Plasmid pRSVL persisted and expressed luciferase for at least 19 months in mouse skeletal muscle after intramuscular injection. Other injected plasmids also stably expressed long-term suggesting that any plasmid DNA could stably persist and express in muscle. Plasmid DNA was demonstrated by quantitative PCR in some of the muscle DNA samples for at least 19 months after injection. The methylation pattern of the plasmid DNA remained in its bacterial form indicating that the foreign DNA did not replicate in the muscle cells. The electroporation of total cellular DNA from injected muscles into bacteria indicated that the plasmid DNA was extrachromosomal. Chromosomal integration of plasmid DNA was searched for by electroporating the injected muscle DNA into bacteria after restriction enzyme digestion and ligation. No plasmids containing plasmid/chromosome junctions were observed in over 1800 colonies examined. Lack of integration increases the theoretical safety of this gene transfer technique. Long-term stability of plasmid DNA in muscle indicates that muscle is an attractive target tissue for the introduction of extrachromosomal plasmid or viral DNA for the purpose of gene therapy.

The nuclear entry of exogenous DNA in mammalian cells is critical for efficient gene transfer. A novel technique was developed for the covalent attachment of cationic peptides to double-stranded DNA using a cyclo-propapyrroloindole cross-linker. The attachment of the SV40 large T antigen nuclear localization signal peptide induced the nuclear accumulation of the conjugated DNA in digitonin-permeabilized cells via the classical pathway for the nuclear transport of karyophilic proteins. Increased nuclear uptake of the modified DNA, however, did not occur after it was microinjected into the cytoplasm of cultured cells. This demonstration that the covalent modification of DNA with a signal peptide alters its behavior and interaction with other cellular factors portends the potential of DNA vector chemistry to enhance the efficiency of cellular gene transfer.

Our previous studies have demonstrated that the intraarterial delivery of naked plasmid DNA leads to high levels of foreign gene expression throughout the muscles of the targeted limb. Although the procedure was first developed in rats and then extended to nonhuman primates, the present study has successfully implemented the procedure in normal mice and the mdx mouse model for Duchenne muscular dystrophy. After intraarterial delivery of plasmid DNA expressing the normal, full-length mouse dystrophin from either the cytomegalovirus promoter or a muscle-specific human desmin gene control region, mdx mouse muscle stably expressed dystrophin in 1-5% of the myofibers of the injected hind limb for at least 6 months. This expression generated an antibody response but no apparent cellular response.

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Our previous study indicated that normal serum contains complement-fixing natural IgM antibodies reacting with a large variety of randomly generated protein carboxy-termini. Here we show that the "carboxy-terminal" IgM (C-IgM) antibodies specifically react with short peptide sequences located immediately at the protein carboxy-terminus. The specificity of C-IgM-peptide interactions is tentatively defined by three to four amino acid residues. All carboxy-terminal peptides in a large peptide library apparently react with C-IgM antibodies. Immobilized synthetic peptides also react with C-IgM antibodies. No interaction of C-IgM antibodies with internal peptide sequences has been observed. C-IgM antibodies are present in germ-free and in athymic adult rats and are absent in newborn rats. The natural ubiquity of protein carboxy-termini in biological structures suggests that C-IgM could play an important role in antigen clearance and presentation to the immune system. From a practical viewpoint, the recognition of carboxy-terminal peptides by complement-fixing C-IgM antibodies has profound implications for the use of peptide- and protein-derivatized delivery vehicles and artificial materials.

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Transfection competent complexes were assembled using a three component system. The constituents of the basic system were plasmid DNA, cationic DNA binding protein (NLS-H1) and anionic liposomes (dioleoyl phosphatidylethanolamine (DOPE) or phosphatidylserine (PS)). In contrast to cationic liposome/DNA binary complexes, all of the DNA in these ternary complexes was sensitive to DNase I degradation and ethidium bromide intercalation. Transmission electron microscopy revealed that these ternary complexes formed unique structures in which the DNA was located either on the outside of individual liposomes or bridging two or more liposomes. This provides evidence that plasmid DNA encapsulation is not essential for transfection competency.