Targeting of the creatine kinase M gene in embryonic stem cells using isogenic and nonisogenic vectors

Homology dependence of targeted recombination at the Chinese hamster APRT locus

Using simple linear fragments of the Chinese hamster adenine phosphoribosyltransferase (APRT) gene as targeting vectors, we have investigated the homology dependence of targeted recombination at the endogenous APRT locus in Chinese hamster ovary (CHO) cells. We have examined the effects of varying either the overall length of targeting sequence homology or the length of 5' or 3' flanking homology on both the frequency of targeted homologous recombination and the types of recombination events that are obtained. We find an exponential (logarithmic) relationship between length of APRT targeting homology and the frequency of targeted recombination at the CHO APRT locus, with the frequency of targeted recombination dependent upon both the overall length of targeting homology and the length of homology flanking each side of the target gene deletion. Although most of the APRT+ recombinants analyzed reflect simple targeted replacement or conversion of the target gene deletion, a significant fraction appear to have arisen by target gene-templated extension and correction of the targeting fragment sequences. APRT fragments with limited targeting homology flanking one side of the target gene deletion yield proportionately fewer target gene conversion events and proportionately more templated extension and vector correction events than do fragments with more substantial flanking homology.

Creatine kinase (CK) in skeletal muscle energy metabolism: a study of mouse mutants with graded reduction in muscle CK expression

To understand better the role of the creatine kinase (CK)/phosphocreatine system in muscle bioenergetics, a series of mouse mutants with subnormal muscle CK (M-CK) expression has been generated. Here we compare the phenotypes of mice deficient in M-CK (M-CK-/-) and M-CK leaky-mutant mice, which carry a targeted insertion of a hygromycin B-poly(A) resistance cassette in the second M-CK intron. Mice homozygous for this M-CK allele (M-CKI/I) have a 3-fold reduction of dimeric muscle CK enzyme activity, whereas compound heterozygotes with the null M-CK allele (M-CKI/-) display a 6-fold reduction. Unlike M-CK-/- mice, these mutants have no increased glycogen content or glycogen consumption in their fast fibers. The intermyofibrillar mitochondrial volume of these fibers is also normal, suggesting that energy transport via the CK/phosphocreatine system may function at low myofibrillar M-band CK levels. Conversely, the flux of energy through the CK reaction is still not visible by means of 31P NMR spectroscopy, indicating that relatively high levels of M-CK expression (> 34% of normal) are required to generate CK fluxes detectable by this technique. The ability of muscles to perform burst activity is also subnormal and closely correlates with the level of M-CK expression.

Creation and phenotypic analysis of alpha-lactalbumin-deficient mice

alpha-Lactalbumin is an abundant milk-specific calcium metalloprotein which has an evolutionary relationship to lysozyme. It modifies the substrate specificity of a Golgi galactosyltransferase by forming the lactose synthetase binary complex. Lactose, together with other sugars and diffusible ions, is responsible for the osmotic pressure of milk. To assess the involvement of alpha-lactalbumin in lactogenesis, alpha-lactalbumin-deficient mice were created by disrupting the gene by homologous recombination in embryonic stem cells. Homozygous mutant mice are viable and fertile but females cannot feed their offspring. They produce a highly viscous milk that pups appear to be unable to remove from the mammary gland. This milk is rich in fat and protein and is devoid of alpha-lactalbumin and lactose. The phenotype of heterozygous mice was found to be intermediate, with a 40% decrease in alpha-lactalbumin but only a 10-20% decrease in the lactose content of their milk compared with wild-type animals. These results emphasize the key function of alpha-lactalbumin in lactogenesis and open new opportunities to manipulate milk composition.

Myelination in the absence of myelin-associated glycoprotein

The hypothesis that myelin-associated glycoprotein (MAG) initiates myelin formation is based in part on observations that MAG has an adhesive role in interactions between oligodendrocytes and neurons. Furthermore, the over- or underexpression of MAG in transfected Schwann cells in vitro leads to accelerated myelination or hypomyelination, respectively. Here we test this idea by creating a null mutation in the mag locus and deriving mice that are totally deficient in MAG expression at the RNA and protein level. In adult mutant animals the degree of myelination and its compaction are normal, whereas the organization of the periaxonal region is partially impaired. Mutant animals show a subtle intention tremor. Our findings do not support the widely held view that MAG is critical for myelin formation but rather indicate that MAG is necessary for maintenance of the cytoplasmic collar and periaxonal space of myelinated fibres.

Approaching the multifaceted nature of energy metabolism: inactivation of the cytosolic creatine kinases via homologous recombination in mouse embryonic stem cells

To study the physiological role of the creatine kinase/phosphocreatine (CK/PCr) system in cells and tissues with a high and fluctuating energy demand we have concentrated on the site-directed inactivation of the B- and M-CK genes encoding the cytosolic CK protein subunits. In our approach we used homologous recombination in mouse embryonic stem (ES) cells from strain 129/Sv. Using targeting constructs based on strain 129/Sv isogenic DNA we managed to ablate the essential exons of the B-CK and M-CK genes at reasonably high frequencies. ES clones with fully disrupted B-CK and two types of M-CK gene mutations, a null (M-CK-) and leaky (M-CK1) mutation, were used to generate chimaeric mutant mice via injection in strain C57BL/6 derived blastocysts. Chimaeras with the B-CK null mutation have no overt abnormalities but failed to transmit the mutation to their offspring. For the M-CK- and M-CK1 mutations successful transmission was achieved and heterozygous and homozygous mutant mice were bred. Animals deficient in MM-CK are phenotypically normal but lack muscular burst activity. Fluxes through the CK reaction in skeletal muscle are highly impaired and fast fibres show adaptation in cellular architecture and storage of glycogen. Mice homozygous for the leaky M-CK allele, which have 3-fold reduced MM-CK activity, show normal fast fibres but CK fluxes and burst activity are still not restored to wildtype levels.

Gene targeting in human somatic cells. Complete inactivation of an interferon-inducible gene

The role, if any, of the human interferon-inducible 6-16 gene in the establishment of a cellular antiviral state is unknown. To address this problem, and as part of a wider investigation of homologous recombination (HR) and its applications in somatic cells, we have been using HR to disrupt the 6-16 gene in human cell lines [Itzhaki, J. E. & Porter, A. C. G. (1991) Nucleic Acids Res. 19, 3835-3842.] We describe here the design and use of insertion and replacement-type targeting constructs based on a promoterless bacterial gpt gene that is activated by HR with the 6-16 gene. In HeLa cells, both targeting constructs underwent extrachromosomal HR with a cotransfected plasmid carrying the 6-16 gene. In a previously targeted clone derived from the fibrosarcoma cell line HT1080, the replacement construct underwent HR with either the modified or the unmodified 6-16 allele. The latter events generated doubly disrupted (6-16-/-) clones that failed to express any detectable 6-16 messenger RNA in response to interferon. Plaque assays of infected 6-16-/- cells showed that expression of the 6-16 gene was not required for the induction by interferon of an antiviral state against encephalomyocarditis virus, semliki forest virus or cocal virus.

Gene targeting in mammalian cells by homologous recombination

Gene targeting by homologous recombination is a genetic tool that permits modification of cellular genes in a precise and predetermined fashion. The methodologies that are currently available permit gene targeting at high efficiency and fidelity. New developments promise large-scale modification of the mammalian genome using these techniques.

Skeletal muscles of mice deficient in muscle creatine kinase lack burst activity

To understand the physiological role of the creatine kinase-phosphocreatine (CK-PCr) system in muscle bioenergetics, a null mutation of the muscle CK (M-CK) gene was introduced into the germline of mice. Mutant mice show no alterations in absolute muscle force, but lack the ability to perform burst activity. Their fast-twitch fibers have an increased intermyofibrillar mitochondrial volume and an increased glycogenolytic/glycolytic potential. PCr and ATP levels are normal in resting M-CK-deficient muscles, but rates of high energy phosphate exchange between PCr and ATP are at least 20-fold reduced. Strikingly, PCr levels decline normally during muscle exercise, suggesting that M-CK-mediated conversion is not the only route for PCr utilization in active muscle.

Site-directed point mutations in embryonic stem cells: a gene-targeting tag-and-exchange strategy

Sequential gene targeting was used to introduce point mutations into one alpha 2 isoform Na,K-ATPase homolog in mouse embryonic stem (ES) cells. In the first round of targeted replacement, the gene was tagged with selectable markers by insertion of a Neor/HSV-tk gene cassette, and this event was selected for by gain of neomycin (G418) resistance. In the second targeted replacement event, the tagged genomic sequence was exchanged with a vector consisting of homologous genomic sequences carrying five site-directed nucleotide substitutions. Embryonic stem cell clones modified by exchange with the mutation vector were selected for loss of the HSV-tk gene by resistance to ganciclovir. Candidate clones were further screened and identified by polymerase chain reaction and Southern blot analysis. By this strategy, the endogenous alpha 2 isoform Na,K-ATPase gene was altered to encode two other amino acids so that the enzyme is resistant to inhibition by cardiac glycosides while maintaining its transmembrane ion-pumping function. Since the initial tagging event and the subsequent mutation-exchange event are independent of one another, a tagged cell line can be used to generate a variety of mutant lines by exchange with various mutation vectors at the tagged locus. This method should be useful for testing specific mutations introduced into the genomes of tissue culture cells and animals and for developing animal models encompassing the mutational variability of known genetic disorders.

Site-directed point mutations in embryonic stem cells: a gene-targeting tag-and-exchange strategy

Sequential gene targeting was used to introduce point mutations into one alpha 2 isoform Na,K-ATPase homolog in mouse embryonic stem (ES) cells. In the first round of targeted replacement, the gene was tagged with selectable markers by insertion of a Neor/HSV-tk gene cassette, and this event was selected for by gain of neomycin (G418) resistance. In the second targeted replacement event, the tagged genomic sequence was exchanged with a vector consisting of homologous genomic sequences carrying five site-directed nucleotide substitutions. Embryonic stem cell clones modified by exchange with the mutation vector were selected for loss of the HSV-tk gene by resistance to ganciclovir. Candidate clones were further screened and identified by polymerase chain reaction and Southern blot analysis. By this strategy, the endogenous alpha 2 isoform Na,K-ATPase gene was altered to encode two other amino acids so that the enzyme is resistant to inhibition by cardiac glycosides while maintaining its transmembrane ion-pumping function. Since the initial tagging event and the subsequent mutation-exchange event are independent of one another, a tagged cell line can be used to generate a variety of mutant lines by exchange with various mutation vectors at the tagged locus. This method should be useful for testing specific mutations introduced into the genomes of tissue culture cells and animals and for developing animal models encompassing the mutational variability of known genetic disorders.

Effect of vector topology on homologous recombination at the CHO aprt locus

The Chinese hamster ovary aprt gene was used as a model for studying the effect of vector topology on gene targeting frequency. A single recombination vector containing 2.7 kb of isogenic DNA homologous to the aprt gene was digested with eight separate restriction enzymes to generate a variety of both replacement- and insertion-type recombination substrates. The frequency of homologous recombination, normalized by cotransfection with a linearized neo' marker, was assayed by the correction of a mutant hemizygous aprt allele and was not found to reflect vector topology. Southern analysis of representative recombination products suggests that the gene targeting events occurred predominantly by double crossover/gene conversion.

Mouse genetics in the 21st century: using gene targeting to create a cornucopia of mouse mutants possessing precise genetic modifications

Over 1500 mouse mutants have been identified, but few of the genes responsible for the defects have been identified. Recent developments in the area of gene targeting are revolutionizing the field of mouse genetics and our understanding of numerous genes, including those thought to be involved in cell proliferation and differentiation. Gene targeting was developed as a method for producing a predetermined mutation in a specific endogenous gene. Advances in the design of targeting vectors and in the use of embryonic stem cells have permitted the production of numerous mutant mice with null mutations in specific genes. These mutant mice will be critical for investigating the in vivo functions of many genes that have been cloned in recent years. This review discusses a wide range of new developments in the field of gene targeting with a focus on issues to be considered by those planning to use this new technology. It also examines some of the lessons learned from recent gene targeting studies and discusses different applications of the technology that are likely to generate scores of new animal models for a wide range of human diseases.