Hormone Status Selects for Spontaneous Somatic Androgen Receptor Variants That Demonstrate Specific Ligand and Cofactor Dependent Activities in Autochthonous Prostate Cancer

Abstract

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.

Early and delayed castrations confer a similar survival advantage in TRAMP mice

“…In this study, AR expression was reduced in epithelial and stromal cells 1-week postcastration, which was the normal short-term response to castration in TRAMP mice. Sometimes AR was not detected in tumour cells, which was possibly because of lack of AR expression or the expression of a mutated AR [22,23]. In this study, AR expression was significantly lower in tumours from the early castration group than in the sham-castrated or delayed castration groups, and the tumours in the delayed castration group showed less AR expression than in the sham-castrated group.…”

Targeted therapy of cancer: new roles for pathologists—prostate cancer

“…Similar mutations in the same region have been reported. 29,30,[39][40][41] The type of treatment given was also shown to influence the kind of mutation seen. 42 Since reports have shown that mutations are present in 20-40% of the advanced PCAs before therapy, 30,31 it could be hypothesized that the mutations were not induced by therapy but were pre-existent.…”

A gene signature identified using a mouse model of androgen receptor‐dependent prostate cancer predicts biochemical relapse in human disease

“…[10][11][12] AR variants identified in clinical prostate tumors typically exhibit promiscuous activation by nonandrogenic ligands and/or enhanced transactivation capacity due to altered interactions with coregulators. 13 The prototypical example of AR promiscuity is AR-T877A (T857A in mice), a variant expressed in the LNCaP prostate cancer cell line that is activated by estrogen, progesterone and the AR antagonist, hydroxyflutamide, in addition to androgens. 14 The AR contains a highly variable amino-terminal domain (NTD) essential for receptor stabilization 15,16 that also regulates its transcriptional activity by providing an interaction surface for key prostate-specific co-regulators and the transcriptional machinery.…”

“…17 The AR-NTD signature (ANTS) sequence, composed of amino acids 234-247 (229-242 in mice), is the most evolutionarily conserved region of the AR but, interestingly, is not found in related steroid receptors. 13,18 Mutations have been identified proximal to the ANTS sequence in clinical prostate cancers 2 and within or near this sequence in castrate-resistant tumors arising in the transgenic adenocarcinoma of the mouse prostate (TRAMP) model. 2,13 Two AR variants arising from such mutations, E231G and A229T, display increased basal activity and response to androgenic and non-androgenic ligands compared to wild-type AR (wtAR).…”

“…13,18 Mutations have been identified proximal to the ANTS sequence in clinical prostate cancers 2 and within or near this sequence in castrate-resistant tumors arising in the transgenic adenocarcinoma of the mouse prostate (TRAMP) model. 2,13 Two AR variants arising from such mutations, E231G and A229T, display increased basal activity and response to androgenic and non-androgenic ligands compared to wild-type AR (wtAR). AR-E231G exhibits increased responsiveness to androgens [testosterone and 5a-dihydrotestosterone (DHT)] and estrogens (estradiol) in the presence of the co-activators, ARA70 and ARA160, 13 and decreased responsiveness to the co-repressor, CHIP.…”