BRCA1-associated protein 1 expression and prognostic role in prostate adenocarcinoma

Prostate cancer (PCa) is diagnosed in 13.5% of male cancer patients worldwide, making it the second most frequently diagnosed cancer in men [1]. PCa usually appears in a localized form, but clinical outcomes vary considerably. Current clinical prognostic factors are very limited in their ability to improve individual triaging of patients to refine prognosis or recurrent risk. Therefore, it is crucial to investigate PCa progression factors and to find novel biomarkers for predicting PCa prognosis.

BRCA1-associated protein 1 (BAP-1), a deubiquitinating protein [2], regulates cell cycles and DNA damage by interacting with the BRCA1/BARD1 tumor-suppressor heterodimer [3]. Also, nuclear-localized BAP-1 functions as an independent inhibitor in cell proliferation and as a regulator of apoptosis [4]. In addition to these growth inhibitory functions, BAP-1 also acts as an activator of cell proliferation. By forming complexes with host cell factor 1 (HCF-1) and through deubiquitination, BAP-1 acts as a cell cycle progression factor at the G1/S transition [5]. Depletion of BAP-1 has even been shown to slow the S phase [3]. Thus, BAP-1 plays dual roles in cell cycle regulation as an activator and deactivator [6].

In recent years, a BAP-1 loss-of-function mutation has been described for several cancers. According to earlier reports, a germline BAP-1 mutation is associated with several hereditary cancer syndromes, including uveal melanoma, malignant pleural mesothelioma, clear cell renal cell carcinoma, and cholangiocarcinoma [7, 8, 9, 10]. Additional studies identified somatic BAP-1 mutations that were also associated with corresponding cases [11, 12, 13]. However, the view of BAP-1 as a prognostic biomarker is controversial. Patients with BAP-1 loss-of-function mutations have shorter overall survival in clear cell renal cell carcinoma and uveal melanoma [14]. In contrast, malignant pleural mesothelioma patients with high BAP-1 expression have shorter overall survival [15]. Thus, the prognostic role of BAP-1 depends on cancer type.

A previous study showed that immunohistochemical staining for BAP-1 protein was a good surrogate marker for locating BAP-1 mutations [16]. Thus, BAP-1 protein analysis by immunohistochemistry (IHC) might be useful for prognostic evaluation during routine work-up. Here, we evaluated BAP-1 expression in PCa and estimated the correlations between BAP-1 and clinicopathologic parameters, as well as disease-free survival.

Ubiquitin attaches to proteins and causes post-translational modification that either alters target protein function or suppresses protein expression. Deubiquitinase removes ubiquitin via hydrolysis. Depending on its target, the function of deubiquitinase on cellular regulation could differ. When tumor suppressors are deubiquitinated, their expression is preserved, preventing uncontrolled cellular proliferation. In this case, deubiquitinase acts as a tumor suppressor. Overall, the role of BAP-1, a nuclear-localized deubiquitinating protein [2], could differ according to target protein.

Earlier studies reported that BAP-1 binds to the RING finger motif of BRCA1 and enhances the BRCA1-mediated cellular growth suppressor function by deubiquitination [2]. By serving as a regulator of the BRCA1 pathway, BAP-1 acts as a tumor suppressor.

However, several other studies reported that BAP-1 participates in cell proliferation. BAP-1 depletion by RNAi inhibits proliferation of breast cancer cells (MCF-7 and T-47D). Furthermore, previous research argued that the interaction between BAP-1 and HCF-1 is critical for BAP-1-mediated cell growth. HCF-1, a cell cycle modulator, is a heterodimer composed of HCF-1N and HCF-1C. In particular, BAP-1 regulates cell proliferation by deubiquitinating HCF-1N, which promotes G1-phase progression and S-phase entry [3, 21]. Another study revealed that BAP-1 loss downregulates the downstream targets of E2F, an important cell cycle progression factor, including cyclin A2, E2F1, p107, and CDC25C [11]. Taken together, these findings indicate that BAP-1 may play a dual role in cellular growth, both preventing excessive growth and ensuring appropriate cell growth.

BAP-1 status can be evaluated by three methods. First, genomic hybridization can inspect for deletions in the BAP-1 gene locus at 3p21. Second, the BAP-1 gene can be sequenced to identify any mutations. Third, IHC can detect BAP-1 proteins [20]. Evaluating BAP-1 protein expression via IHC is considered the superior method for evaluating BAP-1's functional status, rather than evaluating deletions or mutations in coding lesions [16]. Moreover, because most BAP-1 mutations cause rapid degradation of proteins or reduce nuclear localization, BAP-1 mutations are highly associated with negative immunohistochemical staining [22].

To our knowledge, this is the first study to provide evidence of correlations between BAP-1 expression and clinicopathologic factors in PCa cases using IHC. In our study, PCa cases with positive BAP-1 expression had significantly higher T-stage and higher PSA level. Consistently, PCa patients with positive BAP-1 expression had shorter disease-free survival than did patients with negative BAP-1 expression. Also, high preoperative PSA level, high grade group, and lymphovascular invasion were significantly associated with shorter disease-free survival. However, a preexisting staging score, including T-stage, extraprostatic extension, and seminal vesicle invasion, were not correlated with disease-free survival. This result could be explained by the relatively small number of cases. Despite our limited sample size, our study adds to the literature suggesting BAP-1 immunoexpression as a potential prognostic biomarker.

Although BAP-1 has been discussed in a broad spectrum of cancers, consensus has not been reached for it prognostic role. BAP-1 loss is associated with poor prognosis in clear cell renal cell carcinoma and uveal melanoma [14]. In contrast, however, BAP-1 loss was found to have a protective role in malignant mesothelioma, which is consistent with our results for PCa [15]. These conflicting results support the view that BAP-1 may have many yet unknown biological functions, and that its activity might have a tissue-specific pattern. Especially in PCa cases, the BAP-1 gene may play an oncogenic role rather than a generally well-known tumor-suppressor role.

Since BAP-1 functions as a deubiquitinating enzyme, it can act as either an oncogene or a tumor-suppressor gene, depending on the target protein for deubiquitination. As previously mentioned, cell cycle proliferation pathways such as the HCF-1 molecular pathway and E2F related proteins would be affected by BAP-1 oncogenic action [3, 11]. Other molecular pathways associated with BAP-1 need to be clarified in further studies.

BAP-1 was initially discovered as a BRCA1-associated protein [2], and many studies are now attempting to identify the association between BAP-1 and BRCA1, especially the pathways associated with histone H2A, heterochromatin, and DNA damage response [23]. A study by Omari et al. [24] reported that BRCA1 gene loss along with BRCA1 gene gain is associated with ALDH1 expression, which is a stem cell marker. This indicates that BRCA1 gene gain is associated with PCa dedifferentiation. The poor prognosis among patients with positive BAP-1 expression might be explained by BRCA1 gene gain affecting dedifferentiation. Also, the cooperative activity between BRCA1 and BAP-1 could be a future drug target.

Our study limitations included our relatively small cohort study size and the lack of genomic or mutational testing in our analyses. Despite these limitations, ours is the first study to correlate BAP-1 immunoexpression with clinicopathologic factors in PCa, and we revealed that BAP-1 is a potential prognostic marker for PCa. Larger-scaled and molecular studies will further enhance the validity of our research, and they could also serve to elucidate new therapeutic strategies. Finally, IHC for BAP-1 may be used for preoperative biopsy specimens for PCa treatment planning.

The purpose of our study was to determine whether BAP-1 IHC can stratify PCa prognoses in a clinically useful way. In our dataset, BAP-1 immunoexpression predicted poor prognosis in PCa patients. Analyzing BAP-1 expression by IHC may be a valuable way to stratify more aggressive PCa cases from less aggressive cases. The precise role of BAP-1 in PCa deserves further investigation and validation in future studies.