Diamond-Blackfan anemia (DBA) typically presents with red blood cell aplasia that usually manifests in the first year of life. The only gene currently known to be mutated in DBA encodes ribosomal protein S19 (RPS19). Previous studies have shown that the yeast RPS19 protein is required for a specific step in the maturation of 40S ribosomal subunits. Our objective here was to determine whether the human RPS19 protein functions at a similar step in 40S subunit maturation. Studies where RPS19 expression is reduced by siRNA in the hematopoietic cell line, TF-1, show that human RPS19 is also required for a specific step in the maturation of 40S ribosomal subunits. This maturation defect can be monitored by studying rRNA-processing intermediates along the ribosome synthesis pathway. Analysis of these intermediates in CD34 ؊ cells from the bone marrow of patients with DBA harboring mutations in RPS19 revealed a pre-rRNA-processing defect similar to that observed in TF-1 cells where RPS19 expression was reduced. This defect was observed to a lesser extent in CD34 ؉ cells from patients with DBA who have mutations in RPS19. IntroductionDiamond-Blackfan anemia (DBA) typically presents as a red blood cell aplasia that affects children in their first year of life. In addition to anemia, patients with DBA present with a heterogeneous mixture of congenital abnormalities. 1 Craniofacial abnormalities are observed in approximately 50% of patients with DBA, while other defects, including growth failure, thumb malformation, and cardiac and urogenital defects, are observed less frequently.Approximately 25% of patients with DBA have mutations in the gene encoding ribosomal protein S19, 1 of 33 ribosomal proteins that together with 18S rRNA constitutes the 40S ribosomal subunit. [2][3][4] The etiology of the remaining cases of DBA is unknown. DBA is the first and only human disease known to be caused by mutations in a gene encoding a ribosomal protein. Interestingly, several other bone marrow (BM) failure syndromes have been linked to factors involved in ribosome synthesis. 5 These syndromes include dyskeratosis congenita (DC), cartilage hair hypoplasia (CHH), and Shwachman Diamond syndrome (SDS). The proteins and RNAs affected in these diseases include the DKC1 gene in X-linked DC, which encodes a pseudouracil synthase, 6 dyskerin involved in rRNA modification, the gene RMRP involved in CHH, which participates in rRNA processing, 7 and SBDS, the gene affected in SDS which encodes a protein thought to function in RNA metabolism. [8][9][10][11] The exact role of a defect in ribosome synthesis in each of these marrow failure syndromes is obscured by the fact that some of these proteins and RNAs are part of complexes that have multiple functions within cells. Dyskerin is a component of a number of ribonucleoprotein complexes, including telomerase, 12-14 whereas RMRP is a component of an endoribonuclease involved in mRNA decay in addition to rRNA processing. 15 The only other known function for ribosomal protein S19 (RPS19) is as a monoc...

Approximately 25% of cases of Diamond Blackfan anemia, a severe hypoplastic anemia, are linked to heterozygous mutations in the gene encoding ribosomal protein S19 that result in haploinsufficiency for this protein. Here we show that deletion of either of the two genes encoding Rps19 in yeast severely affects the production of 40 S ribosomal subunits. Rps19 is an essential protein that is strictly required for maturation of the 3-end of 18 S rRNA. Depletion of Rps19 results in the accumulation of aberrant pre-40 S particles retained in the nucleus that fail to associate with pre-ribosomal factors involved in late maturation steps, including Enp1, Tsr1, and Rio2. When introduced in yeast Rps19, amino acid substitutions found in Diamond Blackfan anemia patients induce defects in the processing of the pre-rRNA similar to those observed in cells underexpressing Rps19. These results uncover a pivotal role of Rps19 in the assembly and maturation of the pre-40 S particles and demonstrate for the first time the effect of Diamond Blackfan anemiaassociated mutations on the function of Rps19, strongly connecting the pathology to ribosome biogenesis. Diamond Blackfan anemia (DBA)5 is a severe hypoplastic anemia that generally presents early in infancy (1, 2). Other clinical features of DBA are heterogeneous with some patients presenting craniofacial abnormalities, growth failure, predisposition to cancer, and other congenital abnormalities. Most cases of DBA arise spontaneously, with only a small proportion exhibiting familial transmission typically showing autosomal dominant inheritance. Approximately 25% of the DBA cases have been linked to mutations in the gene encoding ribosomal protein S19 (RPS19), and in these cases haploinsufficiency for this ribosomal protein gives rise to the disease (3, 4). The remaining cases are of unknown etiology.The Rps19 protein is a component of the small ribosomal subunit, and as such, defects in some aspect of ribosome structure, function, or synthesis may be an underlying cause of DBA. The human Rps19 protein belongs to a family of ribosomal proteins restricted to eukaryotes and archea. Rps19 does not have a homolog in the eubacterial ribosome where the properties of individual ribosomal proteins have been most extensively studied. As such, little is known regarding the function of members of the eukaryotic Rps19 family. In prokaryotes, ribosomal proteins play critical roles in ribosomal assembly through their interactions with each other and rRNA (5). These interactions promote both steps in rRNA processing important for subunit maturation and the formation of active sites in mature subunits necessary for ribosome function. The precise functions of the ribosomal proteins in the production of the subunits in eukaryotes are poorly characterized, which calls for a more detailed investigation of the role played by Rps19 in ribosome synthesis and function.The yeast Saccharomyces cerevisiae has proven to be an outstanding system to investigate factors involved in eukaryotic ribosome synthesis...

Diamond Blackfan Anemia (DBA) is one of several bone marrow failures that have been linked to defects in ribosome synthesis. 25% of DBA cases are linked to mutations in ribosomal protein S19 (Rps19). The etiology of the remaining cases is unknown. To gain a better understanding of the function of the Rps19 family of proteins we have characterized members of this protein family in the yeast, Saccharomyces cerevisiae. In yeast, Rps19 is encoded by duplicated genes, RPS19A and RPS19B. Yeast cells lacking both RPS19 genes are not viable, whereas those lacking a single gene are viable but have growth defects. These latter strains are defective in a specific step in rRNA processing that preferentially affects the maturation of 40S ribosomal subunits. We scanned other yeast strains with mutations in genes for 40S subunit proteins for processing phenotypes similar to RPS19 mutants. Several have phenotypes that overlap with RPS19 mutants, but only RPS18 stands out as being virtually identical to RPS19 mutants. The human RPS18 gene is therefore a candidate locus for pathogenic mutations in DBA patients with normal RPS19. We are currently developing strategies to sequence RPS18 genes from DBA patients with normal RPS19 to determine if mutations in RPS18 are associated with DBA. We have also developed a yeast system for the functional testing of mutant alleles of RPS19 found in DBA patients. In general, a mutation is considered pathogenic if it is not found in unaffected family members and in the general population. We have found, however, that several missense mutations classified as pathogenic in DBA patients do not affect Rps19 function in the yeast system. The failure of these mutations to affect Rps19 function in yeast points to a need for functional testing of RPS19 mutant alleles in human cells.