Bad neighbours: hypoxia and genomic instability in prostate cancer

Abstract

Prostate cancer (PCa) is a clinically heterogeneous disease and has poor patient outcome when tumours progress to castration-resistant and metastatic states. Understanding the mechanistic basis for transition to late stage aggressive disease is vital for both assigning patient risk status in the localised setting and also identifying novel treatment strategies to prevent progression. Subregions of intratumoral hypoxia are found in all solid tumours and are associated with many biologic drivers of tumour progression. Crucially, more recent findings show the co-presence of hypoxia and genomic instability can confer a uniquely adverse prognosis in localised PCa patients. In-depth informatic and functional studies suggests a role for hypoxia in co-operating with oncogenic drivers (e.g. loss of PTEN) and suppressing DNA repair capacity to alter clonal evolution due to an aggressive mutator phenotype. More specifically, hypoxic suppression of homologous recombination represents a "contextual lethal" vulnerability in hypoxic prostate tumours which could extend the application of existing DNA repair targeting agents such as poly-ADP ribose polymerase inhibitors. Further investigation is now required to assess this relationship on the background of existing genomic alterations relevant to PCa, and also characterise the role of hypoxia in driving early metastatic spread. On this basis, PCa patients with hypoxic tumours can be better stratified into risk categories and treated with appropriate therapies to prevent progression.

Figure 1.

Role of hypoxia in the clinical course of PCa: PCa can be broadly classified into curable castrate sensitive and incurable castrate resistant disease. Patients with curable tumours can be sub stratified into low, intermediate and high risk of progression to a more aggressive disease state, usually according to readouts such as histological-based GS and measurement of serum-based PSA. Hypoxia is prognostic in localised patients, attenuates local treatment strategies, drives androgen independence and can promote metastatic spread. Hypoxia therefore plays a key role in PCa disease progression. GS, Gleason Score; PCa, prostate cancer; PSA, prostate-specific antigen.

Figure 2.

Cellular responses to hypoxia: Under oxic conditions PHD2 adds hydroxyl groups to HIF1α. This facilitates E3-dependent ubiquitination (not shown) and subsequent proteasomal degradation. Hypoxia inhibits PHD2-mediated hydroxylation, allowing HIF1α to dimerise with HIF1β, translocate to the nucleus and mediate transcription of target genes. Hypoxia also induces the unfolded protein response. Under hypoxia the ER has reduced capacity to mediate protein maturation. The ER chaperone BiP binds misfolded proteins in the ER and releases the luminal domain of PERK, facilitating PERK auto-phosphorylation and activation. Subsequent phosphorylation of eIF2α attenuates its role in global translation initiation and leads to activation of the ATF4 transcription factor. Collectively, these hypoxic responses can suppress transcription and translation of components of multiple DNA repair pathways, including HR, MMR and BER. BER, base excision repair; eIF2α, eukaryotic initiation factor 2α; ER, endoplasmic reticulum; HIF1α, hypoxia inducible factor 1α; HR, homologous recombination; MMR, mismatch repair; PERK, PKR-like ER kinase; PHD2, proline hydroxylase D2.

Figure 3.

Synthetic and contextual lethality: Endogenous and exogenous stress induce both SSBs/DSBs. Inhibitors of PARP prevent repair of SSBs, which are further processed to DSBs if encountered at a replication fork. Tumours with HR mutations such as BRCA2 (synthetic lethal), or cancer cells which have disseminated away from the nearest blood vessel and exhibit hypoxic suppression of HR proteins (contextual lethal) are unable to efficiently repair DSBs using HR. This leads to DSB accumulation, mitotic catastrophe and selective tumour cell killing. DSB, double stranded breaks; HR, homologous recombination; PARP, poly ADP ribose polymerase; SSB, single stranded breaks

Figure 4.

HYPROGEN: Illuminating the genomic landscape of hypoxia-driven early metastatic prostate cancer: HYPROGEN is an exploratory biomarker driven study which will investigate hypoxia driven genomic instability in treatment naïve metastatic PCa patients. Patients will be treated with oral PIMO and samples will be taken from the primary tumour and its’ associated metastases. The trial will yield new ex-vivo models including organoids, circulating tumour cells and patient derived xenografts, as well as genomic data from untreated bone/lymph metastases and circulating tumour DNA. PCa, prostate cancer; PIMO, pimonidazole.

Figure 5.

Assays and treatment for hypoxia mediated aggression: (a) Clinical outcome studies following surgery or radiotherapy for localised prostate cancer have shown that the patients whose prostate cancers acquire both high PGA and increased hypoxia have adverse outcomes when compared to patients who have only one or none of these two biological states. (b) Intraprostatic hypoxia subregions can be visualised in-situ using intrinsic (e.g. staining for HIF-1, GLUT-1) or extrinsic (e.g. pimonidazole binding) biomarkers, or metabolic imaging techniques such as OE-MRI and CSI-MRI. To date, imaging with PET hypoxic tracers (e.g. PET-FAZA or PET-MISO) has been less successful. Ascertainment of genetic instability or DNA repair deficiencies can be accomplished using genome sequencing techniques for CNAs or single nucleotide mutations. In-situ staining showing reduced DNA repair protein expression (e.g. reduced RAD51) relative to hypoxic staining may be a biomarker of hypoxia-mediated DNA repair deficiencies. Treatments to target hypoxia-mediated aggressive biology and improve cure in localised prostate cancer with surgery or radiotherapy includes the use of direct hypoxic cell toxins (e.g. evofosfamide and OCT1002) or radiosensitisation during radiotherapy using radiosensitisers (e.g. nimorazole, an oxygen mimetic) or increasing tumour oxygen content (e.g. carbogen and nicotinamide). DNA repair-deficient or cell cycle checkpoint-deficient hypoxic tumours can be targeted with PARP or ATR inhibition. Additionally, androgen deprivation has been shown to increase oxygen content and decrease DNA repair in prostate cancer. (c) These treatments can be used in neoadjuvant or concurrent settings to clear resistant and genetically unstable hypoxic cells within the primary tumour in combination with radiotherapy or surgery. Adjuvant treatments can improve outcomes by targeting occult metastatic disease and therefore prevent the outgrowth of lethal CRPC and NEPC metastases. Note concurrent use of DNA repair inhibitors with radiotherapy is generally too toxic to normal tissues and therefore neoadjuvant or adjuvant use of these agents may be preferred. CNA, copy number alteration; CRPC, castrate-resistant prostate cancer; CS-MRI, chemical shift MRI; NEPC, neuroendocrine prostate cancer; OE-MRI, oxygen-enhanced MRI; PET, positron emmision tomography; PGA, percent genome alteration.