Deregulating the expression of Bcl-x_L, an inhibitor of apoptosis, in an erythropoietin-dependent erythroblast cell line averts apoptosis induced by the withdrawal of erythropoietin. Since in polycythemia vera an abnormal clone of erythroid progenitors is independent of erythropoietin, we investigated whether the endogenous expression of Bcl-x_L was deregulated in these cells.

Erythroid colonies from patients with polycythemia vera and normal subjects were cultured in the presence and absence of erythropoietin and assessed by immunocytochemical and flow-cytometric analysis with anti–Bcl-x antibodies that recognize the two species of Bcl-x (Bcl-x _L and Bcl-x_S). Reverse-transcriptase–polymerase-chain-reaction analysis was used to determine which one of the two species was responsible for anti–Bcl-x staining. Bone marrow mononuclear cells from 8 healthy bone marrow donors, 14 patients with polycythemia vera, 19 patients with other myeloproliferative syndromes, and 12 patients with secondary erythrocytosis were analyzed by flow cytometry with antibodies against Bcl-x and glycophorin A, an erythroid marker.

Erythroid cells from patients with polycythemia vera survived in vitro without erythropoietin, and this finding correlated with the expression of Bcl-x protein (Bcl-x_L messenger RNA was the main species of Bcl-x found), even in mature erythroblasts that normally do not express Bcl-x. The mean (±SD) percentage of cells positive for both glycophorin A and Bcl-x in the 14 patients with polycythemia vera (21.8±3.6 percent) was significantly higher than that in 8 normal donors (6.62±1.58 percent), 12 patients with secondary erythrocytosis (6.87±1.95 percent), 9 patients with essential thrombocythemia (3.81±0.97 percent), and 10 patients with chronic myeloid leukemia (2.7±0.41 percent).

Deregulated expression of Bcl-x may contribute to the erythropoietin-independent survival of erythroid-lineage cells in polycythemia vera and thereby contribute to the pathogenesis of this disease.