Poster Board II-985
Congenital dyserythropoietic anemia type II (CDA II) is an autosomal recessive disorder affecting the normal differentiation-proliferation pathway of the erythroid lineage. It comprises an anemia of variable severity, jaundice, and variable splenomegaly. Erythroid hyperplasia with binuclearity or multinuclearity involving late erythroblasts in the bone marrow (BM) is a key feature of the diagnosis (Iolascon, 2001). In addition, on electron microscopy, vesicles of endoplasmic reticulum (ER) appear to be running beneath the plasma membrane (Alloisio, 1996). The principal biochemical feature is the hypoglycosilation of several proteins, such as transferrin and band 3 (Anselstetter, 1977).
Recently, we identified SEC23B as the CDA II causative gene (Schwarz, 2009). The SEC23B gene encodes the SEC23B component which is part of the cytoplasmic coat protein (COP)II complex. COPII coated vesicles bud from the endoplasmic reticulum to export newly synthesized proteins to the trans Golgi. In yeast, COPII coated vesicles form by the sequential binding of Sar1-GTP, the inner complex proteins Sec23-Sec24 and the outer complex components Sec13-Sec31 on the endoplasmic reticulum (ER) (Fromme, 2008).
Our aim was to characterize the COPII complex in CD34+ progenitor cells during erythroid differentiation by gene expression analysis. Mononuclear cells from the peripheral blood of normal subjects were isolated on a Ficoll-Hypaque gradient and CD34+ progenitors were separated on immuno-affinity columns. For erythroid differentiation, CD34+ cells of three different pools were separately plated on plastic culture dishes in methylcellulose medium containing 3U/mL erythropoietin (EPO) (Pinho, 2008). The cells have been cultured for 7 and 14 days after EPO treatment. Quantitative real time (qRT)-PCR on CD34+ during erythroid differentiation was performed to assess the gene expression level. The relative gene expression was calculated by 2*(-ΔCt) method (Livak, 2001), using GAPDH gene as internal control (figure 1). Mammalian orthologues have been identified for each of the five proteins involved in COPII coat formation. In humans, two isoforms of Sec23, Sar1 and Sec31 and four mammalian isoforms of Sec24 have been reported (Kuge, 1994; Paccaud, 1996; Wendeler, 2007; Mancias, 2008; Shugrue, 1999; Tang, 2000; Stankewich, 2005). We already demonstrated that during normal erythropoiesis a SEC23A down regulation is associated to SEC23B upregulation. These data suggest that SEC23B mutants could disrupt the COPII complex in erythroid lineage, and consequently induce CDA II (Schwarz, 2009). On the contrary, the two isoforms of SAR1 showed the same trend during erythroid differentiation: however, SAR1A isoforms has an higher expression when compared to SAR1B isoform. Among the four SEC24 isoforms, SEC24A, B and C showed the same increased expression after EPO treatment; SEC24D, indeed, showed a decreasing trend during differentiation time. Only one form of mammalian SEC13 has been described (Shaywitz, 1995), and it showed an increased expression after EPO treatment. Between SEC31 mammalian isoforms, SEC31A showed overall an higher gene expression compared to SEC31B gene. The gene expression analysis of SEC12, the transmembrane guanine nucleotide exchange factor that catalyzes the COPII vesicle formation by GDP-GTP exchange on Sar1 (Sato, 2004), revealed an higher mRNA level at 14 days when compared to 7 days after EPO treatment.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.