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

The effects of polymorphisms on human gene targeting

DNA mismatches that occur between vector homology arms and chromosomal target sequences reduce gene targeting frequencies in several species; however, this has not been reported in human cells. Here we demonstrate that even a single mismatched base pair can significantly decrease human gene targeting frequencies. In addition, we show that homology arm polymorphisms can be used to direct allele-specific targeting or to improve unfavorable vector designs that introduce deletions.

Direct cloning of isogenic murine DNA in yeast and relevance of isogenicity for targeting in embryonic stem cells

Efficient gene targeting in embryonic stem cells requires that modifying DNA sequences are identical to those in the targeted chromosomal locus. Yet, there is a paucity of isogenic genomic clones for human cell lines and PCR amplification cannot be used in many mutation-sensitive applications. Here, we describe a novel method for the direct cloning of genomic DNA into a targeting vector, pRTVIR, using oligonucleotide-directed homologous recombination in yeast. We demonstrate the applicability of the method by constructing functional targeting vectors for mammalian genes Uhrf1 and Gfap. Whereas the isogenic targeting of the gene Uhrf1 showed a substantial increase in targeting efficiency compared to non-isogenic DNA in mouse E14 cells, E14-derived DNA performed better than the isogenic DNA in JM8 cells for both Uhrf1 and Gfap. Analysis of 70 C57BL/6-derived targeting vectors electroporated in JM8 and E14 cell lines in parallel showed a clear dependence on isogenicity for targeting, but for three genes isogenic DNA was found to be inhibitory. In summary, this study provides a straightforward methodological approach for the direct generation of isogenic gene targeting vectors.

Hypomorphic mice

The use of genetically engineered mice has become a standard approach in order to study the physiological contribution of genes in a variety of life-science disciplines. Classical and conditional gene-targeting methods aimed at generating knock-out mice that lack gene products have been useful, but may be limited in their scope. If the gene of interest is essential for cell viability, little insight can be gained into the in vivo function of these genes. A hypomorphic approach, utilizing many of the same methods employed for traditional gene targeting, allows one to disrupt the function of genes to a lesser degree and bypass the lethality caused by many gene mutations. The purpose of this chapter is to introduce the concepts behind how hypomorphic alleles impede normal genetic function and provide information necessary to construct a targeting vector successfully for use in ES cells to generate ultimately mice with lower than normal amounts of an endogenous protein of interest.

Generation of human embryonic stem cell reporter knock-in lines by homologous recombination

This unit describes a series of technical procedures to form clonal human embryonic stem cell (hESC) lines that are genetically modified by homologous recombination. To develop a reporter knock-in hESC line, a vector is configured to contain a reporter gene adjacent to a positive selection cassette. These core elements are flanked by homologous sequences that, following electroporation into hESCs, promote the integration of the vector into the appropriate genomic locus. The positive selection cassette facilitates the enrichment and isolation of genetically modified hESC colonies that are then screened by PCR to identify correctly targeted lines. The selection cassette, flanked by loxP sites, is subsequently excised from the positively targeted hESCs via the transient expression of Cre recombinase. This is necessary because the continued presence of the cassette may interfere with the regulation of the reporter or neighboring genes. Finally, these genetically modified hESCs are clonally isolated using single-cell deposition flow cytometry. Reporter knock-in hESC lines are valuable tools that allow easy and rapid identification and isolation of specific hESC derivatives.

Human somatic cell gene targeting

Human somatic cell gene targeting provides a powerful tool to scientists studying gene function in cultured human cells. This technology allows scientists to knock out genes in human somatic cells in a fashion analogous to the creation of knockout mice. Human somatic cell gene targeting brings the power of genetics to the study of human genes in human cells by making it possible to compare cells or individuals that are genetically identical except for a single, well-defined mutation in an endogenous gene. These modified cells can be studied both in vitro and in vivo. This unit presents protocols for human somatic cell gene targeting.

Gene targeting methods for studying nuclear transport factors in mice

Genetically engineered mice have been widely used to study gene function in a variety of life-science disciplines. However, the use of animal models in the field of nucleocytoplasmic transport has been limited, mainly because disruption of individual transport factors is expected to deregulate basic biological processes so severely that the embryo dies at an early stage in development. Early studies in which transport factors were knocked out in mice have confirmed this notion. Recent work has shown that hypomorphic alleles are very useful for studying essential genes at the organismal level. In combination with wild-type and knockout alleles, hypomorphic alleles can be used to generate a series of mice in which the expression of a protein is gradually reduced from normal to zero. Within this series, there is often an allelic combination that yields liveborn mice that develop overt phenotypes as they age, and that can be used to study the physiological relevance of the protein. In this article, we present an efficient method for generating an allelic series of mice. It involves the use of a multi-purpose gene-targeting vector that produces a hypomorphic allele that can also be converted into conditional and knockout alleles within the mouse. This method saves time and provides flexibility in terms of choosing the most appropriate model for studying components of the nucleocytoplasmic machinery at the organismal level.

Pooling and PCR as a method to combat low frequency gene targeting in mouse embryonic stem cells

The introduction of germ line modifications by gene targeting in mouse embryonic stem (ES) cells has proven a fundamental technology to relate genes to mammalian biology. Critical aspects required for successful gene targeting have traditionally been experimental enhancements that increase the frequency or detection of homologous recombination within ES cells; however, the utilization of such methods may still result in the failed isolation of a positively targeted ES cell clone. In this study, we discuss the current enhancement methods and describe an ES cell pooling strategy that maximizes the ability to detect properly targeted ES cells regardless of an inherent low targeting efficiency. The sensitivity required to detect correctly targeted events out of a pool of ES cell clones is provided by polymerase chain reaction (PCR), and only those pools containing positives need to be expanded and screened to find individually targeted clones. This method made it possible to identify targeted clones from a screen of approximately 2,300 ES cell colonies by performing only 123 PCR reactions. This technically streamlined approach bypasses the need to troubleshoot and re-engineer an existing targeting construct that is functionally suitable despite its low targeting frequency.

Fen1does not control somatic hypermutability of the (CTG)n· (CAG)nrepeat in a knock-in mouse model for DM1

The mechanism of trinucleotide repeat expansion, an important cause of neuromuscular and neurodegenerative diseases, is poorly understood. We report here on the study of the role of flap endonuclease 1 (Fen1), a structure-specific nuclease with both 5' flap endonuclease and 5'-3' exonuclease activity, in the somatic hypermutability of the (CTG)(n)*(CAG)(n) repeat of the DMPK gene in a mouse model for myotonic dystrophy type 1 (DM1). By intercrossing mice with Fen1 deficiency with transgenics with a DM1 (CTG)(n)*(CAG)(n) repeat (where 104n110), we demonstrate that Fen1 is not essential for faithful maintenance of this repeat in early embryonic cleavage divisions until the blastocyst stage. Additionally, we found that the frequency of somatic DM1 (CTG)(n)*(CAG)(n) repeat instability was essentially unaltered in mice with Fen1 haploinsufficiency up to 1.5 years of age. Based on these findings, we propose that Fen1, despite its role in DNA repair and replication, is not primarily involved in maintaining stability at the DM1 locus.

Modeling for Lesch-Nyhan disease by gene targeting in human embryonic stem cells

Human embryonic stem (ES) cells are pluripotent cells derived from blastocyst-stage embryos. It has been suggested that these cells should play a major role in transplantation medicine and be able to advance our knowledge in human embryology. We propose that these cells should also play a vital role in the creation of models of human disorders. This aspect would be most valuable where animal models failed to faithfully recapitulate the human phenotype. Lesch-Nyhan disease is caused by a mutation in the HPRT1 gene that triggers an overproduction of uric acid, causing gout-like symptoms and urinary stones, in addition to neurological disorders. Due to biochemical differences between humans and rodents, a mouse lacking the HPRT expression will fail to accumulate uric acid. In this research we demonstrate a model for Lesch-Nyhan disease by mutating the HPRT1 gene in human ES cells using homologous recombination. We have verified the mutation in the HPRT1 allele at the DNA and RNA levels. By using selection media, we show that HPRT1 activity is abolished in the mutant cells, and the HPRT1-cells show a higher rate of uric acid accumulation than the wild-type cells. Therefore, these cells recapitulate to some extent the characteristics of Lesch-Nyhan syndrome and can help researchers further investigate this genetic disease and analyze drugs that will prevent the onset of its symptoms. We therefore suggest that human diseases may be modeled using human ES cells.

Mild impairment of motor nerve repair in mice lacking PTP-BL tyrosine phosphatase activity

Mouse PTP-BL is a large, nontransmembrane protein tyrosine phosphatase of unclear physiological function that consists of a KIND domain, a FERM domain, five PDZ domains, and a COOH-terminal catalytic PTP domain. PTP-BL and its human ortholog PTP-BAS have been proposed to play a role in the regulation of microfilament dynamics, cytokinesis, apoptosis, and neurite outgrowth. To investigate the biological function of PTP-BL enzyme activity, we have generated mice that lack the PTP-BL PTP moiety. These PTP-BLΔP/ΔPmice are viable and fertile and do not present overt morphological alterations. Although PTP-BL is expressed in most hematopoietic cell lineages, no alterations of thymocyte development in PTP-BLΔP/ΔPmice could be detected. Sciatic nerve lesioning revealed that sensory nerve recovery is unaltered in these mice. In contrast, a very mild but significant impairment of motor nerve repair was observed. Our findings exclude an essential role for PTP-BL as a phosphotyrosine phosphatase and rather are in line with a role as scaffolding or anchoring molecule.

Gene Therapy: The Potential Applicability of Gene Transfer Technology to the Human Germline

The theoretical possibility of applying gene transfer methodologies to the human germline is explored. Transgenic methods for genetically manipulating embryos may in principle be applied to humans. In particular, microinjection of retroviral vector appears to hold the greatest promise, with transgenic primates already obtained from this approach. Sperm-mediated gene transfer offers potentially the easiest route to the human germline, however the requisite methodology is presently underdeveloped. Nuclear transfer (cloning) offers an alternative approach to germline genetic modification, however there are major health concerns associated with current nuclear transfer methods. It is concluded that human germline gene therapy remains for all practical purposes a future possibility that must await significant and important advances in gene transfer technology.

Presence of (phospho)creatine in developing and adult skeletal muscle of mice without mitochondrial and cytosolic muscle creatine kinase isoforms

We assessed the relationship between phosphocreatine (PCr) and creatine (Cr) content and creatine kinase (CK) activity in skeletal muscle of mice. The PCr and total Cr (tCr) concentrations, as well as CK activity, in hindlimb muscles of mice, with or without the cytosolic and mitochondrial isoforms of muscle creatine kinase (wild-type or CK--/-- mice), were determined by in vivo magnetic resonance (MR) spectroscopy and by biochemical means during postnatal growth and adulthood. In wild-type muscle the [tCr], PCr/ATP ratio and CK activity increased rapidly in the first 4-7 weeks. Remarkably, CK--/-- mice showed a similar increase in the PCr/ATP ratio during the first month in the presence of only minor brain-type BB-CK activity. Uptake of Cr in muscle was seemingly unrelated to CK activity as tCr increased in the same way in the muscles of both mouse types. At older ages the PCr/ATP ratio decreased in CK--/-- muscles, in contrast to wild-type where it still slowly increased, whereas [tCr] was similar for muscle of both mouse types. Using a new in vivo MR approach with application of [4-13C]Cr, a lower PCr/tCr ratio was also observed in CK--/-- muscle. From these data it follows that in vivo global ATP levels at rest are similar in the presence or absence of CK. Although Cr could still be converted to PCr in mature CK--/-- muscle, the immediate availability of PCr decreased, and PCr became partly inconvertible at older ages. Apparently, catalysis of the CK reaction by BB-CK, although significant in muscles of newborn mice, gradually declines to very low levels in adulthood. Part or all of this BB-CK may arise from satellite cells fusing with myotubes, a process that is most active during the first months of life. Finally, our observation that the MR and chemical assessment of muscle [tCr] and PCr/tCr ratio were similar for all mice does not support the existence of a significant MR-invisible or immobile pool of Cr, with a role for CK in this phenomenon.

A chromosomal position effect on gene targeting in human cells

We describe gene targeting experiments involving a human cell line (RAN10) containing, in addition to its endogenous alleles, two ectopic alleles of the interferon-inducible gene 6-16. The frequency of gene targeting at one of the ectopic 6-16 alleles (H3.7) was 34-fold greater than the combined frequency of gene targeting involving endogenous 6-16 alleles in RAN10. Preference for H3.7 was maintained when the target loci in RAN10 were transcriptionally activated by interferon. Despite the 34-fold preference for H3.7, the absolute gene targeting efficiency in RAN10 was only 3-fold higher than in the parental HT1080 cell line. These data suggest that different alleles can compete with each other, and perhaps with non-homologous loci, in a step which is necessary, but not normally rate-limiting, for gene targeting. The efficiency of this step can therefore be more sensitive to chromosomal position effects than the rate-determining steps for gene targeting. The nature of the position effects involved remains unknown but does not correlate with transcription status, which in our system has a very modest influence on the frequency of gene targeting. In summary, our work unequivocally identifies a position effect on gene targeting in human cells.

Manipulating the mammalian genome by homologous recombination

Gene targeting in mammalian cells has proven invaluable in biotechnology, in studies of gene structure and function, and in understanding chromosome dynamics. It also offers a potential tool for gene-therapeutic applications. Two limitations constrain the current technology: the low rate of homologous recombination in mammalian cells and the high rate of random (nontargeted) integration of the vector DNA. Here we consider possible ways to overcome these limitations within the framework of our present understanding of recombination mechanisms and machinery. Several studies suggest that transient alteration of the levels of recombination proteins, by overexpression or interference with expression, may be able to increase homologous recombination or decrease random integration, and we present a list of candidate genes. We consider potentially beneficial modifications to the vector DNA and discuss the effects of methods of DNA delivery on targeting efficiency. Finally, we present work showing that gene-specific DNA damage can stimulate local homologous recombination, and we discuss recent results with two general methodologies--chimeric nucleases and triplex-forming oligonucleotides--for stimulating recombination in cells.

Adenylate kinase 1 gene deletion disrupts muscle energetic economy despite metabolic rearrangement

Efficient cellular energy homeostasis is a critical determinant of muscle performance, providing evolutionary advantages responsible for species survival. Phosphotransfer reactions, which couple ATP production and utilization, are thought to play a central role in this process. Here, we provide evidence that genetic disruption of AK1-catalyzed ss-phosphoryl transfer in mice decreases the potential of myofibers to sustain nucleotide ratios despite up-regulation of high-energy phosphoryl flux through glycolytic, guanylate and creatine kinase phosphotransfer pathways. A maintained contractile performance of AK1-deficient muscles was associated with higher ATP turnover rate and larger amounts of ATP consumed per contraction. Metabolic stress further aggravated the energetic cost in AK1(-/-) muscles. Thus, AK1-catalyzed phosphotransfer is essential in the maintenance of cellular energetic economy, enabling skeletal muscle to perform at the lowest metabolic cost.

1H and (31)P magnetization transfer studies of hindleg muscle in wild-type and creatine kinase-deficient mice

The authors addressed the hypothesis that interactions with creatine kinase (CK) play a role in the off-resonance magnetization transfer (MT) effect of creatine in skeletal muscle. Toward that aim, (1)H MT studies were done on hindleg muscle in wild-type mice and in transgenic mice, lacking cytoplasmic CK and/or mitochondrial CK. The (1)H MT effect was essentially identical in wild-type muscle and the two single CK knock-out muscles, while moderately decreased in tissue lacking both CK isoforms. (31)P-NMR showed no off-resonance (31)P MT effect in skeletal muscle for PCr in any of the mice, while the enzymatic CK reaction flux was circa 0.2-0.3 sec(-1) in the wild-type muscle and in muscle deficient in mitochondrial CK. The CK enzyme flux was negligible in the other two CK knock-outs. These data suggest that CK plays a minor role in the (1)H MT effect of creatine. Irrespective of the underlying mechanism the creatine MT phenomenon probably has no significant consequences for the thermodynamic availability of total creatine to the CK reaction.

Effects of ischemia on skeletal muscle energy metabolism in mice lacking creatine kinase monitored by in vivo 31P nuclear magnetic resonance spectroscopy

The aim of this study was to provide in vivo experimental evidence for the proposed biological significance of the creatine kinase (CK)/phosphocreatine (PCr) system in the energy metabolism of skeletal muscle. As a test system we compared hindlimb muscle of knockout mice lacking the cytosolic M-type (M-CK(-)/(-)), the mitochondrial ScMit-type (ScCKmit(-)/(-)), or both creatine kinase isoenzymes (CK(-)/(-)), and in vivo 31P-NMR was used to monitor metabolic responses during and after an ischemic period. Although single mutants show some subtle specific abnormalities, in general their metabolic responses appear similar to wild type, in contrast to CK(-)/(-) double mutants. This implies that presence of one CK isoform is both necessary and sufficient for the system to be functional in meeting ischemic stress conditions. The global ATP buffering role of the CK/PCr system became apparent in a 30% decline of ATP in the CK(-)/(-) mice during ischemia. Both M-CK(-)/(-) and CK(-)/(-) showed increased phosphomonoester levels during ischemia, most likely reflecting adaptation to a more efficient utilization of glycogenolysis. While in M-CK(-)/(-) muscle PCr can still be hydrolyzed to provide Pi for this process, in CK(-)/(-) muscle only Pi from ATP breakdown is available and Pi levels increase much more slowly. The experiments also revealed that the system plays a role in maintaining pH levels; the CK(-)/(-) mice showed a faster and more pronounced acidification (pH = 6.6) than muscles of wild type and single knockout mutants (pH = 6.9).

Alternative splicing of CD45 pre-mRNA is uniquely obedient to conditions in lymphoid cells

The leucocyte common antigen (LCA or CD45) consists of various isoforms generated by alternative splicing of variable exons 4, 5 and 6 (or A, B and C). To follow splicing behaviour in different cell types we developed a human CD45 mini-gene and analysed its expression in transfected cell lines and transgenic mouse tissues. In Cos-1, HeLa and 3T3 cells we found distinct expression patterns which could only be modulated slightly by protein synthesis inhibitors but not by variation in culture conditions like pH, serum concentration and cell density, or by stimulation with phorbol ester (TPA). In all non-lymphoid transgenic tissues the default splicing pattern (CD45R0) was found, while the expression profile in lymphoid cells, where all eight isoforms are present, mimics that of the endogenous mouse LCA gene products. Next, to examine the factors involved in alternative exon use we analysed the expression pattern of members of the family of SR proteins, well known splicing regulators with arginine/serine-rich (R/S) domains. Cell lines expressed variable levels of SRp75, SRp30 and SRp20 and constant amounts of SRp40. Mouse tissues expressed large amounts of SRp75, SRp55 and SRp40, additional expression of SRp30s and SRp20 was restricted to lymphoid tissues. Therefore, SRp30 and SRp20 may contribute to forming the appropriate cellular conditions for alternative use of CD45 exons 4-6 in the haematopoietic compartment.

A vital role for glycosphingolipid synthesis during development and differentiation

Glycosphingolipids (GSLs) are believed to be integral for the dynamics of many cell membrane events, including cellular interactions, signaling, and trafficking. We have investigated their roles in development and differentiation by eliminating the major synthesis pathway of GSLs through targeted disruption of the Ugcg gene encoding glucosylceramide synthase. In the absence of GSL synthesis, embryogenesis proceeded well into gastrulation with differentiation into primitive germ layers and patterning of the embryo but was abruptly halted by a major apoptotic process. In vivo, embryonic stem cells deficient in GSL synthesis were again able to differentiate into endodermal, mesodermal, and ectodermal derivatives but were strikingly deficient in their ability to form well differentiated tissues. In vitro, however, hematopoietic and neuronal differentiation could be induced. The results demonstrate that the synthesis of GSL structures is essential for embryonic development and for the differentiation of some tissues and support the concept that GSLs are involved in crucial cell interactions mediating these processes.

Use of Embryonic and Somatic Cells for Production of Transgenic Domestic Animals

In contrast to the highly developed genetic modification systems available for manipulating the mouse genome, at this time only simple gain of function modifications can be undertaken in domestic species. Clearly, the greatest barrier to gene targeting in domestic species has been the unavailability of cell lines that can be modified in vitro and still be used to generate a living organism. In the mouse, the embryonic stem (ES) cells and embryonic germ (EG) cells have fulfilled that role. While the nuclear transfer procedures have solved this problem in sheep and cattle, in swine ES and EG cells are still needed. In addition, targeting in domestic species is affected by the need to develop targeting constructs containing isogenic DNA regions. As a result, it is necessary to isolate the gene of interest, sequence required regions, and develop isogenic targeting constructs by technologies such as long-range PCR. On the positive side, enrichment protocols developed in the mouse can be applied to domestic species, thus facilitating the identification of correctly modified cell lines. Hence, progress in mammalian cloning, the development of EG cell lines, and advances in gene targeting presently allows the introduction of precise genetic modifications into the domestic animal genome.

Prx1 and Prx2 in skeletogenesis: roles in the craniofacial region, inner ear and limbs

Prx1 and Prx2 are closely related paired-class homeobox genes that are expressed in very similar patterns predominantly in mesenchyme. Prx1 loss-of-function mutants show skeletal defects in skull, limbs and vertebral column (Martin, J. F., Bradley, A. and Olson, E. N. (1995) Genes Dev. 9, 1237–1249). We report here that mice in which Prx2 is inactivated by a lacZ insertion had no skeletal defects, whereas Prx1/Prx2 double mutants showed many novel abnormalities in addition to an aggravation of the Prx1 single mutant phenotype. We found defects in external, middle and inner ear, reduction or loss of skull bones, a reduced and sometimes cleft mandible, and limb abnormalities including postaxial polydactyly and bent zeugopods. A single, or no incisor was present in the lower jaw, and ectopic expression of Fgf8 and Pax9 was found medially in the mandibular arch. A novel method to detect β-galactosidase activity in hydroxyethylmethacrylate sections allowed detailed analysis of Prx2 expression in affected structures. Our results suggest a role for Prx genes in mediating epitheliomesenchymal interactions in inner ear and lower jaw. In addition, Prx1 and Prx2 are involved in interactions between perichondrium and chondrocytes that regulate their proliferation or differentiation in the bones of the zeugopods.

Using knockout and transgenic mice to study neurophysiology and behavior

Reverse genetics, in which detailed knowledge of a gene of interest permits in vivo modification of its expression or function, provides a powerful method for examining the physiological relevance of any protein. Transgenic and knockout mouse models are particularly useful for studies of complex neurobiological problems. The primary aims of this review are to familiarize the nonspecialist with the techniques and limitations of mouse mutagenesis, to describe new technologies that may overcome these limitations, and to illustrate, using representative examples from the literature, some of the ways in which genetically altered mice have been used to analyze central nervous system function. The goal is to provide the information necessary to evaluate critically studies in which mutant mice have been used to study neurobiological problems.

Embryonic stem cells with a disrupted GD3 synthase gene undergo neuronal differentiation in the absence of b-series gangliosides

The dramatic changes in the expression of GD3 and other b-series gangliosides during neuronal development and morphogenesis have led to a widely held belief that these gangliosides may be necessary for neuronal differentiation. To determine directly if GD3 and b-series gangliosides are required for neuronal differentiation, we have produced embryonic stem (ES) cells with both alleles of the GD3 synthase gene (GD3S) disrupted by successive rounds of gene targeting. The double-targeted ES cells were deficient in GD3 synthase activity and did not synthesize b-series gangliosides. Despite this deficit, the GD3S(-/-) ES cells could be induced to undergo neuronal differentiation. Neuronally differentiated wild-type and GD3S(-/-) ES cells formed a complex neurite network around the embryoid bodies. Both types of neuronal cells expressed the axon-specific cytoskeletal proteins, neurofilament-M, and growth-associated protein-43 as well as the dendrite-specific marker, microtubule-associated protein-2. Our results indicate that GD3 synthase and b-series gangliosides are not necessary for the neuronal differentiation of uncommitted precursor cells.

Cytoarchitectural and metabolic adaptations in muscles with mitochondrial and cytosolic creatine kinase deficiencies

We have blocked creatine kinase (CK) mediated phosphocreatine (PCr) <==> ATP transphosphorylation in mitochondria and cytosol of skeletal muscle by knocking out the genes for the mitochondrial (ScCKmit) and the cytosolic (M-CK) CK isoforms in mice. Animals which carry single or double mutations, if kept and tested under standard laboratory conditions, have surprisingly mild changes in muscle physiology. Strenuous ex vivo conditions were necessary to reveal that MM-CK absence in single and double mutants leads to a partial loss of tetanic force output. Single ScCKmit deficiency has no noticeable effects but in combination the mutations cause slowing of the relaxation rate. Importantly, our studies revealed that there is metabolic and cytoarchitectural adaptation to CK defects in energy metabolism. The effects involve mutation type-dependent alterations in the levels of AMP, IMP, glycogen and phosphomonoesters, changes in activity of metabolic enzymes like AMP-deaminase, alterations in mitochondrial volume and contractile protein (MHC isoform) profiles, and a hyperproliferation of the terminal cysternae of the SR (in tubular aggregates). This suggests that there is a compensatory resiliency of loss-of-function and redirection of flux distributions in the metabolic network for cellular energy in our mutants.

Modification and repression of genes expressed in the mammary gland using gene targeting and other technologies

Transgenic experiments using oocyte micro-injection methodology are often performed in order to target expression of a foreign gene in a specific tissue or, to a lesser extent, to study the regulation of gene expression. However, the isolation of embryonic stem cells in mice and the development of antisense and ribozyme technologies have allowed more subtle alterations of endogenous gene expression to be achieved. The mammary gland is one of the few organs able to undergo several cycles of development, differentiation and apoptosis through complex multihormonal regulation during adult life. It is thus an attractive model to assess the in vivo function of some genes potentially involved in these mechanisms, either by silencing them or by partially repressing their expression. Furthermore, such alterations of gene expression have also been performed for more applied objectives such as the modification of milk composition for nutritional and technological purposes. This review will describe the experimental procedures used toward these aims and the results already obtained in this field. Some potential new targets will be suggested.

Impaired mammary gland development and function in mice lacking LAR receptor-like tyrosine phosphatase activity

The LAR receptor-like protein tyrosine phosphatase is composed of two intracellular tyrosine phosphatase domains and a cell adhesion molecule-like extracellular region containing three immunoglubulin-like domains in combination with eight fibronectin type-III-like repeats. This architecture suggests that LAR may function in cellular signalling by the regulation of tyrosine phosphorylation through cell-cell or cell-matrix interactions. We used gene targeting in mouse embryonic stem cells to generate mice lacking sequences encoding both LAR phosphatase domains. Northern blot analysis of various tissues revealed the presence of a truncated LAR mRNA lacking the cytoplasmic tyrosine phosphatase domains and indicated that this LAR mutation is not accompanied by obvious changes in the expression levels of one of the LAR-like receptor tyrosine phosphatases PTPdelta or PTPsigma. LAR-/- mice develop and grow normally and display no appreciable histological tissue abnormalities. However, upon breeding we observed an abnormal neonatal death rate for pups from LAR-/- females. Mammary glands of LAR-/- females were incapable of delivering milk due to an impaired terminal differentiation of alveoli at late pregnancy. As a result, the glands failed to switch to a lactational state and showed a rapid involution postpartum. In wild-type mice, LAR expression is regulated during pregnancy reaching maximum levels around Day 16 of gestation. Taken together, these findings suggest an important role for LAR-mediated signalling in mammary gland development and function.

Genealogy of the 129 inbred strains: 129/SvJ is a contaminated inbred strain

The 129 mouse is the most widely used strain in gene targeting experiments. However, numerous substrains exist with demonstrable physiological differences. In this study a set of simple sequence length polymorphisms (SSLPs) was used to determine the relatedness of selected 129 substrains. 129/SvJ was significantly different from the other 129 substrains and is more accurately classified as a recombinant congenic strain (129cX/Sv), being derived from 129/Sv and an unknown strain. This mixed genetic background could complicate gene targeting experiments by reducing homologous recombination efficiency when constructs and ES cells are not derived from the same 129 substrain. Additionally, discrepancies due to different genetic backgrounds may arise when comparing phenotypes of genes targeted in different 129-derived ES cell lines.

Mouse choroideremia gene mutation causes photoreceptor cell degeneration and is not transmitted through the female germline

Choroideremia (CHM) is an X-linked progressive eye disorder which results from defects in the human Rab escort protein-1 (REP-1) gene. A gene targeting approach was used to disrupt the mouse chm/rep-1 gene. Chimeric males transmitted the mutated gene to their carrier daughters but, surprisingly, these heterozygous females had neither affected male nor carrier female offspring. The targeted rep-1 allele was detectable, however, in male as well as female blastocyst stage embryos isolated from a heterozygous mother. Thus, disruption of the rep-1 gene gives rise to lethality in male embryos; in female embryos it is only lethal if the mutation is of maternal origin. This observation can be explained by preferential inactivation of the paternal X chromosome in murine extraembryonic membranes suggesting that expression of the rep-1 gene is essential in these tissues. In both heterozygous females and chimeras the rep-1 mutation causes photoreceptor cell degeneration. Consequently, conditional rescue of the embryonic lethal phenotype of the rep-1 mutation may provide a faithful mouse model for choroideremia.

Combined myofibrillar and mitochondrial creatine kinase deficiency impairs mouse diaphragm isotonic function

Creatine kinase (CK) is an enzyme central to cellular high-energy phosphate metabolism in muscle. To characterize the physiological role of CK in respiratory muscle during dynamic contractions, we compared the force-velocity relationships, power, and work output characteristics of the diaphragm (Dia) from mice with combined myofibrillar and sarcomeric mitochondrial CK deficiency (CK[-/-]) with CK-sufficient controls (Ctl). Maximum velocity of shortening was significantly lower in CK[-/-] Dia (14.1 +/- 0.9 Lo/s, where Lo is optimal fiber length) compared with Ctl Dia (17.5 +/- 1.1 Lo/s) (P < 0.01). Maximum power was obtained at 0.4-0.5 tetanic force in both groups; absolute maximum power (2,293 +/- 138 W/m2) and work (201 +/- 9 J/m2) were lower in CK[-/-] Dia compared with Ctl Dia (2,744 +/- 146 W/m2 and 284 +/- 26 J/m2, respectively) (P < 0.05). The ability of CK[-/-] Dia to sustain shortening during repetitive isotonic activation (75 Hz, 330-ms duration repeated each second at 0.4 tetanic force load) was markedly impaired, with CK[-/-] Dia power and work declining to zero by 37 +/- 4 s, compared with 61 +/- 5 s in Ctl Dia. We conclude that combined myofibrillar and sarcomeric mitochondrial CK deficiency profoundly impairs Dia power and work output, underscoring the functional importance of CK during dynamic contractions in skeletal muscle.

ES cell cycle rates affect gene targeting frequencies

We have investigated the gene targeting frequency at the hprt locus in a range of embryonic stem cell lines selected for variations in cell cycle parameters. Our results show that targeting frequency varies with cell line by as much as 12-fold between nonisogenic lines and 3-fold between isogenic lines and that a nonisogenic line can support homologous recombination events by up to 21-fold more frequently than an isogenic line. This variation is consistent with both insertion and replacement vectors. These results can be explained by an inverse linear correlation of targeting frequencies with cell doubling times. Additionally, by reducing serum concentration in the culture medium the mean cell doubling time for R1 ES cells can be increased from 11.4 to 15.7 h, with a subsequent 15-fold decrease in gene targeting frequency. This change fits the correlation found for the different nonisogenic cell lines. Our observations have important implications when performing gene targeting experiments and explain some of the variation noted between experiments.

Inhibition of ubiquitous mitochondrial creatine kinase expression in HeLa cells by an antisense oligodeoxynucleotide

Antisense strategy has been used to inhibit the synthesis of the human ubiquitous mitochondrial creatine kinase (Mi-CK) in HeLa cells. Indeed, elevated levels of Mi-CK in the serum of some cancer patients seem to be an adverse pronostic indicator (for refs see Wallimann T and Hemmer W, Mol Cell Biochem 133/134: 193-220, 1994). A phosphorothioate oligonucleotide, complementary to the second intron-exon splice junction site of the human ubiquitous Mi-CK pre-mRNA was shown to inhibit Mi-CK synthesis by 80% without modifying F1-ATPase beta subunit expression or hampering HeLa cell growth. This inhibition was correlated to a decrease of the Mi-CK mRNA level that could be determined quantitatively after amplification of reverse transcription products (RT) in the presence of varying concentrations of internal standard competitors. This study also demonstrated that the Mi-CK mRNA copy number was much lower in HeLa cells than that of the cytosolic creatine kinase isoform, B-CK. The antisense-induced decrease in Mi-CK mRNA and protein level influenced neither the expression of B-CK which uses up the phosphocreatine produced by Mi-CK during the phosphocreatine shuttle, nor that of another nuclear encoded mitochondrial gene, the F1-ATPase subunit which provides ATP to Mi-CK. In conclusion, an elevated Mi-CK expression is not required for cancer cell growth and therefore, Mi-CK is not a significant limiting factor for the growth of the cancer cells which contain it. In addition, a decrease in Mi-CK synthesis does not induce a change in the expression of mitochondrial F1-ATPase which provides ATP to Mi-CK or in the expression of cytosolic B-CK which is involved together with Mi-CK in the phosphocreatine shuttle. Therefore, the use of the phosphocreatine shuttle as a process mandatory for the active growth of some cancer cells is questioned.

Targeted gene disruption: applications in neurobiology

In classical gene inactivation approaches by homologous recombination in embryonic stem cells, the resulting knockout mice are genotypically homogeneous. The inactivation of a gene in the complete organism may sometimes lead to early embryonic lethality. The observation that bacterial recombinases can drive site-specific recombination in mammalian cells has allowed for spatiotemporally controlled genetic modifications. Thus, conditional gene inactivation can be achieved in a specific subset of cells, leaving the rest of the organism genotypically unchanged. Another application of bacterial recombinases is the generation of exon-specific knockout mice, allowing for the analysis of the role of tissue-specific splice variants. A combination of the above-mentioned bacterial recombinase technique with inducible promoter systems permits the investigator to choose precisely the onset of recombination. An extension of the above-mentioned techniques is the combination of the bacterial recombinase technique with adenovirus-based technology, which would open vast possibilities of tissue-specific genetic modifications in a controlled time frame.

Efficient targeting of the IL-4 gene in a BALB/c embryonic stem cell line

Embryonic stem (ES) cell lines have been derived from the inner cell mass of day 3.5 blastocysts of the inbred mouse strain BALB/cJ. Twenty-three lines were karyotyped and three were selected for injection into C57BL/6J host blastocysts. Two of the three lines, BALB/c-I and BALB/c-IV, produced germ-line chimaeras. The suitability of the BALB/c-I line for gene targeting experiments was tested by transfecting a targeting construct for the interleukin-4 (IL-4) gene. Transfected BALB/c-I cells exhibited efficient homologous recombination of the targeting vector and transmitted the induced mutation through the germline. This newly-characterized BALB/c-ES cell line thus provides an alternative to the traditional 129-derived and the recently described C57BL/6 embryonic stem cell lines, and will be useful in disrupting genes involved in the immune system. Furthermore, the genetically pure BALB/c IL-4 deficient mice will aid in studying the role of IL-4 in several infectious disease models in which the BALB/c mouse is a susceptible strain.

G2 arrest and impaired nucleocytoplasmic transport in mouse embryos lacking the proto-oncogene CAN/Nup214

The vertebrate nucleopore complex (NPC) is a 125 MDa multiprotein assembly that mediates nucleocytoplasmic transport. One of its components, CAN/Nup214, is an FXFG repeat-containing protein known to be involved in myeloid leukemia in humans. We have devised a powerful genetic approach, using maternally derived protein in murine null embryos, to show that CAN/ Nup214 is essential for NPC function in vivo. We demonstrate that CAN-/- mouse embryonic stem (ES) cells are not viable and that CAN-/- embryos die in utero between 4.0 and 4.5 days postcoitum, following the depletion of their CAN from maternal sources. In 3.5-day-old mutant embryos, cultured in vitro, progressive depletion of CAN leads to cell cycle arrest in G2 phase, and eventually to blastocoel collapse, impaired NLS-mediated protein uptake and nuclear accumulation of polyadenylated RNA. Remarkably, these defective CAN-depleted embryos do not display any gross morphological abnormalities in their nuclear envelopes or NPCs. Our data suggest that CAN is critical to cell cycle progression and required for both nuclear protein import and mRNA export.

Abnormal myotonic dystrophy protein kinase levels produce only mild myopathy in mice

Myotonic dystrophy (DM) is commonly associated with CTG repeat expansions within the gene for DM-protein kinase (DMPK). The effect of altered expression levels of DMPK, which is ubiquitously expressed in all muscle cell lineages during development, was examined by disrupting the endogenous Dmpk gene and overexpressing a normal human DMPK transgene in mice. Nullizygous (-/-) mice showed only inconsistent and minor size changes in head and neck muscle fibres at older age, animals with the highest DMPK transgene expression showed hypertrophic cardiomyopathy and enhanced neonatal mortality. However, both models lack other frequent DM symptoms including the fibre-type dependent atrophy, myotonia, cataract and male-infertility. These results strengthen the contention that simple loss- or gain-of-expression of DMPK is not the only crucial requirement for development of the disease.

AML1, the target of multiple chromosomal translocations in human leukemia, is essential for normal fetal liver hematopoiesis

The AML1-CBF beta transcription factor is the most frequent target of chromosomal rearrangements in human leukemia. To investigate its normal function, we generated mice lacking AML1. Embryos with homozygous mutations in AML1 showed normal morphogenesis and yolk sac-derived erythropoiesis, but lacked fetal liver hematopoiesis and died around E12.5. Sequentially targeted AML1-/-es cell retained their capacity to differentiate into primitive erythroid cells in vitro; however, no myeloid or erythroid progenitors of definitive hematopoietic origin were detected in either the yolk sac or fetal livers of mutant embryos. Moreover, this hematopoietic defect was intrinsic to the stem cells in that AML1-/-ES cells failed to contribute to hematopoiesis in chimeric animals. These results suggest that AML1-regulated target genes are essential for definitive hematopoiesis of all lineages.

An animal model for Norrie disease (ND): gene targeting of the mouse ND gene

In order to elucidate the cellular and molecular processes which are involved in Norrie disease (ND), we have used gene targeting technology to generate ND mutant mice. The murine homologue of the ND gene was cloned and shown to encode a polypeptide that shares 94% of the amino acid sequence with its human counterpart. RNA in situ hybridization revealed expression in retina, brain and the olfactory bulb and epithelium of 2 week old mice. Hemizygous mice carrying a replacement mutation in exon 2 of the ND gene developed retrolental structures in the vitreous body and showed an overall disorganization of the retinal ganglion cell layer. The outer plexiform layer disappears occasionally, resulting in a juxtaposed inner and outer nuclear layer. At the same regions, the outer segments of the photoreceptor cell layer are no longer present. These ocular findings are consistent with observations in ND patients and the generated mouse line provides a faithful model for study of early pathogenic events in this severe X-linked recessive neurological disorder.

Muscle creatine kinase-deficient mice. II. Cardiac and skeletal muscles exhibit tissue-specific adaptation of the mitochondrial function

Functional properties of in situ mitochondria and of mitochondrial creatine kinase were studied in saponin-skinned fibers taken from normal and M-creatine kinase-deficient mice. In control animals, apparent Km values of mitochondrial respiration for ADP in cardiac (ventricular) and slow-twitch (soleus) muscles (137 +/- 16 microM and 209 +/- 10 microM, respectively) were manyfold higher than that in fast-twitch (gastrocnemius) muscle (7.5 +/- 0.5 microM). Creatine substantially decreased the Km values only in cardiac and slow-twitch muscles (73 +/- 11 microM and 131 +/- 21 microM, respectively). As compared to control, in situ mitochondria in transgenic ventricular and slow-twitch muscles showed two times lower Km values for ADP, and the presence of creatine only slightly decreased the Km values. In mutant fast-twitch muscle, a decrease rather than increase in mitochondrial sensitivity to ADP occurred, but creatine still had no effect. Furthermore, in these muscles, relatively low oxidative capacity was considerably elevated. It is suggested that in the mutant mice, impairment of energy transport function in ventricular and slow-twitch muscles is compensated by a facilitation of adenine nucleotide transportation between mitochondria and cellular ATPases; in fast-twitch muscle, mainly energy buffering function is depressed, and that is overcome by an increase in energy-producing potential.

Cre-mediated site-specific translocation between nonhomologous mouse chromosomes

Chromosome rearrangements, such as large deletions, inversions, or translocations, mediate migration of large DNA segments within or between chromosomes, which can have major effects on cellular genetic control. A method for chromosome manipulation would be very useful for studying the consequences of large-scale DNA rearrangements in mammalian cells or animals. With the use of the Cre-loxP recombination system of bacteriophage P1, we induced a site-specific translocation between the Dek gene on chromosome 13 and the Can gene on chromosome 2 in mouse embryonic stem cells. The estimated frequency of Cre-mediated translocation between the nonhomologous mouse chromosomes is approximately 1 in 1200-2400 embryonic stem cells expressing Cre recombinase. These results demonstrate the feasibility of site-specific recombination systems for chromosome manipulation in mammalian cells in vivo, breaking ground for chromosome engineering.

Mice deficient in ubiquitous mitochondrial creatine kinase are viable and fertile

Creatine kinase isoenzymes (CK; EC 2.7.3.2) play a pivotal role in high-energy phosphoryl metabolism through subcellular compartmentation of the creatine-phosphate < = > ATP conversion reaction. In mouse, protein subunits constituting the ubiquitous mitochondrial CK (UbCKmit) and cytosolic B-CK isoforms are co-expressed in various cells and tissues with high and fluctuating energy demands such as brain, retina, smooth muscle, uterus, placenta and spermatozoa. Using targeted mutagenesis via homologous recombination in embryonic stem cells, we have generated mice that are deficient in UbCKmit subunits. These mice are viable and show no overt physical or behavioural abnormalities. Matings between UbCKmit-deficient mice produced normal numbers of offspring, showing that both females and males are completely fertile. Motility patterns of isolated spermatozoa were analyzed and found not to be impaired by absence of UbCKmit. From these results we conclude that UbCKmit is not essential for mouse viability, fertility, maintenance of pregnancy, or delivery.

Mouse ubiquitous mitochondrial creatine kinase: gene organization and consequences from inactivation in mouse embryonic stem cells

Individual members of the creatine kinase isoenzyme family (CK; EC 2.7.3.2), which play a prominent role in energy homeostasis, are encoded by four separate nuclear genes. We have isolated and characterized the complete mouse UbCKmit gene, the product of which is ubiquitously expressed and is located in the intermembrane space of mitochondria. Transcription of this gene is initiated at multiple adjacent positions and the region immediately upstream of these sites shares many features with genes encoding housekeeping proteins. These include a high G/C content, absence of TATA and CCAAT motifs, and presence of SP1 and AP2 recognition sequences. In addition, a binding site for HIP1, hormone-responsive elements, and three Mt-motifs, known as boxes shared between nuclear genes encoding mitochondrial proteins, were identified. To study the functional role of the UbCKmit protein, we have inactivated both UbCKmit alleles in mouse embryonic stem (ES) cells. UbCKmit-deficient cells, obtained by consecutive rounds of gene targeting using homologous recombination and drug selection-driven gene conversion events, show no obvious growth disadvantage or abnormal differentiation potential. Activities of mitochondrial cytochrome c oxidase and citrate synthase, as well as the rate of pyruvate oxidation, showed values equal to wild-type cells, indicating a normal aerobic metabolism. Mitochondria of in vivo differentiated knock-out cells were structurally intact, as demonstrated by electron microscopy. Approaches to study the role of the UbCKmit gene further are discussed.

Integration vectors for antibody chimerization by homologous recombination in hybridoma cells

Gene targeting in hybridoma cells provides a tool for generating chimeric antibodies with great ease and at high yield. We present an evaluation of integration vectors for the chimerization of the immunoglobulin heavy chain locus which are universally applicable to hybridomas of different isotypes and mouse strains. There are three problems arising with vector integration: (i) the frequent persistence of the parental isotype; (ii) an isotype-dependent aberrant replacement-like recombination giving rise to antibodies devoid of the CH1 domain; and (iii) secondary recombinations leading to excision of the integrated sequence. To overcome these problems, we have systematically evaluated the consequences of extending the vector flank. Although the homology length clearly determines the recombination frequency, this effect is counteracted by the secondary recombination, which also correlates to the homology length. In contrast, the truncating recombination events are not dependent on the homology length and never lead to re-excision of the construct. To take advantage of the increased genetic stability obtained with short flanks, we constructed an enrichment vector which yields high recombination efficiencies despite using a short flanking sequence. In addition, irradiation of the cells enhanced homologous recombination. The problem of the co-production of two isotypes was overcome by a two-step targeting reaction.

Increased response to cholesterol feeding in apolipoprotein C1-deficient mice

The function of apolipoprotein (apo) C1 in vivo is not well understood. From in vitro studies it has been reported that an excess of apoC1 relative to apoE inhibits receptor-mediated uptake of remnant lipoproteins [Sehayek and Eisenberg (1991) J. Biol. Chem. 266, 22453-22459]. In order to gain a better understanding of the role of apoC1 in lipoprotein metabolism in vivo, we have generated apoC1-deficient mice by gene targeting in embryonic stem cells. Homozygous mutant mice are viable and do not show overt abnormalities. Serum triacylglycerol levels are increased by 60% on both a standard mouse diet and a mild hypercholesterolaemic diet compared with controls. Total serum cholesterol levels are similar to controls on the two diets. However, the level of high-density lipoprotein cholesterol in the apoC1-deficient mice fed on the mild hypercholesterolaemic diet is slightly decreased, which is accompanied by a 3-fold increase in very-low-density plus low-density lipoprotein (VLDL+LDL) cholesterol. On a severe atherogenic diet, the homozygous apoC1-deficient mice become hypercholesterolaemic, with a serum cholesterol level of 10.7 +/- 3.3 mM compared with 6.7 +/- 1.8 mM and 5.1 +/- 1.6 mM in heterozygous and control mice respectively. The increase in cholesterol is mainly confined to the VLDL+LDL-sized fractions. Binding experiments revealed that lipoproteins lacking apoC1 with d < 1.006 g/ml are poor competitors for 125I-labelled LDL binding to the LDL receptor on HepG2 cells. This suggests that total apoC1 deficiency leads to impaired receptor-mediated clearance of remnant lipoproteins rather than enhanced uptake, as was expected from data reported in the literature.

Normal development, growth and reproduction in cellular retinoic acid binding protein-I (CRABPI) null mutant mice

We have generated mouse null mutants for the cellular retinoic acid (RA) binding protein type I (CRABPI), a protein whose spatio-temporal expression pattern coincides with the target tissues for RA action. Inactivation of the CRABPI gene was accomplished via homologous recombination in embryonic stem cells. Cells carrying the correctly targeted gene were injected into blastocysts and the resulting chimaeras yielded offspring heterozygous for the knockout mutation. Subsequent breeding programs resulted in normal litter sizes containing viable and fertile CRABPI deficient mice. Homozygous mice carrying the knockout mutation were studied in detail to detect possible organ and skeletal anomalies and/or abnormalities of the hematopoietic system. No overt phenotype was evident indicating that a deficiency for CRABPI does not seem to interfere with normal development, growth and reproduction.

Diet-induced hypercholesterolemia and atherosclerosis in heterozygous apolipoprotein E-deficient mice

Apolipoprotein (apo) E is a ligand for the receptor-mediated uptake of lipoprotein remnant particles. Complete absence of apo E in humans leads to a severe form of type III hyperlipoproteinemia. We have used targeted inactivation in murine embryonic stem cells, as also described by others, to specifically study the effects of heterozygous Apoe gene loss on the development of hyperlipidemia. After 6 weeks on a severe semi-synthetic atherogenic diet, heterozygous null mutants, with only one functional Apoe alle, developed hypercholesterolemia as compared with controls (10.1 mM vs. 4.7 mM serum cholesterol). Interestingly, serum cholesterol levels in female heterozygotes were doubled as compared with male heterozygotes (15.0 mM vs. 7.5 mM). On this diet, heterozygous apo E deficient mice also showed an increased susceptibility to atherosclerosis, depending on gender (mean lesion area per section of 9524 microns 2 vs. 61,388 microns 2 for males and females, respectively), whereas wild-type mice displayed far fewer lesions (354 microns 2 and 9196 microns 2 for males and females, respectively). This study indicates that a subnormal expression-level of the Apoe gene leads to hypercholesterolemia and, consequently, to an increased susceptibility to the development of atherosclerosis.

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.