Sharing and archiving of GA mice

Archiving should be part of every institution’s GA breeding/colony management programme, not only to optimise 3Rs practices locally, but also to facilitate the sharing of GA strains which will expand these 3Rs benefits more widely.

There are multiple benefits of archiving mouse strains. These include:

• Reducing the number of GA strains maintained on the shelf at any one time, minimising the overall number of animals used.

• Providing some insurance against loss of valuable stocks or strains caused by adverse events such as environmental disasters, disease outbreaks, genetic drift, transgene silencing and breeding failure.

• Facilitating strain sharing, providing further opportunities for reduction in animal use, and minimising the need for researchers to duplicate strains or models.

• Refining the process of strain sharing by avoiding the need to transport live animals, by shipping cryopreserved gametes or embryos instead. *

• Preventing unnecessary use of tick-over (maintenance) colonies.

• Minimising genetic drift within a strain when used appropriately, maximising data reproducibility by maintaining genetic integrity of the strain over time and ensuring that strains available for sharing are well characterised.

• Avoiding wasting animals at the end of a project or when projects need to be closed down quickly, for example due to staff shortages. This is particularly true for inbred colonies when sperm, embryos and oocytes can be cryopreserved for later use in a range of applications (e.g. microinjections, QC tests), in addition to strain recovery.

• Managing strains with welfare concerns; archiving the strain can reduce the length of time it is maintained as a live colony.

Cryopreservation and maintenance costs vary considerably from institution to institution. Nevertheless, archiving can have financial benefits, avoiding or reducing cage and genotyping costs, and decreasing the staff resource required to maintain animals.

Institutions need to develop a comprehensive archiving strategy. Cryopreserving novel strains soon after genotype and phenotype characterisation brings benefits such as securing the strain against uncontrolled genetic drift and safeguarding against problems with breeding, disease or transgene silencing. When establishing a colony there may be surplus mice from breeding not required for the maintenance of the colony, which could be of a suitable genotype to be used for cryopreservation (e.g. archiving heterozygotes produced during the establishment of a homozygous colony).

Both embryos and sperm can be successfully cryopreserved, and live strains recovered with relative ease on a routine basis. The decision of whether to cryopreserve embryos or sperm depends on a number of factors, including breeding performance of the strain, genotype complexity, local expertise and the availability of specialised equipment. More detailed information on deciding whether to archive sperm or embryos can be found in the archiving best practice page of the NC3Rs Breeding and Colony Management resource (see Archiving best practice). Links to well-established protocols for cryopreservation of different materials from different species can be found in Appendix 2.

If local storage capacity is not a limitation, all unique GA mouse strains should be part of the cryopreservation programme, including strains carrying the same genome modification but on a different genetic background. Tissues and DNA from unique strains should also be archived, as these can be used for quality control and comparison with material from future rounds of recovery of the strain. In addition, they can be useful for addressing specific questions before, or instead of recovery.

Before taking an important strain off the shelf ensure that the strain has either been cryopreserved (and check that the quality control is sufficient) or that the strain is available elsewhere on the required background (see Appendix 2 for online repository databases).

Ideally a strain should be cryopreserved at the following checkpoints:

1. When a new GA strain has been generated – archiving a new strain should only happen after a basic allele quality control has been performed (see Archiving best practice), to ensure that the desired genotype has been achieved.

2. When a new strain has been generated by crossing different strains together (also consider cryopreserving intermediate steps if a complex breeding plan is required to achieve the desired strain).

3. When a strain has been backcrossed onto a particular genetic background.

4. To replace cryopreserved stocks when depleted. For example, after thawing stocks and refreshing strains.

5. To avoid unnecessary tick-over when a colony will not be used for experiments for an extended period of time.

There are two main approaches to archiving mouse strains: archiving in-house or archiving through a centralised facility such as the European Mouse Mutant Archive (EMMA), the Centre for Animal Resources and Development (CARD) or the Mutant Mouse Resource and Research Centre (MMRRC). In-house facilities will require well-funded resources and a robust quality control programme to ensure that cryopreserved embryos and gametes are of high enough quality to enable the consistent recovery of live born offspring. All these requirements are integral to a centralised repository, which are often available at no cost to depositors. Centralised repositories also remove the burden associated with setting up frequent shipments when sharing GA mouse strains that are in high demand. In the UK these services are provided by the MRC Harwell Archive which acts as the UK node for EMMA.

Centralised cryopreservation facilities

Centralised facilities provide free embryo and sperm cryopreservation services, plus long-term storage, on the condition that archived strains can be made available to the scientific community. The intellectual property (IP) remains with the originator and, where necessary, a grace period (e.g. a two-year grace period for EMMA) can be applied which allows investigators to prevent sharing of their mouse strains before their data has been published. An example of a centralised facility is EMMA comprised of 16 archiving centres across Europe.

In-house cryopreservation facilities

Many institutions that generate and breed GA mice have in-house cryopreservation and storage facilities which retain strains for use locally. To fully achieve the benefits of cryopreservation it is important that local cryopreservation and archiving facilities are effectively managed. This includes specific staff training and rigorous record keeping.

Few would argue that the products of publicly funded research, such as GA strains, should not be accessible to the wider scientific community, and that sharing cryopreserved material avoids the stress induced by transporting live animals. However, whilst archiving is relatively straightforward, there are both real and perceived barriers to sharing GA strains. Some of these are resource driven, such as staff time, the cost implications of maintaining and disseminating cryopreserved stocks in-house, and the need to keep records and databases up to date.

A perceived barrier is the need to retain IP rights. When archives are maintained in-house, IP rights are readily managed through appropriate Material Transfer Agreements (MTAs), and the conditions of the MTA are checked before strains are shared. The same protection can also be applied when using centralised facilities such as EMMA. These centralised services ensure that fully executed copies of the originator’s MTAs are exchanged before a strain is distributed. This mechanism operates whether or not a strain was originally submitted with restricted access, and ensures all beneficial rights are retained by the originator.

A significant barrier to sharing is the dissemination of knowledge on what is available to share. Researchers can take a range of steps to ensure that their GA strains are quickly and easily accessible to the wider community. This includes registering GA strains and alleles with MGI which will also confirm the identity of the originating laboratory. As discussed earlier in this document, depositing strains with centralised repositories such as EMMA enables the strain to be cryopreserved free of charge, the only cost is that of shipping the animals or tissue.

When sharing mouse strains, to facilitate reproducible research, it is important that the information contained in the mouse passport is fully updated (see Record keeping and Appendix 1). This also highlights the benefits of acquiring strains directly from a centralised repository where genetic drift is more likely to be controlled and the background characterised.

Centralised repositories

The International Mouse Strain Resource is an umbrella organisation established  to coordinate the archiving and dissemination activities of the public archiving centres, providing links to all public archives around the world. In the UK, information on GA strains held or archived by institutions within in-house facilities is accessible to investigators who register with the Mouse Locator Network.

UK Mouse Locator Network

The Mouse Locator Network (MLN) is a mechanism by which unique locally held or archived mouse strains can be identified and shared, reducing welfare concerns and costs associated with shipping mice long distances [2]. The success of this email network relies on participation from all UK institutions with in-house archives of GA strains, as well as a comprehensive local searchable strain database at each institution, in order that strains can easily be located and that requesters have confidence in the technical details of the strain (e.g. the correct mutation, time since the last backcross). There are similar networks in other countries such as Compartir in Spain [3].

Transportation

Wherever possible, GA strains should be distributed as fresh or cryopreserved embryos or gametes in order to avoid the transport of live mice, which has notable welfare concerns. Live mice may be subject to quarantine at the receiving institution to protect the health statuses of existing stocks and often require rederivation to ensure that any newly introduced strains are clean (e.g. specific pathogen free) prior to entry into the main housing areas of a facility. However, other factors do influence this, and the optimal means of transportation will also depend on how far the animals need to travel, the mode of transport and the competence of the institution to receive live, fresh or cryopreserved stocks. If the transport of live mice cannot be avoided then they should only be carried by approved animal couriers, in accordance with LASA Guidelines for the Transport of Laboratory Animals (see Appendix 2).

Although there is a focus on mice, this guidance applies to all common laboratory animal species and guidelines and repositories are available for rat, zebrafish and Xenopus. It is possible to archive rats using embryos and sperm, and both Xenopus and zebrafish can be archived as sperm. Links to the relevant repositories, including protocols and guidance on nomenclature can be found in Appendix 2.

When cryopreserving and archiving a strain:

• Confirm the genetic modification(s) of the strain through specific genotyping assays and re-genotype the mice providing the gametes (e.g. males for sperm cryopreservation, both males and females for embryo cryopreservation) at the point of cryopreservation.

• Update the mouse passport to include the genetic background and generation details of the cryopreserved material, link to the relevant health screens, and detail the cryopreservation and thawing protocols.

• Quality control the cryopreserved material (e.g. culture to blastocyst stage, see Archiving best practice).

• Store cryopreserved stocks at two or more locations (ideally different sites) for security.

The archiving and sharing of GA strains provides significant opportunities for reduction and refinement of the use of animals in colony management. These can be maximised by implementing the following recommendations:

• Archiving should be part of every institution’s breeding and colony management programme, not only to optimise reduction and refinement practices locally, but also to facilitate the sharing of GA strains which will expand these 3Rs benefits more widely.

• Archive novel GA mouse strains as cryopreserved embryos and/or sperm in order to:
  • Safeguard stocks in the case of unplanned events.
  • Remove unused breeding stocks from the shelf (avoiding tick-over colonies).
  • Minimise genetic drift.
  • Facilitate sharing.
  • Avoid transportation of live animals.
  • Potentially minimise costs.
• Archive GA stocks locally and/or centrally in an accessible repository.
• Maintain good records of archived stocks (see Appendix 1), and include:
  • An inventory of contents.
  • Information on the stocks (including quality control and cryopreservation protocols).
  • An updated mouse passport for each strain containing welfare and scientific information.
• Register stocks with MGI when appropriate.

• All institutions using and/or creating GA strains should be accessible through the UK Mouse Locator Network or local equivalent.

• Distribute GA strains as cryopreserved material when possible.

This list has been generated from information in the mouse passport guidelines [1] and the strain submission form for EMMA (see Submission form).

We gratefully acknowledge all members of the Breeding and Colony Management working group for their contributions to this document. We thank the RSPCA for the original version of this text and for their comments on this updated version.