A new cross-reactive idiotype-defined family in the phthalate humoral immune response of mice. I. Linkage of VH-Xmp to IgCH allotype locus and mapping with respect to other known VH genes

A cross-reactive idiotype family was previously identified from a very large library of phthalate-specific hybridoma clones. The prototype of this idiotype family is the hybridoma, 2E9, secreting an IgM antibody with phthalate specificity. A portion of both primary and secondary anti-phthalate antibodies elicited in all BALB/c mice tested expresses the 2E9 cross-reactive idiotype. This idiotype has now been found in the anti-phthalate antibodies of several other inbred strains of mice (A/HeHa, DBA/2, and C3Hf/HeHa) tested but not in C57BL/6 mice. Anti-phthalate antibodies elicited from congenic mice BC.8, which express the same IgCH allotype as BALB/c mice but possess C57BL/6 genetic background, contain the 2E9 cross-reactive idiotype, whereas this idiotype is not expressed on the anti-phthalate antibodies derived from another congenic mouse CB.20, which expresses a C57BL/6 IgCH allotype and a genetic background of the BALB/c strain. These results indicate that the gene controlling the 2E9 idiotype is closely linked to the IgCH allotype locus. The 2E9 cross-reactive idiotype was also found in all of the F1 mice (BALB/c X C57BL/6) tested, and the level of expression of this idiotype in the F1 mice was quantitatively equivalent to the allotype/idiotype homozygous mice. The expression of the 2E9 idiotype in the phthalate repertoire has been followed in 12 different wild mouse populations. As expected, the 2E9 idiotype was observed in a large proportion of the wild mouse strains. Surprisingly, several examples of nonconcordance in the expression of idiotype and allotype were observed in these mice. One likely explanation for the linkage breakdown is a crossing over of the heavy chain constant and variable region gene complexes. In the SM/J inbred strain of mice, where such a crossover has occurred, nonconcordance between allotype and 2E9 idiotype expression was demonstrated. By using the recombinant inbred BXD strains of mice, the VH gene encoding the 2E9 idiotype has been mapped with respect to other known VH gene families. Relative to other VH genes the VH-Xmp is situated very close to the IgCH gene region.     

Influence of the immunoglobulin heavy chain locus on expression of the VK1GAC light chain

The VK1GAC light chain represents the dominant V kappa structure employed in the antibody response of A/J mice to streptococcal group A carbohydrate ( GAC ). Two anti-idiotypic antisera, anti- Id5 and anti- Id20 , with specificity for the VK1GAC light chain were used to examine anti- GAC antibody responses in a series of inbred mouse strains that differ at the heavy chain constant region ( IgCH ) allotype locus. Both idiotypes were expressed in normal and immune sera from mice of most IgCH allotypes, except IgCHb (C57BL/6J) and IgCHf (CE/J). C57BL/6J mice expressed Id5 , but not Id20 , whereas CE/J mice did not express either idiotype. Testing of recombinant inbred strains between BALB/c and C57BL/6 indicated that the pattern of idiotype expression did not correlate with IgCH allotype. The C X B recombinants expressed all three idiotype patterns that were observed in the panel of inbred strains. Testing of allotype congenic mice between BALB/c and C57BL/6 showed that CB.20 and BC.8 mice were Id20 -, whereas BAB-14 mice were Id20 +, indicating that both VH and background (V kappa or regulatory) loci must be derived from BALB/c to obtain Id20 expression. The difference in the frequency of idiotype expression observed between BALB/c and BAB-14 mice indicates that the IgCH locus may exert a quantitative influence on the expression of this light chain. To examine the Id20 -, Id5 + antibodies of C57BL/6 mice, anti- GAC hybridomas were prepared. Of 16 C57BL/6-derived anti- GAC monoclonal antibodies, six were reactive with anti- Id5 and not with anti- Id20 . Isoelectric focusing of the purified kappa light chains from three of these antibodies revealed two distinct spectrotypes that co-migrated with the two known VK1GAC spectrotypes observed with A/J anti- GAC light chains. Idiotypic analysis of in vitro recombinants between the heavy and light chains of A/J and C57BL/6 monoclonal antibodies demonstrated that the C57BL/6 light chains were idiotypically similar to A/J light chains when they were free in solution or paired with A/J heavy chains. These results demonstrate that C57BL/6 mice can express a light chain that is very similar, if not identical, to the VK1GAC light chain, although the light chain is expressed in lower frequency and is paired with a distinct VH structure, which can mask expression of one of the VK1GAC idiotypes. These effects on V kappa expression map to at least three genetic loci: VH, CH, and an unlinked locus.     

Structural polymorphism of murine complement factor H controlled by a locus located between the Hc and the beta 2M locus on the second chromosome of the mouse

Structural polymorphism of murine factor H protein was demonstrated by using three different methods. 1) By prolonged agarose electrophoresis and immunofixation, factor H protein was visualized in the beta region as a single, distinct protein band in freshly bled EDTA-plasmas from many laboratory and wild mice. Two variants were detected among a large number of tested strains; one, referred to as H.1, moved faster to the anodal region (type strain, BALB/c), and the other, referred to as H.2, moved more slowly to the anodal region (type strain, STR). The F1 hybrid between BALB/c and STR exhibited a combining type of factor H protein, which was observed in each parent. 2) Two-dimensional peptide mapping analysis was carried out with tryptic peptides of these two factor H allotypes. Almost all of the spots in the maps of tryptic peptides were common to both allotypes. However, three distinct spots among the 57 spots detected in the map of tryptic peptides of the H.1 allotypes were not detected in that of H.2 allotype, whereas two spots among the 56 spots in the map of H.2 allotype were unique for this allotype. The F1 hybrid between BALB/c and STR showed a combining type of the map of parent. 3) Alloantisera against each of H allotypes were successfully produced in BALB/c or BALB/c-H.2 (a congenic strain with H.2 allotype) by repeated injection of each purified factor H protein either from the BALB/c or the STR strain. These findings indicated that the observed variants of factor H represent antigenically and structurally distinguishable allotypes. The allotypes of murine factor H protein are controlled by a single codominant locus located between the Hc locus and the beta 2M locus on the second chromosome of the mouse. This was shown by phenotyping the Hc locus and H locus with backcross progenies between A/J (one of strain with H.1) and MoA (one of strain with H.2). The recombination frequency between these two loci was 0.17 +/- 0.046.     

Genetic control of the immune response to staphylococcal nuclease

Summary Antibody responses of inbred strains of mice to staphylococcal nuclease were studied by isoelectric focusing in polyacrylamide gels followed by in situ labeling of focused antibodies with radioactive antigen. All A/J mice examined produced antinuclease antibodies of limited heterogeneity, and although there was individual variation in the focusing patterns observed, a characteristic spectrotype produced by all of the animals could be discerned. In order to determine the possible relationship between this characteristic spectrotype and the cross-reactive idiotypes of A/J antinuclease antibodies previously described (7), focused antibodies were also examined with a radioactively labeled pig anti-(A/J antinuclease) anti-idiotypic antibody preparation. Using this reagent, similar spectrotypes to those observed for antigen binding were seen in all of the individual A/J sera, suggesting that cross-reactive idiotype expression is a reflection of the characteristic spectrotypes observed. The same labeled anti-idiotypic reagent revealed characteristic but different spectrotypes when used to develop focused antinuclease antibodies from individual mice of other strains, suggesting that the use of similar variable region structures may be a common feature of the antinuclease response in mice of different allotypes. These studies thus provide a structural basis for the genetics of idiotype expression defined previously by serologic analysis.     

Genetics of the anti-dextran B512 and the autoanti-idiotypic response: codominant expression in F1 hybrids and dichotomy of response and allotype-linked idiotype

The IgM plaque-forming response to the alpha 1-6 epitope of dextran B512 is linked to the Ig-1 heavy chain allotypes j and b characteristic of CBA and C57BL strains, respectively, and the response typically induces the formation of autoanti-idiotypic antibodies that can distinguish between anti-dextran antibodies of CBA and C57BL origin. Nevertheless, some substrains of Balb/c mice (allotype a) and some Bailey recombinant stains give a PFC response although they do not possess allotypes j or b. The anti-dextran antibodies in these strains lack the idiotypes characteristic of either CBA and C57BL antibodies to dextran, but they possess their own particular idiotype. F1 hybrids between two responder strains possessing different idiotypes on their antibodies against dextran, produce both idiotypes and two different autoanti-idiotypic antibodies. CBA(Ig-1b) mice were high responders to dextran and possessed the idiotype of C57BL, whereas C57BL/6(Ig-1a) mice were low responders. The V(H) recombinant strains BAB.14 and CB-8KN that possess the Ig-1b allotype of C57BL, but have some of the V(H) genes from Balb/c and the rest from C57BL/6 were high responders to dextran, but did not possess the C57BL idiotype, suggesting that the genes determining the response against dextran and the idiotype may have different locations in the heavy chain locus.     

Idiotypic analysis of polyclonal and monoclonal anti-p-azophenylarsonate antibodies of BALB/c mice expressing the major cross-reactive idiotype of the A/J strain

The idiotypic cascade allows the induction of silent idiotypes, and as such, the immune system can be reprogrammed towards predetermined goals. To understand the genetic origin of silent idiotypes, we have used a system in which detailed structural and genetic information is available. The major cross-reactive idiotype (CRIA) of A/J mice (positive strain) immunized with arsonate coupled to a carrier can be regularly induced in BALB/c mice (negative strain) by anti-idiotypic treatment with or without subsequent antigen immunization. By using a panel of monoclonal anti-idiotypic antibodies, we have found that the germline-encoded CRIA displays a mosaic of at least five idiotopes. Polyclonal and monoclonal anti-arsonate antibodies prepared from idiotypically manipulated BALB/c mice have been studied. Four germline idiotopes are shared between the CRIA of the A/J strain and the CRIA-like idiotype induced in BALB/c mice. Furthermore, CRIA-like antibodies can appear "spontaneously" in some BALB/c mice immunized with antigen only. The data suggest that anti-idiotypic treatment in BALB/c mice selects a preexisting subset of antibodies. From the serological analysis, it is predicted that CRIA molecules from A/J and CRIA-like molecules from BALB/c employ different VH subgroups but share some components of the hypervariable regions. These predictions are tested in a forthcoming paper that describes the amino acid sequences of BALB/c monoclonal antibodies displaying the major cross-reactive idiotype of the A/J strain.     

Relationship of idiotypes of the anti-p-azophenylarsonate antibodies of A/J and BALB/c mice

It has previously been shown that A/J anti-Ar antibodies contain 2 different families of cross-reactive idiotypes, referred to as the major and minor idiotypes populations. The present report shows that the minor A/J idiotype is related to a major idiotype of BALB/c anti-Ar antibodies. Anti-idiotype directed against the minor A/J idiotype binds 5 to 10% of A/J anti-Ar but an average of about 40% of BALB/c anti-Ar. This BALB/c population corresponds to the major BALB/c anti-Ar idiotype. For individual BALB/c anti-Ar preparations the maximum percentages of antibody bound by anti-id directed to A/J or BALB/c anti-Ar are very similar. Anti-id reactive with the minor A/J idiotypic population suppressed the formation of the BALB/c major idiotype when injected into BALB/c mice. Adsorption experiments showed that only about one-third of the minor A/J population is related to the BALB/c idiotype and that the expression of this idiotype is highly variable in individual A/J sera. Several types of evidence, obtained with hybridoma products expressing the major A/J idiotype, revealed no detectable relationship between the major A/J and BALB/c anti-Ar idiotypes.     

Contribution of the erythrocyte matrix to the formation of rosettes by idiotype-specific lymphocytes.

Immunosuppression of a cross-reactive idiotype associated with anti-p-azophenylarsonate (anti-Ar) antibodies of A/J mice, followed by hyperimmunization, causes the appearance of substantial percentages of T cells with anti-idiotypic specificity. The T cells were identified through their capacity to form rosettes with autologous (A/J) erythrocytes coated with Fab fragments bearing the idiotype. We report here that the RBC are a significant factor in determining the capacity of the lymphocytes to form rosettes. Relatively low numbers of rosettes were observed when the RBC were derived from species other than the mouse or from certain strains of mice. The property is not determined by the H-2 haplotype but is controlled by one or more genes that are closely linked to the H-2 locus, as shown by data obtained with RBC of several congenic strains. In an F₁ hybrid the property is dominant, suggesting a positive contribution of the RBC rather than the effect of an inhibitor. Also, idiotype-specific lymphocytes from C.AL-20 mice, which possess the allotype of the AL/N strain on a BALB/c genetic background, form rosettes much more efficiently with coated RBC from C.AL-20 or BALB/c mice than with A/J RBC; the opposite is true of idiotype-specific lymphocytes from A/J mice. This provides further evidence for complementary interaction between the lymphocytes and RBC.     

Idiotypic analysis of a monoclonal anti-Sm antibody. II. Strain distribution of a common idiotypic determinant and its relationship to anti-Sm expression

The idiotypes borne by Y2, a monoclonal anti-Sm antibody of MRL-lpr/lpr mouse strain origin, were investigated to elucidate genetic mechanisms in this autoantibody response. An anti-Y2 anti-idiotypic antiserum was raised in a rabbit and was rendered specific for idiotype by extensive absorption with globulins of the B6 and BALB/c strains as well as the BALB/c myeloma UPC 10. By using a sensitive assay for idiotype by inhibition ELISA, the Y2 determinant was found to be commonly expressed in sera of MRL-lpr/lpr and MRL-+/+ mice. Moreover, sera of several normal strain mice also bore the idiotype and, in mice bearing the lpr gene, idiotype levels were increased 1.5 to fivefold, even in the absence of a serum anti-Sm response. The relationship of this idiotype to anti-Sm expression was further assessed by determining the idiotype content of affinity-purified anti-Sm antibodies from MRL-lpr/lpr mice. Anti-Sm from serum pools or individual animals showed no significant enrichment of the Y2 idiotype in comparison to unselected MRL-lpr/lpr IgG. These results suggest that the Y2 idiotype defines only a minor component of the anti-Sm autoantibody response, and that most antibodies with this determinant express other antigenic specificities.     

In vitro T cell-mediated killing of Pseudomonas aeruginosa. III. The role of suppressor T cells in nonresponder mice

T lymphocytes from immune BALB/c mice can adoptively transfer protection against infection with the extracellular Gram-negative bacterium Pseudomonas aeruginosa to nonimmune recipients, and in vitro, immune T cells are able to kill these bacteria. Earlier studies indicated that this killing is mediated by a bactericidal lymphokine. The current studies demonstrate that T cells from immunized CB.20 mice, a strain congenic with BALB/c, fail to kill Pseudomonas aeruginosa in vitro. This nonresponsiveness is attributable to the activity of suppressor T cells of the Lyt-1-, 2,3+, I-J+ phenotype. CB.20 mice are known to differ from BALB/c mice only at a single locus, which includes the Igh-1 allotype CH genes. These results suggest a critical role for this locus or closely linked genes in the control of T cell killing of this extracellular bacterium.     

The role of genetic factors in the determination of self-stimulation behaviour in the mouse: backcross analysis

Intracranial self-stimulation behaviour in the lateral hypothalamus was studied in male mice obtained from a backcross between the F1 generation BALB/c J × DBA/2 J and the strain BALB/c J, recessive for the majority of the self-stimulation parameters.The 48 animals of the backcross were divided into 3 groups at the 40 μ A intensity. One group of 11 mice had mean performances similar to those of the recessive parental strain BALB/c. This result suggests that the genetic determination of the difference of self-stimulation performances observed between BALB/c and DBA/2 strains is not very complex.A negative correlation appeared between the thresholds and the performances of self-stimulation.Finally, stimulation intensities above 40 μ A triggered convulsion with similar frequencies in the three backcross groups.     

Inheritance of idiotype expression unlinked to the H chain allotype locus. Novel features of genetic control in the Ia.7 system

We have observed a pattern of inherited idiotype expression in three mouse strains that is unexpected from the genetics of the strains: a dominant idiotype that was expressed at high levels in two parental strains was expressed only at low levels in a heavy chain allotype congenic strain derived from them. In the C3H.SW strain, the antibody response to the class II MHC Ag I-E is of limited diversity, with dominant expression of an idiotype and the V kappa 21 L chain. The C57BL/10 strain expresses the same idiotype at high levels, whereas the CWB/12 strain, which was derived by replacing the Ig H chain Igh-Cj allele of C3H.SW with the Igh-Cb allele derived from C57B1/10, has been found to express little of this dominant idiotype. CWB/12 responds, with titers equal to those of the parental strains, to the I-E epitope responsible for dominant idiotype expression, and it expresses normal V kappa 21 levels; thus deficiencies in epitope-specific responsiveness or in V kappa 21 expression cannot explain the low Id expression in CWB/12. Furthermore, Southern blot analysis of three VH families gave no evidence of recombination within the the VH locus of CWB/12, which was Igh-Vb throughout. Black-cross analysis demonstrated that expression of the dominant idiotype segregated independently of Ig allotype, and was therefore due to genes unlinked to the H chain gene locus. To our knowledge, this pattern of Id expression is unprecedented, and indicates the need for caution in the interpretation of studies using allotype congenic strains. It also demonstrates a role for genes outside the Igh locus in the control of Id expression.     

Independent regulation of serologically distinct idiotypes in F1 hybrid mice

We have investigated the regulation of expression of two distinct intrastrain cross-reactive idiotypes, CRI_A and CRI_C, characteristic of anti-p-azophenylarsonate (anti-Ar) antibodies of the A/J and BALB/c strains, respectively, in (BALB/c x A/J)F₁ (CAF₁) mice. Such hybrid mice were found to synthesize antibodies with each idiotype when immunized against the Ar hapten group, although the expression of each was significantly reduced as compared with the parental strain. CAF₁ mice were pretreated with idiotypic-specific antibody reagents and subsequently hyperimmunized against the Ar hapten. Analysis of the idiotypes present in immune sera showed that suppression of either CRI did not concomitantly suppress the expression of the other. Alteration of the expression of one idiotype was not, however, without influence on the other; the expression of CRI_C was markedly enhanced in mice suppressed for CRI_A.Abbreviations used in this paper anti-Id(A/J) idiotypic-specific antibodies against A/J serum Ar-specific antibodies - anti-Id(BALB/c) idiotypic-specific antibodies against BALB/c serum Ar-specific antibodies - Ar p-azophenylarsonate - BGG bovine -globulin - BSA bovine serum albumin - CAF₁ F(BALB/c x A/J) - CFA complete Freund's adjuvant - CRIA the major cross-reactive idiotype of A/J Ar-specific antibodies - CRI_C the major cross-reactive idiotype of BALB/c Ar-speck antibodies - CRI_m the minor cross-reactive idiotype of A/J Ar-specific antibodies - DTH delayed-type hypersensitivity - HP hybridoma product(s) - KLH keyhole limpet hemocyanin     

Structural evidence for a polymorphic or allelic form of the heavy chain variable region.

The heavy (H) chains of anti-phosphocholine (PC) antibodies from C57BL/6J and CBA/J were sequenced through the N-terminal 36 residues and compared with previously published sequences of A/J anti-PC antibody and BALB/c PC-binding myeloma proteins T15, M603, and M511. Each of these antibody preparations contained molecules having light (L) chains and idiotypic determinants of T15, M511, and M603 indicating the presence of at least three different anti-PC antibodies in each pool. The structures of the C57BL/6J and CBA/J H chains each revealed a single sequence from positions 1 to 36 (which includes the first complementarity determining region (CDR), and they were identical. The first CDR was identical to that previously found for BALB/c and A/J indicating that this portion of these antibody molecules is highly conserved throughout inbred mice and is probably critical to PC-binding. A surprising finding was that both C57NL/6 and CBA sequence differed from the BALB/c and A/J sequences at two positions, residue 14 and 16. Since each of these strains differs at the allotype locus, the data indicates that the evolution of allotypy in mice occurred after variable region diversity for the particular genes.     

Regulation of clones responding to α 1 → 3 dextran: I. Individual variation in the expression of idiotypes

Balb/c mice were immunized with dextran B1355S and assayed for serum antibodies and direct plaque-forming cells. The earliest detectable anti-dextran response occurred in 2-week-old animals. Anti-idiotypic antisera against MOPC-104E and J558 were raised in A/He mice and rendered individual idiotype specific by cross-absorption. When the amounts of MOPC-104E and J558 idiotypes in immune sera and the PFC response of individual mice were analyzed, the ratio of both idiotypes were found highly variable in all tested animals. This observed individual variability in the expression of two major idiotypes in the Balb/c response to dextran B1355S provides an experimental basis for investigating the mechanisms regulating idiotype expression.     

Variations in genetic control of susceptibility to Theiler's murine encephalomyelitis virus (TMEV)-induced demyelinating disease. I. Differences between susceptible SJL/J and resistant BALB/c strains map near the T cell beta-chain constant gene on chromosome 6

In some susceptible mouse strains, intracerebral (IC) inoculation of Theiler's murine encephalomyelitis virus (TMEV) results in a persistent infection leading to chronic demyelinating disease. Previous genetic analyses between susceptible SJL/J and resistant C57BL/6 mice indicated a role for multiple unlinked genes in the development of clinical and histopathological disease, including a major influence of the D region of the H-2 complex. In this study, genetic analysis of a different strain combination (susceptible SJL/J and resistant BALB/c) also demonstrates the involvement of multiple genes, but the H-2 genotype (H-2s and H-2d, respectively) does not appear to contribute significantly to susceptibility differences. In both segregation studies and recombinant-inbred (R-I) analysis, clinical and histopathological disease occurs in both H-2s homozygotes and H-2d homozygotes (as well as H-2s/H-2d heterozygotes), with the actual frequency related to the proportion of non-H-2 genome from the susceptible strain. There appear to be at least two non-H-2 genes involved in differential susceptibility of SJL/J and BALB/c to TMEV-induced disease. Analysis of R-I strains generated from BALB/c and SJL/J progenitors indicates linkage of at least one of these non-H-2 genes to those encoding the constant portion of the beta-chain of the T cell receptor on chromosome 6. Many genes may actually be involved, but each strain comparison defines a different subset of these loci--only those at which the two strains in question carry "functionally" different alleles. Thus, different strain comparisons may accent the roles of different genes in resistance to the same infectious organism or disease process. In addition to the genes identified thus far, there may be yet other genes contributing to development of TMEV-induced disease, but their recognition may require analysis of still other strain combinations.     

Relationship between genetic differentiation of the thymus in mice of different strains and malignant growth. IV. Genetic analysis of the thymic index in mice]

Relative thymus weight was estimated in C3H/He, C57BL/6 mice, their F1 and backcross hybrids, as well as in the progeny of complete diallel crosses between the BALB/c, C3H/He, C57BL/6 and AKR/J strains. On the basis of the analysis of these measurements, a conclusion is drawn that this character is inherited with incomplete dominance of smaller relative weight. The genes determining greater thymus weight are concentrated in the genetic pool of AKR/J and C57BL/6 strains. These genes are characterized by some degree of recessivity with respect to the genes determining smaller thymus weight which are concentrated in the genetic pool of C3H/He and BALB/c strains. The highest concentration of the "plus" and "minus" genes is found in the genetic pools of AKR/J and C3H/He strains respectively.     

Genetics of the alpha 1,6-dextran response: expression of the QUPC52 idiotype in different inbred and congenic strains of mice

Antibodies to dextran B512 were raised in various strains of mice and were assayed by a radioimmunoassay procedure. Idiotypic antibodies to the IgA(k) dextran B512 binding myeloma proteins QUPC52 and W3129 of BALB/c origin were prepared in rabbits. After adsorption each antiserum was specific for the immunizing myeloma protein and did not react with hundreds of other myeloma proteins; nonetheless, antibodies to dextran B512 from various strains of mice cross-reacted in these test systems. Of the 2 idiotypes tested, the W3129 idiotype was more universally expressed in different strains of mice. The QUPC52 idiotype was the predominant idiotype in BALB/c anti-dextran B512 antibodies and was found in only a few other inbred strains. Using a battery of congenic and inbred strains, it was shown that the QUPC52 idiotype was controlled by genes linked to the Igh complex locus (chromosome 12) and to the Ig kappa complex locus (chromosome 6). The W3129 idiotype was found in a number of stocks of mice in the genus Mus recently isolated from the wild. The QUPC52 idiotype thus far was found only in inbred mice.     

Identification of quantitative trait loci for susceptibility to mouse adenovirus type 1

Adult SJL/J mice are highly susceptible to mouse adenovirus type 1 (MAV-1) infections, whereas other inbred strains, including BALB/cJ, are resistant (K. R. Spindler, L. Fang, M. L. Moore, C. C. Brown, G. N. Hirsch, and A. K. Kajon, J. Virol. 75:12039-12046, 2001). Using congenic mouse strains, we showed that the H-2(s) haplotype of SJL/J mice is not associated with susceptibility to MAV-1. Susceptibility of MAV-1-infected (BALB/cJ x SJL/J)F(1) mice was intermediate between that of SJL/J mice and that of BALB/cJ mice, indicating that susceptibility is a genetically controlled quantitative trait. We mapped genetic loci involved in mouse susceptibility to MAV-1 by analysis of 192 backcross progeny in a genome scan with 65 simple sequence length polymorphic markers. A major quantitative trait locus (QTL) was detected on chromosome 15 (Chr 15) with a highly significant logarithm of odds score of 21. The locus on Chr 15 alone accounts for 40% of the total trait variance between susceptible and resistant strains. QTL modeling of the data indicated that there are a number of other QTLs with small effects that together with the major QTL on Chr 15 account for 54% of the trait variance. Identification of the major QTL is the first step in characterizing host genes involved in susceptibility to MAV-1.     

Genetic locus on mouse chromosome 7 controls elevated heart rate

Elevated heart rate (HR) is a risk factor for cardiovascular diseases. The goal of the study was to map HR trait in mice using quantitative trait locus (QTL) analysis followed by genome-wide association (GWA) analysis. The first approach provides mapping power and the second increases genome resolution. QTL analyses were performed in a C3HeB×SJL backcross. HR and systolic blood pressure (SBP) were measured by the tail-cuff plethysmography. HR was ～80 beats/min higher in SJL compared with C3HeB. There was a wide distribution of the HR (536-763 beats/min) in N2 mice. We discovered a highly significant QTL (logarithm of odds = 6.7, P < 0.001) on chromosome 7 (41 cM) for HR in the C3HeB×SJL backcross. In the Hybrid Mouse Diversity Panel (58 strains, n = 5-6/strain) we found that HR (beats/min) ranged from 546 ± 12 in C58/J to 717 ± 7 in MA/MyJ mice. SBP (mmHg) ranged from 99 ± 6 in strain I/LnJ to 151 ± 4 in strain BXA4/PgnJ. GWA analyses were done using the HMDP, which revealed a locus (64.2-65.1 Mb) on chromosome 7 that colocalized with the QTL for elevated HR found in the C3HeB×SJL backcross. The peak association was observed for 17 SNPs that are localized within three GABA(A) receptor genes. In summary, we used a combined genetic approach to fine map a novel elevated HR locus on mouse chromosome 7.