Genetic variants in the noncoding region of RPS19 gene in Diamond-Blackfan anemia: potential implications for phenotypic heterogeneity.

Mutations in the RPS19 gene have been identified in 25% of individuals affected by Diamond-Blackfan anemia (DBA), a congenital erythroblastopenia characterized by an aregenerative anemia and a variety of malformations. More than 60 mutations in the five coding exons of RPS19 have been described to date. We previously reported a mutation (c.-1 + 26G>T) and an insertion at -631 upstream of ATG (c.-147_-146insGCCA) in the noncoding region. Because DBA phenotype is extremely heterogeneous from silent to severe and because haploinsufficiency seems to play a role in this process, it is likely that genetic variations in the noncoding regions affecting translation of RPS19 can modulate the phenotypic expression of DBA. However, to date, very few studies have addressed this question comprehensively. In this study, we performed detailed sequence analysis of the RPS19 gene in 239 patients with DBA and 110 of their relatives. We found that 6.2% of the patients with DBA carried allelic variations upstream of ATG: 3.3% with c.-1 + 26G>T; 2.5% with c.-147_-146insGCCA; and 0.4% with c.-174G>A. Interestingly, the c.-147_-146insGCCA, which has been found in a black American and French Caribbean control population, was not found in 500 Caucasian control chromosomes we studied. However, it was found in association with the same haplotype distribution of four intronic polymorphisms in our patients with DBA. Although a polymorphism, the frequency of this variant in the patients with DBA and its association with the same haplotype raises the possibility that this polymorphism and the other genetic variations in the noncoding region could play a role in DBA pathogenesis.

Study of the effects of proteasome inhibitors on ribosomal protein S19 (RPS19) mutants, identified in patients with Diamond-Blackfan anemia.

BACKGROUND: Mutations in the ribosomal protein S19 gene (RPS19) have been found in 25% of patients with Diamond-Blackfan anemia, a rare syndrome of congenital bone marrow failure characterized by erythroblastopenia and various malformations. Mechanistic understanding of the role of RPS19 in normal erythropoiesis and in the Diamond-Blackfan anemia defect is still poor. However, defective ribosome biogenesis and, in particular, impaired 18S ribosomal RNA maturation have been documented in association with various identified RPS19 mutations. Recently, new genes, all encoding ribosomal proteins, have been found to be mutated in Diamond-Blackfan anemia, adding further support to the concept that ribosome biogenesis plays an important role in regulating erythropoiesis. We previously showed variability in the levels of expression and subcellular localization of a subset of RPS19 mutant proteins. DESIGN AND METHODS: To define the mechanistic basis for this variability better, we studied a large number of mutant proteins and characterized both RPS19 expression level using a specific antibody against RPS19 and RPS19 subcellular localization after transfection of Cos-7 cells with various green fluorescent protein-RPS19 mutants. To investigate the role of the proteasome in RPS19 degradation, we examined the effect of various proteasome inhibitors, namely lactacystin, MG132, and bortezomib on RPS19 expression and subcellular localization RESULTS: We found two distinct classes of RPS19 protein defects in Diamond-Blackfan anemia based on the stability of the mutant proteins: (i) slightly decreased to normal levels of expression and normal nucleolar localization and (ii) markedly deficient expression and failure to localize to the nucleolus. All the proteasome inhibitors tested were able to restore the expression levels and normal subcellular localization of several unstable mutant proteins. CONCLUSIONS: Our findings demonstrate an important role for the proteasomal degradation pathway in regulating the expression levels and nucleolar localization of certain mutant RPS19 proteins in Diamond-Blackfan anemia.

Impaired ribosome biogenesis in Diamond-Blackfan anemia.

The gene encoding the ribosomal protein S19 (RPS19) is frequently mutated in Diamond-Blackfan anemia (DBA), a congenital erythroblastopenia. The consequence of these mutations on the onset of the disease remains obscure. Here, we show that RPS19 plays an essential role in biogenesis of the 40S small ribosomal subunit in human cells. Knockdown of RPS19 expression by siRNAs impairs 18S rRNA synthesis and formation of 40S subunits and induces apoptosis in HeLa cells. Pre-rRNA processing is altered, which leads to an arrest in the maturation of precursors to the 18S rRNA. Under these conditions, pre-40S particles are not exported to the cytoplasm and accumulate in the nucleoplasm of the cells in perinuclear dots. Consistently, we find that ribosome biogenesis and nucleolar organization is altered in skin fibroblasts from DBA patients bearing mutations in the RPS19 gene. In addition, maturation of the 18S rRNA is also perturbed in cells from a patient bearing no RPS19-related mutation. These results support the hypothesis that DBA is directly related to a defect in ribosome biogenesis and indicate that yet to be discovered DBA-related genes may be involved in the synthesis of the ribosomal subunits.

Sub-lethal hydrops as a manifestation of dehydrated hereditary stomatocytosis in two consecutive pregnancies.

Dehydrated hereditary stomatocytosis (DHS) is a rare congenital hemolytic anemia mapping to 16q23-q24. We showed recently that it is part of a pleiotropic syndrome likely to display pseudohyperkalemia and/or different forms of fetal and placental fluid collections. Here, we report a woman with DHS. She had two consecutive pregnancies associated with severe fetal hydrops. Hydrops would probably have been lethal in the absence of appropriate removal of ascites and excess amniotic fluid. In utero exchange transfusion, performed once, was useless, because anemia was not pronounced enough to be the cause of the hydrops. In both newborns, ascites resolved within a week following birth and never recurred. The association of hydrops and hemolytic anemia suggests the possibility of DHS. Symptomatic treatment of the hydrops assists survival until spontaneous resorption occurs.

Congenital dyserythropoietic anemia type I is caused by mutations in codanin-1.

Congenital dyserythropoietic anemias (CDAs) constitute a rare group of inherited red-blood-cell disorders associated with dysplastic changes in late erythroid precursors. CDA type I (CDAI [MIM 224120], gene symbol CDAN1) is characterized by erythroid pathological features such as internuclear chromatin bridges, spongy heterochromatin, and invagination of the nuclear membrane, carrying cytoplasmic organelles into the nucleus. A cluster of 45 highly inbred Israeli Bedouin with CDAI enabled the mapping of the CDAN1 disease gene to a 2-Mb interval, now refined to 1.2 Mb, containing 15 candidate genes on human chromosome 15q15 (Tamary et al. 1998). After the characterization and exclusion of 13 of these genes, we identified the CDAN1 gene through 12 different mutations in 9 families with CDAI. This 28-exon gene, which is transcribed ubiquitously into 4738 nt mRNA, was reconstructed on the basis of gene prediction and homology searches. It encodes codanin-1, a putative o-glycosylated protein of 1,226 amino acids, with no obvious transmembrane domains. Codanin-1 has a 150-residue amino-terminal domain with sequence similarity to collagens and two shorter segments that show weak similarities to the microtubule-associated proteins, MAP1B (neuraxin) and synapsin. These findings, and the cellular phenotype, suggest that codanin-1 may be involved in nuclear envelope integrity, conceivably related to microtubule attachments. The specific mechanisms by which codanin-1 underlies normal erythropoiesis remain to be elucidated.