Advancements in Avian Genome Engineering: Strategies, Primordial Germ Cell Manipulation, and Emerging Delivery Techniques

Abstract

Genome engineering tools have greatly advanced in the past 5 years, enabling scientists to make accurate modulations to the genome. Systems of delivering these new genome editing tools have been improved in addition to the development of the new tools. Genome engineering instruments are typically deposited to the in vitro fertilized single cell embryos, which are then cultured and implanted to a host animal. This holds no true in the avian species and the alternative procedures have been engineered in genome engineering in birds. The most common is in vitro culturing of primordial germ cells (PGCs), which migrate in the embryonic circulatory system to the developing gonad and colonize the gonad, eventually developing into the gonadocytes that produce sperm or ova. The population can be screened and enriched and then transposed into a recipient embryo, although in culture, the PGCs can be engineered to express new transgenes or alterations in genes. The greatest drawback of PGC culture is that culture procedures cannot be easily interpolated between the avian species; therefore, culture procedures are only reliable on a limited number of species which include the chicken. Two additional newer technologies that appear to be less taxing to a wider range of avian species are direct injection and sperm transfection-assisted gene editing (STAGE). The rationale behind the direct injection technique is the injection of the genome engineering tools into the circulatory system of the developing embryo just before the development stage, when the PGCs are migrating to the gonads. This is facilitated by the combination of the genome engineering equipment with transfection reagents and the transfection of the PGCs in vivo can be done. STAGE is a technique that involves the application of sperm transfection to implant genome engineering vectors into the recently fertilized embryo. Early signs are that each of the two methodologies can be scaled down to attempt use in other bird species than the chicken, but more research in this direction is required.