CK5/6 Expression in Molecular Subtypes of Invasive Ductal Carcinoma

Background Breast cancer (BC) is the leading cause of cancer-related deaths in women worldwide. There has been a significant increase in the incidence of BC in Pakistan. Family history, older age, obesity, tobacco use, oral contraceptive use, early menarche, and hormonal replacement therapy are among the major risk factors. The most common histological subtype of BC is invasive ductal carcinoma (IDC). Molecular subtypes of BC include mainly Luminal A, Luminal B, human epidermal growth factor receptor 2 (HER-2) enriched, and triple-negative BC subtypes, with the triple-negative subtype having the worst prognosis. CK5/6 serves as a basal keratin biomarker. This aimed to assess the expression of CK5/6 in IDC of the breast belonging to different molecular classes and to compare its expression with traditionally defined prognostic factors for different molecular subtypes. Methodology A cross-sectional, observational study was conducted at the Chughtai Institute of Pathology after approval from the Institutional Review Board (approval number: 1198/IRB/CIP). All cases during a period of six months (April 2023 to September 2023) were sampled using non-probability convenient sampling. All mastectomy samples diagnosed as IDC were included in the study. After standard tissue processing, paraffin tissue blocks and slides were prepared followed by hematoxylin & eosin staining. Hormonal receptors (estrogen receptor, progesterone receptor, HER-2) were assessed for cases to segregate them into molecular subtypes. CK5/6 antibody was then applied and the data were collected on a pre-designed proforma. SPSS version 25.0 (IBM Corp., Armonk, NY, USA) was used for data analysis. Results Of a total of 85 cases, 19 (22.3%) were positive for CK5/6. Of these 19 cases, the majority (68%, p = 0.001) belonged to the triple-negative class of tumors, comprising 13 cases. No case from the Luminal A class showed expression for CK5/6 stain (p = 0.028). Overall, four cases of the Luminal B subtype showed CK5/6 positivity (10.8%, p = 0.022) while two cases of the HER-2-enriched subtype were positive for the stain (33.3%, p > 0.05). These results were analyzed in relation to different prognostic factors. The majority of CK5/6-positive cases showed lymphovascular invasion (42%) and belonged to grade 3 tumors (57.8%). Conclusions The expression of CK5/6 in IDC of the breast is associated with poor prognostic factors such as triple-negative molecular subtypes, high histological grade, lymphovascular invasion, positive nodal status, and high pathological stage.