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Journal of Virology, September 1999, p. 7848-7852, Vol. 73, No. 9
Laboratory of Persistent Viral Diseases,
Rocky Mountain Laboratories, National Institute of Allergy and
Infectious Diseases, National Institutes of Health, Hamilton, Montana
598401; Department of Biochemistry,
University of Kentucky, Lexington, Kentucky
40536-00842; Department of Biochemistry,
Molecular Biology, and Cell Biology, Northwestern University, Evanston,
Illinois 602083; Mammalian Genetics
Laboratory, ABL Basic Research Program, NCI Frederick Cancer
Research and Development Center, Frederick, Maryland
217024; and The Jackson Laboratory, Bar
Harbor, Maine 046095
Received 1 April 1999/Accepted 24 May 1999
Rfv3 is a host resistance gene that operates through an
unknown mechanism to control the development of the virus-neutralizing antibody response required for recovery from infection with Friend retrovirus. The Rfv3 gene was previously mapped to an
approximately 20-centimorgan (cM) region of chromosome 15. More refined
mapping was not possible, due to a lack of microsatellite markers and leakiness in the Rfv3 phenotype, which prevented definitive
phenotyping of individual recombinant mice. In the present study, we
overcame these difficulties by taking advantage of seven new
microsatellite markers in the Rfv3 region and by using
progeny tests to accurately determine the Rfv3 phenotype of
recombinant mice. Detailed linkage analysis of relevant crossovers
narrowed the location of Rfv3 to a 0.83-cM region. Mapping
of closely linked genes in an interspecific backcross panel allowed us
to exclude two previous candidate genes, Ly6 and
Wnt7b. These studies also showed for the first time that the Hsf1 gene maps to the Rfv3-linked cluster of genes
including Il2rb, Il3rb, and Pdgfb.
This localization of Rfv3 to a region of less than 1 cM now
makes it feasible to attempt the cloning of Rfv3 by
physical methods.
Infection with Friend virus
complex (FV) induces rapid polyclonal erythroid cell proliferation and
splenomegaly in genetically susceptible adult mice (5, 10).
Mice of strains which mount rapid humoral and cell-mediated FV-specific
immune responses spontaneously recover from FV-induced splenomegaly
without progressing to erythroleukemia. Such recovery from FV disease
is dependent on a number of host genes, including several genes of the
major histocompatibility complex (MHC), which influence critical
CD4+- and CD8+-T-cell responses (5,
13). However, unlike some other viral systems in which either a
cellular or humoral immune response alone is sufficient to resolve
infection (1, 19, 20), spontaneous recovery from FV requires
both FV-specific T-cell responses and virus-neutralizing antibody
responses (6, 12, 21).
The non-MHC gene Rfv3 influences the ability of mice to
mount FV-neutralizing antibody responses following infection
(6). C57BL/10 and C57BL/6 mice have the genotype
Rfv3r/Rfv3r, and BALB/c, A.BY, and
A/WySn mice have the genotype
Rfv3s/Rfv3s (6, 7, 9). At
about 2 weeks postinfection, mice carrying at least one dominant
Rfv3 resistance allele (Rfv3r), such
as (C57BL/10 × A.BY)F1 mice, begin to make
FV-neutralizing antibodies, and they usually clear FV plasma viremia by
30 days postinfection (DPI). In contrast, mice with two sensitive
alleles (Rfv3s/Rfv3s) fail to make
FV-neutralizing antibodies, remain viremic, and eventually
succumb to FV-induced erythroleukemic splenomegaly (6,
9). Interestingly, Rfv3s/Rfv3s
mice have normal antibody responses to other antigens, suggesting that
these mice are not generally immunosuppressed (18). The mechanism whereby Rfv3 controls the FV-specific humoral
response remains unknown. The gene has been mapped to a 20-centimorgan (cM) region of mouse chromosome 15, ruling out linkage to genes such as
MHC genes, immunoglobulin genes, or T-cell receptor genes (14). In this study, we used progeny testing and
microsatellite linkage analysis with seven new markers to define the
location of the Rfv3 gene to a region of less than 1 cM.
These experiments determined that the previous candidate genes,
Ly6 and Wnt7, mapped to regions adjacent to,
rather than within, the Rfv3 gene region.
To map the Rfv3 gene, heterozygous (B10.A × A/Wy)F1 mice
(Rfv3r/Rfv3s) were intercrossed to
produce F2 offspring. Tail tip DNA samples from 181 F2 mice were analyzed by using PCR amplification of simple sequence length polymorphisms (microsatellites) (15).
Initially, two markers, D15Mit28 and D15Mit108,
which flank the 20-cM region containing the Rfv3 gene
(14), were used to identify 45 recombinant F2
mice. DNA from these recombinants with microsatellite markers lying
between the flanking markers was further tested, and nine recombination
locations (groups I through IX) were identified (Fig.
1). Because the Rfv3 phenotype
shows some leakiness even in genetically identical mice (Fig. 1)
(14), we could not rely on the accuracy of phenotyping of
individual recombinant F2 mice. Rather, the
recombinant F2 mice were backcrossed to A/Wy parental mice,
and the resulting progeny were genotyped and tested. A total of 23 of
the 45 recombinant F2 mice were backcrossed, and all progeny were genotyped prior to phenotypic analysis. For determination of the Rfv3 phenotype, recombinant progeny and a
number of nonrecombinant littermate control mice were infected with FV
and tested for plasma viremia at 30 DPI. Viremia has been shown
to inversely correlate with FV-neutralizing antibody production
(9, 22) and is a convenient assay for determining the
Rfv3 phenotype. Mice exhibiting less than 200 focus-forming units (FFU)/ml of plasma were scored as nonviremic,
whereas mice with more than 104 FFU/ml of plasma were
considered highly viremic. Leakiness in the Rfv3
phenotype was manifested by intermediate viremia levels, which were
observed at a low incidence in both
Rfv3r/Rfv3s and
Rfv3s/Rfv3s controls (Fig. 1).
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Fine Mapping of the Friend Retrovirus Resistance
Gene, Rfv3, on Mouse Chromosome 15
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FIG. 1.
Genotypes of recombinant (B10.A × A/Wy)F2 mice and corresponding Rfv3 phenotypes
for (B10.A × A/Wy)F2 × A/Wy backcross progeny.
(Left) F2 mice were typed for D15Mit markers by
PCR. Thirty cycles of PCR were performed with 100 ng of tail DNA
template, 1× PCR buffer (Promega), 0.2 µM deoxynucleoside
triphosphate, 1 mM MgCl2, 1 µM flanking primers, and 0.05 U of Taq polymerase (Promega). Arabic numbers at the top
refer to the markers used in this study. The nine crossover locations
detected on chromosome 15 are shown. The markers are evenly spaced for
convenience, and crossover is arbitrarily shown halfway between each
marker. Black regions denote DNA originating from the B10.A
(Rfv3r/Rfv3r) parent; white regions
denote DNA originating from the A/Wy
(Rfv3s/Rfv3s) parent. Numbers in
parentheses indicate the numbers of recombinant F2 mice
showing crossover in the same region which were progeny tested. (Right)
Plasma viremia data were analyzed at 30 DPI for individual recombinant
(B10.A × A/Wy)F2 × A/Wy backcross progeny from
each crossover group. Viremia data for nonrecombinant
(Rfv3r/Rfv3s or
Rfv3s/Rfv3s) littermates from each
backcross are grouped at the top. Viremia levels between 200 and
104 FFU/ml are plotted on a log10 scale; values
of <200 and values of >104 are grouped. Each dot
represents the FV viremia titer for one mouse as detected by focal
immunoassay on Mus dunni cells (17). The
detection limit of the assay was 200 FFU/ml.
To map Rfv3, we looked for a correlation between the genotypes and phenotypes in the recombinant backcross progeny. High levels of viremia were seen in 35 of 46 (76%) recombinant progeny from groups I, II, III, IV, and V. When the genotypes of these mice were compared, it was observed that the mice had the Rfv3s/Rfv3s genotype at marker D15Mit214 (Fig. 1). In contrast, the low-viremia groups VI, VII, and VIII had the Rfv3r/Rfv3s genotype at this marker (Fig. 1). These results indicated that the Rfv3 gene was located near D15Mit214 in a region between markers D15Mit70 and D15Mit107.
Group IX, which consisted of progeny from three F2 recombinants, R45, R172, and R80, exhibited both high and low levels of viremia (Fig. 1 and 2). This variability prompted further typing of the mice of this group in order to detect possible differences in recombination positions. Typing with three markers located between D15Mit214 and D15Mit107 revealed unique crossover positions in these three recombinants (Fig. 2). Interestingly, 17 of 18 progeny of R45 were either nonviremic or showed only intermediate viremia (<104 FFU/ml of plasma), whereas 20 of 21 progeny of recombinants R172 and R80 were highly viremic (Fig. 2). These data were consistent with a location for Rfv3 that was distal to D15Mit239 and proximal to D15Mit107 (Fig. 2). Progeny testing of another recombinant mouse, R8 (group III), with a recombination in the same region supported this position for Rfv3 (Fig. 2).
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The genotyping of recombinant F2 mice has allowed us to determine the genetic distances between the D15Mit markers used in this study. Our previous results indicated that Rfv3 was located in a 5- to 20-cM region between D15Mit108 and D15Mit93. Here, we genotyped F2 mice with the microsatellite markers D15Mit108 and D15Mit28, which is very closely linked to D15Mit93. We now estimate the distance between these markers to be 12.2 cM (Fig. 3A). Furthermore, our mapping of Rfv3 between D15Mit239 and D15Mit107 localizes the Rfv3 position to a 0.83-cM region of chromosome 15 (Fig. 3A).
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To identify other genes in the interval of the Rfv3 locus, we mapped two closely linked markers, D15Mit214 and D15Mit239, in an interspecific backcross panel derived from the matings of (C57BL/6J × Mus spretus)F1 × C57BL/6J mice (8). This mapping panel has been typed for over 2,700 loci, most of which are genes that are well distributed among all the autosomes as well as the X chromosome. By this analysis, Rfv3 was separable from Ly6 and Wnt7b (Fig. 3B), two genes which cosegregated with Rfv3 in previous experiments (14). However, Rfv3 colocalized with a cluster of genes including the immune-system-related genes Il2rb (interleukin 2 [IL-2] receptor beta), Il3rb1 (IL-3 receptor beta 1), and Pdgfb (platelet-derived growth factor beta) in the central region of mouse chromosome 15 (3, 4, 11). This cluster of loci is 0.9 cM distal to Ly6 and 3.1 cM proximal to Wnt7b (Fig. 3B).
One previously unmapped gene, Hsf1 (heat shock factor 1), was also found to map to the same gene region as Rfv3. The position of Hsf1 was determined by Southern blot analysis with a cDNA probe (16). Major fragments of 11.5 and 2.8 kb were detected in TaqI-digested C57BL/6J DNA, and major fragments of 5.2 and 3.1 kb were detected in TaqI-digested M. spretus DNA. The presence or absence of the M. spretus fragments which cosegregated was monitored in [(C57BL/6 × M. spretus) × C57BL/6]B1 backcross mice. Recombination distances were calculated with Map Manager, version 2.6.5, and the gene order was determined by minimizing the number of recombination events required to explain the allele distribution patterns.
Ideally, it would be desirable to type the critical recombinant mice for crossovers between Rfv3 and the candidate genes Il2rb, Il3rb1, Pdgfb, and Hsf1. However, to our knowledge, no polymorphisms that distinguish the alleles of these genes in the C57BL/10 and A/Wy strains of mice have been reported. Furthermore, no crossovers between any of these genes have been detected in several crosses including Mus musculus and M. spretus, where distinguishing between alleles is more feasible. Because of the obvious importance of IL-2 receptor-mediated signal transduction in many immune responses, we made a preliminary attempt to detect allele-specific polymorphisms in Il2rb mRNAs but found no allelic differences between C57BL/10 and A/Wy mice. Also, we found no significant differences in the levels of expression of Il2rb when spleen RNA was examined by RNase protection assays at multiple time points during the first 3 weeks following infection with FV (data not shown).
The present study has narrowed the location of Rfv3 by a factor of over 20 and has both excluded two previous candidate genes and included Hsf1 as a new candidate. The central region of mouse chromosome 15 has homology with human chromosomes 8q and 22q (Fig. 3B), and further mapping of these regions of the human chromosomes may uncover additional candidate genes. While the analysis of candidates is appealing, the Rfv3 gene region could contain numerous unidentified genes, any of which could be Rfv3. The real advantage of the current fine mapping is that identification of Rfv3 by physical means is now feasible. Bacterial artificial chromosomes containing overlapping sections of the Rfv3 region can be produced and used to breed transgenic mice. The future identification of the Rfv3 gene will aid our understanding of the control of the FV host immune response and may ultimately contribute more generally to understanding human retroviral immunity.
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ACKNOWLEDGMENTS |
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The first two authors contributed equally to this study.
We thank Thomas Malek for the generous gift of the Il2rb cDNA clone and Verity Letts for use of her genetic maps. We also thank Clint Kenley, Deborah Householder, and Mary Barnstead for excellent technical assistance and John Portis and Don Lodmell for critical reviews of the manuscript.
This research was supported, in part, by the National Cancer Institute, DHHS, under contract with ABL.
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FOOTNOTES |
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* Corresponding author. Mailing address: Laboratory of Persistent Viral Diseases, National Institute of Allergy and Infectious Diseases, NIH, Rocky Mountain Laboratories, 903 South 4th St., Hamilton, MT 59840-2999. Phone: (406) 363-9354. Fax: (406) 363-9286. E-mail: bchesebro{at}nih.gov.
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