Page 77 - The Welfare of Cattle

54 the WeLfare of CattLe

including goats, rabbits, and chickens, GE fluorescent fish for aquarium purposes, and also some
trials using GE insects for pest control applications.

aNIMaL WeLFare aSPeCtS OF GeNe eDItING

Genome or gene editing refers to the use of site-directed nucleases to precisely introduce a
double-stranded break (DSB) at a predetermined location in the genome. The cell can repair that
DSB in one of two ways—nonhomologous end joining (NHEJ) or homologous-directed repair
(HDR) using a nucleic-acid template that includes the sequences homologous to either side of the
DSB. The outcomes of these repair processes result in random mutations or precise gene edits,
respectively (Figure 7.2).
As the name “gene editing” suggests, HDR can be employed to precisely add, delete, or replace
letters in the genetic code at the location of the break by providing the appropriate template nucleic
acid. In the case of NHEJ, although the location of the cut site is very precise, the exact change that
occurs when the DNA is repaired is random so a number of different outcomes representing minor
sequence insertions (ins) or deletions (del), termed indels, are possible.
Genome-editing technologies enable breeders to efficiently turn off a gene through NHEJ
or precisely introduce specific allelic variants. This introduction could be as simple as a single-
base-pair change or could conceptually involve entire genes or transgenes, as dictated by the HDR
template nucleic-acid sequence, that breeders would like to introduce into their target population
using editing.
Gene editing has many potential animal welfare applications. For example, it can be used to
correct diseases and disorders that have a genetic basis by altering the error that resulted in the dis-
ease phenotype. It could also be used to change a less desirable allele of a gene to a more desirable

Nuclease-induced
double-strand break
NHEJ

Deletions
Donor
template

Insertions
HDR
Variable length
indels

Precise insertion or modification

Figure 7.2 nuclease-induced double-strand breaks can be repaired by nonhomologous end joining (nheJ)
or homology-directed repair (hdr) pathways. Imprecise nheJ-mediated repair can produce
insertion and/or deletion mutations of variable length at the site of the double-strand break. hdr-
mediated repair can introduce precise point mutations or insertions from a single-stranded or
double-stranded dna donor template. Image from sander Jd, Joung JK. CrIsPr-Cas systems
for editing, regulating and targeting genomes. Nat Biotech 2014; 32:347–355.