Erythrocyte mean corpuscular volume (MCV) provides important information about anemia. Anemias with decreased MCVs, such as iron deficiency or thalassemia, are a consequence of inadequate heme or globin synthesis. Anemias with elevated reticulocytes, like those resulting from blood loss or hemolysis, have increased MCVs because reticulocytes are larger than mature erythrocytes. In macrocytic anemias with an inappropriately low reticulocyte number, increased MCVs are the result of impaired DNA synthesis, as found in megaloblastic anemia, myelodysplasia, Diamond-Blackfan anemia, and Fanconi anemia. Among vertebrates, a reciprocal relationship exists between MCV and RBC counts such that species with lower MCVs have a higher concentration of circulating erythrocytes and those with larger MCVs have a lower concentration of circulating erythrocytes.1,2 Using human genomewide associated studies, Sankaran et al. identified an erythroid-specific enhancer 15 kb upstream of the promoter of CCND3, the gene encoding cyclin D3. The D-type cyclins bind and activate cyclindependent kinases CDK4 and CDK6, which promote cell-cycle progression from the gap1 (G1) to the subsequent DNA-synthesis (S) phase. Cyclin D3 has a role in the development of T and B lymphocytes, and rearrangements of CCND3 have been associated with T- and B-lymphoid malignancies. Sankaran et al. report that cyclin D3 regulates cell division in terminally differentiating erythropoietic cells, thereby controlling final exit from cell cycle and ultimate erythrocyte size (Figure).
A Model of How Reduced or Absent Expression of Cyclin D3 Can Modulate Erythropoiesis and Cause Increased Erythrocyte Size and Reduced RBC Counts. Model of Sankaran et al. in which reduced numbers of cell divisions due to decreased or absent cyclin D3 during terminal erythropoiesis leads to increased MCV and decreased RBC count. Under normal conditions, terminally differentiating proerythroblasts divide three times with a reduction in erythroblast size at each division before they cease dividing and mature into erythrocytes. When cyclin D3 is decreased or absent, the proerythroblasts divide twice before they cease dividing and mature into erythrocytes. Because the cyclin D3-deficient erythroblasts have divided less, the erythroblasts that mature into erythrocytes are fewer and larger than normal, resulting in fewer but larger erythrocytes.Sankaran VG et al. Genes Dev. 2012;26:2075-2087 © 2012 by Cold Spring Harbor Laboratory Press
A Model of How Reduced or Absent Expression of Cyclin D3 Can Modulate Erythropoiesis and Cause Increased Erythrocyte Size and Reduced RBC Counts. Model of Sankaran et al. in which reduced numbers of cell divisions due to decreased or absent cyclin D3 during terminal erythropoiesis leads to increased MCV and decreased RBC count. Under normal conditions, terminally differentiating proerythroblasts divide three times with a reduction in erythroblast size at each division before they cease dividing and mature into erythrocytes. When cyclin D3 is decreased or absent, the proerythroblasts divide twice before they cease dividing and mature into erythrocytes. Because the cyclin D3-deficient erythroblasts have divided less, the erythroblasts that mature into erythrocytes are fewer and larger than normal, resulting in fewer but larger erythrocytes.Sankaran VG et al. Genes Dev. 2012;26:2075-2087 © 2012 by Cold Spring Harbor Laboratory Press
In avian and mammalian erythropoiesis in vitro, progenitors at the colony-forming-unit-erythroid/proerythroblast stages, which proliferate without differentiating, can be induced to terminally differentiate by altering culture conditions.3,4 During terminal differentiation, erythroblasts undergo dramatic changes that transform them into much smaller cells containing high concentrations of hemoglobin. Cell size reductions during terminal erythroid differentiation result from shortening the G1phase of cell cycle while the length of S and gap2/mitosis (G2/M) phases remain unaffected.3 After these accelerated cell divisions, terminally differentiating erythroblasts abruptly stop dividing and mature into reticulocytes. In mammals, this erythroblast maturation includes enucleation. Sankaran et al. demonstrate that murine erythroblasts with Ccnd3 knockout or knockdown or with inhibition of CDK4 and CDK6 have fewer divisions during terminal differentiation. Erythrocytes produced in these in vitro studies and in vivo in Ccnd3 knockout mice had increased MCVs. Compared with wild-type littermates, Ccnd3 knockout mice had a 40 percent increase in MCVs and a 38 percent decrease in RBC counts, with only very mild anemias secondary to slightly decreased mean corpuscular hemoglobin concentrations. Indeed, the deficit in cell divisions during terminal erythroid differentiation appeared to be independent of other events in terminal differentiation, including the capacity to respond to erythropoietin. In vitro knockdown of CCND3 showed similar decreased numbers of cell divisions and increased cell size in terminally differentiating primary human erythroid cells. The human polymorphism that led to identification of the erythroid-specific enhancer in CCND3 has decreased enhancer function, and it is associated with increased MCVs and decreased RBC counts.
In Brief
Sankaran et al. have demonstrated a central role for cyclin D3 in terminal erythroid differentiation where rapidly dividing erythroblasts progressively decrease their cell size and then abruptly cease cell division. Absent or reduced cyclin D3 activity decreases cell divisions and yields fewer but larger erythrocytes. Whether decreased cell divisions are responsible for large MCVs in inherited or acquired macrocytic anemias remains to be determined. However, understanding cyclin D3’s role in the reduction of cell size and the final exit from cell cycle in terminal erythroid differentiation will likely provide information about its role in normal and malignant lymphopoiesis in particular and about terminal differentiation in general.
