In childhood acute lymphoblastic leukemia the hypophosphorylated retinoblastoma protein, p110RB, is diminished, as compared with normal CD34+ peripheral blood progenitor cells.
Acute lymphoblastic leukemia (ALL) of childhood arises from dysregulated clonal expansion of immature lymphoid precursor cells that fail to differentiate into functional lymphocytes. The cell-cycling status of ALL cells shares many common features with that of normal CD34+ hematopoietic progenitor cells, such as low number of resting G0 and cycling S phase cells even though the growth fraction is high. Thus, ALL cells should be in a long G1 phase. Phosphorylation of the retinoblastoma protein is a crucial step in cell-cycle progression from G0/early G1 to late G1/S phase. We therefore analyzed the G1 distribution of these two immature cell populations by immunostaining and Western blot. Bone marrow samples from children with ALL at diagnosis as well as purified CD34+ cells, before and after in vitro stimulation with cytokines, were investigated for the expression of hypophosphorylated p110RB (early G1 phase), total retinoblastoma protein, statin (G0 phase), bromo-deoxyuridine (S phase), proliferating cell nuclear antigen, and p120 (cycling cells). Compared with unstimulated CD34+ cells (95.8 +/- 1.2%) the component of ALL cells containing hypophosphorylated p110RB (16.3 +/- 13.2%) was significantly reduced (p = 0.00018), whereas only a minor difference could be detected for the proportion of cycling cells (p = 0.03), and no difference in G0 and S phase cells (p > 0.05). Our results indicate that, as opposed to unstimulated CD34+ cells, the majority of ALL cells are beyond the restriction point and therefore irreversible committed to DNA replication and mitosis.
Leibundgut
K
K
Department of Pediatrics, University of Bern, Inselspital, Switzerland.
Schmitz
N
N
Tobler
A
A
Lüthy
A R
AR
Hirt
A
A
D018952:Antigens, CD34 / Q000032:analysis; D001402:B-Lymphocytes / Q000276:immunology / Q000378:metabolism; D001853:Bone Marrow / Q000276:immunology / Q000473:pathology; D002453:Cell Cycle; D002648:Child; D006412:Hematopoietic Stem Cells / Q000276:immunology / Q000378:metabolism*; D006801:Humans; D015448:Leukemia, B-Cell / Q000276:immunology / Q000378:metabolism / Q000473:pathology; D015458:Leukemia, T-Cell / Q000276:immunology / Q000378:metabolism / Q000473:pathology; D010766:Phosphorylation; D054198:Precursor Cell Lymphoblastic Leukemia-Lymphoma / Q000276:immunology / Q000378:metabolism* / Q000473:pathology; D016160:Retinoblastoma Protein / Q000378:metabolism*
D018952:Antigens, CD34; D016160:Retinoblastoma Protein