Identification of a congenic mouse line with obesity and body length phenotypes

Our primary objective was to discover simplified mouse models corresponding to human obesity linkages. We used the B10.UW– H3^b we Pax1^un a^t/Sn (B10.UW) congenic strain, a subcongenic strain with a reduced UW strain donor region, and their C57BL/10SnJ background strain. The congenic and subcongenic UW strain donor regions are on mouse Chr 2. We measured body length [anal-nasal (AN) length], summed fat depot weights normalized for body weight (Adiposity Index, AI), and percentage of body weight that is lipid. The B10.UW congenic and subcongenic strains have significantly smaller AN lengths (p < 0.0001) and have a significantly lower AI and percentage of body weight as fat than the background strain (p < 0.0001). In an F₂ intercross of the congenic and background strains, AN and AI were both linked to the distal half of the donor region with LOD scores greater than 19 and 5, respectively. F₂ haplotypes identified a minimal region for AN linkage of 0.8 megabases (Mb) that is estimated to express four genes in the current Celera mouse genome assembly. We narrowed the most likely location of the obesity gene to 15 Mb whose homologous genes are all located on human Chr 20 in the region surrounding the centromere. Since a previous study identified human obesity linkage peaking near the centromere, then the B10.UW mice may exhibit obesity due to the homologous gene.     

Congenic strains provide evidence that a mapped locus on chromosome 15 influences excitotoxic cell death

Inbred strains of mice differ in their susceptibility to excitotoxin‐induced cell death, but the genetic basis of individual variation is unknown. Prior studies with crosses of the FVB/NJ (seizure‐induced cell death susceptible) mouse and the seizure‐induced cell death resistant mouse, C57BL/6J, showed the presence of three quantitative trait loci (QTLs), named seizure‐induced cell death 1 (Sicd1) to Sicd3. To better localize and characterize the Sicd2 locus, two reciprocal congenic mouse strains were created. While the B6.FVB‐Sicd2 congenic mouse was without effect on modifying susceptibility to seizure‐induced excitotoxic cell death, the FVB.B6‐Sicd2 congenic mouse, in which the chromosome (Chr) 15 region of C57BL/6J was introgressed into FVB/NJ, showed reduced seizure‐induced excitotoxic cell death following kainate administration. Phenotypic comparison between FVB and the congenic FVB.B6‐Sicd2 strain confirmed that the Sicd2 interval harbors gene(s) conferring strong protection against seizure‐induced excitotoxic cell death. Interval‐specific congenic lines (ISCLs) that encompass Sicd2 on Chr 15 were generated and were used to fine‐map this QTL. Resultant progeny were treated with kainate and examined for the extent of seizure‐induced cell death in order to deduce the Sicd2 genotypes of the recombinants through linkage analysis. All of the ISCLs exhibited reduced cell death associated with the C57BL/6J phenotype; however, ISCL‐2 showed the most dramatic reduction in seizure‐induced cell death in both area CA3 and in the dentate hilus. These findings confirm the existence of polymorphic loci within the reduced critical region of Sicd2 that regulate the severity of seizure‐induced cell death.     

Loci affecting male fertility in hybrids between <Emphasis Type="Italic">Mus macedonicus</Emphasis> and C57BL/6

Male F₁ hybrids between inbred strains and Mus macedonicus have very small testes and are sterile. Cytological analysis of testes shows very few meioses. To determine the genetic basis for this sterility, (C57BL/6J × Mus macedonics) F₁ females were mated to males from C57BL/10J. In about half the male progeny no meiosis I was observed. About half of the animals that progressed through meiosis I showed other indications of low fertility and the balance appeared fertile. QTL analysis of the progeny suggested that loci on proximal Chrs 17 and X were involved in the sterility and a locus on Chr X in variation of body weight. There is also evidence that X//Y dissociation of the pseudo-autosomal region occurs. The QTLs on Chrs X and 17 together account for about 37% of the variance for testis weight. Congenic lines B.MAC-X(1-38), and B.MAC-17(1-23) have been constructed using a modified speed congenic approach. Testis tubules from B.MAC-X(1-38) are narrow and vacuolated. They contain only Sertoli cells and mitotically dividing spermatogonia. Very occasionally a meiotic metaphase can be observed, but no sperm are produced. Homozygous males from B.MAC-17(1-23) are sterile, producing sperm heads but no complete sperm.     

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.     

Susceptibility to Seizure-Induced Excitotoxic Cell Death Is Regulated by an Epistatic Interaction between Chr 18 (Sicd1) and Chr 15 (Sicd2) Loci in Mice

Seizure-induced cell death is believed to be regulated by multiple genetic components in addition to numerous external factors. We previously defined quantitative trait loci that control susceptibility to seizure-induced cell death in FVB/NJ (susceptible) and C57BL/6J (resistant) mice. Two of these quantitative trait loci assigned to chromosomes 18 (Sicd1) and 15 (Sicd2), control seizure-induced cell death resistance. In this study, through the use of a series of novel congenic strains containing the Sicd1 and Sicd2 congenic strains and different combinations of the Sicd1 or Sicd2 sub region(s), respectively, we defined these genetic interactions. We generated a double congenic strain, which contains the two C57BL/6J differential segments from chromosome 18 and 15, to determine how these two segments interact with one another. Phenotypic comparison between FVB-like littermates and the double congenic FVB.B6-Sicd1/Sicd2 strain identified an additive effect with respect to resistance to seizure-induced excitotoxic cell death. It thus appears that C57BL/6J alleles located on chromosomes 18 and 15 interact epistatically in an additive manner to control the extent of seizure-induced excitotoxic cell death. Three interval-specific congenic lines were developed, in which either segments of C57BL/6J Chr 18 or C57BL/6J Chr 15 were introduced in the FVB/NJ genetic background, and progeny were treated with kainate and examined for the extent of seizure-induced cell death. All of the interval-specific congenic lines exhibited reduced cell death in both area CA3 and the dentate hilus, associated with the C57BL/6J phenotype. These experiments demonstrate functional interactions between Sicd1 and Sicd2 that improve resistance to seizure-induced excitotoxic cell death, validating the critical role played by gene-gene interactions in excitotoxic cell death.     

Mouse H2 congenic intervals: analysis and use for mapping

In this study we exploit the unique genetic resource of inbred mouse major histocompatibility complex (H2) congenic and recombinant strains to construct a high-resolution map of microsatellite loci in and around the H2 region, as well as an independent genetic map of other loci on mouse Chromosome (Chr) 17. Microsatellite loci were analyzed in 11 C57BL/10 (B10) strains to determine the size of the congenic interval in each. The length of the congenic interval found in each strain varied widely. Interestingly, the intervals were generally smaller than statistical expectations. However, the observed congenic intervals were still sufficiently long that these strains and probably wild-derived H2 congenics are an important source of genetic variability. The staggered ends of the various congenic intervals and the recombinants were used to construct the map. This map will be useful for physical cloning and to help localize novel genes. As evidence of the mapping application of congenic strains, locational information was derived about Trp53-ps and Stl.Deceased, Aug. 8, 1994.     

Fine mapping of Ath6, a quantitative trait locus for atherosclerosis in mice

Ath6 is a novel quantitative trait locus associated with differences in susceptibility to atherosclerosis between C57BL/6J (B6) and C57BLKS/J (BKS) inbred mouse strains. Combining data from an intercross and a backcross (1593 meioses) between mice from B6 and BKS strains and from The Jackson Laboratory interspecific backcross panels, (C57BL/6J ×Mus spretus) F₁× C57BL/6J and (C57BL/6J × SPRET/Ei) F₁× SPRET/Ei, we constructed a consensus genetic map and narrowed Ath6 to a 1.07 ± 0.26 cM interval between the anonymous DNA marker D12Pgn4 and the gene Nmyc1. This region is near the proximal end of murine Chromosome (Chr) 12, which is homologous to the human chromosomal region 2p24-p25. Marker order in the Ath6 region was concordant among the two crosses and The Jackson Laboratory interspecific backcross panels. This high resolution map rules out candidate genes encoding apolipoprotein B, syndecan 1, and Adam17. The two Ath6 crosses have a combined potential resolution of 0.06 cM. Received: 12 September 2000 / Accepted: 22 February 2001     

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.     

Genetic Dissection of Quantitative Trait Loci for Hemostasis and Thrombosis on Mouse Chromosomes 11 and 5 Using Congenic and Subcongenic Strains

Susceptibility to thrombosis varies in human populations as well as many inbred mouse strains. Only a small portion of this variation has been identified, suggesting that there are unknown modifier genes. The objective of this study was to narrow the quantitative trait locus (QTL) intervals previously identified for hemostasis and thrombosis on mouse distal chromosome 11 (Hmtb6) and on chromosome 5 (Hmtb4 and Hmtb5). In a tail bleeding/rebleeding assay, a reporter assay for hemostasis and thrombosis, subcongenic strain (6A-2) had longer clot stability time than did C57BL/6J (B6) mice but a similar time to the B6-Chr11^A/J consomic mice, confirming the Hmtb6 phenotype. Six congenic and subcongenic strains were constructed for chromosome 5, and the congenic strain, 2A-1, containing the shortest A/J interval (16.6 cM, 26.6 Mbp) in the Hmtb4 region, had prolonged clot stability time compared to B6 mice. In the 3A-2 and CSS-5 mice bleeding time was shorter than for B6, mice confirming the Hmtb5 QTL. An increase in bleeding time was identified in another congenic strain (3A-1) with A/J interval (24.8 cM, 32.9 Mbp) in the proximal region of chromosome 5, confirming a QTL for bleeding previously mapped to that region and designated as Hmtb10. The subcongenic strain 4A-2 with the A/J fragment in the proximal region had a long occlusion time of the carotid artery after ferric chloride injury and reduced dilation after injury to the abdominal aorta compared to B6 mice, suggesting an additional locus in the proximal region, which was designated Hmtb11 (5 cM, 21.4 Mbp). CSS-17 mice crossed with congenic strains, 3A-1 and 3A-2, modified tail bleeding. Using congenic and subcongenic analysis, candidate genes previously identified and novel genes were identified as modifiers of hemostasis and thrombosis in each of the loci Hmtb6, Hmtb4, Hmtb10, and Hmtb11.     

High-resolution genetic mapping of the saccharin preference locus (Sac) and the putative sweet taste receptor (T1R1) gene (Gpr70) to mouse distal Chromosome 4

The Sac (saccharin preference) locus affecting mouse behavioral and neural responsiveness to sweeteners has been mapped to distal Chr 4. A putative sweet taste receptor, T1R1, has been recently cloned, and the gene encoding it, Gpr70, has also been mapped to mouse distal Chr 4. To assess Gpr70 as a candidate gene for Sac, we compared the Gpr70 sequences of C57BL/6ByJ and 129P3/J mouse strains with different alleles of Sac. Using Gpr70 sequence variation between the C57BL/6ByJ and 129P3/J strains, we conducted a high-resolution analysis of the chromosomal localization of the Gpr70 and Sac loci in the F₂ hybrids and 129.B6-Sac partially congenic mice originating from these two strains. The Gpr70 gene maps proximal to Sac, which demonstrates that they are different loci. Received: 24 April 2000 / Accepted: 14 September 2000     

Hybrid breakdown caused by substitution of the X chromosome between two mouse subspecies

Hybrid breakdown is a type of reproductive failure that appears after the F2 generation of crosses between different species or subspecies. It is caused by incompatibility between interacting genes. Genetic analysis of hybrid breakdown, particularly in higher animals, has been hampered by its complex nature (i.e., it involves more than two genes, and the phenotype is recessive). We studied hybrid breakdown using a new consomic strain, C57BL/6J-X(MSM), in which the X chromosome of C57BL/6J (derived mostly from Mus musculus domesticus) is substituted by the X chromosome of the MSM/Ms strain (M. m. molossinus). Males of this consomic strain are sterile, whereas F1 hybrids between C57BL/6J and MSM/Ms are completely fertile. The C57BL/6J-X(MSM) males showed reduced testis weight with variable defects in spermatogenesis and abnormal sperm head morphology. We conducted quantitative trait locus (QTL) analysis for these traits to map the X-linked genetic factors responsible for the sterility. This analysis successfully detected at least three distinct loci for the sperm head morphology and one for the testis weight. This study revealed that incompatibility of interactions of X-linked gene(s) with autosomal and/or Y-linked gene(s) causes the hybrid breakdown between the genetically distant C57BL/6J and MSM/Ms strains.     

Gene-environment interaction: a significant diet-dependent obesity locus demonstrated in a congenic segment on mouse Chromosome 7

We have previously reported suggestive evidence for a locus on Chromosome (Chr) 7 that affects adiposity in F₂ mice from a CAST/Ei × C57BL/6J intercross fed a high-fat diet. Here we characterize the effect of a high-fat (32.6 Kcal% fat) diet on male and female congenic mice with a C57BL/6J background and a CAST/Ei-derived segment on Chr 7. Adiposity index (AI) and weights of certain fat pads were approximately 50% lower in both male and female congenic mice than in control C57BL/6J mice, and carcass fat content was significantly reduced. The reduction of fat depot weights was not seen, however, in congenic animals fed a low-fat chow diet (12 Kcal% fat). The congenic segment is approximately 25 cM in length, extending from D7Mit213 to D7Mit41, and includes the tub, Ucp2, and Ucp3, genes, all of which are candidate genes for this effect. Some polymorphisms have been found on comparing c-DNA sequences of the Ucp2 gene from C57BL/6J and CAST/Ei mice. These results suggest that one or more genes present in the congenic segment modulate the susceptibility to fat deposition on feeding a high-fat diet. We were unable to show any significant difference between the energy intakes of the congenic and the control C57BL/6J mice on the high-fat diet. Also, measurements of energy expenditure in male mice at 6 weeks of age, during the first 2 weeks of exposure to the high-fat diet, failed to show any differences between control and congenic animals. Received: 30 September 1998 / Accepted: 22 December 1998     

An interstitial telomere array proximal to the distal telomere of mouse chromosome 13

Lambda clones of mouse DNA from BALB/c and C57BL/10, each containing an array of telomere hexamers, were localized by FISH to a region close to the telomere of Chr 13. Amplification of mouse genomic DNA with primers flanking SSRs within the cloned DNA showed several alleles, which were used to type eight sets of RI strains. The two lambda clones contained allelic versions of the interstitial telomere array, Tel-rs4, which is 495 bp in C57BL/10 and which includes a variety of sequence changes from the consensus telomere hexamer. Comparison of the segregation of the amplification products of the SSRs with the segregation of other loci in an interspecies backcross (C57BL/6JEi × SPRET/Ei) F₁× SPRET/Ei shows recombination suppression, possibly associated with ribosomal DNA sequences present on distal Chr 13 in Mus spretus, when compared with recombination in an interstrain backcross, (C57BL/6J × DBA/J) F₁× C57BL/6J, and with the MIT F₂ intercross. Analysis of recombination in females using a second interstrain backcross, (ICR/Ha × C57BL/6Ha) F₁× C57BL/6Ha, also indicates recombination suppression when compared with recombination in males of the same strains, using backcross C57BL/6Ha × (ICR/Ha × C57BL/6Ha) F₁. Thus, more than one cause may contribute to recombination suppression in this region. The combined order of the loci typed was D13Mit37–D13Mit30–D13Mit148–(D13Rp1, 2, 3, 4, Tel-rs4)–D13Mit53–D13Mit196–D13Mit77–(D13Mit78, 35). Data from crosses where apparently normal frequencies of recombination occur suggest that the telomere array is about 6 map units proximal to the most distal loci on Chr 13. This distance is consistent with evidence from markers identified in two YAC clones obtained from the region. Received: 24 September 1996/Accepted: 20 January 1997     

A growth QTL (<Emphasis Type="Italic">Pbwg1</Emphasis>) region of mouse chromosome 2 contains closely linked loci affecting growth and body composition

Previous QTL studies have identified 24 QTLs for body weight and growth from 3 to 10 weeks after birth in an intersubspecific backcross mouse population between C57BL/6J and wild Mus musculus castaneus that has 60% of the body size of C57BL/6J. The castaneus allele at the most potent QTL (Pbwg1) on proximal chromosome 2 retards growth. In this study we have developed a congenic strain with a 44.1-Mb interval containing the castaneus allele at Pbwg1 by recurrent backcrossing to C57BL/6J. The congenic mouse developed was characterized by significantly higher body weight gain between 1 and 3 weeks of age and lower weight of white fat pads at 10 weeks of age than C57BL/6J. However, no clear difference in body weight at 1–10 weeks of age was observed between congenic and C57BL/6J strains. QTL analysis with 269 F₂ mice between the two strains did not identify any QTLs for body weight at 1, 3, 6, and 10 weeks of age, but it discovered eight closely linked QTLs affecting body weight gain from 1 to 3 weeks of age, lean body weight, weight of white fat pads, and body length within the Pbwg1 region. The castaneus alleles at all fat pad QTLs reduced the phenotypes, whereas at the remaining growth and body composition QTLs, they increased the trait values. These results illustrate that Pbwg1, which initially appeared to be a single locus, was resolved into several loci with opposite effects on the composition traits of overall body weight. This gives a reason for the loss of the Pbwg1 effect found in the original backcross population. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Sex-restricted non-Mendelian inheritance of mouse Chromosome 11 in the offspring of crosses between C57BL/6J and (C57BL/6J × DBA/2J)F1 mice

We report on the observation of sex-restricted, non-Mendelian inheritance over a region of mouse Chromosome (Chr) 11, occurring in the offspring of crosses between two commonly used Mus musculus-derived inbred strains, C57BL/6J and DBA/2J. In the surviving backcross progeny of reciprocal matings between (C57BL/6J × DBA/2J)F₁ hybrids and the C57BL/6J parental strain, we observed the preferential appearance of C57BL/6J alleles along a region of Chr 11. The deviation from Mendelian predictions was observed only in female offspring from both reciprocal backcrosses, and not in males from either cross. The sex-specificity of the observed non-Mendelian inheritance points to an explanation based on embryonic or neonatal lethality. Our data add to previously obtained evidence for a Chr 11 locus or loci with sex-specific and allele-specific effects on viability. Received: 19 December 1997 / Accepted: 10 June 1998     

An Interval of the Obesity QTL Nob3.38 within a QTL Hotspot on Chromosome 1 Modulates Behavioral Phenotypes

A region on mouse distal chromosome 1 (Chr. 1) that is highly enriched in quantitative trait loci (QTLs) controlling neural and behavioral phenotypes overlaps with the peak region of a major obesity QTL (Nob3.38), which we identified in an intercross of New Zealand Obese (NZO) mice with C57BL/6J (B6). By positional cloning we recently identified a microdeletion within this locus causing the disruption of Ifi202b that protects from adiposity by suppressing expression of 11β-Hsd1. Here we show that the Nob3.38 segment also corresponds with the QTL rich region (Qrr1) on Chr. 1 and associates with increased voluntary running wheel activity, Rota-rod performance, decreased grip strength, and anxiety-related traits. The characterization of a subcongenic line carrying 14.2 Mbp of Nob3.38 with a polymorphic region of 4.4 Mbp indicates that the microdeletion and/or other polymorphisms in its proximity alter body weight, voluntary activity, and exploration. Since 27 out of 32 QTL were identified in crosses with B6, we hypothesized that the microdeletion and or adjacent SNPs are unique for B6 mice and responsible for some of the complex Qrr1-mediated effects. Indeed, a phylogenic study of 28 mouse strains revealed a NZO-like genotype for 22 and a B6-like genotype for NZW/LacJ and 4 other C57BL strains. Thus, we suggest that a Nob3.38 interval (173.0–177.4 Mbp) does not only modify adiposity but also neurobehavioral traits by a haplotype segregating with C57BL strains.     

Mapping of mouse intracisternal A-particle proviral markers in an interspecific backcross

We present a linkage map of intracisternal A-particle (IAP) proviral loci. The IAP family consists of 2000 endogenous proviral elements that are widely dispersed in the mouse genome. The map was constructed by using an interspecific backcross and markers defined by oligonucleotide probes specific for subclasses of expressed IAP elements. In genomic DNA from C57BL/6J mouse, these probes each detected from 12 to 44 HindIII restriction fragments that represent junctions between proviral and 5-flanking DNA. The fragments have characteristic strain distribution patterns (SDPs) that are particularly polymorphic in the DNAs of C57BL/6J and Mus spretus mice used for the backcross. IAP loci were placed on the map by comparison of their distribution patterns with those of known genetic markers in the backcross. The map includes 51 IAP loci that have not been previously mapped and 23 IAP proviruses that had been previously mapped in recombinant inbred (RI) strains. Comparable map positions were obtained with the IAP markers in the interspecific backcross and the RI strains. The mapped IAP loci were widely dispersed on the X Chromosome (Chr) and all of the autosomes except Chrs 9 and 19, providing useful genetic markers for linkage studies.     

High-resolution maps of the murine Chromosome 2 region containing the cholesterol gallstone locus, Lith1

The Lith1 region on Chromosome (Chr) 2 contains a gene that markedly affects the prevalence of cholesterol gallstones in inbred mice. We report the high-resolution genetic and radiation hybrid maps of the chromosomal region surrounding Lith1, using three resources: a DNA panel from 188 progeny from two reciprocal backcrosses between C57BL/6 and Mus spretus inbred strains; 423 progeny of an N4 generation from backcrossing the susceptible C57L/J alleles at Lith1 into the resistant AKR/J strain; and the newly developed hamster–mouse T31 radiation hybrid panel. We mapped 17 microsatellite markers in the D2Mit182 to D2Mit14 region and two candidate genes for Lith1, the canalicular bile salt export pump (Bsep) also known as sister of P-glycoprotein (Spgp) and the low-density-lipoprotein-receptor-related gene megalin (Gp330). Both genetic maps were in agreement and ordered the microsatellite markers into a 10.4 ± 1.5 cM region. The high-resolution physical map revealed ordering of microsatellite markers and relative distances between markers in almost complete agreement with the genetic maps. Mapping of Bsep revealed its location on Chr 2, homologous to the human chromosomal position (Nature Genet 20, 233–238, 1998). The radiation hybrid results also provided the highest resolution of the area containing the two candidate genes, which both mapped in the Lith1 region with close linkage, being separated by a distance of only 15 cR₃₀₀₀. The total radiation hybrid map length of the region between D2Mit182 and D2Mit14 was 326 cR₃₀₀₀, suggesting that 31 cR₃₀₀₀ is equivalent to 1 cM in this region of Chr 2. Received: 29 April 1999 / Accepted: 21 July 1999     

Genetic characterization of the Dyscalc locus

Calcification occurs frequently in the development of atherosclerotic lesions, and studies in mice have indicated a genetic contribution. We now show that one genetic factor contributing to aortic calcification is the Dyscalc locus, previously shown to contribute to myocardial calcification. Thus, the Dyscalc locus, on proximal mouse Chromosome (Chr) 7, segregated with vascular calcification in a large cross between susceptible strain DBA/2J and resistant strain C57BL/6J. Further evidence was observed by analysis of recombinant inbred strains derived from various susceptible and resistant parental strains. Myocardial and vascular calcifications are importantly influenced by multiple modifier loci as well as the Dyscalc gene, making fine mapping of Dyscalc difficult. In order to allow more detailed genetic and biochemical characterization of Dyscalc, we have identified congenic strains containing the Dyscalc locus from resistant strain C57BL/10 on the background of susceptible strain C3H/DiSnA. The congenic strains exhibit little or no myocardial or vascular calcification, unlike the background HcB C3H strain, and the calcification segregated as a Mendelian factor, allowing finer mapping of Dyscalc.     

Detection of quantitative trait loci for body weight at 10 weeks from Philippine wild mice

A genome-wide scan for quantitative trait loci (QTLs) controlling body weight at 10 weeks after birth was carried out in a population of 387 intersubspecific backcross mice derived from a cross between C57BL/6J inbred mice (Mus musculus domesticus) and wild mice (M. m. castaneus) captured in the Philippines, in order to discover novel QTLs from the wild mice that have about 60% lower body weight than C57BL/6J. By interval mapping, we detected four QTLs: a highly significant QTL on Chromosome (Chr) 2, which was common in both sexes; two significant QTLs on Chr 13, one male-specific and the other female-specific; and a suggestive male-specific QTL on X Chr. By composite interval mapping, we confirmed the presence of the three QTLs on Chrs 2 and 13, but not of the male-specific X-linked QTL. The composite interval mapping analysis newly identified three QTLs: a significant male-specific QTL on Chr 11 and two highly significant female-specific QTLs on Chrs 9 and X. Individual QTLs explained 3.8–11.6% of the phenotypic variance, and all the QTL alleles derived from the wild mice decreased body weight. A two-way analysis of variance revealed a significant epistatic interaction between the Chr 2 QTL and the background marker locus D12Mit4 on Chr 12 only in males. The interaction effect unexpectedly increased body weight. The chromosomal region containing the Chr 2 QTL did not coincide with those of growth or fatness QTLs mapped in previous studies. These results suggest that a population of wild mice may play an important role as new sources of valuable QTLs. Received: 14 January 2000 / Accepted: 14 April 2000