SEC23A Functionally Compensates for SEC23B Deficiency in Mice

Congenital Dyserythropoietic Anemia type II (CDAII) is a disease of ineffective erythropoiesis characterized by moderate anemia and increased bone marrow (BM) bi/multi-nucleated erythroid precursors. CDAII is an autosomal recessive disease resulting from mutations in SEC23B. SEC23 is a core component of COPII vesicles, which transport secretory proteins from the endoplasmic reticulum (ER) to the Golgi. Despite identification of the underlying genetic defect, the molecular mechanism by which SEC23B deficiency produces the unique CDAII phenotype remains unknown. We previously reported that mice homozygous for a Sec23b null allele die perinatally, exhibiting massive pancreatic degeneration, precluding evaluation of the adult erythroid compartment. To examine the impact of SEC23B deficiency on adult murine hematopoiesis, we generated mice with erythroid specific and pan-hematopoietic deficiency for SEC23B by crossing a second, conditional Sec23b allele (Sec23b^fl), in which exons 5 and 6 are flanked by loxP sites, to an EpoR-Cre and Vav1-Cre, respectively. These mice did not exhibit anemia or any other CDAII characteristic. Similarly, mice transplanted with SEC23B-deficient fetal liver cells harvested from E17.5 embryos also failed to recapitulate any features of the CDAII phenotype. We next generated mice deficient in SEC23B exclusively in the pancreas by crossing the Sec23b^fl allele to either p48-Cre or Pdx1-Cre. These mice exhibit a phenotype indistinguishable from mice with germline deletion of Sec23b, indicating that loss of pancreatic SEC23B is sufficient to explain the perinatal-lethality of global SEC23B deficiency in mice. The mammalian genome contains two paralogs for SEC23, SEC23A and SEC23B. These paralogs are highly identical at the amino acid level (~85%). We examined the relative expression of SEC23B/SEC23A in WT tissues from both humans and mice. This ratio is higher in human BM compared to pancreas, while it was higher in mouse pancreas compared to BM. In order to determine if SEC23A can rescue the phenotype of SEC23B deficient mice when expressed under the regulatory control of Sec23b, we genetically engineered the Sec23a cDNA into the endogenous genomic locus of Sec23b (Sec23^a-b) in mouse embryonic stem cells via recombinase mediated cassette exchange. A heterozygous Sec23^a-b intercross yielded the expected number of mice homozygous for the Sec23^a-b allele. These mice exhibited normal survival, development, and fertility. Pancreas tissues dissected from Sec23^a-b/a-b mice had normal weights, were histologically indistinguishable from WT controls, and did not exhibit dilated ER by transmission electron microscopy. Western blot analysis confirmed the absence of SEC23B in pancreata of Sec23^a-b/a-b mice, with high levels of SEC23A expression. These data demonstrate that the SEC23A and SEC23B proteins overlap significantly (or completely) at the level of protein function, and suggest that the distinct phenotypes of human and mouse SEC23 deficiency are the result of an evolutionary shift in the tissue-specific gene expression programs of SEC23A and SEC23B. These findings also suggest that therapies that increase the expression of either SEC23 paralog in erythroid cells might be effective in CDAII.