Disparate SEC23B Deficient Phenotypes in Humans and Mice

Abstract 974

SEC23 is a core component of the COPII coated vesicle that mediates the transport of cargo proteins from the endoplasmic reticulum to the Golgi apparatus. Mutations in human SEC23B, one of two mammalian SEC23 paralogs, results in the autosomal recessive disease, congenital dyserythropoietic anemia type II (CDAII). In contrast, SEC23B deficient mice were recently reported to die perinatally with massive pancreatic degeneration, with no evidence of anemia at birth (Tao, et al., 2012. Proc.Natl.Acad.Sci.U.S.A. 109:E2001–9.) To examine the impact of SEC23B deficiency on hematopoietic function in mice surviving beyond the immediate perinatal period, lethally irradiated C57BL/6J mice were transplanted with fetal liver cells (FLC) collected from E17.5 embryos that were either wild-type (WT) or homozygous for the previously reported Sec23b gene trap allele (Sec23b^gt/gt). Recipients of Sec23b^gt/gt FLC were indistinguishable from their WT transplant controls and exhibited none of the phenotypic characteristics of CDAII (anemia, increased bi/multi-nucleated RBC precursors, narrower band size and increased shift of the membrane protein band 3 on SDS gel electrophoresis, or RBC double membrane appearance by electron microscopy). To test for a more subtle defect in hematopoietic reconstitution, Sec23b^gt/gt FLCs were co-transplanted with WT GFP+ FLC in a 1:1 ratio, with no competitive difference observed over the course of 18 weeks of follow-up. Transplant of marrow from these chimeric animals into secondary recipients demonstrated continued equivalence of Sec23b^gt/gt and WT hematopoietic stem cells. To rule out an incidental mutation in a nearby gene (“passenger gene”) as the cause of the pancreatic phenotype in SEC23B-deficient mice, two Sec23b bacterial artificial chromosome transgenes were crossed into the Sec23b^gt line, with both demonstrating complete rescue of the Sec23b^gt/gt pancreatic phenotype, with normal survival to adulthood. Sec23b^gt/gt murine embryonal fibroblasts express a SEC23B/βGEO fusion protein consistent with the gene trap insertion into Sec23b intron 19, and this fusion protein co-immunoprecipitates with SEC24A (Tao, et al., 2012. Proc.Natl.Acad.Sci.U.S.A. 109:E2001–9). To rule out a contribution of the SEC23B/βGEO fusion protein to the disparate human and mouse SEC23B-deficient phenotypes, a second, conditional SEC23B allele was analyzed, in which exons 5 and 6 are flanked by loxP sites (Sec23b^fl). Deletion of exons 5 and 6 results in frame shift and stop codon in exon 7. Mice with an erythroid-specific deficiency of SEC23B were generated by crossing the Sec23b^fl allele to an EpoR-Cre transgene. Sec23b^fl/-/EpoR-CreTg⁺ mice exhibit no anemia compared to their WT litter mates. Transplant recipients of FLCs from E16.5 embryos homozygous for a germline deletion of Sec23b exons 5 and 6 (Sec23b^−/−) were also indistinguishable from mice receiving WT FLCs. Pancreas-specific knock-out generated by crossing the Sec23b^fl allele to a p48-Cre or Pdx1-Cre transgene confirmed the pancreatic phenotype observed. This suggests that the perinatal lethality in Sec23b deficient mice may be the result of the loss of pancreatic Sec23b expression. To explore the mechanism for the disparate human and mouse SEC23B-deficient phenotypes, the expression patterns for SEC23A and SEC23B were examined in mice and humans by RT PCR and western blotting. The ratio of SEC23B/SEC23A expression is higher in mouse pancreas (12.7) compared to bone marrow (2.6), whereas in humans, the ratio is higher in the bone marrow (7.8) relative to pancreas (5.5) (normalized to liver). Taken together with the high degree of sequence similarity between SEC23A and SEC23B (∼ 85% identity at the amino acid level), we hypothesize that these 2 SEC23 paralogs overlap extensively at the level of protein function, with the disparate deficiency phenotypes due primarily to differences in tissue and developmentally-specific gene expression programs that have shifted extensively during recent mammalian evolution. These findings have important implications for the comparative function of other closely related paralogous genes. Further studies of the overlapping functions of SEC23A and SEC23B and their relevant protein cargos should provide new insight into the pathogenesis of CDAII and potential therapeutic approaches.