Effects of Acute Leukemia Viruses on the Differentiation of Hematopoietic Cells. In: Weissman IL, editor. Leukemia. Berlin: Springer Berlin Heidelberg; 1985. pp. 131-45. [DOI: 10.1007/978-3-642-69722-7_9]

The A-myb transcription factor in neoplastic and normal B cells

The myb family of transcription factors has been strongly implicated in the regulation of cell growth and differentiation in the haematopoietic system. The v-myb oncogene, carried by avian defective retroviruses, causes leukaemias in the chicken and transforms haematopoietic cells in vitro. Its normal cellular equivalent c-myb, has been shown to promote the proliferation and block the differentiation of haematopoietic cells in several experimental models and is required for fetal haematopoiesis. Two other members of the family have been cloned more recently, A-myb and B-myb, which show sequence homology with c-myb in several domains, of which the DNA binding domain as well as other regulatory domains. Both have been shown to be transcription factors. B-myb is also involved in the control of proliferation and differentiation, but, unlike c-myb, it is expressed in many cell types. The third member of the family, A-myb, shows the most restricted pattern of expression, suggesting a very specific role for this transcription factor. A-myb is expressed in a subpopulation of normal B lymphocytes activated in vivo and localised in the germinal center of peripheral lymphoid organs and is not detected at significant levels in all other mature or immature haematopoietic populations studied, including bone marrow cells, T lymphocytes, granulocytes, monocytes, either at rest or after in vitro activation. These studies indicate that A-myb plays a role during a narrow window of normal B cell differentiation. A-myb expression has also been studied in a wide range of neoplastic B cells, representing the whole spectrum of B cell differentiation. A-myb is strongly expressed in Burkitt's lymphomas (BL) and slg+ B-acute lymphoblastic leukaemias (B-ALL) and not in all other leukaemias/lymphomas tested, with the exception of a subset of CLL (about 25% of cases). It is intriguing that the A-myb genome has been localised relatively close to the c-myc gene on chromosome 8, suggesting that the c-myc translocation in BL and B-ALL may affect A-myb transcription. Studies are in progress to investigate the functional relationship between A-myb and c-myc, particularly in the context of BL cells and to determine whether A-myb is deregulated in these cells.

Regulation of hematopoietic cell proliferation and differentiation by the myb oncogene family of transcription factors

The myb family of genes include the virally encoded v-myb oncogene, its normal cellular equivalent c-myb and two related members called A-myb and B-myb. They are all transcription factors that recognize the same DNA sequence (PyAACG/TG) and are all involved in the regulation of proliferation and differentiation in different cell types, including hematopoietic cells. C-myb is most highly expressed in hematopoietic cells and its oncogenic activation leads to transformation of these cells. Several lines of evidence have demonstrated that c-myb regulates both the proliferation and differentiation of hematopoietic cells of different lineages. The mechanisms of action of c-myb and v-myb are becoming clearer, mostly through the study of the different genes that are regulated by these transcription factors and the cofactors with which c-myb and v-myb co-operate. More recently the biological and biochemical functions of the B-myb and A-myb gene products have been investigated. Evidence for the function of the different members of the myb family in relation to hematopoietic proliferation and differentiation is presented, and the different roles of the myb genes are discussed.

Reversibility of differentiation and proliferative capacity in avian myelomonocytic cells transformed by tsE26 leukemia virus

Chicken hematopoietic cells infected with E26 leukemia virus can be transformed into growth factor-dependent, rapidly proliferating cells that exhibit properties of immature myelomonocytic cells. Cells infected with a mutant of E26 that carries a temperature-sensitive lesion, presumably residing in the myb oncogene, differentiate into resting, macrophage-like cells when shifted from 37 degrees to 42 degrees C (Beug et al. 1984). Here we show that differentiated tsE26 cells gradually reacquire an immature phenotype and proliferative capacity when shifted back to 37 degrees C, provided that they are kept at 42 degrees C no longer than 4-8 days. We also show that DNA synthesis inhibitors do not prevent terminal differentiation at 42 degrees C but inhibit the complete reexpression of the immature phenotype in downshift experiments. Our results suggest that the reactivation of the E26 protein function can both induce a "retro-differentiation" and cell proliferation in myelomonocytic target cells.