A mouse beta-globin mutant that is an exact model of hemoglobin Rainier in man

Thrombocytopenia and erythrocytosis in mice with a mutation in the gene encoding the hemoglobin β minor chain

Diverse mutations in the genes encoding hemoglobin (Hb) have been characterized in human disease. We describe here a mutation in the mouse Hbb-b2 gene, denoted Plt12, that precisely mimics the human hemoglobin Hotel Dieu variant. The mutation results in increased affinity of Hb for oxygen and Plt12 mutant mice exhibited reduced partial pressure of O(2) in the blood, accompanied by erythrocytosis characterized by elevated erythropoietin levels and splenomegaly with excess erythropoiesis. Most homozygous Hbb-b2(Plt12/Plt12) mice succumbed to early lethality associated with emphysema, cardiac abnormalities, and liver degeneration. Survivors displayed a marked thrombocytopenia without significant deficiencies in the numbers of megakaryocytes or megakaryocyte progenitor cells. The lifespan of platelets in the circulation of Hbb-b2(Plt12/Plt12) mice was normal, and splenectomy did not correct the thrombocytopenia, suggesting that increased sequestration was unlikely to be a major contributor. These data, together with the observation that megakaryocytes in Hbb-b2(Plt12/Plt12) mice appeared smaller and deficient in cytoplasm, support a model in which hypoxia causes thrombocytopenia as a consequence of an inability of megakaryocytes, once formed, to properly mature and produce sufficient platelets. The Plt12 mouse is a model of high O(2)-affinity hemoglobinopathy and provides insights into hematopoiesis under conditions of chronic hypoxia.

Burrow characteristics of the co-existing sibling species Mus booduga and Mus terricolor and the genetic basis of adaptation to hypoxic/hypercapnic stress

BACKGROUND

The co-existing, sibling species Mus booduga and Mus terricolor show a difference in site-preference for burrows. The former build them in flat portion of the fields while the latter make burrows in earthen mounds raised for holding water in cultivated fields. In northern India which experiences great variation in climatic condition between summer and winter, M. booduga burrows have an average depth of 41 cm, as against 30 cm in southern India with less climatic fluctuation.M. terricolor burrows are about 20 cm deep everywhere. The three chromosomal species M. terricolor I, II and III have identical burrows, including location of the nest which is situated at the highest position. In contrast, in M. booduga burrows, the nest is at the lowest position.

RESULTS

The nest chamber of M. booduga is located at greater depth than the nest chamber of M. terricolor. Also, in the burrows of M. booduga the exchange of air takes place only from one side (top surface) in contrast to the burrows of M. terricolor where air exchange is through three sides. Hence, M. booduga lives in relatively more hypoxic and hypercapnic conditions than M. terricolor.We observed the fixation of alternative alleles in M. booduga and M. terricolor at Superoxide dismutase-1 (Sod-1), Transferrin (Trf) and Hemoglobin beta chain (Hbb) loci. All the three are directly or indirectly dependent on oxygen concentration for function. In addition to these, there are differences in burrow patterns and site-preference for burrows suggesting difference in probable adaptive strategy in these co-existing sibling species.

CONCLUSION

The burrow structure and depth of nest of the chromosomal species M. terricolor I, II and III are same everywhere probably due to the recency of their evolutionary divergence. Moreover, there is lack of competition for the well-adapted 'microhabitats' since they are non-overlapping in distribution. However, the co-existing sibling species M. booduga and M. terricolor exhibit mutual "exclusion" of the 'microhabitats' for burrow construction. Thus, location, structure and depth of the burrows might have been the contributory factors for selection of alternative alleles at three loci Sod-1, Trf and Hbb, which reflect difference in probable adaptive strategy in M. booduga and M. terricolor.

Genomic and Phylogenic Comparisons of theα-Globin andβ-Globin Intergenic Sequences between Zebra Fish (Danio rerio) and Six Closely RelatedCyprinindaeSpecies

The alpha-globin and beta-globin genes of the zebrafish are tightly linked on the same chromosome in a 3'-5' and 5'-3' configuration, respectively. Although the location of the controlling sequences has been mapped to the intergenic region, analysis determining the uniqueness of this unusual arrangement to zebrafish has not been undertaken. To explore this, we isolated, sequenced, and phylogenetically analyzed the intergenic region between globin gene families of seven Cyprinindae species including zebrafish. These species were grouped into an in group (immediate relatives, not so distant relatives), and an out group (distant relative). Cellulose acetate electrophoresis of hemoglobin (Hb) detected multiple variants in each species, but a band with electrophoretic mobility (EM) of 6.7 x 10(-5) cm(2).volt(-1).sec(-1) was shared between species. Polymerase chain reaction (PCR) amplification of the intergenic globin gene region also detected a 1.0-kb fragment that was repeated in the in group and a 1.2-kb fragment in the out group. Sequence comparison confirmed that the genetic orientation and controlling sequences location were conserved throughout this region in all seven species. This phylogenic footprinting indicated that the configuration was not exclusive to zebrafish. To confirm sequence alignment, maximum parsimony phylogenic analysis, was performed. Only one member of that group the giant danio, was not closely clustered, being located almost equidistance between the immediate relative and the species of the other clusters. This may represent an ancestral configuration prior to transposition of the alpha globin and beta-globin genes families to nonsynteny.

Biochemical and recessive visible specific locus responses of C3H/HeH to fractionated, acute radiation

The recessive visible specific locus test has been widely used for many years to investigate the genetic effects of radiation in mice. We devised an electrophoretic-specific locus test so that biochemical mutations leading to alterations in the activity or amount of four enzymes and proteins, as well as charge changes could be detected. We measured the yield of recessive visible and electrophoretic mutations in the same experiment so that a direct comparison of mutation incidence could be made. Dominant visible mutations were also scored. The recessive visible specific locus response of male C3H/HeH to a fractionated dose of 3 + 3 Gy X-irradiation separated by 24 h was similar to that previously reported for the F1 hybrid widely used in mutagenesis studies, and other strains. The response of C3H/HeH was significantly greater for the recessive visible mutations than for the biochemical mutations, supporting the contention that the recessive visible loci are more mutable than others. Mutational analysis of some of the mutants showed that the lesions ranged from a very deletion (30% of chromosome 14 deleted) to a point mutation. The number of loci scored in the electrophoretic test has been reassessed, and it is now considered that six, not four were scored, and this has implications for the calculation of the doubling dose.

Tools for targeted manipulation of the mouse genome

In the postgenomic era the mouse will be central to the challenge of ascribing a function to the 40,000 or so genes that constitute our genome. In this review, we summarize some of the classic and modern approaches that have fueled the recent dramatic explosion in mouse genetics. Together with the sequencing of the mouse genome, these tools will have a profound effect on our ability to generate new and more accurate mouse models and thus provide a powerful insight into the function of human genes during the processes of both normal development and disease.

Using targeted large deletions and high-efficiency N-ethyl-N-nitrosourea mutagenesis for functional analyses of the mammalian genome

The Human Genome Project has generated nucleotide sequences from an estimated 80,000 to 100,000 genes, only a small fraction of which have a known role. Nucleotide sequence information alone is insufficient to predict gene function. One of the most powerful ways of revealing gene function, as demonstrated in bacteria, worms, yeast, and flies, is to generate mutations and characterize them at both the phenotypic and the molecular levels. Given the physiological and anatomical parallels between mouse and human, genotype-phenotype relationships established in mice can be extrapolated to human syndromes. A new method is described for functional genetic analyses in the mouse that uses loxP/Cre engineering to generate coat color-tagged large deletions. The haploid regions can then be dissected by mutagenesis with N-ethyl-N-nitrosourea in phenotype-driven screens to obtain functional information on genes in any desired region of the mouse genome.

Sequence characterization of ENU-induced mutants of glucose phosphate isomerase in mouse

Four of five mutations producing GPI1 null lethal phenotypes in the homozygous state, which were previously identified from the offspring of male mice, spermatogonially treated with N-ethyl N-nitrosourea (ENU), have been characterized at the nucleotide level by reverse transcription of RNA from heterozygotes for mutant and wild-type alleles and cycle sequencing with cDNA-derived primers. In three of the mutations studied, a single nucleotide substitution, altering the predicted amino acid on translation, was observed in the mutant allele. In Gpi1-sam1H amino acid residue 277, TCA Ser (wild type), is altered to CCA Pro, and in Gpi1-sbm3H and Gpi1-sbm4H amino acid residue 510 Asp GAC (wild type) is altered to GGC Gly. These ENU-induced mutations occur at A-T base pairs in agreement with the current view of the mechanism of action for this mutagen. These changes also occur at residues implicated as being important in the catalytic functioning of the enzyme, from crystallographic studies, and may explain the loss of enzyme function. The fourth identified mutation, Gpi1-sbm2H, is a deletion of amino acid residues Arg134 to Leu162 inclusive, which may arise from incorrect splicing of mRNA; a fifth mutation has remained undetermined.

Analysis of mouse hemoglobin alpha-chain locus mutation induced by N-ethyl-N-nitrosourea

In a mouse specific-locus test in stem-cell spermatogonia of mice after treatment with N-ethyl-N-nitrosourea (ENU), a mutation at the hemoglobin alpha-chain locus was obtained. In an original mutant, the mutation was detected as a change in band density using cellulose acetate electrophoresis, e.g. the slow moving band was much higher than that of normal F1 mice. Yet, migration of the mouse bands was similar. In isoelectric focusing, the migration of a band toward cathode was observed, indicating that the variant hemoglobin is caused by mutation at an alpha-globin chain. This mutation was found to be heritable and viable in homozygous condition from a genetic test on the mutant, suggesting that this mutation would be due to a base-pair change. According to the usual nomenclature of mutation at the hemoglobin locus, the allele symbol of this mutation was tentatively designated as Hbacc3.

Temporal and molecular characteristics of mutations induced by ethylnitrosourea in germ cells isolated from seminiferous tubules and in spermatozoa of lacZ transgenic mice

The lacZ transgenic mouse (Muta mouse) model was used to examine the timing of ethylnitrosourea (ENU)-induced mutations in germ cells. The spectrum of mutations was also determined. Animals received five daily treatments with ENU at 50 mg/kg and were sampled at times up to 55 days after treatment. In mixed germ-cell populations isolated from seminiferous tubules, there was little increase in the mutant frequency 5 days after treatment; subsequently, there was a continuous increase until the maximum (17.5-fold above background) was reached by approximately 35 days. In the spermatozoa, an increase in mutant frequency was not seen until 20 days after treatment, with the maximum (4.3-fold above background) being achieved no sooner than approximately 35 days. Based on the timing of sampling, these data demonstrate the detection of both spermatogonial and postspermatogonial, mutations. The most prominent feature of the ENU-induced base-pair mutations in testicular germ cells sampled 55 days after treatment is that 70% are induced in A.T base pairs, compared to only 16% in spontaneous mutations. These findings are consistent with comparable data from ENU studies using assays for inherited germ-cell mutations in mice. This study has demonstrated the utility and potential of the transgenic mouse lacZ model (Muta mouse) for the detection and study of germ-cell mutations and provides guidance in the selection of simplified treatment and sampling protocols.

Mutagenesis and human genetic disease: an introduction

This special issue attempts to provide a fresh perspective on the importance of germ-cell mutagenesis studies and restate the questions and challenges inherent in efforts to minimize the incidence of human genetic diseases. We are working in a time when rapidly advancing molecular technologies provide the tools that permit a more detailed understanding of germ-cell mutagenesis and genetic disease. Meanwhile, discoveries of new genetic disease phenomena challenge our abilities to conceive and develop research models for their study. It is hoped that the collection of articles in this issue will serve to stimulate interest in scientists of varied disciplines and help focus those interests on the issues surrounding the relationship between environmental mutagens and human genetic disease.

Analysis of in vivo mutation induced by N-ethyl-N-nitrosourea in the hprt gene of rat lymphocytes

The rat lymphocyte hprt assay measures in vivo mutagenicity by quantifying the frequency of 6-thioguanine-resistant (TGr) spleen lymphocytes cultured in vitro. In this study we have examined the types of mutations induced in the hprt gene of TGr lymphocyte clones from female Fischer 344 rats exposed to 100 mg/kg N-ethyl-N-nitrosourea (ENU). Hprt exons 3 and 8 were amplified from DNA extracted from each of 249 clones, and the resulting products were screened for mutant:wild-type heteroduplex formation by denaturing gradient gel electrophoresis. The analysis revealed 59 clones with mutations in exon 3, and 20 clones with mutations in exon 8. DNA sequence analysis of the heteroduplexes identified 84 mutations: all of the mutations were base pair substitutions, and 88% were mutations of A:T base pairs. At least 82% were induced independently. These results suggest that the mutations found in TGr rat lymphocytes from ENU-treated rats were due mainly to ethylthymidine adducts. In addition, a comparison of these results with previously reported in vivo ENU mutational profiles indicates that the types of mutation detected by heteroduplex screening of rat hprt exons 3 and 8 are representative of mutation in the entire protein coding sequence.

Characterization of mutations induced by ethylnitrosourea in seminiferous tubule germ cells of transgenic B6C3F1 mice

Transgenic B6C3F1 mice carrying a lacI target gene were exposed to acute and multiple doses of ethylnitrosourea (ENU), and germ cells from the seminiferous tubules were assayed for mutation 3 and 90 days after treatment. Relative to untreated controls, the mutation frequency increased 3.2- and 19.9-fold at 3 and 90 days after treatment, respectively. Mutant lacI genes recovered from untreated and treated groups were sequenced, and the spectra of mutations were determined. Eighty-five percent (11/13) of the spontaneous mutations resulted in G.C-->A.T transitions, all of which occurred at CpG dinucleotides. Fifteen of 22 sites (68%) found mutated 3 days after ENU treatment occurred at G.C base pairs, although some of these are expected to be spontaneous mutations. Ninety days after treatment, 13 of 19 sites (68%) found mutated occurred at A.T base pairs. The mutation spectra seen are consistent with proposed mechanisms of ENU mutagenesis and correlate with the in vivo spectra seen in ENU studies by using transmissibility assays and the hprt gene. These findings represent significant progress toward defining the in vivo spectra of ENU mutagenesis in mammalian germ cells.

A consideration of the advantages and potential difficulties of the use of transgenic mice for the study of germinal mutations

The utility of transgenic mouse systems in the study of germ-cell mutations is discussed. These systems promise to fill a gap in the evaluation of potential genotoxic agents between the identification of mutagens in short-term test systems and evaluation of human genetic risk. A less appreciated major contribution that transgenic systems can make is as research tools for achieving an understanding of the mechanisms of mutation induction in germ cells. Questions concerning the germ-cell mutations using transgenic systems include whether these systems can detect large genetic lesions, whether they can detect mutations in repair-deficient male germ-cell stages, whether it is valid to extrapolate mutational spectra from transgenes to endogenous genes, and whether the transgenic systems can be used to address issues concerning differences in locus sensitivities to mutation. Available shuttle-vector systems are not suitable for the direct detection of mutations in female germ cells. Future directions for development include the use of the present systems in research and testing and the development of systems with new capabilities.

Genetic effects from exposure to hazardous agents

Mammalian germ cell stages exhibit differences in DNA synthesis activity, capability to repair DNA damage, and chromosome-associated proteins. The sensitivity to mutation induction may be influenced by such factors as the accessibility of DNA to chemical mutagens, the interval between DNA damage induction and the next round of DNA replication, and the repair of DNA damage. Such qualitative and quantitative differences indicate the complexities of mutation induction in vivo and emphasize that no single in vitro test system can adequately represent the in vivo situation. Therefore, germ-cell mutagenesis in humans can most adequately be represented by an in vivo mammalian germ-cell test system. Information regarding the mechanisms of mutation induction in germ cells of the mouse, appropriate mutation test systems available in the mouse, as well as principles of chemical mutagenesis in the mouse and their implications for an adequate human genetic risk estimation will be discussed.

ENU-induced mutagenesis at a single A: T base pair in transgenic mice containing phi X174

Transgenic mice containing the bacteriophage phi X174 am3 as a chromosomally integrated and recoverable marker for in vivo mutation have been produced to measure spontaneous and induced substitutions at an A:T base pair among single copies. phi X174 was chosen for its small size (5 kb), unique sequence, and the opportunity to take advantage of previously reported in vitro data on mutation and repair; the am3 site provides sequence specificity in a reversion assay for mutation of an A:T base pair. Inbred C57Bl/6 mice have been made homozygous for approximately 100 copies of the the phage sequence without any apparent detrimental effects on the homozygous individuals. Recoveries of phage from mouse tissues are in the range of 1-5 x 10(7) PFU per micrograms mouse DNA; both recovery and mutation are independent of endogenous CpG methylation. Background mutation frequencies are 2-4 x 10(-7) among phage recovered from liver, brain, spleen, and kidney. Adult mice were treated with 200 mg/kg N-ethyl-N-nitrosourea, and phage were recovered at 2 and 14 days after treatment. At 2 days after treatment we observed a slight increase only among phage isolated from the brain of one mouse out of four. At 14 days after ENU treatment, there were significant increases in mutation frequencies among phage recovered from the liver (6 x) and spleen (10 x). These results demonstrate (1) response of a single A:T base pair to alkylation-induced mutation in a nonexpressed gene, (2) the role of cell proliferation in somatic mutagenesis, and (3) provide a model for a transgenic approach for study of site-specific mutagenesis in vivo in higher eukaryotes.

ENU mutagenesis in the mouse electrophoretic specific-locus test. 2. Mutational studies of mature oocytes

Experiments were conducted using the biochemical specific-locus test to assess the mutagenicity of N-ethyl-N-nitrosourea (ENU) in mature oocytes of mice. C57Bl/6J females were treated with 100 mg/kg ENU by intraperitoneal injection and mated to untreated DBA males for 1 week following treatment. 1447 progeny were screened for evidence of mutations affecting the electrophoretic mobility of 32 different proteins; two mutants were detected by electrophoretic analyses. These results provide evidence that ENU is a germ-cell mutagen in mouse mature oocytes, although the frequency of mutants is somewhat lower than that obtained from spermatogonia treated with the same dose.

Oxygen association-dissociation and stability analysis on mouse hemoglobins with mutant alpha- and beta-globins

Oxygen association-dissociation and hemoglobin stability analysis were performed on mouse hemoglobins with amino acid substitutions in an alpha-globin (alpha 89, His to Leu) and a beta-globin (beta 59, Lys to Ile). The variant alpha-globin, designated chain 5m in the Hbag2 haplotype, had an high oxygen affinity and was stable. The variant beta-globin, (beta s2) of the Hbbs2 haplotype, also had an elevated oxygen affinity and in addition was moderately unstable in 19% isopropanol. Hemoglobins from the expected nine (Hbag2/Hbag2;Hbbs/Hbbs x Hbaa/Hbaa;Hbbs2/Hbbs2) F2 genotypes can be grouped into five classes of P50 values characterized by strict additivity and dependency on mutant globin gene dosage; physiologically, both globin variants gave indistinguishable effects on oxygen affinity. The hemoglobin of normal mice (Hbaa/Hbaa;Hbbs/Hbbs) had a P50 = 40 mm Hg and the hemoglobin of Hbag2/Hbag2;Hbbs2/Hbbs2 F2 mice had a P50 = 25 mm Hg (human P50 = 26 mm Hg). Peripheral blood from Hbag2/Hbag2;Hbbs/Hbbs, Hbaa/Hbaa;Hbbs2/Hbbs2 and Hbag2/Hbag2;Hbbs2/Hbbs2 mice exhibited normal hematological values except for a slightly higher hematocrit for Hbag2/Hbag2;Hbbs/Hbbs and Hbag2/Hbag2;Hbbs2/Hbbs2 mice, slightly elevated red cell counts for mice of the three mutant genotypes, and significantly lower values for the mean corpuscular volume and mean corpuscular hemoglobin for Hbag2/Hbag2;Hbbs2/Hbbs2 mice.

The molecular characterization of an A:T to G:C transition in the Hbb-b1 gene of the murine homologue of hemoglobin Rainier

An N-ethyl-N-nitrosourea (ENU)-induced mutation in the Hbb-b1 gene of the mouse hemoglobin-beta complex (Hbb) has been shown to result in a high-oxygen affinity hemoglobin, homologous with hemoglobin Rainier in man (Peters, J., et al., Genetics 110:709, 1985). Substitution of beta 145 tyrosine by cysteine had occurred in both human and mouse forms, probably as the result of a point mutation. Provided that sufficient sequence information is available, point mutations can be directly and rapidly analyzed by allele-specific amplification (ASA), as this technique is sensitive enough to detect single nucleotide differences. We report the use of ASA to detect and characterize the mutation in the murine beta-globin gene, Hbb-b1d-m1, and find that the codon for beta 145 tyrosine (TAC) has been replaced by the codon for cysteine (TGC). Therefore, ENU induced an A:T----G:C transition.

ENU mutagenesis in the mouse electrophoretic specific-locus test, 1. Dose-response relationship of electrophoretically-detected mutations arising from mouse spermatogonia treated with ethylnitrosourea

The mouse electrophoretic specific-locus test for induced germ-cell mutations, was used to determine the response of spermatogonial stem cells to a series of doses of the germ cell mutagen N-ethyl-N-nitrosourea (ENU). Male DBA/2J and C57B1/6J mice were treated with doses of 50, 100, 200 or 250 mg/kg ENU and their progeny screened for electrophoretically-detectable mutations at 32 separate loci. As expected, increasing doses of ENU led to increasing mutant frequencies. The differences in mutant frequencies between treated DBA/2J and C57B1/6J males were not statistically significant.

The induction of forward and reverse specific-locus mutations and dominant cataract mutations in spermatogonia of treated strain DBA/2 mice by ethylnitrosourea

The mutagenic effectiveness of ethylnitrosurea (ENU) was assessed in treated spermatogonia of DBA/2 mice. In a total of 17,515 offspring examined following 160 mg ENU/kg body weight treatment of parental males, 26 forward specific-locus mutations, 2 reverse specific-locus mutations and 9 dominant cataract mutations were recovered. ENU increased the mutation rate to all 3 genetic endpoints. However, ENU was less effective in treated DBA/2 mice than in the standard experimental protocol employing treated hybrid (102 X C3H)F1 male mice. This observed difference for a direct-acting mutagen such as ENU may result from differences in the detoxification of ENU or from differences in the DNA-repair capabilities of strain DBA/2. The first documented reverse mutation of the b allele is reported. The reversion was shown to be due to an AT to GC transition. To date, in addition to the reverse mutation of the b allele, 5 independent ENU-induced mutations recovered in germ cells of the mouse have been molecularly characterized and all have been shown to be base substitutions at an AT site. This is in contrast to the expected mechanism of ENU mutation induction due to O6-ethylguanine adduct formation which results in a GC to AT base-pair substitution and emphasizes the complexities of mutagenesis in germ cells of mammals.

A new haplotype of the beta-globin gene complex, Hbbw1, in Chinese wild mouse

A new pattern was observed in the electrophoretic survey of the hemoglobin beta chain (Hbb) in Chinese wild mice, Mus musculus. The electrophoretic mobility of the major component of the new Hbb was identical to that of Hbbs on cellulose acetate plate, although it was almost identical to that of Hbbd or Hbbp on acrylamide gel. This suggests that the major component of Chinese Hbb has a unique primary structure. A minor component of the new Hbb was completely different from that of the other three Hbb haplotypes well known. These results indicate that the Hbb-b1 and Hbb-b2 of the new Hbb haplotype, assigned Hbbw1, are unique genes in their molecular structure. So far, Hbbw1 has been observed in northwestern China.

Mast cell growth factor maps near the steel locus on mouse chromosome 10 and is deleted in a number of steel alleles

Many spontaneous, chemical-induced, and radiation-induced dominant white spotting (W) and steel (Sl) mutations have been identified in the mouse. W and Sl mutations have similar phenotypic effects including deficiencies in pigment cells, germ cells, and blood cells, Numerous studies have suggested that W acts within the affected cell while Sl instead exerts its effects in the extracellular environment. Recent findings demonstrating that W encodes the c-kit proto-oncogene, a tyrosine kinase membrane receptor, have suggested that Sl encodes a ligand for c-kit. In the accompanying article we report the identification and purification of mast cell growth factor (MGF), a c-kit ligand. Here we describe the cloning of sequences encoding MGF. Furthermore, we show that Mgf maps near Sl in the distal region of mouse chromosome 10 and is deleted in a number of Sl alleles. These findings strongly support the notion that Sl encodes the mast cell growth factor.

The molecular basis of brown, an old mouse mutation, and of an induced revertant to wild type

The murine b locus encodes the tyrosinase related protein, TRP-1, a putative membrane-bound, copper-containing enzyme having about 40% amino acid identity with tyrosinase. The protein is essential for production of black rather than brown hair pigment. We show that skin of mutant brown mice contains the same amount of TRP-1 mRNA as wild type. On sequencing the coding region of the mutant mRNA we find four nucleotide differences from the wild-type (Black) sequence. Two of these differences result in different amino acid residues encoded by the brown allele. By sequencing the TRP-1 gene from a mouse in which a reversion from brown to Black has been induced by ethylnitrosourea we are able to show that only one of these amino acid changes, which substitutes a tyrosine for a conserved cysteine, is the cause of the brown phenotype. This mutation is adjacent to another cysteine at which, in the analogous position in tyrosinase a mutation results in the albino phenotype. The sequence of the revertant is the first report of DNA sequence of an ethylnitrosourea-induced genetic change in mouse.

ENU-induced allele of brachyury (Tkt1) exhibits a developmental lethal phenotype similar to the original brachyury (T) mutation

New alleles of brachyury (Tkt1, Tkt4) were induced in the mouse complete tw5 haplotype by ethylnitrosourea (ENU). Like the original brachyury (T) mutation, the new alleles cause a short-tailed phenotype in heterozygotes, and interact with the t complex tail interaction factor (tct) in trans to cause phenotypically tailless mice. Because ENU is mainly a point mutagen, it is important to determine that the new alleles are homozygous embryonic lethal mutations like the original T allele, and to characterize their embryonic lethal phenotype. Moreover, the Tkt1 mutation maps to an inverted position relative to quaking (qk) in t haplotypes as compared with its position on normal chromosome 17. The Tkt1 allele was separated from the resident tw5 lethal gene, tclw5, by recombination, allowing embryology studies to be performed. Embryological analyses show that the Tkt1 allele is nearly identical to the classic T allele. At 9 and 10 days of development, homozygous Tkt1/Tkt1 embryos are grossly abnormal with properties including 1) irregular, disorganized somite pairs, 2) a shortened posterior end of the embryo, 3) an irregular neural tube, and 4) an abnormal notochord. In addition, 10 day-old abnormal embryos have anterior limb buds that point dorsally rather than ventrally, and are smaller than normal littermates. We conclude that the Tkt1 mutation is a valuable allele for both mapping and molecular characterization of the brachyury locus.

High resolution metrical analysis applied to the assessment of damage associated with induced mutations in the mouse

Morphometric methods were used to investigate variation in the skeletons of 1030 offspring produced from matings of male DBA/2J by female C57BL/6J mice. 751 offspring originated from males that had received a single intraperitoneal injection of ethyl nitrosourea (EtNU) at a dose of 250 mg/kg. The remainder of the mice served as controls. The male parents of the controls were injected only with the buffer used as vehicle for the EtNU. Offspring were obtained for 3 weeks following injection. The treated males were then sterile for about 8 weeks. Immediately after the sterile period another sample of progeny was obtained. In the treated group, litter sizes at birth and weaning were reduced and survival to adulthood was lower. However, none of the differences were statistically significant. The skeletons were evaluated by two independent approaches. The first relied upon gross observation for unusual phenotypic variation, the second on a series of metrical measurement and coordinate data. A considerable amount of variation was recorded by both approaches. Some of the variants were severe but others were mild and perhaps of little or no importance to the health of the mice. The gross observation method produced no evidence for increased mild or severe variants in any group of offspring from the treated mice. The metrical methods also showed no evidence for treatment-related effects in offspring produced during the first 3 weeks of mating. However, in offspring produced after the sterile period, a pronounced, very highly statistically significant increase in all levels of metrical variation was observed. This treatment group revealed both increased variant measures and increased numbers of mice with variant measures. Much of this variation was so slight that it would have escaped notice were it not for the exacting measurements used in the analysis. Our morphometric approach is an analytically powerful tool, suitable for detecting variation in virtually any biological structure that can be measured. If the increased variation reported here is due to induced mutations, the effects would be consistent with that expected from slightly harmful mutations distributed throughout the mouse genome. It is appropriate to consider such effect in connection with genetic risk estimation.

Biochemical and molecular analysis of spontaneous and induced mutations at the mouse Mod-1 locus

We have analyzed five Mod-1 (malic enzyme) mutants at the molecular and biochemical level. Four of these mutants, three electrophoretic variants and one null mutant, were induced by ethylnitrosourea (ENU). Another null mutant was the result of a spontaneous mutation. All of these mutations were heritable in a Mendelian fashion and viable in the homozygous condition. Restriction endonuclease and Southern blot analysis revealed that the spontaneous null mutant possessed an altered restriction fragment banding pattern. All of the ENU-induced mutants possessed normal restriction fragment banding patterns. All 5 mutants produced normal levels of Mod-1-specific mRNA. Only the spontaneous null mutant produced mRNA with altered size, which was consistent with the altered DNA-banding pattern. MOD-1 enzyme activity levels were normal in the three ENU-induced mutants with altered electrophoretic mobility. Enzyme activity was significantly lower than normal in tissues from animals homozygous for the null alleles, however, using Western blot analysis, low but significant levels of MOD-1 protein in Mod-1 null homozygotes were detected.

Induction of specific locus mutations in mouse spermatogonial stem cells by combined chemical X-ray treatments

Data that demonstrate how the biology of spermatogenesis plays an important role in determining the yield of genetic damage from ionizing radiation are briefly reviewed. It is suggested that for valid extrapolations of data from mouse mutation experiments to man detailed knowledge of the spermatogonial stem cell systems in the two species is required. Two new sets of mouse specific mutation data are presented. (1) When a 2 mg/kg dose of triethylenemelamine (TEM) was used as a conditioning dose and followed 24 h later by 6 Gy X-rays, the mutation yield from spermatogonial stem cells was over twice as high (30.20 X 10(-5)/locus/gamete) as that when the X-ray dose was given alone (13.75 X 10(-5)/locus/gamete). No such effect was found when the TEM was given only 3 h prior to the X-irradiation. Since TEM at the dose used is inefficient at inducing specific-locus mutations, an augmentation of the X-ray response is indicated. It has therefore been concluded that the augmented mutation responses obtained with equal 24 h X-ray fractionations at high doses are attributable to mutation induction by the second dose. The responsive cells would be the formerly resistant component of the stem cell population that had survived the TEM treatment and that had been 'triggered' into a radiosensitive phase by the population depletion. (2) When 2 doses of 500 mg/kg hydroxyurea (HU) were given 3 h apart 3 h prior to 6 Gy X-rays to reduce the numbers of stem cells in the S and G2 phases of the cell cycle exposed to the radiation, the mutation responses was greatly enhanced to a level that is the highest yet recorded per unit X-ray dose (7.10 X 10(-5)/locus/gamete/Gy). No such effect was obtained when the intervals between the HU and X-ray treatments were either shorter (less than 0.5 h) or longer (24 h). It was concluded that X-ray-induced specific-locus mutations derive principally from stem cells in the G1 phase of the cell cycle. The reasons why the X-ray-induced mutation-yields from repopulating stem cells (with a short cell cycle and, hence, short G1 phase) are similar to those from undamaged stem cell populations, in contrast to translocation yields, therefore remains unresolved.

Chromosome maps of man and mouse. IV

Current knowledge of man-mouse genetic homology is presented in the form of chromosomal displays, tables and a grid, which show locations of the 322 loci now assigned to chromosomes in both species, as well as 12 DNA segments not yet associated with gene loci. At least 50 conserved autosomal segments with two or more loci have been identified, twelve of which are over 20 cM long in the mouse, as well as five conserved segments on the X chromosome. All human and mouse chromosomes now have conserved regions; human 17 still shows the least evidence of rearrangement, with a single long conserved segment which apparently spans the centromere. The loci include 102 which are known to be associated with human hereditary disease; these are listed separately. Human parental effects which may well be the result of genomic imprinting are reviewed and the location of the factors concerned displayed in relation to mouse chromosomal regions which have been implicated in imprinting phenomena.

Three ENU-induced alleles of the murine quaking locus are recessive embryonic lethal mutations

The quaking (qk) locus on mouse chromosome 17 has been defined by a single viable quaking allele. Three new alleles of quaking were selected after ENU mutagenesis by their failure to complement the quaking phenotype. Theqkk2allele was induced on wild-type chromatin and theqkkt1andqkkt4alleles were induced ont-chromatin. Each is a recessive embryonic lethal mutation. They fail to complement each other and are not complemented by the deletion,TtOrl. Homozygotes and hemizygotes die at 8–9·5 days gestation, but not at a single precise time. Because the classical quaking mutation complements the lethality of these new alleles, but they fail to complement its quaking phenotype (myelination defect), we conclude that the quaking+function is required for embryonic survival as well as for myelination.

N-ethyl-N-nitrosourea-induced null mutation at the mouse Car-2 locus: an animal model for human carbonic anhydrase II deficiency syndrome

Electrophoretic screening of (C57BL/6J x DBA/2J)F1 progeny of male mice treated with N-ethyl-N-nitrosourea revealed a mouse that lacked the paternal carbonic anhydrase II (CA II). Breeding tests showed that this trait was heritable and due to a null mutation at the Car-2 locus on chromosome 3. Like humans with the same inherited enzyme defect, animals homozygous for the new null allele are runted and have renal tubular acidosis. However, the prominent osteopetrosis found in humans with CA II deficiency could not be detected even in very old homozygous null mice. A molecular analysis of the deficient mice shows that the mutant gene is not deleted and is transcribed. The CA II protein, which is normally expressed in most tissues, could not be detected by immunodiffusion analysis in any tissues of the CA II-deficient mice, suggesting a nonsense or a missense mutation at the Car-2 locus.

Chromosome maps of man and mouse, III

Data on loci whose positions are known in both man and mouse are presented in the form of chromosomal displays, a table, and autosomal and X-chromosomal grids. At least 40 conserved autosomal segments with two or more loci, as well as 17 homologous X-linked loci, are now known in the two species, in which mitochondrial DNA is also highly conserved. Apart from the Y, the only chromosome now lacking a conserved group is human 13. Human 17 has a single conserved group which includes both short and long arms, and so may have remained largely intact in mammalian evolution. Human and mouse chromosomal maps show the approximate locations of homologous genes while the mouse map also shows the positions of translocations used in gene location.