Faculty Bibliography

Recurrent somatic ASXL1 mutations occur in patients with myelodysplastic syndrome, myeloproliferative neoplasms, and acute myeloid leukemia, and are associated with adverse outcome. Despite the genetic and clinical data implicating ASXL1 mutations in myeloid malignancies, the mechanisms of transformation by ASXL1 mutations are not understood. Here, we identify that ASXL1 mutations result in loss of polycomb repressive complex 2 (PRC2)-mediated histone H3 lysine 27 (H3K27) tri-methylation. Through integration of microarray data with genome-wide histone modification ChIP-Seq data, we identify targets of ASXL1 repression, including the posterior HOXA cluster that is known to contribute to myeloid transformation. We demonstrate that ASXL1 associates with the PRC2, and that loss of ASXL1 in vivo collaborates with NRASG12D to promote myeloid leukemogenesis.

Hematopoietic stem cells (HSCs) are defined by their ability to self-renew and reconstitute all elements of the hematopoietic system. Acute myeloid leukemia (AML) is thought to arise from, and be maintained by, leukemic stem cells (LSCs), which exhibit similar features to HSCs, including the abilities to self-renew and differentiate into non-self-renewing cells. Acquisition of stem-cell-like characteristics by the LSCs is likely mediated in part by molecular mechanisms that normally regulate HSC function. Thus, understanding the shared and unique aspects of the molecular regulation of these cell populations will be important to understanding the relationship between normal hematopoiesis and leukemogenesis. MicroRNAs (miRNAs) are small noncoding RNAs that act at the posttranscriptional level to regulate protein expression. Unfortunately, most investigations of the role of miRNAs in normal hematopoiesis have been restricted to studies of their effects on lineage commitment in progenitors and mature effector cell function, but not on HSCs. Recent studies have identified miRNAs that enhance HSC function, and an abundance of profiling studies using primary AML samples have identified dysregulated miRNAs that may target genes implicated in self-renewal (HOX genes, P53, and PTEN), thus providing a potential link between normal and malignant stem cells. While these studies as well as recent in vivo models of miRNA-induced leukemogenesis (e.g. miR-29a, miR-125b) suggest a role for miRNAs in the development of AML, future studies using serial transplantation of primary AML blasts, from both mouse models and primary human AML specimens, will be necessary to assess the roles of miRNAs in LSC biology.

Reduced gene dosage of ribosomal protein subunits has been implicated in 5q- myelodysplastic syndrome and Diamond Blackfan anemia, but the cellular and pathophysiologic defects associated with these conditions are enigmatic. Using conditional inactivation of the ribosomal protein S6 gene in laboratory mice, we found that reduced ribosomal protein gene dosage recapitulates cardinal features of the 5q- syndrome, including macrocytic anemia, erythroid hypoplasia, and megakaryocytic dysplasia with thrombocytosis, and that p53 plays a critical role in manifestation of these phenotypes. The blood cell abnormalities are accompanied by a reduction in the number of HSCs, a specific defect in late erythrocyte development, and suggest a disease-specific ontogenetic pathway for megakaryocyte development. Further studies of highly purified HSCs from healthy patients and from those with myelodysplastic syndrome link reduced expression of ribosomal protein genes to decreased RBC maturation and suggest an underlying and common pathophysiologic pathway for additional subtypes of myelodysplastic syndrome.

Hematopoietic stem cells (HSCs) self-renew to maintain the lifelong production of all blood populations. Here, we show that the proliferating cell nuclear antigen-associated factor (Paf) is highly expressed in cycling bone marrow HSCs and plays a critical role in hematopoiesis. Mice lacking Paf exhibited reduced bone marrow cellularity; reduced numbers of HSCs and committed progenitors; and leukopenia. These phenotypes are caused by a cell-intrinsic blockage in the development of long-term (LT)-HSCs into multipotent progenitors and preferential loss of lymphoid progenitors caused by markedly increased p53-mediated apoptosis. In addition, LT-HSCs from Paf(-/-) mice had increased levels of reactive oxygen species (ROS), failed to maintain quiescence, and were unable to support LT hematopoiesis. The loss of lymphoid progenitors was likely due the increased levels of ROS in LT-HSCs caused by treatment of Paf(-/-) mice with the anti-oxidant N-acetylcysteine restored lymphoid progenitor numbers to that of Paf(+/+) mice. Collectively, our studies identify Paf as a novel and essential regulator of early hematopoiesis.

Acute lymphoblastic leukemia (ALL) is the most common pediatric malignancy and constitutes 15% of adult leukemias. Although overall prognosis for pediatric ALL is favorable, high-risk pediatric patients and most adult patients have significantly worse outcomes. Multiagent chemotherapy is standard of care for both pediatric and adult ALL, but is associated with systemic toxicity and long-term side effects and is relatively ineffective against certain ALL subtypes. Recent efforts have focused on the development of targeted therapies for ALL including monoclonal antibodies. Here, we report the identification of CD47, a protein that inhibits phagocytosis, as an antibody target in standard and high-risk ALL. CD47 was found to be more highly expressed on a subset of human ALL patient samples compared with normal cell counterparts and to be an independent predictor of survival and disease refractoriness in several ALL patient cohorts. In addition, a blocking monoclonal antibody against CD47 enabled phagocytosis of ALL cells by macrophages in vitro and inhibited tumor engraftment in vivo. Significantly, anti-CD47 antibody eliminated ALL in the peripheral blood, bone marrow, spleen, and liver of mice engrafted with primary human ALL. These data provide preclinical support for the development of an anti-CD47 antibody therapy for treatment of human ALL.

Under normal physiological conditions, cellular homeostasis is partly regulated by a balance of pro- and anti-phagocytic signals. CD47, which prevents cancer cell phagocytosis by the innate immune system, is highly expressed on several human cancers including acute myeloid leukemia, non-Hodgkin's lymphoma, and bladder cancer. Blocking CD47 with a monoclonal antibody results in phagocytosis of cancer cells and leads to in vivo tumor elimination, yet normal cells remain mostly unaffected. Thus, we postulated that cancer cells must also display a potent pro-phagocytic signal. Here, we identified calreticulin as a pro-phagocytic signal that was highly expressed on the surface of several human cancers, but was minimally expressed on most normal cells. Increased CD47 expression correlated with high amounts of calreticulin on cancer cells and was necessary for protection from calreticulin-mediated phagocytosis. Blocking the interaction of target cell calreticulin with its receptor, low-density lipoprotein receptor-related protein, on phagocytic cells prevented anti-CD47 antibody-mediated phagocytosis. Furthermore, increased calreticulin expression was an adverse prognostic factor in diverse tumors including neuroblastoma, bladder cancer, and non-Hodgkin's lymphoma. These findings identify calreticulin as the dominant pro-phagocytic signal on several human cancers, provide an explanation for the selective targeting of tumor cells by anti-CD47 antibody, and highlight the balance between pro- and anti-phagocytic signals in the immune evasion of cancer.

MicroRNAs profoundly impact hematopoietic cells by regulating progenitor cell-fate decisions, as well as mature immune effector function. However to date, microRNAs that regulate hematopoietic stem cell (HSC) function have been less well characterized. Here we show that microRNA-125b (miR-125b) is highly expressed in HSCs and its expression decreases in committed progenitors. Overexpression of miR-125b in mouse HSC enhances their function, demonstrated through serial transplantation of highly purified HSC, and enriches for the previously described Slamf1(lo)CD34(-) lymphoid-balanced and the Slamf1(neg)CD34(-) lymphoid-biased cell subsets within the multipotent HSC (CD34-KLS) fraction. Mature peripheral blood cells derived from the miR-125b-overexpressing HSC are skewed toward the lymphoid lineage. Consistent with this observation, miR-125b overexpression significantly increases the number of early B-progenitor cells within the spleen and induces the expansion and enrichment of the lymphoid-balanced and lymphoid-biased HSC subset via an antiapoptotic mechanism, reducing the mRNA expression levels of two proapoptotic targets, Bmf and KLF13. The antiapoptotic effect of miR-125b is more pronounced in the lymphoid-biased HSC subset because of their intrinsic higher baseline levels of apoptosis. These effects of miR-125b are associated with the development of lymphoproliferative disease, marked by expansion of CD8(+) T lymphocytes. Taken together, these data reveal that miR-125b regulates HSC survival and can promote lymphoid-fate decisions at the level of the HSC by preferentially expanding lymphoid-balanced and lymphoid-biased HSC.

Monoclonal antibodies are standard therapeutics for several cancers including the anti-CD20 antibody rituximab for B cell non-Hodgkin lymphoma (NHL). Rituximab and other antibodies are not curative and must be combined with cytotoxic chemotherapy for clinical benefit. Here we report the eradication of human NHL solely with a monoclonal antibody therapy combining rituximab with a blocking anti-CD47 antibody. We identified increased expression of CD47 on human NHL cells and determined that higher CD47 expression independently predicted adverse clinical outcomes in multiple NHL subtypes. Blocking anti-CD47 antibodies preferentially enabled phagocytosis of NHL cells and synergized with rituximab. Treatment of human NHL-engrafted mice with anti-CD47 antibody reduced lymphoma burden and improved survival, while combination treatment with rituximab led to elimination of lymphoma and cure. These antibodies synergized through a mechanism combining Fc receptor (FcR)-dependent and FcR-independent stimulation of phagocytosis that might be applicable to many other cancers.