Tumour and cellular distribution of activated forms of PR in breast cancers: a novel immunohistochemical analysis of a large clinical cohort

Abstract

Background: The progesterone receptor (PR) is expressed by ∼70% of early breast tumours and is implicated in the progression of breast cancer. In cancerous tissues PR may be activated in the absence of a ligand, or when ligand concentrations are very low, resulting in aberrantly activated PR (APR). The presence of APR may indicate that patients with breast cancer are more likely to respond to antiprogestins. The aims of this study were to describe and classify the histological subnuclear morphology of active and inactive PR in archival breast cancer samples.

Methods: Archived tumour specimens from 801 women with invasive breast cancer were collected. Tissue samples (n=789) were analysed for PR isoforms A and B (PRA and PRB), Ki67 and estrogen receptors (ERα) status, using immunohistochemistry. Medical records were used to determine human epidermal growth factor 2 (HER2) status, tumour stage and grade.

Results: A total of 79% of tumours stained positive for either PRA or PRB, and of these 25% of PRA-positive and 23% of PRB-positive tumours had PR present in the activated form. APRA was associated with higher tumour grade (p=0.001). APRB was associated with a higher tumour grade (p=0.046) and a trend for a more advanced stage. Patients with PR-positive tumours treated with antiestrogens had better disease-free survival (DFS) than those with PR-negative tumours (p<0.0001). Cumulative progression rate and DFS were similar irrespective of APR status. Both APRA and APRB were independent of HER2, ERα and Ki67 expression.

Conclusions: APR had a binary mode of expression in the breast cancer specimens tested, allowing separation into two tumour subsets. APR is an independent target at the cellular and tumour level and may therefore be a suitable predictive marker for antiprogestins, such as onapristone. Using the described technique, a companion diagnostic is under development to identify APR in solid tumours.

Keywords: activated; breast cancer; onapristone; progesterone receptor.

Figure 1

Progesterone receptor (PR)A and PRB isoform receptor domains and post-translational modifications. PRs exist as either A (97 kDa) or B (120 kDa) translated from the same gene by the use of alternate promoters. Each isoform contains a C-terminal ligand-binding domain (LBD), a DNA-binding domain (DBD) and at least two transcriptional activation function (AF) domains. AF1 and AF2 are located within the aminoterminal domain (ATD) and the LBD, respectively. PRB contains an additional AF (AF3) within the unique 164-amino acid B-upstream segment. Adapted from Lange; Hill KK, et al.

Figure 2

Progesterone receptor nuclear morphology patterns in breast cancer. (A) Diffuse pattern: homogeneous and diffuse fine granular distribution in the nuclei of tumour cells (×100). (B) Diffuse pattern: homogeneous and diffuse fine granular distribution in the nuclei of tumour cells (×40). (C) Aggregated pattern: heterogeneous distribution of moderate and large spots in the nuclei of tumour cells (×100). (D) Aggregated pattern: heterogeneous distribution of moderate and large spots in the nuclei of tumour cells (×40).

Figure 3

Per cent progesterone receptor (PR) positivity as a function of activated PR pattern. Each box corresponds to the range of percentage values of PRA (A) or PRB (B) stained tumour cells. The limits of the boxes show the SD of the distribution, and the line the extremes, while the line inside the box is the average. The left panel compared the distribution of the percentages for aggregated pattern (activated) and diffuse pattern (inactive). The same is shown on the right for PRA. It is apparent that the boxes for each PR are not at the same levels, and in particular that the average lines are at different levels, for example, the average percentages are different. The tests used in a Kruskal-Wallis test confirm that the distributions are different between aggregated and diffuse. (A) p=0.0003; (B) p=0.001.