Predictive Value of Ectopic HORMAD1 Tumor Expression for High-Dose Platinum-Based Chemotherapy Benefit in Patients with High-Risk HER2-negative Breast Cancer.

541 Background: The meiotic DNA break regulator HORMAD1 is aberrantly expressed in many cancers and is associated with increased genomic instability. The susceptibility of HORMAD1 expressing tumors to agents targeting DNA damage repair (DDR) pathways is poorly understood since clinical data within the context of a randomized clinical trial (RCT) is lacking. Here, we retrospectively studied HORMAD1 expression as a putative predictive biomarker in an RCT for benefit of adjuvant high-dose platinum-based chemotherapy (HDCT) with autologous stem cell support in patients with high-risk HER2-negative early breast cancer (BC). Methods: Patients with stage III BC participated in an RCT comparing HDCT to conventional chemotherapy (CDCT; Rodenhuis et all, NEJM, 2003; Steenbruggen et all, JAMA Oncol, 2020). We studied the subgroup with HER2-negative BC for whom tumor BRCA1-like classification was previously determined using a validated DNA comparative genomic hybridization algorithm (Vollebergh et all, BCR, 2014). Tumor HORMAD1 expression was determined on FFPE samples using RNAscope, an RNA in situ hybridization method, and classified as negative (no expression) or positive (any expression detected). Results: For 195/246 (79.3%) HER2-negative patients treated according to protocol, HORMAD1 RNAscope status was available; dropout was due to absence or insufficient quality of tumor specimens. HORMAD1 positivity was enriched in triple-negative breast cancer (TNBC) (23/47; 48.9%). Furthermore, in all HER2-negative BCs, HORMAD1 positivity (45/195; 23.1%) was associated with age ≤40 years, histological grade III, <10 positive lymph nodes, breast-conserving surgery, BRCA1-like profile, and tumor-infiltrating lymphocytes (TILs) >10%. Such association, although not significant, was also observed within TNBC. During a median follow-up of 20.3 years, 124 (63.6%) recurrences and 115 deaths (59.0%) occurred. The prognostic effect of HORMAD1 positivity on overall survival (OS) varied with follow-up time and was borderline significant at 10 years and significant thereafter (10-year: adjusted (adj.) HR 0.47, 95% CI 0.21-1.04; 15-year: adj. HR 0.25, 95% CI 0.07-0.91). Benefit on RFS from HDCT over CDCT was stronger in patients with HORMAD1-positive tumors (adj. HR 0.18, 95% CI 0.06-0.54) than in patients with HORMAD1-negative tumors (adj. HR 0.69, 95% CI 0.46-1.02) (P-interaction = 0.02). Similar results were observed for OS. Conclusions: In this retrospective sub study of 195 patients with high-risk HER2-negative BC participating in an RCT, tumor HORMAD1 expression is predictive for benefit of high-dose platinum-based chemotherapy. Our observations are consistent with the prior observations that HORMAD1 expression is associated with genomic instability and impaired DDR pathways. Further research is warranted to validate our findings. Clinical trial information: NCT03087409.