Overlap of SEC23A and SEC23B Function Suggests a Novel Therapeutic Approach for Congenital Dyserythropoietic Anemia Type II

Congenital Dyserythropoietic Anemia type II (CDAII) is an autosomal recessive disease characterized by anemia and increased bone marrow (BM) bi/multi-nucleated erythroblasts. CDAII results from loss of function mutations in SEC23B encoding a core component of coat complex protein II (COPII) vesicles, which transport secretory proteins from the endoplasmic reticulum to the Golgi apparatus. Despite the identification of the genetic cause of CDAII, the pathophysiology of the disease remains unknown.

Morpholino-induced SEC23B deficiency in zebrafish (ZF) has been previously reported to result in an erythroid phenotype mimicking CDAII (Shwartz et al, Nature genetics 2009), suggesting conservation of the underlying CDAII mechanism from fish to humans. Thus, we were puzzled to observe the absence of anemia or other CDAII characteristics in mice with erythroid specific (EpoR -Cre) and pan-hematopoietic (Vav1 -Cre) SEC23B deficiency (Khoriaty et al, MBC and Khoriaty et al, Sci Rep).

To re-examine the ZF phenotype, we injected the morpholino targeting Sec23b into one-cell stage ZF embryos demonstrating no increase in circulating bi-nucleated erythroid cells, in contrast to the previous report. Given the variable knock-down that can result from morpholinos, we next generated ZF heterozygous for a 53 bp deletion (Sec23b^+/-) using CRISPR/Cas9 genome editing. Intercrosses between Sec23b^+/- ZF demonstrated lethality of Sec23b^-/- ZF between days 17-21. However, the percentage of circulating bi-nucleated erythrocytes observed at day 16 was indistinguishable between Sec23b^-/- ZF and wildtype (WT) clutch mate controls.

Mammals and fish express two paralogs for SEC23, SEC23A and SEC23B, encoding highly related (~85%) proteins. To investigate the different functions of SEC23A and SEC23B, we defined the SEC23A and SEC23B interactomes using "BioID" (proximity dependent biotinylation) in HEK293 cells expressing BirA*-tagged SEC23A, SEC23B, or GFP control. Surprisingly, SEC23A and SEC23B exhibit indistinguishable interactomes. We also demonstrated that both mouse and human SEC23 paralogs can complement SEC23 deficiency in yeast. Similarly, rescue of the Sec23b^-/- lethal phenotype in ZF by a Sec23a transgene demonstrated at least partial functional overlap of SEC23A/SEC23B function in vertebrates.

To extend these observations to mammals, we genetically engineered the murine Sec23a cDNA into the endogenous mouse genomic locus of Sec23b . We demonstrated that SEC23B-deficient mice (previously shown to die perinatally from pancreatic degeneration) are rescued by SEC23A, exhibiting normal survival and pancreas histology, with no abnormalities apparent on detailed hematologic and anatomic examination.

The expression of SEC23A and SEC23B mRNAs in human and mouse BMs were examined by qRT-PCR. SEC23B is the predominantly expressed paralog in human BM, with greater levels of SEC23A and reduced SEC23B in mouse BM. We therefore hypothesized that mice with erythroid deficiency of SEC23A alone or combined SEC23A/SEC23B deficiency might exhibit an erythroid defect. We first generated mice with erythroid-specific SEC23A deficiency, with the latter mice exhibiting no anemia or other CDAII characteristic. In contrast, mice with combined erythroid SEC23A and SEC23B deficiency die at ~E12.5, exhibiting reduced size and appear white in color compared to their WT litter mate controls, consistent with requirement of SEC23 in the erythroid compartment.

Taken together, these data suggest complete (or near complete) overlap in function between SEC23A and SEC23B, and suggest that therapies that increase the expression of either SEC23 paralog might prove effective in treating CDAII. This paradigm might also apply to other disorders due to mutations in paralogous genes. Finally, our findings also suggest that a switch in paralog expression could account for other disparate disease phenotypes observed between animal models and humans.