Knockout results for mouse genetics
The utility of these cells was further improved by re-activating the expression of a dominant coat colour gene (agouti). This genetic change permits the visual assessment of ES cell contribution to the germline and simplifies the breeding required to generate pure inbred lines of mice.
Historically genetic manipulation of the mouse genome has been carried out mainly in embryonic stem cells derived from the 129 strain of mice. While 129 ES cells have proven to be very robust, many genetic experiments are better carried out in C57BL/6 mice, one of the best characterised inbred strains of mice and the reference strain for the mouse genome sequence.
With the completion of the mouse genome, international programmes have started that aim to knockout more than 21,000 mouse genes in C57BL/6 inbred mice. To make this feasible, robust, highly germline competent ES cells derived from C57BL/6 are required to support large-scale mouse production and phenotyping efforts that are to follow.
In their quest for suitable C57BL/6 embryonic stem cells, the team established a male cell line (JM8) from the C57BL/6N sub-strain that contained a normal complement of chromosomes and exhibited normal undifferentiated morphology when cultured on feeder cells and on gelatine-treated plates. When early passage JM8 cells were injected into blastocysts from albino mice, pups were produced that showed high coat colour chimaerism, a sex distortion in favour of males (80 per cent) and a high proportion of chimaeras with 100 per cent contribution to both somatic tissues and the germline.
Two further selected sub-lines, JM8.F6 (feeder-dependent) and JM8.N4 (feeder-free) were then tested for their performance in high-throughput gene targeting experiments. In these experiments the clonal germline transmission rate was reliably better than 65 per cent. Critically, culturing JM8 cells under feeder-free conditions did not compromise their pluripotency.
Experience has shown that injection of C57BL/6N embryonic stem cells into albino (C57BL/6 Tyrc-Brd) blastocysts is a particularly favourable combination for germline transmission. A disadvantage of this combination, however, is that mice of a mixed genetic background are produced in testcrosses to albino mice (C57BL/6 Tyrc-Brd X C57BL/6N [F1]). Thus, to simply the breeding strategy required to obtain pure C57BL/6N inbred mice, the non-agouti mutation was repaired in JM8 cells.
The non-agouti mutation in C57BL/6 strains is due to an 11.8 Kb pair retrotransposon in the first intron of the agouti gene which abolishes expression. The team realised that restoring agouti function to C57BL/6N embryonic stem cells would allow visualisation of embryonic stem cell-derived mice by coat colour and permit the recovery of pure inbred mice from test crosses with C57BL6/N mice. The team therefore designed a targeting strategy to delete the retrotransposon from the locus and restore agouti gene function.
When they injected three of these ‘restored’ clones (JM8A) into albino blastocysts, all three produced chimeras with a high percentage agouti coat colour contribution and germline colonisation, with one clone, JM8A3, showing particularly favourable results. Since these JM8A lines are heterozygous for the corrected agouti allele, testcrosses with C57BL/6N mice yield embryonic stem cell–derived offspring with either agouti or black coats.
Finally, to assess the suitability of these JM8A3 cells for high-throughput gene targeting, the team performed targeting experiments and measured the clonal transmission rate of targeted clones as above. They found a clonal germline transmission rate of 80% from the injection of 11 targeted clones.All of the cell lines produced in this project are available on request from the Knock Out Mouse Project repository and the European Conditional Mouse Mutagenesis repository.
More information
Funding
- This work was funded by the Wellcome Trust Sanger Institute, grants from the National Institute of Health and a grant from the Sixth Framework Programme of the EU.
Participating Centres
- Wellcome Trust Sanger Institute, Wellcome Trust Genome Campus, Hinxton, Cambridge, UK
- Center for Comparative Medicine, School of Veterinary medicine, University of California, California, USA
- Brigham and Women’s Hospital, Genetics Division, Harvard Medical School, Boston, Massachusetts, USA
Publications:
Selected websites
Brigham and Women's Hospital
Brigham and Women’s Hospital is a 747-bed nonprofit teaching affiliate of Harvard Medical School and a founding member of Partners HealthCare System, an integrated health care delivery network. BWH is committed to excellence in patient care with expertise in virtually every specialty of medicine and surgery. The BWH medical preeminence dates back to 1832 and today that rich history in clinical care is coupled with its national leadership in quality improvement and patient safety initiatives, dedication to educating and training health care professionals, and strength in biomedical research. With $370M in funding and more than 500 research scientists, BWH is an acclaimed leader in clinical, basic and epidemiological investigation – including the landmark Nurses Health Study, Physicians Health Studies, and the Women’s Health Initiative.
The University of California Davis Center for Comparative Medicine (CCM)
The University of California Davis Center for Comparative Medicine (CCM) is a cooperative, interdisciplinary research and teaching center that is co-sponsored by the School of Medicine and the School of Veterinary Medicine. CCM Faculty members have academic appointments in one or both Schools.
The CCM Research Mission is to investigate the pathogenesis of human and animal disease, using animal models or naturally occurring animal diseases. Areas of emphasis include host-agent interactions during infectious disease, intervention and prevention strategies for infectious diseases, cancer, and mouse biology. CCM faculty contribute a broad range of expertise to these areas, including the disciplines of immunology, genomics, pathology, biochemistry, physiology, microbiology, molecular virology, and informatics.
The Wellcome Trust Sanger Institute
The Wellcome Trust Sanger Institute, which receives the majority of its funding from the Wellcome Trust, was founded in 1992. The Institute is responsible for the completion of the sequence of approximately one-third of the human genome as well as genomes of model organisms and more than 90 pathogen genomes. In October 2006, new funding was awarded by the Wellcome Trust to exploit the wealth of genome data now available to answer important questions about health and disease.
The Wellcome Trust
The Wellcome Trust is a global charitable foundation dedicated to achieving extraordinary improvements in human and animal health. We support the brightest minds in biomedical research and the medical humanities. Our breadth of support includes public engagement, education and the application of research to improve health. We are independent of both political and commercial interests.
