Whole-exome sequencing and functional studies identify RPS29 as a novel gene mutated in multicase Diamond-Blackfan anemia families.

Diamond-Blackfan anemia (DBA) is a cancer-prone inherited bone marrow failure syndrome. Approximately half of DBA patients have a germ-line mutation in a ribosomal protein gene. We used whole-exome sequencing to identify disease-causing genes in 2 large DBA families. After filtering, 1 nonsynonymous mutation (p.I31F) in the ribosomal protein S29 (RPS29[AUQ1]) gene was present in all 5 DBA-affected individuals and the obligate carrier, and absent from the unaffected noncarrier parent in 1 DBA family. A second DBA family was found to have a different nonsynonymous mutation (p.I50T) in RPS29. Both mutations are amino acid substitutions in exon 2 predicted to be deleterious and resulted in haploinsufficiency of RPS29 expression compared with wild-type RPS29 expression from an unaffected control. The DBA proband with the p.I31F RPS29 mutation had a pre-ribosomal RNA (rRNA) processing defect compared with the healthy control. We demonstrated that both RPS29 mutations failed to rescue the defective erythropoiesis in the rps29(-/-) mutant zebra fish DBA model. RPS29 is a component of the small 40S ribosomal subunit and essential for rRNA processing and ribosome biogenesis. We uncovered a novel DBA causative gene, RPS29, and showed that germ-line mutations in RPS29 can cause a defective erythropoiesis phenotype using a zebra fish model.

Calmodulin inhibitors improve erythropoiesis in Diamond-Blackfan anemia.

Diamond-Blackfan anemia (DBA) is a rare hematopoietic disease characterized by a block in red cell differentiation. Most DBA cases are caused by mutations in ribosomal proteins and characterized by higher than normal activity of the tumor suppressor p53. Higher p53 activity is thought to contribute to DBA phenotypes by inducing apoptosis during red blood cell differentiation. Currently, there are few therapies available for patients with DBA. We performed a chemical screen using zebrafish ribosomal small subunit protein 29 (rps29) mutant embryos that have a p53-dependent anemia and identified calmodulin inhibitors that rescued the phenotype. Our studies demonstrated that calmodulin inhibitors attenuated p53 protein amount and activity. Treatment with calmodulin inhibitors led to decreased p53 translation and accumulation but does not affect p53 stability. A U.S. Food and Drug Administration-approved calmodulin inhibitor, trifluoperazine, rescued hematopoietic phenotypes of DBA models in vivo in zebrafish and mouse models. In addition, trifluoperazine rescued these phenotypes in human CD34+ hematopoietic stem and progenitor cells. Erythroid differentiation was also improved in CD34+ cells isolated from a patient with DBA. This work uncovers a potential avenue of therapeutic development for patients with DBA.

Drug discovery for Diamond-Blackfan anemia using reprogrammed hematopoietic progenitors.

Diamond-Blackfan anemia (DBA) is a congenital disorder characterized by the failure of erythroid progenitor differentiation, severely curtailing red blood cell production. Because many DBA patients fail to respond to corticosteroid therapy, there is considerable need for therapeutics for this disorder. Identifying therapeutics for DBA requires circumventing the paucity of primary patient blood stem and progenitor cells. To this end, we adopted a reprogramming strategy to generate expandable hematopoietic progenitor cells from induced pluripotent stem cells (iPSCs) from DBA patients. Reprogrammed DBA progenitors recapitulate defects in erythroid differentiation, which were rescued by gene complementation. Unbiased chemical screens identified SMER28, a small-molecule inducer of autophagy, which enhanced erythropoiesis in a range of in vitro and in vivo models of DBA. SMER28 acted through autophagy factor ATG5 to stimulate erythropoiesis and up-regulate expression of globin genes. These findings present an unbiased drug screen for hematological disease using iPSCs and identify autophagy as a therapeutic pathway in DBA.

Hematopoietic defects in rps29 mutant zebrafish depend upon p53 activation.

Disruption of ribosomal proteins is associated with hematopoietic phenotypes in cell culture and animal models. Mutations in ribosomal proteins are seen in patients with Diamond Blackfan anemia, a rare congenital disease characterized by red cell aplasia and distinctive craniofacial anomalies. A zebrafish screen uncovered decreased hematopoietic stem cells in embryos with mutations in ribosomal protein rps29. Here, we determined that rps29(-/-) embryos also have red blood cell defects and increased apoptosis in the head. As the p53 pathway has been shown to play a role in other ribosomal protein mutants, we studied the genetic relationship of rps29 and p53. Transcriptional profiling revealed that genes upregulated in the rps29 mutant are enriched for genes upregulated by p53 after irradiation. p53 mutation near completely rescues the rps29 morphological and hematopoietic phenotypes, demonstrating that p53 mediates the effects of rps29 knockdown. We also identified neuronal gene orthopedia protein a (otpa) as one whose expression correlates with rps29 expression, suggesting that levels of expression of some genes are dependent on rps29 levels. Together, our studies demonstrate a role of p53 in mediating the cellular defects associated with rps29 and establish a role for rps29 and p53 in hematopoietic stem cells and red blood cell development.