Daniel N. Weinberg

Studies of human cancer metastasis have been limited by a lack of experimental assays in which cancer cells from patients metastasize in vivo in a way that correlates with clinical outcome. This makes it impossible to study intrinsic differences in the metastatic properties of cancers from different patients. We recently developed an assay in which human melanomas readily engraft in nonobese diabetic/severe combined immunodeficient interleukin-2 receptor-γ chain null (NSG) mice. We show that melanomas from 25 patients exhibited reproducible differences in the rate of spontaneous metastasis after transplantation into NSG mice and that these differences correlated with clinical outcome in the patients. Stage IIIB/C melanomas that formed distant metastases within 22 months in patients also formed tumors that metastasized widely in NSG mice, whereas stage IIIB/C melanomas that did not form distant metastases within 22 to 50 months in patients metastasized more slowly in NSG mice. These differences in the efficiency of metastasis correlated with the presence of circulating melanoma cells in the blood of NSG mice, suggesting that the rate of entry into the blood is one factor that limits the rate of metastasis. The study of NSG mice can therefore yield information about the metastasis of human melanomas in vivo, in this case revealing intrinsic differences among stage III melanomas in their ability to circulate/survive in the blood and to metastasize.

A high percentage of pediatric gliomas and bone tumors reportedly harbor missense mutations at glycine 34 in genes encoding histone variant H3.3. We find that these H3.3 G34 mutations directly alter the enhancer chromatin landscape of mesenchymal stem cells by impeding methylation at lysine 36 on histone H3 (H3K36) by SETD2, but not by the NSD1/2 enzymes. The reduction of H3K36 methylation by G34 mutations promotes an aberrant gain of PRC2-mediated H3K27me2/3 and loss of H3K27ac at active enhancers containing SETD2 activity. This altered histone modification profile promotes a unique gene expression profile that supports enhanced tumor development in vivo. Our findings are mirrored in G34W-containing giant cell tumors of bone where patient-derived stromal cells exhibit gene expression profiles associated with early osteoblastic differentiation. Overall, we demonstrate that H3.3 G34 oncohistones selectively promote PRC2 activity by interfering with SETD2-mediated H3K36 methylation. We propose that PRC2-mediated silencing of enhancers involved in cell differentiation represents a potential mechanism by which H3.3 G34 mutations drive these tumors.