Enforced Expression of the Toll/Interleukin-1 Receptor Associated Protein in Murine Marrow Results in MDS-Like Bone Marrow Failure Mediated by Interferon Gamma

Abstract 555

The Myelodysplastic Syndromes (MDS) are the most common hematological malignancies arising from stem/progenitor cells. MDS is characterized by ineffective hematopoiesis in one or more lineage of the bone marrow, resulting in peripheral cytopenias and the propensity to develop into either acute myeloid leukemia (AML) or bone marrow failure (BMF). The molecular and cellular events involved in MDS development are yet to be elucidated. Large scale cytogenetic aberrations such as del(5q) have been associated with many types of MDS. Previous work from our lab has shown that the chromosomal band 5q32–33 harbours microRNAs (miR-145 and miR-146a), haploinsufficiency of which recapitulates the megakaryocytic dysplasia and neutropenia associated with del(5q) MDS. Ingenuity pathway analysis predicts that innate immune signaling is one of the most highly deregulated pathways by loss of these two miRNAs. Two innate immune signaling molecules, TNF receptor associated factor 6 (TRAF6) and Toll/interleukin-1 receptor associated protein (TIRAP), are targets ofmiR-146a and miR-145 respectively. While the role of TRAF6 in del(5q) MDS has been investigated, little is known about the role of TIRAP in MDS pathogenesis.

To investigate the role of TIRAP in aberrant hematopoiesis, we transplanted lethally irradiated mice with bone marrow cells expressing TIRAP or vector control. TIRAP transplanted mice develop an MDS-like bone marrow failure characterized by anemia, neutropenia, and thrombocytopenia as early as 4 weeks post-transplant (unlike TRAF6 transplanted mice which succumb to BMF or AML approximately 4 months post-transplant). The discrepancy between TIRAP and TRAF6 disease progression models suggests that TRAF6 independent signaling originating from TIRAP may be responsible for the rapid bone marrow failure onset. Furthermore, TIRAP transplanted mice display hypocellular marrows characterized by increased apoptosis as measured by Annexin V/PI staining, similar to low risk MDS patients.

In MDS, normal hematopoiesis is blocked in the non-disease fraction of bone marrow by autoimmunity associated cytokines. To identify factors that may be responsible for suppression of normal hematopoiesis in our BMF model, we performed cytokine expression profiling of TIRAP expressing marrow. Quantitative RT-PCR showed increased expression of both IL-10 and IFNγ in TIRAP expressing marrow compared to vector control. Interestingly, IFNγ is known to suppress hematopoiesis has been shown to be overexpressed in BM mononuclear cells in MDS and other BMF conditions. Also, IL-10 levels have been shown to be elevated in high risk MDS patients. To investigate the role of IL-10 and IFNγ in TIRAP mediated marrow failure, we transduced IL-10 −/− and IFNγ −/− marrow with TIRAP or vector control, and transplanted wildtype leathally irradiated mice. Loss of IFNγ but not IL-10 partially rescues the pancytopenia phenotype and prevents early death due to bone marrow failure. Interestingly however, mice transplanted with TIRAP expressing IFNγ −/− bone marrow succumb to a myeloproliferative disorder at later time points, suggesting that the factors responsible for myelosuppression are the same ones responsible for progression to AML.