Use of gene transfer and gene delivery for controlling fertility in cats and dogs
In human gene transfer, genetic material is introduced into the human body to cure a disease. The introduced gene (DNA) may produce a required protein that is in low levels or entirely absent in the affected individual. Alternatively, the introduced gene may regulate the expression of other genes, either increasing or decreasing their expression to produce its effect.
In most cases, this treatment is used in people that are born with genetic defects that result in a “missing” protein, which can be restored by delivering the gene that codes for that protein, in essence, reversing the genetic defect. Recent successes in the field of human gene transfer include treatments for lipoprotein lipase deficiency (1), hemophilia B (2) and Leber's congenital amaurosis (LCA) (3). Currently there are hundreds of human gene transfer clinical trials underway for a wide variety of conditions, including cancer (4), HIV (5), heart disease (6) and muscular dystrophy (7). The effects of gene transfer in animal models and human clinical trials have been observed for at least eight years (8) and suggest that some of these alterations may be permanent.
The most common method of delivery of a gene into humans is using a virus that has been engineered for that purpose (Viruses used in gene transfer trials include adenovirus, lentivirus and adeno-associated virus (AAV) (9). These viral vectors deliver the gene but do not replicate, reducing the possibility of safety issues. Many types of AAV have been identified, which are able to target a wide variety of tissues (10).
In the case of animal contraception, gene transfer could be used to introduce genes into the animal, which could suppress reproduction in a variety of ways, depending on the gene used. The advantage of this method is that the transfer could produce long-term term or even permanent effects eliminating the need for re-treatment. Recently, the results of two gene transfer studies to produce contraception in female cats were reported. In both studies, adeno-associated virus (AAV) was used to deliver relevant genes. In the first, the gene encoding Mullerian Inhibiting Substance (MIS), which inhibits the activation of primordial follicles, was used. In female mice, a single treatment with AAV-MIS produced superphysiological levels of MIS, and after allowing time for the protein to take effect resulted in contraception during the 24 weeks of the study (11). The MIS gene was introduced into the thigh muscle of three female cats using AAV. No adverse effects were observed at the site of the injection and blood chemistry parameters remained normal for all cats, indicating the treatment was safe. The treatment suppressed estrus cycles in the females, but after a few months cats returned to normal, likely because the MIS gene used was not the feline gene, and cats may have mounted an immune response to it, making it ineffective (12). A follow up study is planned using the feline MIS.
Other studies targeted Gonadotropin Releasing Hormone (GnRH), which is required for both gametogenesis and sex-steroid production, and controls fertility in both males and females. The approach was to deliver a gene that codes for a monoclonal antibody against GnRH. Many studies of GnRH vaccines have shown that if serum levels of anti-GnRH antibody can be produced (usually requiring several booster injections) reproduction is suppressed. In this gene transfer approach, instead of a vaccine, the gene is delivered that codes for the antibody, which then could be expressed for a long period of time after a single injection.
A study in mice demonstrated that a gene encoding a high affinity anti-GnRH monoclonal antibody (SMI41) expressed using a recombinant AAV vector was both safe and effective in producing long-term (1 year) infertility (13). To test this approach in cats, three female cats were given an intramuscular injection of AAV containing the gene encoding this antibody. No injection site reactions or adverse effects were observed in the cats. Relative to the mouse, only slight increases in SMI41 antibody titers were seen and in two of the three cats these levels rapidly returned to baseline (14). Again, it is likely that the monoclonal antibody was seen as foreign by the cats, and inactivated due to the cats mounting an immune response.
Although neither cat study achieved long-term contraception, both demonstrated that gene transfer could be used to express proteins regulating animal reproduction. Given the promising rodent data, studies are progressing that use genes that are more feline specific, with the goal of extending the duration of effect in cats, and in the future, dogs. The potential for this method to deliver a ‘single shot’ lifetime suppression of fertility is there.
- Gaudet, D, et. al. (2013). Efficacy and long-term safety of alipogene tiparvovec (AAV1-LPLS447X) gene therapy for lipoprotein lipase deficiency: an open-label trial. Gene Ther. 20(4): 361-369.
- Doshi, BS, et. al. (2018). Gene therapy for hemophilia: what does the future hold? Ther. Adv. Hematol. 9(9): 273–293.
- Sharif, W, et. al. (2017). Leber’s congenital amaurosis and the role of gene therapy in congenital retinal disorders. Int. J. Ophthalmol. 10(3): 480-484.
- Wirth, T, et. al. (2014). Gene therapy used in cancer treatment. Biomedicines 2(2): 149-162.
- DiGusto, D, et. al. (2010). RNA-based gene therapy for HIV with lentiviral vector–modified CD34+ cells in patients undergoing transplantation for AIDS-related lymphoma. Science Trans. Med. 2(36): 36-43.
- Tilemann, L, et. al. (2012). Gene therapy for heart failure. Circ. Res. 110(5): 777-793.
- Ramos, J, and Chamberlain, JS. (2015). Gene therapy for Duchenne muscular dystrophy. Expert Opin. Orphan Drugs 3(11): 1255-1266.
- Niemeyer GP et.al. (2009). Long-term correction of inhibitor-prone hemophilia B dogs treated with liver-directed AAV2-mediated factor IX gene therapy. Blood 113(4): 796-806.
- Dunbar, C.E. et. al. (2018). Gene therapy comes of age. Science 359(6372): eaan4672.
- Naso, MF, et. al. (2017). Adeno-associated virus (AAV) as a vector for gene therapy. BioDrugs 31(4): 317-334.
- Kano, M, et. al. (2017). AMH/MIS as contraceptive that protects the ovarian reserve during chemotherapy. Proc. Natl. Acad. Sci. USA. 114(9): E1688-E1697.
- Pépin, D. Gene therapy using AAV9-delivery of an MIS transgene inhibits estrus in female cats. Presentation at the 6th International Symposium on Non-Surgical Methods of Pet Population Control, Boston, MA. View presentation recording and PowerPoint slides.
- Li, J, et. al. (2015). Vectored antibody gene delivery mediates long-term contraception. Curr. Bio. 25(19): R820-R822.
- Vansandt, L, et al. (2018, July). AAV-vectored generation of GnRH-binding immunoglobulins for non-surgical sterilization of domestic cats. Presentation at the 6th International Symposium on Non-Surgical Methods of Pet Population Control, Boston, MA. View presentation recording and PowerPoint slides.