ERG oncoprotein expression in prostate carcinoma patients of different ethnicities
- Authors:
- Published online on: September 18, 2014 https://doi.org/10.3892/mco.2014.418
- Pages: 23-30
Abstract
Introduction
Prostate carcinoma (CaP) is the fourth most frequently diagnosed cancer among Malaysian men, preceded by lung, colorectal and nasopharyngeal cancers (1). However, consistent with the global tendency of an increasing median age of patient populations due to increased overall longevity, the incidence of CaP in Malaysia is also on the increase (2). Among the major ethnic groups in Malaysia, the incidence of CaP was found to increase after the age of 45 years and is highest among Malaysian Chinese (MC) compared to Malaysian Indian (MI) and Malay men (1).
There is a clear disparity in CaP incidence and mortality worldwide and the major determining factors are being actively investigated. Although socioeconomic status and access to healthcare are often associated with disparities in the diagnosis, treatment and survival of CaP patients of different ethnic backgrounds, contributing genetic differences have also been identified (3, 4). Some of the tools used to characterize gene alterations and identify potential driver genes include genome-wide association studies, karyotyping of chromosomal copy number, as well as exome and whole-genome sequencing. In addition to the search for underlying genetic events that initiate cancer or distinguish aggressive from indolent tumors, the search for genetic alterations that may explain the ethnic disparities in CaP is currently actively pursued (3, 5, 6).
The variant allele on 8q24, which increases the risk for CaP, particularly in men of African ancestry, is one of the most convincing risk alleles for CaP (7, 8). Another allele associated with an increased risk of CaP men of African ancestry is the rs743572 single-nucleotide polymorphism of CYP17 (9). More recently, evaluations of the transmembrane protease, serine 2 (TMPRSS2)-erythroblast transformation-specific (ETS)-related gene (ERG) fusion in different populations have highlighted the differences in frequency between ethnic groups. Recurrent gene fusions between regulatory sequences of an androgen receptor (AR)-regulated gene, such as TMPRSS2, solute carrier family 45 member 3 or N-myc downstream-regulated gene 1, and an ETS gene family member, such as ERG, ETS translocation variant (ETV)1, 4 and 5 as the 3′; fusion partner, result in androgen-dependent expression of ETS transcription factors. Among these genetic alterations, the TMPRSS2-ERG fusion, detected in 50–70% of CaP patients from Western countries, is the most prevalent (10, 11). The frequency of TMPRSS2-ERG gene fusions detected in CaPs of African Americans (AA) (31–43%) is often lower compared to that of Caucasian Americans (CA) (50–66%) (5, 12). Interestingly, ERG overexpression is more frequently detected in the index tumors of CA (63.3%), compared to those of AA patients (28.6%) (5). However, evaluations of TMPRSS2-ERG gene fusions, either by immunohistochemistry (IHC) detection of ERG expression alone or in combination with fluorescence in situ hybridization (FISH), in different populations worldwide demonstrated lower frequencies compared to that detected CA and Europeans (12–20).
The aberrant overexpression of an ERG oncoprotein as a result of TMPRSS2-ERG fusion exerts a profound effect on cellular pathways associated with cancer initiation and progression (10, 21–24). Evidence of the association between ERG-positive prostatic intraepithelial neoplasia lesions and ERG-positive prostate tumors highlights the significance of ERG activation in the early stages of tumor development (25). ERG overexpression inhibits prostate epithelial differentiation, while promoting epithelial-to-mesenchymal transition (26, 27). In addition, ERG regulates target genes with functions in DNA damage repair, epigenetic silencing and inflammation, which affect pathways associated with tumor cell growth, proliferation and invasion (24). For example, the cooperation of ERG with phosphatase and tensin homolog (PTEN) deletion and activation of AKT has been shown to promote neoplastic transformation (28, 29). A better understanding of how ERG interacts with cancer genes that contribute to cancer progression has led to the development of various treatment strategies that target ERG and its downstream effectors (30). The ability to clearly detect ERG expression in prostate tumors in contrast to normal glands by IHC using specific monoclonal antibodies (MAbs) has improved the diagnosis of the majority of CaPs (25, 31). The high concordance between the evaluations of TMPRSS2-ERG fusion by FISH and ERG protein expression by IHC supports the reliability and accuracy of ERG IHC as a surrogate for FISH detection (25, 31–33). Furthermore, the evaluation of prostate tumors for ERG expression, together with PTEN deletion and integrity of AR signaling pathways, may help the prognostic stratification of patients and the selection of treatment options (34, 35).
To date, no study has evaluated the frequency of ERG alterations in CaP patients in Malaysia, which has a population comprising diverse ethnic groups. The major ethnic groups in Malaysia are Malays (55%), Chinese (24%) and Indians (7.2%) (36). In order to better understand the role of ERG in the etiology of CaP initiation and progression, we used the detection of ERG by IHC as a surrogate for ERG fusion events to evaluate the prevalence of ERG expression in a multiethnic cohort of Malaysian CaP patients.
Materials and methods
Specimens
Transrectal ultrasound (TRUS)-guided biopsies were performed on 120 patients who were diagnosed with CaP based on clinical findings at the Sime Darby Medical Centre, Subang Jaya, Malaysia. The specimens were collected between 2011 and 2013, following approval by an independent Ethics Committee of Sime Darby Healthcare (ethics reference no. 201309.5). The TRUS-guided biopsy entails targeting the suspected prostatic lesion, as well as random sampling of the prostatic gland. Typically, 12–18 biopsy cores were collected from each prostate. Occasionally, 24–36 biopsy cores were collected from patients with significantly larger prostates, or from whom a second biopsy was required. The biopsy specimens were fixed with formalin and embedded in paraffin blocks.
IHC detection for ERG expression
Anti-ERG-MAb 9FY was obtained (cat. no. CM421C; Biocare Medical, Concord, CA, USA). Sections (4-µm) were cut from formalin-fixed paraffin-embedded (FFPE) blocks, mounted on slides and deparaffinized. IHC was performed using a Ventana Benchmark Ultra autostainer (Ventana Medical Systems, Inc., Tucson, AZ, USA) using Ventana reagents. Briefly, the sectioned specimens were processed for antigen retrieval using CC1 antigen retrieval solution prediluted in Tris/borate/EDTA buffer (pH 8.0–8.5) and incubated at 95°C for 48 min. The sections were then put through peroxidase inhibition prior to incubation with ERG-MAb at a dilution of 1:100 for 20 min at room temperature. ERG expression was detected by using OptiView HQ universal Linker and OptiView HRP Multimer (Ventana Medical Systems, Inc.), incubated consecutively at room temperature for 8 min. The color was developed using Bluing reagent for 4 min and the sections were counterstained with hematoxylin. The ERG protein expression status and Gleason scores of the prostate sections were evaluated by a trained pathologist. Depending on the amount and intensity of the ERG IHC staining, the specimens were scored as follows: 0, negative; 1+, mild; 2+, moderate; and 3+, strong staining. Positive staining of endothelial cells in the specimens served as a built-in control for the staining.
Statistical analysis
Statistical analyses were performed using SPSS 16.0 software for Windows (IBM, Inc., New York, NY, USA). The Pearson's Chi-square and Kruskal-Wallis tests were used to determine the statistical associations of ERG expression with ethnicity, age and Gleason sum score. P<0.05 was considered to indicate a statistically significant difference.
Results
Patient demographics and Gleason score of prostate specimens
The ERG oncoprotein expression status was evaluated in tumor specimens from 120 patients by demographic distribution and tumor Gleason scores (Table I). The mean age of the patients was 69 years (range, 52–91 years). MC patients represented the largest ethnic group in this study (68.3%), followed by MI (25%) and Malay patients (6.7%). Both the left and right lobes of the prostate were biopsied in all the patients. Among the 120 patients, 71 were found to have tumors in both lobes, whereas 20 patients had tumors in only the left lobe and 29 had tumors in only the right lobe of the prostate. Specifically, of the 191 tumor sections that were evaluated, 91 were from the left lobe and 100 were from the right lobe of the prostate (Table I and Fig. 1). The ERG expression status for each patient was scored as positive if ERG oncoprotein expression was detected in tumor sections from either lobe, taking into account inter-tumoral heterogeneity within the same prostate (25).
 | Table IDemographics of prostate cancer patients (n=120) and Gleason scores of tumor specimens (n=191). |
Prevalence of ERG expression in different ethnic groups
The evaluation of ERG oncoprotein expression by IHC in the multiethnic cohort of Malaysian CaP patients revealed an overall frequency of 39.2%, with positive ERG expression in 47 of the 120 patients. ERG-positive tumors were detected in 31.4% (60/191) of the individual tumor sections examined. The status and intensity of ERG staining are detailed in the heatmap in Fig. 1 and sections representative for each level of expression are shown in Fig. 2.
Among the MC patients, who formed the majority ethnic group of this study, 27 of 82 cases (32.9%) were ERG-positive (Table II). Prostate tumor sections were evaluated from either the right or left lobe of the prostate in 29 cases and from both lobes in 53 cases (Fig. 1). Positive ERG expression was detected in 35 of the 135 sections (25.9%) examined (Table II). Of the 27 cases positive for ERG expression, ERG was detected in either the right or the left lobe of the prostate in 19 and in both lobes in 8 cases.
| Table IIAssociation of erythroblast transformation-specific-related gene (ERG) oncoprotein expression status with ethnicity, age and Gleason score, as evaluated by patient and by individual tumor sections. |
Surprisingly, 19 of 30 (63.3%) MI patients were positive for ERG expression (Table II). Biopsy specimens were examined from either the right or left lobe of the prostate in 18 and from both lobes in 12 cases. Positive ERG expression was detected in 24 of 42 (57.1%) individual tumor sections (Table II). Of the 19 MI patients with a positive ERG expression status, ERG was detected in either the right or the left lobe of the prostate in 14 patients and in both lobes in 5 cases.
Among the 8 Malay patients evaluated, only 1 (12.5%) was positive for ERG (Table II). Prostate tumor sections from both lobes of the prostate were evaluated for 6 of the 8 Malay patients. Only 1 of the 14 (7.1%) sections examined was positive for ERG expression (Table II).
Analysis of the association of the ERG expression status of patients with age and Gleason score
The association of ERG expression status of patients with age and Gleason score of tumors was evaluated by statistical analysis. The results revealed a positive correlation between positive ERG expression of tumors and younger patients, when evaluated either by patient (P=0.04) or by individual tumor sections (P=0.015; Table II). We also observed a correlation between higher intensity of ERG staining with younger patients as a whole (P=0.032; Table III). The evaluation of the association between ERG expression status and Gleason score, either by patient or by individual tumor sections, did not reveal a significant correlation (Table II).
| Table IIIAssociation of erythroblast transformation-specific-related gene (ERG) oncoprotein staining intensity in the examined sections with ethnicity, age and Gleason score. |
Discussion
In this study, we evaluated the expression of ERG oncoprotein in a multiethnic cohort of patients as a surrogate for the detection of TMPRSS2-ERG fusion events. We examined 191 sections of FFPE prostate tumor specimens isolated by TRUS-guided biopsy from 120 patients. The ethnic distribution of this study cohort, which consisted of 82 MC (68.3%), 30 MI (25.0%) and 8 Malay men (6.7%), is representative of patient enrollment at the hospital where this study was conducted and does not mirror the ethnic distribution of the overall Malaysian population. However, it does represent the overall incidence of CaP diagnosed in the country, with the highest incidence among MC, followed by MI, and the lowest among Malays (1).
The overall frequency of ERG oncoprotein expression in the cohort of Malaysian CaP patients, as determined by IHC, was 39.2%, which was considerably lower compared to the frequency of 50–70% detected in Western countries. The prevalence of ERG among MC, the largest ethnic group analyzed, was 32.9%. Although this frequency of ERG-positive expression in the MC population is marginally higher compared to the frequencies of 15.9–29.7% reported for populations from Korea, Japan and China, it remains within a similar range (12, 14, 15, 18, 20) (Table IV).
| Table IVSummary of the frequency of erythroblast transformation-specific-related gene (ERG) oncoprotein expression status in different populations worldwide. |
Interestingly, we detected a disproportionately higher frequency (63.3%) of ERG-positive tumors among MI patients in this study. This is in comparison to a previous study on Indian CaP patients without prior hormonal treatment from New Delhi, India, in which ERG-positive tumors were detected in 8 of the 30 cases (27%) examined (19). However, the higher prevalence of ERG-positive cases in this study may be attributed to the limitations inherent in a small sample size. Whether the higher prevalence of ERG-positive tumors among MI patients indicates a regional variation where TMPRSS2-ERG fusion contributes more significantly to the progression of the disease compared to other populations of the same ethnicity, requires confirmation by studies on larger populations. Among the three ethnic groups, Malay CaP patients exhibited the lowest frequency (12.5%) of ERG-positive tumors. However, the results obtained from the small sample of Malay patients analyzed in this study require further confirmation in studies involving larger cohorts.
Efforts to identify the correlation of TMPRSS2-ERG fusion or ERG overexpression with clinicopathological characteristics have yielded variable results, which is likely due to the heterogeneity of patient cohorts evaluated in different studies. In certain studies, a higher Gleason score and a lower tumor differentiation exhibited a significant correlation with ERG gene alterations or with ERG-positive immunostaining (25, 37–39). Other studies have reported the association of a lower Gleason score with a higher number of TMPRSS2-ERG fusion events (13, 40). However, other studies have reported a significant association of TMPRSS2-ERG fusion with tumors of higher stage and lymph node metastasis (41) or higher pathological stage (42), but no association between TMPRSS2-ERG fusion and Gleason score. In a comparison between patients of different ethnic backgrounds, Rosen et al (5) reported a correlation between ERG-negative status and high-grade CaP tumors among AA but not among CA patients. There was no significant correlation between Gleason score and ERG expression or intensity when evaluated against either tumor sections or patients in our study. However, a significant association between younger patients (aged <69 years) and a positive ERG expression status, as well as ERG intensity, was observed in our Malaysian cohort as a whole. This correlation was also observed in studies among Japanese and European CaP patients (17, 43), which suggests that ERG rearrangement may be particularly important in patients with early-onset CaP.
The effect of multiple factors, including diet, genetics and environmental factors, may contribute to the significant disparity in the frequency of CaP globally. The TMPRSS2-ERG gene fusion alteration, which is frequent among Western Caucasian populations, has been found to be less frequent among South Asian and East Asian populations. Whether other genomic alteration events typified by the fusion of other ETS gene family members, such as ETV1 and ETV4, to androgen-regulated promoters (10, 23), amplification of the 8q24 loci (44), PTEN deletion (20), or yet to be identified genetic events, are more prevalent in Asian populations remains to be investigated. A more comprehensive study, including a larger number of Malay and MI patients should be undertaken, not only to confirm the frequency of TMPRSS2-ERG fusion events, but to gain better understanding of the underlying genetics of CaP in the Malaysian population.
Acknowledgements
This study was funded by the sponsors of the Cancer Research Initiatives Foundation (CARIF). We are grateful to Mr. Vijaya Kumar T. Krishnan and Ms. Mary Catherine D. Cruz from the Histopathology Laboratory, Sime Darby Medical Centre, for their technical assistance. The Henry M. Jackson Foundation for the Advancement of Military Medicine has filed a patent application on the mouse monoclonal anti-ERG antibody, 9FY, on which S.T., A.D. and S.S. are co-inventors and have been licensed to the Biocare Medical. This study was conducted independently of any involvement from Biocare Medical.
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