Murine albino-deletion complex: high-resolution microsatellite map and genetically anchored YAC framework map

Creating a "hopeful monster": mouse forward genetic screens

One of the most straightforward approaches to making novel biological discoveries is the forward genetic screen. The time is ripe for forward genetic screens in the mouse since the mouse genome is sequenced, but the function of many of the genes remains unknown. Today, with careful planning, such screens are within the reach of even small individual labs. In this chapter we first discuss the types of screens in existence, as well as how to design a screen to recover mutations that are relevant to the interests of a lab. We then describe how to create mutations using the chemical N-ethyl-N-nitrosourea (ENU), including a detailed injection protocol. Next, we outline breeding schemes to establish mutant lines for each type of screen. Finally, we explain how to map mutations using recombination and how to ensure that a particular mutation causes a phenotype. Our goal is to make forward genetics in the mouse accessible to any lab with the desire to do it.

Quantitative Trait Loci Specifying the Response of Body Temperature to Dietary Restriction

Dietary restriction (DR) retards aging and mortality across a variety of taxa. In homeotherms, one of the hallmarks of DR is lower mean body temperature (T(b)), which might be directly responsible for some aspects of DR-mediated life extension. We conducted a quantitative trait locus (QTL) analysis of the response of T(b) to DR in mice using a panel of 22 LSXSS recombinant inbred strains, tested in two cohorts. T(b) in response to DR had a significant genetic component, explaining approximately 35% of the phenotypic variation. We mapped a statistically significant QTL to chromosome 9 and a provisional QTL to chromosome 17, which together accounted for about two thirds of the genetic variation. Such QTLs could be used to critically test whether the response of T(b) to DR also affects the response of life extension. In addition, this study demonstrates the feasibility of trying to map QTLs that affect other physiological responses to DR, including the life extension response. Importantly, the genes underlying such QTLs would be causal factors affecting these responses and could be identified by positional cloning.

Mutations in the clathrin-assembly gene Picalm are responsible for the hematopoietic and iron metabolism abnormalities in fit1 mice

Recessive N-ethyl-N-nitrosourea (ENU)-induced mutations recovered at the fitness-1 (fit1) locus in mouse chromosome 7 cause hematopoietic abnormalities, growth retardation, and shortened life span, with varying severity of the defects in different alleles. Abnormal iron distribution and metabolism and frequent scoliosis have also been associated with an allele of intermediate severity (fit14R). We report that fit14R, as well as the most severe fit15R allele, are nonsense point mutations in the mouse ortholog of the human phosphatidylinositol-binding clathrin assembly protein (PICALM) gene, whose product is involved in clathrin-mediated endocytosis. A variety of leukemias and lymphomas have been associated with translocations that fuse human PICALM with the putative transcription factor gene AF10. The Picalmfit1-5R and Picalmfit1-4R mutations are splice-donor alterations resulting in transcripts that are less abundant than normal and missing exons 4 and 17, respectively. These exon deletions introduce premature termination codons predicted to truncate the proteins near the N and C termini, respectively. No mutations in the genes encoding Picalm, clathrin, or components of the adaptor protein complex 2 (AP2) have been previously described in which the suite of disorders present in the Picalmfit1 mutant mice is apparent. These mutants thus provide unique models for exploring how the endocytic function of mouse Picalm and the transport processes mediated by clathrin and the AP2 complex contribute to normal hematopoiesis, iron metabolism, and growth.

Experimental murine colitis is regulated by two genetic loci, including one on chromosome 11 that regulates IL-12 responses

BACKGROUND & AIMS

Immunogenetic analysis of experimental colitis may contribute to the further unraveling of the complex genetic basis of the inflammatory bowel diseases, Crohn's disease and ulcerative colitis.

METHODS

Genetic regions associated with susceptibility to trinitrobenzene sulfonic acid (TNBS)-induced colitis were identified in a genome-wide linkage analysis in F2 progeny of colitis-susceptible SJL/J and -resistant C57BL/6 mice. An immunogenetic approach was then used to further study the pathophysiologic role of one of the identified loci.

RESULTS

We identified susceptibility loci on chromosomes 9 (Tnbs1) and 11 (Tnbs2). Tnbs2 harbors the interleukin (IL)-12 p40 gene, a likely candidate gene because IL-12 is a known central mediator for both experimental colitis and human Crohn's disease. We therefore tested the ability of colitis-susceptible and -resistant strains to mount IL-12 responses to lipopolysaccharide (LPS), a strong inducer of IL-12 that is abundantly present in the intestine. We observed a remarkably higher serum IL-12 response to LPS in susceptible SJL/J mice. Subsequently, we showed that the genetic region regulating the IL-12 response to LPS colocalizes with Tnbs2.

CONCLUSIONS

These data strongly suggest that the tendency to mount a high LPS-induced IL-12 response and susceptibility to TNBS-induced colitis are related and that in fact a genetically determined high IL-12 response is involved in the immunologic basis of susceptibility to colitis.

Molecular and functional mapping of the piebald deletion complex on mouse chromosome 14

The piebald deletion complex is a set of overlapping chromosomal deficiencies surrounding the endothelin receptor B locus collected during the Oak Ridge specific-locus-test mutagenesis screen. These chromosomal deletions represent an important resource for genetic studies to dissect the functional content of a genomic region, and several developmental defects have been associated with mice homozygous for distinct piebald deletion alleles. We have used molecular markers to order the breakpoints for 20 deletion alleles that span a 15.7-18-cM region of distal mouse chromosome 14. Large deletions covering as much as 11 cM have been identified that will be useful for regionally directed mutagenesis screens to reveal recessive mutations that disrupt development. Deletions identified as having breakpoints positioned within previously described critical regions have been used in complementation studies to further define the functional intervals associated with the developmental defects. This has focused our efforts to isolate genes required for newborn respiration and survival, skeletal patterning and morphogenesis, and central nervous system development.

Physical localization of the mesoderm development (mesd) functional region

The mesoderm development (mesd) functional interval is essential for primitive streak formation and mesoderm induction. Mesd is defined by overlapping albino (c) deletions on chromosome 7. We have constructed a bacterial artificial chromosome (BAC) contig that spans the mesd functional region. BAC end-sequence identifies three segments that recognize novel expressed sequences. Localization of the proximal breakpoints from Del(7)Tyr(c-3YPSd) and Del(7)Tyr(c-112K) within the contig defines a deletion interval of 310-350 kb that is essential for mesd function. Importantly, using BAC transgene rescue, we define a 75-kb mesd critical region containing at least one expressed sequence.

Self-tolerance to the murine homologue of a tyrosinase-derived melanoma antigen: implications for tumor immunotherapy

The human tyrosinase-derived peptide YMDGTMSQV is presented on the surface of human histocompatibility leukocyte antigen (HLA)-A*0201(+) melanomas and has been suggested to be a tumor antigen despite the fact that tyrosinase is also expressed in melanocytes. To gain information about immunoreactivity and self-tolerance to this antigen, we established a model using the murine tyrosinase-derived homologue of this peptide FMDGTMSQV, together with transgenic mice expressing the HLA-A*0201 recombinant molecule AAD. The murine peptide was processed and presented by AAD similarly to its human counterpart. After immunization with recombinant vaccinia virus encoding murine tyrosinase, we detected a robust AAD-restricted cytotoxic T lymphocyte (CTL) response to FMDGTMSQV in AAD transgenic mice in which the entire tyrosinase gene had been deleted by a radiation-induced mutation. A residual response was observed in the AAD(+)tyrosinase(+) mice after activation under certain conditions. At least some of these residual CTLs in AAD(+)tyrosinase(+) mice were of high avidity and induced vitiligo upon adoptive transfer into AAD(+)tyrosinase(+) hosts. Collectively, these data suggest that FMDGTMSQV is naturally processed and presented in vivo, and that this presentation leads to substantial but incomplete self-tolerance. The relevance of this model to an understanding of the human immune response to tyrosinase is discussed.

Mechanisms of mutation induction in germ cells of the mouse as assessed by the specific locus test

Mouse germ cell specific locus mutagenesis data and a molecular characterization of mutant alleles have been reviewed to arrive at an understanding of the mechanism of mutation induction in mammals. (a) The spermatogenic stage specificity for the sensitivity to mutation induction by 20 chemical mutagens is considered. (b) The effects of a saturable repair process and its recovery over time are examined for the mutagenic efficiency of ethylnitrosourea. (c) The mutagenic events following methylnitrosourea and chlorambucil are shown to be mainly deletions. In contrast the mutations recovered after ethylnitrosourea treatment are almost exclusively base pair substitutions. (d) It is emphasized that to date very few specific locus experiments have been designed to test for mutagenic events outside the interval stem cell spermatogonia-mature spermatozoa. A specific locus mutation has recently been shown to be due to loss of heterozygosity via mitotic recombination in an early zygote stage and suggests a broader range of possible mechanisms of mutation when these stages are considered. (e) With the cloning of all 7 marker loci mutation analysis at the molecular level will allow a more direct assessment of the mutation process in future studies.

N-ethyl-N-nitrosourea mutagenesis of a 6- to 11-cM subregion of the Fah-Hbb interval of mouse chromosome 7: Completed testing of 4557 gametes and deletion mapping and complementation analysis of 31 mutations

An interval of mouse chromosome (Chr) 7 surrounding the albino (Tyr; c) locus, and corresponding to a long 6- to 11-cM Tyr deletion, has been the target of a large-scale mutagenesis screen with the chemical supermutagen N-ethyl-N-nitrosourea (ENU). A segment of Chr 7, from a mutagenized genome bred from ENU-treated males, was made hemizygous opposite the long deletion for recognition and recovery of new recessive mutations that map within the albino deletion complex. Over 6000 pedigrees were analyzed, and 4557 of these were completely tested for mutations specifying both lethal and gross visible phenotypes. Thirty-one nonclustered mutations were identified and assigned to 10 complementation groups by pairwise trans-complementation crosses. Deletion-mapping analyses, using the extensive series of radiation-induced Tyr deletions, placed the loci defined by each of these complementation groups into defined intervals of the Tyr-region deletion map, which facilitates the identification of each locus on physical and transcription maps of the region. These mutations identified seven new loci and provided new ENU-induced alleles at three previously defined loci. Interestingly, no mutations were recovered that recapitulated three phenotypes defined by analysis of homozygous or partially complementing albino deletions. On the basis of our experience with this screen, we discuss a number of issues (e.g., locus mutability, failure to saturate, number of gametes to screen, allelic series) of concern when application of chemical mutagenesis screens to megabase regions of the mouse genome is considered.

Physical localization of the mouse aryl hydrocarbon receptor nuclear translocator-2 (Arnt2) gene within the c112K deletion

The albino deletions identify at least seven functional intervals essential for pre- and postnatal development in the 6- to 10-cM region surrounding the albino coat color (c = tyrosinase) locus on mouse chromosome 7. The c112K deletion identifies a putative thymus functional region not removed by the overlapping c3H deletion. Cloning the c3H proximal breakpoint provided a starting point for construction of an 840-kb BAC contig spanning the c112K and c3H (D7Ssb3Hp) proximal breakpoints. These breakpoints are separated by 320-350 kb. The aryl hydrocarbon receptor nuclear translocator-2 (Arnt2) is completely removed by the c112K deletion and spans 130-170 kb of the interval. Although Arnt2 is a candidate for the thymus defects in c112K homozygotes, the possibility that other as yet unidentified genes in the c112K deletion are responsible for the abnormalities has not been ruled out. Arnt2 is a member of the bHLH-PAS (Per, Ahr, Arnt, Sim) family of transcription factors and shares the highest similarity with Arnt. The survival of c112K homozygotes markedly contrasts the embryonic lethality observed in Arnt-deficient embryos and suggests distinct roles for these related transcription factors during embryogenesis.