Treatment of Zebrafish Models of Ribosomopathies (Diamond Blackfan Anemia (DBA) and 5q- Syndrome) with L-Leucine Results In An Improvement of Anemia and Developmental Defects: Evidence for a Common Pathway?

Abstract 195

A growing number of disorders associated with impaired ribosome biogenesis and function have been recently recognized and termed ribosomopathies. The first of these to be identified was Diamond Blackfan Anemia (DBA), a congenital autosomal dominant bone marrow failure syndrome of childhood manifested as profound macrocytic anemia. DBA is characterized by enhanced sensitivity of hematopoietic progenitors to apoptosis with evidence of stress erythropoiesis. In addition to bone marrow defects, DBA patients often have craniofacial, genitourinary, cardiac and limb abnormalities and are at an increased risk of developing hematopoietic malignancies and osteosarcoma. Twenty-five percent of patients with DBA have heterozygous mutations in the ribosomal protein S19 (RPS19) gene, which encodes a component of the 40S ribosomal subunit. Additionally, a growing percentage of DBA patients lacking a mutation in the RPS19 gene have been shown to have mutations in other ribosomal protein genes. A second ribosomopathy, 5q- syndrome is a subtype of myelodysplasia, with an erythroid phenotype strikingly similar to DBA. The erythroid phenotype of 5q- syndrome has been recently demonstrated to be attributable to acquired haploinsufficiency of the ribosomal protein RPS14 gene associated with clonal loss of the long arm of chromosome 5. In both DBA and 5q- syndrome, it is unclear why defects in ribosomal proteins, which would be expected to have widespread consequences, have such a specific effect on erythroid maturation. It has been demonstrated that the efficiency of mRNA translation is significantly depressed in cells derived from DBA patients, consistent with a pathogenic ribosomal defect. L-leucine is an essential branched chain amino acid that is known to modulate protein synthesis by enhancing translation. L-leucine has been used to treat DBA patients, and has been shown to lead to improvement in hemoglobin levels and to transfusion independence in some patients. Based on this observation we hypothesized that a common pathway associated with defective mRNA translation underlies both DBA and 5q- MDS, and that activation of translation by L-Leucine would alleviate the anemia associated with both ribosomopathies. In order to address this hypothesis, we have modeled DBA and 5q- MDS in zebrafish embryos using an antisense morpholino appraoach and have demonstrated that similar to the human diseases, Rps19 and Rps14 deficiency lead to defective erythropoiesis and to developmental abnormalities (specific to DBA). Treatment of Rps19 and Rps14 deficient embryos with L-Leucine resulted in partial reversal of both the anemia and the developmental defects. L-Leucine is thought to regulate protein synthesis by acting as a nutrient signal involving the mTOR (mammalian target of rapamycin) signaling pathway. It has been suggested that L-Leucine enhances translation by activating translation initiation factors and by specific up-regulation of the ribosomal protein S6 kinase. Activation of the mTOR pathway is currently being analyzed in the L-Leucine treated Rps19 and Rps14 deficient zebrafish embryos. Additionally, our observations are also being validated in in vitro human models for DBA and 5q-MDS using human hematopoietic progenitor CD34+ cells and shRNAs specific for Rps19 and Rps14 genes followed by L-Leucine treatment. Our studies should provide evidence for activation of pathways involved in mRNA translation and validate the use of the orally administered amino acid L-Leucine as a therapeutic agent in the treatment of DBA as well as 5q- syndrome without the side-effects associated with current therapies for both diseases.