Enza Mariani

[Danny Hosford]

Hormonaltherapies including androgen deprivation therapy and androgen receptor (AR) pathway inhibitors such as abiraterone and enzalutamide have been widely used to treat advanced prostate cancer. However, treatment resistance emerges after hormonal manipulation in most prostate cancers, and it is attributable to a number of mechanisms, including AR amplification and overexpression, AR mutations, the expression of constitutively active AR variants, intra-tumor androgen synthesis, and promiscuous AR activation by other factors. Although various AR mutations have been reported in prostate cancer, specific AR mutations (L702H, W742L/C, H875Y, F877L, and T878A/S) were frequently identified after treatment resistance emerged. Intriguingly, these hot spot mutations were also revealed to change the binding affinity of ligands including steroids and antiandrogens and potentially result in altered responses to AR pathway inhibitors. Currently, precision medicine utilizing genetic and genomic data to choose suitable treatment for the patient is becoming to play an increasingly important role in clinical practice for prostate cancer management. Since clinical data between AR mutations and the efficacy of AR pathway inhibitors are accumulating, monitoring the AR mutation status is a promising approach for providing precision medicine in prostate cancer, which would be implemented through the development of clinically available testing modalities for AR mutations using liquid biopsy. However, there are few reviews on clinical significance of AR hot spot mutations in prostate cancer. Then, this review summarized the clinical landscape of AR mutations and discussed their potential implication for clinical utilization.

Prostate cancer is one of the most frequently diagnosed cancers in developed countries. Characteristically, prostate cancer depends on androgen receptor (AR) signaling for its carcinogenesis, development, and progression (Basu & Tindall 2010). Meanwhile, androgen deprivation therapy has been a standard treatment for metastatic hormone-sensitive prostate cancer (mHSPC) (Shiota & Eto 2016). However, most prostate cancers eventually progress to castration-resistant prostate cancer (CRPC). Recently, novel AR pathway inhibitors (ARPIs) such as the cytochrome P17 (CYP17) inhibitor abiraterone acetate and second-generation antiandrogens such as enzalutamide, apalutamide, and darolutamide have proven to prolong survival in patients with mHSPC and CRPC (Shiota & Eto 2016, Harada et al. 2021).

Prostate cancer can acquire resistance to hormonal treatments including surgical or medical castration (leuprorelin, goserelin, and degarelix), antiandrogens (bicalutamide, flutamide, enzalutamide, apalutamide, and darolutamide), CYP17 inhibitors (abiraterone), and several steroidal agents (estrogen and glucocorticoid), and aberrant activation of AR signaling plays a crucial role in this process (Shiota et al. 2011a,, 2012). Aberrant activation of the AR signaling pathway in prostate cancer has been attributed to a number of mechanisms, including AR amplification and overexpression, AR mutations, expression of constitutively active AR variants, intra-tumor androgen synthesis, and promiscuous AR activation caused by other factors (Shiota et al. 2011a,b, 2012).

In 1990, Veldscholte et al. at Erasmus University discovered a point mutation (T868A in codon 910 of AR cDNA; this mutation is equivalent to T878A in codon 920 of AR cDNA based on the human reference genome Hg19 and codon numbering was based on the human reference genome Hg19 thereafter) in the ligand-binding domain (LBD) of the AR gene in LNCaP prostate cancer cells and reported that this AR mutant was activated in vitro by androgens as well as progesterone, estrogen, and the antiandrogen cyproterone acetate (Table 1) (Veldscholte et al. 1990). Subsequently, AR mutations such as H875Y and L702H/T878A were found in CWR22 and MDA PCa 2a prostate cancer cell lines, respectively (Tan et al. 1997, Zhao et al. 1999).

In 1992, Newmark et al. at Johns Hopkins University reported the first case of a somatic mutation in the LBD of AR (V731M) among 26 specimens of untreated organ-confined prostate cancer (Table 1) (Newmark et al. 1992). Subsequently, Culig et al. at University of Innsbruck, Suzuki et al. at Chiba University, and others found somatic mutations in the tumor tissues from patient who showed refractory to endocrine treatment, indicating that AR mutations were associated with disease progression and resistant to endocrine therapy (Table 1) (Culig et al. 1993, Suzuki et al. 1993, Gaddipati et al. 1994, Taplin et al. 1995, 1999, Suzuki et al. 1996, Tilley et al. 1996, Marcelli et al. 2000, Tepper et al. 2002).

In addition to treatment resistance, AR mutations such as T878A and W742C/L were reported to be associated with antiandrogen withdrawal syndrome (AWS), suggesting the clinical importance of AR mutations in treatment navigation (Table 1) (Suzuki et al. 1996, Hara et al. 2003). Meanwhile, in the Cancer and Leukemia Group B Study 9663, AR mutations were detected in 5 of 48 CRPC tumors, but there was no association between AR mutations and antiandrogen withdrawal response or survival (Table 1) (Taplin et al. 2003). This null result may be because all AR mutations were analyzed together even though each AR mutation may have a different clinical impact, suggesting the importance of evaluating AR mutations individually. In addition, promiscuous activation of mutated AR by steroids and antiandrogens were shown to contribute cancer cell growth non-cognate ligand (Table 1) (Zhao et al. 2000). Thus, it was demonstrated that AR mutations were closely associated with tumor response to endocrine therapy in prostate cancer.

In the 2000s, NGS was developed, and it allowed the sequences of entire genomes or targeted regions of DNA or RNA to be determined. After the discoveries of AR mutations using traditional techniques such as Sanger sequencing, various studies were performed by NGS using tissues and blood from patients, and they revealed the landscape of gene mutations, in which AR mutations were detected reproducibly (Tolkach & Kristiansen 2019).

In 2012, 159 AR mutations were found in prostate cancer tissues according to the AR gene mutations database, and almost all are single-base substitutions due to somatic mutations rather than germline mutations (Gottlieb et al. 2012). Although a substantial minority of mutations arise in N-terminal domain (NTD) and the fewest in the DNA-binding domain, the majority occur in the LBD (approximately 45%) (Gottlieb et al. 2012). Common mutations in the LBD broaden ligand specificity of the AR, which can be activated not only by androgens but also by non-androgenic steroids and antiandrogens (Gottlieb et al. 2012). Meanwhile, some of AR mutations in NTD such as E252G, E255K, and W435L were reported to increase AR activity through various mechanisms such as protein stability, nuclear translocation, intramolecular interaction, and recruitment of coactivator (Han et al. 2005, Steinkamp et al. 2009).

The scheme for AR (androgen receptor) gene and protein structures. The human AR gene consists of eight exons, which are coding AR protein, including the N-terminal domain (NTD), DNA-binding domain (DBD), and the ligand-binding domain (LBD). Pathogenic mutations were indicated with their position and sample numbers, which was retrieved from cBioPortal ( ) at 30 April 2022. A full color version of this figure is available at -22-0140.

Missense mutations in the LBD of AR can alter the binding affinity of ligands and result in activation by ligands other than testosterone and dihydrotestosterone (DHT). As indicated in Fig. 2A, the residues of hot spot mutation are located close to the binding site of the cognate ligand testosterone (Lallous et al. 2016). Lallous et al. determined the in vitro effects of four antiandrogens (enzalutamide, hydroxyflutamide, bicalutamide, and apalutamide) and four steroids (DHT, progesterone, estradiol, and hydrocortisone) on AR variants with mutations in the LBD (Lallous et al. 2016). Intriguingly, as presented in Table 3, several mutants responded differentially to antiandrogens and steroids (Lallous et al. 2016). In addition, recent studies demonstrated an antagonistic effect of darolutamide on AR mutants (Moilanen et al. 2015, Sugawara et al. 2019, Lallous et al. 2021). Because abiraterone is administered with prednisone and it blocks the catalysis of progesterone into androstenedione, abiraterone may result in increased levels of glucocorticoid and progesterone (Attard et al. 2008). Therefore, the alterations of steroid levels following abiraterone treatment may affect tumor response through aberrant AR signaling caused by AR mutation.

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