Mice with skin-specific DNA repair gene (Ercc1) inactivation are hypersensitive to ultraviolet irradiation-induced skin cancer and show more rapid actinic progression

Ercc1 has an essential role in the nucleotide excision repair (NER) pathway that protects against ultraviolet (UV)-induced DNA damage and is also involved in additional repair pathways. The premature death of simple Ercc1 mouse knockouts meant that we were unable to study the role of Ercc1 in the skin. To do this, we have used the Cre-lox system to generate a skin-specific Ercc1 knockout. With a Cre transgene under control of the bovine keratin 5 promoter we achieved 100% recombination of the Ercc1 gene in the epidermis. Hairless mice with Ercc1-deficient skin were hypersensitive to the short-term effects of UV irradiation, showing a very low minimal erythemal dose and a dramatic hyperproliferative response. Ultraviolet-irradiated mice with Ercc1-deficient skin developed epidermal skin tumours much more rapidly than controls. These tumours appeared to arise earlier in actinic progression and grew more rapidly than tumours on control mice. These responses are more pronounced than have been reported for other NER-deficient mice, demonstrating that Ercc1 has a key role in protecting against UV-induced skin cancer.

Lack of involvement of nucleotide excision repair gene polymorphisms in colorectal cancer

DNA repair has an essential role in protecting the genome from damage by endogenous and environmental agents. Polymorphisms in DNA repair genes and differences in repair capacity between individuals have been widely documented. For colorectal cancer, the loss of mismatch repair gene activity is a key genetic determinant. Nucleotide excision repair (NER), recombination repair (RR) and base excision repair (BER) pathways have critical roles in protection against other cancers, and we wished to investigate their role in colorectal cancer. We have compared the frequency of polymorphisms in the NER genes, XPD, XPF, XPG, ERCC1; in the BER gene, XRCC1; and in the RR gene, XRCC3; in colorectal cancer patients and in a control group. No significant associations were found for any of the NER gene polymorphisms or for the XRCC1 polymorphism. The C allele (position 18067) of the XRCC3 gene was weakly but significantly associated with colorectal cancer (odds ratio 1.52, 95% confidence interval 1.04-2.22, P=0.03). For all patients who were heterozygous for any of the repair genes studied, tumour tissue was investigated for loss of heterozygosity (LOH). Only one example of LOH was found for all the genes examined. From the association and LOH data, we conclude that these genes do not have an important role in protection against colorectal carcinogenesis.

Correction of liver dysfunction in DNA repair-deficient mice with an ERCC1 transgene

The ERCC1 gene is essential for the repair of UV-induced DNA damage. Unlike most genes in the nucleotide excision repair (NER) pathway, ERCC1 is also involved in recombinational repair. Perhaps for this reason, ERCC1 knockout mice are not a model for the human NER deficiency disorder, xeroderma pigmentosum. Instead, ERCC1 null mice are severely runted and die before weaning from liver failure with accelerated hepatocyte polyploidy that is more reminiscent of a premature ageing disorder. To permit study of the role of ERCC1 in other tissues we have corrected the liver ERCC1 deficiency with a transgene under the control of a liver-specific promoter. The transgene alleviated runting and extended the lifespan. The elevated level of oxidative DNA damage and premature liver polyploidy were reversed and liver function was corrected. A widespread mitochondrial dysfunction was identified and an essential role for ERCC1 in the kidney was also revealed with transgene-containing ERCC1-deficient animals going on to die of renal failure. The nuclei of kidney proximal tubule cells became polyploid in a similar way to the premature liver polyploidy observed in younger ERCC1-deficient animals. We believe that this is a response to the accumulation of endogenous DNA damage in these particularly susceptible tissues which cannot be repaired in ERCC1-deficient animals.

Induction of HO-1 and NOS in doppel-expressing mice devoid of PrP: implications for doppel function

Ectopic expression of the doppel (Dpl) protein, a homologue of the prion protein (PrP), was recently associated with cerebellar Purkinje cell degeneration observed in two aging prion protein knock-out (Prnp(0/0)) mouse lines. We investigated the possible role of Dpl in oxidative metabolism. Two Prnp(0/0) mouse lines of similar genetic background were studied. One line expresses Dpl in the brain and displays Dpl-associated cerebellar abnormalities. The other has no elevated expression of Dpl and no cerebellar abnormalities. We observed a correlation between Dpl expression and the induction of both heme oxygenase 1 (HO-1) and nitric oxide synthase systems (nNOS and iNOS). These responses are suggestive of increased oxidative stress in the brains of the Dpl-expressing Prnp(0/0) mice. No induction was observed with Hsp-60, indicating a specific response by the HO/NOS system. We proposed that Dpl expression exacerbates oxidative damage that is antagonistic to the protective function of wild-type PrP.

A one-step gene amplification system for use in cultured mammalian cells and transgenic animals

Gene amplification is widely used for the production of pharmaceuticals and therapeutics in situations where a mammalian system is essential to synthesise a fully active product. Current gene amplification systems require multiple rounds of selection, often with high concentrations of toxic chemicals, to achieve the highest levels of gene amplification. The use of these systems has not been demonstrated in specialised mammalian cells, such as embryonic-stem cells, which can be used to generate transgenic animals. Thus, it has not yet proved possible to produce transgenic animals containing amplified copies of a gene of interest, with the potential to synthesise large amounts of a valuable gene product. We have developed a new amplification system, based around vectors encoding a partially disabled hypoxanthine phosphoribosyltransferase (HPRT) minigene, which can achieve greater than 1000-fold amplification of HPRT and the human growth hormone gene in a single step in Chinese hamster-lung cells. The amplification system also works in mouse embryonic-stem cells and we have used it to produce mice which express 30-fold higher levels of human protein C in milk than obtained with conventional transgenesis using the same protein C construct. This system should also be applicable to large animal transgenics produced by nuclear transfer from cultured cell lines.

Nucleotide excision repair gene XPD polymorphisms and genetic predisposition to melanoma

The nucleotide excision repair pathway has evolved to deal with UV light-induced DNA damage. Individuals with the rare inherited nucleotide excision repair deficiency disease xeroderma pigmentosum have a 1000-fold increased incidence of skin cancer. We are interested in the possibility that more subtle changes in nucleotide excision repair genes, resulting in either a reduced capacity for repair or in altered interactions between repair proteins and components of the cell cycle control machinery, might constitute important genetic risk factors for the development of skin cancer in the general population. To investigate this hypothesis we have compared the frequency of polymorphisms in exons 6, 22 and 23 of the XPD gene in melanoma patients and a control group. For each of these two allele polymorphisms one of the alleles was over-represented in the melanoma group and there was a significant association with melanoma. Importantly, this association did not extend to markers immediately flanking the XPD gene, thus providing evidence that XPD gene polymorphisms might predispose to melanoma in the general population. There is a report that one of the polymorphic XPD alleles (exon 23 Lys), which is over-represented in the melanoma group, has reduced repair proficiency and we discuss the possibility that this is the causal change to the XPD gene that predisposes to melanoma.

Nucleotide excision repair gene (ERCC1) deficiency causes G(2) arrest in hepatocytes and a reduction in liver binucleation: the role of p53 and p21

A wide range of DNA lesions, both UV and chemically induced, are dealt with by the nucleotide excision repair (NER) pathway. Defects in NER result in human syndromes such as xeroderma pigmentosum (XP), where there is a 1000-fold increased incidence of skin cancer. The ERCC1 protein is essential for NER, but ERCC1 knockout mice are not a model for XP. In the absence of exogenous DNA-damaging agents, these mice are runted and die before weaning, with dramatically accelerated liver polyploidy and elevated levels of p53. Here we present a morphological, immunological, and molecular study to understand the mechanism for the unusual liver pathology in ERCC1-deficient mice. We show that the enlarged ERCC1-deficient hepatocytes are arrested in G(2) and that DNA replication and the normal process of binucleation are both reduced. This is associated with a p53-independent increase in expression of the cyclin-dependent kinase inhibitor p21. The most dramatic feature of the ERCC1-deficient liver phenotype, the accelerated polyploidy, is not rescued by p53 deficiency, but we show that p53 is responsible for the reduced DNA replication and binucleation. We consider that the liver phenotype is a response to unrepaired endogenous DNA damage, which may reflect an additional non-NER-related function for the ERCC1 protein.

A single amino acid alteration (101L) introduced into murine PrP dramatically alters incubation time of transmissible spongiform encephalopathy

A mutation equivalent to P102L in the human PrP gene, associated with Gerstmann-Straussler syndrome (GSS), has been introduced into the murine PrP gene by gene targeting. Mice homozygous for this mutation (101LL) showed no spontaneous transmissible spongiform encephalopathy (TSE) disease, but had incubation times dramatically different from wild-type mice following inoculation with different TSE sources. Inoculation with GSS produced disease in 101LL mice in 288 days. Disease was transmitted from these mice to both wild-type (226 days) and 101LL mice (148 days). In contrast, 101LL mice infected with ME7 had prolonged incubation times (338 days) compared with wild-type mice (161 days). The 101L mutation does not, therefore, produce any spontaneous genetic disease in mice but significantly alters the incubation time of TSE infection. Additionally, a rapid TSE transmission was demonstrated despite extremely low levels of disease-associated PrP.

Ataxia in prion protein (PrP)-deficient mice is associated with upregulation of the novel PrP-like protein doppel

The novel locus Prnd is 16 kb downstream of the mouse prion protein (PrP) gene Prnp and encodes a 179 residue PrP-like protein designated doppel (Dpl). Prnd generates major transcripts of 1.7 and 2.7 kb as well as some unusual chimeric transcripts generated by intergenic splicing with Prnp. Like PrP, Dpl mRNA is expressed during embryogenesis but, in contrast to PrP, it is expressed minimally in the CNS. Unexpectedly, Dpl is upregulated in the CNS of two PrP-deficient (Prnp(0/0)) lines of mice, both of which develop late-onset ataxia, suggesting that Dpl may provoke neurodegeneration. Dpl is the first PrP-like protein to be described in mammals, and since Dpl seems to cause neurodegeneration similar to PrP, the linked expression of the Prnp and Prnd genes may play a previously unrecognized role in the pathogenesis of prion diseases or other illnesses.

Efficient BLG-Cre mediated gene deletion in the mammary gland

Using the phage P1-derived Cre/loxP recombination system, we have developed a strategy for efficient mammary tissue specific inactivation of floxed genes. Transgenic mice were generated which express Cre DNA-recombinase under the control of the mammary gland specific promoter of the ovine beta-lactoglobulin (BLG) gene. To test the specificity of Cre mediated recombination, we crossed these mice to animals harbouring a floxed DNA ligase I allele. We show that the BLG-Cre construct specifies mammary specific gene deletion, and furthermore that it is temporally regulated, predominantly occurring during lactation. We fully characterised the extent of gene deletion in one line (line 74). In this strain the virgin gland is characterised by low levels (7%) of Cre mediated deletion, whereas 70-80% of cells within the lactating mammary gland have undergone recombination. Immunohistochemistry and indirect in situ PCR were used respectively to demonstrate that both Cre protein and Cre activity were evenly distributed throughout the population of secretory epithelial cells. The level of background recombination in non-mammary tissues was found to be < or = 1.1%, irrespective of mammary gland developmental status. Crossing the transgenic BLG-Cre strain described here to mice harbouring other floxed alleles will facilitate the functional analysis of those genes during differentiation and development of the mammary gland.

Lessons from keratin 18 knockout mice: formation of novel keratin filaments, secondary loss of keratin 7 and accumulation of liver-specific keratin 8-positive aggregates

Here, we report on the analysis of keratin 18 null mice. Unlike the ablation of K8, which together with K18 is expressed in embryonic and simple adult epithelia, K18 null mice are viable, fertile, and show a normal lifespan. In young K18 null mice, hepatocytes were completely devoid of keratin filaments. Nevertheless, typical desmosomes were formed and maintained. Old K18 null mice, however, developed a distinctive liver pathology with abnormal hepatocytes containing K8-positive aggregates. These stained positively for ubiquitin and MM120-1 and were identified as Mallory bodies, one hallmark of human alcoholic hepatitis. This is the first demonstration that the ablation of one keratin leads to the accumulation of its single partner. Another striking finding was the absence or drastic down regulation of K7 in several tissues despite its ongoing transcription. Moreover, K18 null mice revealed new insights in the filament-forming capacity of the tail-less K19 in vivo. Due to the unexpected secondary loss of K7, only K8/19 are expressed in the uterine epithelium of K18 null mice. Immunoelectron microscopy of this tissue demonstrated the presence of typical K8/19 IF, thus highlighting in vivo that K19 is a fully competent partner for K8.

Cells from ERCC1-deficient mice show increased genome instability and a reduced frequency of S-phase-dependent illegitimate chromosome exchange but a normal frequency of homologous recombination

The ERCC1 protein is essential for nucleotide excision repair in mammalian cells and is also believed to be involved in mitotic recombination. ERCC1-deficient mice, with their extreme runting and polyploid hepatocyte nuclei, have a phenotype that is more reminiscent of a cell cycle arrest/premature ageing disorder than the classic DNA repair deficiency disease, xeroderma pigmentosum. To understand the role of ERCC1 and the link between ERCC1-deficiency and cell cycle arrest, we have studied primary and immortalised embryonic fibroblast cultures from ERCC1-deficient mice and a Chinese hamster ovary ERCC1 mutant cell line. Mutant cells from both species showed the expected nucleotide excision repair deficiency, but the mouse mutant was only moderately sensitive to mitomycin C, indicating that ERCC1 is not essential for the recombination-mediated repair of interstrand cross links in the mouse. Mutant cells from both species had a high mutation frequency and the level of genomic instability was elevated in ERCC1-deficient mouse cells, both in vivo and in vitro. There was no evidence for an homologous recombination deficit in ERCC1 mutant cells from either species. However, the frequency of S-phase-dependent illegitimate chromatid exchange, induced by ultra violet light, was dramatically reduced in both mutants. In rodent cells the G1 arrest induced by ultra violet light is less extensive than in human cells, with the result that replication proceeds on an incompletely repaired template. Illegitimate recombination, resulting in a high frequency of chromatid exchange, is a response adopted by rodent cells to prevent the accumulation of DNA double strand breaks adjacent to unrepaired lesion sites on replicating DNA and allow replication to proceed. Our results indicate an additional role for ERCC1 in this process and we propose the following model to explain the growth arrest and early senescence seen in ERCC1-deficient mice. In the absence of ERCC1, spontaneously occurring DNA lesions accumulate and the failure of the illegitimate recombination process leads to the accumulation of double strand breaks following replication. This triggers the p53 response and the G2 cell cycle arrest, mediated by increased expression of the cyclin-dependent kinase inhibitor p21(cip1/waf1). The increased levels of unrepaired lesions and double strand breaks lead to an increased mutation frequency and genome instability.

Mice with gene targetted prion protein alterations show that Prnp, Sinc and Prni are congruent

Classical genetic analysis has identified Sinc/Prni as the major gene controlling mouse scrapie incubation time. Sinc/Prni is linked to Prnp, the gene encoding the prion protein (PrP). Prnp alleles express distinct PrP protein variants, PrP A and PrP B, which arise from codon 108L/F and 189 T/V dimorphisms. Prnp genotype segregates with incubation time length which suggests, but does not prove, that incubation time is controlled by PrP dimorphisms, and that the Sinc/Prni and Prnp loci are congruent. We have used gene targetting to construct mice in which the endogenous Prnp allele has been modified to express PrP B instead of PrP A. Challenge with a mouse-adapted BSE strain results in dramatically shortened incubation times and demonstrates that PrP dimorphisms at codon 108 and/or 189 control incubation time, and that Sinc/Prni and Prnp are congruent.

Stability of HPRT marker gene expression at different gene-targeted loci: observing and overcoming a position effect

For sophisticated gene targeting procedures requiring two sequential selective steps to operate efficiently it is essential that the marker genes used are not prone to position effects. The double replacement gene targeting procedure, to produce mice with subtle gene alterations, is based on the use of hypoxanthine phosphoribosyltransferase ( HPRT) minigenes in HPRT-deficient embryonic stem cells. Our standard HPRTminigene, under the control of the mouse phosphoglycerate kinase-1 gene promoter, was stably expressed at five of six target loci examined. At the remaining locus, DNA ligase I (Lig1), expression of this minigene was highly unstable. A different minigene, under the control of the mouse HPRT promoter and embedded in its natural CpG-rich island, overcame this position effect and was stably expressed when targeted to the identical site in the Lig1 locus. The promoter region of the stably expressed minigene remained unmethylated, while the promoter of the unstably expressed minigene rapidly became fully methylated. The difference in the stability of HPRT minigene expression at the same target locus can be explained in the context of the different lengths of their CpG-rich promoter regions with associated transcription factors and a resulting difference in their susceptibility to DNA methylation, rather than by differences in promoter strength.

Transgenic analysis of prion diseases

Prion diseases are fatal transmissible neurological disorders afflicting a range of mammalian species. Although still controversial, a large body of data suggests that the causative agent may be composed entirely of a small glycoprotein. The brains of infected animals have accumulations of a pathogenic protease-resistant isoform (PrPsc) of a normal host-encoded glycoprotein, PrPc or prion protein. A number of lines of biochemical evidence implicate the disease-specific isoform, PrPsc, as the transmissible agent and genetic analysis has shown tight linkage between PrP gene mutations and polymorphisms and differential susceptibility to prion diseases, Perhaps the strongest evidence for a protein-only model of the agent is that PrP gene-ablated mice are resistant to scrapie and that mice with PrP mutation, corresponding to those found in a human familial prion disease, spontaneously develop a transmissible prion disease. This review describes the critical role that transgenic technology has played in the study of the biology of prion diseases and considers the issues raised by this work.

Mice with adenine phosphoribosyltransferase deficiency develop fatal 2,8-dihydroxyadenine lithiasis

Deficiencies in different steps of purine metabolism give rise to a number of human inherited disorders. Lesch-Nyhan syndrome is a severe neurological disorder, caused by a deficiency in the purine salvage enzyme hypoxanthine phosphoribosyltransferase (HPRT). HPRT-deficient mice have been generated, but have proved to be an unsuccessful model of the human disease. We have suggested that this may be due to a greater dependency in rodents on the other purine salvage enzyme, adenine phosphoribosyltransferase (APRT). We have generated an APRT-deficient mouse line by gene targeting, with a phenotype that closely resembled the symptoms of APRT deficiency in man. APRT null mice were viable, but 90% died prematurely before 6 months of age, displaying highly abnormal kidney morphology, with pathology characteristic of tubule obstruction. These mice have elevated urinary levels of adenine and 2,8-dihydroxyadenine, a highly insoluble adenine derivative, plus birefringent crystalline deposits and calculi within tubules throughout the kidney. A standard therapy for APRT-deficient human patients is the administration of the xanthine oxidase inhibitor, allopurinol. This has proved an effective therapy for APRT null mice, preventing accumulation of 2,8-dihydroxyadenine and much of the resultant renal obstruction, allowing us to establish a breeding line. We believe that these mice should provide a useful model for further study of APRT deficiency in humans. Furthermore, by generating APRT and HPRT double mutants, we will be able to test our hypothesis that both genes must be inactivated in mice before a model for Lesch-Nyhan syndrome can be obtained.

DNA ligase I is required for fetal liver erythropoiesis but is not essential for mammalian cell viability

Four distinct DNA ligase activities (I-IV) have been identified within mammalian cells. Evidence has indicated that DNA ligase I is central to DNA replication, as well as being involved in DNA repair processes. A patient with altered DNA ligase I displayed a phenotype similar to Bloom's syndrome, being immunodeficient, growth retarded and predisposed to cancer. Fibroblasts isolated from this patient (46BR) exhibited abnormal lagging strand synthesis and repair deficiency. It has been reported that DNA ligase I is essential for cell viability, but here we show that cells lacking DNA ligase I are in fact viable. Using gene targeting in embryonic stem (ES) cells, we have produced DNA ligase I-deficient mice. Embryos develop normally to mid-term when haematopoiesis usually switches to the fetal liver. Thereupon acute anaemia develops, despite the presence of erythroid-committed progenitor cells in the liver. Thus DNA ligase I is required for normal development, but is not essential for replication. Hence a previously unsuspected redundancy must exist between mammalian DNA ligases.

Gene targeting at the mouse cytokeratin 10 locus: severe skin fragility and changes of cytokeratin expression in the epidermis

Bullous congenital ichthyosiform erythroderma (BCIE) is a dominantly inherited blistering skin disorder caused by point mutations in the suprabasal cytokeratins 1 or 10. Targeting the murine cytokeratin 10 gene in ES cells resulted in mice with different phenotypes in the homozygotes and heterozygotes; both of which exhibit similarities to specific clinical characteristics of BCIE. Homozygotes suffered from severe skin fragility and died shortly after birth. Heterozygotes were apparently unaffected at birth, but developed hyperkeratosis with age. In both genotypes, aggregation of cytokeratin intermediate filaments, changes in cytokeratin expression, and alterations in the program of epidermal differentiation were observed. In addition we demonstrate, for the first time, the existence of the murine equivalent of human cytokeratin 16.

Double replacement gene targeting for the production of a series of mouse strains with different prion protein gene alterations

We have developed a double replacement gene targeting strategy which enables the production of a series of mouse strains bearing different subtle alterations to endogenous genes. This is a two-step process in which a region of the gene of interest is first replaced with a selectable marker to produce an inactivated allele, which is then re-targeted with a second vector to reconstruct the inactivated allele, concomitantly introducing an engineered mutation. Five independent embryonic stem cell lines have been produced bearing different targeted alterations to the prion protein gene, including one which raises the level of expression. We have constructed mice bearing the codon 101 proline to leucine substitution linked to the human familial prion disease, Gerstmann-Straussler-Scheinker syndrome. We anticipate that this procedure will have applications to the study of human inherited diseases and the development of therapies.

Comparison between cDNA clones encoding murine DNA ligase I

A 3172-nucleotide (nt) cDNA clone encoding mouse DNA ligase I (LigI) was isolated from an embryonic stem cell cDNA library. Another mouse LigI cDNA clone has been recently described. Six single-amino-acid alterations have been identified between the two mouse LigI clones.

Gene targeting in the mouse

Mice with alterations to specific endogenous genes can be produced by gene targeting in embryonic stem cells. The field has developed rapidly over the past decade, so that large numbers of mice with different gene deficiencies have been generated. Knockout mice provide an ideal opportunity to analyse the function of individual mammalian genes and to model a range of human inherited disorders. This powerful approach has also identified numerous examples of gene redundancy and has highlighted the need to consider metabolic differences between man and mouse in disease modelling. More sophisticated gene-targeting methods are now being used to introduce subtle gene alterations. In the future, more refined genetic analysis and genome, rather than individual gene, alterations will be achieved by incorporating site-specific recombination into targeting strategies. Gene targeting could also make a contribution to improved protocols for gene therapy.

Use of double-replacement gene targeting to replace the murine alpha-lactalbumin gene with its human counterpart in embryonic stem cells and mice

The mouse alpha-lactalbumin gene has been replaced with the human gene by two consecutive rounds of gene targeting in hypoxanthine phosphoribosyltransferase (HPRT)-deficient feeder-independent murine embryonic stem (ES) cells. One mouse alpha-lactalbumin allele was first replaced by an HPRT minigene which was in turn replaced by human alpha-lactalbumin. The end result is a clean exchange of defined DNA fragments with no other DNA remaining at the target locus. Targeted ES cells at each stage remained capable of contributing efficiently to the germ line of chimeric animals. Double replacement using HPRT-deficient ES cells and the HPRT selection system is therefore a powerful and flexible method of targeting specific alterations to animal genes. A typical strategy for future use would be to generate a null mutation which could then be used to produce multiple second-step alterations at the same locus.

Mice with DNA repair gene (ERCC-1) deficiency have elevated levels of p53, liver nuclear abnormalities and die before weaning

Defects in nucleotide excision repair are associated with the human condition xeroderma pigmentosum which predisposes to skin cancer. Mice with defective DNA repair were generated by targeting the excision repair cross complementing gene (ERCC-1) in the embryonic stem cell line, HM-1. Homozygous ERCC-1 mutants were runted at birth and died before weaning with liver failure. Examination of organs revealed polyploidy in perinatal liver, progressing to severe aneuploidy by 3 weeks of age. Elevated p53 levels were detected in liver, brain and kidney, supporting the hypothesised role for p53 as a monitor of DNA damage.

Phenotypic correction of a human cell line (46BR) with aberrant DNA ligase I activity

Ligation of DNA after replication and repair is a prerequisite for the preservation of DNA and chromosome structure and function. Biochemical studies with Bloom's syndrome cells have revealed an abnormal DNA ligase I activity. However, genetic analysis has not revealed any differences in transcript levels or in the cDNA sequences of DNA ligase I between Bloom's syndrome and normal cells. Another human cell line, 46BR, derived from an immunodeficient patient, also has an abnormal DNA ligase I. This cell line has recently been demonstrated to harbour two different missense mutations, one at each allele of DNA ligase I. These mutations resulted in a decreased ability of partially purified cell extracts to form an enzyme-adenylate reaction intermediate. We show that 46BR hypersensitivity to an alkylating agent, ethyl methanesulphonate, and to the polyADP-ribose polymerase inhibitor 3-aminobenzamide, is rescued by transfection of wild-type DNA ligase I sequences. This provides additional genetic evidence that the defect in 46BR is at the DNA ligase I locus.

Production of a model for Lesch-Nyhan syndrome in hypoxanthine phosphoribosyltransferase-deficient mice

The inherited disease Lesch-Nyhan syndrome, which is caused by a deficiency of the enzyme hypoxanthine phosphoribosyltransferase (HPRT), is characterized by behavioural alterations, including self-injurious behaviour and mental retardation. Although HPRT-deficient mice have been generated using the embryonic stem cell system, no spontaneous behavioural abnormalities had been reported. We examined whether mice were more tolerant of HPRT deficiency because they were more reliant on adenine phosphoribosyltransferase (APRT) than HPRT for their purine salvage. The administration of an APRT inhibitor to HPRT-deficient mice induced persistent self-injurious behaviour. This combined genetic and biochemical model will facilitate the study of Lesch-Nyhan syndrome and the evaluation of novel therapies.

A position- and orientation-dependent element in the first intron is required for expression of the mouse hprt gene in embryonic stem cells

The gene (hprt) coding for mouse HPRT (hypoxanthine phosphoribosyltransferase) is transcribed from a promoter lacking CAAT and TATAA boxes. It is expressed ubiquitously, albeit at different levels, in all tissues and cultured cells. During investigations to characterise hprt transcription control elements required in embryonic stem (ES) cells and to develop compact hprt minigenes for gene-targeting strategies, we discovered a requirement for intron-1 sequences for expression in ES cells. The essential intron-1 element, which is 420 bp long, is located 230 bp downstream from the transcription start point and is shown to increase transcription from the hprt promoter in a position- and orientation-dependent manner. We propose that this element is an integral downstream part of the hprt promoter.

Gene targeting using a mouse HPRT minigene/HPRT-deficient embryonic stem cell system: inactivation of the mouse ERCC-1 gene

A convenient system for gene targeting that uses hypoxanthine phosphoribosyltransferase (HPRT) minigenes as the selectable marker in HPRT-deficient mouse embryonic stem (ES) cells is described. Improvements to the expression of HPRT minigenes in ES cells were achieved by promoter substitution and the provision of a strong translational initiation signal. The use of minigenes in the positive-negative selection strategy for gene targeting was evaluated and the smaller minigenes were found to be as effective as a more conventional marker--the herpes simplex virus thymidine kinase gene. Minigenes were used to target the DNA repair gene ERCC-1 in ES cells. A new HPRT-deficient ES cell line was developed that contributes with high frequency to the germ line of chimeric animals. The ability to select for and against HPRT minigene expression in the new HPRT-deficient ES cell line will make this system useful for a range of gene-targeting applications.

Mouse models of hypoxanthine phosphoribosyltransferase deficiency

Lesch--Nyhan syndrome is an X-linked disease caused by the deficiency of hypoxanthine phosphoribosyltransferase, an enzyme involved in the purine salvage pathways. It is characterized by severe gout, choreoathetosis, self-mutilatory behaviour and mental retardation. The derivation of mice genetically deficient in this enzyme may help to elucidate the pathogenesis of the neurological abnormality where previously models using drug administration to mimic the disorder have had to suffice.

Germ line transmission and expression of a corrected HPRT gene produced by gene targeting in embryonic stem cells

The deletion mutation in the HPRT-deficient mouse embryonic stem (ES) cell line E14TG2a has been corrected by gene targeting. The presence of plasmid sequences in the correcting vector DNA did not affect the frequency of correction. We have characterized three different HPRT gene structures in correctants. Cells from one corrected clone have been introduced into mouse blastocysts, and germ line transmission of the ES cell-derived corrected gene has been achieved. The corrected gene has the same pattern of expression as the wild-type gene, with the characteristic elevated level of expression in brain tissue. Hence, we have demonstrated the feasibility of introducing targeted modifications into the mouse germ line by homologous recombination in ES cells.

Expression of injected HPRT minigene DNA in mouse embryos and its inhibition by antisense DNA

We have used a highly sensitive biochemical microassay to monitor the expression of a cloned minigene for hypoxanthine phosphoribosyl transferase (HPRT, EC.2.4.2.8) in preimplantation mouse embryos. The mouse HPRT promoter and the mouse metallothionein promoter (MT-I) function equally well in embryos at the 2-cell stage whereas the viral SV40 promoter does not allow HPRT expression. Induced HPRT activity from the MT-I HPRT minigene construct occurs in cleavage embryos cultured in the presence of cadmium. In contrast, negation of enzyme expression from the injected minigene DNA is mediated by simultaneous injection of HPRT antisense DNA.

Characterization, evolutionary relationships, and chromosome location of processed mouse HPRT pseudogene

Studies on a cell line with amplified copies of the mouse hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene and HPRT gene transfer experiments revealed the existence of a nonfunctional HPRT-related sequence in the mouse genome. This sequence was isolated and found to be a processed HPRT pseudogene. With the exception of a small internal deletion, the pseudogene is believed to comprise a complete reverse transcript of HPRT mRNA, although the 3' end of the pseudogene was lost in the cloning process. A probe from a region flanking the mouse pseudogene was used to investigate the evolutionary relationships of mammalian HPRT pseudogenes. The pseudogenes in mouse and Chinese hamster appear to have a common origin, but no homology to any of the four known human HPRT pseudogenes was detected. A pseudogene-linked restriction fragment length polymorphism was used to map the pseudogene to the distal end of mouse chromosome 17.

Characterisation of cDNA clones for hypoxanthine-guanine phosphoribosyltransferase from the human malarial parasite, Plasmodium falciparum: comparisons to the mammalian gene and protein

The isolation of cDNA clones for hypoxanthine-guanine phosphoribosyltransferase (HPRT) from the human malarial parasite, Plasmodium falciparum, is described. Northern analysis indicates that P. falciparum HPRT mRNA is the same size as that coding for mammalian HPRT. The predicted amino acid sequence of the P. falciparum HPRT protein shows extensive homology to the mammalian enzyme. Homology between the two proteins occurs in distinct blocks and a putative catalytic binding domain in the centre of the protein is also conserved. Five out of the seven characterised mammalian HPRT missense mutations map to regions which are conserved in the P. falciparum protein.

Targetted correction of a mutant HPRT gene in mouse embryonic stem cells

Two recent developments suggest a route to predetermined alterations in mammalian germlines. These are, first, the characterization of mouse embryonic stem (ES) cells that can still enter the germline after genetic manipulation in culture and second, the demonstration that homologous recombination between a native target chromosomal gene and exogenous DAN can be used in culture to modify specifically the target locus. We here use gene targetting functionally to correct the mutant hypoxanthine-guanine phosphoribosyl transferase (HPRT) gene in the ES cell line which has previously been isolated and used to produce an HPRT-deficient mouse. This modification of a chosen gene in pluripotent ES cells demonstrates the feasibility of this route to manipulating mammalian genomes in predetermined ways.

Structure and mapping of the DNA of human parvovirus B19

DNA from human parvovirus B19 was prepared from infected serum and examined by electron microscopy. Double-stranded molecules were seen, often with characteristic 'fold-back' ends that were assumed to be due to the inverted terminal repeats of the genome DNA. This double-stranded DNA was mapped with 13 restriction enzymes. More than 40 isolates, including the virus from the original B19 serum, were compared. Although isolates could be grouped by this method, there was no correlation between a particular restriction endonuclease map and any of the several disease presentations of the virus.

Methylation of the mouse hprt gene differs on the active and inactive X chromosomes

It has been proposed that DNA methylation is involved in the mechanism of X inactivation, the process by which equivalence of levels of X-linked gene products is achieved in female (XX) and male (XY) mammals. In this study, Southern blots of female and male DNA digested with methylation-sensitive restriction endonucleases and hybridized to various portions of the cloned mouse hprt gene were compared, and sites within the mouse hprt gene were identified that are differentially methylated in female and male cells. The extent to which these sites are methylated when carried on the active and inactive X chromosomes was directly determined in a similar analysis of DNA from clonal cell lines established from a female embryo derived from a mating of two species of mouse, Mus musculus and Mus caroli. The results revealed two regions of differential methylation in the mouse hprt gene. One region, in the first intron of the gene, includes four sites that are completely unmethylated when carried on the active X and extensively methylated when carried on the inactive X. These same sites are extensively demethylated in hprt genes reactivated either spontaneously or after 5-azacytidine treatment. The second region includes several sites in the 3' 20kilobases of the gene extending from exon 3 to exon 9 that show the converse pattern; i.e., they are completely methylated when carried on the active X and completely unmethylated when carried on the inactive X. At least one of these sites does not become methylated after reactivation of the gene. The results of this study, together with the results of previous studies by others of the human hprt gene, indicate that these regions of differential methylation on the active and inactive X are conserved between mammalian species. Furthermore, the data described here are consistent with the idea that at least the sites in the 5' region of the gene play a role in the X inactivation phenomenon and regulation of expression of the mouse hprt gene.

Expression of the mouse HPRT gene: deletional analysis of the promoter region of an X-chromosome linked housekeeping gene

The mouse hypoxanthine phosphoribosyltransferase gene, like several other housekeeping genes, lacks many of the features associated with promoters of RNA polymerase II-transcribed genes. HPRT transcripts have multiple initiation sites and an HPRT minigene was used to show that only 49 bases of 5' flanking sequence was necessary for normal expression in cultured cells. The essential region, which occurs within a complex series of direct repeats, is homologous to sequences upstream of other housekeeping genes. When this sequence was deleted, cryptic upstream initiation sites were revealed. Similar aberrant patterns of initiation were seen with all minigenes assayed in Xenopus oocytes. We speculate that this region of the HPRT promoter is involved in a different interaction with the transcriptional machinery to that occurring at more conventional promoters.

Structure, expression, and mutation of the hypoxanthine phosphoribosyltransferase gene

The wild-type mouse hypoxanthine phosphoribosyltransferase (HPRT; IMP:pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) gene has been isolated from genomic libraries and its structure has been determined. This X chromosome-linked gene is greater than 33 kilobases long and is split into nine exons. All the exon sequences have been determined, and a single-base substitution in the HPRT cDNA coding sequence from a mouse neuroblastoma cell line that overproduces a mutant HPRT protein has been identified. The 5' end of the gene has been defined, both by nuclease S1 protection and primer extension studies and by a functional assay in which an HPRT minigene, capable of expression in cultured cells, was created by ligating the 5' end of the gene onto wild-type human HPRT cDNA. Sequences normally associated with eukaryotic promoters are not present in the immediate 5'-flanking region of the HPRT gene, which is instead highly G+C rich. This observation is discussed in relation to the possible link between DNA methylation and X-chromosome inactivation.

Cloned cDNA sequences of the hypoxanthine/guanine phosphoribosyltransferase gene from a mouse neuroblastoma cell line found to have amplified genomic sequences

Cloned cDNA sequences of the murine hypoxanthine/guanine phosphoribosyltransferase (HPRT; EC 2.4.2.8) gene have been isolated by using a mouse neuroblastoma cell line containing increased levels of a variant HPRT protein. We have used these sequences as probes to demonstrate that protein overproduction in this cell line is a consequence of at least a 20-fold increase in HPRT mRNA levels resulting from approximately 50-fold amplification of HPRT genomic sequences. The largest cDNA insert so far characterized represents about 70% of the HPRT mRNA sequence. This cDNA is shown to possess regions of homology with mRNA and DNA from Chinese hamster, baboon, and human, thus facilitating detailed analysis of this locus in these four species.

In vitro translation of hypoxanthine/guanine phosphoribosyltransferase mRNA: characterization of a mouse neuroblastoma cell line that has elevated levels of hypoxanthine/guanine phosphoribosyltransferase protein

Antibody specific for the native form of Chinese hamster hypoxanthine/guanine phosphoribosyltransferase (HPRT; IMP:pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) was used to detect the synthesis of HPRT protein in a rabbit reticulocyte lysate translation system primed with mRNA from Chinese hamster tissues and cultured cells. Electrophoretic analysis of the immunopurified products from the translation of mRNA from wild-type and a series of mutant Chinese hamster cells indicated that HPRT synthesis in vitro qualitatively and quantitatively corresponded to synthesis in vivo. The translation system was used to identify two mRNA sources producing high levels of HPRT protein: Chinese hamster brain and a mouse neuroblastoma HPRT revertant cell line, NBR4. Translation of NBR4 mRNA generated 25-50 times more HPRT protein than mRNA from wild-type cells. The basis for HPRT overproduction is considered in view of an X chromosome alteration found in NBR4 cells.

Cell fusion-induced mouse neuroblastomas HPRT revertants with variant enzyme and elevated HPRT protein levels

Fusion of 6-thioguanine-resistant mouse neuroblastoma to HeLa whole and minicells generated neuroblastoma HPRT revertants in addition to true cell hybrids. All revertants contained HPRT with decreased electrophoretic mobility and heat stability relative to wild-type mouse enzyme. Revertant HPRT expression was dependent on continuous HAT selection. Quantitative immunoadsorption experiments showed that revertants with low HPRT specific activity had wild-type levels of HPRT protein while a revertant with high apparent activity (NBR4) contained elevated levels of variant protein. HPRT extracted from NBR4 had decreased affinity of both hypoxanthine and PRPP relative to wild type. Evidence is presented that HPRT elevation is dependent on the reversion process itself.