Use of intercross outbred mice and single nucleotide polymorphisms to map skin cancer modifier loci

Car-R and Car-S outbred mouse lines, phenotypically selected for resistance and susceptibility to skin carcinogenesis respectively, show significant linkage disequilibrium (LD) at genetic markers mapping on chromosomal regions where skin cancer modifier loci (Skts3, Skts1, and Psl1 on Chrs 5, 7, and 9 respectively) have been mapped in standard crosses. Analysis of these regions for genetic linkage with skin cancer phenotypes in 245 (Car-R × Car-S)F₂ intercross mice, by using single nucleotide polymorphisms (SNPs), revealed significant linkage at a possible allelic form of the Skts1 locus, whose mapping region was shortened to a <5.5-cM interval near the Tyr locus. The Car-derived Skts1 locus was linked with papilloma multiplicity and latency by a recessive inheritance of the susceptibility allele. Putative loci on Chr 5 (Skts3) and 9 (Psl1) showed no significant linkage. These results point to the important role of the Stks1 locus in mouse skin tumorigenesis in independent crosses. The shortened Skts1 mapping region should facilitate the identification of candidate genes. Received: 23 June 2000 / Accepted: 22 November 2000     

The genetic control of chemically and radiation-induced skin tumorigenesis: a study with carcinogenesis-susceptible and carcinogenesis-resistant mice

Outbred carcinogenesis-resistant (Car-R) and carcinogenesis-susceptible (Car-S) mouse lines were generated by phenotypic selection for resistance or susceptibility to two-stage skin carcinogenesis. These two Car mouse lines differ by >100-fold in susceptibility. In the present study, we tested the hypothesis that a subset of genetic loci responsible for susceptibility or resistance to chemical skin tumorigenesis may also be involved in radiation-induced skin tumorigenesis. Skin tumorigenesis was tested in groups of Car-S/R mice after X-ray initiation and 12-O-tetradecanoylphorbol-13-acetate (TPA) promotion. We found that ionizing radiation can initiate skin tumors in Car-S mice but not in Car-R mice. In Car-S mice, the most effective radiation doses (6 and 10 Gy given in four fractions) gave a threefold increase in tumor multiplicity and a twofold increase in tumor incidence compared to a TPA-only control group. We performed a molecular analysis of Hras gene mutations in skin tumors of Car-S mice induced by X-ray initiation/TPA promotion or by TPA promotion alone. The most notable difference emerging from the comparison of these mutation patterns is the high incidence ( approximately 50%) of papillomas lacking Hras gene mutations in X-ray-initiated/TPA-promoted papillomas compared to 13% in papillomas induced by TPA alone, suggesting that lack of Hras gene mutations is a consistent feature of radiation-induced papillomas.     

Integrating QTL and high-density SNP analyses in mice to identify Insig2 as a susceptibility gene for plasma cholesterol levels

The use of inbred strains of mice to dissect the genetic complexity of common diseases offers a viable alternative to human studies, given the control over experimental parameters that can be exercised. Central to efforts to map susceptibility loci for common diseases in mice is a comprehensive map of DNA variation among the common inbred strains of mice. Here we present one of the most comprehensive high-density, single nucleotide polymorphism (SNP) maps of mice constructed to date. This map consists of 10,350 SNPs genotyped in 62 strains of inbred mice. We demonstrate the utility of these data via a novel integrative genomics approach to mapping susceptibility loci for complex traits. By integrating in silico quantitative trait locus (QTL) mapping with progressive QTL mapping strategies in segregating mouse populations that leverage large-scale mapping of the genetic determinants of gene expression traits, we not only facilitate identification of candidate quantitative trait genes, but also protect against spurious associations that can arise in genetic association studies due to allelic association among unlinked markers. Application of this approach to our high-density SNP map and two previously described F2 crosses between strains C57BL/6J (B6) and DBA/2J and between B6 ApoE(-/-) and C3H/HeJ ApoE(-/-) results in the identification of Insig2 as a strong candidate susceptibility gene for total plasma cholesterol levels.     

Genetic control of susceptibility to carcinogen-induced colorectal cancer in mice: The Ccs3 and Ccs5 loci regulate different aspects of tumorigenesis

Colorectal cancer (CRC) is a multistep disease that involves a two-way interaction between a complex genetic pre-disposition component, and a set of poorly understood extrinsic environmental factors. In mice, CRC can be induced by treatment with azoxymethane (AOM). Using a set of AcB/BcA recombinant congenic strains derived from CRC-susceptible A/J and CRC-resistant C57Bl/6J (B6) progenitors, we previously detected the Ccs3 locus (colon cancer susceptibility locus 3) as a major regulator of CRC susceptibility. Phenotyping of additional AcB/BcA strains for susceptibility to AOM-induced CRC has refined the Ccs3 interval to a 6.7 Mb segment on chromosome 3. In addition, the presence of intermediate susceptibility phenotypes in individual AcB/BcA strains suggested additional gene effects regulating CRC susceptibility in A/J and B6 strains. Those were investigated by linkage analysis and whole genome scanning in a set of 208 informative (B6 x A/J)F2 progeny, using tumor multiplicity as a quantitative measure of susceptibility. This analysis validated the important role of Ccs3 in regulating this trait, and additionally detected contribution from a second locus on the distal portion of chromosome 9 (LOD = 3.76), that was given the temporary designation of Ccs5. Ccs5 modulates tumor multiplicity in F2 animals bearing at least one A/J-derived susceptibility allele at Ccs3, with A/J-derived Ccs5 susceptibility alleles being inherited in a recessive manner. There is a strong additive effect of Ccs3 and Ccs5 on tumor multiplicity in F2 mice: mice doubly homozygotes for A/J or B6 alleles at Ccs3 and Ccs5 show tumor numbers similar to those of parental A/J and B6, respectively. Interestingly, the Ccs5 region overlaps several quantitative trait loci previously reported to regulate intestinal homeostasis and susceptibility to intestinal colitis in mice and humans. Our findings identify a novel two-locus system regulating CRC susceptibility in mice, of which the relevance to human CRC can now be tested experimentally.     

Pas1 is a common lung cancer susceptibility locus in three mouse strains

Inherited predisposition to lung cancer is a phenotypic trait shared by different mouse inbred strains that show either a high or an intermediate predisposition. Other strains are instead genetically resistant. The Pas1 locus is the major determinant of lung cancer predisposition in the A/J strain (Gariboldi et al. 1993). To define the determinants of susceptibility to lung tumorigenesis in the highly susceptible SWR/J and in the intermediately susceptible BALB/c mice, we analyzed (BALB/c × SWR/J)F₂ and (BALB/c × C3H/He)F₂ crosses by genetic linkage experiments. The present results provide unequivocal evidence that the same Pas1/+ allele that leads to lung cancer predisposition is shared by A/J, SWR/J, and BALB/c strains. The intermediate susceptibility of the BALB/c strain would result by interaction of Pas1 locus with lung cancer resistance loci. Received: 18 April 1997 / Accepted: 15 June 1997     

Genetic association of a GABAA receptor γ2 subunit variant with severity of acute physiological dependence on alcohol

The ultimate goal of quantitative trait locus (QTL) mapping is to identify the genes affecting complex traits. Using animal models, we recently identified QTLs on mouse Chromosomes (Chrs) 1, 4, and 11 affecting genetic predisposition to acute alcohol withdrawal. Among mice derived from the C57BL/6J (B6) and DBA/2J (D2) inbred strains, the locus identified on Chr 11 (∼20 cM) accounted for 12% of the genetic variability in withdrawal liability. Candidate genes in proximity to this QTL encode the γ₂, α₁, and α₆ subunits of GABA_A receptors. The present studies identify a polymorphism between the B6 and D2 strains in the γ₂ subunit gene, Gabrg2, and expand genotypic analysis to their BXD recombinant inbred strains. This polymorphism predicts a difference in amino acid sequence (Ala-11 vs. Thr-11) within the extracellular amino-terminal region of the γ₂ subunit. Analysis using BXD strain means for acute alcohol withdrawal severity suggests that the γ₂ subunit polymorphism is genetically correlated with alcohol withdrawal severity. This is the first report that QTL mapping for an alcohol-related trait has successfully led to the identification of a polymorphic candidate gene product that is genetically associated with the trait. Received: 15 September 1998 / Accepted: 8 October 1998     

Susceptibility to a mouse acquired immunodeficiency syndrome is influenced by theH-2

The development of a mouse acquired immunodeficiency syndrome (MAIDS) induced following LP-BM5 MuLV infection depends on host genetic factors. Susceptible mice, such as C57BL/6J mice, develop a profound impairment of lymphoproliferative response to mitogens and hyperplasia of lymphoid organs and succumb to infection within 6 months. These changes do not occur in resistant mice, such as A/J mice. Resistance to MAIDS is a dominant trait since (C57BL/6JxA/J)F₁ hybrid mice did not develop any immune dysfunctions following infection. Genetic regulation of the trait of resistance/susceptibility to MAIDS was determined in AXB/BXA recombinant inbred (RI) mouse strains (derived from resistant A/J and susceptible C57BL/6J progenitors). Two different criteria were used to determine their resistance or susceptibility to developing MAIDS: the gross pathologic evaluation of lymphoid organs at 13–15 weeks of infection, and survival. RI mouse strains segregated into two non-overlapping groups. The first group did not develop any significant pathology, and these mouse strains were considered as resistant to MAIDS. The second group showed the virus-induced pathological changes as well as an immunological dysfunction as seen in C57BL/6J progenitor mice, and these strains were thus considered as susceptible to MAIDS. This bimodal strain distribution pattern of resistance/susceptibility to MAIDS among the RI strains suggests that this phenotype is controlled by a single gene. Linkage analysis with other allelic markers showed a strong association between resistance/susceptibility to MAIDS and theH-2 complex. Possession of theH-2 ^b haplotype derived from C57BL/6J mice was associated with susceptibility to MAIDS, while theH-2 ^a haplotype conferred resistance to the disease. This finding was confirmed by demonstrating thatH-2 ^a congenics on the susceptible C57BL/10 background were as resistant to MAIDS as A/J mice which donated theH-2 ^a locus. Gene(s) within theH-2 complex thus represent the major regulatory mechanism of resistance/susceptibility to MAIDS.     

Identification and characterization of a novel conserved DNA repeat

Homozygous In(10)17Rk mice contain a paracentric inversion within Chromosome (Chr) 10 and exhibit a pygmy phenotype, suggesting that the distal inversion breakpoint is within the pygmy (pg) locus. In order to obtain the pygmy gene by positional cloning procedures, In(10)17Rk DNA was subjected to RFLP analysis with single-copy probes derived from the wild-type pygmy locus. This analysis identified a DNA polymorphism in the DBA/2J mouse strain on which the In(10)17Rk mutation was originally induced. A detailed characterization of this polymorphism revealed the presence of a novel, tandemly repeated DNA element. Copy number estimation experiments indicate that there are approximately 100,000 copies of this element in the haploid DBA/2J genome. PCR typing studies revealed the presence of the repeat at the pygmy locus of 6 of the 18 Mus domesticus strains analyzed. The absence of the repeat from the pygmy locus of 12 strains of the M. domesticus species and from the M. caroli, M. spretus, M. castaneus, and M. molossinus species suggests that the repeat could serve as a strain-specific hybridization probe in genetic mapping studies. Finally, the novel tandem DNA repeat is conserved in both rat and human genomes as indicated by Southern hybridization experiments.     

Chromosomal mapping of host susceptibility loci to Angiostrongylus costaricensis nematode infection in mice

Angiostrongylus costaricensis is a nematode found mainly as a rodent parasite. Laboratory mice were experimentally infected with this parasite. It is known that there is great variability in mortality among inbred mouse strains after infection with this nematode. The survival rate at 5 weeks after infection of A/J mice was 90.5%, whereas that of SM/J mice was only 33.3%, with severe anemia and decreased body weight about 3 weeks after infection. To identify host susceptibility genes for infection with this nematode, we undertook chromosomal mapping by a whole-genome scanning approach in (A/JxSM/J)F2 mice. We mapped a host susceptibility locus (here designated Acsns, for Angiostrongylus costaricensis nematode susceptibility locus) to the telomeric portion of Chromosome 19 (peak LOD=4.35). We also identified two loci on Chr 13 and Chr 17 that have epistatic effects on host survival. This is the first report on host susceptibility loci for helminth infection mapped by whole-genome scanning.     

SJL/J Mice Are Highly Susceptible to Infection by Mouse Adenovirus Type 1

Mouse adenovirus type 1 (MAV-1) targets endothelial and monocyte/macrophage cells throughout the mouse. Depending on the strain of mouse and dose or strain of virus, infected mice may survive, become persistently infected, or die. We surveyed inbred mouse strains and found that for the majority tested the 50% lethal doses (LD(50)s) were >10(4.4) PFU. However, SJL/J mice were highly susceptible to MAV-1, with a mean LD(50) of 10(-0.32) PFU. Infected C3H/HeJ (resistant) and SJL/J (susceptible) mice showed only modest differences in histopathology. Susceptible mice had significantly higher viral loads in the brain and spleen at 8 days postinfection than resistant mice. Infection of primary macrophages or mouse embryo fibroblasts from SJL/J and C3H/HeJ mice gave equivalent yields of virus, suggesting that a receptor difference between strains is not responsible for the susceptibility difference. When C3H/HeJ mice were subjected to sublethal doses of gamma irradiation, they became susceptible to MAV-1, with an LD(50) like that of SJL/J mice. Antiviral immunoglobulin G (IgG) levels were measured in susceptible and resistant mice infected by an early region 1A null mutant virus that is less virulent that wild-type virus. The antiviral IgG levels were high and similar in the two strains of mice. Taken together, these results suggest that immune response differences may in part account for differences in susceptibility to MAV-1 infection.     

Association of a lithogenic Abcg5/Abcg8 allele on Chromosome 17 (Lith9) with cholesterol gallstone formation in PERA/EiJ mice

To examine further the genetic determinants of cholesterol gallstone susceptibility in inbred mice, we performed quantitative trait locus (QTL) analysis of an intercross of gallstone-susceptible PERA/EiJ and gallstone-resistant DBA/2J inbred mice. Three hundred twenty-four F₂ offspring were phenotyped for cholelithiasis during consumption of a lithogenic diet and genotyped using microsatellite markers. Linkage analysis was performed by interval mapping. In addition, we analyzed the combined datasets from this cross and from an independent cross of strain PERA and gallstone-resistant I/Ln mice. QTL mapping detected one significant new gallstone susceptibility (Lith) locus on Chromosome 13 (Lith15). A second significant QTL on Chr 6 (Lith16) confirmed a previous QTL. Furthermore, suggestive QTLs confirmed Lith loci from previous crosses on Chromosomes 1, 2, 5, 16 and X. QTL analysis of the dataset derived from the combined crosses increased the detection power and narrowed confidence intervals of Lith loci on Chromosomes 2, 6, 13, and 16. Moreover, the analysis of combined datasets revealed a shared QTL between both crosses on Chromosome 17 (Lith9). Significantly higher mRNA expression of Abcg5 and Abcg8 in strain PERA compared with strains I/Ln and DBA/2 further substantiated that the PERA allele of Abcg5/Abcg8 was responsible for lithogenicity underlying Lith9.     

Genetic control of neuroadapted sindbis virus replication in female mice maps to chromosome 2 and associates with paralysis and mortality

Neuroadapted Sindbis virus (NSV) infection of mice causes hindlimb paralysis and 100% mortality in the C57BL/6 mouse strain, while adults of the BALB/cBy mouse strain are resistant to fatal encephalomyelitis. Levels of viral RNA are higher in the brains of infected C57BL/6 mice than in BALB/cBy mice (D. C. Thach et al., J. Virol. 74:6156-6161, 2000). These phenotypic differences between the two strains allowed us to map genetic loci involved in mouse susceptibility to NSV and to find relationships between mortality, paralysis, and viral RNA levels. Analysis of percent mortality in H2-congenic and F(1) mice suggested that the H2 locus, sex linkage, and imprinting were not involved in determining susceptibility and that resistance was partially dominant over susceptibility. Segregation analysis using CXB recombinant inbred (RI) mice indicated that the percent mortality was multigenic. Interval mapping detected a suggestive quantitative trait locus (QTL) on chromosome 2 near marker D2Mit447. Analysis of paralysis in the RI mice detected the same suggestive QTL. Viral RNA level in F(1) mice was intermediate. Interval mapping using viral RNA levels in RI mice detected a significant QTL near marker D2Mit447 that explained 69% of the genetic variance. This QTL was confirmed in F2 mice and was designated as Nsv1. Viral RNA level, percent paralyzed, and percent mortality were linearly correlated (r = 0.8 to 0.9). These results indicate that mortality, paralysis, and viral RNA levels are related complex traits and that Nsv1 controls early viral load and determines the likelihood of paralysis and death.     

Fine mapping of Ath6, a quantitative trait locus for atherosclerosis in mice

Ath6 is a novel quantitative trait locus associated with differences in susceptibility to atherosclerosis between C57BL/6J (B6) and C57BLKS/J (BKS) inbred mouse strains. Combining data from an intercross and a backcross (1593 meioses) between mice from B6 and BKS strains and from The Jackson Laboratory interspecific backcross panels, (C57BL/6J ×Mus spretus) F₁× C57BL/6J and (C57BL/6J × SPRET/Ei) F₁× SPRET/Ei, we constructed a consensus genetic map and narrowed Ath6 to a 1.07 ± 0.26 cM interval between the anonymous DNA marker D12Pgn4 and the gene Nmyc1. This region is near the proximal end of murine Chromosome (Chr) 12, which is homologous to the human chromosomal region 2p24-p25. Marker order in the Ath6 region was concordant among the two crosses and The Jackson Laboratory interspecific backcross panels. This high resolution map rules out candidate genes encoding apolipoprotein B, syndecan 1, and Adam17. The two Ath6 crosses have a combined potential resolution of 0.06 cM. Received: 12 September 2000 / Accepted: 22 February 2001     

Spink2 Modulates Apoptotic Susceptibility and Is a Candidate Gene in the Rgcs1 QTL That Affects Retinal Ganglion Cell Death after Optic Nerve Damage

The Rgcs1 quantitative trait locus, on mouse chromosome 5, influences susceptibility of retinal ganglion cells to acute damage of the optic nerve. Normally resistant mice (DBA/2J) congenic for the susceptible allele from BALB/cByJ mice exhibit susceptibility to ganglion cells, not only in acute optic nerve crush, but also to chronic inherited glaucoma that is characteristic of the DBA/2J strain as they age. SNP mapping of this QTL has narrowed the region of interest to 1 Mb. In this region, a single gene (Spink2) is the most likely candidate for this effect. Spink2 is expressed in retinal ganglion cells and is increased after optic nerve damage. This gene is also polymorphic between resistant and susceptible strains, containing a single conserved amino acid change (threonine to serine) and a 220 bp deletion in intron 1 that may quantitatively alter endogenous expression levels between strains. Overexpression of the different variants of Spink2 in D407 tissue culture cells also increases their susceptibility to the apoptosis-inducing agent staurosporine in a manner consistent with the differential susceptibility between the DBA/2J and BALB/cByJ strains.     

Detection of quantitative trait loci for body weight at 10 weeks from Philippine wild mice

A genome-wide scan for quantitative trait loci (QTLs) controlling body weight at 10 weeks after birth was carried out in a population of 387 intersubspecific backcross mice derived from a cross between C57BL/6J inbred mice (Mus musculus domesticus) and wild mice (M. m. castaneus) captured in the Philippines, in order to discover novel QTLs from the wild mice that have about 60% lower body weight than C57BL/6J. By interval mapping, we detected four QTLs: a highly significant QTL on Chromosome (Chr) 2, which was common in both sexes; two significant QTLs on Chr 13, one male-specific and the other female-specific; and a suggestive male-specific QTL on X Chr. By composite interval mapping, we confirmed the presence of the three QTLs on Chrs 2 and 13, but not of the male-specific X-linked QTL. The composite interval mapping analysis newly identified three QTLs: a significant male-specific QTL on Chr 11 and two highly significant female-specific QTLs on Chrs 9 and X. Individual QTLs explained 3.8–11.6% of the phenotypic variance, and all the QTL alleles derived from the wild mice decreased body weight. A two-way analysis of variance revealed a significant epistatic interaction between the Chr 2 QTL and the background marker locus D12Mit4 on Chr 12 only in males. The interaction effect unexpectedly increased body weight. The chromosomal region containing the Chr 2 QTL did not coincide with those of growth or fatness QTLs mapped in previous studies. These results suggest that a population of wild mice may play an important role as new sources of valuable QTLs. Received: 14 January 2000 / Accepted: 14 April 2000     

Quantitative trait loci in a bacterially induced model of inflammatory bowel disease

Inflammatory bowel diseases (IBDs) are complex disorders caused by a combination of environmental, microbial, and genetic factors. Genome-wide association studies in humans have successfully identified multiple genes and loci associated with disease susceptibility, but the mechanisms by which these loci interact with each other and/or with environmental factors (i.e., intestinal microbiota) to cause disease are poorly understood. Helicobacter hepaticus-induced intestinal inflammation in mice is an ideal model system for elucidating the genetic basis of IBD susceptibility in a bacterially induced system, as there are significant differences in H. hepaticus-induced disease susceptibility among inbred mouse strains. Infected A/J mice develop acute overexpression of proinflammatory cytokines followed 2?C3?months later by chronic cecal inflammation, whereas infected C57BL/6 mice fail to develop cecal inflammation or increased cytokine expression. The goal of this project was to use quantitative trait locus (QTL) mapping to evaluate genetic factors that contribute to the differential disease susceptibility between these two mouse strains. Using acute cecal IL-12/23p40 expression as a biomarker for disease susceptibility, QTL analysis of H. hepaticus-infected F₂ mice revealed involvement of multiple loci. The loci with the strongest association were located on Chromosome 3 and Chromosome 17, with logarithm of odds (LOD) scores of 6.89 and 3.09, respectively. Cecal expression of IL-12/23p40 in H. hepaticus-infected C57BL/6J-Chr3^A/J/NaJ chromosome substitution mice had an intermediate phenotype, significantly higher than in resistant C57BL/6 but lower than in susceptible A/J mice, confirming the importance of this locus to the immune response to H. hepaticus infection.     

Polymorphism of the mouse gene for the interleukin 10 receptor alpha chain (Il10ra) and its association with the autoimmune phenotype

Several studies suggest that interleukin (IL)-10 pathway is involved in murine lupus, while no linkage of IL-10 gene polymorphism to disease susceptibility has been reported in studies with lupus-prone mice. Since IL-10 functions through the specific IL-10 receptor alpha (IL-10RA) chain and the IL-10RA gene (Il10ra) is linked to the susceptibility loci of atopic dermatitis and Crohn's disease identified using mouse models, we supposed that IL-10RA might be involved in murine lupus. By flow cytometry analysis, we found that NZW mice, one of the parental strains of lupus-prone (NZB×NZW) F1 mice, express extremely low levels of IL-10RA compared with NZB mice, the other parental strain, and the healthy BALB/c and C57BL/6 mice. Sequence analyses of Il10ra cDNA of NZW mice showed multiple nucleotide mutations compared with that of NZB and C57BL/6 strains, some of which would result in amino acid substitutions in the IL-10RA protein. Lupus-prone MRL mice shared the same polymorphism with NZW. Analyses using (NZB×NZW) F1×NZB backcross mice showed that high serum levels of IgG antichromatin antibodies were regulated by a combinatorial effect of the NZW Il10ra allele and a heterozygous genotype for Tnfa microsatellite locus. Our data suggest that the polymorphic NZW-type Il10ra may be involved in the pathologic production of antichromatin antibodies and, if so, may contribute in part to the development of systemic lupus erythematosus as one susceptibility allele. The Il10ra polymorphism data reported in this paper have been submitted to the Mouse Genome Informatics database and have been assigned the accession number MGI: 3528086.     

Genetic characterization of the Dyscalc locus

Calcification occurs frequently in the development of atherosclerotic lesions, and studies in mice have indicated a genetic contribution. We now show that one genetic factor contributing to aortic calcification is the Dyscalc locus, previously shown to contribute to myocardial calcification. Thus, the Dyscalc locus, on proximal mouse Chromosome (Chr) 7, segregated with vascular calcification in a large cross between susceptible strain DBA/2J and resistant strain C57BL/6J. Further evidence was observed by analysis of recombinant inbred strains derived from various susceptible and resistant parental strains. Myocardial and vascular calcifications are importantly influenced by multiple modifier loci as well as the Dyscalc gene, making fine mapping of Dyscalc difficult. In order to allow more detailed genetic and biochemical characterization of Dyscalc, we have identified congenic strains containing the Dyscalc locus from resistant strain C57BL/10 on the background of susceptible strain C3H/DiSnA. The congenic strains exhibit little or no myocardial or vascular calcification, unlike the background HcB C3H strain, and the calcification segregated as a Mendelian factor, allowing finer mapping of Dyscalc.     

Assignment of a locus for mouse lung tumor susceptibility to proximal Chromosome 19

Previous studies have hypothesized that at least three genetic loci contribute to differences in pulmonary adenoma susceptibility between mouse strains A/J and C57BL/6J. One gene that may confer susceptibility to lung tumorigenesis is the Kras protooncogene. To identify other relevant loci involved in this polygenic trait, we determined tumor multiplicity in 56 randomly chosen N-ethyl-N-nitrosourea-treated (A/J×C57BL/6J) N1×C57BL/6 backcross (AB6N2) progeny and correlated it with genotypes at 77 microsatellite markers spanning the genome. A correlation of lung tumor multiplicity phenotypes with genotypes of microsatellite markers on distal Chromosome (Chr) 6 in the Kras region (Pas1) was confirmed, and a new region on Chr 19 (designated Pas3) was identified that also contributes to susceptibility. Linkage analysis on Chr 19 with 270 AB6N2 mice localized the region flanked by D19Mit42 and D19Mit19 that is most closely associated with lung tumor susceptibility. The Pas3 locus may be an enhancer of the susceptibility locus on Chr 6.     

Immune responses of different mouse strains after challenge with equivalent lethal doses of Toxoplasma gondii

Most immunological studies that utilize different strains of inbred mice following T. gondii infection fail to compensate for differences in host susceptibility to the size of the parasite innoculum. To address this concern, susceptible C57BL/6 and resistant CBA/J mice were orally infected with either an equivalent 50% lethal dose (LD50) of brain cysts of the 76K strain of T. gondii (15 cysts in C57BL/6, 400 cysts in CBA/J) or the same dose of parasites in each mouse strain. C57BL/6 mice receiving 400 cysts (LD50 of CBA/J mice) died post infection, whereas CBA/J mice that received 15 cysts (LD50 of C57BL/6 mice) survived. Parasite loads in the brains and serum Toxoplasma-specific IgG1 titers of LD50-infected C57BL/6 mice were significantly higher than those in LD50- or 15 cysts-infected CBA/J mice, whereas splenocyte proliferation to Toxoplasma antigen and the percentage of CD8 alpha+ T cells were reduced in LD50-infected C57BL/6 mice. In contrast, serum IgG2a and IgM titers, the percentage of gamma delta T cells and IFN-gamma expression of spleen of LD50-infected CBA/J mice were higher than those of either 15 cysts-infected CBA/J mice or LD50-infected C57BL/6 mice. These observations demonstrate that the immune response between LD50-infected C57BL/6 and CBA/J mice was more prominent when compared to C57BL/6 or CBA/J mice receiving the same parasite inoculum. These observations would suggest that caution must be excersized in the planning and interpretation of data when the size of the parasite inoculum has not been adjusted for mouse strain.