Heterogeneity of ERG expression in prostate cancer: a large section mapping study of entire prostatectomy specimens from 125 patients

Abstract

Background: TMPRSS2:ERG fusions are frequent in prostate cancer, and occur predominantly in young patients. Several studies had proposed intratumoral heterogeneity of these fusions. This study was designed to determine frequency and extent of ERG fusion heterogeneity in early-onset prostate cancer (EO-PCA, <50 years) and in elderly patients.

Methods: The prostates from 63 EO-PCA and 62 elderly prostate cancer patients were thoroughly reviewed for presence of cancer foci. All 1592 tumor-containing sections were analyzed by immunohistochemistry for ERG expression.

Results: The prostates included in this study contained one tumor focus in 44, two tumor foci in 21, three tumor foci in 32, four tumor foci in 15, and five or more tumor foci in 13 patients. Among 59 cancer foci with ≤3 mm, 19 (32.2 %) were homogeneously ERG positive, 39 66.1 %) were homogeneously ERG negative, and one case (1.7 %) showed a heterogeneous ERG status. The fraction of homogeneously ERG positive cancer foci remained largely constant (14-37 %) with increasing tumor focus diameter but the fraction of heterogeneous ERG findings continuously increased with tumor size and reached 39 % in cancer foci larger than 22 mm. On a patient level, ERG expression was markedly more frequent in EO-PCA than in elderly patients: 13 % of EO-PCA were homogeneously and 62 % were heterogeneously ERG positive. In elderly patients, 3 % of cancers were homogeneously and 57 % were heterogeneously ERG positive (p = 0.0721).

Conclusion: These data show that about 20-30 % of prostate cancer foci have early ERG fusions. ERG fusions further occur in about 50 % of initially ERG negative cancer foci during cancer progression. The vast majority of cancers are heterogeneous for TMPRSS2:ERG fusions on a patient level, challenging the concept of classifying prostate cancer patients into "fusion type" and "non-fusion type" prostate cancer.

Keywords: ERG; Heterogeneity; Prostate cancer.

Fig. 1

Distribution of the tumor focus size (mm) in patients in unifocal (1 focus per patient, n = 44) and multifocal cancers (≥2 foci per patient, n = 273)

Fig. 2

Association between the number of tumor foci and the level of ERG heteogeneity (p = 0.0238) on a patient basis

Fig. 3

ERG heterogeneity in prostatectomies. a Association between the tumor focus size and the level of ERG heterogeneity (p<0.0001). b Example of a prostate with two separate tumor foci marked in red and green color

Fig. 4

Representative images of ERG immunostainings. a Negative ERG immunostaining from a homogeneous ERG negative prostate cancer. The blue arrow indicates positive ERG immunostaining in endothelial cells as a positive control, b positive ERG immunostaining from a homogeneous ERG positive prostate cancer, c positive ERG immunostaining (red circle) and negative ERG immunostaining (green circle) from an intrafocal heterogeneous prostate cancer; the blue arrow indicates positive ERG immunostaining in endothelial cells as a positive control, d false heterogeneity, positive cancer (left), false negative cancer (right), the blue arrow indicates endothelial cells also lacking ERG immunostaining (d)

Fig. 5

Association between the Gleason grade and the level of ERG heterogeneity (p = 0.5694) on a tumor focus basis

Fig. 6

Association between patient age and the level of ERG heterogeneity on the basis of all 125 patients (a) and on a tumor focus basis in all 317 foci (b), as well as in the subsets of tumor foci with Gleason ≤3 + 4 (c) and ≥4 + 3 (d). Chi2 p-value was calculated across all groups (ERG homogenous negative, ERG homogenous positive and ERG heterogeneous positive)

Fig. 7

Representative images of ERG immunostainings. (a–d) Positive ERG immunostaining in non-neoplastic appearing prostate epithelium (a and c) with corresponding H&E staining (b and d). The blue arrow indicates normal prostate epithelium, the green arrow indicates cancer cells. (e) Positive staining in high-grade prostatic intraepithelial neoplasia (HGPIN, red box) and negative staining in prostate cancer (green box). (f) Heterogeneous ERG immunostaining in HGPIN (green asterisk). Red asterisk indicates invasive tumor cells