From mice to humans: identification of commonly deregulated genes in mammary cancer via comparative SAGE studies

Abstract

Genetically engineered mouse mammary cancer models have been used over the years as systems to study human breast cancer. However, much controversy exists on the utility of such models as valid equivalents to the human cancer condition. To perform an interspecies gene expression comparative study in breast cancer we used a mouse model that most closely resembles human breast carcinogenesis. This system relies on the transplant of p53 null mouse mammary epithelial cells into the cleared mammary fat pads of syngeneic hosts. Serial analysis of gene expression (SAGE) was used to obtain gene expression profiles of normal and tumor samples from this mouse mammary cancer model (>300,000 mouse mammary-specific tags). The resulting mouse data were compared with 25 of our human breast cancer SAGE libraries (>2.5 million human breast-specific tags). We observed significant similarities in the deregulation of specific genes and gene families when comparing mouse with human breast cancer SAGE data. A total of 72 transcripts were identified as commonly deregulated in both species. We observed a systematic and significant down-regulation in all of the tumors from both species of various cytokines, including CXCL1 (GRO1), LIF, interleukin 6, and CCL2. All of the mouse and most human mammary tumors also displayed decreased expression of genes known to inhibit cell proliferation, including NFKBIA (IKBalpha), GADD45B, and CDKN1A (p21); transcription-related genes such as CEBP, JUN, JUNB, and ELF1; and apoptosis-related transcripts such as IER3 and GADD34/PPP1R15A. Examples of overexpressed transcripts in tumors from both species include proliferation-related genes such as CCND1, CKS1B, and STMN1 (oncoprotein 18); and genes related to other functions such as SEPW1, SDFR1, DNCI2, and SP110. Importantly, abnormal expression of several of these genes has not been associated previously with breast cancer. The consistency of these observations was validated in independent mouse and human mammary cancer sets. This is the first interspecies comparison of mammary cancer gene expression profiles. The comparative analysis of mouse and human SAGE mammary cancer data validates this p53 null mouse tumor model as a useful system closely resembling human breast cancer development and progression. More importantly, these studies are allowing us to identify relevant biomarkers of potential use in human studies while leading to a better understanding of specific mechanisms of human breast carcinogenesis.

Fig. 1

Representative Northern blot analysis of commonly deregulated genes in p53 null mouse mammary tumors. A, transcripts with increased expression in p53 null mammary tumors as compared with normal mouse mammary epithelium. B, significantly down-regulated transcripts in p53 null mammary tumors relative to normal mouse mam-mary epithelium. *2310004N11Rik, homologue of human MGC4796 gene.

Fig. 2

Heat map of commonly deregulated genes in mouse and human tumors displayed in fold change relative to normal tissue counterparts. Color scale at the bottom depicts the approximate fold change in expression for each transcript and library relative to normal mammary epithelium: negative fold change (transcripts with decreased expression in tumors) is represented in green, and positive fold change (transcripts with over-expression in tumors) is represented in red.

Fig. 3

Real-time PCR validation of 9 deregulated genes in 12 additional human breast carcinomas. 18S rRNA was used as control. Bar charts were plotted based on the mean values ±2 SEM of the ratio between target and control genes in normal human breast epithelium and invasive carcinomas. Values shown in the top left corner of each plot represent the equivalent fold change between the mean of tumors and normal tissue values. *, expression not detected in normal breast tissue.