Identification of Mom7, a novel modifier of Apc(Min/+) on mouse chromosome 18

The Apc(Min) mouse model of colorectal cancer provides a discrete, quantitative measurement of tumor multiplicity, allowing for robust quantitative trait locus analysis. This advantage has previously been used to uncover polymorphic modifiers of the Min phenotype: Mom1, which is partly explained by Pla2g2a; Mom2, a spontaneous mutant modifier; and Mom3, which was discovered in an outbred cross. Here, we describe the localization of a novel modifier, Mom7, to the pericentromeric region of chromosome 18. Mom7 was mapped in crosses involving four inbred strains: C57BL/6J (B6), BTBR/Pas (BTBR), AKR/J (AKR), and A/J. There are at least two distinct alleles of Mom7: the recessive, enhancing BTBR, AKR, and A/J alleles and the dominant, suppressive B6 allele. Homozygosity for the enhancing alleles increases tumor number by approximately threefold in the small intestine on both inbred and F(1) backgrounds. Congenic line analysis has narrowed the Mom7 region to within 7.4 Mb of the centromere, 28 Mb proximal to Apc. Analysis of SNP data from various genotyping projects suggests that the region could be as small as 4.4 Mb and that there may be five or more alleles of Mom7 segregating among the many strains of inbred mice. This has implications for experiments involving Apc(Min) and comparisons between different or mixed genetic backgrounds.     

Analysis of reciprocal congenic lines reveals the C3H/HeJ genome to be highly resistant to ApcMin intestinal tumorigenesis

Genetic background affects polyp development in the Multiple intestinal neoplasia (Apc(Min)) mouse model. The Modifier of Min 1 (Mom1) locus accounts for approximately 50% of the variation in polyp multiplicity. We generated reciprocal congenic lines, such that the recipient C57BL/6J (B6) strain carries a donor C3H/HeJ (C3H) Mom1 allele, and the recipient C3H strain carries a donor B6 Mom1 allele. Hybrid progeny from congenic females mated to B6 Apc(Min/+) males were analyzed. A single C3H Mom1 locus on the B6 background reduced small intestinal polyp numbers by 50% and colon polyp incidence by 66% compared to their susceptible B6 Mom1(S/S)Apc(Min/+) siblings. These findings show that the C3H genome contains a resistant Mom1(R) locus. The reciprocal congenic line, which carries the susceptible B6 Mom1(S) locus on the C3H background, reduced small intestinal polyp numbers by 80% and colon polyp incidence by 95% compared to B6 Mom1(S/S)Apc(Min/+) mice. These data demonstrate that unidentified modifiers in the C3H strain can suppress intestinal polyp multiplicity in Apc(Min/+) mice, and act in the absence of a resistant Mom1(R) locus.     

Identification of Mom12 and Mom13, two novel modifier loci of ApcMin-mediated intestinal tumorigenesis

Colorectal cancer is a heterogeneous disease resulting from a combination of genetic and environmental factors. The C57BL/6J (B6) Apc^Min/+ mouse develops polyps throughout the gastrointestinal tract and has been a valuable model for understanding the genetic basis of intestinal tumorigenesis. Apc^Min/+ mice have been used to study known oncogenes and tumor suppressor genes on a controlled genetic background. These studies often utilize congenic knockout alleles, which can carry an unknown amount of residual donor DNA. The Apc^Min model has also been used to identify modifer loci, known as Modifier of Min (Mom) loci, which alter Apc^Min-mediated intestinal tumorigenesis. B6 mice carrying a knockout allele generated in WW6 embryonic stem cells were crossed to B6 Apc^Min/+ mice to determine the effect on polyp multiplicity. The newly generated colony developed significantly more intestinal polyps than Apc^Min/+ controls. Polyp multiplicity did not correlate with inheritance of the knockout allele, suggesting the presence of one or more modifier loci segregating in the colony. Genotyping of simple sequence length polymorphism (SSLP) markers revealed residual 129X1/SvJ genomic DNA within the congenic region of the parental knockout line. An analysis of polyp multiplicity data and SSLP genotyping indicated the presence of two Mom loci in the colony: (1) Mom12, a dominant modifier linked to the congenic region on chromosome 6 and (2) Mom13, which is unlinked to the congenic region and whose effect is masked by Mom12. The identification of Mom12 and Mom13 demonstrates the potential problems resulting from residual heterozygosity present in congenic lines.Key words: adenomatous polyposis coli, modifier of min, congenic mice, caveolin-1, cancer susceptibility     

Identification of Mom12 and Mom13, two novel modifier loci of ApcMin-mediated intestinal tumorigenesis

Colorectal cancer is a heterogeneous disease resulting from a combination of genetic and environmental factors. The C57BL/6J (B6) Apc^Min/+ mouse develops polyps throughout the gastrointestinal tract and has been a valuable model for understanding the genetic basis of intestinal tumorigenesis. Apc^Min/+ mice have been used to study known oncogenes and tumor suppressor genes on a controlled genetic background. These studies often utilize congenic knockout alleles, which can carry an unknown amount of residual donor DNA. The Apc^Min model has also been used to identify modifer loci, known as Modifier of Min (Mom) loci, which alter Apc^Min-mediated intestinal tumorigenesis. B6 mice carrying a knockout allele generated in WW6 embryonic stem cells were crossed to B6 Apc^Min/+ mice to determine the effect on polyp multiplicity. The newly generated colony developed significantly more intestinal polyps than Apc^Min/+ controls. Polyp multiplicity did not correlate with inheritance of the knockout allele, suggesting the presence of one or more modifier loci segregating in the colony. Genotyping of simple sequence length polymorphism (SSLP) markers revealed residual 129X1/SvJ genomic DNA within the congenic region of the parental knockout line. An analysis of polyp multiplicity data and SSLP genotyping indicated the presence of two Mom loci in the colony: 1) Mom12, a dominant modifier linked to the congenic region on chromosome 6, and 2) Mom13, which is unlinked to the congenic region and whose effect is masked by Mom12. The identification of Mom12 and Mom13 demonstrates the potential problems resulting from residual heterozygosity present in congenic lines.     

Sex reversal in C57BL/6J XY mice caused by increased expression of ovarian genes and insufficient activation of the testis determining pathway

Sex reversal can occur in XY humans with only a single functional WT1 or SF1 allele or a duplication of the chromosome region containing WNT4. In contrast, XY mice with only a single functional Wt1, Sf1, or Wnt4 allele, or mice that over-express Wnt4 from a transgene, reportedly are not sex-reversed. Because genetic background plays a critical role in testis differentiation, particularly in C57BL/6J (B6) mice, we tested the hypothesis that Wt1, Sf1, and Wnt4 are dosage sensitive in B6 XY mice. We found that reduced Wt1 or Sf1 dosage in B6 XY(B6) mice impaired testis differentiation, but no ovarian tissue developed. If, however, a Y(AKR) chromosome replaced the Y(B6) chromosome, these otherwise genetically identical B6 XY mice developed ovarian tissue. In contrast, reduced Wnt4 dosage increased the amount of testicular tissue present in Sf1+/- B6 XY(AKR), Wt1+/- B6 XY(AKR), B6 XY(POS), and B6 XY(AKR) fetuses. We propose that Wt1(B6) and Sf1(B6) are hypomorphic alleles of testis-determining pathway genes and that Wnt4(B6) is a hypermorphic allele of an ovary-determining pathway gene. The latter hypothesis is supported by the finding that expression of Wnt4 and four other genes in the ovary-determining pathway are elevated in normal B6 XX E12.5 ovaries. We propose that B6 mice are sensitive to XY sex reversal, at least in part, because they carry Wt1(B6) and/or Sf1(B6) alleles that compromise testis differentiation and a Wnt4(B6) allele that promotes ovary differentiation and thereby antagonizes testis differentiation. Addition of a "weak" Sry allele, such as the one on the Y(POS) chromosome, to the sensitized B6 background results in inappropriate development of ovarian tissue. We conclude that Wt1, Sf1, and Wnt4 are dosage-sensitive in mice, this dosage-sensitivity is genetic background-dependant, and the mouse strains described here are good models for the investigation of human dosage-sensitive XY sex reversal.     

Differences in spermatogenesis in cryptorchid testes among various strains of mice

The purpose of the study reported here was to define strain differences in spermatogenesis in cryptorchid testes in mice. Mice of strains A/J, BALB/c, CBA/N, C3H/He, C57BL/6 (B6), ddY and ICR were found to be sensitive to heat stress attributable to experimentally induced cryptorchidism. In contrast, mice of strains AKR/N (AKR), MRL/MpJ-+/+ (M+) and MRL/MpJ-lpr/lpr (lpr) were resistant to heat stress. Relative increases of apoptotic cells were detected in the sensitive group, but not in the resistant group. A decrease of proliferating cell nuclear antigen-immunoreactive cells after experimentally induced cryptorchidism was observed only in the sensitive group. These results suggested that heat stress-resistant germ cells were present in MRL and AKR strains, possibly originating from the genetic background.     

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.     

Macronutrient diet selection in thirteen mouse strains

The strain distribution for macronutrient diet selection was described in 13 mouse strains (AKR/J, NZB/B1NJ, C57BL/6J, C57BL/6ByJ, DBA/2J, SPRET/Ei, CD-1, SJL/J, SWR/J, 129/J, BALB/cByJ, CAST/Ei, and A/J) with the use of a self-selection protocol in which separate carbohydrate, fat, and protein diets were simultaneously available for 26-30 days. Relative to carbohydrate, nine strains consumed significantly more calories from the fat diet; two strains consumed more calories from carbohydrate than from fat (BALB/cByJ, CAST/Ei). Diet selection by SWR/J mice was variable over time, resulting in a lack of preference. One strain (A/J) failed to adapt to the diet paradigm due to inadequate protein intake. Comparisons of proportional fat intake across strains revealed that fat selection/consumption ranged from 26 to 83% of total energy. AKR/J, NZB/B1NJ, and C67BL/6J mice self-selected the highest proportion of dietary fat, whereas the CAST/Ei and BALB/cByJ strains chose the lowest. Finally, epididymal fat depot weight was correlated with fat consumption. There were significant positive correlations in AKR/J and C57BL/6J mice, which are highly sensitive to dietary obesity. However, absolute fat intake was inversely correlated with epididymal fat in two of the lean strains: SWR/J and CAST/Ei. We hypothesize that the SWR/J and CAST/Ei strains are highly sensitive to a negative feedback signal generated by increasing body fat, but the AKR/J and C67BL/6J mice are not. The variation in dietary fat selection across inbred strains provides a tool for dissecting the complex genetics of this trait.     

Identification of Hepatocarcinogen-Resistance Genes in Dba/2 Mice

Male DBA/2J mice are ~20-fold more susceptible than male C57BL/6J mice to hepatocarcinogenesis induced by perinatal treatment with N,N-diethylnitrosamine (DEN). In order to elucidate the genetic control of hepatocarcinogenesis in DBA/2J mice, male BXD recombinant inbred, D2B6F(1) X B6 backcross, and D2B6F(2) intercross mice were treated at 12 days of age with DEN and liver tumors were enumerated at 32 weeks. Interestingly, the distribution of mean tumor multiplicities among BXD recombinant inbred strains indicated that hepatocarcinogen-sensitive DBA/2 mice carry multiple genes with opposing effects on the susceptibility to liver tumor induction. By analyzing D2B6F(1) X B6 backcross and D2B6F(2) intercross mice for their liver tumor multiplicity phenotypes and for their genotypes at simple sequence repeat marker loci, we mapped two resistance genes carried by DBA/2J mice, designated Hcr1 and -2, to chromosomes 4 and 10, respectively. Hcr1 and Hcr2 resolved the genetic variance in the backcross population well, indicating that these resistance loci are the major determinants of the variance in the backcross population. Although our collection of 100 simple sequence repeat markers allowed linkage analysis for ~95% of the genome, we failed to map any sensitivity alleles for DBA/2J mice. Thus, it is likely that the susceptibility of DBA/2J mice is the consequence of the combined effects of multiple sensitivity loci.     

Reduced frequencies of Mitomycin-C induced sister chromatid exchanges in AKR Mice

Summary The frequencies of base-line and Mitomycin-C (MMC) induced sister chromatid exchanges (SCE) were surveyed in four inbred strains of mice. In contrast to the C57B1/6J, CBA/J, and A/J strains where frequencies of SCE increased linearly with increasing dose of MMC, levels of SCE were significantly lower in AKR/J mice at high MMC concentrations. At a dose of 5 mg/kg MMC, chromosomal aberrations were more frequent in bone marrow cells of AKR/J mice than in C57B1/6J mice. These observations suggest an altered response to DNA damage in the AKR mouse strain.     

Modulation of F1 cytotoxic potentials by GvHR: role and mode of action of non-MHC genes that determine the hybrid resistance to GvHR-associated suppression of F1 cytotoxic potential

The resistance of unirradiated F1 mice against graft-vs-host reaction (GvHR) induced by lymphocytes from certain parental strains is apparently a violation of the basic law in classical transplantation immunity. To explore genetic mechanisms of this peculiar phenomenon, GvHR-associated immunosuppression was examined on various kinds of F1 mice undergoing GvHR induced by parental lymphocytes. In F1 mice raised by crossing DBA/2 mice with various H-2-congeneic B10-series strains, parental lymphocytes having non-H-2 genetic background of DBA (DBA/2 and DBA/1) invariably could not induce GvHR-associated immunosuppression, irrespective of the H-2 haplotype incompatibility involved, whereas lymphocytes of the partner parental strain induced the immunosuppression. The number of the relevant loci in the DBA non-H-2 was assessed to be three recessive loci by examination of the capability to induce the GvHR-associated immunosuppression on lymphocytes from individual (B 10.D2 X DBA/2)F1 X DBA/2 backcross mice. On the other hand, in F1 mice raised by crossing C3H/He or AKR/J mice with various H-2-congeneic B10-series strains, parental lymphocytes of H-2k haplotype, irrespective of their non-H-2 haplotype, invariably could not induce the GvHR-associated immunosuppression. Furthermore, it was revealed that non-H-2 genes of parental C3H or AKR incorporated in the F1 mice determine the resistance of the F1 mice against the H-2k-induced GvHR. The results of examination of the resistance on individual (B10 X [B10.BR X C3H/He]F1) and (B10 X [B10.BR X AKR/J]F1) mice suggested that three non-H-2 loci of C3H/He or two non-2 loci of AKR/J incorporated in F1 hybrids could determine the resistance of the respective F1 mice.     

Genetic control of polyamine-dependent susceptibility to skin tumorigenesis

Overexpression of an ornithine decarboxylase (ODC) transgene greatly increases the susceptibility of mouse skin to carcinogen-induced tumor development. Like many phenotypes in transgenic models, this enhanced susceptibility phenotype is strongly influenced by genetic background. We have mapped tumor-modifier genes in intraspecific crosses between transgenic K6/ODC mice on a susceptible strain background (C57Bl/6J), a moderately resistant background (FVB), or a highly resistant background (C3H/HeJ). We identified several quantitative trait loci that influenced either tumor multiplicity or predisposition to the development of squamous cell carcinoma, but not both phenotypes. Because we did not use a tumor-promotion protocol to induce tumors, most of the quantitative trait loci mapped in this study are distinct from skin tumor-susceptibility loci identified previously. The use of a combined transgenic-standard strain approach to genetic analysis has resulted in detection of previously unknown genetic loci affecting skin tumor susceptibility.     

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) induces genetic changes in murine intestinal tumours and cells with ApcMin mutation

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is one of the mutagenic heterocyclic amines derived from cooked meat. In previous animal studies, spontaneous tumour formation in B6(Min/+) mice was associated with somatic loss of the wild-type Apc+ allele by loss of the entire chromosome 18 or by recombination. The objective of this study was to examine genetic changes caused by PhIP-exposure in a mouse intestinal cell line and in tumours from hybrid mice by keeping track of the chromosomes carrying the two Apc alleles. We transformed the SV40 T-immortalised intestinal epithelial cell line IMCE, derived from the B6(Min/+) mice by exposure to N-OH-PhIP, and studied the effect on Apc status and chromosome 18. Eighteen transformed cultures were obtained and all of them had retained the Apc+ allele. Five of seven transformed cultures were tumorigenic after implantation in nude mice. Chromosomal analysis of these five cultures and the parent IMCE cell line showed that the IMCE cells were near-tetraploid with an average of 77 chromosomes/cell, while the tumorigenic cell cultures were all triploid to hyper-triploid with a range of 61-69 chromosomes/cell. The number of copies of chromosome 18 was about four in the IMCE line and this copy number was retained in the transformed lines derived from IMCE. Changes in chromosome 18 and Apc during tumour development in vivo were examined in spontaneously formed and PhIP-induced intestinal tumours from two hybrid mice strains, i.e. B6(Min/+) - a murine FAP model - crossed with either AKR/J or A/J. We evaluated the allelic status of Apc, and the heterogenic microsatellite markers D18Mit19 and D18Mit4, located at the upper and lower ends of chromosome 18, respectively. In tumours from untreated animals, instability in the D18Mit19 and Apc was observed. Upon PhIP exposure, the B6(Min/A+) hybrid mouse tumours differed distinctly in genetic profile from those obtained from untreated animals and we detected three genetically different tumour groups, all of which had apparently retained Apc+. One group had allelic balance between the Apc(Min) and Apc+, the second had allelic imbalance between the Apc and D18Mit4 alleles, indicative of chromosomal stability in the first group and instability in the lower end of chromosome 18 in the second group, respectively. The third group showed variable allelic status of the three markers. A similar change in genetic profile was also seen in intestinal tumours of PhIP-exposed B6(Min/AKR+) hybrid mice, but it was less pronounced. Chromosomal breaks and/or recombinational events could be alternative explanations for the observed allelic imbalances in chromosome 18 markers in intestinal tumours from PhIP-exposed mice.     

Genetic factors at the enterocyte level account for variations in intestinal cholesterol absorption efficiency among inbred strains of mice.

Interindividual and interstrain variations in cholesterol absorption efficiency occur in humans and animals. We investigated physiological biliary and small intestinal factors that might determine variations in cholesterol absorption efficiency among inbred mouse strains. We found that there were significant differences in cholesterol absorption efficiency measured by plasma, fecal, and lymphatic methods: <25% in AKR/J, C3H/J, and A/J strains; 25-30% in SJL/J, DBA/2J, BALB/cJ, SWR/J, and SM/J strains; and 31-40% in C57L/J, C57BL/6J, FVB/J, and 129/SvJ strains. In (AKRxC57L)F1 mice, the cholesterol absorption efficiency (31 +/- 6%) mimicked that of the C57L parent (37 +/- 5%) and was significantly higher than in AKR mice (24 +/- 4%). Although biliary bile salt compositions and small intestinal transit times were similar, C57L mice displayed significantly greater bile salt secretion rates and pool sizes than AKR mice. In examining lymphatic cholesterol transport in the setting of a chronic biliary fistula, C57L mice displayed significantly higher cholesterol absorption rates compared with AKR mice. Because biliary and intestinal transit factors were accounted for, we conclude that genetic variations at the enterocyte level determine differences in murine cholesterol absorption efficiency, with high cholesterol absorption likely to be a dominant trait. This study provides baseline information for identifying candidate genes that regulate intestinal cholesterol absorption at the cellular level.     

Effects of FVB/NJ and C57Bl/6J strain backgrounds on mammary tumor phenotype in inducible nitric oxide synthase deficient mice

The ability to genetically manipulate mice has led to rapid progress in our understanding of the roles of different gene products in human disease. Transgenic mice have often been created in the FVB/NJ (FVB) strain due to its high fecundity, while gene-targeted mice have been developed in the 129/SvJ-C57Bl/6J strains due to the capacity of 129/SvJ embryonic stem cells to facilitate germline transmission. Gene-targeted mice are commonly backcrossed into the C57Bl/6J (B6) background for comparison with existing data. Genetic modifiers have been shown to modulate mammary tumor latency in mouse models of breast cancer and it is commonly known that the FVB strain is susceptible to mammary tumors while the B6 strain is more resistant. Since gene-targeted mice in the B6 background are frequently bred into the polyomavirus middle T (PyMT) mouse model of breast cancer in the FVB strain, we have sought to understand the impact of the different genetic backgrounds on the resulting phenotype. We bred mice deficient in the inducible nitric oxide synthase (iNOS) until they were congenic in the PyMT model in the FVB and B6 strains. Our results reveal that the large difference in mean tumor latencies in the two backgrounds of 53 and 92 days respectively affect the ability to discern smaller differences in latency due to the Nos2 genetic mutation. Furthermore, the longer latency in the B6 strain enables a more detailed analysis of tumor formation indicating that individual tumor development is not stoichastic, but is initiated in the #1 glands and proceeds in early and late phases. NO production affects tumors that develop early suggesting an association of iNOS-induced NO with a more aggressive tumor phenotype, consistent with human clinical data positively correlating iNOS expression with breast cancer progression. An examination of lung metastases, which are significantly reduced in PyMT/iNOS^−/− mice compared with PyMT/iNOS^+/+ mice only in the B6 background, is concordant with these findings. Our data suggest that PyMT in the B6 background provides a useful model for the study of inflammation-induced breast cancer.     

Synchronizing allelic effects of opposing quantitative trait loci confirmed a major epistatic interaction affecting acute lung injury survival in mice

Increased oxygen (O(2)) levels help manage severely injured patients, but too much for too long can cause acute lung injury (ALI), acute respiratory distress syndrome (ARDS) and even death. In fact, continuous hyperoxia has become a prototype in rodents to mimic salient clinical and pathological characteristics of ALI/ARDS. To identify genes affecting hyperoxia-induced ALI (HALI), we previously established a mouse model of differential susceptibility. Genetic analysis of backcross and F(2) populations derived from sensitive (C57BL/6J; B) and resistant (129X1/SvJ; X1) inbred strains identified five quantitative trait loci (QTLs; Shali1-5) linked to HALI survival time. Interestingly, analysis of these recombinant populations supported opposite within-strain effects on survival for the two major-effect QTLs. Whereas Shali1 alleles imparted the expected survival time effects (i.e., X1 alleles increased HALI resistance and B alleles increased sensitivity), the allelic effects of Shali2 were reversed (i.e., X1 alleles increased HALI sensitivity and B alleles increased resistance). For in vivo validation of these inverse allelic effects, we constructed reciprocal congenic lines to synchronize the sensitivity or resistance alleles of Shali1 and Shali2 within the same strain. Specifically, B-derived Shali1 or Shali2 QTL regions were transferred to X1 mice and X1-derived QTL segments were transferred to B mice. Our previous QTL results predicted that substituting Shali1 B alleles onto the resistant X1 background would add sensitivity. Surprisingly, not only were these mice more sensitive than the resistant X1 strain, they were more sensitive than the sensitive B strain. In stark contrast, substituting the Shali2 interval from the sensitive B strain onto the X1 background markedly increased the survival time. Reciprocal congenic lines confirmed the opposing allelic effects of Shali1 and Shali2 on HALI survival time and provide unique models to identify their respective quantitative trait genes and to critically assess the apparent bidirectional epistatic interactions between these major-effect loci.     

A Specific Nuclear DNA Background Is Required for High Frequency Lymphoma Development in Transmitochondrial Mice with G13997A mtDNA

We previously found that mouse mitochondrial DNA (mtDNA) with a G13997A mutation (G13997A mtDNA) controls not only the transformation of cultured lung carcinoma cells from poorly metastatic into highly metastatic cells, but also the transformation of lymphocytes into lymphomas in living C57BL/6 (B6) mice. Because the nuclear genetic background of the B6 strain makes the strain prone to develop lymphomas, here we examined whether G13997A mtDNA independently induces lymphoma development even in mice with the nuclear genetic background of the A/J strain, which is not prone to develop lymphomas. Our results showed that the B6 nuclear genetic background is required for frequent lymphoma development in mice with G13997A mtDNA. Moreover, G13997A mtDNA in mice did not enhance the malignant transformation of lung adenomas into adenocarcinomas or that of hepatocellular carcinomas from poorly metastatic into highly metastatic carcinomas. Therefore, G13997A mtDNA enhances the frequency of lymphoma development under the abnormalities in the B6 nuclear genome, and does not independently control tumor development and tumor progression.     

AKR/J gene(s) unlinked to H-2 determines dominant inheritance of lymphocyte hyporesponsiveness to acetylcholine receptor

Mice with the H-2b major histocompatibility complex haplotype are high immune responders to nicotinic acetylcholine receptors (AChR), whereas mice with the H-2k haplotype are generally low responders. F1 progeny of C57BL/6 (H-2b) mice crossed with mice of most H-2k strains are high responders to AChR in standard conditions of testing helper T cell proliferation in vitro (4 X 10(5) lymph node cells/microwell, 1 wk after primary challenge in vivo). In contrast, the F1 progeny of AKR/J (H-2k) crossed with high responder (H-2b) strains (B6, A.BY, or C3H.SW) were all hyporesponsive to AChR when lymphocytes were tested at 4 X 10(5) cells/well. However, at a density of 1 X 10(6) or greater/well, a high level of antigen-specific responsiveness was demonstrable in the F1 hybrid lymphocytes. A shift from low to high responsiveness to AChR at high cell densities was observed also in the H-2b strain AKR.B6. Other strains previously demonstrated to be low responders to AChR did not become responsive to AChR when lymphocyte numbers were increased to 1.4 X 10(6)/well. The N2 generation yielded by backcrossing (AKR X B6)F1 mice to AKR/J were all low responders, whereas N2 progeny derived by backcrossing F1 to B6 were high or low responders in a ratio of approximately 1:1 (independent of their H-2 phenotype). Results consistent with this observation were obtained in (AKR X B6) F2 mice. These data suggest that at least one AKR/J gene outside of the H-2 complex exerts a hyporesponsive influence on the I-A-dependent helper T cell response to AChR in H-2b mice.     

Mouse T lymphocyte response to acetylcholine receptor determined by T cell receptor for antigen V beta gene products recognizing Mls-1a.

Mice of strain B6, but not AKR/J, respond to immunization with Torpedo acetylcholine receptor (AChR) by manifesting in vitro an Ag-specific T lymphocyte proliferative response. Our analysis of (AKR x B6)F1 mice reveals that the T cell unresponsiveness of AKR/J is inherited as a dominant trait, possibly associated with expression of the Mls-1a allele. Mice derived from backcrossing (AKR x B6)F1 x B6 were selected for H-2b homozygosity and were classified as Mls-1a or Mls-1b according to the relative numbers of peripheral blood T cells that expressed the TCR V beta 6 gene product. After challenge by injection with AChR in CFA, lymph node cells from mice classified as having less than 2% of V beta 6+ peripheral T cells had low responsiveness to AChR, whereas mice with greater than 7% V beta 6+ peripheral T cells had high T cell responsiveness to AChR. These results are consistent with the notion that regulation of the T cell repertoire by Mls loci may be a determinant of susceptibility to autoimmunity.