The prognostic value of SUMO1/Sentrin specific peptidase 1 (SENP1) in prostate cancer is limited to ERG-fusion positive tumors lacking PTEN deletion

Immunohistochemically detectable SENP1 expression was found in about 35 % of prostate cancers in our study. This frequency is lower than what has been observed in two earlier IHC studies, reporting positive SENP1 staining in 76.5 % of 115 [16] and high SENP1 expression in 47 % of 117 [17] analyzed prostate cancers from Asian patients. These earlier studies also analyzed tissue microarrays. Although both previous studies utilized a slightly larger core diameter (1 mm) than in our study (0.6 mm), it seems unlikely that the lower fraction of SENP1 positive cancers in our study was caused by sampling bias due to this small difference in core diameter. Rather, different antibodies, immunohistochemistry protocols, and scoring criteria might have contributed to the slightly variable results between these studies. Given the paramount impact of IHC protocols on the positivity rates in TMA studies [18] we would not view our data as strong evidence in favor of possible ethnical differences in SENP1 expression in prostate cancers.

Our analysis revealed weak cytoplasmic SENP1 staining in secretory cells of normal prostate epithelium, while more intense cytoplasmic and nuclear staining was rare and occurred in only about 10 % of normal tissues. Finding a markedly higher fraction of cytoplasmic/nuclear SENP1 staining in cancer as compared to normal prostate suggests that SENP1 becomes upregulated in a fraction of tumors. Comparable to our observation, Li et al. [16] reported a gradual increase of SENP1 positivity from normal prostate (4.2 %) to prostatic intraepithelial neoplasia (PIN, 57.9 %) and cancer (76.5 %). SENP1 expression was significantly linked to adverse tumor features including advanced stage, high Gleason grade, and presence of lymph node metastases, preoperative PSA levels, and early biochemical recurrence in our analysis. These findings are in line with earlier studies in prostate cancer reporting significant associations with advanced and high-grade cancers as well as poor prognosis in Asian patients [16, 17]. Similar results have also been observed in analyses of other solid cancer types, including cancers of the colon [11], bladder [12], head & neck [13], and lung [14], where SENP1 overexpression was consistently linked to advanced and high-grade cancers and in some studies also with adverse clinical outcome [11, 13]. A relevant tumor biological role of SENP1 is also supported by our observation that SENP1 expression was linked to increased cell proliferation. Known biological functions of SENP1 are consistent with a role in cancer development and progression. SENP1 activity affects the homeostasis of post-transcriptional SUMO modification of various target proteins required for normal cell physiology. While both loss of SUMO conjugation as well as excessive SUMOylation results in embryonic lethality [28, 29], more subtle changes of the SUMOylation machinery lead to deregulation of multiple cellular pathways including those with relevance for cell proliferation and differentiation [10]. Genes and pathways known to be targeted by SENP1 include histone deacetylases [7], c-Jun- and ERK-dependent transcription [30, 31], cyclin D1 activity [32], Pi3K/AKT signaling pathway [33, 34], and HIF1α-dependent angiogenesis [29].

The high number of tumors in our TMA enabled us to profoundly evaluate SENP1 in the context of key genomic alterations of prostate cancer. Gene fusions involving the androgen-regulated serine protease TMPRSS2 and ERG, a member of the ETS family of transcription factors, occur in about 50 % of prostate cancers and result in strong AR-driven ERG protein overexpression [35, 36] and massive transcriptional changes [37‐40]. The increased SENP1 expression levels in ERG positive cancers detected by two independent approaches (i.e. ERG-IHC and -FISH) in our study apparently reflects the AR dependency of both SENP1 and ERG, since SENP1 functions both as a transcriptional target as well as an inducer of AR expression in a positive feedback loop [32, 41].

Further subgroup analyses targeted highly recurrent chromosomal deletions that are tightly linked to the ERG status and that may delineate important molecular subgroups within ERG positive and ERG negative cancers. For example, 3p13 and PTEN deletions are linked to ERG positivity and deletions at 5q21 and 6q15 to ERG negativity and all these deletions have high prognostic impact within these subgroups [23‐25, 42‐44]. This analysis revealed that SENP1 expression was not only linked to a positive ERG status but to an even stronger extent to PTEN deletions. The classical function of PTEN involves control of the PI3K/AKT signaling pathway by antagonizing PI3K activity [45]. A functional relationship of PTEN and SENP1 is conceivable because SENP1 induced SUMOylation is known to occur and to have biological impact in the PTEN/PI3K/AKT signaling pathway [33, 34]. Comparison of large enough molecularly defined subgroups with clinical data is one approach to further interrogate functional interrelationships “in vivo”. The complete lack of a difference in clinical outcome between PTEN deleted cancers with and without SENP1 expression argues against a clinically relevant cooperative effect of reduced PTEN function and SENP1 activation. The very strong association between SENP1 overexpression and PTEN would, however, be consistent with models suggesting a role of SENP1 activation for development of PTEN deletions. This could be driven by the effect of SENP1 on histone modification and its impact on the epigenetic machinery. Both histone configuration and epigenetic events are thought to predispose to the development of specific genomic alterations including deletions [46‐48]. In such a scenario, the additional PTEN deletion would result in such a strong disruption of cancer cell physiology that SENP1 expression no longer has a critical additional effect on tumor aggressiveness.

The overall prognostic impact of SENP1 expression was – although statistically highly significant - rather small in absolute numbers. Several models for multivariate analyses were used in this study in order to - as much as possible - model the application of prognostic features in pre- and postoperative scenarios. Unfortunately, in the real world, prognostic molecular features can hardly be analyzed on preoperative biopsies because these are typically distributed among many different pathology laboratories, and even if they were available for analyses such precious collection of tissues would be used up after only a few studies. The application of multivariate models revealed that SENP1 largely lacked independent prognostic value if all tumors and the classical molecular subgroups of ERG positive and ERG negative cancers were analyzed. However, our subgroup analyses demonstrated that the significant impact of SENP1 expression on outcome was entirely driven by the subgroup of ERG positive PTEN non-deleted cancers. Accordingly, independent prognostic relevance was seen for SENP1 expression in this particular subgroup. In earlier studies, we have identified other molecular markers that seemed to exert their prognostic impact only in specific molecularly defined subgroups such as in ERG positive and PTEN deleted cancers [49, 50], ERG negative cancers lacking PTEN deletion [51], ERG positive cancers [25], ERG negative cancers [52], cancers lacking PTEN deletion [53, 54], or in all cancers irrespective of ERG and PTEN status [55].

The frequent finding of subtype specific prognostic features challenges the concept of molecular classifiers that apply to all prostate cancers. For example, several multiparametric prognostic tests were recently suggested in prostate cancer [56‐59] and several tests are now commercially available to patients [60, 61]. It might be interesting to see, how these tests perform in molecularly defined prostate cancer subgroups.

With SENP1 being one of the most important de-SUMOylating enzymes, it has been hypothesized that targeting SENP1 with inhibitory drugs may restore the balance of the SUMO modification system [10], and several experimental SENP1 specific inhibitors have been successfully designed as to yet [8, 62‐64]. Such inhibitors may even cooperate with other treatment modalities that are commonly used in prostate cancer. Recently, Wang et al. used RNAi for depletion of SENP1 in lung cancer cell lines and found that inhibition of SENP1 markedly enhanced the radiosensitivity of lung carcinoma by promoting irradiation-induced cell cycle arrest, γ-H2AX expression and apoptosis [14]. Although clinical studies are so far lacking, these first attempts emphasize the potential druggability of SENP1 in human cancers. Given that prostate cancer is characterized by AR-driven SENP1 expression, it is possible that drugs targeting SENP1, possibly in combination with anti-androgenic therapy, will also be effective in prostate cancer.