Haploinsufficiency of ribosomal proteins (RP) has been shown to be the common basis for the anemia observed in ribosomopathies such as Diamond-Blackfan anemia (DBA) and myelodysplastic syndrome with loss of chromosome 5q (del(5q) MDS). DBA is a congenital bone marrow failure syndrome characterized by a profound macrocytic anemia. More than half the patients with DBA have been shown to have a heterozygous loss of an RP gene, with RPS19 being the most frequently mutated. The “5q- syndrome” is a subtype of myelodysplastic syndrome (MDS) also characterized by severe anemia that is caused by heterozygous loss of the RPS14 gene on chromosome 5q. The p53 pathway is known to play a critical role in the pathophysiology of the ribosomopathies. The leading hypothesis is that ribosomal haploinsufficiency leads to disrupted ribosome biogenesis with an accumulation of free ribosomal proteins that bind MDM2. MDM2 is an E3 Ubiquitin ligase that normally binds to and targets p53 for proteosomal degradation. The consequent accumulation of p53 leads to cell cycle arrest and apoptosis, which ultimately results in anemia. Several animal models have shown that the anemia associated with RP haploinsufficiency is almost completely alleviated in a p53 null background. However, we and others have shown that p53-independent pathway(s) also contribute to the anemia associated with RP haploinsufficiency.
We have previously modeled DBA and del (5q) MDS in zebrafish using antisense morpholinos to rps19 and rps14 respectively, and have demonstrated that, as in humans, haploinsufficient levels of these proteins lead to a profound anemia. We have further demonstrated that treatment of Rps19 and Rps14 deficient embryos with the amino acid L-Leucine, a known activator of mRNA translation, results in a marked improvement in anemia. This observation was confirmed in primary human CD34+ cells, following shRNA knockdown of RPS19 and RPS14. Furthermore, we showed that L-leucine treatment activates the mTOR pathway in zebrafish embryos deficient in Rps19 or Rps14. In order to determine if the effect of L-Leucine on RP deficient erythroid cells is p53 dependent, we injected rps19 and rps14 morpholinos into zebrafish embryos and treated them with L-Leucine. Total RNA was collected 48hpf and evaluated for expression of p53 by qPCR analysis. As expected, the expression of p53 and its downstream targets (p21 and PUMA) were upregulated in Rps14 and Rps19 deficient embryos. P53 expression levels remained elevated even after L-Leucine treatment. Levels of p21, a direct transcriptional target of p53, remained unchanged in L-Leucine treated RP deficient zebrafish embryos; however, expression of PUMA increased following L-Leucine treatment. The expression of the PUMA gene has previously been shown to have a p53-independent regulatory component. Preliminary studies in the A549 cell line, which harbors wild type p53, also showed increased levels of p53 expression upon shRNA mediated downregulation of both RPS19 and RPS14, which remained unaltered following L-Leucine treatment. These observations are currently being confirmed in primary human CD34+ cells, following shRNA knockdown of RPS19 and RPS14.
Our preliminary studies show that the effect of L-Leucine in improving the anemia in models of DBA and del(5q) MDS occurs independently of p53. This supports our hypothesis that the erythroid phenotype in these disorders has a p53-independent component. Our finding that L-Leucine treated RP deficient cells are likely to express elevated rather than diminished levels of p53 in spite of improved anemia also has important implications for the clinical management of patients, since p53 inactivation is associated with tumor growth. A trial using L-Leucine for patients with DBA will be opening soon in the United States.
Ebert:Celgene: Consultancy; Genoptix: Consultancy.
Asterisk with author names denotes non-ASH members.