We have used the autochthonous transgenic adenocarcinoma of mouse prostate (TRAMP) model to investigate the relationship between somatic mutation in the androgen receptor (AR) and the emergence of androgenindependent prostate cancer. Here we report the identification, isolation, and characterization of distinct classes of AR variants from spontaneous prostate tumors in the TRAMP model. Using cDNA cloning, single stranded conformation polymorphism and sequencing strategies, 15 unique somatic mutations in the AR were identified in prostate tumors obtained from eight TRAMP mice between 24 and 29 weeks of age. At least one mutation was isolated from each mouse. All mutations were single base substitutions, 10 were missense and 5 were silent. Nine mutations in the AR were identified in tumors of four mice that were castrated at 12 weeks of age. Interestingly, the majority of mutations (seven out of nine, 78%) identified in the androgen-independent tumors colocalized in the AR transactivation domain. The remaining mutations colocalized in the AR ligand binding domain. In general, the AR variants demonstrated promoter-, cell-, and cofactor-specific activities in response to various hormones. All AR variants isolated in this study maintained strong sensitivity for androgens, and four AR variants isolated from castrated mice demonstrated increased activities in the absence of ligand. The K638M and F677S variants demonstrated increased activities in response to androgen, and K638M also demonstrated increased response to estradiol. In the presence of AR coactivator ARA70 the E231G variant demonstrated increased activity in response to both androgen and estradiol. However, in the presence of AR coactivator ARA160 the E231G variant was selectively responsive to androgen. Collectively these analyses not only indicate that somatic mutations in the AR gene occur spontaneously in TRAMP tumors but also how changes in the hormonal environment may drive the selection of spontaneous somatic mutations that provide a growth advantage.

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