ATG4A regulates human erythroid maturation and mitochondrial clearance.

Autophagy is a self-degradation pathway that is essential for erythropoiesis. During erythroid differentiation, autophagy facilitates the degradation of macromolecules and the programmed clearance of mitochondria. Impaired mitochondrial clearance results in anemia and alters the lifespan of red blood cells in vivo. While several essential autophagy genes contribute to autophagy in erythropoiesis, little is known about erythroid-specific mediators of this pathway. Genetic analysis of primary human erythroid and nonerythroid cells revealed the selective upregulation of the core autophagy gene ATG4A in maturing human erythroid cells. Because the function of ATG4A in erythropoiesis is unknown, we evaluated its role using an ex vivo model of human erythropoiesis. Depletion of ATG4A in primary human hematopoietic stem and progenitor cells selectively impaired erythroid but not myeloid lineage differentiation, resulting in reduced red cell production, delayed terminal differentiation, and impaired enucleation. Loss of ATG4A impaired autophagy and mitochondrial clearance, giving rise to reticulocytes with retained mitochondria and autophagic vesicles. In summary, our study identifies ATG4A as a cell type-specific regulator of autophagy in erythroid development.

Coordinated missplicing of TMEM14C and ABCB7 causes ring sideroblast formation in SF3B1-mutant myelodysplastic syndrome.

SF3B1 splicing factor mutations are near-universally found in myelodysplastic syndromes (MDS) with ring sideroblasts (RS), a clonal hematopoietic disorder characterized by abnormal erythroid cells with iron-loaded mitochondria. Despite this remarkably strong genotype-to-phenotype correlation, the mechanism by which mutant SF3B1 dysregulates iron metabolism to cause RS remains unclear due to an absence of physiological models of RS formation. Here, we report an induced pluripotent stem cell model of SF3B1-mutant MDS that for the first time recapitulates robust RS formation during in vitro erythroid differentiation. Mutant SF3B1 induces missplicing of ∼100 genes throughout erythroid differentiation, including proposed RS driver genes TMEM14C, PPOX, and ABCB7. All 3 missplicing events reduce protein expression, notably occurring via 5' UTR alteration, and reduced translation efficiency for TMEM14C. Functional rescue of TMEM14C and ABCB7, but not the non-rate-limiting enzyme PPOX, markedly decreased RS, and their combined rescue nearly abolished RS formation. Our study demonstrates that coordinated missplicing of mitochondrial transporters TMEM14C and ABCB7 by mutant SF3B1 sequesters iron in mitochondria, causing RS formation.