Ribosomal Protein Mutations Result in Constitutive p53 Protein Degradation through Impairment of the AKT Pathway.

Ana T Antunes,Alyson W Macinnes,Dorien Hermkens,Tamara C Pereboom,Yvonne J Goos,Lydie Da Costa,Marcin W Wlodarski

doi:10.1371/journal.pgen.1005326

Abstract

Mutations in ribosomal protein (RP) genes can result in the loss of erythrocyte progenitor cells and cause severe anemia. This is seen in patients with Diamond-Blackfan anemia (DBA), a pure red cell aplasia and bone marrow failure syndrome that is almost exclusively linked to RP gene haploinsufficiency. While the mechanisms underlying the cytopenia phenotype of patients with these mutations are not completely understood, it is believed that stabilization of the p53 tumor suppressor protein may induce apoptosis in the progenitor cells. In stark contrast, tumor cells from zebrafish with RP gene haploinsufficiency are unable to stabilize p53 even when exposed to acute DNA damage despite transcribing wild type p53 normally. In this work we demonstrate that p53 has a limited role in eliciting the anemia phenotype of zebrafish models of DBA. In fact, we find that RP-deficient embryos exhibit the same normal p53 transcription, absence of p53 protein, and impaired p53 response to DNA damage as RP haploinsufficient tumor cells. Recently we reported that RP mutations suppress activity of the AKT pathway, and we show here that this suppression results in proteasomal degradation of p53. By re-activating the AKT pathway or by inhibiting GSK-3, a downstream modifier that normally represses AKT signaling, we are able to restore the stabilization of p53. Our work indicates that the anemia phenotype of zebrafish models of DBA is dependent on factors other than p53, and may hold clinical significance for both DBA and the increasing number of cancers revealing spontaneous mutations in RP genes.

Highlights

The stabilization of the p53 tumor suppressor is a pivotal event in the programmed cell death response
We previously reported that one of these mechanisms is found in tumor cells with ribosomal protein (RP) gene mutations
While Fanconi anemia is predominantly linked to mutations in DNA repair enzymes, several genes found mutated in dyskeratosis congenita patients have a known role in the rRNA maturation steps of early ribosome biogenesis

Summary

Introduction

The stabilization of the p53 tumor suppressor is a pivotal event in the programmed cell death response. While Fanconi anemia is predominantly linked to mutations in DNA repair enzymes, several genes found mutated in dyskeratosis congenita patients have a known role in the rRNA maturation steps of early ribosome biogenesis. The mutation of these latter genes in zebrafish stabilizes p53, as does the mutation of several other genes important for the processing of rRNA [5,6,7]. In human bone marrow failures syndromes linked to RP haploinsufficiency such as Diamond-Blackfan anemia (DBA) and 5qmyelodysplastic syndrome, the loss of hematopoietic progenitor CD34+ cells by p53-induced apoptosis is believed by some to be the major cause of cytopenia [8]. The contribution of p53 stabilization to the loss of erythrocytes in DBA patients is possibly less significant than previously thought

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Discussion

Conclusion