High expression of two diverged homeobox genes, HB24 and HB9, in acute leukemias: molecular markers of hematopoietic cell immaturity.

Homeobox genes encode for sequence-specific DNA-binding proteins which have been implicated in the control of gene expression during development and in certain adult tissues. Two recently characterized human homeobox-containing genes, HB9 and HB24, are known to be expressed in hematopoietic progenitors and to be involved in the regulation of growth and differentiation of progenitor cells to mature hematopoietic cell types. In this study, elevated levels of HB24 and HB9 mRNA expression were detected in bone marrow and peripheral blood mononuclear cells (PBMC) isolated from patients with acute myelogenous or acute lymphocytic leukemia. While the levels of both mRNAs were elevated in all the patients with acute leukemias, the levels of HB9 mRNA were more variable than those of HB24. Immunohistochemical analysis utilizing an HB24 polyclonal antiserum demonstrated elevated levels of HB24 protein in cytopreparations of acute leukemic cells. Nuclear run-on experiments showed that the increases of HB9 and HB24 mRNA transcripts in patients' cells were, at least in part, secondary to increased transcription. The expression of HB9 and HB24 correlated with the clinical status of the patient. No significant level of expression of either HB9 or HB24 was detected in PBMC isolated from patients in remission. In contrast to the findings with cells isolated from patients with acute leukemias, no significant increase in either HB9 or HB24 transcript levels were found in cells from patients with chronic lymphocytic or chronic myelogenous leukemia when compared to normal controls. These findings demonstrate that high levels of HB9 and HB24 expression are common features of acute leukemia and suggest the possibility that the dysregulated expression of these two genes may contribute to leukemogenesis. However, since these two genes are markers of immature hematopoietic cells they may not have an etiologic role in leukemogenesis.