Genetic variation in Glp1r expression influences the rate of gastric emptying in mice

We demonstrated previously that food intake traits map to a quantitative trait locus (QTL) on proximal chromosome 17, which encompasses Glp1r (glucagon-like peptide 1 receptor), encoding an important modulator of gastric emptying. We then confirmed this QTL in a B6.CAST-17 congenic strain that consumed 27% more carbohydrate and 17% more total calories, yet similar fat calories, per body weight compared with the recipient C57BL/6J. The congenic strain also consumed greater food volume. The current aims were to 1) identify genetic linkage for total food volume in F(2) mice, 2) perform gene expression profiling in stomach of B6.CAST-17 congenic mice using oligonucleotide arrays, 3) test for allelic imbalance in Glp1r expression, 4) evaluate gastric emptying rate in parental and congenic mice, and 5) investigate a possible effect of genetic variation in Glp1r on gastric emptying. A genome scan revealed a single QTL for total food volume (Tfv1) (log of the odds ratio = 7.6), which was confirmed in B6.CAST-17 congenic mice. Glp1r exhibited allelic imbalance in stomach, which correlated with accelerated gastric emptying in parental CAST and congenic B6.CAST-17 mice. Moreover, congenic mice displayed an impaired gastric emptying response to exendin-(9-39). These results suggest that genetic variation in Glp1r contributes to the strain differences in gastric emptying rate.     

Dissection of a genetically complex cluster of growth and obesity QTLs on mouse chromosome 2 using subcongenic intercrosses

In a previous study we characterized the B6.CAST-(D2Mit329-D2Mit457)N(6) (B62D) congenic strain, which possesses CAST/EiJ (CAST) chromosome 2 donor alleles from 74 to 180 Mbp on a C57BL6/J (B6) background. This strain exhibited significant decreases in body weight and adiposity attributable to the weight gain 2 (Wg2) quantitative trait locus (QTL). To refine the location of Wg2, we used a two-stage genetic dissection strategy consisting of a B62D × B6 backcross, which mapped Wg2 to the proximal portion of the B62D donor region, followed by the development of seven overlapping subcongenic F₂ intercrosses targeting the Wg2 genomic interval. Surprisingly, five of the seven intercrosses displayed significant differences, dependent on genotype, in body weight and/or fat pad mass. These effects were the result of at least four independent QTLs that were named Wg2a, b, c, and d. In contrast to the lean and low body weight phenotype of the B62D parental strain, mice homozygous for CAST congenic alleles (cast/cast) at Wg2a were significantly heavier at 6 and 9 weeks of age, while cast/cast mice at Wg2c had higher levels of total fat. Consistent with the prior observed effects of Wg2, cast/cast mice at Wg2b displayed significant decreases in 6- and 9-week body weight as well as a decrease in total fat pad mass. All of the QTLs had additive effects on body composition except Wg2d, which displayed underdominance for total fat mass. Significant differences in weight and adiposity were also observed in genetically identical b6/b6 homozygous mice across the panel of subcongenics, suggesting either maternal or paternal contributions to body composition. These data represent a significant advancement toward the identification of mouse chromosome 2 growth and obesity quantitative trait genes.     

Gene expression between a congenic strain that contains a quantitative trait locus of high bone density from CAST/EiJ and its wild-type strain C57BL/6J

Peak bone density is an important determining factor of future osteoporosis risk. We previously identified a quantitative trait locus (QTL) that contributes significantly to high bone density on mouse chromosome 1 from a cross between C57BL/6J (B6) and CAST/EiJ (CAST) mouse strains. We then generated a congenic strain, B6.CAST-1T, in which the chromosomal fragment containing this QTL had been transferred from CAST to the B6 background. The congenic mice have a significantly higher bone density than the B6 mice. In this study we performed cDNA microarray analysis to evaluate the gene expression profile that might yield insights into the mechanisms controlling the high bone density by this QTL. This study led to several interesting observations. First, approximately 60% of 8,734 gene accessions on GEM I chips were expressed in the femur of B6 mice. The expression and function of two-thirds of these expressed genes and ESTs have not been documented previously. Second, expression levels of genes related to bone formation were lower in congenic than in B6 mice. These data are consistent with a low bone formation in the congenic mice, a possibility that is confirmed by reduced skeletal alkaline phosphatase activity in serum compared with B6 mice. Third, expression levels of genes that might have negative regulatory action on bone resorption were higher in congenic than in B6 mice. Together these findings suggest that the congenic mice might have a lower bone turnover rate than B6 mice and raise the possibility that the high bone density in the congenic mice could be due to reduced bone resorption rather than increased bone formation. Electronic Publication     

Genome-wide gene expression profiling of human mast cells stimulated by IgE or FcεRI-aggregation reveals a complex network of genes involved in inflammatory responses

Background

High growth (hg) modifier and background independent quantitative trait loci (QTL) affecting growth, adiposity and carcass composition were previously identified on mouse chromosomes (MMU) 1, 2, 5, 8, 9, 11 and 17. To confirm and further characterize each QTL, two panels of speed congenic strains were developed by introgressing CAST/EiJ (CAST) QTL alleles onto either mutant C57Bl/6J-hg/hg (HG) or wild type C57Bl/6J (B6) genetic backgrounds.

Results

The first speed congenic panel was developed by introgressing four overlapping donor regions spanning MMU2 in its entirety onto both HG and B6 backgrounds, for a total of eight strains. Phenotypic characterization of the MMU2 panel confirmed the segregation of multiple growth and obesity QTL and strongly suggested that a subset of these loci modify the effects of the hg deletion. The second panel consisted of individual donor regions on an HG background for each QTL on MMU1, 5, 8, 9, 11 and 17. Of the six developed strains, five were successfully characterized and displayed significant differences in growth and/or obesity as compared to controls. All five displayed phenotypes similar to those originally attributed to each QTL, however, novel phenotypes were unmasked in several of the strains including sex-specific effects.

Conclusion

The speed congenic strains developed herein constitute an invaluable genomic resource and provide the foundation to identify the specific nature of genetic variation influencing growth and obesity.     

Quantitative trait loci for carbohydrate and total energy intake on mouse chromosome 17: congenic strain confirmation and candidate gene analyses (Glo1, Glp1r)

Quantitative trait loci (QTL) for carbohydrate (Mnic1) and total energy (Kcal2) intake on proximal mouse chromosome 17 were identified previously from a C57BL/6J (B6) X CAST/Ei (CAST) intercross. Here we report that a new congenic strain developed in our laboratory has confirmed this complex locus by recapitulating the original linked phenotypes: B6.CAST-17 homozygous congenic mice consumed more carbohydrate (27%) and total energy (17%) compared with littermate wild-type mice. Positional gene candidates with relevance to carbohydrate metabolism, glyoxalase I (Glo1) and glucagon-like peptide-1 receptor (Glp1r), were evaluated. Glo1 expression was upregulated in liver and hypothalamus of congenic mice when compared with B6 mice. Analyses of Glp1r mRNA and protein expression revealed tissue-specific strain differences in pancreas (congenic>B6) and stomach (B6>congenic). These results suggest the possibility of separate mechanisms for enhanced insulin synthesis and gastric accommodation in the presence of high carbohydrate intake and larger food volume, respectively. Sequence analysis of Glp1r found a G insert at nt position 1349, which results in earlier termination of the open reading frame, thus revealing an error in the public sequence. Consequently, the predicted length of GLP-1R is 463 aa compared with 489 aa, as previously reported. Also, we found a polymorphism in Glp1r between parental strains that alters the amino acid sequence. Variation in Glp1r could influence nutrient intake in this model through changes in the regulatory or protein coding regions of the gene. These congenic mice offer a powerful tool for investigating gene interactions in the control of food intake.     

High-Resolution Mapping of a Genetic Locus Regulating Preferential Carbohydrate Intake,Total Kilocalories,and Food Volume on Mouse Chromosome 17

The specific genes regulating the quantitative variation in macronutrient preference and food intake are virtually unknown. We fine mapped a previously identified mouse chromosome 17 region harboring quantitative trait loci (QTL) with large effects on preferential macronutrient intake-carbohydrate (Mnic1), total kilcalories (Kcal2), and total food volume (Tfv1) using interval-specific strains. These loci were isolated in the [C57BL/6J.CAST/EiJ-17.1-(D17Mit19-D17Mit50); B6.CAST-17.1] strain, possessing a ∼40.1 Mb region of CAST DNA on the B6 genome. In a macronutrient selection paradigm, the B6.CAST-17.1 subcongenic mice eat 30% more calories from the carbohydrate-rich diet, ∼10% more total calories, and ∼9% more total food volume per body weight. In the current study, a cross between carbohydrate-preferring B6.CAST-17.1 and fat-preferring, inbred B6 mice was used to generate a subcongenic-derived F₂ mapping population; genotypes were determined using a high-density, custom SNP panel. Genetic linkage analysis substantially reduced the 95% confidence interval for Mnic1 (encompassing Kcal2 and Tfv1) from 40.1 to 29.5 Mb and more precisely established its boundaries. Notably, no genetic linkage for self-selected fat intake was detected, underscoring the carbohydrate-specific effect of this locus. A second key finding was the separation of two energy balance QTLs: Mnic1/Kcal2/Tfv1 for food intake and a newly discovered locus regulating short term body weight gain. The Mnic1/Kcal2/Tfv1 QTL was further de-limited to 19.0 Mb, based on the absence of nutrient intake phenotypes in subcongenic HQ17IIa mice. Analyses of available sequence data and gene ontologies, along with comprehensive expression profiling in the hypothalamus of non-recombinant, cast/cast and b6/b6 F₂ controls, focused our attention on candidates within the QTL interval. Zfp811, Zfp870, and Btnl6 showed differential expression and also contain stop codons, but have no known biology related to food intake regulation. The genes Decr2, Ppard and Agapt1 are more appealing candidates because of their involvement in lipid metabolism and down-regulation in carbohydrate-preferring animals.     

Dissection of multigenic obesity traits in congenic mouse strains

Previous quantitative trait locus mapping (QTL) identified multigenic obesity (MOB) loci on mouse Chromosome (Chr) 2 that influence the interrelated phenotypes of obesity, insulin resistance, and dyslipidemia. To better localize and characterize the MOB locus, three congenic mouse strains were created. Overlapping genomic intervals from the lean CAST/Ei (CAST) strain were introgressed onto an obesity-susceptible C57BL/6 (BL6) background to create proximal (15 Mb–73 Mb), middle (63 Mb–165 Mb), and distal (83 Mb–182 Mb) congenic strains. The congenic strains showed differences in obesity, insulin, and lipid traits consistent with the original QTL analysis for the locus. Importantly, characterization of the MOB congenics localized the effects of genes that underlie obesity-related traits to an introgressed interval (73–83 Mb) unique to the middle MOB congenic. Conversely, significant differences between the lipid and insulin profiles of the middle and distal MOB congenics implicated the presence of at least two genes that underlie these traits. When fed an atherogenic diet, several traits associated with metabolic syndrome were observed in the distal MOB congenic, while alterations in plasma lipoproteins were observed in the middle MOB congenic strain.     

Quantitative trait loci that regulate plasma lipid concentration in hereditary obese KK and KK-Ay mice

To identify quantitative trait loci (QTLs) responsible for regulating plasma lipid concentration associated with obesity, linkage analysis was carried out on the 190 F2 progeny of a cross between C57BL/6J female and KK-Ay (Ay allele at the agouti locus congenic) male. In F2 a/a (agouti locus genotype) mice, two QTLs were identified on chromosome 1 and a QTL on chromosome 3 for total-cholesterol. A QTL for HDL-cholesterol was identified on chromosome 1 and a QTL for NEFA on chromosome 9. In F2 Ay/a mice, two QTLs for HDL-cholesterol were found on chromosome 1. Loci for other lipids with suggestive linkage were also identified. In both F2 mice, one QTL on chromosome 1 for total- and HDL-cholesterol was mapped near D1Mit150, in the vicinity of the apolipoprotein A-II (Apoa2) locus. Seven nucleotide substitutions out of 309 nucleotide apolipoprotein A-II cDNA sequences were identified between KK and C57BL/6J. The Ay allele may be an indication of the plasma lipid levels, but its influence was less apparent than in the case of weight control. The loci for lipids were not on identical chromosomes with those previously identified for obesity, suggesting that hyperlipidemia in KK does not coincidentally occur with obesity.     

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.     

Functional mapping of quantitative trait loci that interact with the hg mutation to regulate growth trajectories in mice

The high growth (hg) mutation increases body size in mice by 30-50%. Given the complexity of the genetic regulation of animal growth, it is likely that the effect of this major locus is mediated by other quantitative trait loci (QTL) with smaller effects within a web of gene interactions. In this article, we extend our functional mapping model to characterize modifier QTL that interact with the hg locus during ontogenetic growth. Our model is derived within the maximum-likelihood context, incorporated by mathematical aspects of growth laws and implemented with the EM algorithm. In an F2 population founded by a congenic high growth (HG) line and non-HG line, a highly additive effect due to the hg gene was detected on growth trajectories. Three QTL located on chromosomes 2 and X were identified to trigger significant additive and/or dominant effects on the process of growth. The most significant finding made from our model is that these QTL interact with the hg locus to affect the shapes of the growth process. Our model provides a powerful means for understanding the genetic architecture and regulation of growth rate and body size in mammals.     

Genetic analysis of the psychostimulant effects of nicotine in chromosome substitution strains and F2 crosses derived from A/J and C57BL/6J progenitors

Previous research utilizing the AcB/BcA recombinant congenic strains (RCS) of mice mapped provisional quantitative trait loci (QTLs) for the psychostimulant effects of nicotine to multiple regions on chromosomes 7, 11, 12, 14, 16, and 17. The current study was designed to confirm these QTLs in an A/J (A) × C57Bl/6J (B6) F2 cross and a panel of B6.A chromosome substitution strains (CSS). The panel of B6.A CSS consists of 21 strains, each carrying a different A/J chromosome on a B6 background. The A × B6 F2, CSS, A, and B6 mice were tested for sensitivity to the effects of nicotine on locomotor activity using a computerized open-field apparatus. In A × B6 F2 mice two QTLs were identified which confirm those previously observed in the AcB/BcA RCS. Significant differences in the expression of nicotine-induced activity were associated with loci on chromosome 11 (D11Mit62) and chromosome 16 (D16Mit131) in the A × B6 F2. At the chromosome 11 QTL, an A allele was associated with lower nicotine-induced activity scores relative to the B6. In contrast, the A allele was associated with greater relative nicotine activity values for the chromosome 16 QTL. A survey of the CSS panel confirmed the presence of QTLs for nicotine activation on chromosomes 2, 14, 16, and 17 previously identified in the AcB/BcA RCS. In the informative CSS strains, A alleles were consistently associated with greater nicotine-induced activity scores compared to the B6. The results of the present study are the first to validate QTLs for sensitivity to the effects of nicotine across multiple strains of mice. QTLs on chromosomes 2, 11, 14, 16, and 17 were confirmed in CSS and/or F2 mice. Significantly, the identification of a QTL on chromosome 16 has now been replicated in three crosses derived from the A and B6 progenitors.     

Epistasis affecting litter size in mice

Litter size is an important reproductive trait as it makes a major contribution to fitness. Generally, traits closely related to fitness show low heritability perhaps because of the corrosive effects of directional natural selection on the additive genetic variance. Nonetheless, low heritability does not imply, necessarily, a complete absence of genetic variation because genetic interactions (epistasis and dominance) contribute to variation in traits displaying strong heterosis in crosses, such as litter size. In our study, we investigated the genetic architecture of litter size in 166 females from an F2 intercross of the SM/J and LG/J inbred mouse strains. Litter size had a low heritability (h2 = 12%) and a low repeatability (r = 33%). Using interval-mapping methods, we located two quantitative trait loci (QTL) affecting litter size at locations D7Mit21 + 0 cM and D12Mit6 + 8 cM, on chromosomes 7 and 12 respectively. These QTL accounted for 12.6% of the variance in litter size. In a two-way genome-wide epistasis scan we found eight QTL interacting epistatically involving chromosomes 2, 4, 5, 11, 14, 15 and 18. Taken together, the QTL and their interactions explain nearly 49% (39.5% adjusted multiple r2) of the phenotypic variation for litter size in this cross, an increase of 36% over the direct effects of the QTL. This indicates the importance of epistasis as a component of the genetic architecture of litter size and fitness in our intercross population.     

Increased physical activity cosegregates with higher intake of carbohydrate and total calories in a subcongenic mouse strain

C57BL/6 J (B6) and CAST/EiJ (CAST), the inbred strain derived from M. musculus castaneus, differ in nutrient intake behaviors, including dietary fat and carbohydrate consumption in a two-diet-choice paradigm. Significant quantitative trait loci (QTLs) for carbohydrate (Mnic1) and total energy intake (Kcal2) are present between these strains on chromosome (Chr) 17. Here we report the refinement of the Chr 17 QTL in a subcongenic strain of the B6.CAST- ^{D17Mit19-D17Mit91} congenic mice described previously. This new subcongenic strain possesses CAST Chr 17 donor alleles from 4.8 to 45.4 Mb on a B6 background. Similar to CAST, the subcongenic mice exhibit increased carbohydrate and total calorie intake per body weight, while fat intake remains equivalent. Unexpectedly, this CAST genomic segment also confers two new physical activity phenotypes: 22% higher spontaneous physical activity levels and significantly increased voluntary wheel-running activity compared with the parental B6 strain. Overall, these data suggest that gene(s) involved in carbohydrate preference and increased physical activity are contained within the proximal region of Chr 17. Interval-specific microarray analysis in hypothalamus and skeletal muscle revealed differentially expressed genes within the subcongenic region, including neuropeptide W (Npw); glyoxalase I (Glo1); cytochrome P450, family 4, subfamily f, polypeptide 1 (Cyp4f15); phospholipase A2, group VII (Pla2g7); and phosphodiesterase 9a (Pde9a). This subcongenic strain offers a unique model for dissecting the contributions and possible interactions among genes controlling food intake and physical activity, key components of energy balance.     

Genetic analysis of alcohol intake in recombinant inbred and congenic strains derived from A/J and C57BL/6J progenitors

The objective of the present study was to map quantitative trait loci (QTL) for alcohol intake using A × B/B × A recombinant inbred (RI) and AcB/BcA recombinant congenic (RC) strains of mice that were independently derived from the A/J and C57BL/6J progenitors. Mice were screened for levels of alcohol consumption with four days of forced exposure to alcohol, followed by three weeks of free choice between water and a 10% alcohol solution. Alcohol consumption data previously collected for 27 A × B/B × A RI strains were reanalyzed using a larger marker set and composite interval mapping. The reanalysis found markers on Chromosome 2 (D2Mit74, 107 cM) (males and females) and on Chromosome 11 (Pmv22, 8 cM) (females only) that exceeded the threshold for significant loci, and found suggestive loci (in males) on Chromosomes 10 (D10 Mit126, 21 cM), 12 (D12Mit37, 1 cM), 15 (Pdgfb, 46.8 cM), and 16 (D16Mit125, 29 cM). An additional suggestive locus was identified in female RI mice on Chromosome 11 (D11Mit120, 47.5 cM). Composite interval mapping (CIM) analysis indicated that there was a significant association between loci at Pdgfb and D2Mit74 in both males and females. Analysis of the AcB/BcA RC strains identified 11 QTL on Chromosomes 2, 3, 5,6, 7, 8, 9, 10, 12, 13, and 15. QTL on Chromosomes 7, 10, 12, and 15 were identified in both the A × B/B × A RI and AcB/BcA RC strains of mice. Additional QTLs identified on Chromosomes 2, 3, 7, 11, and 15 overlap with those previously identified in the literature using strains of mice with a C57BL/6J progenitor.     

Genetic studies of B-lymphocyte deficiency and mastocytosis in strain A/WySnJ mice

The A/WySnJ mouse, but not the related A/J strain, has peripheral B-lymphocyte deficiency and mastocytosis. Minimally, two quantitative trait loci (QTLs) control the B-cell deficiency in (A/WySnJ x CAST/Ei)F2 intercross mice; one of them, Bcmd-1, mapped to Chromosome (Chr) 15. Several QTLs controlled the mastocytosis in this intercross, and it was not possible to determine whether any of them co-segregated with Bcmd-1. We have now mapped a second QTL controlling the B-cell deficiency, Bcmd-2, to Chr 4. Furthermore, we narrowed the map position of Bcmd-1 to <2.0 cM. Both QTLs have been confirmed through the construction of AW. Bcmd-1(c), AW. Bcmd-2(c), and AW. Bcmd-1(c)Bcmd-2(c) recombinant congenic strains. The Bcmd-1 locus is the major regulator of B-cell homeostasis, while Bcmd-2 is the minor regulator, and their effects are additive, as shown by splenic B-cells analysis in these congenic strains. In addition, Bcmd-2 or a linked locus controls mastocytosis, while Bcmd-1 does not, as indicated by splenic mast cell analysis in the congenic strains. Thus, the major genetic controls on B-cell homeostasis and mast cell homeostasis in A/WySnJ mice are asserted by distinct genes.     

Detection of modifier loci influencing the lung phenotype of cystic fibrosis knockout mice

The variable severity of lung disease associated with cystic fibrosis (CF) cannot be explained by the genotype of the cystic fibrosis transmembrane conductance regulator (CFTR) locus alone. Lung disease has been reported in a congenic CF mouse model of C57BL/6J genetic background (B6 CF), in the absence of detectable infection, but not in CF mice of mixed genetic background, nor in wild-type animals maintained in identical environments. In this report, studies are presented to show that the same CF mutation in mice of a BALB/c background (BALB CF) results in minimal lung disease. By 12 weeks of age B6 CF mice developed a lung disease consisting of mononuclear cell interstitial infiltrate and fibrosis, and BALB CF or littermate control mice developed minimal histopathology. Therefore, it is possible to identify the chromosomal locations of genes that can contribute to the susceptibility to lung disease in B6 CF mice compared with BALB CF mice by means of a quantitative trait loci (QTL) mapping strategy based on the variable histology of the (B6 × BALB) F2 CF mice. Significant linkage of the fibrotic lung phenotype was detected for a region on Chromosome (Chr) 6, defined by markers D6Mit194 to D6Mit201, and suggestive linkage was found for regions on Chr 1, 2, 10, and 17. Additional loci, suggestive of linkage, were also detected for the interstitial thickening phenotype. Most of these putative loci are specific to the sex of the animals. These results suggest that multiple genes can influence the severity of CF lung disease in mice.     

Mapping of genetic factors influencing the weight of cystic fibrosis knockout mice

One of the poorly understood clinical manifestations of cystic fibrosis (CF) is low body weight. Mice in which the CF causative gene, cystic fibrosis transmembrane conductance regulator (Cftr), has been knocked out reflect this as they are smaller than age-matched littermates. The variable weight of F2 Cftr -/- (CF) mice derived from a cross between congenic C57BL/6J and BALB/cJ Cftr heterozygotic mice permits the mapping of modifiers of this cystic fibrosis phenotype. In this report, quantitative trait loci (QTL) mapping was used to identify the chromosomal locations of genes that contribute to the body weight of 12-week-old F2 CF mice. Five loci of CF body weight were detected with four of the five acting in a sex-specific manner. Significant linkage of the phenotype to a region of Chromosome (Chr) 13 from D13Mit179 to D13Mit254 (LOD = 4.2) was established in female mice; and suggestive loci on Chrs 7 and 10 were identified. The weights of F2 male CF mice were suggestively linked to regions of Chrs 1 and 6, and to the same locus on Chr 7 as in female mice. The suggestive loci did not influence the weight of the limited set of control mice and thus are presumed to be CF specific in their effects. Further study of these putative CF body weight modifiers may provide insight on the pathogenesis of cystic fibrosis.     

A novel mouse Chromosome 2 congenic strain with obesity phenotypes

Linkage studies have identified many chromosomal regions containing obesity genes in mice. However, only a few of these quantitative trait loci (QTLs) have been used to guide the production of congenic mouse strains that retain obesity phenotypes. We seek to identify chromosomal regions containing obesity genes in the BSB model of spontaneous obesity because the BSB model is a multigenic obesity model. Previous studies identified QTLs on Chromosomes (Chrs) 2, 6, 7,12, and 15. BSB mice are made by backcross of lean C57BL/6J × Mus spretus. F₁s were backcrossed to C57BL/6J mice to produce BSB progeny. We have constructed a new BSB cross and produced congenic mice with obesity phenotypes by marker-directed selection called B6.S–D2Mit194–D2Mit311. We found a highly significant QTL for percentage body lipid on Chr 2 just proximal to the Agouti locus. Chr 2 congenics were constructed to determine whether the main effects would be detectable. We observed highly significant linkage of the Chr 2 congenic containing Agouti and containing markers distal to D2Mit311 and proximal to D2Mit194. Thus, this congenic contains approximately 14.6 cM or 30 Mb (about 1.1% of the spretus mouse genome) and several hundred genes. The obesity phenotype of the QTL is retained in the congenic. The congenic can now be used to model the genetic and physiological basis for a relatively simple, perhaps monogenic, obesity.     

Mapping genetic loci that regulate lipid levels in a NZB/B1NJxRF/J intercross and a combined intercross involving NZB/B1NJ, RF/J, MRL/MpJ, and SJL/J mouse strains

The NZB/B1NJ (NZB) mouse strain exhibits high cholesterol and HDL levels in blood compared with several other strains of mice. To study the genetic regulation of blood lipid levels, we performed a genome-wide linkage analysis in 542 chow-fed F2 female mice from an NZBxRF/J (RF) intercross and in a combined data set that included NZBxRF and MRL/MpJxSJL/J intercrosses. In the NZBxRF F2 mice, the cholesterol and HDL concentrations were influenced by quantitative trait loci (QTL) on chromosome (Chr) 5 [logarithm of odds (LOD) 17-19; D5Mit10] that was in the region identified earlier in crosses involving NZB mice, but two QTLs on Chr 12 (LOD 4.7; D12Mit182) and Chr 19 (LOD 5.7; D19Mit1) were specific to the NZBxRF intercross. Triglyceride levels were affected by two novel QTLs at D12Mit182 (LOD 8.7) and D15Mit13 (LOD 3.5). The combined-cross linkage analysis (1,054 mice, 231 markers) 1) identified four shared QTLs (Chrs 5, 7, 14, and 17) that were not detected in one of the parental crosses and 2) improved the resolution of two shared QTLs. In summary, we report additional loci regulating lipid levels in NZB mice that had not been identified earlier in crosses involving the NZB strain of mice. The identification of shared loci from multiple crosses increases confidence toward finding the QTL gene.     

Differential role of three major New Zealand Black-derived loci linked with Yaa-induced murine lupus nephritis

By assessing the development of Y-linked autoimmune acceleration (Yaa) gene-induced systemic lupus erythematosus in C57BL/6 (B6) x (New Zealand Black (NZB) x B6.Yaa)F(1) backcross male mice, we mapped three major susceptibility loci derived from the NZB strain. These three quantitative trait loci (QTL) on NZB chromosomes 1, 7, and 13 differentially regulated three different autoimmune traits: anti-nuclear autoantibody production, gp70-anti-gp70 immune complex (gp70 IC) formation, and glomerulonephritis. Contributions to the disease traits were further confirmed by generating and analyzing three different B6.Yaa congenic mice, each carrying one individual NZB QTL. The chromosome 1 locus that overlapped with the previously identified Nba2 (NZB autoimmunity 2) locus regulated all three traits. A newly identified chromosome 7 locus, designated Nba5, selectively promoted anti-gp70 autoantibody production, hence the formation of gp70 IC and glomerulonephritis. B6.Yaa mice bearing the NZB chromosome 13 locus displayed increased serum gp70 production, but not gp70 IC formation and glomerulonephritis. This locus, called Sgp3 (serum gp70 production 3), selectively regulated the production of serum gp70, thereby contributing to the formation of nephritogenic gp70 IC and glomerulonephritis, in combination with Nba2 and Nba5 in NZB mice. Among these three loci, a major role of Nba2 was demonstrated, because B6.Yaa Nba2 congenic male mice developed the most severe disease. Finally, our analysis revealed the presence in B6 mice of an H2-linked QTL, which regulated autoantibody production. This locus had no apparent individual effect, but most likely modulated disease severity through interaction with NZB-derived susceptibility loci.