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.     

New quantitative trait loci for the genetic variance in circadian period of locomotor activity between inbred strains of mice

Provisional quantitative trait loci (QTL) for circadian locomotor period and wheel-running period have been identified in recombinant inbred (RI) mouse strains. To confirm those QTL and identify new ones, the genetic component of variance of the circadian period was partitioned among an F2 intercross of RI mouse strains (BXD19 and CXB07). First, a genomic survey using 108 SSLP markers with an average spacing of 15 cM was carried out in a population of 259 (BXD19 x CXB07)F2 animals. The genome-wide survey identified two significant QTL for period of locomotor activity measured by infrared photobeam crossings on mouse chromosomes 1 (lod score 5.66) and 14 (lod score 4.33). The QTL on distal chromosome 1 confirmed a previous report based on congenic B6.D2-Mtv7a/Ty mice. Lod scores greater than 2.0 were found on chromosomes 1, 2, 6, 12, 13, and 14. In a targeted extension study, additional genotyping was performed on these chromosomes in the full sample of 341 F2 progeny. The 6 chromosome-wide surveys identified 3 additional QTL on mouse chromosomes 6, 12, and 13. The QTL on chromosome 12 overlaps with circadian period QTL identified in several prior studies. For wheel-running period, the chromosome-wide surveys identified QTL on chromosomes 2 and 13 and one highly suggestive QTL on proximal chromosome 1. The results are compared to other published studies of QTL of circadian period.     

Differences in DBA/1J and DBA/2J reveal lipid QTL genes

Recent advances in mouse genomics have revealed considerable variation in the form of single-nucleotide polymorphisms (SNPs) among common inbred strains. This has made it possible to characterize closely related strains and to identify genes that differ; such genes may be causal for quantitative phenotypes. The mouse strains DBA/1J and DBA/2J differ by just 5.6% at the SNP level. These strains exhibit differences in a number of metabolic and lipid phenotypes, such as plasma levels of triglycerides (TGs) and HDL. A cross between these strains revealed multiple quantitative trait loci (QTLs) in 294 progeny. We identified significant TG QTLs on chromosomes (Chrs) 1, 2, 3, 4, 8, 9, 10, 11, 12, 13, 14, 16, and 19, and significant HDL QTLs on Chrs 3, 9, and 16. Some QTLs mapped to chromosomes with limited variability between the two strains, thus facilitating the identification of candidate genes. We suggest that Tshr is the QTL gene for Chr 12 TG and HDL levels and that Ihh may account for the TG QTL on Chr 1. This cross highlights the advantage of crossing closely related strains for subsequent identification of QTL genes.     

Identification of quantitative trait loci for locomotor activation and anxiety using closely related inbred strains

We carried out a quantitative trait loci (QTL) mapping experiment in two phenotypically similar inbred mouse strains, C57BL/6J and C58/J, using the open‐field assay, a well‐established model of anxiety‐related behavior in rodents. This intercross was initially carried out as a control cross for an ethylnitrosurea mutagenesis mapping study. Surprisingly, although open‐field behavior is similar in the two strains, we identified significant QTL in their F₂ progeny. Marker regression identified a locus on Chr 8 having associations with multiple open‐field measures and a significant interaction between loci on Chr 13 and 17. Together, the Chr 8 locus and the interaction effect form the core set of QTL controlling these behaviors with additional loci on Chr 1 and 6 present in a subset of the behaviors.     

Two quantitative trait loci for prepulse inhibition of startle identified on mouse chromosome 16 using chromosome substitution strains

Prepulse inhibition (PPI) of acoustic startle is a genetically complex quantitative phenotype of considerable medical interest due to its impairment in psychiatric disorders such as schizophrenia. To identify quantitative trait loci (QTL) involved in mouse PPI, we studied mouse chromosome substitution strains (CSS) that each carry a homologous chromosome pair from the A/J inbred strain on a host C57BL/6J inbred strain background. We determined that the chromosome 16 substitution strain has elevated PPI compared to C57BL/6J (P = 1.6 x 10(-11)), indicating that chromosome 16 carries one or more PPI genes. QTL mapping using 87 F(2) intercross progeny identified two significant chromosome 16 loci with LODs of 3.9 and 4.7 (significance threshold LOD is 2.3). The QTL were each highly significant independently and do not appear to interact. Sequence variation between B6 and A/J was used to identify strong candidate genes in the QTL regions, some of which have known neuronal functions. In conclusion, we used mouse CSS to rapidly and efficiently identify two significant QTL for PPI on mouse chromosome 16. The regions contain a limited number of strong biological candidate genes that are potential risk genes for psychiatric disorders in which patients have PPI impairments.     

Mapping of clinical and expression quantitative trait loci in a sex-dependent effect of host susceptibility to mouse-adapted influenza H3N2/HK/1/68

Seasonal influenza outbreaks and recurrent influenza pandemics present major challenges to public health. By studying immunological responses to influenza in different host species, it may be possible to discover common mechanisms of susceptibility in response to various influenza strains. This could lead to novel therapeutic targets with wide clinical application. Using a mouse-adapted strain of influenza (A/HK/1/68-MA20 [H3N2]), we produced a mouse model of severe influenza that reproduces the hallmark high viral load and overexpression of cytokines associated with susceptibility to severe influenza in humans. We mapped genetic determinants of the host response using a panel of 29 closely related mouse strains (AcB/BcA panel of recombinant congenic strains) created from influenza-susceptible A/J and influenza-resistant C57BL/6J (B6) mice. Combined clinical quantitative trait loci (QTL) and lung expression QTL mapping identified candidate genes for two sex-specific QTL on chromosomes 2 and 17. The former includes the previously described Hc gene, a deficit of which is associated with the susceptibility phenotype in females. The latter includes the phospholipase gene Pla2g7 and Tnfrsf21, a member of the TNFR superfamily. Confirmation of the gene underlying the chromosome 17 QTL may reveal new strategies for influenza treatment.     

Quantitative trait loci associated with short-term intake of sucrose, saccharin and quinine solutions in laboratory mice.

The goal of this study was simultaneously to map two genetic loci which, collectively, have a large effect on intake of sucrose, saccharin and quinine solutions in mice. These loci had been previously identified using long-term measurements with the traditional two-bottle test, but the present study used a short-term, one-bottle test. Intake of distilled water, 100 mM sucrose, 10 mM sodium saccharin and 1.1 mM quinine HCl over 6 h was measured on two occasions from a non-deprived group of 61 male and 72 female F2 mice derived from a cross of the C57BL/6J and DBA/2J mouse strains and used to detect quantitative trait loci (QTL). DNA from each animal was typed for polymorphisms in anonymous microsatellite markers on mouse chromosomes 4 and 6. Saccharin and sucrose relevant QTL were detected on distal chromosome 4 and a quinine relevant QTL was detected on medial/distal chromosome 6 in the region of Prp. The location of these QTL and the proportion of phenotypic variance they accounted for were similar to those arrived at following previous determinations using the two-bottle test. Measurement stability for the three gustatory phenotypes was high, product-moment correlation coefficients between first and second determinations varying between approximately 0.80 for sucrose and saccharin and 0.73 for quinine. QTL parameters assessed independently for first and second presentations of sucrose and saccharin were stable, but the location of the quinine QTL differed between presentations. The present experiment illustrates the utility of a 6 h fluid intake test in the mapping of Sac and Qui loci. The short duration of the test provides a simple means of measuring variation in gustatory processes and the discovery that these loci influence short-term as well as long-term fluid intake extends understanding of the mechanism of gene action.     

Genetic basis for the psychostimulant effects of nicotine: a quantitative trait locus analysis in AcB/BcA recombinant congenic mice

Genetic differences in sensitivity to nicotine have been reported in both animals and humans. The present study utilized a novel methodology to map genes involved in regulating both the psychostimulant and depressant effects of nicotine in the AcB/BcA recombinant congenic strains (RCS) of mice. Locomotor activity was measured in a computerized open-field apparatus following subcutaneous administration of saline (days 1 and 2) or nicotine on day 3. The phenotypic measures obtained from this experimental design included total basal locomotor activity, as well as total nicotine activity, nicotine difference scores, nicotine percent change and nicotine regression residual scores. The results indicated that the C57BL/6J (B6) were insensitive to nicotine over the entire dose-response curve (0.1, 0.2, 0.4 and 0.8 mg/kg). However, the 0.8-mg/kg dose of nicotine produced a significant decrease in the locomotor activity in the A/J strain and a wide and continuous range of both locomotor excitation and depression among the AcB/BcA RCS. Single-locus association analysis in the AcB RCS identified quantitative trait loci (QTL) for the psychostimulant effects of nicotine on chromosomes 11, 12, 13, 14 and 17 and one QTL for nicotine-induced depression on chromosome 11. In the BcA RCS, nicotine-induced locomotor activation was associated with seven putative regions on chromosomes 2, 7, 8, 13, 14, 16 and 17. There were no overlapping QTL and no genetic correlations between saline- and nicotine-related phenotypes in the AcB/BcA RCS. A number of putative candidate genes were in proximity to regions identified with nicotine sensitivity, including the alpha2 subunit of the nicotinic acetylcholine receptor and the dopamine D3 receptor.     

Quantitative trait loci (QTL) for lean body mass and body length in MRL/MPJ and SJL/J F(2) mice

Studies on the genetic mechanisms involved in the regulation of lean body mass (LBM) in mammals are minimal, although LBM is associated with a competent immune system and an overall good (healthy) body functional status. In this study, we performed a high-density genome-wide scan using 633 (MRL/MPJ × SJL/J) F₂ intercross to identify the quantitative trait loci (QTL) involved in the regulation of LBM. We hypothesized that additional QTL can be identified using a different mouse cross (MRL/SJL cross). Ten QTL were identified for LBM on chromosomes (chrs) 2, 6, 7, 9,13 and 14. Of those ten, QTL on chrs 6, 7 and 14 were exclusive to LBM, while QTL on chrs 4 and 11 were exclusively body length. LBM QTL on chrs 2 and 9 overlap with those of size. Altogether, the ten LBM QTL explained 41.2% of phenotypic variance in F₂ mice. Five significantly interacting loci that may be involved in the regulation of LBM were identified and accounted for 24.4% of phenotypic variance explained by the QTL. Five epistatic interactions, contributing 22.9% of phenotypic variance, were identified for body length. Interacting loci on chr 2 may influence LBM by regulating body length. Therefore, epistatic interactions as well as single QTL effects play an important role in the regulation of LBM. Electronic Publication     

Sex-specific quantitative trait loci govern susceptibility to Theiler's murine encephalomyelitis virus-induced demyelination

Susceptibility to Theiler's murine encephalomyelitis virus-induced demyelination (TMEVD), a mouse model for multiple sclerosis (MS), is genetically controlled. Through a mouse-human comparative mapping approach, identification of candidate susceptibility loci for MS based on the location of TMEVD susceptibility loci may be possible. Composite interval mapping (CIM) identified quantitative trait loci (QTL) controlling TMEVD severity in male and female backcross populations derived from susceptible DBA/2J and resistant BALBc/ByJ mice. We report QTL on chromosomes 1, 5, 15, and 16 affecting male mice. In addition, we identified two QTL in female mice located on chromosome 1. Our results support the existence of three linked sex-specific QTL on chromosome 1 with opposing effects on the severity of the clinical signs of TMEV-induced disease in male and female mice.     

Sensitivity to the locomotor-stimulant effects of ethanol and allopregnanolone: a quantitative trait locus study of common genetic influence

Previous studies have suggested that common genetic mechanisms influence sensitivity to the locomotor-stimulant effects of ethanol and allopregnanolone. We conducted two quantitative trait locus (QTL) studies to identify chromosomal regions that harbor genes that influence locomotor response to ethanol (2 g/kg) and allopregnanolone (17 mg/kg) using F2 crosses between C57BL/6J and DBA/2J mice. Because our previous data from the BXD recombinant inbred strains had indicated that chromosome 2 contained QTL for sensitivity to the locomotor-stimulant effects of both ethanol and allopregnanolone, we also tested reciprocal chromosome 2 congenic strains for sensitivity to the locomotor-stimulant effects of both drugs. The F2 analysis for ethanol sensitivity identified significant QTL on chromosomes 1 and 2 and suggestive QTL on chromosomes 5 and 9. The analysis of the allopregnanolone F2 study identified suggestive QTL on chromosomes 3, 5 and 12. Suggestive evidence for a female-specific QTL on chromosome 2 was also found. The studies of congenic mouse strains indicated that both the congenic strains captured one or more QTL for sensitivity to the locomotor-stimulant effects of both ethanol (2 g/kg) and allopregnanolone (17 mg/kg). When Fisher's method was used to combine the P values for the RI, F2 and congenic studies of the chromosome 2 QTL, cumulative probability scores of 9.6 x 10(-15) for ethanol and 7.7 x 10(-7) for allopregnanolone were obtained. These results confirm the presence of QTL for ethanol and allopregnanolone sensitivity in a common region of chromosome 2 and suggest possible pleiotropic genetic influence on sensitivity to these drugs.     

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.     

Genomic survey of prepulse inhibition in mouse chromosome substitution strains

Prepulse inhibition (PPI) is a measure of sensorimotor gating, a pre-attentional inhibitory brain mechanism that filters extraneous stimuli. Prepulse inhibition is correlated with measures of cognition and executive functioning, and is considered an endophenotype of schizophrenia and other psychiatric illnesses in which patients show PPI impairments. As a first step toward identifying genes that regulate PPI, we performed a quantitative trait locus (QTL) screen of PPI phenotypes in a panel of mouse chromosome substitution strains (CSSs). We identified five CSSs with altered PPI compared with the host C57BL/6J strain: CSS-4 exhibited decreased PPI, whereas CSS-10, -11, -16 and -Y exhibited higher PPI compared with C57BL/6J. These data indicate that A/J chromosomes 4, 10, 11, 16 and Y harbor at least one QTL region that modulates PPI in these CSSs. Quantitative trait loci for the acoustic startle response were identified on seven chromosomes. Like PPI, habituation of the startle response is also disrupted in schizophrenia, and in the present study CSS-7 and -8 exhibited deficits in startle habituation. Linkage analysis of an F₂ intercross identified a highly significant QTL for PPI on chromosome 11 between positions 101.5 and 114.4 Mb (peak LOD = 4.54). Future studies will map the specific genes contributing to these QTLs using congenic strains and other genomic approaches. Identification of genes that modulate PPI will provide insight into the neural mechanisms underlying sensorimotor gating, as well as the psychopathology of disorders characterized by gating deficits.     

New quantitative trait loci that regulate wound healing in an intercross progeny from DBA/1J and 129×1/SvJ inbred strains of mice

Wound healing/regeneration mouse models are few, and studies performed have mainly utilized crosses between MRL/MPJ (a good healer) and SJL/J (a poor healer) or MRL/lpr (a good healer) and C57BL/6J (a poor healer). Wound healing is a complex trait with many genes involved in the expression of the phenotype. Based on data from previous studies that common and additional quantitative trait loci (QTL) were identified using different crosses of inbred strains of mice for various complex traits, we hypothesized that a new cross would identify common and additional QTL, unique modes of inheritance, and interacting loci, which are responsible for variation in susceptibility to fast wound healing. In this study, we crossed DBA/1J (DBA, a good healer) and 129/SvJ (129, a poor healer) and performed a genome-wide scan using 492 (DBA×129) F2 mice and 98 markers to identify QTL that regulate wound healing/regeneration. Four QTL on chromosomes 1, 4, 12, and 18 were identified which contributed toward wound healing in F2 mice and accounted for 17.1% of the phenotypic variation in ear punch healing. Surprisingly, locus interactions contributed to 55.7% of the phenotype variation in ear punch healing. In conclusion, we have identified novel QTL and shown that minor interacting loci contribute significantly to wound healing in DBA×129 mice cross. The authors Masinde, Li, and Nguyen contributed equally to this article.     

A novel strategy for genetic dissection of complex traits: the population of specific chromosome substitution strains from laboratory and wild mice

The mouse is an irreplaceable model for understanding the precise genetic mechanisms of mammalian physiological pathways. Thousands of quantitative trait loci (QTLs) have been mapped onto the mouse genome during the last two decades. However, only a few genes’ underlying complex traits have been successfully identified, and rapid fine mapping of QTL genes still remains a challenge for mouse geneticists. Currently, the Collaborative Cross (CC) has proceeded to the goal of establishing more than 1,000 recombinant inbred strains for the sub-centimorgan mapping resolution of QTL loci. In this article, a novel complementary strategy, designated as population of specific chromosome substitution strains or PSCSS, is proposed for rapid fine mapping of QTLs on the substituted chromosome. One specific chromosome (Chr 1) of recipient mouse strain C57BL/6 J has been substituted by homologous counterparts from five different inbred strains (C3H/He, FVB/N, AKR, NOD/LtJ, NZW/LacJ), an outbred line Kunmin mouse in China, and 23 wild mice captured in different localities. The primary genetic studies on the structure of these wild donor chromosomes (Chr 1) show that these donor chromosomes harbor extensive genetic polymorphisms, with a high density of SNP distribution, abundant variations of STR alleles, and a high level of historical recombination accumulation. These specific chromosome substitution strains eventually form a special population that has the identical genetic background of the recipient strain and differs only in the donor chromosomes. With simple association studies, known QTLs on the donor chromosome can be rapidly mapped in high resolution without requirement of further crosses. This approach, taking advantage of the extensive genetic polymorphisms of wild resources and chromosome substitution strategy, brings a new outlook for genetic dissection of complex traits.     

Quantitative trait loci that determine lipoprotein cholesterol levels in DBA/2J and CAST/Ei inbred mice

To investigate genetic contributions to individual variations of lipoprotein cholesterol concentrations, we performed quantitative trait locus/loci (QTL) analyses of an intercross of CAST/Ei and DBA/2J inbred mouse strains after feeding a high-cholesterol cholic acid diet for 10 weeks. In total, we identified four QTL for HDL cholesterol. Three of these were novel and were named Hdlq10 [20 centimorgans (cM), chromosome 4], Hdlq11 (48 cM, chromosome 6), and Hdlq12 (68 cM, chromosome 6). The fourth QTL, Hdl1 (48 cM, chromosome 2), confirmed a locus discovered previously using a breeding cross that employed different inbred mouse strains. In addition, we identified one novel QTL for total and non-HDL cholesterol (8 cM, chromosome 9) that we named Chol6. Hdlq10, colocalized with a mutagenesis-induced point mutation (Lch), also affecting HDL. We provide molecular evidence for Abca1 as the gene underlying Hdlq10 and Ldlr as the gene underlying Chol6 that, coupled with evidence generated by other researchers using knockout and transgenic models, causes us to postulate that polymorphisms of these genes, different from the mutations leading to Tangier's disease and familial hypercholesterolemia, respectively, are likely primary genetic determinants of quantitative variation of lipoprotein levels in mice and, by orthology, in the human population.     

Recalculation of 23 mouse HDL QTL datasets improves accuracy and allows for better candidate gene analysis

In the past 15 years, the quantitative trait locus (QTL) mapping approach has been applied to crosses between different inbred mouse strains to identify genetic loci associated with plasma HDL cholesterol levels. Although successful, a disadvantage of this method is low mapping resolution, as often several hundred candidate genes fall within the confidence interval for each locus. Methods have been developed to narrow these loci by combining the data from the different crosses, but they rely on the accurate mapping of the QTL and the treatment of the data in a consistent manner. We collected 23 raw datasets used for the mapping of previously published HDL QTL and reanalyzed the data from each cross using a consistent method and the latest mouse genetic map. By utilizing this approach, we identified novel QTL and QTL that were mapped to the wrong part of chromosomes. Our new HDL QTL map allows for reliable combining of QTL data and candidate gene analysis, which we demonstrate by identifying Grin3a and Etv6, as candidate genes for QTL on chromosomes 4 and 6, respectively. In addition, we were able to narrow a QTL on Chr 19 to five candidates.     

Quantitative trait locus analysis for obesity reveals multiple networks of interacting loci

Obesity is a highly heritable and genetically complex trait with hundreds of potential loci identified. An intercross of 513 F₂ progeny between the SM/J × NZB/BINJ inbred mouse strains was generated to identify quantitative trait loci (QTL) that are involved in the weight of four fat pads: mesenteric, inguinal, gonadal, and retroperitoneal. Sex and lean body weight were treated as covariates in the analysis of these fat pads. This analysis uncoupled genetic effects related to overall body size from those influencing the adiposity of a mouse. We identified multiple significant QTL. QTL alleles associated with increased lean body weight and individual fat pad weights are contributed by the NZB background. Adiposity loci are distinct from these body size QTLs and high-adiposity alleles are contributed by the SM background. An extended network of epistatic QTL is also observed. A QTL on Chr 19 is the center of a network of eight interacting QTL, Chr 4 is the center of six, and Chr 17 the center of four interacting QTL. We conclude that interacting networks of multiple genes characterize the regulation of fat pad depots and body weight. Haplotype patterns and a literature-driven approach were used to generate hypotheses regarding the identity of the genes and pathways underlying the QTL.     

Segregation of a QTL cluster for home-cage activity using a new mapping method based on regression analysis of congenic mouse strains

Recent genetic studies have shown that genetic loci with significant effects in whole-genome quantitative trait loci (QTL) analyses were lost or weakened in congenic strains. Characterisation of the genetic basis of this attenuated QTL effect is important to our understanding of the genetic mechanisms of complex traits. We previously found that a consomic strain, B6-Chr6C^MSM, which carries chromosome 6 of a wild-derived strain MSM/Ms on the genetic background of C57BL/6J, exhibited lower home-cage activity than C57BL/6J. In the present study, we conducted a composite interval QTL analysis using the F2 mice derived from a cross between C57BL/6J and B6-Chr6C^MSM. We found one QTL peak that spans 17.6 Mbp of chromosome 6. A subconsomic strain that covers the entire QTL region also showed lower home-cage activity at the same level as the consomic strain. We developed 15 congenic strains, each of which carries a shorter MSM/Ms-derived chromosomal segment from the subconsomic strain. Given that the results of home-cage activity tests on the congenic strains cannot be explained by a simple single-gene model, we applied regression analysis to segregate the multiple genetic loci. The results revealed three loci (loci 1–3) that have the effect of reducing home-cage activity and one locus (locus 4) that increases activity. We also found that the combination of loci 3 and 4 cancels out the effects of the congenic strains, which indicates the existence of a genetic mechanism related to the loss of QTLs.