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Department of Veterinary Pathobiology (R.J.R., C.T.) and University Laboratory High School (A.W.M., R.E.B.), University of Illinois at Urbana-Champaign, Urbana, Illinois 61802; the Department of Microbiology, Brigham Young University (J.S.G.), Provo, Utah 84602; the Department of Obstetrics and Gynecology, University of Pennsylvania School of Medicine (C.R.L.), Philadelphia, Pennsylvania 19104; and the Departments of Agronomy and Statistics, Purdue University (R.W.D.), West Lafayette, Indiana 47907
Address all correspondence and requests for reprints to: Dr. Cory Teuscher, Department of Veterinary Pathobiology, 2001 South Lincoln Avenue, Urbana, Illinois 61802. E-mail: cteusche{at}staff.uiuc.edu
| Abstract |
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| Introduction |
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Recently, we showed that the number of eosinophils infiltrating the uterus of ovariectomized (Ovx) E2-treated mice is a genetically controlled phenotype (6). Estrogen-dependent pituitary growth has also been shown to be genetically controlled in rats (10). The extent of eosinophilic infiltration into the uterus is governed by Est1, a quantitative trait locus (QTL) that maps to chromosome 4 and two minor loci on chromosomes 10 and 16 (6). Given that the E2-regulated uterine inflammatory response is genetically controlled, we undertook the present study to address whether uterine growth, as determined by wet weight, is also a genetically controlled phenotype. Using a whole genome scan on a (C57BL/6J x C3H/HeJ) x C3H/HeJ backcross population segregating for the high and low responder wet weight phenotypes, we report the identification of two QTL, Est2 on chromosome 5 and Est3 on chromosome 11, controlling E2-regulated uterine growth. Additionally, we present evidence for genetic interaction between loci involved in the genetic response to E2 and identified an interaction locus, Est4, on chromosome 10. Characterization of the genes at the molecular level will undoubtedly provide greater insight into the complex mechanisms by which E2 regulates growth in both normal and pathological states.
| Materials and Methods |
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Ovariectomies (Ovx) and hormonal stimulation
Parental, F1 hybrid, and BC1 female mice were Ovx
(6) at 57 weeks of age. After Ovx, all animals were rested for 1
week, at which time they underwent E2 stimulation. All mice
received 40.0 µg/kg BW 17ß-estradiol, sc, in 0.1 ml saline
containing 20% (vol/vol) ethanol on days 0 and 1. All animals were
killed 24 h after the second injection. The uteri were removed,
cleaned of fat and adventitia, blotted to remove luminal fluid, and
weighed.
Genotyping
Genomic DNA was isolated from liver tissue, as previously
described (6). The genotype of each of the 94 BC1 animals was
determined at 178 microsatellite marker loci (11, 12, 13) that
distinguished the C57BL/6J and C3H/HeJ parental strains. These markers
were spaced 20 centimorgans (cM) apart across the entire genome and
more densely in regions of association with the phenotype. PCR
parameters were optimized as previously described (11, 12, 13, 14).
Linkage analysis
A linkage map of the microsatellite markers was estimated using
the Kosambi map function of the MAPMAKER/EXP computer package (15, 16).
Likelihood ratio tests (LRT) were used to test association of single
markers or marker intervals with the uterine wet weight phenotype in
QTL Cartographer (17, 18, 19). When significant linkage was
indicated by the LRT value, a new QTL was proposed (17, 20). Single
marker analysis used each of the markers to detect QTL linked to the
phenotype (17, 19). Additionally, interval mapping was used to identify
and locate single QTL across the genome in 2-cM intervals (18, 19).
Multiple trait analysis was performed using interval mapping based on
the phenotypes of uterine wet weight and eosinophilic infiltration (19, 21).
Permutation-derived critical values
Significant linkage of QTL to genetic loci for all analyses was
determined by permutation threshold theory (17, 20, 22). This method of
analysis takes into account the specifics of the experimental
situation, such as the number of animals used, genome size, and missing
data, and also satisfies the multiple testing issues implicit in genome
scans (marker density, number of tests used, and independence of tests)
(20). For this experiment, 1000 permutations of the actual data were
generated to provide the sampling distribution of test statistics under
a null hypothesis of no linkage. Each permutation was performed by
randomly shuffling and reassigning phenotypes from the BC1 animals to
one of the specific genotypes of the population as defined by the
microsatellite markers. Linkage analysis was then performed for each
permuted set of data, and new test statistics (LRT) were generated for
that permutation. A distribution of test statistics from the 1000
permutations was created, and significant linkage was declared
according to the values of the pertinent distribution. Experimentwise
threshold values included all data points in the genome scan and were
obtained by using a distribution of the maximum test statistic from
each of the 1000 permutations of the data. Comparisonwise thresholds
used all of the permutation test statistics at a single marker and were
identified using a distribution composed of the test statistics from
1000 permutations for the single marker (20, 23). Using 1000
permutations of the data, we are able to accurately define 90% (
=
0.10) and 95% (
= 0.05) thresholds of linkage, whereas higher
thresholds would be estimated with substantially more permutations
(20).
Interaction analysis
Interaction between QTL was investigated by using a general
linear model in SAS (PROC GLM) (10, 24). No additional genetic map
information was incorporated because all QTL tested were found on
separate chromosomes. Models were constructed and assumptions were
verified for each model. Interaction between Est2 and
Est3 was tested using a model containing Est2,
Est3, the interaction term between these QTL, and the
dependent variable of uterine wet weight.
Models with QTL for both the uterine weight and number of infiltrating eosinophils and their interactions were constructed using either of the two phenotypes as the dependent variable. Independent variables representing QTL linked to D4Mit6 (Est1), D5Mit296 (Est2), D11Mit67 (Est3), D10Mit180 (Est4), and D16Mit144 (6) as well as all possible interactions among these variables were included in the model. The independent variables were selected in the following manner. The two known markers linked to the QTL for uterine weight, D5Mit296 (Est2) and D11Mit67 (Est3), or number of eosinophils [D4Mit6 (Est1), D10Mit180 (Est4), and D16Mit144] respectively, remained in the model while the other independent variables were selected using stepwise regression in SAS (24).
| Results and Discussion |
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< 0.05). In the carrier-treated animals, no significant
difference in uterine wet weight was observed among the three groups.
These results are consistent with the E2-regulated high
responder uterine growth as seen in C57BL/6J mice being a genetically
controlled, dominant trait. Thus, although all strains displayed a
response to E2 stimulation, it was clear that the magnitude
of the response was genetically determined. This finding, as in other
genetically controlled phenotypes, can lead to the identification of
loci regulating such responses.
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=
0.05; threshold value = 12.85) on chromosome 5 at
D5Mit296, D5Mit148, and D5Mit388 and
on chromosome 11 at D11Mit67 and D11Mit132.
Significant comparisonwise linkage (
= 0.05) was detected on
chromosome 5 for markers D5Mit387-D5Mit80 and on
chromosome 11 for markers D11Mit86-D11Mit61
(Table 2
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= 0.05;
threshold value = 13.39; see Fig. 1
= 0.10; threshold value = 11.67; see Fig. 2
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= 0.05) from those of the other loci
combinations. No differences between other loci combinations, (C/C,
C/C), (C/B, C/C), and (C/C, C/B), were statistically significant. This
suggests epistasis or interaction between Est2 and
Est3.
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Multiple interaction factors in the signaling pathways leading to the E2-regulated responses have been suggested (25, 26, 27, 28). We hypothesized that the QTL controlling eosinophilic infiltration in the uterus after E2 stimulation may also be involved in the uterine growth phenotype and that these QTL may interact to control these E2-dependent phenotypes. Our rational for creating new interactive independent variables was that some of the interaction terms may encode transcription factors, activation factors, or coactivators (25, 26, 27, 28) that are common to multiple pathways in E2 responses. Most simply, these interactions may represent heterodimers that function as a single entity and therefore are represented effectively as new, independent interaction terms.
For uterine weight, a model including the independent variables D5Mit296 (Est2) and D11Mit67 (Est3) as well as the interaction term between D11Mit67 (Est3) and D10Mit180 was significant in explaining the quantitative trait (F = 14.256; P < 0.0001). The model with these three independent variables explained 33.0% of the variation in uterine weight, and when adjusted for the number of parameters in the model, the explained variation was reduced to 30.7%. A model based on linear multiple regression with uterine eosinophils counted in the stroma (6) as the dependent variable found the independent variables D4Mit6 (Est1), D10Mit180, and D16Mit144 as well as the interaction terms between D10Mit180 and D4Mit6 (Est1) and between D10Mit180 and D5Mit296 (Est3) to be significant (F = 9.026; P < 0.0001). The model with these three independent variables explained 34.7% of the variation in uterine weight, and when adjusted for the number of parameters in the model, the explained variation was reduced to 30.9%. The interaction analysis supports our hypothesis of interaction between QTL controlling E2-regulated responses.
To understand the overall genetic control of the
E2-regulated uterine response, we performed multiple trait
analysis (21) for both quantitative traits (uterine weight and number
of infiltrating eosinophils) together (6). Linkage was found to marker
loci D4Mit6 (Est1; LRT = 15.05),
D5Mit296 (Est2; LRT = 17.84),
D10Mit180 (LRT = 16.02), and D11Mit67
(Est3; LRT = 14.38;
= 0.10 and
= 0.05
thresholds equal 14.60 and 16.72, respectively). Based on the
significance of D10Mit180 in eosinophil infiltration, the
significant interaction of D10Mit180 in both
E2-regulated responses, and the significance of this locus
in multiple trait analysis, we have designated the QTL linked to
D10Mit180 as Est4. These results corroborate the
conclusions of the interaction analysis.
Interestingly, each of the QTL identified in this study controlling
E2-regulated uterine growth maps to a region known to
encode other E2-regulated genes or loci that may
potentially influence E2-regulated responses
(www.informatics.jax.org/locus.html). On chromosome 5 these include
serotonin receptor 5a (Htr5a) (29) and interleukin-6
(Il6) (30, 31, 32). Genes linked to Est3 on
chromosome 11 include procollagen, type I,
1 (Cola1)
(33), integrin
3 (Itga3) (34, 35),
granulocyte colony-stimulating factor (Csfg) (36), retinoic
acid receptor-
(Rara) (37, 38, 39), thyroid hormone
receptor-
(Thra) (38), and one of the genes linked to
human familial breast cancer (Brca1). Although the interval
encoding each QTL may contain several known E2-regulated
genes, the exact role of any of these candidate genes in the hormonal
response can only be suggested by the current data.
The colocalization of Brca1 with Est3 is, however, worth noting given the current status of the role of E2 in the regulation of Brca1 (40). Human Brca1 is known to contain E2 response elements, but the murine form of the gene has no homolog to these Alu sequences (27). It has been postulated that the expression of Brca1 in numerous tissues, including the ovary of both humans and mice, is controlled by E2 (25, 28, 41), yet the current consensus is that Brca1 is not directly influenced by E2. Rather, E2 is thought to indirectly influence the expression of Brca1 and is under complex control (26, 27, 42, 43, 44).
The control of the expression of Brca1 is interesting in light of the interaction analysis in this study. When uterine weight was used as a dependent variable, the interaction between Est3 and Est4 was significant. Moreover, Est4 was also involved in interactions controlling uterine eosinophilic infiltration (6). We hypothesize that Est4 may encode a transcription factor, activation factor, or coactivation factor that is important in the overall genetic response to E2 treatment and that only in combination with additional factors is quantitative variation observed in the phenotype (25, 26, 27, 28). As such, Est4 may encode the putative factor involved in the E2-regulated expression of Brca1. It is also interesting that neither Est3 nor Est4 interacts with D5Mit169, the closest marker linked to Brca2 in our model (data not shown; www.genome.wi.mit.edu/cgi-bin/mouse/index). This suggests independent estrogen regulation of Brca1 and Brca2 (44). We have shown that interaction and multiple trait analysis may prove useful to identify additional QTL, provide a model to describe the phenotypic effect, and elucidate mechanisms of biochemical interaction.
Similar mapping approaches have identified QTL on chromosomes 2, 3, 5,
9, and 14 controlling diethylstilbestrol-dependent pituitary tumor
growth in the rat (10). From the current comparative map of mouse and
rat (www.informatics.jax.org/homology.html), it appears that
Est2, Est3, and Est4 are not
represented among these five QTL. Est2 on mouse chromosome 5
is syntenic with rat chromosome 4, whereas Est3 on mouse
chromosome 11 is syntenic with rat chromosome 10
(www.informatics.jax.org/homology.html). However, Est1, the
locus involved in controlling the E2-regulated uterine
eosinophilic inflammatory response is probably syntenic with
Edpm5 on rat chromosome 5
(www.informatics.jax.org/homology.html). It is worth noting that in all
three models epistasis plays a role in the genetic control.
Additionally, none of the QTL mapped colocalize with estrogen
receptor-
(Estra; chromosome 10 at 12 cM) or estrogen
receptor-ß (Estrb; chromosome 12 at 33 cM;
www.informatics.jax.org/locus.html), suggesting that polymorphisms in
these two E2 receptor genes do not underlie the phenotypic
variations observed.
Taken together, our results suggest that the regions of chromosomes 5 and 11 encoding Est2 and Est3 contain a variety of E2-regulated genes and that many of these genes have the clear potential to diversify or amplify E2-regulated responses. Thus, it is conceivable that Est2 and Est3 may represent single genes or loci within conserved gene complexes that play fundamental roles in mediating the effects of E2. These genes or loci may also be involved in the general reproductive performance and characteristics of mice. Interestingly, C57BL/6J mice have larger litter sizes, more litters in a lifetime, and greater relative fecundity than C3H/HeJ mice. C57BL/6J mice also have a high response to superovulation, whereas C3H/HeJ mice are low responders (45). Characterization of the Est loci at the molecular level will undoubtedly lead to a greater understanding of the genetic components underlying both genomic and uterotropic responses elicited by E2. Finally, our results suggest that other E2-regulated/dependent responses, such as the immunological (46, 47), carcinogenic (3), skeletal (4), and developmental (5), may also have a genetic component that plays a role in the phenotypic spectra observed within these response groups, and that mapping studies can be used for the eventual identification of genes involved in hormonally regulated responses.
| Acknowledgments |
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| Footnotes |
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Received May 28, 1998.
| References |
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