Androgen receptor variants and prostate cancer in humanized AR mice

Abstract

Androgen, acting via the androgen receptor (AR), is central to male development, differentiation and hormone-dependent diseases such as prostate cancer. AR is actively involved in the initiation of prostate cancer, the transition to androgen independence, and many mechanisms of resistance to therapy. To examine genetic variation of AR in cancer, we created mice by germ-line gene targeting in which human AR sequence replaces that of the mouse. Since shorter length of a polymorphic N-terminal glutamine (Q) tract has been linked to prostate cancer risk, we introduced alleles with 12, 21 or 48 Qs to test this association. The three "humanized" AR mouse strains (h/mAR) are normal physiologically, as well as by cellular and molecular criteria, although slight differences are detected in AR target gene expression, correlating inversely with Q tract length. However, distinct allele-dependent differences in tumorigenesis are evident when these mice are crossed to a transgenic prostate cancer model. Remarkably, Q tract variation also differentially impacts disease progression following androgen depletion. This finding emphasizes the importance of AR function in androgen-independent as well as androgen-dependent disease. These mice provide a novel genetic paradigm in which to dissect opposing functions of AR in tumor suppression versus oncogenesis.

Figure 1. Structure of the humanized AR gene

The vector for gene targeting in mouse embryonic stem cells encompassed all of mAR exon 1 with surrounding 5′ flank and downstream intron 1 sequences. Into the construct, human AR cDNA (hatched region) encoding amino acids 35 to 466 (dependent on length of polyamino acid tracts) was substituted for the corresponding mouse sequence. The human AR sequence differs about 15% in this N-terminal region, including an extensive glycine tract (G) and a variable glutamine tract (12, 21 or 48 Qs were inserted) that is also farther 5′-ward than the mouse Q tract. The DBD and LBD are nearly identical between man and mouse in protein sequence. Approximate intron locations are marked by bars below the receptor map. The recombination essentially reconstructs the human AR under control of mouse regulatory sequences.

Figure 2. Prostate gene expression is sensitive to AR Q-tract length

Prostate mRNAs from 3–8 mice at 6 months of age per genotype were quantified by real-time RT-PCR. Results are relative to levels in wild type mAR mice. Nkx3.1 and probasin are up-regulated by AR, clusterin is repressed. Modified from data in Albertelli et al. [37].

Figure 3. Q tract length in h/mARxTRAMP mice affects prostate tumorigenesis

Mice were abdominally palpated weekly to track tumor development. The status of each genotype at 29 weeks of age is shown for intact mice (A) and mice castrated at 12 weeks of age (B). Mice that died prior to 29 weeks of age are represented by the black portion of the bar, those with a palpable tumor are represented by hatching, and those with no palpable tumor are in white. Modified in part from data in Albertelli et al. [37] and unpublished studies.

Figure 4. h/mAR mice respond differentially to treatment

Mice were abdominally palpated weekly to track tumor development. The status of each treatment group at 29 weeks of age is shown for intact mice, mice castrated at 12 weeks of age, or mice fed the AR antagonist bicalutamide in chow from the age of 12 weeks. Mice that died prior to 29 weeks are represented by the black portion of the bar, those with a palpable tumor are represented by hatching, and those with no palpable tumor are in white.