The Epigenetic Control of Gene Expression in Leukaemia and Haematopoiesis

There are many specialized cells in our body designed to carry out different specific tasks, and yet they all contain the same genetic information. During development, cells receive information that they translate into specific developmental outputs, usually without altering their DNA sequence. Changes in cell phenotype that do not alter the DNA sequence are referred to as “epigenetic”. Human diseases such as cancer often result from changes in gene expression patterns that are not always associated with DNA mutations, thus epigenetic changes are also a key mechanism in human disease. In living cells, genes do not exist as “naked” DNA, but as a highly conserved protein/DNA complex termed chromatin. The basic subunit of chromatin is the nucleosome, which consists of DNA wrapped around an octamer core of globular histone proteins, two each of H2A, H2B, H3, and H4.The N terminal "tails" of these histone proteins are chemically modified with "marks" such as methylation or acetylation. The specific carriers of epigenetic information have not been completely worked out, but emerging work over the past decade has suggested that epigenetic information is established in part by the modification of histone tails. We are specifically interested in how changes in histone methylation lead to transcriptional misregulation in human disease. As a system for asking specific questions about this very complex problem, I work on the Mixed Lineage Leukemia 1 (MLL1) protein, a histone methyltransferase that controls gene activation during development. Mutations in MLL1 also cause aggressive leukaemias in both children and adults. Specific information about MLL1 activity will not only provide potential therapeutic information for this subset of human leukemias, but will also have broader implications for stem cell development and the epigenetic regulation of gene expression in other human diseases.

The Mixed Lineage Leukaemia 1 protein (MLL1) protein is important for the epigenetic regulation of gene expression during stem and progenitor cell development but is also mutated in a subset of aggressive human leukaemias. The most common leukaemic disruptions of the MLL1 gene are chromosome translocations that fuse the N terminus of MLL1 in frame with over 40 different partner genes creating novel fusion proteins. Recent work has suggested that wild type MLL1 and MLL1 fusion proteins cooperate in leukaemogenesis by together causing aberrant epigenetic profiles at target genes in vivo. Epigenetic changes are often defined as heritable changes in gene expression or chromosome stability that don’t alter the underlying DNA sequence. We are interested in identifying the key molecular events in MLL1 mediated leukaemogenesis in order to fully understand the epigenetic basis for this disease. This major goal has been divided into three key questions: 1) What key downstream gene targets are essential for MLL1 mediated leukaemogenesis?; 2) how do MLL1 and MLL1 fusion proteins control epigenetic gene regulation on a molecular level?; and 3) how are MLL1 and MLL1 fusion proteins recruited to important gene targets in the cell? To answer these questions, we are using xenograft transplant assays to identify Leukaemic Stem Cells (LSCs) in MLL1 patient samples, ChIP-seq to identify and characterise direct gene targets in specific haematopoietic cell populations, small molecular inhibitors and siRNA combined with ChIP and ChIP-seq to determine how MLL1 fusion proteins regulate gene targets in the cell and MLL1 domain analysis coupled with genome wide techniques to determine how MLL1 is recruited to gene targets in the cell. Answering these questions may not only be useful for future therapeutic strategies, but will also inform our basic understanding of epigenetic gene regulation during normal stem and progenitor cell development.