RESEARCH

Ongoing Projects

Developing New Models of Human Blood

Our lab has devoted considerable time and effort to generating new genetic mouse systems to more accurately recapitulate the genetic progression of human blood cancer. This involves mice with multiple genetic alleles of human blood cancer driver genes, with each potentially inducible in a cell-type- and temporal-dependent manner. We have also optimized CRISPR/Cas9 targeting in primary cells from human patients to translate our findings from basic research into clinical settings. Furthermore, we have developed a novel transplant system to study primary stem/progenitor cells from MPN patients in vivo using new immunodeficient mouse strains. All these complementary tools put us in a unique position to translate findings from basic biology into clinical applications for human blood disorders

The role of epigenetic modifications in hematopoietic stem cell fate decision

Almost all cells in the human body contain the exact same DNA sequence. Obviously, a skin cell is very different from a neuron, even though they both have the exact same genes. How a cell determines which genes are switched on or off in any given cell type of the body is controlled by epigenetic modifications. Epigenetic modifications are marks or “flags” on the DNA genome, which occur outside of changes in the DNA sequence. The combinations of the epigenetic “flags” are established in stem cells during development and tell each specific cell type which genes are to be switched on or off. Hematopoietic stem cells (HSCs) are responsible for sustaining the blood and bone marrow for the lifespan of vertebrate animals. HSCs achieve this by a process known as asymmetric cell division, whereby each cell division of a HSC may produce one of two outcomes: (1) differentiation,which produces daughter cells that are committed to generating more blood cells, or (2) self-renewal, which allows the HSC to produce exact copies of itself so the population is maintained for as long as the animal is alive. The decision-making process behind this HSC choice is very poorly understood, but is now recognized to be, in part, under epigenetic control. A major focus of the lab is defining the roles of epigenetic modifications in the regulation of HSC fate decisions, so that one day we might be able to manipulate them to improve HSC function for therapies such as bone marrow transplantation.

The functions of epigenetic mutations in hematopoietic cancers

Hematopoietic cancers such as acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS) arise from genetic mutations in HSCs that impair differentiation and alter proliferation of the mutant cells, leading to a clonal expansion in the bone marrow. High-throughput genome sequencing studies have revealed that almost half of all AML and MDS patients harbor diseases driven by genetic mutations in some component of the epigenetic machinery that affects DNA methylation or chromatin structure. Importantly, patients with mutations in epigenetic pathways have significantly worse clinical outcomes. Epigenetic modifications present attractive therapeutic targets as unlike genetic mutations, the marks are reversible, underscored by the relative effectiveness of the DNA hypomethylating agents 5-azacitidine and decitabine in a subset of MDS patients. We use complex genetic mouse models to understand how genetic mutations in epigenetic components cause stem cell dysfunction and ultimately lead to pathogenic transformation.

Modifying the epigenome for somatic cell reprogramming

Technological breakthroughs that facilitate global mapping of epigenetic marks, such as DNA methylation, histone modifications and nucleosome positioning, has unveiled how aberrant placement of these epigenetic marks is involved in human disease. A comprehensive understanding of epigenetic mechanisms in health and disease has become a priority in biomedical research. It is now known that a number of human disorders are associated with abnormal distribution of epigenetic marks, suggesting that reversal of these modifications may present a target for therapeutic intervention. As epigenetic modifications are malleable, they are able to be dynamically transformed (unlike genetic mutations).However, no current therapy is able to alter the epigenomic landscape at particular disease-promoting loci while leaving the rest of the normal marks intact in the cell. Currentepigenetic therapies cause global epigenetic remodeling, which may provide a short-term benefit, but have long-lasting unwarranted side-effects. We are developing tools for locus-specific epigenomic remodeling. Our hypothesis is that CRISPRs can be used as finely-honed molecular scalpels to edit the epigenome without off-target effects. The technology could be easily adaptable for any disease with a characterized epigenomic defect and provide powerful tools for the study of epigenetics.

- A Primer on Genome Editing - https://en.wikipedia.org/wiki/Epigenome_editing


Summary

Epigenetic regulation of hematopoietic stem cells (HSCs) ensures lifelong production of blood and bone marrow. Recently, we reported that loss of de novo DNA methyltransferase Dnmt3a results in HSC expansion and impaired differentiation. Here, we report conditional inactivation of Dnmt3b in HSCs either alone or combined with Dnmt3a deletion. Combined loss of Dnmt3a and Dnmt3b was synergistic, resulting in enhanced HSC self-renewal and a more severe block in differentiation than in Dnmt3a-null cells, whereas loss of Dnmt3b resulted in a mild phenotype. Although the predominant Dnmt3bisoform in adult HSCs is catalytically inactive, its residual activity in Dnmt3a-null HSCs can drive some differentiation and generates paradoxical hypermethylation of CpG islands. Dnmt3a/Dnmt3b-null HSCs displayed activated β-catenin signaling, partly accounting for the differentiation block. These data demonstrate distinct roles for Dnmt3b in HSC differentiation and provide insights into complementary de novo methylation patterns governing regulation of HSC fate decisions.

http://www.ncbi.nlm.nih.gov/pubmed/25130491