H. Joachim Deeg

Myelodysplastic syndrome and programmed cell death

The term myelodysplastic syndrome (MDS) describes a broad spectrum of clonal marrow disorders. Our research is focused on the characterization of the underlying pathophysiology and mechanisms of marrow failure, with the aim of developing biologically rational therapeutic strategies. TNF-alpha, Fas/Fas-ligand, and TRAIL (TNF-related apoptosis inducing ligand) and its agonistic and decoy receptors are upregulated MDS, and marrow cells from patients with early stages of the disease show increased rates of programmed cell death (apoptosis) in both clonal and non-clonal hemopoietic precursors. With disease progression, the ratio of TNF receptors R1 and R2 on clonal cells shifts in favor of R2. Since R1 can trigger both pro-apoptotic (via caspase-8) and cytoprotective signals (via NF-kB), R2, however, which lacks a death domain, only cytoprotective signals, the result is a decline in apoptosis in clonal precursors. This appears to be one factor contributing to treatment resistance and clonal expansion with advanced MDS. Hence, differential manipulation of R1 and R2 initiated signals may offer one therapeutic strategy.

Additional studies are aimed at characterizing the role of cytoplasmatic adaptor molecules such as FLIP (long and short), which controls Fas- and TRAIL-induced apoptosis, and which is regulated via NF-kB. One rather controversial question is that of the role of the marrow microenvironment in the dysregulation of hemopoiesis in MDS. There is evidence, however, that gene expression in marrow stroma is modified by TNF-alpha, and preliminary data suggest effects on jagged1 (a ligand for Notch), and the insulin-like growth factor (IGF) binding protein 3 (BP3). These molecules are involved in myeloid differentiation and proliferation, respectively. Therefore, another series of experiments is directed at defining the relevance of stroma-derived signals for the propagation of clonal as compared to non-clonal hemopoietic precursors. Those studies are being pursued, using wild type and genetically modified human stroma cells in vitro and in a murine model (NOD/SCID/β2mnull) in vivo. In addition to functional studies, this work also includes genomic and proteomic analyses.

A second focus is on newly recognized properties of Transferrin (Tf), a central component of iron transport and metabolism. Our studies show that Tf exerts potent cytoprotective/anti-apoptotic functions. Tf protects lympho-hemopoietic cells against apoptosis mediated by TNF-alpha, Fas, or gamma-irradiation. Tf also protects mice against otherwise fatal Fas-mediated hepatic failure. The mechanism involves modulation of the mitochondrial pathway of apoptosis (BID translocation, cytochrome c release, caspase-9 activation).The observed effects are iron independent; in fact holo-Tf triggers pro-apoptotic signals. The role of changes in redox status of the cell is unclear, although changes in GSH/GSSG levels are observed. The relative importance of Tf receptor 1 (CD71) as compared to Tf receptor 2 remains to be determined.

Graft-versus-Host Disease; hemopoietic stem cell transplantation.