PTEN loss is associated with upgrading of prostate cancer from biopsy to radical prostatectomy

Abstract

When distinguishing between indolent and potentially harmful prostate cancers, the Gleason score is the most important variable, but may be inaccurate in biopsies due to tumor under-sampling. This study investigated whether a molecular feature, PTEN protein loss, could help identify which Gleason score 6 tumors on biopsy are likely to be upgraded at radical prostatectomy. Seventy one patients with Gleason score 6 tumors on biopsy upgraded to Gleason score 7 or higher at prostatectomy (cases) were compared with 103 patients with Gleason score 6 on both biopsy and prostatectomy (controls). A validated immunohistochemical assay for PTEN was performed, followed by fluorescence in situ hybridization (FISH) to detect PTEN gene deletion in a subset. PTEN protein loss and clinical-pathologic variables were assessed by logistic regression. Upgraded patients were older than controls (61.8 vs 59.3 years), had higher pre-operative PSA levels (6.5 vs 5.3 ng/ml) and a higher fraction of involved cores (0.42 vs 0.36). PTEN loss by immunohistochemistry was found in 18% (13/71) of upgraded cases compared with 7% (7/103) of controls (P=0.02). Comparison between PTEN immunohistochemistry and PTEN FISH showed the assays were highly concordant, with 97% (65/67) of evaluated biopsies with intact PTEN protein lacking PTEN gene deletion, and 81% (13/16) of the biopsies with PTEN protein loss showing homozygous PTEN gene deletion. Tumors with PTEN protein loss were more likely to be upgraded at radical prostatectomy than those without loss, even after adjusting for age, preoperative PSA, clinical stage and race (odds ratio=3.04 (1.08-8.55; P=0.035)). PTEN loss in Gleason score 6 biopsies identifies a subset of prostate tumors at increased risk of upgrading at radical prostatectomy. These data provide evidence that a genetic event can improve Gleason score accuracy and highlight a path toward the clinical use of molecular markers to augment pathologic grading.

Figure 1. Gleason Score 6 prostate biopsies that are upgraded to Gleason Score 7 at radical prostatectomy show PTEN protein loss by immunohistochemistry

(A) Low power (200× magnification) photomicrograph of PTEN immunohistochemistry in a Gleason score 6 needle biopsy specimen demonstrates PTEN protein loss in tumor glands (arrow) with preservation of PTEN staining in intermingled benign glands (arrowheads). (B) High power (630x) image highlights infiltrating tumor glands with PTEN loss and surrounding benign glands with intact staining. Note the presence of apical membrane staining in glands with cytoplasmic PTEN loss (arrowhead). The biological significance of this membrane staining remains unclear. (C) Low power (200× magnification) photomicrograph of tumor with heterogeneous PTEN protein loss, demonstrating PTEN protein loss in some tumor glands (arrow) and intact PTEN protein in adjacent tumor (arrowhead). (D) High power (630x) image showing tumor glands with and without PTEN protein loss. As in B, note the presence of apical membrane staining in glands with cytoplasmic PTEN loss (arrowhead). The biological significance of this membrane staining remains unclear.

Figure 2. PTEN FISH results are highly concordant with PTEN immunohistochemistry

(A) PTEN immunohistochemistry image (200× magnification) demonstrating PTEN protein loss in tumor glands (arrow) and PTEN protein retention in nearby benign glands (arrowhead) (B) PTEN FISH image captured from the tumor in (A) demonstrates glands with homozygous PTEN gene loss (yellow signal; arrow and upper right panel) intermixed with glands with PTEN intact (yellow signal; arrowhead, lower right panel). Peri-centromeric control probes (red) as well as flanking gene probes WAPAL (green) and FAS (aqua) are intact in all cells. (C) PTEN immunohistochemistry image (200× magnification) from a separate Gleason score 6 biopsy that was not upgraded at radical prostatectomy demonstrating focal loss of cytoplasmic PTEN protein in tumor glands (arrow). Adjacent tumor glands stain positively. (D) PTEN FISH image captured from tumor in (C) demonstrates lack of PTEN deletion in tumor cells throughout (inset). (E) PTEN immunohistochemistry image (100x) demonstrates PTEN protein loss in tumor glands (red outline) with PTEN protein retention in adjacent benign glands (blue outline). (F) Higher power (630x) image of boxed area from (E). Note the presence of apical membrane staining in glands with cytoplasmic PTEN loss (arrowhead). The biological significance of this membrane staining remains unclear in the context of homozygous deletion of the PTEN gene. (G) PTEN FISH image captured from region delineated in (E) demonstrates intermixed glands with hemizygous PTEN, WAPAL and FAS deletion (arrow, inset) and glands lacking PTEN deletion (arrowhead).