The SMXA: a new set of recombinant inbred strain of mice consisting of 26 substrains and their genetic profile

Neonatal lethality of mouse A/J-7SM consomic strain is caused by an insertion mutation in the Dchs1 gene

Homosomic mice of the A/J-7^SM consomic mouse strain that introduced the entire chromosome 7 (Chr 7) of SM/J into the A/J strain exhibited neonatal lethality. We tentatively maintained segregating inbred strains (A/J-7A^SM and A/J-7D^SM) in which the central portion of Chr 7 was heterozygous for the A/J and SM/J strains, and the centromeric and telomeric sides of Chr 7 were homozygous for the SM/J strain, instead of the A/J-7^SM strain. Based on the chromosomal constitution of Chr 7 in A/J-7A^SM and A/J-7D^SM mice, the causative gene for neonatal lethality in homosomic mice was suggested to be located within an approximately 1.620 Mb region between D7Mit125 (104.879 Mb) and D7Mit355 (106.499 Mb) on Chr 7. RT-PCR analysis revealed that homosomic mice lacked dachsous cadherin-related 1 (Dchs1), which is located within the D7Mit125 to D7Mit355 region and functions in the regulation of planar cell polarity. Screening for mutations in Dchs1 indicated that homosomic mice possessed an early transposable (ETn)-like sequence in intron 1 of Dchs1. Moreover, an allelism test between Dchs1 ETn-like-insertion alleles detected in homosomic mice and CRISPR/Cas9-induced Dchs1 deletion alleles revealed that Dchs1 is a causative gene for neonatal lethality in homosomic mice. Based on these results, we concluded that in the A/J-7^SM strain, ETn-like elements were inserted into intron 1 of SM/J-derived Dchs1 during strain development, which dramatically reduced Dchs1 expression, thus resulting in neonatal lethality in homosomic mice. Additionally, it was suggested that the timing of lethality in Dchs1 mutant mice is influenced by the genetic background.

Ablation of Iah1, a candidate gene for diet-induced fatty liver, does not affect liver lipid accumulation in mice

Nonalcoholic fatty liver disease (NAFLD) is a pathological condition caused by excess triglyceride deposition in the liver. The SMXA-5 severe fatty liver mouse model has been established from the SM/J and A/J strains. To explore the genetic factors involved in fatty liver development in SMXA-5 mice, we had previously performed quantitative trait locus (QTL) analysis, using (SM/J×SMXA-5)F2 intercross mice, and identified Fl1sa on chromosome 12 (centromere-53.06 Mb) as a significant QTL for fatty liver. Furthermore, isoamyl acetate-hydrolyzing esterase 1 homolog (Iah1) was selected as the most likely candidate gene for Fl1sa. Iah1 gene expression in fatty liver-resistant A/J-12^SM mice was significantly higher than in fatty liver-susceptible A/J mice. These data indicated that the Iah1 gene might be associated with fatty liver development. However, the function of murine Iah1 remains unknown. Therefore, in this study, we created Iah1 knockout (KO) mice with two different backgrounds [C57BL/6N (B6) and A/J-12^SM (A12)] to investigate the relationship between Iah1 and liver lipid accumulation. Liver triglyceride accumulation in Iah1-KO mice of B6 or A12 background did not differ from their respective Iah1-wild type mice under a high-fat diet. These results indicated that loss of Iah1 did not contribute to fatty liver. On the other hands, adipose tissue dysfunction causes lipid accumulation in ectopic tissues (liver, skeletal muscle, and pancreas). To investigate the effect of Iah1 deficiency on white adipose tissue, we performed DNA microarray analysis of epididymal fat in Iah1-KO mice of A12 background. This result showed that Iah1 deficiency might decrease adipokines Sfrp4 and Metrnl gene expression in epididymal fat. This study demonstrated that Iah1 deficiency did not cause liver lipid accumulation and that Iah1 was not a suitable candidate gene for Fl1sa.

Genetic mapping of a male factor subfertility locus on mouse chromosome 4

Male reproductive anomalies are widely distributed among mammals, and male factors are estimated to contribute to approximately 50% of cases of human infertility. The B10.M/Sgn (B10.M) mouse strain exhibits two adverse reproductive phenotypes: severe teratospermia and male subfertility. Although teratospermia is known to be heritable, the relationship between teratospermia and male subfertility has not been well characterized. The fertility of B10.M male mice is considerably lower (~ 30%) than that of standard laboratory mouse strains (~ 70%). To genetically analyze male subfertility, F2 males were produced by intercrossing the F1 progeny of female B10.M and male C3H/HeN mice. The fertility of each F2 male mouse was assessed based on the outcomes of matings with five females. Statistical analysis of correlations between the two reproductive phenotypes (teratospermia and subfertility) in F2 males (n = 177) revealed that teratospermia is not the cause of male subfertility. Quantitative trait loci (QTL) analysis of the male subfertility phenotype (n = 128) using GigaMUGA markers mapped one significant QTL peak to chromosome 4 at 62.9 centimorgans (cM) with a logarithm of odds score of 11.81 (P < 0.05). We named the QTL locus Mfsf1 (male factor subfertility 1). Further genetic analysis using recombinant males restricted the physical area to 1.53 megabasepairs (Mbp), encompassing 22 protein-coding genes. In addition, we found one significant QTL and one indicative QTL on chromosome 5 and 12, respectively, that interacted with the Mfsf1 locus. Our results demonstrate that genetic dissection of male subfertility in the B10.M strain is a useful model for characterizing the complex genetic mechanisms underlying reproduction and infertility.

Existence of NEU1 sialidase on mouse thymocytes whose natural substrate is CD5

Membrane-bound sialidases in the mouse thymus are unique and mysterious because their activity at pH 6.5 is equal to or higher than that in the acidic region. The pH curve like this has never been reported in membrane-bound form. To clarify this enzyme, we studied the sialidase activities of crude membrane fractions from immature-T, mature-T and non-T cells from C57BL/6 mice and from SM/J mice, a strain with a defect in NEU1 activity. Non-T cells from C57BL/6 mice had high activity at pH 6.5, but those from SM/J mice did not. Neu1 and Neu3 mRNA was shown by real-time PCR to be expressed in T cells and also in non-T cells, whereas Neu2 was expressed mainly in non-T cells and Neu4 was scarcely expressed. However, the in situ hybridization study on the localization of four sialidases in the thymus showed that Neu4 was clearly expressed. We then focused on a sialidase on the thymocyte surface because the possibility of the existence of a sialidase on thymocytes was suggested by peanut agglutinin (PNA) staining after incubation of the cells alone in PBS. This activity was inhibited by NEU1-selective sialidase inhibitor C9-butyl-amide-2-deoxy-2,3-dehydro-N-acetylneuraminic acid. The natural substrate for the cell surface sialidase was identified as clustered differentiation 5 (CD5) by PNA-blot analysis of anti-CD5 immunoprecipitate. We conclude that NEU1 exists on the cell surface of mouse thymocytes and CD5 is a natural substrate for it. Although this is not the main reaction of the membrane-bound thymus-sialidases, it must be important for the thymus.

Congenic mapping and candidate gene analysis for streptozotocin-induced diabetes susceptibility locus on mouse chromosome 11

Streptozotocin (STZ) has been widely used to induce diabetes in rodents. Strain-dependent variation in susceptibility to STZ has been reported; however, the gene(s) responsible for STZ susceptibility has not been identified. Here, we utilized the A/J-11^SM consomic strain and a set of chromosome 11 (Chr. 11) congenic strains developed from A/J-11^SM to identify a candidate STZ-induced diabetes susceptibility gene. The A/J strain exhibited significantly higher susceptibility to STZ-induced diabetes than the A/J-11^SM strain, confirming the existence of a susceptibility locus on Chr. 11. We named this locus Stzds1 (STZ-induced diabetes susceptibility 1). Congenic mapping using the Chr. 11 congenic strains indicated that the Stzds1 locus was located between D11Mit163 (27.72 Mb) and D11Mit51 (36.39 Mb). The Mpg gene, which encodes N-methylpurine DNA glycosylase (MPG), a ubiquitous DNA repair enzyme responsible for the removal of alkylated base lesions in DNA, is located within the Stzds1 region. There is a close relationship between DNA alkylation at an early stage of STZ action and the function of MPG. A Sanger sequence analysis of the Mpg gene revealed five polymorphic sites in the A/J genome. One variant, p.Ala132Ser, was located in a highly conserved region among rodent species and in the minimal region for retained enzyme activity of MPG. It is likely that structural alteration of MPG caused by the p.Ala132Ser mutation elicits increased recognition and excision of alkylated base lesions in DNA by STZ.

Genetic dissection of the fatty liver QTL Fl1sa by using congenic mice and identification of candidate genes in the liver and epididymal fat

BACKGROUND

Nonalcoholic fatty liver disease (NAFLD) is a multifactorial disease caused by interactions between environmental and genetic factors. The SMXA-5 mouse is a high-fat diet-induced fatty liver model established from SM/J and A/J strains. We have previously identified Fl1sa, a quantitative trait locus (QTL) for fatty liver on chromosome 12 (centromere-53.06 Mb) of SMXA-5 mice. However, the chromosomal region containing Fl1sa was too broad. The aim of this study was to narrow the Fl1sa region by genetic dissection using novel congenic mice and to identify candidate genes within the narrowed Fl1sa region.

RESULTS

We established two congenic strains, R2 and R3, from parental A/J-12^SM and A/J strains. R2 and R3 strains have genomic intervals of centromere-29.20 Mb and 29.20-46.75 Mb of chromosome 12 derived from SM/J, respectively. Liver triglyceride content in R2 and R3 mice was significantly lower than that in A/J mice fed with a high-fat diet for 7 weeks. This result suggests that at least one of the genes responsible for fatty liver exists within the two chromosomal regions centromere-29.20 Mb (R2) and 29.20-46.75 Mb (R3). We found that liver triglyceride accumulation is inversely correlated with epididymal fat weight among the parental and congenic strains. Therefore, the ectopic fat accumulation in the liver may be due to organ-organ interactions between the liver and epididymal fat. To identify candidate genes in Fl1sa, we performed a DNA microarray analysis using the liver and epididymal fat in A/J and A/J-12^SM mice fed with a high-fat diet for 7 weeks. In epididymal fat, mRNA levels of Zfp125 (in R2) and Nrcam (in R3) were significantly different in A/J-12^SM mice from those in A/J mice. In the liver, mRNA levels of Iah1 (in R2) and Rrm2 (in R2) were significantly different in A/J-12^SM mice from those in A/J mice.

CONCLUSIONS

In this study, using congenic mice analysis, we narrowed the chromosomal region containing Fl1sa to two regions of mouse chromosome 12. We then identified 4 candidate genes in Fl1sa: Iah1 and Rrm2 from the liver and Zfp125 and Nrcam from epididymal fat.

Detection of differentially expressed candidate genes for a fatty liver QTL on mouse chromosome 12

Background

The SMXA-5 mouse is an animal model of high-fat diet-induced fatty liver. The major QTL for fatty liver, Fl1sa on chromosome 12, was identified in a SM/J × SMXA-5 intercross. The SMXA-5 genome consists of the SM/J and A/J genomes, and the A/J allele of Fl1sa is a fatty liver-susceptibility allele. The existence of the responsible genes for fatty liver within Fl1sa was confirmed in A/J-12^SM consomic mice. The aim of this study was to identify candidate genes for Fl1sa, and to investigate whether the identified genes affect the lipid metabolism.

Results

A/J-12^SM mice showed a significantly lower liver triglyceride content compared to A/J mice when fed the high-fat diet for 7 weeks. We detected differences in the accumulation of liver lipids in response to the high-fat diet between A/J and A/J-12^SM consomic mice. To identify candidate genes for Fl1sa, we performed DNA microarray analysis using the livers of A/J-12^SM and A/J mice fed the high-fat diet. The mRNA levels of three genes (Iah1, Rrm2, Prkd1) in the chromosomal region of Fl1sa were significantly different between the strains. Iah1 mRNA levels in the liver, kidney, and lung were significantly higher in A/J-12^SM mice than in A/J mice. The hepatic Iah1 mRNA level in A/J-12^SM mice was 3.2-fold higher than that in A/J mice. To examine the effect of Iah1 on hepatic lipid metabolism, we constructed a stable cell line expressing the mouse Iah1 protein in mouse hepatoma Hepa1-6 cells. Overexpression of Iah1 in Hepa1-6 cells suppressed the mRNA levels of Cd36 and Dgat2, which play important roles in triglyceride synthesis and lipid metabolism.

Conclusions

These results demonstrated that Fl1sa on the proximal region of chromosome 12 affected fatty liver in mice on a high-fat diet. Iah1 (isoamyl acetate-hydrolyzing esterase 1 homolog) was identified as one of the candidate genes for Fl1sa. This study revealed that the mouse Iah1 gene regulated the expression of genes related to lipid metabolism in the liver.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-016-0385-2) contains supplementary material, which is available to authorized users.

SMXA-5 Mouse as a Diabetic Model Susceptible to Feeding a High-fat Diet

The SMXA-5 strain, a new mouse model for type 2 diabetes, is a recombinant inbred strain derived from non-diabetic SM/J and A/J strains. As dietary fat is a key component in the development of diabetes, we compared the glucose tolerance and diabetes-related traits among the SMXA-5, SM/J, and A/J strains while feeding a high-fat diet for 10 weeks. SMXA-5 fed on a high-fat diet showed an increased serum insulin concentration. Judging from the hyperinsulinemia in SMXA-5, this strain showed insulin resistance, an inability of peripheral tissues to respond to insulin, which was strengthened by feeding with a high-fat diet. When fed on a high-fat diet for 5 weeks, the SMXA-5 mice showed severely impaired glucose tolerance. On the other hand, SM/J showed mildly impaired glucose tolerance, even when fed on a high-fat diet for 10 weeks. These results indicate that SMXA-5 would be available for use as a diabetic model susceptible to a high-fat diet.

Searching for genomic region of high-fat diet-induced type 2 diabetes in mouse chromosome 2 by analysis of congenic strains

SMXA-5 mice are a high-fat diet-induced type 2 diabetes animal model established from non-diabetic SM/J and A/J mice. By using F2 intercross mice between SMXA-5 and SM/J mice under feeding with a high-fat diet, we previously mapped a major diabetogenic QTL (T2dm2sa) on chromosome 2. We then produced the congenic strain (SM.A-T2dm2sa (R0), 20.8–163.0 Mb) and demonstrated that the A/J allele of T2dm2sa impaired glucose tolerance and increased body weight and body mass index in the congenic strain compared to SM/J mice. We also showed that the combination of T2dm2sa and other diabetogenic loci was needed to develop the high-fat diet-induced type 2 diabetes. In this study, to narrow the potential genomic region containing the gene(s) responsible for T2dm2sa, we constructed R1 and R2 congenic strains. Both R1 (69.6–163.0 Mb) and R2 (20.8–128.2 Mb) congenic mice exhibited increases in body weight and abdominal fat weight and impaired glucose tolerance compared to SM/J mice. The R1 and R2 congenic analyses strongly suggested that the responsible genes existed in the overlapping genomic interval (69.6–128.2 Mb) between R1 and R2. In addition, studies using the newly established R1A congenic strain showed that the narrowed genomic region (69.6–75.4 Mb) affected not only obesity but also glucose tolerance. To search for candidate genes within the R1A genomic region, we performed exome sequencing analysis between SM/J and A/J mice and extracted 4 genes (Itga6, Zak, Gpr155, and Mtx2) with non-synonymous coding SNPs. These four genes might be candidate genes for type 2 diabetes caused by gene-gene interactions. This study indicated that one of the genes responsible for high-fat diet-induced diabetes exists in the 5.8 Mb genomic interval on mouse chromosome 2.

Detection of loci for allergic asthma using SMXA recombinant inbred strains of mice

Asthma is regarded as a multifactorial inflammatory disorder arising as a result of inappropriate immune responses in genetically susceptible individuals to common environmental antigens. However, the precise molecular basis is unknown. To identify genes for susceptibility to three asthma-related traits, airway hyperresponsiveness (AHR), eosinophil infiltration, and allergen-specific serum IgE levels, we conducted a genetic analysis using SMXA recombinant inbred (RI) strains of mice. Quantitative trait locus analysis detected a significant locus for AHR on chromosome 17. For eosinophil infiltration, significant loci were detected on chromosomes 9 and 16. Although we could not detect any significant loci for allergen-specific serum IgE, analysis of consomic strains showed that chromosomes 17 and 19 carried genes that affected this trait. We detected genetic susceptibility loci that separately regulated the three asthma-related phenotypes. Our results suggested that different genetic mechanisms regulate these asthma-related phenotypes. Genetic analyses using murine RI and consomic strains enhance understanding of the molecular mechanisms of asthma in human.

Establishment of consomic strains derived from A/J and SM/J mice for genetic analysis of complex traits

Consomic strains, in which one chromosome is derived from a donor strain and the other chromosomes are derived from the recipient strain, provide a powerful tool for the dissection of complex genetic traits. In this study we established ten consomic strains (A-2(SM), A-6(SM), A-11(SM), A-12(SM), A-13(SM), A-15(SM), A-17(SM), A-18(SM), A-19(SM), A-Y(SM)) using the SM/J strain as the donor and the A/J strain as the recipient; these are the parental strains of a set of SMXA recombinant inbred (RI) strains that we had developed previously. We analyzed body weights and blood lipid levels in the consomic and parental strains. The mean values for each trait showed a continuous range of variation in the consomic strains suggesting that they are controlled by multiple genes. We previously identified suggestive QTLs for body weight on chromosome 6 in SMXA RI strains and (SM/J × A/J)F(2) mice. The observation that the A-6(SM) consomic strain had a significantly lower mean body weight than the A/J strain supports the presence of this QTL on chromosome 6. Similarly, the higher blood triglyceride level in the A-11(SM) strain shows the existence of a previously mapped QTL on chromosome 11, and the A-12(SM) strain provides evidence of a QTL for blood total cholesterol level on chromosome 12. These consomic strains, along with the previously developed set of SMXA RI strains from A/J and SM/J mice, offer an invaluable and powerful resource for the analysis of complex genetic traits in mice.

Genetic analysis of abdominal fat distribution in SM/J and A/J mice

Each abdominal fat depot, such as mesenteric or epididymal, differently contributes to the development of insulin resistance. The aim of this study was to identify the genetic regions that contribute to fat accumulation in epididymal/mesenteric fat and to examine whether or not the genetic regions that affect glucose metabolism and body fat distribution are coincident. We previously mapped a major quantitative trait locus (QTL) (T2dm2sa) for impaired glucose tolerance on chromosome 2 and revealed that SM.A-T2dm2sa congenic mice showed not only glucose tolerance but also fat accumulation. In the present study, to identify the loci/genes that control the accumulation of abdominal fat, we performed QTL analyses of epididymal/mesenteric fat weight by using (A/J x SM.A-T2dm2sa)F2 mice in which the effect of T2dm2sa was excluded. As a result, two highly significant QTLs for mesenteric fat, as well as three significant QTLs for epididymal/mesenteric fat, were mapped on the different chromosomal regions. This suggests that the fat accumulations in individual fat depots are controlled by distinct genomic regions. Our comparison of these QTLs for abdominal fat distribution with those for glucose metabolism revealed that the major genetic factors affecting body fat distribution do not coincide with genetic factors affecting glucose metabolism in (A/J x SM.A-T2dm2sa)F2.

Body and major organ weights of A/J-Chr 11(SM) consomic mice

The body and major organ weights of A/J-Chr 11(SM) consomic mice were compared to those of the progenitor strains, A/J and SM/J. The weights of the body and organs, except for those of the brain and uterus, were significantly greater in A/J mice than in SM/J mice. However, those of consomic mice were highly variable. For example, the average body weight of consomic mice was significantly greater than that of SM/J mice and nearly equal to that of A/J mice. Chromosome 11 of SM/J mice induced various significant changes of the organ weights of A/J mice, especially those of the brain, lung, kidney, adrenal, and ovary, demonstrating the importance of this chromosome in determining the phenotypes.

Confirmation of diabetes-related quantitative trait loci derived from SM/J and A/J mice by using congenic strains fed a high-carbohydrate or high-fat diet

The interaction between causative genes and diet is known to influence the onset of obesity and diabetes in humans, although it has remained difficult to identify diabetogenic gene(s) because humans are genetically and environmentally heterogeneous. Mouse SMXA recombinant inbred (RI) strains are established from parental inbred strains (SM/J and A/J) and have been shown to be beneficial tools for analyzing polygenic traits. We previously mapped a significant quantitative trait locus (QTL, T2dm1sa) on Chromosome (Chr.) 10 and suggestive QTLs on Chr. 2, 6, and 18 for diabetes-related traits by using SMXA RI strains fed a high-carbohydrate diet. As a first step in identifying the responsible gene among QTLs for glucose tolerance mapped on Chr. 10 and 18, we established new strains of A.SM-T2dm1sa and SM.A-D18Mit19-D18Mit7 congenic mice. Each congenic strain bears the diabetogenic allele of an introgressed chromosomal region on a genetic background strain carrying the non-diabetogenic allele. The diabetogenic effect of T2dm1sa mapped on Chr. 10 was not supported by studies of A.SM-T2dm1sa congenic mice when the mice were fed a high-carbohydrate or high-fat diet. SM.A-D18Mit19-D18Mit7 congenic mice showed impaired glucose tolerance not only when they were fed a high-carbohydrate diet, but also when they were fed a high-fat diet. Thus, it appears that gene(s) affecting diabetes-related traits under either dietary condition may be present on Chr. 18.

Quantitative trait loci for impaired glucose tolerance in nondiabetic SM/J and A/J mice

The SMXA-5 recombinant inbred strain, which was established from nondiabetic parental SM/J and A/J mice, develops diabetic phenotypes such as impaired glucose tolerance. The combination of diabetogenic genes in the SM/J and A/J genomes impairs glucose tolerance in SMXA-5 mice. Using (SM/J x SMXA-5)F2 mice fed a high-fat diet, we previously detected a diabetogenic locus, T2dm2sa, on chromosome (Chr) 2. The A/J allele at this locus is diabetogenic. The SM.A-T2dm2sa congenic mouse, in which the Chr 2 region of A/J including T2dm2sa was introgressed into SM/J, showed obviously impaired glucose tolerance. These results indicate that SM.A-T2dm2sa mice develop diabetogenic traits due to T2dm2sa with the A/J allele and unknown diabetogenic loci with the SM/J allele. The aim of this study was to dissect these unknown loci, using quantitative trait locus (QTL) analysis in the (A/J x SM.A-T2dm2sa) F2 intercross fed a high-fat diet. The results revealed a highly significant QTL, T2dm4sa, for glucose tolerance on Chr 6 and a significant QTL, T2dm5sa, for glucose tolerance on Chr 11. These loci with the SM/J allele were diabetogenic. The diabetogenic effect of T2dm4sa or T2dm5sa was verified by the impairment of glucose tolerance in the A/J-6(SM) or A/J-11(SM) consomic strain, in which Chr 6 or Chr 11 of SM/J is introgressed into A/J, respectively. These results demonstrate that diabetogenic loci exist in the genomes of nondiabetic A/J and SM/J mice and suggest that T2dm2sa with the A/J allele and T2dm4sa and/or T2dm5sa with the SM/J allele elicit impaired glucose tolerance in SM.A-T2dm2sa mice.

Low expression of Neu2 sialidase in the thymus of SM/J mice-existence of neuraminidase positive cells "Neu-medullocyte" in the murine thymus

We have already reported that the homogenate of the A/J mouse thymus shows a high sialidase activity at the neutral pH region and that in both soluble and membrane fractions optimal pH was 6.5-7 (Kijimoto-Ochiai et al., Glycoconj. J., 20:375-384, 2004). In the present study, we investigated the level of sialidase activities in the thymus of the SM/J mouse, a mouse strain that we know to have a Neu1(a) allele that reveals a low level of sialidase activity in the liver. We found that while in the A/J thymus the soluble sialidase activity at pH 6.5 was high, the SM/J thymus lacked all such activity. A QTL analysis of SMXA recombinant inbred strains showed that soluble sialidase activity correlated well with the D1Mit8/9 marker on chromosome 1. The murine whole DNA-sequence data and the results of our FISH analysis (Kotani et al., Biochem. Biophys. Res. Comm., 286:250-258, 2001) showed that this location is consistent with the position of Neu2 gene. We confirmed that it is hard to detect the Neu2 enzyme of the SM/J mouse thymus by an anti-Neu2 antibody using a Western blot analysis. We also found that while the mRNA expression of Neu2 was quite normal in the SM/J mouse liver, it was very low in the SM/J mouse thymus. We therefore conclude that the lack of soluble sialidase activity in the SM/J mouse thymus is due to the thymus-specific low expression level of the Neu2 gene. We have previously shown that the sialidase positive cell which contains the Mac-1 and immunoglobulin, and which is located sparsely in the corticomedullar region or medullary region of the A/J mouse thymus (Kijimoto-Ochiai et al., Glycoconj. J., 20:375-384, 2004). We showed now in this paper that the detection of this cell in the SM/J mouse thymus at pH 7.0 was difficult. We propose, therefore, to name the cell "Neu-medullocyte".

Searching for genetic factors of fatty liver in SMXA-5 mice by quantitative trait loci analysis under a high-fat diet

Fatty liver is strongly associated with the metabolic syndrome characterized by obesity, insulin resistance, and type 2 diabetes, but the genetic basis and functional mechanisms linking fatty liver with the metabolic syndrome are largely unknown. The SMXA-5 mouse is one of the SMXA recombinant inbred substrains established from SM/J and A/J strains and is a model for polygenic type 2 diabetes, characterized by moderately impaired glucose tolerance, hyperinsulinemia, and mild obesity. SMXA-5 mice also developed fatty liver, and a high-fat diet markedly worsened this trait, although SM/J and A/J mice are resistant to fatty liver development under a high-fat diet. To dissect loci for fatty liver in the A/J regions of the SMXA-5 genome, we attempted quantitative trait loci (QTLs) analysis in (SM/JxSMXA-5)F2 intercross mice fed a high-fat diet. We mapped a major QTL for relative liver weight and liver lipid content near D12Mit270 on chromosome 12 and designated this QTL Fl1sa. The A/J allele at this locus contributes to the increase in these traits. We confirmed the effect of Fl1sa on lipid accumulation in liver using the A/J-Chr12(SM) consomic strain, which showed significantly less accumulation than A/J mice. This suggests that the SM/J and A/J strains, neither of which develops fatty liver, possess loci causing fatty liver and that the coexistence of these loci causes fatty liver in SMXA-5 mice.

Genetic characterization of a new set of recombinant inbred lines (LGXSM) formed from the inter-cross of SM/J and LG/J inbred mouse strains

A new set of LGXSM recombinant inbred (RI) strains is presented. The RI strain panel consists of 18 remaining strains of the original 55 founding strains. Strain characterization is based on 506 polymorphic microsatellites and 4,289 single nucleotide polymorphisms (SNPs) distributed across the genome. Average microsatellite inter-marker distance is 4.80+/-4.84 Mb or 2.91+/-3.21 F(2) cM. SNPs are more densely spaced at 0.57+/-1.27 Mb. Ninety-five percent of all microsatellite inter-marker intervals are separated by less than 15.00 Mb or 8.50 F(2) cM, while 95% of the SNPs are less than 0.95 Mb apart. Strains show expected low levels of nonsyntenic association among loci and complete genomic independence. During inbreeding, the RI strains went through strong natural selection on the agouti locus on Chromosome 2, especially when the epistatically interacting tyrosinase locus on Chromosome 7 carried the wild-type allele. The LG/J and SM/J strains differ in a large number of biomedically important traits, and they and their inter-cross progeny have been used in multiple mapping studies. The LGxSM RI strain panel provides a powerful new resource for mapping the genetic bases of complex traits and should prove to be of great biomedical utility in modeling complex human diseases such as obesity and diabetes.

Major quantitative trait locus on chromosome 2 for glucose tolerance in diabetic SMXA-5 mouse established from non-diabetic SM/J and A/J strains

AIMS/HYPOTHESIS

The SMXA-5 mouse is one of the SMXA recombinant inbred substrains established from the non-diabetic SM/J and A/J strains, and is a model for polygenic type 2 diabetes, characterised by moderately impaired glucose tolerance and hyperinsulinaemia. These diabetic traits are worsened by feeding a high-fat diet. The aim of this study was to dissect the diabetogenic loci in the A/J regions of the SMXA-5 genome that contribute to diabetes-related traits.

MATERIALS AND METHODS

We analysed the quantitative trait loci (QTL) for diabetes-related traits and obesity in (SM/JxSMXA-5)F(2) intercross mice fed a high-fat diet. To verify the function of the responsible locus that was mapped in the present study, we constructed a congenic strain and characterised its diabetes-related traits.

RESULTS

A major QTL for glucose tolerance, free-fed blood glucose concentration and BMI was mapped on chromosome 2. This locus existed near D2Mit15, with the highest logarithm of the odds score (12.6) for glucose concentration at 120 min in a glucose tolerance test, and was designated T2dm2sa. The diabetogenic allele of T2dm2sa originated in the A/J strain. SM.A-T2dm2sa, a congenic strain that introgressed the T2dm2sa region of A/J genome into SM/J, exhibited overt impaired glucose tolerance and hyperinsulinaemia.

CONCLUSIONS/INTERPRETATION

The development of impaired glucose tolerance in SM.A-T2dm2sa mice confirmed the results of QTL analysis for diabetes-related traits in F(2) intercross mice. The present results suggest that there are latent diabetogenic loci in the genomes of non-diabetic A/J and SM/J mice, and that the coexistence of these loci, including T2dm2sa, causes impaired glucose tolerance in SMXA-5 and SM.A-T2dm2sa mice.

High expression of interleukin-1beta in the corneal epithelium of MRL/lpr mice is under the control of their genetic background

MRL/Mp mice bearing the Fas deletion mutant gene, lpr (MRL/lpr), spontaneously develop polyarthritis, sialoadenitis and dacryoadenitis, resembling rheumatoid arthritis (RA), and also corneal involvement such as keratopathy and scleritis, which is a major complication in RA patients. In this study, we found that the expression levels of IL-1beta and MMP-1 mRNAs in cornea were high in both MRL/lpr and MRL/Mp-+/+ strains of mice at an age younger than when they develop any inflammatory lesions. This was not true of other inbred strains, even those bearing the lpr gene, and also not of (NZB x NZW) F1 lupus mice. There was no significant difference in the expression of IL-1alpha and TGFbeta in cornea in these strains. Using crosses between MRL/lpr and C3H/HeJ-lpr/lpr (C3H/lpr) mice, at least the expression of IL-1beta was found to be under the control of the MRL genetic background, likely with a recessive mode of inheritance. Considering that IL-1beta in cornea was detected particularly in the epithelial layer, the high expression of IL-1beta in cornea is most likely involved in the genetic predisposition for corneal involvement and possibly also for arthritis in an MRL strain of mice.

Genetic analysis of crown size in the first molars using SMXA recombinant inbred mouse strains

Tooth crown size may be determined by both genetic and environmental factors. The aim of this study was to identify quantitative trait loci (QTLs) affecting dental crown size and determine whether there is genetic independence between upper and lower teeth, using SMXA recombinant inbred strains of mice. Mesiodistal and buccolingual crown diameters (MD and BL, respectively) of the upper and lower first molars (M(1) and M(1), respectively) were measured. For each trait, mean values of substrains showed a continuous spectrum of distribution. Genome-wide scan detected QTLs exceeding suggestive threshold levels for MD of M(1) (chromosomes 7, 13, and 17), BL of M(1) (chromosomes 8 and 13), MD of M(1) (chromosomes 7 and 13), and BL of M(1) (chromosomes 3 and 15). These findings suggest that tooth crown size is controlled by multiple genes, and that there is some independence of genetic control between M(1) and M(1).

Improved strain distribution patterns of SMXA recombinant inbred strains by microsatellite markers

To develop SMXA recombinant inbred (RI) strains as more valuable genetic resources, 302 microsatellite (Mit) loci were added to the strain distribution patterns (SDP) reported previously. The improved SDP were constructed in a total of 1085 loci containing 484 Mit markers, 571 restriction landmark genomic scanning (RLGS) spot markers and 30 others. This substantially improved SDP can be freely accessed on our homepage (http://www.med.nagoya-u.ac.jp/sisetu/SDP.htm).

Quantitative trait locus analysis of serum insulin, triglyceride, total cholesterol and phospholipid levels in the (SM/J x A/J)F2 mice

Quantitative trait locus (QTL) analysis of serum insulin, triglyceride, total cholesterol and phospholipid levels at 10 weeks of age was performed in 321 F2 offspring from SM/J and A/J mice. Interval mapping revealed a total of 22 suggestive QTLs affecting the four traits: two insulin QTLs on Chromosomes (Chrs) 6 and 8; six triglyceride QTLs on Chrs 4, 8, 9, 11, 12 and 19; six total-cholesterol QTLs on Chrs 1, 3, 4, 14, 17 and 19; and eight phospholipid QTLs on Chrs 2, 3, 4, 6, 8, 10 and 19. Gender influenced the expression of eight of the suggestive QTLs. The total-cholesterol QTLs on Chrs 4, 14 and 17, the triglyceride QTL on Chr 9 and the phospholipid QTL on Chr 4 were specific to females. The phospholipid QTLs on Chrs 2 and 6 and the insulin QTL on Chr 8 were specific to males. In addition, common QTLs involved in the regulation of some of the traits were identified. The female-specific QTL on Chr 4 appeared to be involved in the regulation of total cholesterol and phospholipid levels. The QTL on Chr 8 affected insulin and phospholipid levels, whereas the Chr 19 QTL was common to the three lipid parameters.

Combinations of nondiabetic parental genomes elicit impaired glucose tolerance in mouse SMXA recombinant inbred strains

Type 2 diabetes in humans is not a single gene disorder but a multifactorial disease caused by the interaction of multiple genes and environmental factors. Recombinant inbred (RI) strains are a powerful tool for analyzing not only single genetic traits but also multifactorial genetic traits. By using the SMXA RI mice, we genetically dissected diabetes-related traits (BMI, nonfasting blood glucose concentration, and blood glucose concentration during intraperitoneal glucose tolerance tests). For minimizing the variation of glucose tolerance in each strain, all mice were fed the high-carbohydrate diet and subjected to phenotypic and genetic analyses. The parental strains, SM/J and A/J, were nondiabetic, and the differences of the mean values of diabetes-related traits were small. In contrast, an impaired glucose tolerance was observed in (SM x A)F1 mice, and marked differences in diabetes-related traits were observed in 19 SMXA RI strains. In particular, several SMXA RI strains showed markedly impaired glucose tolerance and hyperglycemia. Quantitative trait locus (QTL) analysis revealed a locus on chromosome (Chr) 10 contributing significant effect on nonfasting blood glucose concentration, as well as six diabetes-related loci on four chromosomes with suggestive evidence of linkage with diabetes-related phenotypes. The A/J-derived QTLs on Chr 2 and 18 and an SM/J-derived QTL on Chr 10 contributed to the impairment of glucose tolerance and/or the increase of blood glucose concentration. The present study indicates that QTLs derived from parental SM/J and A/J genomes, both of which are nondiabetic, interact in the RI genomes, leading to the development of hyperglycemia and diabetic phenotypes. Genetic dissection of this kind of diabetogenesis will increase our understanding of the complex gene-gene interaction and mode of inheritance in human type 2 diabetes.

Quantitative trait loci on chromosomes 10 and 11 influencing mandible size of SMXA RI mouse strains

Predicting the mandible size before the termination of growth of the maxillofacial bones is essential in pedodontics as well as for the predictions needed for genetic analysis. Here, Quantitative Trait Locus (QTL) analysis was used to detect the chromosomal regions responsible for the mandible length between the menton and gonion in an SMXA recombinant inbred strain of mice. Around the region 60 cM from the centromere in chromosome 10, the logarithm of the odds score showed a higher than suggestive level. Around the regions 13 cM and 16 cM in chromosome 11, two significant QTLs were detected. Analysis of genotypes from loci corresponding to those QTLs revealed a large mandible when the region between the markers Hba and D11Mit163 and D10Mit70 and D10Mit136 indicated the genotype from the A/J and SM/J alleles, respectively. These results suggest that the major gene(s) responsible for mandible length are located in these regions.

Genetic identification of multiple susceptibility genes involved in the development of endometrial carcinoma in a rat model

There are clear indications that inheritance plays an essential role in certain cases of human endometrial cancer, and there are at least 2 forms of early-onset heritable endometrial adenocarcinomas (EACs). Females of the BDII inbred rat strain are known to be genetically predisposed to endometrial carcinoma, and we have performed a genetic analysis of susceptibility to endometrial cancer in this strain. F(2) populations were generated by crossing BDII females with males from 2 different strains with a low incidence of EAC, and the occurrence of endometrial cancer was studied. Three chromosome regions associated to EAC susceptibility were identified, and the susceptibility genes in these regions were designated Ecs1, Ecs2 and Ecs3. Our results indicate that the genes affecting susceptibility to EAC are different in the 2 crosses, suggesting that the genes behind the susceptibility in BDII animals may interact with different genes in different genetic backgrounds.

Quantitative trait loci for body weight in the intercross between SM/J and A/J mice

We performed a genome-wide quantitative trait locus (QTL) analysis of body weight at 10 weeks of age in a population of 321 intercross offspring from SM/J and A/J mice, progenitor strains of SMXA recombinant inbred strains. Interval mapping revealed two significant QTLs, Bwq3 (body weight QTL3) and Bwq4, on Chromosomes (Chrs) 8 and 18 respectively, and five suggestive QTLs on Chrs 2, 6, 7, 15 and 19. Bwq3 and Bwq4 explained 6% of the phenotypic variance. The SM/J alleles at both QTLs increased body weight, though the SM/J mouse was smaller than the A/J mouse. On the other hand, four of the five suggestive QTLs detected had male-specific effects on body weight and the remainder was female-specific. These suggestive QTLs explained 5-6% of the phenotypic variance and all the SM/J alleles decreased body weight.

Genetic analysis of mortality in murine angiostrongyliasis costaricensis using SMXA recombinant inbred mouse strains

The SMXA recombinant inbred mouse strain set was produced by systematic inbreeding from the F2 generation of a cross between two progenitor inbred strains, A/J and SM/J, which differed markedly with respect to the patterns of infection with Angiostrongylus costaricensis. We have applied this set to genetic analysis of mouse susceptibility to this nematode infection. The mortality was variable among substrains of the SMXA RI strains, indicating the involvement of multiple genes. Linkage analysis showed several chromosomal regions closely linked to mortality; chromosome 6 (D6Rik86, 87; P<==0.001), 10 (D10Rik66-D10Mit12; P=0.0058), 13 (D13Rik79, 80; P=0.0096) and 17 (D17Mit28-D17Rik76; P=0.0088).

Distribution of body weight, blood insulin and lipid levels in the SMXA recombinant inbred strains and the QTL analysis

In the SMXA recombinant inbred (RI) strains, we measured body weight, blood insulin and lipid (triglyceride, total cholesterol and phospholipid) levels in each strain. In the five traits, mean values of substrains varied remarkably and showed a continuous spectrum of distribution, suggesting control by multiple genes at distinct loci for each trait. We also screened for quantitative trait loci (QTLs) involved in the five traits. Suggestive QTLs for body weight (Chromosomes 1 and 6), insulin (Chromosomes 1, 3, 10 and 17), triglyceride (Chromosomes 4 and 11) and phospholipid (Chromosome 18) levels were detected. The SMXA RI strains are unique tools for analyzing genetic factors that influence body weight, blood insulin and lipids levels.

Characterization of hyperinsulinemic recombinant inbred (RI) strains (SMXA-5 and SMXA-9) derived from normoinsulinemic SM/J and A/J mice

We discovered two mouse strains (SMXA-5 and SMXA-9) with hyperinsulinemia among the substrains and progenitor strains (SM/J and A/J) of the SMXA recombinant inbred (RI) strains, and characterized the two strains at 20 weeks of age. SMXA-5 (mean +/- S.E.M: 9.6 +/- 1.7 ng/ml) and SMXA-9 (7.7 +/- 1.3 ng/ml) males had higher serum immunoreactive insulin levels than SM/J (1.4 +/- 0.3 ng/ml) and A/J (1.1 +/- 0.1 ng/ml) males in the nonfasting condition. The hypoglycemic response to insulin at 30 min after injection was significantly less in SMXA-5 males than in SM/J mice. Glucose tolerance test revealed that the incidence of impaired glucose tolerant males was 58% (11/19) in SMXA-5 and 42% (10/24) in SMXA-9 strains, but none in SM/J and A/J strains. SMXA-5 (209 +/- 29 mg/dl) and SMXA-9 (235 +/- 31 mg/dl) had higher serum triglyceride levels than SM/J (126 +/- 14 mg/dl) and A/J (89 +/- 5 mg/dl) males in the nonfasting condition. Histologic examination revealed enlarged islets in the pancreas of hyperinsulinemic SMXA-5 male mice. Moreover, SMXA-5 and SMXA-9 mice exhibited mild obesity. SMXA-5 and SMXA-9 males were therefore characterized by hyperinsulinemia, impaired glucose tolerance, hypertriglyceridemia and mild obesity which resembled some of the phenotypes of human Syndrome X, although both progenitor strains were normal so far as we examined. Since the RI strains are a powerful tool to facilitate polygenic-trait analysis, SMXA-5 and SMXA-9 mice will be useful materials to investigate the genetic basis of complex diseases, and are possible new metabolic models in relation to hyperinsulinemia.

Differential responses of SM/J and A/J mice to experimental Angiostrongylus costaricensis infection

Angiostrongylus costaricensis matures in mice, but shows variation in mouse mortality and worm burden among inbred strains. Differences in response to infection may be controlled genetically. The patterns of infection with A. costaricensis in SM/J and A/J mouse strains differed markedly in terms of level of haematocrit and the magnitude of splenomegaly.

The LEXF: a new set of rat recombinant inbred strains between LE/Stm and F344

A new set of rat RI strains consisting of 11 independent strains and 13 of their substrains was established by inbreeding F2 rats between F344/DuCrj and LE/Stm. The strain distribution pattern was examined for 66 microsatellite loci, 8 biochemical genetic markers, 2 histocompatibility loci, and 2 coat color genes. A rat salivary protein gene Spe1 was newly mapped on Chr 1.