Generation and breeding of genetically altered mice

There are a number of methods available to generate genetically altered (GA) mice. The most frequently used are pronuclear injection, embryonic stem cells, chemical mutagenesis and the new genome-editing techniques such as CRISPR and TALENS.

Over-expression or the introduction of recombinant genes (transgenics), elimination of gene expression (knock-out), including in a spatial and temporal-specific way (conditional knock-outs) and replacement of genes (knock-in, e.g. humanisation) are all well-established techniques in many laboratories.

All of the techniques are associated with animal welfare challenges which can be divided into:

• Procedures used in generating the genetic alteration per se.
• Adverse phenotypes that manifest as a result of the genetic alteration or subsequent breeding on to different background strains or intercrossing with different GA lines which may modify or alter the phenotype.

Generating GA mice and associated procedures

Many of the procedures used from the superovulation of females to provide fertilised eggs through to the collection of tissue for genotyping have the potential to cause the mice pain, suffering and distress.

Opportunities for minimising this and improving animal welfare are documented in the Sixth report of the BVAAWF/FRAME/RSPCA/UFAW Joint Working Group on Refinement (PDF, 2003).

Adverse phenotypes of GA animals

The phenotypes of GA mice can be extremely variable, ranging from no observable phenotype to one which causes significant suffering. This diversity is compounded by the unpredictable nature of novel genetic alterations and necessitates careful observation and monitoring of new/unknown strains. Although many GA mice appear phenotypically normal and have no appearance of compromised welfare it is important to note that as prey species they conceal signs of suffering.

It is good practice on receipt or generation of a new line to perform an extensive welfare assessment, as outlined in this working group report, to ensure that housing, husbandry and specialist care can be provided accordingly. General information on conducting welfare assessments is also available from the RSPCA.

This can require increased frequency of observation and monitoring until a detailed phenotypic analysis is accomplished. When GA mice are transferred to other facilities or institutions it is important to provide information on phenotypes and any specialist care required. A mouse passport has been developed to facilitate this.

MRC Harwell and the Wellcome Trust Sanger Institute Cambridge have produced a standard set of terminologies for welfare assessments of GA mouse lines.