Abstract PO2-13-02: Detection of SPEN mutations in advanced breast cancer by circulating tumor cell-free DNA

Abstract Background: Alterations in the SPEN gene are rare in primary breast cancer (~3%, ref: TCGA). SPEN is a hormone-inducible transcriptional repressor with known functions in orchestrating X-inactivation in females. Additionally, SPEN has been implicated as a potential estrogen receptor co-repressor and molecular partner of NCOR2 as well as epigenetic modifiers such as KMT2D and HDACs via C-terminal interactions. SPEN loss-of-function is associated with tamoxifen resistance in preclinical models. However, the landscape of SPEN alterations in advanced breast cancer remains poorly described. The primary objective of this study was to evaluate the frequency and different types of SPEN alterations in metastatic breast cancer (MBC). Methods: A query was performed on all patients at an academic institution with available genomic testing from Guardant360, a next-generation sequencing cell-free DNA (cfDNA) assay, evaluating 500 genes in total. Patients with MBC were identified, and specific alterations in SPEN as well as co-alterations were extracted. Silent and germline mutations were excluded from analysis. Retrospective review was conducted to determine clinicopathologic characteristics such as age, hormone and HER2 receptor status, menopausal status, and therapy at time of the detected mutation. Results: Among 366 patients with MBC and available cfDNA, 8.5% (n=31) had an alteration detected in the SPEN gene. Median age of patients with SPEN mutations was 66 years (range 33-92). A majority of patients (90%) had HR+ disease (n=28/31), with HR-/HER2+ disease and TNBC additionally representing 3% (n=1/31) and 6% (n=2/31) of cases, respectively. SNVs were most frequent, comprising 89% (n=42/47) of all alterations. Indels were additionally identified in 11% (n=5/47), and no CNVs or fusions were noted. Nonsense and frameshift mutations represented 8/47 (17%) and 2/47 (4%) of all alterations respectively. No recurrent mutational hotspots were detected. The most common co-alterations in descending order were PIK3CA, TP53, ERBB2, ARID1A, MDM4, PPM1D, NF1, DNMT3A, ATM, and ESR1. Association between SPEN mutations and clinical outcomes will be presented at the meeting. Conclusions: SPEN mutations were detected mostly in patients with HR+/HER2- MBC and at a higher frequency than reported in primary disease via TCGA. SNVs were common, though it remains unclear whether these comprise driver versus passenger mutations. While no mutational hotspots were identified, several novel loss-of-function truncating mutations were identified, which warrant further validation of functional and clinical significance. SPEN alterations in metastatic breast cancer Alterations detected by cell-free DNA sequencing in patients with metastatic breast cancer and annotated by type of alteration and receptor subtype. Citation Format: Charles Dai, Haley Barnes, Arielle Medford, Annika Putur, Jennifer Keenan, Beverly Moy, Seth Wander, Ryan Corcoran, Aditya Bardia. Detection of SPEN mutations in advanced breast cancer by circulating tumor cell-free DNA [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-13-02.