CpG-free DNA Review
CpGs and the Immune Response
Bacterial DNA is rich in unmethylated 2’-deoxyribo (cytidine-phosphateguanosine) (CpG) dinucleotides, in contrast to mammalian DNA which contains a low frequency of CpG dinucleotides that are mostly methylated.
Unmethylated CpGs in specific sequence contexts activate the vertebrate immune system via Toll-Like Receptor (TLR) 9. TLR9 recognizes CpG DNA and initiates a signaling cascade leading to the production of proinflammatory cytokines such as IL-6 and IL-12 [1].
Plasmids used for in vivo experiments are produced in E. coli and therefore their CpGs are unmethylated and induce immune responses through this host defense mechanism, which represents a limitation for the clinical development of DNA vaccines and gene therapy vectors.
CpGs and Gene Silencing
A major limitation of gene delivery vectors for gene therapy applications is the rapid decline of transgene expression in vivo. Methylation of CpG dinucleotides within the promoter is a major factor limiting long-lasting gene expression.
Indeed, the transcriptional activity of the widely used and CpG-rich cytomegalovirus immediate early gene promoter (CMV) is highly robust but prone to inactivation within a few weeks [2]. Replacement of the CMV promoter with a cellular promoter combined with the use of a CpG-reduced backbone was shown by our team and other labs to increase the duration of expression and lower the inflammatory response [3, 4].
Recently, CpGs in plasmid DNA (pDNA) were found to enhance the clearance of PEG-coated pDNA-lipoplexes, a phenomenon that could be reduced by the use of CpG-free pDNA allowing repeated dosing [5].
DNA-induced immune response. Mice were injected i.p. with 500 μg CpG-rich or CpG-free DNA and the levels of serum TNF-α assessed 4h postinjection.
As expected CpG-free DNA (pCpGfree-mcs and genomic calf DNA) induced negligeable amounts of TNF-α whereas CpG-rich DNA (pDNA-CMV, CpG ODN1826 and E. coli DNA) induced significant amounts of TNF-α. All DNAs injected were endotoxin-free.
Construction of CpG-free Plasmids
InvivoGen has developed a new family of plasmids that are completely devoid of CpG dinucleotides, named pCpGfree. pCpGfree plasmids contain elements that either naturally lack CpG dinucleotides, were modified to remove all CpGs, or entirely synthesized such as genes encoding selectable markers or reporters. These plasmids yield high levels of transgene expression both in vitro and in vivo, and in contrast to CMV-based plasmids allow sustained expression in vivo [6].
Applications of CpG-free Plasmids
A major application of CpG-free plasmids is the treatment of inherited diseases cause by a single gene defect, such as cystic fibrosis and hemophilia, through gene therapy. Hyde et al. have reported promising results for the treatment of cystic fibrosis using a pCpGfree-derived plasmid [4]. They show that a CpG-free pDNA can achieve sustained lung transgene expression in contrast to a pDNA containing even a single CpG. These results have led to a clinical trial that has started in February 2009.
pCpGfree plamids represent valuable tools to study the effects of CpGs on gene expression using cell lines expressing TLR9 [7], as well as their effects on the innate and acquired immune systems. Furthermore, pCpGfree plasmids are also useful when analyzing promoter methylation [8, 9].
LacZ expression using a pCpGfree plasmid. C57Bl6 mice were injected with 30 μg pCpGfree-mcs (A) or pCpGfree-LacZ (B) plasmids using hydrodynamic delivery. The livers were harvested 4 days post-inoculation and stained to determine the expression of the CpG-free LacZ gene.
REFERENCES:
1. Bauer S. et al., 2001. Human TLR9 confers responsiveness to bacterial DNA via species-specific CpG motif recognition. PNAS USA. 98(16):9237-42.
2. Scharfmann R. et al., 1991. Long-term in vivo expression of retrovirus-mediated gene transfer in mouse fibroblast implants. PNAS U S A. 88(11):4626-30.
3. Yew NS. et al., 2001. High and sustained transgene expression in vivo from plasmid vectors containing a hybrid ubiquitin promoter. Mol Ther. 4(1):75-82.
4. Hyde SC. et al., 2008. CpG-free plasmids confer reduced inflammation and sustained pulmonary gene expression. Nat Biotechnol. 26(5):549-51.
5. Tagami T. et al., 2010. CpG motifs in pDNA-sequences increase anti-PEG IgM production induced by PEG-coated pDNAlipoplexes. J Control Release. 142(2):160-6
6. Hattori K. et al., 2010. Sustained Exogenous Expression of Therapeutic Levels of IFN-{gamma} Ameliorates Atopic Dermatitis in NC/Nga Mice via Th1 Polarization. J Immunol. 184(5):2729-35.
7.Yasuda K. et al., 2009. Requirement for DNA CpG content in TLR9-dependent dendritic cell activation induced by DNA-containing immune complexes. J Immunol. 183(5):3109-17.
8. Klug M & Rehli M., 2006. Functional analysis of promoter CpG methylation using a CpG-free luciferase reporter vector. Epigenetics. 1(3):127-30.
9. van Straten EM. et al., 2009. The Liver X-Receptor (LXR) gene promoter is hypermethylated in a mouse model of prenatal protein restriction. Am J Physiol Regul Integr Comp Physiol. 282(2):R275-82.