The development of vaccine components or recombinant therapeutics critically depends on sustained expression of the corresponding transgene. This study aimed to determine the contribution of intragenic CpG content to expression efficiency in transiently and stably transfected mammalian cells. Based upon a humanized version of green fluorescent protein (GFP) containing 60 CpGs within its coding sequence, a CpG-depleted variant of the GFP reporter was established by carefully modulating the codon usage. Interestingly, GFP reporter activity and detectable protein amounts in stably transfected CHO and 293 cells were significantly decreased upon CpG depletion and independent from promoter usage (CMV, EF1α). The reduction in protein expression associated with CpG depletion was likewise observed for other unrelated reporter genes and was clearly reflected by a decline in mRNA copy numbers rather than translational efficiency. Moreover, decreased mRNA levels were neither due to nuclear export restrictions nor alternative splicing or mRNA instability. Rather, the intragenic CpG content influenced de novo transcriptional activity thus implying a common transcription-based mechanism of gene regulation via CpGs. Increased high CpG transcription correlated with changed nucleosomal positions in vitro albeit histone density at the two genes did not change in vivo as monitored by ChIP.
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.
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