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.     

C1q deficiency and autoimmunity: the effects of genetic background on disease expression

Gene-targeted C1q-deficient mice have been shown to develop a syndrome reminiscent of human systemic lupus erythematosus with antinuclear Abs and proliferative glomerulonephritis. Initial phenotypic analysis conducted in (129 x C57BL/6) hybrid mice showed that background genes were a significant factor for the full expression of the autoimmune disease. To assess the contribution of background genes in the expression of the autoimmune phenotype, the disrupted C1qa gene was backcrossed for seven generations onto C57BL/6 and MRL/Mp(+/+) strains. These were intercrossed with C57BL/6.lpr/lpr and MRL/Mp-lpr/lpr strains to generate C1q-deficient substrains. In C1q-deficient C57BL/6 mice, no evidence of an autoimmune phenotype was found, and C1q deficiency in both the C57BL/6.lpr/lpr and MRL/Mp-lpr/lpr strains did not modify the autoimmune phenotype observed in wild-type controls. However, in C1q-deficient MRL/Mp(+/+) animals an acceleration of both the onset and the severity of antinuclear Abs and glomerulonephritis was seen. Disease was particularly pronounced in females, which developed severe crescentic glomerulonephritis accompanied by heavy proteinuria. In addition, the C1q-deficient MRL/Mp(+/+) mice had an impairment in the phagocytic clearance of apoptotic cells in vivo. These data demonstrate that the expression of autoimmunity in C1q-deficient mice is strongly influenced by other background genes. The work also highlights the potential value of the C1q-deficient MRL/Mp(+/+) strain as a tool with which to dissect further the underlying mechanisms of the autoimmune syndrome associated with C1q deficiency.     

Genetic dissection of spontaneous autoimmunity driven by 129-derived chromosome 1 Loci when expressed on C57BL/6 mice

Extensive evidence indicates that genetic predisposition is a central element in susceptibility to systemic lupus erythematosus both in humans and animals. We have previously shown that a congenic line carrying a 129-derived chromosome 1 interval on the C57BL/6 background developed humoral autoimmunity. To further dissect the contribution to autoimmunity of this 129 interval, we have created six subcongenic strains carrying fractions of the original 129 region and analyzed their serological and cellular phenotypes. At 1 year of age the congenic strain carrying a 129 interval between the microsatellites D1Mit15 (87.9 cM) and D1Mit115 (99.7 cM) (B6.129chr1b) had high levels of autoantibodies, while all the other congenic lines were not significantly different from the C57BL/6 controls. The B6.129chr1b strain displayed only mild proliferative glomerulonephritis despite high levels of IgG and C3 deposited in the kidneys. FACS analysis of the spleens revealed that the B6.129chr1b mice had a marked increase in the percentage of activated T cells associated with a significant reduction in the proportion of CD4(+)CD25(high) regulatory T cells. Moreover, this analysis showed a significantly reduced percentage of marginal zone B cells that preceded autoantibody production. Interestingly the 129chr1b-expressing bone marrow-derived macrophages displayed an impaired uptake of apoptotic cells in vitro. Collectively, our data indicate that the 129chr1b segment when recombined on the C57BL/6 genomic background is sufficient to induce loss of tolerance to nuclear Ags. These findings have important implication for the interpretation of the autoimmune phenotype associated with gene-targeted models.     

Deletion of p21 (WAF-1/Cip1) does not induce systemic autoimmunity in female BXSB mice

Cell cycle, apoptosis, and replicative senescence are all influenced by the cyclin-dependent kinase inhibitor, p21. It was previously reported that deletion of p21 in 129/Sv x C57BL/6 mixed genetic background mice induced a severe lupus-like disease, almost exclusively in females. However, we did not confirm this finding in an independently derived stock of 129/Sv x C57BL/6 p21(-/-) mice. To further address this discrepancy, we examined the effects of p21 deletion in BXSB female mice that develop late-life, mild lupus-like disease. Survival, polyclonal Igs, anti-chromatin Abs, and kidney histopathology in these mice were unremarkable and identical to wild-type littermates for up to 14 mo of age. We conclude that p21 deficiency does not promote autoimmunity even in females of a predisposed strain. The findings indicate that the use of mixed background 129/Sv x C57BL/6 mice to study effects of gene deletions in systemic autoimmunity may be confounded by the genetic heterogeneity of this cross. We suggest that studies addressing gene deletion effects in systemic autoimmunity should use sufficiently backcrossed mice to attain genetic homogeneity, include wild-type littermate controls, and preferentially use congenic inbred strains with late-life lupus predisposition to emulate the polygenic nature of this disease.     

Genetic variation in C57BL/6 ES cell lines and genetic instability in the Bruce4 C57BL/6 ES cell line

Genetically modified mouse strains derived from embryonic stem (ES) cells are powerful tools for gene function analysis. ES cells from the C57BL/6 mouse strain are not widely used to generate mouse models despite the advantage of a defined genetic background. We assessed genetic variation in six such ES cell lines with 275 SSLP markers. Compared to C57BL/6, Bruce4 differed at 34 SSLP markers and had significant heterozygosity on three chromosomes. BL/6#3 and Dale1 ES cell lines differed at only 3 SSLP makers. The C2 and WB6d ES cell lines differed at 6 SSLP markers. It is important to compare the efficiency of producing mouse models with available C57BL/6 ES cells relative to standard 129 mouse strain ES cells. We assessed genetic stability (the tendency of cells to become aneuploid) in 110 gene-targeted ES cell clones from the most widely used C57BL/6 ES cell line, Bruce4, and 710 targeted 129 ES cell clones. Bruce4 clones were more likely to be aneuploid and unsuitable for ES cell-mouse chimera production. Despite their tendency to aneuploidy and consequent inefficiency, use of Bruce4 ES cells can be valuable for models requiring behavioral studies and other mouse models that benefit from a defined C57BL/6 background. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Derivation and comparison of C57BL/6 embryonic stem cells to a widely used 129 embryonic stem cell line

Typically, embryonic stem (ES) cells derived from 129 mouse substrains are used to generate genetically altered mouse models. Resulting chimeric mice were then usually converted to a C57BL/6 background, which takes at least a year, even in the case of speed congenics. In recent years, embryonic stem cells have been derived from various mouse strains. However, 129 ES cells are still widely used partially due to poor germline transmission of ES cells derived from other strains. Availability of highly germline-competent C57BL/6 ES cells would enormously facilitate generation of genetically altered mice in a pure C57BL/6 genetic background by eliminating backcrossing time, and thus significantly reducing associated costs and efforts. Here, we describe establishment of a C57BL/6 ES cell line (LK1) and compare its efficacy to a widely used 129SvJ ES cell line (GSI-1) in generating germline chimeras. In contrast to earlier studies, our data shows that highly germline-competent C57BL/6 ES cell lines can be derived using a simple approach, and thus support broader use of C57BL/6 ES cell lines for genetically engineered mouse models. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Autoimmune-prone mice share a promoter haplotype associated with reduced expression and function of the Fc receptor FcgammaRII

Human autoimmune diseases thought to arise from the combined effects of multiple susceptibility genes include systemic lupus erythematosus (SLE) and autoimmune diabetes. Well-characterised polygenic mouse models closely resembling each of these diseases exist, and genetic evidence links receptors for the Fc portion of immunoglobulin G (FcR) with their pathogenesis in mice and humans [1] [2] [3]. FcRs may be activatory or inhibitory and regulate a variety of immune and inflammatory processes [4] [5]. FcgammaRII (CD32) negatively regulates activation of cells including B cells and macrophages [6]. FcgammaRII-deficient mice are prone to immune-mediated disease [7] [8] [9]. The gene encoding FcgammaRII, Fcgr2, is contained in genetic susceptibility intervals in mouse models of SLE such as the New Zealand Black (NZB) contribution to the (NZB x New Zealand White (NZW)) F1 strain [1] [10] [11] and the BXSB strain [12], and in human SLE [1] [2] [3]. We therefore sequenced Fcgr2 and identified a haplotype defined by deletions in the Fcgr2 promoter region that is present in major SLE-prone mouse strains (NZB, BXSB, SB/Le, MRL, 129 [13]) and non-obese diabetic (NOD) mice but absent in control strains (BALB/c, C57BL/6, DBA/2, C57BL/10) and NZW mice. The autoimmune haplotype was associated with reduced cell-surface expression of FcgammaRII on macrophages and activated B cells and with hyperactive macrophages resembling those of FcgammaRII-deficient mice, and is therefore likely to play an important role in the pathogenesis of SLE and possibly diabetes.     

Behavioral differences among fourteen inbred mouse strains commonly used as disease models

We compared the behavior of 14 inbred mouse strains and an F1 hybrid commonly used in transgenic and knockout production. These strains were 129P3/J, 129S1/SvImJ, 129S6/SvEvTac, 129T2/SvEmsJ, 129X1/SvJ (formerly 129/J, 129/Sv-p+Tyr+Kitl+/J, 129/SvEvTac, 129SvEmsJ, and 129/SvJ, respectively), A/JCrTac, BALB/cAnNTac, C3H/HeNTac, C57BL/6J, C57BL/6NTac, DBA/2NTac, FVB/NTac, NOD/MrkTac, SJL/JCrNTac, and the hybrid B6129S6F1Tac. Performance in three behavioral tests (rotorod, open-field activity-habituation, and contextual and cued fear conditioning) was determined. On the rotorod assay, SJL/JCrNTac mice had the shortest latencies to fall on the first day of testing, and DBA/2NTac mice showed impaired motor learning. Open-field behavior was analyzed using the parameters total distance, center distance, velocity, and vertical activity. 129T2/EvEmsJ and A/JCrTac were least active in the open field, whereas NOD/MrkTac mice were most active. Contrary to earlier studies, we found that all strains habituated to the open field in at least one of these parameters. In contextual and cued fear conditioning, all strains displayed activity suppression. However, FVB/NTac mice reacted less strongly to both context and cue than did most of the other strains. There were no significant behavioral differences between C57BL/6J and C57BL/6NTac, except for higher open-field activity in C57BL/6J female mice. These findings illustrate the importance of the appropriate selection of background strain for transgenic, gene targeting, or drug research.     

品系对小鼠胚胎干细胞分离效率的影响

为了充分利用小鼠胚胎干(ES)细胞,就必须从众多小鼠品系中分离ES细胞系。本研究通过传统的成纤维细胞饲养层法,从CD-1、129/Sv、C57BL/6J和129/Sv×C57BL/6J四种不同遗传背景的小鼠中分离得到12个ES细胞系,而从KM小鼠没有得到ES细胞系。所有的ES细胞系都具有典型的ES细胞特征,AKP染色呈阳性。从四种不同遗传背景的ES细胞系得到了包含多种组织的畸胎瘤;与桑椹胚聚合后,都得到了生殖系嵌合体。结果表明:品系对小鼠ES细胞的分离有显著影响,利用129小鼠以及包含129小鼠遗传背景的杂交小鼠都较容易分离ES细胞,由ES细胞得到生殖系嵌合体的效率在不同品系间有显著差异,从杂交ES细胞比近交ES细胞中更容易得到生殖系嵌合体。     

Behavioral profiles of inbred strains on novel olfactory,spatial and emotional tests for reference memory in mice

Studying the behavior of genetic background strains provides important information for the design and interpretation of cognitive phenotypes in mutant mice. Our experiments examined the performance of three commonly used strains (C57BL/6J, 129S6, DBA/2J) on three behavioral tests for learning and memory that measure very different forms of memory, and for which there is a lack of data on strain differences. In the social transmission of food preference test (STFP) all three strains demonstrated intact memory for an odor-cued food that had been sampled on the breath of a cagemate 24 hours previously. While C57BL/6J and 129S6 mice showed good trace fear conditioning, DBA/2J mice showed a profound deficit on trace fear conditioning. In the Barnes maze test for spatial memory, the 129S6 strain showed poor probe trial performance, relative to C57BL/6J mice. Comparison of strains for open field exploratory activity and anxiety-like behavior suggests that poor Barnes maze performance reflects low exploratory behavior, rather than a true spatial memory deficit, in 129S6 mice. This interpretation is supported by good Morris water maze performance in 129S6 mice. These data support the use of a C57BL/6J background for studying memory deficits in mutant mice using any of these tasks, and the use of a 129S6 background in all but the Barnes maze. A DBA/2J background may be particularly useful for investigating the genetic basis of emotional memory using fear conditioning.     

Genetic and behavioral differences among five inbred mouse strains commonly used in the production of transgenic and knockout mice

Five strains of mice commonly used in transgenic and knockout production were compared with regard to genetic background and behavior. These strains were: C57BL/6J, C57BL/6NTac, 129P3/J (formerly 129/J), 129S6/SvEvTac (formerly 129/SvEvTac) and FVB/NTac. Genotypes for 342 microsatellite markers and performance in three behavioral tests (rotorod, open field activity and habituation, and contextual and cued fear conditioning) were determined. C57BL/6J and C57BL/6NTac were found to be true substrains; there were only 12 microsatellite differences between them. Given the data on the genetic background, one might predict that the two C57BL/6 substrains should be very similar behaviorally. Indeed, there were no significant behavioral differences between C57BL/6J and C57BL/6NTac. Contrary to literature reports on other 129 strains, 129S6/SvEvTac often performed similarly to C57BL/6 strains, except that it was less active. FVB/NTac showed impaired rotorod learning and cued fear conditioning. Therefore, both 129S6/SvEvTac and C57BL/6 are recommended as background strains for targeted mutations when researchers want to evaluate their mice in any of these three behavior tests. However, any transgene on the FVB/NTac background should be transferred to B6. Habituation to the open field was analyzed using the parameters: total distance, center distance, velocity and vertical activity. Contrary to earlier studies, we found that all strains habituated to the open field in at least two of these parameters (center distance and velocity).     

Identification of 129S6/SvEvTac-specific polymorphisms on mouse chromosome 11

Polymorphisms such as single-nucleotide polymorphisms (SNPs) and insertions/deletions (Indels) can be associated with phenotypic traits and be used as markers for disease diagnosis. Identification of these genetic variations within laboratory mice is crucial to improve our understanding of the genetic background of the mice used for research. As part of a positional cloning project, we sequenced six genes (Mettl16, Evi2a, Psmd11, Cct6d, Rffl, and Ap2b1) within a 6.8-Mb domain of mmu chr 11 in the C57BL/6J and 129S6/SvEvTac inbred strains. Although 129S6/SvEvTac is widely used in the mouse community, there is very little current (or projected future) sequence information available for this strain. We identified 6 Indels and 21 novel SNPs and confirmed genotype information for 114 additional SNPs in these 6 genes. Mettl16 and Ap2b1 contained the largest numbers of variants between the C57BL/6J and 129S6/SvEvTac strains. In addition, we found five new SNPs between 129S6/SvEvTac and 129S1/SvImJ within the Ap2b1 locus. Although we did not detect differences between C57BL/6J and 129S6/SvEvTac within Evi2a, this locus contains a relatively high SNP density compared with the surrounding sequence. Our study highlights the genetic differences among three inbred mouse strains (C57BL/6J, 129S6/SvEvTac, and 129S1/SvImJ) and provides valuable sequence information that can be used to track alleles in genomics-based studies.     

Genetic susceptibility to testicular autoimmunity: comparison between postthymectomy and postvasectomy models in mice

The genetic susceptibility was compared in six mouse strains to both postthymectomy (post-Tx) autoimmune orchitis, induced by thymectomy at Day 3, and postvasectomy (post-Vx) autoimmune testicular lesions, induced by bilateral vasectomy with ligation at 2 months of age. Results from mice examined at 4 months of age showed that the strain distribution of the post-Vx testicular lesion was similar to that of post-Tx orchitis. The SWR/J strain was susceptible, the DBA/1J, C57L/J, BALB/cJ and probably C57BL/10J strains were resistant, and the A/J strain was intermediate. Anti-sperm antibodies (IgG) were produced to varying degrees in all strains of vasectomized mice irrespective of the testicular lesion. Neither the development of the testicular lesion nor antibody formation was associated with the particular H-2 haplotypes of mice in either system. Neonatal thymectomy significantly enhanced anti-sperm antibody formation in the post-Vx system and resulted in a mild testicular atrophy in a resistant strain BALB/cBy. All these data suggest that a gene(s) outside the major histocompatibility complex may determine the genetic susceptibility to testicular autoimmunity, possibly at the testicular level.     

Semen quality parameters and embryo lethality in mice deficient for Trp53 protein

Trp53 is a protein which is able to control semen parameters in mice, but the extent of that control depends on the genetic background of the mouse strain. Males from C57BL/6Kw, 129/Sv, C57BL×129 -p53+/+ (wild type controls) and C57BL×129-p53-/- (mutants) strains were used in the study, and histology and light microscopy were applied to evaluate the influence of genetic background and Trp53 (p53) genotype on testes morphology and semen quality in male mice. We showed that sperm head morphology, maturity and tail membrane integrity were controlled only by the genetic background of C57BL/6Kw and 129/Sv males, while testes weight and sperm concentration depended on both the genetic background and p53 genotype. Cell accumulation in seminiferous tubules may be responsible for heavier testes of p53-deficient males. In addition, to examine the effect of sex and p53 genotype on embryo lethality, pairs of control (C57BL×129-p53+/+) and heterozygous (C57BL×129-p53+/-) mice were examined. Before day 7 post coitum (dpc), female and male embryos were equally resorbed in both crosses types. After 7 dpc, preferential female embryo lethality in the heterozygote pairs was responsible for the skewed sex ratio in their progeny. Also, mutant female and male newborns were underrepresented in the litters of the heterozygous breeding pairs.     

Strain-specific modification of lethality in fucose-deficient mice

The FX locus encodes an essential enzyme in the de novo pathway of GDP-fucose biosynthesis. Mice homozygous for a targeted mutation of the FX gene manifest a host of pleiotropic abnormalities including a lethal phenotype that is almost completely penetrant in heterozygous intercrosses on a mixed genetic background. Here we have investigated genetic suppression of FX-mediated lethality. Reduced recovery of heterozygous mice was observed while backcrossing the null FX allele to C57BL/6J (B6), but was less dramatic in an outcross to CASA/Rk and absent in an outcross to 129S1/SvImJ, indicating that genetic background modifies survival of FX+/- progeny. Substantial strain-specific differences in pre- and postnatal survival of FX-/- progeny were also detected in heterozygous crosses of C57BL/6J congenic, 129S1B6F1, and B6CASAF1 mice. Specifically, intrauterine survival of FX-/- mice was greatly increased during a heterozygous intercross on a uniform C57BL/6J genetic background compared with survival on a hybrid genetic background consisting of a mixture of C57BL/6J and 129S2/SvPas. In addition, statistically significant clustering of FX-/- progeny into litters and specific breeding cages was noted during a B6CASAF1 FX+/- intercross, suggesting a rare mechanism for modifier gene action in which parentally expressed genes define the phenotype, in this case the survival potential, of mutant offspring. Our results disclose that lethality in FX mutant mice is determined by one or more strain-specific modifier loci.     

Lens density tracking in mice by Scheimpflug imaging

Scheimpflug imaging has recently been established for in vivo imaging of the anterior eye segment and quantitative determination of lens transparency in the mouse. This enables more effective investigations of cataract formation with the mouse model, including longitudinal studies. In order to enable recognition of disease-associated irregularities, we performed Scheimpflug measurements with the common laboratory inbred lines C57BL/6J, C3HeB/FeJ, FVB/NCrl, BALB/cByJ, and 129/SvJ in a period between 2 and 12 months of age. C57BL/6J mice showed lowest mean lens densities during the test period. Progressive cortical lens opacification was generally observed, with the earliest onset in C57BBL/6J, C3HeB/FeJ, and 129/SvJ, between 2 and 6 months after birth. Moreover, lenses of these inbred lines developed nuclear opacities. Calculated mean lens density significantly increased between 6 and 12 months of age in all inbred strains except 129/SvJ. Lens densities (and the corresponding standard deviations) of FVB/NCrl and 129/SvJ increased most likely because of differences in the genetic background. Albinism as confounder might be excluded since the albino Balb/cByJ mice are more similar to the C57BL/6J or C3Heb/FeJ mice. We further identified strain-specific anterior lens opacities (C57BL/6J) and cloudy corneal lesions (C57BL/6J, FVB/NCrl, and BALB/cByJ) at later stages. In conclusion, our results indicate that there are lifelong opacification processes in the mouse lens. The highest lens transparency and a dark coat color, which prevents interference from light reflections, make mice with the C57BL/6J background most suitable for cataract research by Scheimpflug imaging. We show that lens densitometry by Scheimpflug imaging in mouse eyes can resolve differences of less than 1 %, making it possible to detect differences in cataract development in different mouse strains, even if they are small.     

Unexpected Rescue of Alpha-synuclein and Multimerin1 Deletion in C57BL/6JOlaHsd Mice by Beta-adducin Knockout

Uniform genetic background of inbred mouse strains is essential in experiments with genetically modified mice. In order to assess Add2 (beta-adducin) function, its null mutation was produced in embryonic stem cells derived from 129Sv mouse and the subsequently obtained mouse mutants were backcrossed for 6 generations with C57BL/6JOlaHsd strain. Comparison of brain proteins between mutated and control animals by two-dimensional gels linked to mass spectroscopy analysis showed expression of Snca (alpha-synuclein) in the mutated animals, but unexpectedly not in the control C57BL/6JOlaHsd mice. Comparison between C57BL/6JOlaHsd and C57BL/6NCrl mice confirmed the presence of a deletion encompassing Snca and in addition Mmrn1 (multimerin1) loci in C57BL/6JOlaHsd strain. The segregation of mutated Add2 together with an adjacent part of the chromosome 6 derived from 129Sv mice, rescued the loss of these two genes in knockout mice on C57BL/6JOlaHsd background. The fact that Add2 knockout was compared with the C57BL/6JOlaHsd mouse strain, which is actually a double knockout of Snca and Mmrn1 emphasizes a need for information provided by commercial suppliers and of exact denominations of substrains used in research.     

Genetic factors influence cataract formation in alpha 3 connexin knockout mice

Connexin alpha 3 (Cx46 or Gja3) gene targeted null mice developed lens nuclear cataracts shortly after birth. A large variance in the cataracts was observed in alpha 3 null sibs on a mixed 129SvJae X C57BL/6J F3 background. This suggested that the genetic background might influence the cataract phenotype. Therefore, we placed the alpha 3 null mutation into a 129SvJae background, and also backcrossed the mutation for six generations into 129SvJ and C57BL/6J backgrounds. While alpha 3 nulls on the two 129 backgrounds contained severe cataracts associated with gamma crystallin cleavage, alpha 3 nulls on the C57B16 background had far milder cataracts with no detectable gamma crystallin cleavage. These findings suggest that a genetic modifier exists that influences gamma crystallin stability, and that gamma crystallin breakdown is associated with severe nuclear cataracts.     

The inhibiting Fc receptor for IgG, FcγRIIB, is a modifier of autoimmune susceptibility

FcγRIIB-deficient mice generated in 129 background (FcγRIIB(129)(-/-)) if back-crossed into C57BL/6 background exhibit a hyperactive phenotype and develop lethal lupus. Both in mice and humans, the Fcγr2b gene is located within a genomic interval on chromosome 1 associated with lupus susceptibility. In mice, the 129-derived haplotype of this interval, named Sle16, causes loss of self-tolerance in the context of the B6 genome, hampering the analysis of the specific contribution of FcγRIIB deficiency to the development of lupus in FcγRIIB(129)(-/-) mice. Moreover, in humans genetic linkage studies revealed contradictory results regarding the association of "loss of function" mutations in the Fcγr2b gene and susceptibility to systemic lupus erythematosis. In this study, we demonstrate that FcγRIIB(-/-) mice generated by gene targeting in B6-derived ES cells (FcγRIIB(B6)(-/-)), lacking the 129-derived flanking Sle16 region, exhibit a hyperactive phenotype but fail to develop lupus indicating that in FcγRIIB(129)(-/-) mice, not FcγRIIB deficiency but epistatic interactions between the C57BL/6 genome and the 129-derived Fcγr2b flanking region cause loss of tolerance. The contribution to the development of autoimmune disease by the resulting autoreactive B cells is amplified by the absence of FcγRIIB, culminating in lethal lupus. In the presence of the Yaa lupus-susceptibility locus, FcγRIIB(B6)(-/-) mice do develop lethal lupus, confirming that FcγRIIB deficiency only amplifies spontaneous autoimmunity determined by other loci.     

Genetic background determines metabolic phenotypes in the mouse

To evaluate the contribution of genetic background to phenotypic variation, we compared a large range of biochemical and metabolic parameters at different ages of four inbred mice strains, C57BL/6J, 129SvPas, C3HeB/FeJ, and Balb/cByJ. Our results demonstrate that important metabolic, hematologic, and biochemical differences exist between these different inbred strains. Most of these differences are gender independent and are maintained or accentuated throughout life. It is therefore imperative that the genetic background is carefully defined in phenotypic studies. Our results also argue that certain backgrounds are more suited to study a given physiologic phenomenon, as distinct mouse strains have a different propensity to develop particular biochemical, hematologic, and metabolic abnormalities. These genetic differences can furthermore be exploited to identify new genes/proteins that contribute to phenotypic abnormalities. The choice of the genetic background in which to generate and analyze genetically engineered mutant mice is important as it is, together with environmental factors, one of the most important contributors to the variability of phenotypic results.