Metastasis to distant organs is an ominous feature of most malignant tumours but the natural history of this process varies in different cancers. The cellular origin, intrinsic properties of the tumour, tissue affinities and circulation patterns determine not only the sites of tumour spread, but also the temporal course and severity of metastasis to vital organs. Striking disparities in the natural progression of different cancers raise important questions about the evolution of metastatic traits, the genetic determinants of these properties and the mechanisms that lead to the selection of metastatic cells.

The molecular basis for breast cancer metastasis to the brain is largely unknown 1,2 . Brain relapse typically occurs years after the removal of a breast tumour [2][3][4] , suggesting that disseminated cancer cells must acquire specialized functions to overtake this organ. Here we show that breast cancer metastasis to the brain involves mediators of extravasation through non-fenestrated capillaries, complemented by specific enhancers of blood-brain barrier crossing and brain colonization. We isolated cells that preferentially infiltrate the brain from patients with advanced disease. Gene expression analysis of these cells and of clinical samples, coupled with functional analysis, identified the cyclooxygenase COX2 (also known as PTGS2), the epidermal growth factor receptor (EGFR) ligand HBEGF, and the α2,6-sialyltransferase ST6GALNAC5 as mediators of cancer cell passage through the blood-brain barrier. EGFR ligands and COX2 were previously linked to breast cancer infiltration of the lungs, but not the bones or liver 5,6 , suggesting a sharing of these mediators in cerebral and pulmonary metastases. In contrast, ST6GALNAC5 specifically mediates brain metastasis. Normally restricted to the brain 7 , the expression of ST6GALNAC5 in breast cancer cells enhances their adhesion to brain endothelial cells and their passage through the blood-brain barrier. This co-option of a brain sialyltransferase highlights the role of cell-surface glycosylation in organspecific metastatic interactions.Brain metastasis affects an estimated 10% of cancer patients with disseminated disease 2,8,9 . Even small lesions can cause neurological disability, and the median survival time of patientsCorrespondence and requests for materials should be addressed to J.M. (E-mail: j-massague@ski.mskcc.org). † Present addresses: Institut de Malalties Hemato-Oncològiques, Hospital Clínic, 08036 Barcelona, Spain (C.N.); Oncology Programme, Institute for Research in Biomedicine, 08028 Barcelona, Spain (R.R.G.). Author InformationThe clinical microarray data on the brain metastatic cell lines have been deposited in NCBI's Gene Expression Omnibus (GEO, http://www.ncbi.nlm.nih.gov/geo) under the GEO series accession number GSE12237.Supplementary Information is linked to the online version of the paper at www.nature.com/nature. Full Methods and any associated references are available in the online version of the paper at www.nature.com/nature. 11 and also by tight junctions and astrocyte foot processes in the blood-brain barrier (BBB) 2,8 , whereas the capillaries in the bone marrow and the liver are fenestrated 11,12 . The composition of the parenchyma also varies extensively between these organs. The protracted progression of disseminated cancer cells in different environments may give rise to metastatic speciation, as suggested by the coexistence of malignant cells with different organ tropisms in fluids from patients with advanced disease 5,13 . Analysis of such malignant cell populations has revealed genes that selectively mediate breast cance...

Summary The dissemination of cancer cells from a primary tumor is conventionally viewed as a unidirectional process that culminates with the metastatic colonization of distant organs. Here we show that circulating tumor cells (CTCs) can also colonize their tumors of origin, in a process that we call “tumor self-seeding”. Self-seeding of breast cancer, colon cancer, and melanoma tumors in mice is preferentially mediated by aggressive CTCs, including those with bone, lung or brain metastatic tropism. The tumor-derived cytokines IL-6 and IL-8 acted as CTC attractants and the poor-prognosis markers MMP1/collagenase-1 and the actin cytoskeleton component fascin-1 as mediators of CTC infiltration into mammary tumors. Self-seeding can accelerate tumor growth, angiogenesis, and stromal recruitment through seed-derived factors including, in a breast cancer model, the chemokine CXCL1. Tumor self-seeding could explain the relationships between anaplasia, tumor size, vascularity and prognosis, and local recurrence seeded by disseminated cells following ostensibly complete tumor excision.

Metastasis can be viewed as an evolutionary process, culminating in the prevalence of rare tumour cells that overcame stringent physiological barriers as they separated from their original environment and developmental fate. This phenomenon brings into focus long-standing questions about the stage at which cancer cells acquire metastatic abilities, the relationship of metastatic cells to their tumour of origin, the basis for metastatic tissue tropism, the nature of metastasis predisposition factors and, importantly, the identity of genes that mediate these processes. With knowledge cemented in decades of research into tumour-initiating events, current experimental and conceptual models are beginning to address the genetic basis for cancer colonization of distant organs.