Expression of RPIP9 (Rap2 interacting protein 9) is activated in breast carcinoma and correlates with a poor prognosis.

MDR1 is upregulated in many tumors. We have previously detected activation of the MDR1 upstream promoter in metastatic breast cancer cells. MDR1 overlaps with an uncharacterized gene transcribed from the opposite strand, coding for Rap2 interacting protein 9 (RPIP9). Rap2 belongs to the Ras superfamily of GTPases, whose role in breast cancer remains unknown. We developed sensitive methods for detecting and quantifying RPIP9 mRNA and used it to identify these transcripts in normal human tissues, 60 biopsies of primary breast carcinoma, in isolated epithelial cells both from the primary tumor and from associated lymph nodes, and from bone marrow biopsies of 74 breast cancer patients. RPIP9 is expressed at high levels in normal testis, brain and adrenal gland, and at very low levels in normal breast. Tumorigenic breast carcinoma cell lines expressed RPIP9, whereas MCF-10A and HBL-100 that do not form tumors in nude mice had undetectable levels of RPIP9 mRNA. RPIP9 was activated in a high proportion of breast carcinomas (61.6%; n = 60) and a significant correlation with metastatic lymph node invasion (N = 0-3 vs. N > 3, where N = number of lymph nodes invaded; p = 0.031) was found. RPIP9 mRNA could be detected in malignant epithelial cells isolated from the primary tumor and from metastasized lymph nodes as well as in the bone marrow of significantly more poor-prognosis (N > 3) than better-prognosis (N = 0-3) patients (p = 0.001). Therefore, activation of RPIP9 occurs during the malignant breast epithelial transformation and increases with progression toward an invasive phenotype.