Tracy E. Strecker

Genetic bases of estrogen-induced tumorigenesis in the rat: Mapping of loci controlling susceptibility to mammary cancer in a Brown Norway × ACI intercross

Exposure to estrogens is associated with an increased risk of breast cancer. Our laboratory has shown that the ACI rat is uniquely susceptible to 17beta-estradiol (E2)-induced mammary cancer. We previously mapped two loci, Emca1 and Emca2 (estrogen-induced mammary cancer), that act independently to determine susceptibility to E2-induced mammary cancer in crosses between the susceptible ACI rat strain and the genetically related, but resistant, Copenhagen (COP) rat strain. In this study, we evaluate susceptibility to E2-induced mammary cancer in a cross between the ACI strain and the unrelated Brown Norway (BN) rat strain. Whereas nearly 100% of the ACI rats developed mammary cancer when treated continuously with E2, BN rats did not develop palpable mammary cancer during the 196-day course of E2 treatment. Susceptibility to E2-induced mammary cancer segregated as a dominant or incompletely dominant trait in a cross between BN females and ACI males. In a population of 251 female (BN x ACI)F(2) rats, we observed evidence for a total of five genetic determinants of susceptibility. Two loci, Emca4 and Emca5, were identified when mammary cancer status at sacrifice was evaluated as the phenotype, and three additional loci, Emca6, Emca7, and Emca8, were identified when mammary cancer number was evaluated as the phenotype. A total of three genetic interactions were identified. These data indicate that susceptibility to E2-induced mammary cancer in the BN x ACI cross behaves as a complex trait controlled by at least five loci and multiple gene-gene interactions.

Genetic control of estrogen action in the rat: mapping of QTLs that impact pituitary lactotroph hyperplasia in a BN × ACI intercross

Estrogens are important regulators of growth and development and contribute to the etiology of several types of cancer. Different inbred rat strains exhibit marked, cell-type-specific differences in responsiveness to estrogens as well as differences in susceptibility to estrogen-induced tumorigenesis. Regulation of pituitary lactotroph homeostasis is one estrogen-regulated response that differs dramatically between different inbred rat strains. In this article we demonstrate that the growth response of the anterior pituitary gland of female ACI rats to 17β-estradiol (E2) markedly exceeds that of identically treated female Brown Norway (BN) rats. We further demonstrate that pituitary mass, a surrogate indicator of absolute lactotroph number, behaves as a quantitative trait in E2-treated F2 progeny generated in a genetic cross originating with BN females and ACI males. Composite interval mapping analyses of the (BN×ACI)F2 population revealed quantitative trait loci (QTLs) that exert significant effects on E2-induced pituitary growth on rat chromosome 4 (RNO4) (Ept5) and RNO7 (Ept7). Continuous treatment with E2 rapidly induces mammary cancer in female ACI rats but not BN rats, and QTLs that impact susceptibility to E2-induced mammary cancer in the (BN×ACI)F2 population described here have been mapped to RNO3 (Emca5), RNO4 (Emca6), RNO5 (Emca8), RNO6 (Emca7), and RNO7 (Emca4). Ept5 and Emca6 map to distinct regions of RNO4. However, Ept7 and Emca4 map to the same region of RNO7. No correlation between pituitary mass and mammary cancer number at necropsy was observed within the (BN×ACI)F2 population. This observation, together with the QTL mapping data, indicate that with the exception of the Ept7/Emca4 locus on RNO7, the genetic determinants of E2-induced pituitary growth differ from the genetic determinants of susceptibility to E2-induced mammary cancer.

Genetic mapping of loci controlling diethylstilbestrol-induced thymic atrophy in the Brown Norway rat

Chronic estrogen administration can lead to thymic atrophy in rodents. In this article we report that the Brown Norway (BN) rat is sensitive to thymic atrophy induced by the estrogen diethylstilbestrol (DES). By contrast, DES does not induce significant thymic atrophy in the August × Copenhagen-Irish (ACI) strain. The sensitivity of the BN rat to DES-induced thymic atrophy appears to segregate as an incompletely dominant trait in crosses between the BN and ACI strains. In a (BN × ACI)F2 population, we find strong evidence for three major genetic determinants of sensitivity to DES-induced thymic atrophy on rat Chromosome (RNO) 10 and RNO2. Genotypes at these loci, termed Esta1, 2, and 3, do not have a significant impact on the ability of DES to induce pituitary tumorigenesis or inhibit growth of these F2 rats. These data indicate that the genetic factors that control DES-induced thymic atrophy are distinct from those that control the effects of DES on pituitary mass and body mass. The Esta intervals on RNO10 and RNO2 overlap with loci that control sensitivity to radiation-induced thymocyte apoptosis, as well as susceptibility to a variety of allergic and autoimmune pathologies, including allergic encephalitis, arthritis, and glomerulonephritis in rodents. These observations suggest that common genetic determinants may control sensitivity to estrogen-induced thymic atrophy, maintenance of thymocyte homeostasis, and immune function.

Diet-Gene Interactions in Estrogen-Induced Mammary Carcinogenesis in the ACI Rat

It is well accepted that hormonal, dietary and genetic factors each influence breast cancer risk. However, the underlying mechanisms and the extent to which these factors interact are largely unknown. We have demonstrated that the female ACI rat exhibits a unique genetically conferred propensity to develop mammary cancers when treated with physiological levels of 17beta-estradiol (E2). More recently, we have mapped to rat chromosome 5 a strong genetic modifier of susceptibility to E2-induced mammary cancers, termed estrogen-induced mammary cancer 1 (Emca1), and have identified potential Emca1 candidate genes. Because estrogens have been inextricably linked to the genesis of breast cancer in humans, the ACI rat model has the potential to reveal novel physiologically relevant insights into how the contributory actions of E2 are modified by specific dietary factors. In the present study, we have examined the ability of a 40% restriction of dietary energy consumption to inhibit E2-induced mammary carcinogenesis. The hypothesis tested was that energy restriction will inhibit mammary carcinogenesis even when circulating E2 remains elevated through administration of exogenous hormone. The data presented herein strongly suggest that energy restriction inhibits E2-induced mammary carcinogenesis in the ACI rat at least partly by retarding progression of atypical hyperplastic foci to carcinoma.

Genetic bases of renal agenesis in the ACI rat: mapping of Renag1 to chromosome 14

Unilateral renal agenesis (URA) is a common developmental defect in humans, occurring at a frequency of approximately 1 in 500–1000 births. Several genetic syndromes include bilateral or unilateral renal agenesis as an associated phenotype. However, URA frequently occurs in individuals not afflicted by these syndromes and is often asymptomatic. Although it is clear that genetic factors contribute to the etiology of URA, the genetic bases of URA are poorly defined at this time. ACI rats, both males and females, exhibit URA at an incidence of 5%–15%. In this article we characterize the incidence of URA in female and male F1, F2, and backcross (BC) progeny from reciprocal genetic crosses between the ACI strain and the unaffected Brown Norway (BN) strain. Through interval mapping analyses of 353 phenotypically defined female F2 progeny, we mapped to rat Chromosome 14 (RNO14) a genetic locus, designated Renag1 (Renal agenesis 1), that serves as the major determinant of URA in these crosses. Further genotypic analyses of URA-affected female and male F2 and BC progeny localized Renag1 to a 14.4-Mb interval on RNO14 bounded by markers D14Rat50 and D14Rat12. The data from these genetic studies suggest that the ACI allele of Renag1 acts in an incompletely dominant and incompletely penetrant manner to confer URA.

Dietary Energy Restriction Inhibits Estrogen Induced Pituitary Tumorigenesis in a Rat Strain Specific Manner

We are investigating modulation by dietary energy consumption of estrogen action in the regulation of cell proliferation and survival and induction of prolactin (PRL)-producing pituitary tumors in different inbred rat strains. Summarized herein are data which indicate that a 40% restriction of energy consumption virtually abolishes development of estrogen induced pituitary tumors in the inbred Fischer 344 (F344) rat strain and that this inhibition occurs through modulation of estrogen regulation of pituitary cell survival, not inhibition of estrogen stimulated cell proliferation. Data are also presented which indicate that the inhibitory effect of energy restriction on estrogen induced pituitary tumorigenesis is rat strain specific. Whereas energy restriction markedly inhibited development of pituitary tumors in the F344 and Copenhagen (COP) rat strains, no inhibitory effect was observed in the ACI strain. Genetic studies have been initiated to elucidate the molecular bases of the strain specific inhibitory actions of dietary energy restriction on development of estrogen induced pituitary tumors in the genetically related COP and ACI rat strains.

Genetic determinants of susceptibility to estrogen-induced mammary cancer in the rat.

Estrogens are inextricably linked to the etiology of breast cancer. We have demonstrated that the ACI rat strain exhibits a unique propensity to develop mammary cancer when treated continuously with 17β-estradiol (E2). Treatment of ovary intact ACI rats with E2 results in virtually a 100% incidence of mammary carcinoma with a median latency of approximately 140 days. These mammary cancers are dependent upon exogenous E2 and exhibit genomic instability, a hallmark of breast cancers in humans. In contrast, the Copenhagen (COP) and Brown Norway (BN) rat strains are resistant to E2-induced mammary carcinogenesis. Susceptibility to E2-induced mammary cancer behaves as an incompletely dominant or dominant trait in crosses between the ACI strain and the COP or BN strains. Genetic linkage analyses of several hundred phenotypically defined F2 progeny from reciprocal crosses between the ACI and COP or BN strains revealed a total of seven genetic determinants of susceptibility to E2-induced mammary cancer on chromosomes 2, 3, 4, 5, 6, 7 and 18. The chromosome 5 locus, designated Emca1, determines susceptibility to E2-induced mammary cancer in reciprocal crosses between the highly susceptible ACI strain and the resistant COP or BN strains. Potential candidate genes residing within the Emca1 locus include Cdkn2a, Cdkn2b and Cdkn2c. Studies on Cdkn2a indicate that expression of the p16Ink4a protein product of the Cdkn2a locus is downregulated at an early stage of E2-induced mammary carcinogenesis in the ACI rat. We are currently evaluating these genes further to determine whether and how they contribute to mammary cancer etiology in this model.