Transcriptional networks in organogenesis: Microarray analysis of mammary gland development

Breast cancer is the second leading cause of cancer-related mortality among women in the United States and as such is a problem of tremendous clinical significance. Given the relationship between the timing of normal developmental events and breast cancer risk, elucidation of the regulatory networks underlying mammary gland organogenesis may yield important clues to mechanisms of carcinogenesis. Oligonucleotide microarrays were used to survey the expression of ∼5300 genes in the murine mammary gland during neonatal development, puberty, pregnancy, lactation, and postlactational involution. To address the inherent difficulty of interpreting gene expression patterns within a complex tissue, an automated approach for the unbiased identification of biologically relevant patterns of gene expression was developed. This approach was applied to mammary gland development and validated through the identification of cellular processes and pathways of known importance. Additionally, this approach permitted the identification of genetic pathways with previously unelucidated patterns of developmental regulation and suggested a novel role for the mammary gland in adaptive thermogenesis. The results of this study suggest that this approach will be broadly applicable to studies of vertebrate development. To confirm and extend these results, the expression of ∼9500 genes was determined at each of the previously examined developmental stages. These data were used to identify the relationship between global patterns of gene expression at a variety of points in mammary gland development. Additionally, triplicate data were used to identify genes that are differentially regulated during the rapid ductal growth of puberty. Finally, microarray analysis was used to analyze the effect of inducibly overexpressing c-MYC within the murine mammary epithelium. In addition to identifying genes involved in proliferation, cell growth, and apoptosis, expression data suggested a potential role for c-MYC in promoting mammary epithelial differentiation. Comparisons of mammary gene expression patterns resulting from c-MYC overexpression with those found during normal mammary gland development suggests the possibility that chronic c-MYC expression is sufficient to activate a developmental program the mimics features of normal pregnancy and involution. These studies demonstrate the utility of large-scale expression profiling for the analysis of developmental perturbations