Inference in linkage analysis of multifactorial traits using recombinant inbred strains of mice

Multigenic control and sex bias in host susceptibility to spore-induced pulmonary anthrax in mice

Mechanisms underlying susceptibility to anthrax infection are unknown. Using a phylogenetically diverse panel of inbred mice and spores of Bacillus anthracis Ames, we investigated host susceptibility to pulmonary anthrax. Susceptibility profiles for survival time and organ pathogen load differed across strains, indicating distinct genetic controls. Tissue infection kinetics analysis showed greater systemic dissemination in susceptible DBA/2J (D) mice but a higher terminal bacterial load in resistant BALB/cJ (C) mice. Interestingly, the most resistant strains, C and C57BL/6J (B), demonstrated a sex bias for susceptibility. For example, BALB/cJ females had a significantly higher survival time and required 4-fold more spores for 100% mortality compared to BALB/cJ males. To identify genetic regions associated with differential susceptibility, survival time and extent of organ infection were assessed using mice derived from two susceptibility models: (i) BXD advanced recombinant inbred strains and (ii) F2 offspring generated from polar responding C and D strains. Genome-wide analysis of BXD strain survival identified linkage on chromosomes 5, 6, 9, 11, and 14. Quantitative trait locus (QTL) analysis of the C×DF2 population revealed a significant QTL (designated Rpai1 for resistance to pulmonary anthrax infection, locus 1) for survival time on chromosome 17 and also identified a chromosome 11 locus for lung pathogen burden. The striking difference between genome-wide linkage profiles for these two mouse models of anthrax susceptibility supports our hypothesis that these are multigenic traits. Our data provide the first evidence for a differential sex response to anthrax resistance and further highlight the unlikelihood of a single common genetic contribution for this response across strains.

Genetic regulation of hypothalamic cocaine and amphetamine-regulated transcript (CART) in BxD inbred mice

Cocaine and Amphetamine-Regulated Transcript (CART) peptides are implicated in a wide range of behaviors including in the reinforcing properties of psychostimulants, feeding and energy balance and stress and anxiety responses. We conducted a complex trait analysis to examine natural variation in the regulation of CART transcript abundance (CARTta) in the hypothalamus. CART transcript abundance was measured in total hypothalamic RNA from 26 BxD recombinant inbred (RI) mouse strains and in the C57BL/6 (B6) and DBA/2J (D2) progenitor strains. The strain distribution pattern for CARTta was continuous across the RI panel, which is consistent with this being a quantitative trait. Marker regression and interval mapping revealed significant quantitative trait loci (QTL) on mouse chromosome 4 (around 58.2 cM) and chromosome 11 (between 20-36 cM) that influence CARTta and account for 31% of the between strain variance in this phenotype. There are numerous candidate genes and QTL in these chromosomal regions that may indicate shared genetic regulation between CART expression and other neurobiological processes referable to known actions of this neuropeptide.

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.

Identification and chromosomal localization of ecogenetic components of electrolyte excretion

OBJECTIVE

To determine to what extent urinary excretion of blood pressure-modulating electrolytes is genetically determined, and to identify their chromosomal localization.

DESIGN AND METHODS

Twenty-six rat recombinant inbred strains (RIS) originating from reciprocal crosses of normotensive Brown Norway (BN.Lx) and spontaneously hypertensive rats (SHR) were used. A pilot experiment on a subset of strains determined that fasting decreases the impact of environmental noise and increases that of heritability. Twenty-four-hour urinary collections were obtained from fasting rats aged 6-12 weeks (3-8 rats per strain). Sodium (Na), potassium (K) and calcium (Ca) excretions were measured, and the Na/K ratio calculated. These phenotypes served as quantitative traits for the search of quantitative trait loci (QTLs) by scanning the RIS genome that was mapped with 475 polymorphic markers.

RESULTS

Constant Na intake resulted in a low heritability for Na excretion, reflecting the environmental impact (intake = excretion), whereas fasting revealed a gradient among RIS indicative of the genetic component of the traits. In the fasting state, a locus on chromosome 14 was found to be significantly associated with K excretion (Alb, P = 0.00002, r = -0.69, logarithm of the odds score (LOD) 3.9), whereas another locus on chromosome 10 (D10Cebrp97s5, P = 0.0003, r = -0.69, LOD 3.0) and one on chromosome 6 (D6Cebrp97s14, P = 0.0007, r = -0.65, LOD 1.9) were more significantly associated with Na excretion and the Na/K ratio respectively. The observed correlations were all negative for Na, K and Na/K, indicating a higher excretion of Na and K and a greater Na/K ratio in rats bearing BN.Lx alleles at these loci, i.e. salt retention in fasting SHR. These three loci accounted for 47-55% of variance of their associated trait, suggesting that they are the main genetic determinants for these phenotypes in basal fasting conditions. Rats bearing the Y chromosome of SHR origin had significantly higher K excretion that, in turn, led to a significantly lower Na/K ratio. Finally, a locus on chromosome 7 was linked to Ca excretion, explaining 46% of the trait variance (D7Mit10, LOD score 3.0).

CONCLUSION

RIS enabled us to determine QTLs for environmentally modulated traits such as Na, K and Ca excretions. We demonstrated that whereas urinary electrolytes are determined mainly by intake (environment) in a steady state, their excretion in an adaptive state (fasting) is predominantly genetically determined by distinct QTL on autosomes as well as the Y chromosome. Furthermore, the loci responsible for Na and K excretions act independently of the locus governing the relative excretion of Na/K. Thus, the salt-retaining aspects of some hypertensives may be, in large part, determined by genes responsible for renal excretion, the impact of which is predominant over the environment under acute challenge.

Sensory and motor development in mice: genes, environment and their interactions

Sensory and motor developmental tests were designed to characterize spontaneous mutations in rodents. These tests are currently used to investigate developmental abnormalities associated with gene overexpression or gene targeting in mice. Here, we present an overview of our studies focused on 15 tests designed to measure sensory and motor development from birth to weaning in mice. Psychometric characteristics and factorial structure of these measures are considered first. The genetic correlates of these measures obtained with neurological mutants and gene mapping are compared. As a general rule, the contribution of genotype to the phenotypic variance of sensory and motor measures of development is low, inviting exploration of other sources of variation. Results from ovary transplantation, embryo transfer and fostering methods indicate that different components of maternal environment (cytoplasmic, uterine or postnatal) contribute to the behavioral phenotype. Although more difficult to detect, interactions between genotype and environment are involved.

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.

Mapping of quantitative trait loci (QTL) of differential stress gene expression in rat recombinant inbred strains

OBJECTIVE

Stress has been shown to be a major environmental contributor to cardiovascular diseases through its effects on blood pressure variability and cardiac function. The cellular stress response is characterized by the expression of specific heat stress genes (hsps), under the transcriptional control of heat shock transcription factors (HSTFs). The levels of hsp mRNA depend on the severity of the stress, with hstf1 acting as a stress sensor. The aim of this work was to evaluate the genetic contribution of the variability in hsp expression, and to identify its putative quantitative trait loci (QTL).

METHODS

Twenty recombinant inbred rat strains (RIS) were studied. The animals underwent a standardized, identical 1 h immobilization stress in restraint cages, followed by 1 h of rest before sacrifice. Total RNA was extracted from the heart kidneys and adrenals, and the mRNA levels of hsp27, hsp70, hsp84, hsp86 and hsp105 were measured. The strain distribution pattern (SDP) of hsp expression was correlated with that of 475 polymorphic markers distributed throughout the RIS genome. A polymorphism of rat hstf1 in RIS was used for its mapping in RIS.

RESULTS

Despite an identical stress being applied to all strains, hsp expression showed up to a 1 2-fold gradient with little intra-strain variability, indicative of a strong genetic contribution to the trait Heritability ranged from 50 to 77% for most hsp genes in the three target organs. The continuous SDP of stress gene expression indicated the polygenic nature of the trait A common locus on chromosome 7 (at D7Cebrp187s3 marker) was consistently associated with all hsp expression in most of the organs [with a likelihood of odds (LOD) score of 3.0 for hsp27 expression]. We have mapped rat hstf1 on chromosome 7 at the same locus. Finally, the D4Mit19 marker was significantly associated with hsp84 expression in the heart (LOD score of 3.1).

CONCLUSION

Two loci were linked with the differential expression of HSPs in response to immobilization stress in target organs of RIS. The chromosome 7 locus unveiled for all HSPs could explain up to 42% of the observed inter-strain variability of hsp levels in response to stress. We propose hstf1 as a positional candidate at this locus.

Contribution of autosomal loci and the Y chromosome to the stress response in rats

Stress is a critical contributor to cardiovascular diseases through its impact on blood pressure variability and cardiac function. Familial clustering of reactivity to stress has been demonstrated in human subjects, and some rodent models of hypertension are hyperresponsive to stress. Therefore, the present study was designed to uncover the genetic determinants of the stress response. We performed a total genome linkage search to identify the loci of the body temperature response to immobilization stress in a set of recombinant inbred strains (RIS) originating from reciprocal crosses of spontaneously hypertensive rats (SHR) with a normotensive Brown Norway Lx strain. Two quantitative trait loci (QTLs) were revealed on chromosomes (Chrs) 10 and 12 (logarithm of odds scores, 2.2 and 1. 3, respectively). The effects of these QTLs were enhanced by a high sodium diet (logarithm of odds scores, 4.0 and 3.3 for Chrs 10 and 12, respectively), which is suggestive of a salt-sensitive component for the phenotype. Congenics for Chr 10 confirmed both the QTL and the salt effect in RIS. Negatively associated loci were also identified on Chrs 8 and 11. Interaction between the loci of Chrs 10 and 12 was demonstrated, with the rat strains bearing SHR alleles at both loci having the highest thermal response to stress. Furthermore, the Y Chr of SHR origin enhanced the response to immobilization stress, as demonstrated in 2 independent models, RIS and Y Chr consomics. However, its full effect requires autosomes of the SHR strain. These findings provide the first evidence for the genetic determination of reactivity to stress with interactions between autosomal loci and between the Y and autosomal Chrs that contribute to the explanation of the 46% of variance in the stress response.

Sex-exclusive quantitative trait loci influences in alcohol-related phenotypes

During the past half century, researchers have identified and examined sex differences in alcohol-related phenotypes, focusing more recently on understanding of the mechanisms underlying these differences. In general, the genetic contributions influencing these differences are not consistent with an interpretation of sex linkage and must, therefore, reflect some form of sex limitation in which allelic differences at particular autosomal loci have different consequences in males and females. Significant sex differences in measures of alcohol consumption in mice have been demonstrated in previous work in our laboratory. To investigate these differences further, we explore the limiting case of sex-exclusive effects using data from (BXD) recombinant inbred (RI) strains of mice and from an intercross derived from the same progenitors, C57BL/6J (B) and DBA/2J (D). By the use of two statistical approaches (examination of residual scores as a sex-exclusive phenotypic value for the RI strains and multivariate regression on sex and genotype in the F(2)) we have identified and confirmed female-exclusive markers for alcohol acceptance on chromosomes 9 and 12 and one marker for alcohol preference on chromosome 2. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 88:647-652, 1999.

Mapping quantitative trait loci for seizure response to a GABAA receptor inverse agonist in mice

To define the genetic contributions affecting individual differences in seizure threshold, a beta carboline [methyl-beta-carboline-3-carboxylate (beta-CCM)]-induced model of generalized seizures was genetically dissected in mice. beta-CCM is a GABAA receptor inverse agonist and convulsant. By measuring the latency to generalized seizures after beta-CCM administration to A/J and C57BL6/J mice and their progeny, we estimated a heritability of 0.28 +/- 0.10. A genome wide screen in an F2 population of these parental strains (n = 273) mapped quantitative trait loci (QTLs) on proximal chromosome 7 [logarithm of the likelihood for linkage (LOD) = 3.71] and distal chromosome 10 (LOD = 4.29) for seizure susceptibility, explaining approximately 22 and 25%, respectively, of the genetic variance for this seizure trait. The best fitting logistic regression model suggests that the A/J allele at each locus increases the likelihood of seizures approximately threefold. In a subsequent backcross population (n = 223), we mapped QTLs on distal chromosome 4 (LOD = 2.88) and confirmed the distal chromosome 10 QTLs (LOD = 4.36). In the backcross, the C57BL/6J allele of the chromosome 10 QTL decreases the risk of seizures approximately twofold. These QTLs may ultimately lead to the identification of genes influencing individual differences in seizure threshold in mice and the discovery of novel anticonvulsant agents. The colocalization on distal chromosome 10 of a beta-CCM susceptibility QTL and a QTL for open field ambulation and vertical movement suggests the existence of a single, pleiotropic locus, which we have named Exq1.

Provisional QTL for circadian period of wheel running in laboratory mice: quantitative genetics of period in RI mice

Wheel running was monitored in B x D recombinant inbred (RI) mice under dark-dark (DD) conditions, and the mean circadian period was calculated for each strain. There were significant differences for this trait among B x D recombinant inbred strains (p < .0001) and a narrow-sense heritability of 21%. Analysis of strain means and variances indicates that at least four segregating loci contribute to the genetic variance for the free-running circadian period in this population. Correlation of the strain means for the circadian period of wheel running for each RI strain against the distribution of markers at over 1500 loci along the mouse genome identified a number of provisional quantitative trait loci (QTL). There were provisional QTL for wheel running at p < .001 on chromosome 11 and at p < .01 on chromosomes 1, 6, 9, 17, and 19. Most were in agreement with a second analysis done under similar conditions.

Animal models sensitive to anti-anxiety agents

Animal models of anxiety serve two primary purposes in psychopharmacological research. (i) The pharmacological profile observed in the model provides a basis which, depending on its predictive validity, permits an estimation of clinical activity. (ii) Insofar as the 'anxiety' occurring in the animal model is homologous to human anxiety disorders, it is possible to investigate the underlying cellular and molecular mechanisms. The most commonly used laboratory procedures for the identification of anxiolytic effects of drugs are described in this paper.

Genetics of age-related hearing loss in mice. IV. Cochlear pathology and hearing loss in 25 BXD recombinant inbred mouse strains

The effects of three putative genes which contribute to age-related hearing loss (AHL genes) were evaluated using auditory brainstem response (ABR) thresholds and post-mortem cochlear histopathology in 25 recombinant BXD inbred mouse strains, originally derived from C57BL/6J (B6) and DBA/2J (D2) progenitor strains. All BXD strains showed substantial elevation of ABR thresholds and loss of spiral ganglion cells (SGCs) during the first year of life. The findings are consistent with our genetic model in which D2 and B6 inbred strains both possess the Ahl (age-related hearing loss) gene, whereas D2 possesses two additional chromosomal loci with AHL genes (Ahl2 and Ahl3). The between-strain distribution in the severity of SGC loss and ABR threshold elevations suggests that the severity of hearing loss is determined in large part by the number of AH L genes an animal possesses and by additional genetic background effects. The present findings also demonstrate that, because BXD strains vary substantially in the rate and severity of progressive hearing loss (but are genetically closely related), they can provide powerful animal models for developmental studies of AHL.

Localization of genes mediating acute and sensitized locomotor responses to cocaine in BXD/Ty recombinant inbred mice

Sensitization to the psychostimulant effects of cocaine has received widespread attention because concomitant changes occur in neurochemical pathways that are part of the brain reward pathway. The current study was undertaken with the purpose of mapping genes determining sensitivity to the acute stimulant and sensitizing effects of cocaine. Sensitivity and sensitization to cocaine (5, 10, and 40 mg/kg) were measured in 25 BXD/Ty recombinant inbred (BXD RI) strains and the progenitor C57BL/6J (B6) and DBA/2J (D2) strains. Quantitative trait locus (QTL) mapping provisionally localized cocaine sensitivity genes to regions on all chromosomes except 6, 11, 17, and X; sensitization QTLs were localized to chromosomes 1-10, 13, 15, 18, 19, and X. Provisional QTLs for locomotion after saline injection in a novel setting were mapped to chromosomes 1, 3-6, 9, 12, 13, 18, and 19 and in a familiar setting to chromosomes 4-7, 9, 13, and 19. There were both common and unique QTL regions across the phenotypes. Evidence for a genetic association between magnitude of acute cocaine response and sensitization was obtained for only the 10 mg/kg dose. Some common QTL regions for cocaine, ethanol, and methamphetamine responses suggest the possibility that these drugs induce stimulant effects or sensitization through some common mechanisms. However, independent mechanisms were also indicated. Many candidate genes reside near the provisional QTLs mapped for cocaine responses, including genes coding a variety of neurotransmitter and hormone receptors. These data, once confirmed, should prove useful for directing investigations of acute and chronic cocaine effects down already suspected and novel avenues.

Newborn organ weight and spontaneous hypertension: recombinant inbred strain study

Heart and kidney weight of newborn rats was studied in two progenitor strains (Brown Norway-BN, and spontaneously hypertensive rats-SHR) and in 31 recombinant inbred (RI) strains developed by inbreeding of F2 cross derived from these two progenitors. The relative weight of both organs was significantly higher in SHR newborns than in BN ones. No differences in relative DNA, protein and water contents were detected in hearts from SHR and BN newborns. On the other hand, in SHR kidneys there was lower DNA and protein content accompanied by a higher water content. This suggested that kidneys of SHR babies had less cells with higher water content. The average body weight of newborns in individual RI strains was continuously distributed between both progenitor strains but more RI strains resembled values of SHR newborns. The opposite was true for relative heart and kidney weights where the predominant influence of BN genes was visible. Moreover, there was an important difference between two reciprocal crosses of RI strains because the relative heart weight was clustered around SHR values only in BxH but not in HxB cross. This was, however, not observed for body weight and relative kidney weight. No significant correlation between blood pressure of adult males and body weight or relative organ weights of newborns of RI strains was found. On the other hand, relative heart and kidney weights of newborns correlated positively with the same parameters in adult males of respective RI strains. Additionally, relative heart weight of newborns was related positively to blood pressure of their mothers only in HxB cross that was derived from SHR females. It can be concluded that 1) kidney enlargement observed in newborn SHR is due to increased water content, 2) organ weight in adulthood can be predicted from newborn organ weight, and 3) heart weight in newborns correlated significantly with maternal blood pressure in HxB but not in BxH cross. It should be noted that the relationship between heart and/or kidney weight at birth and blood pressure in adulthood has not been proven to be significant (at p < 0.05 level) in Prague set of recombinant inbred strains.

Confirmation of quantitative trait loci for ethanol sensitivity in long-sleep and short-sleep mice

Initial insensitivity to alcohol is a strong predictor of human alcoholism, a widespread and heritable health problem. The Long Sleep and Short Sleep lines of mice were developed by genetic selection for high or low alcohol sensitivity. We have identified seven quantitative trait loci (QTLs) specifying differences in alcohol sensitivity using intercross progeny from these selected strains. These QTLs (Lorel-Lore7) together account for approximately 60% of the total genetic variance for this trait. This represents the first report of linkages for genes influencing alcohol action in any mammalian system using stringent, genome-wide mapping criteria.

Quantitative trait loci influencing cholesterol and phospholipid phenotypes map to chromosomes that contain genes regulating blood pressure in the spontaneously hypertensive rat

The frequent coincidence of hypertension and dyslipidemia suggests that related genetic factors might underlie these common risk factors for cardiovascular disease. To investigate whether quantitative trait loci (QTLs) regulating lipid levels map to chromosomes known to contain genes regulating blood pressure, we used a genome scanning approach to map QTLs influencing cholesterol and phospholipid phenotypes in a large set of recombinant inbred strains and in congenic strains derived from the spontaneously hypertensive rat and normotensive Brown-Norway (BN.Lx) rat fed normal and high cholesterol diets. QTLs regulating lipid phenotypes were mapped by scanning the genome with 534 genetic markers. A significant relationship (P < 0.00006) was found between basal HDL2 cholesterol levels and the D19Mit2 marker on chromosome 19. Analysis of congenic strains of spontaneously hypertensive rat indicated that QTLs regulating postdietary lipid phenotypes exist also on chromosomes 8 and 20. Previous studies in the recombinant inbred and congenic strains have demonstrated the presence of blood pressure regulatory genes in corresponding segments of chromosomes 8, 19, and 20. These findings provide support for the hypothesis that blood pressure and certain lipid subfractions can be modulated by linked genes or perhaps even the same genes.

Quantitative trait loci for ethanol sensitivity in the LS x SS recombinant inbred strains: interval mapping

We are mapping the genes (quantitative trait loci or QTLs) that are responsible for individual differences in ethanol sensitivity, measured as the duration of loss of righting, reflex (LORR) and blood ethanol concentrations upon recovery of the righting reflex (BEC). The Long-Sleep (LS) and Short-Sleep (SS) selected lines of mice manifest an 18-fold difference in LORR and serve as a rodent model for ethanol sensitivity. The LS x SS recombinant inbred (RI) series, developed from LS and SS lines, are an important resource for QTL mapping of ethanol-related responses. The current report summarizes the initial QTL analysis of LORR and BEC in the LS x SS strains and compares the results of correlational analysis with an interval-mapping approach. The data provide strong evidence for QTLs that influence ethanol sensitivity on mouse chromosomes 1 and 2 and possible QTLs on chromosomes 1, 3, 4, 5, 6, 7, 12, 13, 16, and 18. These results are compared to those from an F2 cross which confirms QTLs on chromosomes 1, 2, 4, and 18.

Quantitative trait locus analyses of sleep-times induced by sedative-hypnotics in LSXSS recombinant inbred strains of mice

The long-sleep (LS) and short-sleep (SS) selected lines of mice show highly significant differences in sleep-time for many sedative-hypnotic drugs, and the quantitative genetic nature of these differences has been well-established. Using an interval-mapping approach, quantitative trait locus (QTL) analyses of LSXSS recombinant inbred (RI) strains have been applied to sleep-time responses for various classes of sedative-hypnotic drugs: alcohols (ethanol, n-propanol, and n-butanol), the atypical anesthetic chloral hydrate, barbiturates (pentobarbital and secobarbital), and benzodiazepines (chlordiazepoxide and flurazepam). Several provisional QTLs were mapped to similar locations within and between drug classes, suggesting that some common loci are involved in sleep-times elicited by these drugs. Consistent with correlations of strain mean sleep-times between drugs tested in the LSXSS recombinant inbred strains, the number of provisional QTLs mapping to the locations of highest significance for ethanol decreases when the lipid solubility of a particular drug becomes less similar to that of ethanol. Provisional QTLs mapped for the benzodiazepines, however, revealed considerable overlap with those mapped for ethanol, although these drugs represented the most lipid-soluble category of sedative-hypnotics tested. Provisional QTLs for pentobarbital and secobarbital differed from most of those mapped for the alcohols, which supports the hypothesis that alcohols and barbiturates exert their effects mainly through different biological mechanisms in the LS and SS lines. Blood ethanol concentrations at regaining the righting reflex also mapped to several provisional QTLs corresponding to ethanol-induced sleep-times that support the contention that sleep-time is a reasonable index of the observed differences in central nervous system sensitivities to ethanol between LS and SS mice.

Type I and type II error rates for quantitative trait loci (QTL) mapping studies using recombinant inbred mouse strains

Effective mapping strategies for quantitative trains must allow for the detection of the more important quantitative trait loci (QTLs) while minimizing false positives. Type I (false-positive) and Type II (false-negative) error rates were estimated from a computer simulation of QTL mapping in the BXD recombinant inbred (RI) set compromising 26 strains of mice, and comparisons made with theoretical predictions. The results are generally applicable to other RI sets when corrections are made for differing strain numbers and marker densities. Regardless of the number or magnitude of simulated QTLs contributing to the trait variance, the p value necessary to provide adequate protection against both Type I (alpha=.0001) and Type II (beta=.2) errors, a QTL would have to account for more than half of the between-strain (genetic) variance if the BXD or similar set was used alone. In contrast, a two-step mapping strategy was also considered, where RI strains are used as a preliminary screen for QTLs to be specifically tested (confirmed) in an F2 (or other) population. In this case, QTLs accounting for approximately 16% of the between-strain variance could be detected with an 80% probability in the BXD set when alpha = 0.2. To balance the competing goals of minimizing Type I and II errors, an economical strategy is to adopt a more stringent alpha initially for the RI screen, since this requires only a limited genome search in the F2 of the RI-implicated regions (approximately 10% of the F2 genome when p < .01 in the RIs). If confirmed QTLs do not account in the aggregate for a sufficient proportion of the genetic variance, then a more relaxed alpha value can be used in the RI screen to increase the statistical power. This flexibility in setting RI alpha values is appropriate only when adequate protection against Type I errors comes from the F2 (or other) confirmation test(s).

A genetic linkage map of the rat derived from recombinant inbred strains

We have constructed a genetic linkage map in the rat by analyzing the strain distribution patterns of 500 genetic markers in a large set of recombinant inbred strains derived from the spontaneously hypertensive rat and the Brown-Norway rat (HXB and BXH recombinant inbred strains). 454 of the markers could be assigned to specific chromosomes, and the amount of genome covered by the mapped markers was estimated to be 1151 centimorgans. By including a variety of morphologic, biochemical, immunogenetic, and molecular markers, the current map integrates and extends existing linkage data and should facilitate rat gene mapping and genetic studies of hypertension and other complex phenotypes of interest in the HXB and BXH recombinant inbred strains.

Taking stock of complex trait genetics in mice

The mapping of complex trait loci in mice has recently become very popular thanks to dense genetic maps, better approaches to linkage analysis and the continued value of the mouse as a key model organism for human disease. Nevertheless, the ultimate goal remains very difficult to identify genes that underlie complex traits and to understand their function at a molecular level. In assessing the prospects of current efforts, it helps to review the findings of earlier studies of complex traits and, despite all the technology, to be reminded of the inherent benefits and limitations at the source of genetic variation: the laboratory mouse. With the right perspective it should be possible for geneticists analysing complex traits to take full advantage of the resources that the genome project will provide.

Genetic analysis of anxiety-related behaviors and responses to benzodiazepine-related drugs in AXB and BXA recombinant inbred mouse strains

Recombinant inbred (RI) strains derived from the C57BL/6J and A/J mouse strains were used for behavioral studies designed to estimate the number and location of chromosomal loci responsible for anxiety-related behaviors and differential sensitivity to agonists and inverse agonists of the gamma-aminobutyric acidA (GABAA)/benzodiazepine receptor complex. The phenotypes of the parental inbred strains and of 28 RI strains were characterized for the number of transitions in the light<-->dark exploratory model, anxiolytic response to diazepam, vertical and ambulatory activities in an open field, and sensitivity to the convulsant properties of methyl-beta-carboline-3-carboxylate (beta-CCM). The strain distribution patterns and estimates of the minimal number of loci obtained for each trait suggest that multiple chromosomal loci contribute to differences in anxiety-related behavioral phenotypes and the behavioral responses to diazepam and beta-CCM between C57BL/6J and A/J mice. The best probabilities of linkage were found between the variables characterizing response to diazepam and loci on chromosomes 1 (Xmv-41) and 10 (D10Mit2) and between the sensitivity to the convulsant actions of beta-CCM and locus D15Mit5 on chromosome 15.

Mapping of quantitative trait loci for blood pressure and cardiac mass in the rat by genome scanning of recombinant inbred strains

In the HXB and BXH recombinant inbred strains derived from the spontaneously hypertensive rat and the normotensive Brown Norway rat, we determined the strain distribution patterns of 500 genetic markers to scan the rodent genome for quantitative trait loci regulating cardiac mass and blood pressure. The markers spanned approximately 1,139 cM of the genome and were tested for correlations with left ventricular mass adjusted for body weight, and with systolic, diastolic, and mean arterial pressures. The marker for the dopamine 1A receptor (Drd1a) on chromosome 17 showed the strongest correlation with left ventricular heart weight (P = .00038, r = -0.59) and the relationship to heart weight was independent of blood pressure. The markers showing the strongest correlations with systolic, diastolic, and mean arterial pressure were D19Mit7 on chromosome 19 (P = .0012, r = .55), D2N35 on chromosome 2 (P = .0008, r = .56), and Il6 on chromosome 4 (P = .0018, r = .53), respectively. These studies demonstrate that the HXB and BXH strains can be effectively used for genome scanning studies of complex traits and have revealed several chromosome regions that may be involved in the genetic control of blood pressure and cardiac mass in the rat.

Quantitative trait loci associated with the behavioral response of B x D recombinant inbred mice to restraint stress: a preliminary communication

Quantitative trait loci (QTL) analysis was used to make provisional identification of loci containing genes influencing vulnerability to stress. The effect of restraint stress on open-field activity was measured in C57BL/6J and DBA/2J inbred strains of mice and in 22 B x D recombinant inbred strains of mice. QTL analyses were performed by correlating the behavioral delta scores for each group with the strain distribution pattern of 1300 markers for the B x D mice. A significant association was found between postrestraint rearings during min 5 through 8 in the open field and the Lamb2 marker on chromosome 1 (r = .718, p < .0001). Significant associations at the p < .0001 level were also found between baseline open-field rearings of control mice during min 0 through 5 and the Zp3, Ache, and Mr66-1 markers on chromosome 5, baseline open-field rearings of control mice during min 5 through 8 and the Pmv42 marker on chromosome 15, and open-field rearings of experimental mice during min 0 through 5 and the D11Ncvs61 marker on chromosome 11.

Use of recombinant inbred strains for evaluation of intermediate phenotypes in spontaneous hypertension

1. The HXB/BXH recombinant inbred (RI) strains, derived from the spontaneously hypertensive rat (SHR) and the normotensive Brown Norway (BN.1x) rat, represent a very useful system for gene mapping and for genetic analysis of certain model diseases, such as spontaneous hypertension. 2. These RI strains were genotyped in multiple genetic polymorphisms and characterized in blood pressure and some intermediate phenotypes. 3. The analysis of RI strains has revealed that (i) a gene in the vicinity of the major histocompatibility complex (RT1) on chromosome 20, a kallikrein-related gene on chromosome 4 and the renin gene on chromosome 13 were significantly associated with blood pressure, and (ii) Na+ leak in red blood cells correlated with blood pressure whereas relative heart and kidney weights as well as platelet aggregation did not.

Analysis of two measures of paw preference in a large population of inbred mice

Two measures of lateralized forelimb usage, the Collins paw preference test and the Lateral Paw Preference test (LPP), were tested in 693 mice of 29 inbred strains and F1 crosses, and 2 embryo transfer groups. These strains included NZB, SM, and the NXSM recombinant inbred (RI) strains; RF and the NXRF RI strains; BXSB and the Y-consomic BXSB-Yaa+; DBA/2 and F1s of DBA/2 and BXSB and BXSB-Yaa; and NZB x NZW F1s. The findings indicated that (1) the Collins and LPP tests were independent in terms of direction of lateralization, (2) there were significant population biases of 60.96% rightward on the LPP test, and 54.39% leftward on the Collins test, (3) there were significant strain differences on measures of absolute asymmetry on both tests, (4) there were strain differences for direction of asymmetry on the Collins test, but not on LPP, (5) on the basis of the NXSM RI strains, 3 genetic loci contribute to strength of laterality on the LPP test, and (6) there was a strong correlation among strain means for strength of lateralization on the two tests. These results extend previous findings on the strength and direction of laterality, showing that two independent systems for direction of laterality may depend on the same mechanism for magnitude, and establish that multiple factors of handedness, previously identified in humans and other primates, also exist in mice.

Quantitative trait loci mapping of three loci controlling morphine preference using inbred mouse strains

Quantitative trait loci mapping was used to identify the chromosomal location of genes which contribute to oral morphine preference (in a two-bottle choice paradigm) of C57BL/6J mice, compared to DBA/2J mice. An F2 intercross of these two strains (606 mice) was phenotyped for morphine preference and those mice demonstrating extreme values for morphine consumption (the highest and lowest 7.7%) were genotyped for 157 murine microsatellite polymorphisms. Maximum likelihood methods revealed three loci on murine chromosomes 1, 6 and 10 which are responsible for nearly 85% of the genetic variance observed between the two parental strains.

Multifactorial inheritance of neural tube defects: localization of the major gene and recognition of modifiers in ct mutant mice

Neural tube defects (NTD) in humans have been considered to have a multifactorial aetiology, however the participating genes have not been identified. The curly-tail (ct) mutant mouse develops NTD that resemble the human malformations in location, pathology and associated abnormalities. Moreover, there appears to be multifactorial influence on the incidence of NTD in offspring of curly-tail mice. We now describe a linkage analysis that localizes the ct gene to distal chromosome 4 in mice. Further analysis using recombinant inbred strains demonstrates the presence of at least three modifier loci that influence the incidence of NTD. This study provides definitive evidence for multifactorial inheritance in a mouse model of human NTD.

Characterization of benzodiazepine-sensitive behaviors in the A/J and C57BL/6J inbred strains of mice

Exploratory behaviors as well as pharmacological actions of gamma-aminobutyric acidA (GABAA)/benzodiazepine receptor agonists and inverse agonists were characterized in C57BL/6J and A/J strains of mice. C57BL/6J mice displayed higher levels of exploratory behavior than A/J mice in the light in equilibrium with dark exploration model of anxiety and in an open-field test, suggesting that C57BL/6J mice are less "emotional" and more active than A/J mice, respectively. However, C57BL/6J mice were more sensitive than A/J mice to the anxiolytic effects of diazepam in the light in equilibrium with dark exploration model. In contrast, A/J mice were more sensitive than C57BL/6J mice to the convulsant effects of methyl-beta-carboline-3-carboxylate. C57BL/6J mice showed no evidence of acquisition of a passive avoidance task, while A/J readily acquired this memory task at low levels of footshock. C57BL/6J and A/J mice should be useful parental strains in recombinant inbred lines for investigating the genetic determinants of benzodiazepine-sensitive behaviors and sensitivity to drugs acting on the GABAA/benzodiazepine receptor complex.

Further evidence that a mouse chromosome 4 locus influences cerebellar folial pattern

The cerebellar folial pattern of mice displays marked variation among inbred strains. Previous studies of progeny of crosses of C57BL/6 mice with DBA/2J and BALB/cByJ mice indicated a multifactorial mode of inheritance in both cases. The cross with DBA/2J led to the mapping, to Chromosome 4, of a locus (Cfp-1) that influences the frequency of the intraculminate fissure. Here we report that Cfp-1 influences the frequency of the declival sulcus, and appears to influence the frequency of the intraculminate and precentral fissures, in F2 hybrid offspring from crosses of C57BL/6J and BALB/cByJ mice.

Genetic mapping of tumor susceptibility genes involved in mouse plasmacytomagenesis

Plasmacytomas (PCTs) were induced in 47% of BALB/cAnPt mice by the intraperitoneal injection of pristane, in 2% of (BALB/c x DBA/2N)F1, and in 11% of 773 BALB/cAnPt x (BALB/cAnPt x DBA/2N)F1 N2 backcross mice. This result indicates a multigenic mode of inheritance for PCT susceptibility. To locate genes controlling this complex genetic trait, tumor susceptibility in backcross progeny generated from BALB/c and DBA/2N (resistant) mice was correlated with alleles of 83 marker loci. The genotypes of the PCT-susceptible progeny displayed an excess homozygosity for BALB/c alleles within a 32-centimorgan stretch of mouse chromosome 4 (> 95% probability of linkage) with minimal recombination (12%) near Gt10. Another susceptibility gene on mouse chromosome 1 may be linked to Fcgr2 (90% probability of linkage); there were excess heterozygotes for Fcgr2 among the susceptible progeny and excess homozygotes among the resistant progeny. Regions of mouse chromosomes 4 and 1 that are correlated with PCT susceptibility share extensive linkage homology with regions of human chromosome 1 that have been associated with cytogenetic abnormalities in multiple myeloma and lymphoid, breast, and endocrine tumors.

Genetic analysis of cerebellar folial pattern in crosses of C57BL/6J and DBA/2J inbred mice

Variation in the cerebellar folial pattern of mice is influenced by genetic elements [Inouye, M. and Oda, S., J. Comp. Neurol., 190 (1980) 357-362]. In crosses of C57BL/6J and DBA/2J inbred mice, the presence or absence of a specific fissure, the intraculminate fissure, is largely determined by a single genetic locus (Cfp-1), which is located on distal Chromosome 4 [Neumann et al., Brain Res., 524 (1990) 85-89]. In the present study, the mid-sagittal cerebellar folial pattern has been examined in crosses of C57BL/6J and DBA/2J mice and in BXD recombinant inbred strains. At least three loci, including Cfp-1, are involved in variation in vermian pattern formation. Genetic variation in thyroid hormone function may be involved in the inheritance of folial pattern. A locus (Cfp-2) that appears to be partially responsible for this negative genetic correlation in mice may be linked to Afp on Chromosome 5. This hypothesis was suggested by the negative correlation between neonatal serum T4 level and the number of folia in rats given neonatal injections of thyroxine or propylthiouracil [Lauder, J.M. et al., Brain Res., 76 (1974) 33-40].

Genetics of life span in mice

Thymic involution that occurs earlier in some individuals than others may be the result of complex interactions between genetic factors and the environment. Such interactions may produce defects of thymus-dependent immune regulation associated with susceptibility to developing autoimmune diseases, malignancy, and an increased number of infections associated with aging. The major histocompatibility complex may be important in determining profiles of cause of death and length of life in mice. Genetic influences on life span involve interactions between loci and allelic interactions during life which may change following viral infections or exposure to other environmental factors. We have used different experimental protocols to study the influence of H-2 on life span and found that interactions between genetic regions, are inconsistent, particularly when comparing mice infected or not infected with Sendai virus. Genes important for life span need to be studied against many genetic backgrounds and under differing environmental conditions because of the complexity of the genetics of life span. Several genetic models were used to demonstrate that the MHC is a marker of life span in backcross and intercross male mice of the H-2d and H-2b genotypes in B10 congenic mice. Females lived longer than males in backcross and intercross mice, while males lived longer than females in B10 congenics. H-2d was at a disadvantage for life span in backcross mice of the dilute brown and brown males exposed to Sendai infection, but intercross mice not exposed to Sendai virus of the same genotype were not at a disadvantage. H-2d mice were not disadvantaged when compared to H-2b in B10 congenics that had not been exposed to Sendai virus infection but the reverse was true when they were exposed. Overall, all our studies suggest that genetic influences in life span may involve interactions between loci and many allelic interactions in growing animals or humans. These genetic influences on life span may vary after they are exposed to infections or other environmental conditions. This paper emphasizes the need to use several genetic models, especially animals that have been monitored for infections, to study the genetics of life span.

Use of recombinant inbred strains to map genes of aging

Recombinant inbred strains have been used in a number of organisms for segregation and linkage analysis of quantitative traits. One major advantage of the recombinant inbred (RI) methodology is that the genetic identity of individuals within a strain permits replicate measures of the same recombinant genotype. Such replicability is important for traits such as aging in Drosophila, where phenotypic expression is highly influenced by different environmental conditions. RI strain methodology has an added advantage for DNA marker-based linkage analysis of traits measured over the lifespan of the organism. The DNA can be extracted from individuals of the same genotype as those measured in a longevity study. In this paper an argument is presented for the use of a set of recombinant inbred strains to map the quantitative trait loci involved in the aging process in Drosophila. A unique use of a set of stable, transposable molecular markers to trace the quantitative trait loci involved is suggested.

Empirical estimates of Bonferroni corrections for use in chromosome mapping studies with the BXD recombinant inbred strains

Most chromosome mapping efforts with the BXD recombinant inbred (RI) strains involve comparisons between a trait of interest and each of a large number of marker loci for evidence of linkage. Such multiple tests or comparisons greatly increase the Type I error rate compared to the single-test situation. Perhaps the most direct way to obtain multiple-test error rates is to employ a Bonferroni correction, where the single-test alpha is multiplied by the number of independent (nonredundant) comparisons (k) to yield a multiple-test alpha that protects against even one fortuitous association with any of the markers. Several empirical estimates of k are discussed for the published BXD marker set of 142 mapped loci and the newer unpublished marker set (October 1991) comprised of 352 marker loci. Reasonable estimates of k appear to be roughly 40 and 65 for these two marker sets, respectively.