Random mutagenesis in the mouse as a tool in drug discovery

Investigation of TRPV1 loss-of-function phenotypes in TRPV1 Leu206Stop mice generated by N-ethyl-N-nitrosourea mutagenesis

N-ethyl-N-nitrosourea (ENU) random mutagenesis was used to generate a mouse model for the analysis of the transient receptor potential vanilloid 1 (TRPV1) cation channel. A transversion from T→A in exon 4 led to a Leu206Stop mutation generating a loss-of-function mutant. The TRPV1 agonist capsaicin was used to analyze functional and nociceptive parameters in vitro and in vivo in TRPV1 Leu206Stop mice and congenic C3HeB/FeJ controls. Capsaicin-induced [Ca²⁺]_i changes in small diameter DRG neurons were significantly diminished in TRPV1 Leu206Stop mice and administration of capsaicin induced neither hypothermia nor nocifensive behaviour in vivo. TRPV1 Leu206Stop mice were tested in the spinal nerve ligation of mononeuropathic pain and developed mechanical hypersensitivity two weeks after nerve injury. In the open field test, a significant increase in spontaneous locomotion was detected in TRPV1 Leu206Stop mice as compared to wildtype controls. TRPV1 knockout mice have been reported to carry a similar phenotype regarding capsaicin-evoked responses in vitro and in vivo. However, in contrast to TRPV1 Leu206Stop mice, TRPV1 knockout mice did not differ in spontaneous locomotion as compared to congenic C57BL/6 mice, suggesting subtle ENU-dependent or independent strain differences between TRPV1 Leu206Stop mice and their wildtype controls. In summary, these data revealed a target-related (i.e. capsaicin-evoked) phenotype of TRPV1 Leu206Stop mice closely resembling that of published TRPV1 knockout mice. However, since ENU-mutant mice are congenic with the mouse strain initially used in random mutagenesis, direct phenotypic comparison with the respective wildtype controls is possible, and the time-consuming backcrossing in lines with targeted mutations is avoided.

The canonical Notch signaling pathway: unfolding the activation mechanism

Notch signaling regulates many aspects of metazoan development and tissue renewal. Accordingly, the misregulation or loss of Notch signaling underlies a wide range of human disorders, from developmental syndromes to adult-onset diseases and cancer. Notch signaling is remarkably robust in most tissues even though each Notch molecule is irreversibly activated by proteolysis and signals only once without amplification by secondary messenger cascades. In this Review, we highlight recent studies in Notch signaling that reveal new molecular details about the regulation of ligand-mediated receptor activation, receptor proteolysis, and target selection.

Diabetes models by screen for hyperglycemia in phenotype-driven ENU mouse mutagenesis projects

More than 150 million people suffer from diabetes mellitus worldwide, and this number is expected to rise substantially within the next decades. Despite its high prevalence, the pathogenesis of diabetes mellitus is not completely understood. Therefore, appropriate experimental models are essential tools to gain more insight into the genetics and pathogenesis of the disease. Here, we describe the current efforts to establish novel diabetes models derived from unbiased, phenotype-driven, large-scale N-ethyl-N-nitrosourea (ENU) mouse mutagenesis projects started a decade ago using hyperglycemia as a high-throughput screen parameter. Mouse lines were established according to their hyperglycemia phenotype over several generations, thereby revealing a mutation as cause for the aberrant phenotype. Chromosomal assignment of the causative mutation and subsequent candidate gene analysis led to the detection of the mutations that resulted in novel alleles of genes already known to be involved in glucose homeostasis, like glucokinase, insulin 2, and insulin receptor. Additional ENU-induced hyperglycemia lines are under genetic analysis. Improvements in screen for diabetic animals are implemented to detect more subtle phenotypes. Moreover, diet challenge assays are being employed to uncover interactions between genetic and environmental factors in the pathogenesis of diabetes mellitus. The new mouse mutants recovered in phenotype-driven ENU mouse mutagenesis projects complement the available models generated by targeted mutagenesis of candidate genes, all together providing the large resource of models required for a systematic dissection of the pathogenesis of diabetes mellitus.

Comparative genetic analysis: the utility of mouse genetic systems for studying human monogenic disease

One of the long-term goals of mutagenesis programs in the mouse has been to generate mutant lines to facilitate the functional study of every mammalian gene. With a combination of complementary genetic approaches and advances in technology, this aim is slowly becoming a reality. One of the most important features of this strategy is the ability to identify and compare a number of mutations in the same gene, an allelic series. With the advent of gene-driven screening of mutant archives, the search for a specific series of interest is now a practical option. This review focuses on the analysis of multiple mutations from chemical mutagenesis projects in a wide variety of genes and the valuable functional information that has been obtained from these studies. Although gene knockouts and transgenics will continue to be an important resource to ascertain gene function, with a significant proportion of human diseases caused by point mutations, identifying an allelic series is becoming an equally efficient route to generating clinically relevant and functionally important mouse models.

Screening for increased plasma urea levels in a large-scale ENU mouse mutagenesis project reveals kidney disease models

Kidney diseases lead to the failure of urinary excretion of metabolism products. In the Munich ethylnitrosourea (ENU) mouse mutagenesis project, which is done on a C3H inbred genetic background, blood samples of more than 15,000 G1 offspring and 500 G3 pedigrees were screened for alterations in clinical-chemical parameters. We identified 44 animals consistently exhibiting increased plasma urea concentrations. Transmission analysis of the altered phenotype of 23 mice to subsequent generations led to the establishment of five mutant lines. Both sexes were affected in these lines. Urinary urea levels were decreased in the mutants. In addition, most mutants showed increased plasma and decreased urinary creatinine levels. Pathological investigation of kidneys from the five mutant lines revealed a broad spectrum of alterations, ranging from no macroscopic and light microscopic kidney alterations to decreased kidney weight-to-body weight ratio, dilation of the renal pelvis, and severe glomerular lesions. Thus screening for elevated plasma urea levels in a large-scale ENU mouse mutagenesis project resulted in the successful establishment of mouse strains which are valuable tools for molecular studies of mechanisms involved in urea excretion or which represent interesting models for kidney diseases.

N-ethyl-N-nitrosourea-based generation of mouse models for mutant G protein-coupled receptors

Chemical random mutagenesis techniques with the germ line supermutagen N-ethyl-N-nitrosourea (ENU) have been established to provide comprehensive collections of mouse models, which were then mined and analyzed in phenotype-driven studies. Here, we applied ENU mutagenesis in a high-throughput fashion for a gene-driven identification of new mutations. Selected members of the large superfamily of G protein-coupled receptors (GPCR), melanocortin type 3 (Mc3r) and type 4 (Mc4r) receptors, and the orphan chemoattractant receptor GPR33, were used as model targets to prove the feasibility of this approach. Parallel archives of DNA and sperm from mice mutagenized with ENU were screened for mutations in these GPCR, and in vitro assays served as a preselection step before in vitro fertilization was performed to generate the appropriate mouse model. For example, mouse models for inherited obesity were established by selecting fully or partially inactivating mutations in Mc4r. Our technology described herein has the potential to provide mouse models for a GPCR dysfunction of choice within <4 mo and can be extended to other gene classes of interest.

Genetic analysis of synaptotagmin 2 in spontaneous and Ca2+-triggered neurotransmitter release

Synaptotagmin 2 resembles synaptotagmin 1, the Ca2+ sensor for fast neurotransmitter release in forebrain synapses, but little is known about synaptotagmin 2 function. Here, we describe a severely ataxic mouse strain that harbors a single, destabilizing amino-acid substitution (I377N) in synaptotagmin 2. In Calyx of Held synapses, this mutation causes a delay and a decrease in Ca2+-induced but not in hypertonic sucrose-induced release, suggesting that synaptotagmin 2 mediates Ca2+ triggering of evoked release in brainstem synapses. Unexpectedly, we additionally observed in synaptotagmin 2 mutant synapses a dramatic increase in spontaneous release. Synaptotagmin 1-deficient excitatory and inhibitory cortical synapses also displayed a large increase in spontaneous release, demonstrating that this effect was shared among synaptotagmins 1 and 2. Our data suggest that synaptotagmin 1 and 2 perform equivalent functions in the Ca2+ triggering of action potential-induced release and in the restriction of spontaneous release, consistent with a general role of synaptotagmins in controlling 'release slots' for synaptic vesicles at the active zone.

Data and animal management software for large-scale phenotype screening

The mouse N-ethyl-N-nitrosourea (ENU) mutagenesis program at the Genomics Institute of the Novartis Research Foundation (GNF) uses MouseTRACS to analyze phenotype screens and manage animal husbandry. MouseTRACS is a Web-based laboratory informatics system that electronically records and organizes mouse colony operations, prints cage cards, tracks inventory, manages requests, and reports Institutional Animal Care and Use Committee (IACUC) protocol usage. For efficient phenotype screening, MouseTRACS identifies mutants, visualizes data, and maps mutations. It displays and integrates phenotype and genotype data using likelihood odds ratio (LOD) plots of genetic linkage between genotype and phenotype. More detailed mapping intervals show individual single nucleotide polymorphism (SNP) markers in the context of phenotype. In addition, dynamically generated pedigree diagrams and inventory reports linked to screening results summarize the inheritance pattern and the degree of penetrance. MouseTRACS displays screening data in tables and uses standard charts such as box plots, histograms, scatter plots, and customized charts looking at clustered mice or cross pedigree comparisons. In summary, MouseTRACS enables the efficient screening, analysis, and management of thousands of animals to find mutant mice and identify novel gene functions. MouseTRACS is available under an open source license at http://www.mousetracs.sourceforge.net.

Genotype-specific environmental impact on the variance of blood values in inbred and F1 hybrid mice

Mice are important models for biomedical research because of the possibility of standardizing genetic background and environmental conditions, which both affect phenotypic variability. Inbred mouse strains as well as F1 hybrid mice are routinely used as genetically defined animal models; however, only a few studies investigated the variance of phenotypic parameters in inbred versus F1 hybrid mice and the potential interference of the genetic background with different housing conditions. Thus, we analyzed the ranges of clinical chemical and hematologic parameters in C3H and C57BL/6 inbred mice and their reciprocal F1 hybrids (B6C3F1, C3B6F1) in two different mouse facilities. Two thirds of the blood parameters examined in the same strain differed between the facilities for both the inbred strains and the F1 hybrid lines. The relation of the values between inbred and F1 hybrid mice was also affected by the facility. The variance of blood parameters in F1 hybrid mice compared with their parental inbred strains was inconsistent in one facility but generally smaller in the other facility. A subsequent study of F1 hybrid animals derived from the parental strains C3H and BALB/c, which was done in the latter housing unit, detected no general difference in the variance of blood parameters between F1 hybrid and inbred mice. Our study clearly demonstrates the possibility of major interactions between genotype and environment regarding the variance of clinical chemical and hematologic parameters.

N-ethyl-N-nitrosourea (ENU) mutagenesis and male fertility research

Male infertility affects about 1 in 25 men in the western world. Conversely, there is an urgent requirement for additional male-based contraceptives, yet progress in both areas has been severely hampered by a lack of knowledge of the biochemistry and physiology of male reproductive function. It is only through a thorough knowledge of these processes that we can hope to insightfully regulate male reproductive function. Without doubt, mouse models will form an important foundation in any future process. In recent years, the chemical mutagen N-ethyl-N-nitrosourea (ENU) has been used widely to identify genes essential for a range of biological systems including male infertility. These studies have shown random mutagenesis is an attractive means of identifying key genes for male fertility. This technique has distinct, but complementary advantages compared to knockout technologies. Specifically, it allows the removal of researcher bias whereby only pre-conceived genes are tested for function; it produces mice with a guaranteed phenotype and allows for the production of allelic series of mice to dissect all aspects of gene function. ENU mouse mutagenesis programs will enable advances in the diagnosis and treatment of human male infertility and ultimately aid in the development of novel male-based contraceptives.

Efficient and fast targeted production of murine models based on ENU mutagenesis

Mice with targeted genetic alterations are the most effective tools for deciphering organismal gene function. We generated an ENU-based parallel C3HeB/FeJ sperm and DNA archive characterized by a high probability to identify allelic variants of target genes as well as high efficiencies in allele retrieval and model revitalization. Our archive size of over 17,000 samples contains approximately 340,000 independent alleles (20 functional mutations per individual sample). Based on an estimated number of approximately 30,000 mouse genes, the parallel sperm/DNA archive should permit the identification and recovery of ten or more alleles per average target gene which translates to a calculated 99% success rate in the discovery of five allelic variants for any given average gene. The low rate of unrelated ENU-induced passenger mutations has no practical impact on the analysis of the allele-specific phenotype at the G3 generation because of dilution and free segregation of such unrelated passenger mutations. To date 39 mouse models representing 33 different genes have been recovered from our archive using in vitro fertilization techniques. The generation time for a murine model heterozygous for a mutation in a gene of interest is less than 2 months, i.e., three to four times faster compared with current embryonic stem-cell-based technologies. We conclude that ENU-based targeted mutagenesis is a powerful tool for the fast and high-throughput production of murine gene-specific models for biomedical research.

Large-Scale Albuminuria Screen for Nephropathy Models in Chemically Induced Mouse Mutants

BACKGROUND/AIM

Phenotype-driven screening of a great pool of randomly mutant mice and subsequent selection of animals showing symptoms equivalent to human kidney diseases may result in the generation of novel suitable models for the study of the pathomechanisms and the identification of genes involved in kidney dysfunction.

METHODS

We carried out a large-scale analysis of ethylnitrosourea (ENU)-induced mouse mutants for albuminuria by using qualitative SDS-polyacrylamide gel electrophoresis.

RESULTS

The primary albuminuria screen preceded the comprehensive phenotypic mutation analysis in a part of the mice of the Munich ENU project to avoid loss of mutant animals as a consequence of prolonged suffering from severe nephropathy. The primary screen detected six confirmed phenotypic variants in 2,011 G1 animals screened for dominant mutations and no variant in 48 G3 pedigrees screened for recessive mutations. Further breeding experiments resulted in two lines showing a low phenotypic penetrance of albuminuria. The secondary albuminuria screen was carried out in mutant lines which were established in the Munich ENU project without preceding primary albuminuria analysis. Two lines showing increased plasma urea levels were chosen to clarify if severe kidney lesions are involved in the abnormal phenotype. This analysis revealed severe albuminuria in mice which are affected by a recessive mutation leading to increased plasma urea and cholesterol levels.

CONCLUSION

Thus, the phenotypic selection of ENU-induced mutants according to the parameter proteinuria in principle demonstrates the feasibility to identify nephropathy phenotypes in ENU-mutagenized mice.

Tools for target identification and validation

Reliable technologies for addressing target identification and validation are the foundation of successful drug development. Microarrays have been well utilized in genomics/proteomics approaches for gene/protein expression profiling and tissue/cell-scale target validation. Besides being used as an essential step in analyzing high-throughput experiments such as those involving microarrays, bioinformatics can also contribute to the processes of target identification and validation by providing functional information about target candidates and positioning information to biological networks. Antisense technologies (including RNA interference technology, which is recently very 'hot') enable sequence-based gene knockdown at the RNA level. Zinc finger proteins are a DNA transcription-targeting version of knockdown. Chemical genomics and proteomics are emerging tools for generating phenotype changes, thus leading to target and hit identifications. NMR-based screening, as well as activity-based protein profiling, are trying to meet the requirement of high-throughput target identification.

The dominant alopecia phenotypes Bareskin, Rex-denuded, and Reduced Coat 2 are caused by mutations in gasdermin 3

Reduced Coat 2 (Rco2) is an ENU-induced mutation affecting hair follicle morphogenesis by an abnormal and protracted catagen. We describe chromosomal mapping and molecular identification of the autosomal dominant Rco2 mutation. The Rco2 critical region on mouse chromosome 11 encompasses the alopecia loci, Bareskin (Bsk), Rex-denuded (Re(den)), Recombination induced mutation 3 (Rim3), and Defolliculated (Dfl). Recently, the gasdermin (Gsdm) gene was described as predominantly expressed in skin and gastric tissues. We provide evidence for a murine-specific gene cluster consisting of Gsdm and two closely related genes which we designate as Gsdm2 and Gsdm3. We show that Gsdm3 reflects a mutation hotspot and that Gsdm3 mutations cause alopecia in Rco2, Re(den), and Bsk mice. We infer a role of Gsdm3 during the catagen to telogen transition at the end of hair follicle morphogenesis and the formation of hair follicle-associated sebaceous glands.

Hypercholesterolemia in ENU-induced mouse mutants

Hypercholesterolemia is caused by multiple environmental factors and genetic predispositions, and plays an important role in the development and pathogenesis of various human diseases. In this study, we aimed to establish randomly mutant mouse lines showing hypercholesterolemia for their further use in the detection of novel causative alleles. In the Munich ENU Mouse Mutagenesis Project, clinical chemistry blood analysis was performed on more than 15,000 G1 mice and 230 G3 pedigrees of chemically mutagenized mice to detect dominant and recessive mutations leading to an increased plasma total cholesterol level. Using inbred C3HeB/FeJ mice we identified more than 100 animals consistently showing hypercholesterolemia. Transmission of the altered phenotype to the subsequent generations led to the production of nine hypercholesterolemic lines. A single line showed further obvious deviations in the analysis of additional clinical chemistry blood parameters. Thus, the lines produced will contribute to the search for alleles that selectively cause primary hypercholesterolemia.

Vestibular defects in head-tilt mice result from mutations in Nox3, encoding an NADPH oxidase

The vestibular system of the inner ear is responsible for the perception of motion and gravity. Key elements of this organ are otoconia, tiny biomineral particles in the utricle and the saccule. In response to gravity or linear acceleration, otoconia deflect the stereocilia of the hair cells, thus transducing kinetic movements into sensorineural action potentials. Here, we present an allelic series of mutations at the otoconia-deficient head tilt (het) locus, affecting the gene for NADPH oxidase 3 (Nox3). This series of mutations identifies for the first time a protein with a clear enzymatic function as indispensable for otoconia morphogenesis.

A forward-genetic approach for analysis of the immune system

The completion of the genome sequences of both humans and mice challenges biologists to determine gene function on a vast, whole-organism scale. Both phenotype-based ('forward') and gene-based ('reverse') strategies are being developed to approach this issue. Forward-genetic approaches, however, provide the unique ability of assigning function to genes in an unbiased, global manner that is independent of previous assumptions about gene function. In this article, we compare various genetic technologies for their potential role in dissecting immune-system development and function, with particular emphasis on the worldwide efforts that use chemical mutagenesis as a forward-genetic strategy.