgene and genomic gene expression levels in mammalian cells. As shown for the Macrophage Inflammatory Protein 1α, de novo RNA synthesis correlates with the number of CpG dinucleotides, whereas RNA splicing, stability, nuclear export and translation are not affected by the sequence modification. Differences in chromatin accessibility in vivo and altered nucleosome positioning in vitro suggest that increased CpG levels destabilize the chromatin structure. Moreover, enriched CpG levels correlate with increased RNA polymerase II elongation rates in vivo. Interestingly, elevated CpG levels particularly at the 5′ end of the gene promote efficient transcription. We show that this is a genome-wide feature of highly expressed genes, by identifying a domain of ∼700 bp with high CpG content downstream of the transcription start site, correlating with high levels of transcription. We suggest that these 5′ CpG domains are required to distort the chromatin structure and to increase gene activity.

Successful therapeutic protein production in vitro and in vivo requires efficient and long-term transgene expression supported by optimized vector and transgene cis-regulatory sequence elements. This study provides a comparative analysis of CpG-rich, highly expressed, versus CpG-depleted, poorly expressed green fluorescent protein (GFP) reporter transgenes, transcribed by various promoters in two different cell systems. Long-term GFP expression from a defined locus in stable Chinese hamster ovary cells was clearly influenced by the combination of transgene CpG content and promoter usage, as shown by differential silencing effects on selection pressure removal among the cytomegalovirus (CMV) promoter and elongation factor (EF)-1α promoter. Whereas a high intragenic CpG content promoted local DNA methylation, CpG depletion rather accelerated transgene loss and increased the local chromatin density. On lentiviral transfer of various expression modules into epigenetically sensitive P19 embryonic pluripotent carcinoma cells, CMV promoter usage led to rapid gene silencing irrespective of the intragenic CpG content. In contrast, EF-1α promoter-controlled constructs showed delayed silencing activity and high-level transgene expression, in particular when the CpG-rich GFP reporter was used. Notably, GFP silencing in P19 cells could be prevented completely by the bidirectional, dual divergently transcribed A2UCOE (ubiquitously acting chromatin-opening element derived from the human HNRPA2B1-CBX3 locus) promoter. Because the level of GFP expression by the A2UCOE promoter was entirely unaffected by the intragenic CpG level, we suggest that A2UCOE can overcome chromatin compaction resulting from intragenic CpG depletion due to its ascribed chromatin-opening abilities. Our analyses provide insights into the interplay of the intragenic CpG content with promoter sequences and regulatory sequence elements, thus contributing toward the design of therapeutic transgene expression cassettes for future gene therapy applications.