The adaptor protein Shb has been implicated in the signaling of several tyrosine kinase receptors and previous studies have suggested a role for Shb in the signal transduction of T cells. Shb associates with the T cell receptor (TCR) and partakes in the signal propagation of activated T lymphocytes. In order to explore Shb’s influence on TCR signaling in vivo, T cell development and function was studied in a Shb knockout mouse. The loss of Shb led to aberrant TCR signaling in both thymocytes and peripheral CD4+ TH cells, with elevated basal phosphorylation of key components in the signal cascade. Shb was found to be dispensable for thymocyte development, but its absence resulted in a TH2 bias in in vitro stimulated peripheral CD4+ TH cells. As imbalances in TH2 responses are linked to allergic diseases, we further explored Shb’s role in immune regulation in a mouse model of atopic dermatitis. Shb knockout mice exhibit more aggravated signs of atopic dermatitis, including increased immune cell recruitment to the affected areas and elevated mRNA levels of typical TH2 cytokines. The effect of Shb on hematopoiesis in general was determined by examining populations of long-term hematopoietic stem cells (LT-HSCs) and hematopoietic progenitor cells in bone marrow of Shb knockout and wild type mice. Shb deficient bone marrow was found to contain significantly fewer relative numbers of LT-HSCs due to a proliferative defect. The reduced cell cycle activity of Shb LT-HSCs could further be linked to an abnormal regulation of the focal adhesion kinase/Rac1/p21-activated kinase pathway. Since alterations in LT-HSC proliferative abilities may have implications for leukemia development, BCR-Abl induced myeloid neoplasia was investigated in the absence of Shb. Shb deficiency confers a more aggressive progression of BCR-Abl induced myeloid neoplasia characterized by an increased peripheral blood neutrophilia and a deregulated cytokine profile. In addition, focal adhesion kinase and STAT3 signaling is hyperactivated in Shb knockout leukemic cells. In conclusion, Shb appears to be a multifunctional signaling mediator that controls several responses in hematopoietic cells, under homeostatic as well as disease conditions.

The azanucleosides decitabine and azacytidine are used widely in the treatment of myeloid neoplasia and increasingly in the context of combination therapies. Although they were long regarded as being largely interchangeable in their function as hypomethylating agents, the azanucleosides actually have different mechanisms of action; decitabine interferes primarily with the methylation of DNA and azacytidine with that of RNA. Here, we examine the role of DNA methylation in the lineage commitment of stem cells during normal hematopoiesis and consider how mutations in epigenetic regulators such as DNMT3A and TET2 can lead to clonal expansion and subsequent neoplastic progression. We also consider why the efficacy of azanucleoside treatment is not limited to neoplasias carrying mutations in epigenetic regulators. Finally, we summarise recent data describing a role for azacytidine-sensitive RNA methylation in lineage commitment and in the cellular response to stress. By summarising and interpreting evidence for azanucleoside involvement in a range of cellular processes, our review is intended to illustrate the need to consider multiple modes of action in the design and stratification of future combination therapies.

The azanucleosides decitabine and azacytidine are used widely in the treatment of myeloid neoplasia and increasingly in the context of combination therapies. Although they were long regarded as being largely interchangeable in their function as hypomethylating agents, the azanucleosides actually have different mechanisms of action; decitabine interferes primarily with the methylation of DNA and azacytidine with that of RNA. Here, we examine the role of DNA methylation in the lineage commitment of stem cells during normal hematopoiesis and consider how mutations in epigenetic regulators such as DNMT3A and TET2 can lead to clonal expansion and subsequent neoplastic progression. We also consider why the efficacy of azanucleoside treatment is not limited to neoplasias carrying mutations in epigenetic regulators. Finally, we summarise recent data describing a role for azacytidine-sensitive RNA methylation in lineage commitment and in the cellular response to stress. By summarising and interpreting evidence for azanucleoside involvement in a range of cellular processes, our review is intended to illustrate the need to consider multiple modes of action in the design and stratification of future combination therapies.