Abstract P5-08-21: Expression of EZH2 and its downstream effectors pEZH2 and H3K27 in pregnancy associated breast cancer (PABC)
Authors: Blanco LZ , Parini V , Dhamne SA , Siziopikou KP 
Published: May 2016

Background: Pregnancy associated breast cancer (PABC) arises during or after pregnancy, is typically triple negative, and is associated with a poor prognosis. Enhancer of Zeste Homolog 2 (EZH2), a core protein of the polycomb-repressive complex, plays a vital role in the epigenetic maintenance of histone H3 lysine (H3K27) repressive chromatin mark and has been found to be aberrantly expressed in various cancers including the breast. EZH2 is involved in stem cell renewal and is involved the initiation, progression and maintenance of cancer cells. EZH2 has been reported to be highly expressed in triple negative breast cancers and to be associated with poor prognosis. In this study, we assessed expression of EZH2 and its downstream markers phosphorylated EZH2 (pEZH2) and H3K27 and correlated their expression with clinicopathologic characteristics in this aggressive type of breast carcinoma.

Design: 23 patients diagnosed with PABC within 2 years of pregnancy (mean age=35.8, range=26-48) and control age-/stage-matched nulliparous women (mean age=37.5, range=29-51) were evaluated. Slides were reviewed and pathologic tumor characteristics were noted. Immunohistochemical stains for EZH2, pEZH2, H3K27 and the cancer stem cell marker ALDH1 were performed on 23 PABC and 15 control cases. Extent (1=1-25% positive tumor cells, 2=26-50%, 3=51-75% or 4=76-100%) and intensity (1=weak, 2=moderate or 3=strong) of staining were assessed. A composite score (CS) was calculated by multiplying the extent by intensity (range=0-9; weak=1-3; moderate=4-6 and strong=7-9).

Results: PABCs were more likely than controls to have moderate to strong immunoreactivity for EZH2 (69.6% vs. 33.3%, p=0.04). All PABC and control cases expressed EZH2 and pEZH2, while 60.9% of PABC and 40% of control cases expressed H3K27. Within the PABC group, moderate to strong EZH2 expression was seen more frequently in grade 3 tumors (86.7% in grade 3 tumors vs. 37.5% in grades 1-2, p=0.03). Of interest, all triple negative (TN) PABC cases had moderate to strong EZH2 (100% vs. 43.8%, p=0.02). All five PABC cases with the strongest EZH2 (CS=9) were grade 3 tumors (3 TN, 1 HER2+ and 1 luminal B), 60% of which had positive lymph node metastasis. Although 83.3% of ALDH1+ cases also had moderate to strong EZH2, no significant correlation was observed.

Conclusions: 1. PABCs express EZH2 and pEZH2 consistently. 2. PABCs are more likely than controls to have moderate to strong immunoreactivity for EZH2. 3. Within the PABC group, all triple negative tumors and the majority of grade 3 tumors have high EZH2 expression. 4. PABC cases with the strongest EZH2 are grade 3 tumors that have positive lymph nodes. Increased expression of EZH2 in PABC may contribute to the poor prognosis in these cases. Our findings add to the understanding of the molecular pathways that operate in different subtypes of breast carcinomas.

Citation Format: Blanco Jr LZ, Parini V, Dhamne SA, Siziopikou KP. Expression of EZH2 and its downstream effectors pEZH2 and H3K27 in pregnancy associated breast cancer (PABC). [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P5-08-21.