The pathogenesis of atypical proliferative Brenner tumor: an immunohistochemical and molecular genetic analysis

Abstract

Brenner tumors are ovarian tumors, usually benign, containing epithelium that resembles transitional epithelium. As with other epithelial tumors there exist frankly malignant tumors and tumors that display greater proliferation than the benign Brenner tumors but lack destructive infiltrative growth, and these have been designated 'atypical proliferative' (borderline) Brenner tumors. There have been no well-documented cases of atypical proliferative Brenner tumors that have exhibited malignant behavior. Based on shared morphologic features it is generally believed that atypical proliferative Brenner tumors develop from benign Brenner tumors. The aim of the present study was to confirm this impression by investigating the immunohistochemical and molecular genetic features of benign and atypical proliferative Brenner tumors. Immunohistochemical staining for p16, fluorescence in-situ hybridization (FISH) for CDKN2A (p16-encoding gene) and mutational analysis of the genes commonly mutated in ovarian tumors were performed. p16 immunostaining was positive in the epithelial component of 12 (92%) of 13 benign Brenner tumors, but completely negative in all 7 atypical proliferative Brenner tumors. FISH identified homozygous deletion of CDKN2A in the epithelial component of all atypical proliferative Brenner tumors, but CDKN2A was retained in all benign Brenner tumors. Two PIK3CA somatic mutations were detected in the stromal component in 1 (5%) of 20 Brenner tumors and 3 somatic mutations (1 in KRAS and 2 in PIK3CA) were identified in the atypical epithelial component of 2 (29%) of 7 atypical proliferative Brenner tumors. In summary, our findings suggest that loss of CDKN2A and, to a lesser extent, KRAS and PIK3CA somatic mutations have a role in the progression of a benign to an atypical proliferative Brenner tumor.

Figure 1

Representative images of histological appearance (hematoxylin–eosin, HE), p16 immunostaining, and fluorescence in-situ hybridization for CDKN2A of benign Brenner tumor (BT) and atypical proliferative Brenner tumor (APBT). Benign BT shows intense and diffuse epithelial p16 positivity, both cytoplasmic and nuclear, and normal copy number (either 1 or 2) for CDKN2A (red) as compared with centromere region (CEP9, green) in both epithelial cells (filled arrow) and stromal cells (empty arrow). APBT is consistently negative for p16 and shows complete loss of CDKN2A (red) and retention of centromere 9 (CEP9, green) in the majority of epithelial cells (filled arrow) as compared with stromal cells, where both signals are identified in normal number (empty arrow). Higher magnification in the insets.

Figure 2

Histological appearance (hematoxylin–eosin, HE), p16 immunostaining, and fluorescence in-situ hybridization for CDKN2A of benign Brenner component in the context of atypical proliferative Brenner tumor. In the upper panels, the benign Brenner tumor component does not express p16, and shows complete loss of CDKN2A signals (red) and retention of centromere 9 signals (CEP9, green) in the majority of epithelial cells (filled arrow) as compared with stromal cells, where both signals are identified in normal number (empty arrow). In the lower panels, the benign Brenner tumor component associated with the atypical proliferative Brenner tumor shows focal p16 positivity, limited to one epithelial nest, retention of CDKN2A signals (red), and centromere signals (green; empty arrow), and adjacent p16 negative atypical proliferative Brenner tumor component, with loss of CDKN2A signals (red; filled arrow). Higher magnification in the insets.

Figure 3

Chromatograms and annotations show PIK3CA somatic mutations in the stromal component (below) of one benign Brenner tumor (BT11) compared with the wild-type sequences of the epithelial component (above). Chromatograms and annotations of the three somatic mutations identified in the epithelial component (below) of two atypical proliferative Brenner tumors (APBT2 and APBT6), two in PIK3CA, and one in KRAS, compared with the wild-type sequence of the stromal component (above).