Abstract:Objective To explore the key genes mediating trastuzumab resistance in human epidermal growth factor receptor 2-positive（HER2+）breast cancer and their molecular mechanisms. Methods Trastuzumab-resistant（SK-BR-3R）and sensitive（SK-BR-3）cell lines were established，followed by transcriptome sequencing to identify upregulated key genes and key enriched pathways in the resistant group. Quantitative real-time reverse transcription- polymerase chain reaction（qRT-PCR）and Western blot were used to validate tectonic family member 1（TCTN1）expression levels in SK-BR-3R. SK-BR-3 cells overexpressing TCTN1 were constructed to assess their resistance to trastuzumab and the expression levels of epithelial-mesenchymal transition（EMT）-related proteins. Immunohistochemistry（IHC）was performed on 20 HER2 + breast cancer tissue specimens to analyze the correlation between TCTN1 expression and patient prognosis or clinical indicators. Results Transcriptome sequencing revealed that TCTN1 was the most significantly upregulated gene in SK-BR- 3R，with enriched pathways including EMT. Both RNA and protein levels confirmed TCTN1 upregulation in SK-BR-3R. Overexpression of TCTN1 downregulated E-cadherin and upregulated N-cadherin protein，promoting EMT and conferring trastuzumab resistance. High TCTN1 expression was associated with poor prognosis and advanced clinical stages in HER2+ breast cancer patients. Conclusion TCTN1 mediates trastuzumab resistance in HER2+ breast cancer by promoting EMT. TCTN1 may serve as a potential prognostic biomarker and novel therapeutic target for HER2+ breast cancer.