Natural genotypes via genetic engineering.

Since livestock were first domesticated, humans have been selecting particular phenotypes as mating stock for each new generation. As a result, humans have been selecting for combinations of alleles that result in animals that better meet our needs: increased production, disease resistance, docility, and so forth. As purpose-bred animals became standard in animal agriculture, breeds began to diverge such that a mating produced between differently purposed breeds (dairy vs. beef, layers vs. broilers) resulted in offspring that were less desirable than individuals from either breed. As causative mutations are discovered that affect agriculturally important traits, their utility is limited if they are exclusive to a singly purposed breed. As an example, a disease resistance gene from a beef-type breed is limited to beef animals because the loss of milk production in a dairy-type breed prevents the crossbred animal from being economically viable. In PNAS, Tan et al. remedy this problem from a technical point of view (1). However, do technical views really matter? The impact of Tan et al. depends on perceptions. As regulatory agencies across the globe have unsuccessfully searched for real-world safety issues with genetically engineered (GE) food animals, and peripheral groups have predicted various forms of calamity from GE animals, genetic engineers have trudged forward. Shortly after the production of the first transgenic mammal that displayed a phenotype of potential agricultural utility (2), techniques were adapted for production of transgenic livestock (3). Over the years, several examples of transgenic animals have emerged that have agricultural potential: more efficient fish (4), sows with greater milk production for their piglets (5), mastitis-resistant cows (6), goats that produce milk that can prevent diarrhea (7), and chickens that do not propagate influenza (8). However, because there is no direct evidence that any regulatory agency on the planet is capable of approving a GE animal for food production, it has been difficult to move forward. The basic premise for regulation appears to be that any genotype produced by breeding is safe, and that any genotype produced intentionally via recombinant DNA technologies cannot be allowed to go to market. Because most GE food animals have been based thus far on the introduction of either extra copies of genes or additions of genes that could not be naturally inherited, genetic engineers have conceded that the engineered genotypes are different from historic genotypes. However, unique genotypes do not routinely trigger regulatory evaluation. The fact is that every animal produced by natural mating (excluding identical twins) also has a unique genotype, and therefore is also different from every historic genotype that has ever been consumed as food. In PNAS, Tan et al. (1) not only demonstrate that livestock genomes can be efficiently edited with precision; they also demonstrate that genotypes, which could have arisen from natural mating, can be produced efficiently and intentionally. The value of this technique in agriculture is that Fig. 1. Simplified representation of the repair of double-strand breaks. Sequence-specific endonucleases can be designed to cleave genome targets (represented by scissors). In general, cleaved sites can be rejoined to correct the double-strand break. This cycle can continue until the nuclease is no longer present or until an error occurs during repair that prevents recleavage. Exonuclease activity can truncate the free ends (bases that have been removed are shown in white). When both strands are truncated, deletions can be produced by nonhomologous end-joining (NHEJ). The change in local spacing produces a sequence that cannot be recleaved (Left pathway). If exonuclease activity removes bases from one strand, a gap can be created after NHEJ. The missing bases can be replaced by polymerase activity (Center pathway). Although this process generally proceeds with high fidelity, errors can occur (C > T error shown in red). If homologous DNA is present (shown in green), homology-directed repair (HDR) can result in the introduction of specific modifications (C > T modification shown in red). The result shown here (Right pathway) produces a product that is identical to the error shown for gap repair. After further processing through mismatch repair, the C > T substitution can be corrected or can retain the substitution. In either case, the resulting products may be recleaved. Larger modifications can be made than shown here. In addition, larger repair templates can be used to facilitate homologous recombination. Author contributions: K.D.W. wrote the paper.