Physical analysis of murine albino deletions that disrupt liver-specific gene regulation or mesoderm development

Mouse fzd4 maps within a region of chromosome 7 important for thymus and cardiac development

The cardiac neural crest (CNC) plays a central role in development of the thymus gland and cardiovascular system. Through morphological and histological characterization of embryos homozygous for the Del(7)Tyr(c-112K) and Del(7)Tyr(c-3H) albino deletions, we identified abnormalities that are consistent with aberrant development of tissues requiring CNC contributions. The defects include incompletely penetrant heart and great vessel patterning defects and hypoplastic thymus glands. The CNC phenotype is complemented by the partially overlapping deletion Del(7)Tyr(c-23DVT). Combined, these results suggest that a functional region necessary for development of CNC derived tissues is located between the Del(7)Tyr(c-23DVT) and Del(7)Tyr(c-112K) distal deletion breakpoints. This interval encompasses a functional region previously identified as important for juvenile survival (juvenile development and fertility, jdf). Using deletion mapping, we localized the Frizzled4 (Fzd4) gene to the jdf/thymus and cardiac development intervals.

A high-resolution map of the brown (b, Tyrp1) deletion complex of mouse chromosome 4

For over 40 years germ-cell mutagenesis experiments have generated many new mutations at the brown (b or Tyrp1) locus on mouse Chromosome (Chr) 4. These mutations, many of which are deletions, were recovered by the specific-locus mutagenesis technique. Previous analysis of a panel of brown deletions, generated at Oak Ridge, has enabled both a preliminary molecular and a functional map around the locus to be generated. We have used a panel of hybrid DNA from 25 Oak Ridge deletions, where the deleted chromosome was heterozygous with a Mus spretus chromosome, to map polymorphic markers including microclones, microsatellites, and cloned DNA markers. We have generated a fine structure map, based on 25 new markers, of an 8.5-cM region surrounding the brown locus. This map will prove useful in future mapping studies of this region and in the isolation of the genes that lie within it.

Genetic and physical mapping of the fitness 1 (fit1) locus within the Fes-Hbb region of mouse chromosome 7

Mutations at the fit1 locus affect normal pre- and post-natal development by retarding growth and reducing viability. We report mapping of the fit1 locus, by trans-complementation crosses to mice carrying deletions of the albino (c) locus in Chromosome (Chr) 7, to a subregion of the c-deletion complex within the Mod2-sh1 interval. The fit1 locus, which is currently defined by five N-ethyl-N-nitrosourea (ENU)-induced mutations, was found to map in a subregion between the eed and exed loci. A restriction fragment containing a deletion breakpoint that genetically defines the proximal border of fit1 was cloned, providing a DNA probe (RN302) that maps proximal to fit1. Long-range mapping with this probe, and with a DNA probe that maps distal to the fit1 interval, established that the region containing at least part of the fit1 gene is 530 kb or less. Positioning of fit1 between deletion breakpoints, and the isolation and mapping of a DNA probe proximal to it, should facilitate the cloning and molecular characterization of fit1, as well as of the eed locus and the tightly linked l(7)5Rn and l(7)6Rn loci.

msd is required for mesoderm induction in mice

Mesoderm induction is fundamental for establishing the basic body plan of the vertebrate embryo and mutations are critical for dissecting this process. Mouse embryos lacking msd (mesoderm deficiency) do not produce mesoderm but have well-defined extraembryonic and thickened embryonic ectoderm. Distribution of transcripts indicate that temporal regulation of gene expression relevant to gastrulation has begun but primitive-streak formation and mesoderm induction are blocked. Both msd-deficient embryos and embryonic stem (ES) cells fail to form highly differentiated structures of mesoderm origin, but are capable of ectodermal differentiation. Thus, the effects of the msd mutation are restricted to mesoderm formation and could result from the inability to respond to an inducing signal.

A molecular model for the genetic and phenotypic characteristics of the mouse lethal yellow (Ay) mutation

Lethal yellow (Ay) is a mutation at the mouse agouti locus in chromosome 2 that causes a number of dominant pleiotropic effects, including a completely yellow coat color, obesity, an insulin-resistant type II diabetic condition, and an increased propensity to develop a variety of spontaneous and induced tumors. Additionally, homozygosity for Ay results in preimplantation lethality, which terminates development by the blastocyst stage. The Ay mutation is the result of a 170-kb deletion that removes all but the promoter and noncoding first exon of another gene called Raly, which lies in the same transcriptional orientation as agouti and maps 280 kb proximal to the 3' end of the agouti gene. We present a model for the structure of the Ay allele that can explain the dominant pleiotropic effects associated with this mutation, as well as the recessive lethality, which is unrelated to the agouti gene.

Role of mouse germ-cell mutagenesis in understanding genetic risk and in generating mutations that are prime tools for studies in modern biology

Highlights are presented on (1) the role mouse germ-cell mutagenesis has played in assessing the genetic harm from radiations and chemicals, and (2) the contributions to the field of modern biology that are being made by the products of this research--the propagated mutations. Among the numerous findings in radiation mutagenesis were the humped dose-effect curve for spermatogonial stem cells, the major differences between the sexes and between germ-cell stages of each sex in both yield and nature of mutations, the dose-rate effect, which provided the first evidence for repair of mutational (or premutational) damage, the augmenting effect of certain regimes of dose fractionation, and many others. Chemical mutagenesis studies that followed revealed at least three patterns of mutation yield and demonstrated that germ-cell stage--much more than the nature of the chemical--governs the nature of the DNA lesions induced. Two "supermutagens," one for intragenic mutations and one for deletions and other rearrangements, have become very useful in the manufacture of mutations for specific purposes. The mutations propagated from radiation- and chemical-mutagenesis experiments are providing prime resources for basic studies in genome organization, gene structure, and function. DNA lesions that involve specific loci have made possible increasingly detailed characterization of extensive deletion complexes that facilitate high-intensity physical and functional mapping within them. Numerous loci associated with interesting developmental anomalies have been identified and have become accessible to positional cloning. Several of the genes accessed with the aid of induced mutations (deletions, other rearrangements, and point mutations) are furnishing prime reagents for elucidating human disease conditions.

Physical mapping of the albino-deletion complex in the mouse to localize alf/hsdr-1, a locus required for neonatal survival

The albino-deletion complex in the mouse defines a genetically well-characterized region of chromosome 7 in which a number of loci essential for normal development and viability reside. One locus, designated alf or hsdr-1, is necessary for neonatal survival. Its absence results in hypoglycemia associated with biochemical and ultrastructural abnormalities in hepatocytes and proximal tubule cells of the kidney. We constructed a long-range physical map of the region defined by the proximal segment of the albino-deletion complex as a step toward localizing alf/hsdr-1. Sixteen markers, including 11 whose isolation is described here and in the accompanying paper (A. Schedl et al., 1992, Genomics 14, 288-297), were ordered on a panel of albino-deletion DNAs and their distribution was examined by pulsed-field gel electrophoresis. The resulting approximately 4300-kb physical map covers the entire region absent from the prototypic alf/hsdr-1 deletion c14CoS, estimated as approximately 3600 kb. Since the deletion c11DSD complements and overlaps most of c14CoS, alf/hsdr-1 was mapped at the proximal extreme of c14CoS, approximately 3000 kb from the albino locus. The density of CpG islands was found to be very heterogeneous across the region mapped.

Murine fumarylacetoacetate hydrolase (Fah) gene is disrupted by a neonatally lethal albino deletion that defines the hepatocyte-specific developmental regulation 1 (hsdr-1) locus

Homozygous deletion of the hepatocyte-specific developmental regulation 1 (hsdr-1) locus in mouse chromosome 7 results in perinatal death and a pleiotropic syndrome characterized by ultrastructural abnormalities of the liver and kidney, failure of induction of a number of specific transcription units in the liver and kidney during late gestation, and marked overexpression of an enzyme that defends against oxidative stress. Previously, the breakpoints of two albino (c) deletions (c14CoS and c1FAFyh) that genetically define hsdr-1 were localized, on a long-range map, in the vicinity of the distal breakpoint of a viable albino deletion (c24R75M) that breaks proximally within the c locus. Here we report the use of a probe derived from a deletion breakpoint fusion fragment cloned from c24R75M/c24R75M DNA to clone a breakpoint fusion fragment caused by the c14CoS deletion. The proximal breakpoint of the c14CoS deletion was discovered to disrupt a gene (Fah) encoding fumarylacetoacetate hydrolase, the last enzyme in the tyrosine degradation pathway. All of the extant c deletions eliciting the hsdr-1 phenotype prevent expression of the Fah gene in the liver, and all but one disrupt the coding segment of the gene. Therefore, the Fah gene maps within or proximal to the hsdr-1 locus, as defined by deletion breakpoints, and disruption of this gene may be partially or completely responsible for the phenotypes associated with the hsdr-1 deletion syndrome. These mouse mutants may also provide models for the human genetic disorder hereditary tyrosinemia, which is associated with fumarylacetoacetate hydrolase deficiency and liver and kidney dysfunction.

Genomic mapping within the albino-deletion complex using individual early postimplantation mouse embryos

Sensitive methods for analysis of DNA from limited amounts of tissue are often difficult, error prone, and time consuming. Here, we describe a procedure for molecular analysis of individual early postimplantation mouse embryos by Southern analysis. The procedure involves embedding single embryos in agarose before lysing and deproteinizing in situ. The embedded DNA can be digested with restriction enzymes and analyzed by standard Southern-blotting procedures. The procedure is sensitive enough to detect single-copy sequences in embryos as early as day 6.5 of development. We have used the technique to genotype embryos homozygous for an embryonic lethal deletion. Normally, the lethal phenotype associated with such mutations is identified by a retrospective statistical analysis of abnormal embryos produced from a heterozygous cross as compared to those produced from a control cross. Now, if associated with a detectable DNA abnormality, the mutant embryo can be genotyped directly. We also report the use of this method for mapping cloned markers relative to deletion breakpoints. This approach can save considerable time since mapping would conventionally be done using restriction fragment length polymorphisms (RFLPs) detected in Mus musculus/Mus spretus interspecies hybrids. Using this procedure, we have been able to redefine the distal limits of the region of Chromosome (Chr) 7 containing a gene (eed) needed for development of the embryonic ectoderm.