A Case Report: A Rare Casr Mutation Associated with HER2-Positive Breast Cancer

Although highly penetrant genes such as BRCA1, BRCA2, TP53, PALB2, CHEK2, and ATM are well known in hereditary breast cancer, genes with low penetrance may also contribute to the disease risk. The CASR (Calcium-Sensing Receptor) gene, located on chromosome 3q13.3-q21, encodes a G-protein-coupled receptor that regulates calcium homeostasis and parathyroid hormone secretion. here we describe the clinical course and genetic findings in a breast cancer patient diagnosed with hER2-positive (hER2/neu +++) and ER/PR-negative infiltrating ductal carcinoma with a rare CASR mutation and explore its potential role in the hereditary breast cancer predisposition. Next-generation sequencing re- vealed a heterozygous CASR variant, c.2265G>t (p.Glu755Asp), classified as a variant of uncertain significance (VUS). No pathogenic mutations were found in BRCA1/2, CHEK2, or PALB2. The patient underwent neoadjuvant chemothera- py, right-sided mastectomy, and trastuzumab therapy, achieving complete pathological regression (pCR) with no recur- rence after ten months of follow-up. Although the CASR p.Glu755Asp variant's pathogenic role is unproven, emerging evidence links CASR overexpression with hER2-positive breast cancers, promoting tumor progression via calcium- dependent signaling pathways (PI3K/AKt, MAPK). This case highlights the potential role of CASR as a low-penetrance susceptibility gene in breast cancer and supports further functional and genomic studies to clarify its clinical impact.