Paternal inheritance of egg traits in mice: a case of genomic imprinting

Eggs from reciprocal hybrids between the C57BL/6By and BALB/cBy strains were tested for their susceptibility to attack by hyaluronidase and pronase. There were significant reciprocal differences between the F1 females in the responses of their unfertilized eggs to both enzymes. The F1 hybrids from BALB mothers showed the increased susceptibility characteristic of C57BL whilst the F1 hybrids with C57BL mothers were more resistant to both enzymes, like BALB mice. Eggs from the four kinds of reciprocal F2 hybrid females also showed patroclinous patterns of susceptibility. A patroclinous difference was found between reciprocal crosses of the CXBD and CXBE recombinant inbred strains but not in crosses between recombinant inbred strains with similar phenotypes. Cross fostering did not alter the phenotypes of the C57BL and BALB females or those of their reciprocal F1 hybrids. The findings are interpreted in terms of differential genomic imprinting of paternally inherited information. The possible general usefulness of patroclinous differences between reciprocal F1 females in revealing differences in imprinting is noted.     

Major genes control hormone-induced ovulation rate in mice

The present study examined the magnitude of genetic variation, mode of inheritance and number of loci controlling major genetic differences in hormone-induced ovulation rate in mice. Mice were injected with 5 i.u. PMSG at 28 days of age and 5 i.u. hCG at 30 days, and hormone-induced ovulation rate was determined from counts of oviducal eggs in cumulus the next morning. Six-fold genetic differences in induced ovulation rate were detected amongst strains, ranging from a low mean (+/- s.e.) value of 8.8 (+/- 0.9) in A/J up to 53.5 (+/- 2.2) in C57BL/6J. The number of ova differed significantly amongst strains and amongst F1 crosses (P less than 0.0001): 70% of the variation in hormone-induced ovulation rate was amongst strains. Of 9 F1 crosses examined, 4 showed positive heterosis, 3 showed no heterosis and 2 showed negative heterosis for hormone-induced ovulation rate. Analysis of parental, F1 and F2 generations revealed that the induced ovulation rate of A/J and C57BL/6J mice differed due to the action of about 3 or 4 loci, and A.SW/SnJ and SJL/J mice differed due to the action of about 2 to 3 loci. Analysis of recombinant inbred strains formed from A/J and C57BL/6J confirmed that these strains differed due to the action of a small number of loci. This study demonstrates the existence of a small number of major genes controlling hormone-induced ovulation rate in young mice.     

Dispersion of the cumulus oophorus and dissolution of the zona pellucida in oocytes of DDK and other strains of mice

The time needed in vitro for granulosa cell dispersal with hyaluronidase and for lysis of the zona pellucida with α-chymotrypsin was estimated for unfertilized eggs from three inbred strains of mice, DDK, C57BL, and C3H, and one outbred strain, Q. Granulosa cells were dispersed most rapidly in the DDK and most slowly in the C3H strain. The time needed for dissolution of the zona pellucida was also shortest for DDK eggs and longest for C3H eggs. The results suggest that the high sensitivity to enzymes of the granulosa layer and the zona pellucida of DDK eggs may allow more than one spermatozoon to penetrate through these barriers before the block against polyspermy is completed. The percentage of unfertilized eggs containing spermatozoa under the zona pellucida on the day of copulation was abnormally high in matings of females from the DDK inbred strain with males from DDK and C57BL strains. This may suggest some disturbances in establishing contact between the egg plasma membrane and fertilizing spermatozoa.     

Susceptibility to raf and raf/myc retroviruses is governed by different genetic loci.

The susceptibility to tumors induced by raf and raf/myc retroviruses was investigated in BALB/c, C57BL/6, (BALB/c x C57BL/6)F1 and (BALB/c x C57BL/6) backcross mice. Newborn mice were susceptible to neoplasms generated by both viruses, but resistance to raf-induced leukemia developed rapidly in all mice as they matured. Older C57BL/6 mice were also resistant to raf/myc lymphomas, whereas BALB/c mice remained susceptible to the virus at all ages, indicating that different genes control susceptibility to raf and raf/myc tumors. From these data and the susceptibility of C x B recombinant inbred strains, it appears that very few genes (perhaps even a single gene) may govern susceptibility to raf/myc lymphomas and that resistance is the dominant trait.     

Identification of quantitative trait loci controlling activation of TRBV4 CD8+ T cells during murine gamma-herpesvirus-induced infectious mononucleosis

The murine gamma-herpesvirus, MHV-68, shares important biological and genetic features with the human gamma-herpesvirus, Epstein-Barr virus. Following intranasal infection, mice develop an infectious mononucleosis-like syndrome accompanied by increased numbers of activated CD8+ T cells in the blood. A consistent feature of the CD8+ T-cell activation is a marked increase in the frequency of cells expressing a TRBV4+ T-cell receptor. Previous studies suggested that the magnitude of TRBV4 expansion varied significantly among mouse strains, and was influenced by both MHC and non-MHC genes. Detailed analysis of strains with high (C57BL/6) or low (DBA/2) TRBV4 CD8+ T-cell expansion showed that differences in the degree of expansion were not a consequence of variation in genetic susceptibility to the viral infection. Rather, the magnitude of the TRBV4 CD8+ T-cell expansion correlated with differences in expression of the unidentified stimulatory ligand on activated, latently infected B cells. In the present study, analysis of TRBV4 expansion in C57BL/6, DBA/2, B6D2 F1 mice, BXD recombinant inbred strains, and the progeny of C57BL/6xDBA/2 F1 hybrids backcrossed to C57BL/6 demonstrated strong cumulative dominance of the low DBA/2 trait and moderately high heritability (h2 approximately 0.5). Two quantitative trait loci (QTLs) strongly associated with variance in TRBV4 expansion were identified using simple and composite mapping procedures. The first QTL is located on Chromosome (Chr) 17, near or proximal to H2. The second QTL is located on Chr 6 in a region spanning the Tcrb and Cd8a loci.     

Differences between C57BL/6 and BALB/cBy mice in mortality and virus replication after intranasal infection with neuroadapted Sindbis virus

Neuroadapted Sindbis virus (NSV), given intranasally, caused fatal encephalitis in 100% of adult C57BL/6 mice and 0% of BALB/cBy mice. Most C57BL/6 mice developed severe kyphoscoliosis followed by hind-limb paralysis, while BALB/cBy mice did not. In situ hybridization for detecting NSV RNA and immunohistochemistry for detecting NSV antigen indicated that virus delivered by this route infected neurons of the olfactory region and spread caudally without infection of ependymal cells. Virus antigen was more abundant and infectious virus increased more rapidly and reached higher levels in C57BL/6 mice than in BALB/cBy mice. Surprisingly, infectious virus was cleared faster in C57BL/6 mice, and this was associated with more rapid production of neutralizing antibody. However, viral RNA was cleared more slowly in C57BL/6 mice. In both mouse strains, more infectious virus was present in the lumbar spinal cord than in the cervical spinal cord. These data suggest that genetic susceptibility to fatal NSV encephalomyelitis is determined at least in part by the efficiency of viral replication and spread in the central nervous system. The differences identified in this study provide possible phenotypes for mapping genetic loci involved in susceptibility.     

Genetic control of red cell osmotic fragility

Among normal mouse strains, natural genetic variation offers the potential to investigate the structure and function of cell membranes. One such polymorphism between C57BL/6J and DBA/2J is a difference in erythrocyte sensitivity to osmotic lysis. The genetic basis for erythrocyte osmotic fragility differences between mouse strains C57BL/6 and DBA/2 was examined through analyses of their serial backcross progeny, recombinant inbred (ri) strains (BXD), and congenic C57BL/6 strains with allelic differences at Hbb or Fv-2. The data indicate that the fragility difference between C57BL/6 and DBA/2 is the result of allelic differences at a minimum of two segregating loci. One of these might be linked to, but is not identical with the gene encoding the beta chain of hemoglobin (Hbb). Allelic differences at Fv-2, a gene known to control the proportion of erythroid precursors in the S phase, and at Hba, the structural locus of hemoglobin alpha chain also appear to exert no major influence on red cell osmotic fragility. Furthermore, the fact that red cells from one of the RI strans (BXD-31) are strikingly more resistant than those from the resistant parental strain DBA/2 leads to the conclusion that the degree of resistance/susceptibility for either strain is determined by the combined contributions of gene effects not all of which act in the same direction. We also found that red cells from strans C57BL/6 and DBA/2 differ in their uptake of 51Cr. This result suggests the possibility that red cell osmotic fragility differences may be due in part to differences in ion metabolism or membrane transport.     

Susceptibility/resistance to mouse hepatitis virus strain 3 and macrophage procoagulant activity are genetically linked and controlled by two non-H-2-linked genes

The mode of inheritance of susceptibility/resistance to mouse hepatitis strain 3 (MHV-3) was determined by typing the set of AXB/BXA recombinant inbred (RI) strains derived from resistant A/J (A) and susceptible C57BL/6J (B) progenitors for susceptibility to infection as determined by the severity of liver pathology. The strain distribution pattern for susceptibility showed a discontinuous variation: one strain was fully resistant (A-like), four strains were fully susceptible (B-like), and 16 strains showed an intermediate degree of susceptibility. The fully susceptible strains developed fulminant hepatitis and died; the fully resistant strain developed no liver disease, whereas a range of disease ranging from mild focal hepatitis to widespread hepatocellular necrosis was seen in the semisusceptible strains. This SDP best fits the two-recessive-gene model of inheritance, and neither of these two loci is linked to the H-2 complex. Macrophage procoagulant activity (PCA) segregated among the RI strains in a strain distribution pattern identical to that of susceptibility/resistance. PCA levels were greater than sevenfold elevated in fully susceptible RI mice and fourfold elevated in semisusceptible mice with no increase in resistant mice. These observations suggest genetic linkage of susceptibility/resistance to MHV-3 infection and macrophage PCA.     

Detection of a new cerebral malaria susceptibility locus,using CBA mice

Human cerebral malaria (CM) during acute Plasmodium falciparum infection is a serious neurological complication that leads to coma and death. P. berghei ANKA infection of CBA mice is a useful experimental model of CM. To identify host susceptibility loci, we performed chromosomal mapping in crossbred populations of both CM-susceptible CBA and CM-resistant DBA/2 mice. One significant region for a CM-susceptible locus in CBA mice was mapped to H2 region on Chromosome 17, tentatively designated cmsc. cmsc was mapped to a different chromosomal region from that previously reported in the C57BL/6 mouse model of CM. It is possible that different loci contribute to CM in CBA and C57BL/6 mouse strains. Comparison of the function of CM susceptibility loci between CBA and C57BL/6 mice could have important implications for the study of the complex pathogenesis of CM in humans.     

Chromosomal locations and gonadal dependence of genes that mediate resistance to ectromelia (mousepox) virus-induced mortality.

Four genetic loci were tested for linkage with loci that control genetic resistance to lethal ectromelia virus infection in mice. Three of the loci were selected because of concordance with genotypes assigned to recombinant inbred (RI) strains of mice derived from resistant C57BL/6 and susceptible DBA/2 (BXD) mice on the basis of their responses to challenge infection. Thirty-six of 167 male (C57BL/6 x DBA/2)F1 x DBA/2 backcross (BC) mice died (22%), of which 27 (75%) were homozygous for DBA/2 alleles at Hc and H-2D. Twenty-eight percent of sham-castrated and 6% of sham-ovariectomized BC mice were susceptible to lethal mousepox, whereas 50% of gonadectomized mice were susceptible. There was no linkage evident between Hc or H-2D and loci that controlled resistance to lethal ectromelia virus infection in 44 castrated BC mice. Mortality among female mice of BXD RI strains with susceptible or intermediate male phenotypes was strongly correlated (r = 0.834) with male mortality. Gonadectomized C57BL/6 mice were as resistant as intact mice to lethal ectromelia virus infection. These results indicate that two gonad-dependent genes on chromosomes 2 and 17 and one gonad-independent gene control resistance to mousepox virus infection, that males and females share gonad-dependent genes, and that the gonad-independent gene is fully protective.     

Different genes control the susceptibility of mice to Moloney or Abelson murine leukemia viruses.

The susceptibility of mice to lymphoma induction by Moloney or Abelson murine leukemia virus has been compared in BALB/c, C57BL/6, and BALB/cXC57BL/6 recombinant inbred strains. BALB/c mice were found to be susceptible to lymphoma induction by either virus, and C57BL/6 mice were found to be relatively resistant to lymphoma induction by either virus. The genes that control these patterns of susceptibility to each virus are not the same because susceptibility to each virus segregated independently in CXB recombinant inbred strains. We also found, as reported by Cook (W. Cook, Proc. Natl. Acad. Sci. U.S.A. 79:2917-2921, 1982), when injected intrathymically that Abelson murine leukemia virus rapidly induced thymomas in weanling B6 mice. Examination of the cellular phenotypes of the tumors induced by Abelson murine leukemia virus or by Moloney murine leukemia virus indicated that different lymphocyte subpopulations were the targets for tumor induction by each virus.     

Synergistic gene interactions control the induction of the mitochondrial uncoupling protein (Ucp1) gene in white fat tissue

Among a selected group of mouse strains susceptible to dietary obesity, those with an enhanced capacity for Ucp1 and brown adipocyte induction in white fat preferentially lost body weight following adrenergic stimulation. Based on the generality of this mechanism for reducing obesity, a genetic analysis was initiated to identify genes that control brown adipocyte induction in white fat depots in mice. Quantitative trait locus (QTL) analysis was performed using the variations of retroperitoneal fat Ucp1 mRNA expression in progeny of genetic crosses between the A/J and C57BL/6J parental strains and selected AXB recombinant inbred strains. Three A/J-derived loci on chromosomes 2, 3, and 8 and one C57BL/6J locus on chromosome 19 were linked to Ucp1 induction in retroperitoneal fat. Although A/J-derived alleles seemed to contribute to elevated Ucp1 expression, the C57BL/6J allele on chromosome 19 increased Ucp1 mRNA to levels higher than parental values. Thus, novel patterns of C57BL/6J and A/J recombinant genotypes among the four mapped loci resulted in a transgressive variation of Ucp1 phenotypes. Although the extent of the interchromosomal interactions have not been fully explored, strong synergistic interactions occur between a C57BL/6J allele on chromosome 19 and an A/J allele on chromosome 8. In addition to selective synergistic interactions between loci, variations in recessive and dominant effects also contribute to the final levels of Ucp1 expression.     

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.     

Inheritance of susceptibility to the myeloproliferative sarcoma virus: effect of the Fv-2 locus and evidence for a myeloproliferative sarcoma virus resistance locus.

Myeloproliferative sarcoma virus (MPSV) causes a generalized stem cell leukemia with erythroid and myeloid hyperplasia in adult mice. MPSV also transforms fibroblasts. Mice congenic for the Fv-2 locus showed marked differences in susceptibility to MPSV according to the Fv-2 genotype. MPSV was injected into C57BL/6 Fvs and C57BL/6 Fv-2r mice congenic except for the Fv-2 locus. C57BL/6 mice with the Fvs genotype were much more susceptible to MPSV than were those with the Fvr genotype. Both DDD Fv-2r mice congenic with DDD Fv-2s mice except for the Fv-2 locus and DDD Fv-2s mice, however, were sensitive to spleen focus formation by MPSV. These data indicate that at least one additional resistance locus to MPSV is present in C57BL/6 mice but not in DDD mice. Both the Fv-2 locus and the putative MSPV resistance locus (loci) Mpsvr appear to be epistatic to either of the sensitivity loci. Fibroblast focus formation by MPSV was obtained well in C57BL/6 Fv-2r and C57BL/6 Fvs fibroblasts, indicating that the genes for MPSV resistance (Fv-2r and Mpsvr) were not operating in fibroblast cells. A model is proposed which may account for the differences in response of genetically different mice to MPSV and Friend spleen focus-forming virus.     

Genetic characterization of strain differences in the ability to mediate CD40/CD28-independent rejection of skin allografts

Simultaneous blockade of the CD40 and CD28 T cell costimulatory pathways effectively promotes skin allograft survival in C3H/HeJ mice, extending median survival times (MSTs) beyond 100 days. This strategy is markedly less effective in C57BL/6 mice, with MSTs ranging between 20 and 30 days. In this study, we investigate the underlying genetic causes of these distinct phenotypes. Using H-2 congenic mice, we show that the genetic basis for the varied responses between these two strains is independent of the H-2 locus and T cell precursor frequency. C57BL/6 mice treated with costimulation blockade are able to generate allospecific CTL- and IFN-gamma-producing T cells within 3-4 wk posttransplant, whereas mice with a C3H background generate neither CTL- nor IFN-gamma-producing cells. Thus, differences appear to be in the generation of the immune response and not T cell homing. Strain differences in costimulation blockade-induced hyporesponsiveness persist in the absence of CD4(+) T cells, implying a direct effect on CD8(+) T cells. We demonstrate that genetic differences are important in cells of hemopoietic origin and that the costimulation blockade-resistant phenotype is dominant. Analysis of BXH recombinant inbred strains indicates that multiple loci contribute to the phenotype, and that the blockade resistance loci are preliminarily linked to 17 markers on four chromosomes. We conclude that strain variation in allograft MSTs following CD40/CD28 blockade results from the ability of CD8(+) T cells in some strains to use alternative modes of costimulation to mount an effective alloresponse.     

Genetic regulation of early survival and cyst number after peroral Toxoplasma gondii infection of A x B/B x A recombinant inbred and B10 congenic mice

Genetics of two traits, survival and brain cyst number after peroral Toxoplasma gondii infection, were studied by using recombinant inbred strains of mice derived from resistant A/J (A) and susceptible C57BL/6J (B) progenitors, F1 progeny of crosses between A/J and C57BL/6J mice, and congenic mice (B10 background). Analysis of strain distribution pattern of survival of A x B/B x A recombinant mice indicated that survival is regulated by a minimum of five genes. One of these genes appears to be linked to the H-2 complex and another is related to an as yet unmapped gene controlling resistance to Ectromelia virus. Associations of defined traits with resistance or susceptibility to Toxoplasma cyst formation were also analyzed. Cyst number is regulated by a locus on chromosome 17 within 0 to 4 centimorgans of the H-2 complex (p = 0.001). Mice with the H-2a haplotype are resistant and those with the H-2b haplotype are susceptible. This analysis also indicated that the Bcg locus on chromosome 1 may effect cyst number (map distance = 12 centimorgans, p = 0.05). Resistance to cyst formation is a dominant trait. To analyze relative roles of H-2 and Bcg loci on cyst numbers, C57BL10 (B10)-derived congenic strains of mice with known H-2 and Bcg type were studied. These studies indicated that the H-2 complex locus has the primary effect on cyst number.     

Genetic analysis of spontaneous and induced palatal slit in C57BL/6 mice

The results of experiments designed to elucidate the nature of the genetic factors underlying the palatal slit defect in C57BL/6 mice indicate that probably two loci in females and four in males determine the difference in susceptibility to the spontaneous defect between the C57BL/6 and SWV/BcTa strains of mice. Moreover, it appears that these are not the same genes determining the difference between these same strains in palatal slit response to maternal treatment with triamcinolone. Additional evidence is presented to support the view that in the C57BL/6 strain the triamcinolone-induced palatal slit and cleft palate responses are not correlated.     

Spontaneous autoimmunity in 129 and C57BL/6 mice-implications for autoimmunity described in gene-targeted mice

Systemic lupus erythematosus (SLE) is a multisystem autoimmune disorder in which complex genetic factors play an important role. Several strains of gene-targeted mice have been reported to develop SLE, implicating the null genes in the causation of disease. However, hybrid strains between 129 and C57BL/6 mice, widely used in the generation of gene-targeted mice, develop spontaneous autoimmunity. Furthermore, the genetic background markedly influences the autoimmune phenotype of SLE in gene-targeted mice. This suggests an important role in the expression of autoimmunity of as-yet-uncharacterised background genes originating from these parental mouse strains. Using genome-wide linkage analysis, we identified several susceptibility loci, derived from 129 and C57BL/6 mice, mapped in the lupus-prone hybrid (129 × C57BL/6) model. By creating a C57BL/6 congenic strain carrying a 129-derived Chromosome 1 segment, we found that this 129 interval was sufficient to mediate the loss of tolerance to nuclear antigens, which had previously been attributed to a disrupted gene. These results demonstrate important epistatic modifiers of autoimmunity in 129 and C57BL/6 mouse strains, widely used in gene targeting. These background gene influences may account for some, or even all, of the autoimmune traits described in some gene-targeted models of SLE.     

An investigation of genetic variation within a series of congenic strains of mice

2 congenic strains of mice, B6N.AKN-Ahk and D2N.B6N-Ahb, imported from the USA, were found to be either segregating or fixed for an incorrect allele at a number of biochemical loci. B6N.AKN-Ahk, supposedly congenic with C57BL/6N, had the wrong genotype at 6 out of 12 biochemical loci; D2N.B6N-Ahb, supposedly congenic with DBA/2N, was segregating at 3 out of 9 loci. There was genetic variation in mandible shape within the 2 strains but no abnormal coat colours were found and no hybrid vigour in breeding performance was detected. Analyses in the USA confirmed these results and showed that 2 other congenic strains, C3N.D2N-Ahd and AKN.B6J-Ahb, were also segregating at a number of loci. Some of the alleles found in the C3N.D2N-Ahd mice must be the result of a genetic contamination. The simplest explanation for this breakdown in the backcrossing programme is genetic contamination with other congenic strains or recombinant inbred lines under development in the same laboratory. These findings emphasize the importance of continual genetic monitoring of all genetic stocks at regular intervals and in particular during the development of congenic and recombinant lines.     

Susceptibility of several strains of mice to Echinostoma hortense infection

Susceptibilities of 5 different mice strains, including C3H/HeN, BALB/c, C57BL6, FvB and ICR, to Echinostoma hortense infection, was evaluated. The worm expulsion rate, worm size and egg production were observed from 1 to 8 weeks after infection with 30 metacercariae. C3H/HeN and ICR mice showed the highest worm maturation rates. The worm recovery rate and the number of eggs per gram (EPG) of feces was also higher in C3H/HeN and ICR mice than in BALB/c, C57BL6, and FvB mice. It is suggested that E. hortense is highly infectious to ICR and C3H/HeN mice, but not to the other strains of mice. Based on the results obtained, we believe that the susceptibility of different mouse strains to E. hortense infection is dependent on the genetic and immunologic background of mice.