NIH

Pu Paul Liu, M.D., Ph.D.

Senior Investigator, Translational and Functional Genomics Branch
Head, Oncogenesis and Development Section
Deputy Scientific Director, NHGRI

Scientific Summary

Dr. Liu's laboratory investigates the molecular mechanisms of leukemia, with the long-term goal of translating research findings to improved clinical practices, including better diagnosis and treatment of leukemia and related hematological diseases. A group of diseases that strikes approximately 43,000 Americans each year, leukemias are frequently associated with chromosome abnormalities such as translocations, inversions and deletions. Two genes, CBFB and RUNX1, which encode proteins (CBFb and RUNX1, respectively) that form a dimer for DNA binding and gene expression regulation, are frequent targets of such chromosome abnormalities.  Most of Dr. Liu's work focuses on these two genes and the core binding factor (CBF) leukemia, a subset of leukemia caused by chromosome abnormalities in these two genes.

One form of CBF leukemia is associated with an inversion of chromosome 16. Dr. Liu found that this inversion generates a fusion gene between CBFB and MYH11, the gene encoding smooth muscle myosin heavy chain (SMMHC) (Liu et al., Science 1993). To study the fusion gene CBFB-MYH11, Dr. Liu's group generated a knock-in mouse model, which demonstrated that CBFB-MYH11 blocks normal hematopoiesis through dominant repression of Cbfb and Runx1 (Castilla et al., Cell 1996). Using the Cbfb-MYH11 knock-in mice (in which human MYH11 was inserted into mouse Cbfb to recreate the leukemia fusion gene), they demonstrated that CBFB-MYH11 is necessary but not sufficient for leukemogenesis (Castilla et al., Nature Genetics 1999).  Dr. Liu's group identified cooperating genetic changes for leukemogenesis with a retroviral insertional mutagenesis approach (Castilla et al., PNAS 2004), and demonstrated that the fusion protein CBFβ-SMMHC blocks lymphoid differentiation, which may explain how the fusion protein only causes myeloid leukemia (Zhao et al., Blood 2007).

More recently, Dr. Liu's group made the novel observation that Cbfb-MYH11 induces acute myeloid leukemia (AML) without dominant repression of Runx1, which was previously believed to be the only function of this fusion gene (Kamikubo et al., Cancer Cell 2010). They also identified novel Cbfb-MYH11 target genes during leukemia development by microarray analysis with cells from Cbfb-MYH11 knock-in mice, and identified leukemia-initiating cells using these new targets (Hyde et al., Blood 2010). With a clinically relevant transgenic mouse model, they demonstrated leukemogenic cooperation between Cbfb-MYH11 and mutationally activated KIT, which are frequently found in human AML cases with chromosome 16 inversion (Zhao et al., Blood 2012). Current and future efforts in Dr. Liu's lab include more functional studies of CBFβ-SMMHC with novel transgenic mouse models.

Mouse, zebrafish, iPS cells, small chemical screensThe zebrafish is a useful model for studying embryonic development, and is easily amenable to large-scale genetic studies. Dr. Liu's group, with the help of the NHGRI Zebrafish Core, headed by Dr. Raman Sood, has used zebrafish to study the roles of several transcription factor genes in hematopoiesis, such as runx1, cbfb, and gata1 (Blake et al., Blood 2000; Lyons et al., PNAS 2002, and Sood et al., Blood 2010). Using a positional cloning/candidate gene approach, they identified a non-sense mutation in gata1 as the cause for a zebrafish mutant with a "bloodless" phenotype (Lyons et al., PNAS 2002). As the first gata1 mutation identified in the zebrafish, this finding demonstrated significant functional conservation between mammalian and zebrafish hematopoiesis. Using a high-throughput reverse genetic screening system they generated a fish line carrying a truncation mutation in runx1, which led to discoveries of novel functions of the gene during hematopoiesis (Sood et al., Blood 2010). Dr. Liu's group also generated fish lines with novel mutations in gata1, which uncovered differential roles of gata1 between primitive and definitive stages of hematopoiesis (Belele et al., Blood 2009). More recently Dr. Liu's lab has generated fish lines with null mutations in cbfb and made novel findings regarding its role in hematopoietic stem cell production (Bresciani, Blood in press). Overall the zebrafish has proven to be an excellent model for dissecting the genetic control of early hematopoiesis, especially the formation of hematopoietic stem cells, from fresh angles and in great detail.

A major current and future project in Dr. Liu's lab is to develop targeted treatments for leukemia. Current treatments for CBF leukemia are associated with significant morbidity and mortality, with a 5-year survival rate of 50-60 percent. It was hypothesized that the interaction between RUNX1 and CBFβ is critical for CBF leukemia and can be targeted for drug development. In collaboration with the NIH Chemical Genomics Center (NCGC), part of the National Center for Advancing Translational Sciences (NCATS), Dr. Liu's group developed high-throughput methods to quantify the RUNX1-CBFβ interaction and screened 243,398 compounds. A lead compound (Ro5-3335) was confirmed as an inhibitor of RUNX1-CBFβ interaction in zebrafish embryos and transcriptional reporter assays.  It preferentially killed human CBF leukemia cell lines, and reduced leukemia burden in the mouse CBF leukemia models (Cunningham et al., PNAS 2012). The group is currently conducting structure-activity relationship (SAR) studies to identify more potent analogs and performing additional preclinical tests for eventual clinical trials with human CBF leukemia patients, with support from the Therapeutics for Rare and Neglected Diseases (TRND) Program in NCATS.

The Liu lab is taking a new direction toward application of induced pluripotent stem cell (iPSC) technology to model disease and development of new treatments, especially for diseases without suitable animal models. Supported by the NIH Center for Regenerative Medicine (NCRM) and in collaboration with NIH Intramural Sequencing Center (NISC), the group performed whole genome sequencing of several iPSC lines to demonstrate that the reprogramming process does not generate significant changes at the genomic level (Cheng et al., Cell Stem Cell, 2012). The Liu lab is using the technology to model familial platelet disorder with propensity to acute myeloid leukemia (FPD/AML), caused by mutations in RUNX1, for which good animal models are not available. Dr. Liu hopes to develop cell-based therapies for FPD/AML patients, which may serve as a model for other hematological diseases.

Oncogenesis and Development Section Members

Halah Alkadi, M.S., Graduate Student (Special Volunteer)
Erica Bresciani, Ph.D., Postdoctoral Fellow
Jamie Diemer, Ph.D., Biologist
Erika Kwon, Ph.D., Postdoctoral Fellow
Raman Sood, Ph.D., Associate Investigator and Director, Zebrafish Core
Ling Zhao, Ph.D., Staff Scientist
Tao Zhen, Ph.D., Postdoctoral Fellow

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Posted: June 3, 2015