MolecularChanges inLobularBreastCancers inResponse to Endocrine Therapy

Invasive lobular carcinoma (ILC) accounts for approximately 10% to 15% of breast carcinomas, and although it responds poorly to neoadjuvant chemotherapy, it appears to respond well to endocrine therapy. Preand ontreatment (after 2 weeks and 3 months) biopsies and surgical samples were obtained from 14 postmenopausal women with estrogen receptor–positive (ERþ) histologically confirmed ILC who responded to 3 months of neoadjuvant letrozole and were compared with a cohort of 14 responding invasive ductal carcinomas (IDC) matched on clinicopathologic features. RNA was extracted and processed for whole human genome expression microarray. Dynamic clinical response was assessed using periodic three-dimensional ultrasoundmeasurements performed during treatment and defined as a reduction of >70% in tumor volume by 3 months. Pretreatment profiles of ILC and IDC tumors showed distinctive expression of genes associated with E-cadherin signaling, epithelial adhesion, and stromal rearrangement. The changes in gene expression in response to letrozole were highly similar between responding ILC and IDC tumors; genes involved in proliferation were downregulated and those involvedwith immune function and extracellularmatrix remodelingwere upregulated. However, molecular differences between the histologic subtypes were maintained upon treatment. This is the first study of molecular changes in ILC in response to endocrine therapy to date. The genes that change on letrozole are highly consistent between ILC and IDC. Differences in gene expression between ILC and IDC at diagnosis are maintained at each time point on treatment. Cancer Res; 74(19); 5371–6. 2014 AACR. Introduction Invasive lobular carcinoma (ILC) accounts for approximately 10% to 15% of newly diagnosed breast cancers (1, 2), affecting roughly 30,000 women annually in the United States (2). ILC is classically characterized by small, regular uniform neoplastic cells that invade the stroma in a single-file pattern with cells encircling normal breast tissue (1). Inactivation of E-cadherin (CDH1) by a variety of molecular mechanisms is considered a defining characteristic of ILC. When compared with the more common invasive breast cancers of no special type, also known as invasive ductal carcinomas (IDC), ILC is more likely to be estrogen receptor–positive (ERþ) and of lower nuclear grade (3). ILC is often large at diagnosis and there have been numerous reports on the response to neoadjuvant chemotherapy. It was thought that patients with ILC are significantly less likely to have a pathologic complete response than patients with IDC; however, a recent study concluded that ILC represents a heterogeneous group of tumors and the difference in response to neoadjuvant chemotherapy is largely explained by differences in molecular characteristics, particularly hormone receptor andHER2, and is independent of lobular histology (4). ILC have been shown to respond well to endocrine therapy (2) and we recently described the clinical response to neoadjuvant letrozole in a series of 61 patients (3). The lack of understanding of how lobular breast cancer responds to treatment is compounded by the paucity of research models (reviewed in ref. 2); however, a very recent study suggested that E2 and antiestrogens differentially regulate ERa-mediated gene expression in ILC versus IDC cell lines and xenografts (5). Previous microarray studies of ILC tumors have focused on transcriptional differences between lobular and ductal histology before treatment (6–10). To our knowledge, there have been no previous studies on the molecular response to endocrine therapy in ILC patient samples. We and others have characterized the molecular response to endocrine therapy in breast tumors in previous studies (11–13), but have not previously considered the effects of histologic subtype. Comparing preand posttreatment biopsies from the same patients, using the "window of opportunity" afforded with neoadjuvant therapy is a powerful approach that can improve statistical power due to reducing patient– patient variation (14). However, these studies are challenging to perform and are dependent on analyzing sufficient numbers of suitably appropriate samples. Edinburgh Breast Unit, University of Edinburgh, Western General Hospital, Edinburgh, United Kingdom. Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/). Corresponding Authors: Andrew H. Sims, Edinburgh Breast Unit, University of Edinburgh, Western General Hospital, Crewe Road South, Edinburgh, EH4 2XR, United Kingdom. Phone: 44-131-777-3516; Fax: 44-131777-3520; E-mail: andrew.sims@ed.ac. uk; and J. Michael Dixon, mike.dixon@ed.ac. uk doi: 10.1158/0008-5472.CAN-14-062