David A. Ingram Jr, MD
Professor: Department of Pediatrics
Professor: Department of Pediatrics
Clinical Section: Neonatal-Perinatal Medicine
Basic Science Joint Appointment: Biochemistry and Molecular Biology
Postdoctoral Fellowship: Pediatric Scientist Development Program Indiana University School of Medicine
Fellowship: Neonatal-Perinatal Medicine, Indiana University School of Medicine
Residency: University of California at San Francisco
MD: Vanderbilt University School of Medicine
Current Research Interests:
To understand how signaling pathways control the development of hematopoietic stem and progenitor cells and how aberrations in these pathways cause human blood disorders and leukemias.
Neurofibromatosis type I (NF1) is a common genetic disorder caused by mutations in the NF1 tumor suppressor gene. Children with NF1 are strongly predisposed to developing juvenile myelomonocytic leukemia (JMML). Neurofibromin, the protein product of NF1, is a GTPase activating protein (GAP) for p21ras that accelerates the conversion of active p21ras-GTP to inactive p21ras-GDP. Though Nf1 -/- mice die in utero, mice reconstituted with Nf1 -/- fetal hematopoietic cells develop a myeloproliferative disease (MPD) that is remarkably reminiscent of the JMML observed in NF1 patients. While neurofibromin functions as a GAP for p21ras, alterations in specific p21ras effector pathways in NF1 deficient cells are largely unknown. A hallmark of Nf1 deficient cells is their propensity to hyperproliferate and survive in response to low doses of cytokines. Previous studies argue that loss of neurofibromin results in increased activation of the classical p21ras-Raf-Mek-ERK pathway which leads to hyperproliferation of Nf1 deficient cells. However, we have preliminary data to support an alternative biochemical model. Utilizing pharmacological inhibitors of PI-3 kinase, our data indicate that hyperactivation of the p21ras-PI-3 kinase pathway is responsible for the hyperproliferation and increased survival of Nf1 -/- hematopoietic cells. These findings are remarkably consistent with other murine models where hyperactivation of PI-3 kinase leads to an abnormal expansion of myeloid cells and progression of myeloid leukemia in vivo.
Though these data are encouraging, interpretation of results using pharmacologic inhibitors is limited because there are four classes of PI-3 kinase, and the inhibitors we utilized inactivate all classes. We are currently using genetic models to directly determine whether hyperactivation of class IA PI-3 kinase specifically alters the growth characteristics of Nf1 -/- hematopoietic cells. These results would be a major step forward in understanding the biology of these cells and permit a more rational design of experimental therapeutics. Our rationale for studying class IA PI-3 kinase is twofold. First, in contrast to other PI-3 kinases, all class IA PI-3 kinase catalytic subunits contain a p21ras-binding domain. Second, p21ras directly interacts with these catalytic subunits to augment kinase activity in vitro, and no evidence exists to show that p21ras augments the activity of other classes of PI-3 kinase. Recently, a p85a ?a regulatory subunit of class IA PI-3 kinase) knockout strain was generated which results in a 97 percent reduction in class IA PI-3 kinase activity in myeloid cells. Given our preliminary data and observations by others, we hypothesize that hyperactivation of the p21ras-class IA PI-3 kinase pathway alters the proliferation and survival of Nf1 -/- hematopoietic progenitors and contributes to the progression of the MPD in mice transplanted with Nf1 -/- fetal hematopoietic cells. To formally test this hypothesis, a series of in vivo and in vitro experiments utilizing a genetic intercross of Nf1 +/- and p85a knockout mice are being conducted.
Ongoing projects include:
- To generate a murine model to test the hypothesis that hyperactivation of class IA PI-3 kinase directly contributes to the myeloproliferative disease in mice reconstituted with Nf1 -/- hematopoietic stem cells by altering biochemical pathways which control hematopoietic cell proliferation and survival.
- To examine how genetic inactivation of class IA PI-3 kinase alters the proliferation and growth factor responsiveness of committed, multipotential, and primitive hematopoietic Nf1 +/+ and Nf1 -/- progenitor cells in vitro and in vivo.
- To examine how neurofibromin and class IA PI-3 kinase regulate cell cycle progression and survival of phenotypically defined cells in different compartments of the hematopoietic system in vivo.