The Frequency and Characteristics of the Ribosomal Protein L5 (RPL5) and Ribosomal Protein L11 (RPL11) Gene Mutations in Patients with Diamond-Blackfan Anemia in the Czech National Registry

Introduction: Diamond-Blackfan anemia (DBA) is a congenital red cell aplasia that is usually diagnosed during early infancy. Apart from defects in red cell maturation, the disorder is also associated with various physical anomalies in 40% of patients. Mutations in the ribosomal protein (RP) S19 were found in 25% of patients, while mutations in other proteins of a small ribosomal subunit (RPS17 and RPS24) were published only in a small fraction of patients. Recently, mutations in RPL5, RPL11 and RPL35a of a large ribosomal subunit were also disclosed in several DBA patients.

Results: The Czech DBA registry currently comprises 31 patients. Mutations in RPL5 were identified in 8/31 patients (26%), and mutations in RPL11 in 2/31 patients (6.5%), implying that mutations in RPL5 account for more Czech DBA cases than mutations in RPS19 (22.6%). As for the classification of mutations, all types were identified, including a nonsense mutation (in RPL11), point mutations (in RPL11 and RPL5), a supposed splicing defect and a small insertion and deletions (all in RPL5). Except for point mutations, all other changes were predicted to cause frameshift with premature stop codon. Since identified alterations were found neither in dbSNP nor in 52 healthy controls, and in two families mutations segregate with the disease, we conclude that they represent true DBA-causative mutations. Although the Czech DBA Registry is rather small, we performed a direct comparison of the group of patients with RPS19 mutations (n=7) with the group of patients with RPL5 mutations (n=8). No differences were found in sex ratio, steroid responsiveness, severity or course of the disease or the treatment outcome. However, patients with RPL5 mutations were generally born small for gestational age (SGA) compared with patients from the RPS19-mutated group. Only one patient (12.5%) with an RPL5 mutation was born with normal birth weight compared to four patients (57.1%) with RPS19 mutations. The second difference was even more striking: all patients with RPL5 mutations had flat thenar and some also an additional thumb anomaly, while no thumb anomalies were observed in patients with RPS19 mutations. It is questionable whether normal RPL5 function is in some way more important for proper thumb development than RPS19.

Discussion: The identification of mutations in the genes in DBA patients is also interesting from another point of view. Both proteins RPL5 and RPL11 have been reported to be implicated in the activation of p53 through the interaction with the MDM2 protein, suppressing its E3 ubiquitin ligase function that otherwise directs p53 to a rapid degradation. It is noteworthy that yet another RP of a large ribosomal subunit was described, having exactly the same function – RPL23. Because no RPL23 mutations in our DBA patients were found, the primary function of RPL5 and RPL11 in ribosome biogenesis and/or translation underlies DBA phenotype rather than the conjoint role of RPL5, RPL11 and RPL23 in the p53 regulation.

Conclusions: We identified 6 and 2 different mutations in the RPL5 and RPL11 genes, respectively, expanding the repertoire of known DBA-associated mutations. No mutations in the RPL23 were identified, suggesting that aberrant p53 activation due to mutations in RPL5 and RPL11 seems unlikely to be the primary cause of DBA. Patients with RPL5 mutations are more commonly born SGA and have higer frequency of thumb anomalies.