RNA splicing and splicing regulator changes in prostate cancer pathology

Abstract

Changes in mRNA splice patterns have been associated with key pathological mechanisms in prostate cancer progression. The androgen receptor (abbreviated AR) transcription factor is a major driver of prostate cancer pathology and activated by androgen steroid hormones. Selection of alternative promoters by the activated AR can critically alter gene function by switching mRNA isoform production, including creating a pro-oncogenic isoform of the normally tumour suppressor gene TSC2. A number of androgen-regulated genes generate alternatively spliced mRNA isoforms, including a prostate-specific splice isoform of ST6GALNAC1 mRNA. ST6GALNAC1 encodes a sialyltransferase that catalyses the synthesis of the cancer-associated sTn antigen important for cell mobility. Genetic rearrangements occurring early in prostate cancer development place ERG oncogene expression under the control of the androgen-regulated TMPRSS2 promoter to hijack cell behaviour. This TMPRSS2-ERG fusion gene shows different patterns of alternative splicing in invasive versus localised prostate cancer. Alternative AR mRNA isoforms play a key role in the generation of prostate cancer drug resistance, by providing a mechanism through which prostate cancer cells can grow in limited serum androgen concentrations. A number of splicing regulator proteins change expression patterns in prostate cancer and may help drive key stages of disease progression. Up-regulation of SRRM4 establishes neuronal splicing patterns in neuroendocrine prostate cancer. The splicing regulators Sam68 and Tra2β increase expression in prostate cancer. The SR protein kinase SRPK1 that modulates the activity of SR proteins is up-regulated in prostate cancer and has already given encouraging results as a potential therapeutic target in mouse models.

Keywords: Abiraterone; Androgen Deprivation Therapy; Androgen Receptor; Prostate Cancer; Prostate Cancer Cell.

Fig. 1

Different kinds of splicing pattern and their effect on prostate cancer cell biology. The most common form of alternative splicing in human cells is shown, with key examples from prostate cancer (not shown, whole introns can also be left in the mRNA)

Fig. 2

Prostate tissue visualised using tissue biopsies. a, b. Histological sections made from benign prostatic hyperplasia (BPH, with normal glandular structure embedded in stroma). Prostate cancer development is clinically described as a series of Gleason grades (1–5, with 1 corresponding to well-differentiated tissue containing a glandular structure and 5 being the most advanced with only few glands still visible) (Gleason and Mellinger ; Mellinger et al. 1967). c, d Histological sections made from prostate cancer (Gleason grade 5, notice breakdown of glandular structure). Left panels, sections processed using H&E staining. Right panels, sections processed by staining with haematoxylin and counterstaining with the RNA-binding protein Sam68 (brown stain). Figure adapted from (Rajan et al. 2008) with permission from the Journal of Pathology

Fig. 3

Transcriptional control by a the full-length androgen receptor and b constitutively active AR isoforms made by splice variants. In (a), testosterone enters the prostate cancer cell and becomes modified to dihydroxytestosterone (DHT) by 5α-reductase. DHT binds to the androgen receptor (AR), displacing heat shock protein 90 (HSP) and resulting in AR translocation into the nucleus. Once inside the nucleus, the AR binds to consensus DNA sequence elements called androgen response elements (AREs) to control target gene expression. In (b), an androgen receptor variant protein (AR-V) lacking the ligand-binding domain is able to directly translocate into the nucleus without binding to DHT, resulting in androgen-independent control of gene expression

Fig. 4

Exon–intron organisation of the AR gene and frequent pathogenic AR mRNA splice isoforms. a The AR protein is encoded by the 8-exon AR gene on the X chromosome. The full-length AR mRNA is made by splicing of exons 1–8. ARv7 mRNA is made by splicing cryptic exon 3 (CE3) after exons 1–3. Splicing of CE3 is associated with transcriptional termination, so this makes a truncated mRNA and truncated AR protein (aberrant splicing pathway shown above the AR gene). ARv567 is made by skipping of exons 5–7 in the AR mRNA (aberrant splicing pathway shown below the AR gene). Exons 1–8 are spliced together to produce the canonical AR splice isoform that encodes a full-length AR protein isoform. Aberrant splicing patterns include splicing of exon 3 to cryptic exon CE3, which is linked to premature termination of transcription (using an intron 3-internal polyA site) and a truncated mRNA, and skipping of exons 5–7. b Both full-length and AR variants are translated from different mRNA isoforms. Full-length AR protein contains an N-terminal domain (important for transcriptional activation) encoded by exon 1, a C4-type zinc finger DNA-binding domain encoded by exons 2–3 and a ligand-binding domain (which binds to androgens) encoded by exons 4–8. ARv7 is lacking the ligand-binding domain. ARv567es is lacking most of the ligand-binding domain