Decreased Expression of Stem Cell-Specific Genes in Aplastic Anemia; Possible Regulation of HOXB4 Gene by GATA-2.

Aplastic anemia (AA) is characterized by reduced hematopoiesis resulting in pancytopenia. It is suggested that a certain immunological attack to hematopoietic stem cells play an important role in developing AA. However, limited information is available for the intrinsic characteristics of stem cells in AA. Previous work in our laboratory showed decreased expression of GATA-2 gene in CD34 positive cells in AA, suggesting that there is an aberrant expression of stem cell-specific genes in stem cells in AA. Recently it is emerged that some genes such as HOXB4 and BMI-1, function for the proliferation and maintenance of stem cells. In this study, we examined expression levels of HOXB4 and BMI-1 in CD34 positive cells by quantitative PCR in 10 patients with AA and 13 with idiopathic thrombocytopenic purpura (ITP). Between these two factors, the expression level of HOXB4 was markedly decreased in AA compared with in ITP, whereas that of BMI-1 was not. Moreover, the expression level of GATA-2 in these populations was significantly correlated to HOXB4 gene expression (Spearman’s rank correlation, r=0.6573 p<0.01) compared to that of BMI-1(r=0.4107, p>0.05). As they functions as a transcription factor, these results raise the possibility that GATA-2 and HOXB4 regulate each other. To explore this possibility, first, we cloned HOXB4 5′flanking region by PCR and performed promoter analysis. Since the previous report showed that the region from −164 to −116 was important for promoter activity of HOXB4 gene, we focused on a GATA element located at −160. When this element was deleted, the reporter activity was decreased to 60% of wild-type in K562 cells. Furthermore, co-transfection of GATA-2 expression vector significantly activates the reporter gene in a dose dependent manner. EMSA revealed that GATA-2 binds specifically to this element. On the other hand, the active region both of exon 1S and 1G promoter of GATA-2 gene, which was identified by the promoter analysis, did not contain the consensus sequence recognized by HOXB4. These findings suggest that in stem cells in AA, the decreased expression of GATA-2 gene lead to the reduced HOXB4 gene expression, which may responsible for the development of the disease.