The function of DNMT1 for NPM-ALK driven Lymphomagenesis

Aberrant DNA methylation is widely observed in tumors, which can result (i) in genomic instability due to global hypomethylation and (ii) silencing of tumor suppressor genes due to localized hypermethylation of CpG island promoters, respectively. Although the recent boost of epigenetic research has nourished our understanding of the consequences of epigenetic alterations, we still dont know the cause of aberrant epigenetic patterning and the effects of external or cellular signals that might direct such changes. Receptor tyrosine kinases (RTKs) have important functions for cellular proliferation or differentiation, and their non-physiological activation has been implicated in various cancers. As a member of the insulin receptor superfamily of RTks the anaplastic lymphoma kinase (ALK) was discovered almost twenty years ago as an oncogenic translocation product in anaplastic large cell lymphoma (ALCL). More than 20 different ALK translocation partners have been identified and aside from its role as an oncogenic driver in ALCL ALK fusion proteins were discovered in several epithelial tumors and in neuroblastoma indicating a broad relevance of ALK activity in tumorigenesis. Recent work has suggested that ALK signaling can directly impact on epigenetic alterations in tumor cells. There is evidence that the key downstream ALK mediator STAT3 can upregulate the methyltransferase DNMT1 and target methyltransferases to promoters, which induces silencing of different tumor suppressor genes. Our preliminary results demonstrate that deletion of the maintenance methyltransferase gene Dnmt1 can inhibit tumor formation in a transgenic NPM-ALK mouse model. Thus, we hypothesize that aberrant DNA methylation is critically involved in ALK dependent lymphomagenesis. Here, we aim to use a NPM-ALK transgenic mouse model to study the function of DNMT1 for tumor development and progression. We will use both a genetic and a pharmacologic approach to examine the relation between ALK signaling and aberrant DNA methylation in vivo. We intend to perform genome-wide methylation analyses to define targets of epigenetic silencing in the mouse model. We will define genome-wide STAT3 binding in NPM-ALK tumors and integrate these data with gene expression and DNA methylation patterns, to define the in vivo link between DNA methylation and ALK dependent downstream signaling. We expect that our project will provide mechanistic insights as to how oncogenic signaling can induce and target epigenetic alterations in vivo and provide a rationale for epigenetic therapy of ALK dependent malignancies. Furthermore, we expect to discover novel epigenetic drivers of lymphomagenesis, which might be essential for ALK dependent tumors in general.

DNA methyltransferases are enzymes that attach methyl groups to specific bases of DNA and, through this chemical change, affect the packaging of DNA and its gene expression. In addition to mutations in DNA, changes in DNA methylation, so-called epigenetic changes, make an important contribution to the development and progression of various cancers. The loss of DNA methyl groups results in genomic instability, while increased DNA methylation of genes can lead to their silencing. In this project, we investigated the role of the DNA methyltransferase DNMT1 in the development of a rare T-cell lymphoma. Anaplastic large cell lymphoma is often due to the expression of an oncogene, the NPM-ALK fusion gene. Using a mouse model, we were able to show that DNMT1 is essential for tumorigenesis. Transformation of T cells is associated with increased expression of the DNMT1 protein. Deletion of the Dnmt1 gene in T cells completely prevented ALK-dependent lymphoma formation. Although the T cells of the transgenic mice expressed the NPM-ALK oncogene, they could no longer grow unrestrictedly after loss of the DNA methyltransferase. Furthermore, we were able to show that both human lymphoma cell lines and the transgenic animals reacted very sensitively to the treatment with DNMT inhibitors and that in the future these therapeutics could possibly represent an important alternative to conventional therapies.