PTPN9 is differentially expressed in the tumors of breast cancer patients treated with trastuzumab.

Trastuzumab (Herceptin) is a monoclonal antibody targeting the extracellular domain of the human epidermal growth factor receptor 2 (HER2) (1) utilized for the treatment of adjuvant and metastatic breast cancer (2) in the United States and worldwide. We mined published microarray data (3, 4) to discover in an unbiased manner the most significant transcriptional changes associated with trastuzumab treatment. We identified PTPN9 among the genes most differentially expressed in the primary tumors of patients with breast cancer treated with trastuzumab. The primary tumors of breast cancer patients treated with trastuzumab expressed higher levels of PTPN9 messenger RNA than did patients not treated with trastuzumab, and a single administration of trastuzumab was sufficient to result in differential expression of PTPN9 in primary tumors of the breast, demonstrating decreased expression of a negative regulator of the molecular target of trastuzumab (5) as an apparent transcriptional consequence of trastuzumab administration in patients with breast cancer.