Alternatively spliced androgen receptor variants

Abstract

Alternative splicing is an important mechanism for increasing functional diversity from a limited set of genes. Deregulation of this process is common in diverse pathologic conditions. The androgen receptor (AR) is a steroid receptor transcription factor with functions critical for normal male development as well as the growth and survival of normal and cancerous prostate tissue. Studies of AR function in androgen insensitivity syndrome (AIS) and prostate cancer (PCa) have demonstrated loss-of-function AR alterations in AIS and gain-of-function AR alterations in PCa. Over the past two decades, AR gene alterations have been identified in various individuals with AIS, which disrupt normal AR splicing patterns and yield dysfunctional AR protein variants. Recently, altered AR splicing patterns have been identified as a mechanism of PCa progression and resistance to androgen depletion therapy. Several studies have described the synthesis of alternatively spliced transcripts encoding truncated AR isoforms that lack the ligand-binding domain, which is the ultimate target of androgen depletion. Many of these truncated AR isoforms function as constitutively active, ligand-independent transcription factors that can support androgen-independent expression of AR target genes, as well as the androgen-independent growth of PCa cells. In this review, we will summarize the various alternatively spliced AR variants that have been discovered, with a focus on their role and origin in the pathologic conditions of AIS and PCa.

Figure 1

Modular organization of the AR gene, mRNA, and protein. (A) Exon organization of the ~180kb AR gene located on chromosome Xq11-12. (B) Normal splicing of AR exons 1–8 yields a 10.6kb mRNA species with large 5’ and 3’ untranslated regions (UTR) and a 2.7kb open reading frame (ORF). (C) The AR protein consists of a large AR NH₂-terminal domain (NTD) harboring transcriptional activation unit-1 (TAU-1) and TAU-5, a central DNA binding domain (DBD) consisting of two zinc fingers, and a COOH-terminal domain (CTD) harboring the AR ligand binding domain (LBD) and transcriptional activation function-2 (AF-2).

Figure 2

Alternative splicing of AR exon 1B gives rise to the AR45 mRNA isoform. (A) Schematic of the AR gene locus. The location of exon 1B is illustrated in red. (B) The AR45 mRNA is predicted to encode a NH₂-terminally truncated protein with an intact DNA binding domain and COOH-terminal domain.

Figure 3

Mutations and structural alterations in the AR gene disrupt mRNA splicing patterns in androgen insensitivity syndrome (AIS). (A) Schematic of the AR gene locus with intronic point mutations and structural alterations that have been identified in individuals with complete AIS (CAIS) or partial AIS (PAIS). Predominant AR mRNA isoforms expressed in individuals with these specific gene alterations are illustrated in (B).

Figure 4

Alternatively spliced AR isoforms identified in prostate cancer (PCa). (A) Schematic of the AR gene locus with locations of alternatively spliced, cryptic exons illustrated in red. The location of an intragenic tandem duplication identified in 22Rv1 cells is indicated. (B) Alternative splicing of cryptic exons in the AR locus or exon skipping gives rise to COOH-terminally truncated AR mRNA isoforms that encode constitutively active, ligand-independent transcription factors. CE2’ denotes the use of an alternative splice acceptor site for splicing of exon CE2 downstream from exon 3. Additional alternatively-spliced mRNA isoforms which have not been cloned or characterized are not illustrated in this figure, but are discussed in the text.