Altered expression of G1-specific genes in human malignant myeloid cells

We have studied the expression of cell-cycle genes specific to the G1 (2A9, 2F1, 4F1, c-myc) and S (histone H3) phases of the cell cycle in normal and malignant human myeloid cycling cells. The levels of expression were determined by measuring the amounts of specific RNA in blot hybridization assays. Levels of expression of the G1 genes were compared to the level of expression of the S-phase-specific H3 gene. This method can distinguish whether an increased expression of G1 genes is truly due to deregulation or simply reflects an increase in the fraction of proliferating cells. In a normal asynchronous system provided by the bone marrow cells of three normal donors, the expressions of the four G1-specific genes 2A9, 2F1, 4F1, and c-myc, and of the S-phase-specific gene H3 were in ratios that differed little from one individual to another. In the total RNA of eight patients in the chronic phase of chronic myelogenous leukemia, a high level of expression of G1 cell-cycle genes was paralleled by a high level of expression of the S-phase H3 gene, simply reflecting an increase in the fraction of proliferating cells. In patients with acute myelogenous leukemia (AML), the RNA levels of 2F1 and 4F1 paralleled the expression of H3-i.e., the ratios of expression 2F1/H3 and 4F1/H3 were the same as in normal bone marrow cells. However, in 9 of 10 patients with AML we found that the expression of c-myc was elevated with respect to H3 expression. The expression of 2A9 (with respect to H3) was also elevated in some of these AML patients. Two important conclusions can be drawn from these findings: increased levels of a G1-specific RNA in a tumor may not indicate overexpression of that gene but may instead simply reflect the fraction of proliferating cells; and in some patients with AML, however, the expression of certain G1 genes is truly deregulated and might contribute to the impairment of proliferative control that is associated with this phenotype.