Recent advances in gene targeting technologies in the mouse have taken us one leap closer to understanding the genetic pathways that operate during normal mammary gland development and tumorigenesis. The possibility to delete or mutate genes specifically in mammary epithelial cells and at predetermined time points permits investigators to analyze the fates of defined cell types in the absence of confounding systemic effects. Gene deletion (knockout) and transgenic mice, both alone and in combination, can be used to address specific questions in developmental and cancer biology. The genetic ablation of steroid (estrogen and progesterone) and peptide (prolactin, epidermal growth factor) hormone receptors and their ligands has provided a deep insight into their function during ductal and alveolar development and has shed light on their redundancy and parallel pathways. Finally, the deletion of transcription factors, including those that mediate peptide hormone signaling, has revealed distinct roles in epithelial cell proliferation, differentiation, and death (for a detailed assessment of genetic approaches to study mammary development, see ). Rather than describing individual models (an array of mouse models will be presented in depth in the January 2000 issue of the journal Oncogene), herein I discuss some of the lessons we have learned during the past 15 years from the mice models that are at hand, and the technological hurdles we now encounter. Like in many explorations, the initial concepts, approaches, and tools are rather crude and need to be further developed and refined as new information streams in and new hypotheses are articulated. On the basis of this need I present contemporary approaches that should aid our quest to identify and understand molecular pathways of pathogenesis.
Experiments conducted by Philip Leder and coworkers 15 years ago represent a milestone in breast cancer research . They fused the long terminal repeat (LTR) of the mouse mammary tumor virus (MMTV) to the human c-myc proto-oncogene and incorporated this hybrid gene into mice. These transgenic mice expressed the human myc protein in their mammary glands, which resulted in the development of breast tumors . This landmark paper helped to establish an entirely new research arena poised to identify genetic pathways that control breast cancer. After decades of research on tissue culture cells, both federal and private funding agencies saw the opportunity to extend investigations into settings that more closely resembled the human condition. Fifteen years after the study by Leder and coworkers, research by Deng (a former student of Leder) and coworkers set another milestone towards this goal. These investigators succeeded in inactivating the breast cancer gene Brca1 specifically in mammary epithelial cells of mice, and they demonstrated that mammary tumors coincided with genome instability . The distinct lesson learned from these studies was that the wrongful expression of an oncogene and the inactivation of a tumor suppressor gene in mice can cause cancer, just like in humans. However, the myc and Brca1 mice differ in two fundamental aspects from the human situation. In the myc mice oncogene activity occurs as early as puberty, whereas in humans genetic changes leading to cancer may occur later in life. The appearance of tumors in Brca1 conditional mice depends on the loss of both alleles, whereas in humans only one BRCA1 allele is altered (for discussion, see ).
Understanding genetic pathways was considered to be, and still remains the prerequisite for the development of molecular and pharmacological agents to treat and prevent cancer. Over the past 15 years almost 100 mouse models have been generated that permit the investigation of defined aspects of tumorigenesis. The impact of transgenic mouse models on breast cancer research was the topic of recent conferences in Annapolis, Maryland (March 3-5, 1999) and Bar Harbor, Maine, USA (The Jackson Laboratory Conference on 'Cancer of the Mammary Gland', October 5-8, 1999) . It is fair to say that not a single model by itself covers the full spectrum of this disease, but that individual models address distinct aspects. Each transgene targets different signaling pathways outside and inside the mammary cell, and disrupts these pathways at different time points during development. In addition, the concomitant disruption of some physiologic parameters provided insight into the cellular requirements for cellular transformation to occur.
At the Annapolis conference, pathologists and basic researchers convened and assessed different mouse models. Specifically, they asked the following key question: How similar are mouse models to the human condition? A panel of nine medical and veterinary pathologists with expertise in mammary gland biology reviewed material representing more than 90% of the mouse models. A nomenclature was developed and recommendations for future analyses were drafted. The consensus report from the Annapolis meeting, including the 'Annapolis guidelines' will be published in an upcoming issue of the journal Oncogene . It is suggested that the Annapolis nomenclature is adopted by the research community and in federally funded research. In addition, the recent development of a web-based interactive histology atlas  now permits the comparison of high-resolution images from mouse models and human breast cancer, and researchers in different locations can view, discuss, annotate, and compare histologic images in real time. The histology atlas in conjunction with the database for genetically engineered mice  will provide in depth information on genetic pathways in human breast cancer and corresponding mouse models.
In comparing the biology from human breast tumors with that of mammary tumors in genetically engineered mice, the Annapolis pathologists identified similarities and differences (Table (Table66 in ). Among the similarities identified are as follows: molecular lesions that cause breast cancer in humans can also cause cancer in genetically engineered mice; lesions in both species display similar morphologic patterns; multi-hit kinetics of cancer development; mammary cancers in both species are metastatic; and mammary cancer is frequently hormone independent. Among the differences are as follows: some molecular lesions that cause mammary cancer in mice have not been found in human mammary cancer; the morphology of most mouse tumors does not resemble that of common human cancers; some transgenes in mouse appear to be associated with single-hit kinetics; most mouse tumors metastasize to the lung, whereas most human tumors metastasize to the lymph nodes; and half of the human cancers are hormone independent, but most mouse tumors are hormone dependent. Although many transgenic mice display dissimilarities to the human condition, it is likely that their usefulness extends into understanding molecular pathways that lead to cancer initiation and progression. For example, the viral oncogene that encodes the SV40 T antigen cannot be linked to human breast cancer, but the respective transgenic mice provide insight into cell-cycle control during hyperplasia and tumor progression (see below).