Abstract P5-05-15: Transgenic Mice Expressing Either p200, p110 or p75 CUX1 Isoform Develop Mammary Tumors of Various Histological Types
Authors: Ramzdan ZM , Cadieux C , Drossos M , Nepveu A. 
Published: April 2011

Abstract
Short isoforms of the CUX1 homeodomain protein are overexpressed in breast tumors and cancer cell lines. The full-length CUX1 protein, p200 CUX1, is proteolytically processed into p110 CUX1 by a nuclear isoform of cathepsin L. Another isoform, p75 CUX1, was found to be aberrantly expressed in breast tumors. The full-length and short CUX1 isoforms exhibit distinct biochemical and cellular activities. They bind DNA differently and regulate transcription in different ways. Over-expression of p110 or p75 CUX1 accelerates the start of DNA replication, promotes genetic instability and stimulates cell motility. In contrast, the full-length p200 CUX1 isoform appears to be inactive in these cell-based assays. To assess and compare the ability of the full-length and short CUX1 isoforms in driving mammary tumor development, we used site-specific transgenesis into the hprt locus to generate transgenic mice expressing p200, p110 or 75 CUX1 under the control of the mouse mammary tumor virus (MMTV) long terminal repeat. We report that mammary tumors of various histopathologies developed in all transgenic lines after a long latency period. Mammary tumors were classified as solid carcinomas with or without papillary differentiation, adenosquamous carcinomas, adenomyoepithelioma or tubular/acinar adenoma and many displayed pathophysiological features similar to that of human basal-like breast cancers.
Interestingly, higher expression of erbB2 mRNA was seen in most tumors, not only solid carcinomas but also adenosquamous carcinomas, while higher expression of various Wnt genes and activation of the β-catenin pathway was observed primarily in adenosquamous carcinomas. Activation of erbB2 expression appeared to represent a cooperating event that occurred independently of CUX1. In contrast, chromatin immunoprecipitation, shRNA-mediated knockdown and reporter assays established that CUX1 is involved in the transcriptional regulation of several Wnt genes. Together these results support the notion that oncogenic activity of CUX1 can facilitate the establishment of a Wnt/β-catenin autocrine loop. To our surprise, transgenic mice expressing p200 CUX1 developed mammary tumors with a penetrance equivalent to that of p75 and p110 CUX1 transgenic mice. Preliminary results showed that the expression and activity of p110 CUX1 was elevated in tumors from p200 CUX1 transgenics. Accordingly, cathepsin L was over-expressed in 7 out of 8 tumors from p200 CUX1 transgenic mice, but in only 1 out 11 tumors from p110 CUX1 mice. Moreover, in a p200 CUX1 tumor an activating point mutation has been identified at codon 61 of the k-ras gene. Activation of the ras pathway has previously been shown to lead to over-expression of cathepsin L. These results suggest that molecular events that lead to cathepsin L overexpression cooperate in tumorigenicity by stimulating the proteolytic processing of p200 CUX1 into the active p110 CUX1 isoform.
Overall, these results confirm the oncogenic potential of CUX1 in the mammary gland and reveal some of the cooperating events and molecular pathways involved in mammary tumorigenesis.
Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P5-05-15.