Who is in the driver's seat in 8p12 amplifications? ZNF703 in luminal B breast tumors.

Two recent reports identify ZNF703 as an oncogene driving selection of frequent chromosome 8p12 amplifications in luminal B breast tumors. The estrogen-responsive ZNF703 gene encodes a transcriptional cofactor that, when overexpressed, induces cell proliferation and interferes with transforming growth factor beta signaling. In MCF7 cells, increased ZNF703 expression results in activation of genes involved in stem cell self-renewal - while in primary human mammary epithelial cells, ZNF703 increases the ratio of luminal to basal progenitors. Expression of the murine homolog of ZNF703 reduces cell adhesion and promotes metastasis. ZNF703 overexpression thus alters regulation of proliferation and differentiation in luminal B tumors.


Introduction
Five major breast cancer subtypes -basal, luminal A, luminal B, ERBB2-positive, and normal breast-like -are distinguishable on the basis of molecular profi ling [1]. Both luminal A and luminal B subtypes are estrogen receptor (ER)-positive, but luminal B tumors are more metastatic [2] and have poorer prognoses [3]. Each of the breast cancer subtypes is characterized by a specifi c pattern of genomic abnormalities [4]. Luminal B tumors often contain high-level amplifi cations of chromosome region 8p12, and patients bearing tumors with ampli fi cations in this region exhibit signifi cantly poorer outcomes than patients whose tumors do not contain such amplifi cations [4]. Th e most commonly amplifi ed 1 Mb seg ment in the 8p12 region contains only fi ve genes for which amplifi cation correlates with the gene expression: ZNF703, ERLIN2, PROSC, BRF2, and RAB11FIP1 [5,6].
Until now, however, the identity of the driver oncogene within this group remained elusive.

Articles
Two recent reports describe high-density array comparative genomic hybridization analyses of the 8p12 region using independent panels of breast tumors and cell lines [6,7]. In one case, 1,001 primary breast cancers were used to defi ne the boundaries of the minimal amplicon [6]. Both sets of these comparative genomic hybridization results implicate ZNF703 as the main gene whose amplifi cation is selected for in luminal B cancers. Th e evidence includes two tumors in which ZNF703 was the only gene amplifi ed within the 8p12 region. High levels of ZNF703 amplifi cation and mRNA expression were associated with poor outcomes in ER-positive and luminal tumors.
Th e association studies are supplemented by experimen tal data obtained using cell culture models. Transfection of ZNF703 resulted in transformation of NIH 3T3 cells, and induced proliferation of both nonmalignant human mammary epithelial cells and malignant MCF7 cells. Holland and coworkers [6] further demonstrated that increased levels of ZNF703 resulted in decreased expres sion of transforming growth factor beta (TGFβ) receptor II and prevented TGFβ from inhibiting prolifera tion of MCF7 cells. Binding of ZNF703 to the TGFβ receptor II promoter was demonstrated by chromatin immuno precipitation, and was associated with repressive chromatin modifi cations. ZNF703 contains a single zinc fi nger domain and is unlikely to bind to DNA directly, however [8]. Sircoulomb and colleagues [7] found that ZNF703 forms complexes with DCAF7, PBH2, and NCOR2 factors involved in transcriptional repression, in agreement with the proposed function of the ZNF703 homolog Nlz1 in zebrafi sh development [9].
While the ZNF703 promoter contains estrogenrespon sive elements and its expression is responsive to estrogen, ZNF703 transfection led to reduced ER expression and activity in MCF7 cells, indicating the existence of negative feedback regulation [7]. ZNF703 transfection, however, also resulted in reduced expression of cell cycle inhibitors p27 and p15, increased pRb phosphorylation,

Abstract
Two recent reports identify ZNF703 as an oncogene driving selection of frequent chromosome 8p12 amplifi cations in luminal B breast tumors. The estrogenresponsive ZNF703 gene encodes a transcriptional cofactor that, when overexpressed, induces cell proliferation and interferes with transforming growth factor beta signaling. In MCF7 cells, increased ZNF703 expression results in activation of genes involved in stem cell self-renewal -while in primary human mammary epithelial cells, ZNF703 increases the ratio of luminal to basal progenitors. Expression of the murine homolog of ZNF703 reduces cell adhesion and promotes metastasis. ZNF703 overexpression thus alters regulation of proliferation and diff erentiation in luminal B tumors.

© 2010 BioMed Central Ltd
Who is in the driver's seat in 8p12 amplifi cations? ZNF703 in luminal B breast tumors Alexey V Bazarov 1,2 and Paul Yaswen 2 *

V I E W P O I N T
*Correspondence: P_Yaswen@lbl.gov 2 Life Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Full list of author information is available at the end of the article and increased E2F1 transcriptional activity. Based on these data, the authors of this work propose that ZNF703 expression leads to a shift from an ER-regulated to an E2F1-regu lated transcriptional program, characteristic of luminal B tumors. Additional data indicate that ZNF703 expression leads to activation of pathways involved in stem cell self-renewal, and that it increases the ratio of luminal to basal progenitors, suggesting that, in addition to promoting proliferation, its oncogenic function includes altering diff erentiation kinetics.
Th e new fi ndings in human tissues and cells complement another recent study showing that Zeppo1 (Zpo1), the murine homolog of ZNF703, reduces cell-cell adhesion and increases invasiveness as well as proliferation in three-dimensional cultures [10]. At the molecular level, Zeppo1 represses E-cadherin expression and causes increased expression of the promigratory p120-catenin isoform. Consequently, Zeppo1 overexpression promotes metastasis in a murine tumor model. Th ese results may explain invasiveness and poor prognosis in human luminal B tumors.

Viewpoint
While ZNF703 has been largely unknown in the breast cancer fi eld, the new reports fi rmly establish it as a functional contributor to luminal B tumors. Th e preponderance of clinical correlations and experimental data are commensurate with the classical defi nition of an oncogene. As elegantly pointed out [7], however, the oncogene concept is evolving and expanding to accommodate context-dependent genetic and epigenetic features, as well as novel functions. For example, while ZNF703 transfection by itself can cause anchorage-independent growth of NIH 3T3 cells -a classical demonstration of oncogenicity -it does not cause transformation of MCF10A cells unless p53 is also compromised [11]. Under normal circumstances, aberrant proliferation is insuffi cient for malignancy. Regulation of proliferation, however, is intimately connected to diff erentiation. Th e major oncogenic role of amplifi ed/overexpressed ZNF703 may be dysregulation of diff erentiation rather than abrogation of cell cycle checkpoints. Now that ZNF703 has been fi rmly implicated in the pathogenesis of luminal B breast cancers, the next challenges will be to determine how expression/function of the encoded protein can be regulated and to determine the major downstream eff ectors of its oncogenic functions. For example, cyclin D 1 , located at chromosome 11q13, induces ZNF703 expression in an E2F1-dependent fashion [11]. As ZNF703 is in turn involved in E2F signaling, these results may explain frequent coamplifications of 8p12 and 11q13 regions. By mapping ZNF703 connections to other biochemical pathways, it may be possible to fi nd pharmacologically tractable points at which the eff ects of ZNF703 amplifi cation might be mitigated.