Characterization of Congenital Sideroblastic Anemia Model Due to ABCB7 Defects: How Do Defects in Iron-Sulfur Cluster Metabolism Lead to Ring Sideroblast Formation?

Background: The sideroblastic anemias (SAs) are a group of congenital and acquired bone marrow disorderscharacterized by bone marrow ring sideroblasts (RSs). The disease commonly presents as myelodysplastic syndrome with RS (MDS-RS), known as an acquired clonal SA that is strongly correlated with a specific somatic mutation inSF3B1 (splicing factor 3b subunit 1), which is involved in RNA splicing machinery. Thus far, several studies have consistently revealed compromised splicing and/or expression of ABCB7 (ATP-binding cassette subfamily B member 7) in MDS-RS harboring the SF3B1 mutation. ABCB7 encodes an ATP-binding cassette family transporter localizing to the inner mitochondrial membrane, and its loss-of-function mutation causes a syndromic form of congenital SA, which is associated with cerebellar ataxia. The substrates transported by ABCB7 are predicted to be iron-sulfur clusters (ISCs), which are essential for the function of multiple mitochondrial and extramitochondrial proteins, such as ferrochelatase and aconitase (its apo-form without ISC is called IRP1; iron regulatory protein 1). However, the detailed molecular mechanisms by which defects in ISC metabolism resulting from ABCB7 defects contribute to RS formation remains to be fully elucidated.

Methods: Endogenous ABCB7 was depleted based on pGIPZ lentiviral shRNAmir (Dharmacon) in human umbilical cord blood-derived erythroid progenitor (HUDEP)-2 cells (Kurita et al., PLoS ONE, 2013). Puromycin (Sigma) was used for the selection of transduced cells. To induce terminal erythroid differentiation, HUDEP-2 cells were co-cultured with OP9 stromal cells (ATCC) in Iscove's modified Dulbecco's medium supplemented with fetal bovine serum, erythropoietin, dexamethasone, monothioglycerol, insulin-transferrin-selenium, ascorbic acid, and sodium ferrous citrate (Saito and Fujiwara et al., MCB, 2019). For transcription profiling, Human Oligo Chip 25K (Toray) was used.

Results: We first conducted ABCB7 knockdown in HUDEP-2 cells based on two independent shRNA plasmids. When the knockdown cells were induced to undergo erythroid differentiation,the majority of the erythroblasts exhibited aberrant mitochondrial iron deposition. Thus, we sought to clarify the potential causative link between ABCB7 defects and RS formation. Expression profiling revealed >1.5-fold up- and down-regulation of 33 and 44 genes, respectively, caused by the ABCB7 knockdown. Intriguingly, 43% of the downregulated gene ensemble (19/44 genes) included multiple ribosomal genes, such as RPS2, RPL11,and RPS12. The downregulated genes also included HMOX1 (heme oxygenase 1), implying that heme biosynthesis would be compromised by the knockdown. Gene ontology (GO) analysis revealed significant (p< 0.01) enrichment of genes associated with nuclear-transcribed mRNA catalytic process, cytoplasmic translation, and cellular iron ion homeostasis. Whereas the mRNA expression for ALAS2 (erythroid-specific 5-aminolevulinate synthase), encoding a rate-limiting enzyme of heme biosynthesis and one of the responsible genes for congenital SA, was not affected, its protein expression was noticeably decreased by ABCB7 knockdown, indicating that compromised transport of ISC from mitochondria to the cytosol may result in decreased ALAS2 translation by the binding of IRP1 to the iron-responsive element located in the 5'-UTR of ALAS2 mRNA.We are currently conducting detailed biological analyses to elucidate the causative link between defects in ISC metabolism due to ABCB7 defects and RS formation.

Conclusion: We have first demonstrated the emergence of RS by ABCB7 depletion in human erythroblasts. Further characterization of the established SA model would aid in the clarification of its molecular etiology and the establishment of novel therapeutic strategies. Furthermore, our results may lead to a better understanding of the role of ISC in affecting cerebellar symptoms.