How Genome Editing Can Synergistically Accelerate Animal Genetic Improvement Programs

 Although genetics is not often connected with sustainability, the impact of selective breeding programs on the footprint of agriculture and food production is difficult to overstate. The number of US dairy cows dropped from a high of 25.6 million in 1944 to approximately 9 million today, despite a concurrent 1.6 fold increase in total milk production. This means a glass of milk today is associated with approximately ⅓ of the greenhouse gas (GHG) emissions it was in the 1940s. It has been estimated that historic genetic improvement in selected livestock production traits (e.g. milk/meat output, feed efficiency) has resulted in a 1% per year reduction in GHG emissions per unit animal product produced. A number of breeding methods have been employed in genetic improvement programs to achieve these gains including artificial insemination, embryo transfer, crossbreeding, and more recently genomic selection. These technologies are limited by the fact that identifying desirable new genetic variation is a matter of chance, selection may inadvertently leave behind favorable variants that existed in founder populations, and breeders may unintentionally propagate deleterious mutations because they are closely linked to advantageous variants. Genome editing offers a way to avoid these difficulties by precisely introducing desirable genetic variation into livestock breeding programs. Genome editing has already been successfully used to target disease resistance (e.g. virus resistance in pigs) and animal welfare (e.g. cattle dehorning) traits. Future applications include the production of single gender offspring in species where only one gender produces the desired product (e.g. layer hens). The regulatory status of such animals is unclear as the trigger for the mandatory, pre-market regulatory evaluation of genetically engineered (GE) animals in the United States is the use of recombinant DNA (rDNA) techniques in the genetic modification process. However, the FDA exerts regulatory authority over GE animals as new animal drugs based on the presence of the novel rDNA sequence in their genome. In the case of animals modified by genome editing, there may be no novel rDNA present in their genome, or the introduced DNA sequence may be identical to existing, naturally-occurring DNA sequences. The prospect of differential regulation for such animals as compared to those carrying indistinguishable genetic modifications obtained through conventional breeding concerns animal breeders who are eager to employ genome editing to complement traditional genetic improvement programs.