Regulation of globin-heme balance in Diamond-Blackfan anemia by HSP70/GATA1

Abstract

K E Y P O I N T Sl Mutations in genes other than RPS19 result in significant imbalance between globin and heme synthesis, leading to excess free heme.l Decreased levels of HSP70 and GATA1 account for excess free heme in DBA erythroblasts; HSP70 reexpression restores the globin/heme synthesis.Diamond-Blackfan anemia (DBA) is a congenital erythroblastopenia that is characterized by a blockade in erythroid differentiation related to impaired ribosome biogenesis. DBA phenotype and genotype are highly heterogeneous. We have previously identified 2 in vitro erythroid cell growth phenotypes for primary CD34 1 cells from DBA patients and following short hairpin RNA knockdown of RPS19, RPL5, and RPL11 expression in normal human CD34 1 cells. The haploinsufficient RPS19 in vitro phenotype is less severe than that of 2 other ribosomal protein (RP) mutant genes. We further documented that proteasomal degradation of HSP70, the chaperone of GATA1, is a major contributor to the defect in erythroid proliferation, delayed erythroid differentiation, increased apoptosis, and decreased globin expression, which are all features of the RPL5 or RPL11 DBA phenotype. In the present study, we explored the hypothesis that an imbalance between globin and heme synthesis may be involved in pure red cell aplasia of DBA. We identified disequilibrium between the globin chain and the heme synthesis in erythroid cells of DBA patients. This imbalance led to accumulation of excess free heme and increased reactive oxygen species production that was more pronounced in cells of the RPL5 or RPL11 phenotype. Strikingly, rescue experiments with wild-type HSP70 restored GATA1 expression levels, increased globin synthesis thereby reducing free heme excess and resulting in decreased apoptosis of DBA erythroid cells. These results demonstrate the involvement of heme in DBA pathophysiology and a major role of HSP70 in the control of balanced heme/globin synthesis. (Blood.

Regulation of GATA1 levels in erythropoiesis

“…In DBA, it was recently shown that HSP70 protein is downregulated due to its increased ubiquitination and proteasomal degradation in RPL11‐ and RPL5‐deficient human primary erythroid cells, but not in RPS19‐deficient cells, resulting in proliferation and differentiation defects and increased apoptosis in erythroid cells, resembling DBA features . A subsequent study showed a disequilibrium of globin chain and heme synthesis in RPL11‐ and RPL5‐deficient human erythroid cells, resulting in excessive free heme which downregulated HSP70 protein . The study by Doty et al also suggests that aberrant accumulation of heme, contributes to the amplification of RP imbalance, thus causing premature downregulation of GATA1 levels, responsible for the IE that is characteristic of DBA and MDS .…”

“…153 A subsequent study showed a disequilibrium of globin chain and heme synthesis in RPL11-and RPL5-deficient human erythroid cells, resulting in excessive free heme which downregulated HSP70 protein. 154 The study by Doty et al also suggests that aberrant accumulation of heme, contributes to the amplification of RP imbalance, thus causing premature downregulation of GATA1 levels, responsible for the IE that is characteristic of DBA and MDS. 152 Hence, it is clear in that in DBA, HSP70 downregulation results in a significant drop in GATA1 protein levels due to caspase-mediated cleavage, which underlies the observed erythroid maturation defects.…”

The Role of α1-Microglobulin (A1M) in Erythropoiesis and Erythrocyte Homeostasis—Therapeutic Opportunities in Hemolytic Conditions

“…The phenotypic similarities between DBA and FLVCR mutant mice and FeLV-C-infected cats suggest a common pathophysiology involving heme toxicity. There is a significant induction of free heme and ROS in normal erythroid cells transduced with shRNA constructs targeting the three DBA genes, RPS19 , RPL5 and RPL11 [ 72 ]. The increased production of ROS in ribosomal protein-deficient erythroid cells leads to increased expression of FLVCR1 and decreased expression of BACH1 [ 72 ].…”

GATA1 mutations in red cell disorders

“…Later studies provided new insights into GATA1‐globin‐heme feedback loop in the pathogenesis of DBA. Two recent papers suggest that decrease of GATA1 full‐length protein resulting from RP haploinsufficiency and deficiency of HSP70 can disturb the balance of globin‐heme and leads to the accumulation of free cytoplasmic heme in erythroid progenitors, which can cause increase of P53‐dependent apoptosis of DBA erythroid cells . Of note, unlike in the presence of RP mutations in DBA patients, accumulation of heme upon only FLVCR1 depletion causes P53‐independent apoptosis …”