NOL11, implicated in the pathogenesis of North American Indian childhood cirrhosis, is required for pre-rRNA transcription and processing

The ribosome biogenesis factor Nol11 is required for optimal rDNA transcription and craniofacial development in Xenopus

The production of ribosomes is ubiquitous and fundamental to life. As such, it is surprising that defects in ribosome biogenesis underlie a growing number of symptomatically distinct inherited disorders, collectively called ribosomopathies. We previously determined that the nucleolar protein, NOL11, is essential for optimal pre-rRNA transcription and processing in human tissue culture cells. However, the role of NOL11 in the development of a multicellular organism remains unknown. Here, we reveal a critical function for NOL11 in vertebrate ribosome biogenesis and craniofacial development. Nol11 is strongly expressed in the developing cranial neural crest (CNC) of both amphibians and mammals, and knockdown of Xenopus nol11 results in impaired pre-rRNA transcription and processing, increased apoptosis, and abnormal development of the craniofacial cartilages. Inhibition of p53 rescues this skeletal phenotype, but not the underlying ribosome biogenesis defect, demonstrating an evolutionarily conserved control mechanism through which ribosome-impaired craniofacial cells are removed. Excessive activation of this mechanism impairs craniofacial development. Together, our findings reveal a novel requirement for Nol11 in craniofacial development, present the first frog model of a ribosomopathy, and provide further insight into the clinically important relationship between specific ribosome biogenesis proteins and craniofacial cell survival.

Determinants of mammalian nucleolar architecture

The nucleolus is responsible for the production of ribosomes, essential machines which synthesize all proteins needed by the cell. The structure of human nucleoli is highly dynamic and is directly related to its functions in ribosome biogenesis. Despite the importance of this organelle, the intricate relationship between nucleolar structure and function remains largely unexplored. How do cells control nucleolar formation and function? What are the minimal requirements for making a functional nucleolus? Here we review what is currently known regarding mammalian nucleolar formation at nucleolar organizer regions (NORs), which can be studied by observing the dissolution and reformation of the nucleolus during each cell division. Additionally, the nucleolus can be examined by analyzing how alterations in nucleolar function manifest in differences in nucleolar architecture. Furthermore, changes in nucleolar structure and function are correlated with cancer, highlighting the importance of studying the determinants of nucleolar formation.

Two orthogonal cleavages separate subunit RNAs in mouse ribosome biogenesis

Ribosome biogenesis is a dynamic multistep process, many features of which are still incompletely documented. Here, we show that changes in this pathway can be captured and annotated by means of a graphic set of pre-rRNA ratios, a technique we call Ratio Analysis of Multiple Precursors (RAMP). We find that knocking down a ribosome synthesis factor produces a characteristic RAMP profile that exhibits consistency across a range of depletion levels. This facilitates the inference of affected steps and simplifies comparative analysis. We applied RAMP to examine how endonucleolytic cleavages of the mouse pre-rRNA transcript in the internal transcribed spacer 1 (ITS1) are affected by depletion of factors required for maturation of the small ribosomal subunit (Rcl1, Fcf1/Utp24, Utp23) and the large subunit (Pes1, Nog1). The data suggest that completion of early maturation in a subunit triggers its release from the common pre-rRNA transcript by stimulating cleavage at the proximal site in ITS1. We also find that splitting of pre-rRNA in the 3' region of ITS1 is prevalent in adult mouse tissues and quiescent cells, as it is in human cells. We propose a model for subunit separation during mammalian ribosome synthesis and discuss its implications for understanding pre-rRNA processing pathways.

Ribosomopathies: mechanisms of disease

Ribosomopathies are diseases caused by alterations in the structure or function of ribosomal components. Progress in our understanding of the role of the ribosome in translational and transcriptional regulation has clarified the mechanisms of the ribosomopathies and the relationship between ribosomal dysfunction and other diseases, especially cancer. This review aims to discuss these topics with updated information.

Diamond-Blackfan anemia with mandibulofacial dystostosis is heterogeneous, including the novel DBA genes TSR2 and RPS28

Patients with physical findings suggestive of Treacher Collins syndrome (TCS) or mandibulofacial dysostosis (MFD) and macrocytic anemia diagnostic of Diamond-Blackfan anemia (DBA) have been reported. Disease-causing genes have been identified for TCS and other MFDs. Mutations in several ribosomal protein genes and the transcription factor GATA1 result in DBA. However, no disease-causing mutation had been identified in the reported patients with the combination of TCS/MFD and DBA phenotype, and we hypothesized that pathogenic mutations in a single gene could be identified using whole exome analysis. We studied probands from six unrelated families. Combining exome analysis and Sanger sequencing, we identified likely pathogenic mutations in 5/6 families. Two mutations in unrelated families were seen in RPS26, the known DBA10 gene. One variant was predicted to affect mRNA splicing, and the other to lead to protein truncation. In another family a likely pathogenic X-linked mutation affecting a highly conserved residue was found in TSR2, which encodes a direct binding partner of RPS26. De novo mutations affecting the RPS28 start codon were found in two unrelated probands, identifying RPS28 as a novel disease gene. We conclude that the phenotype combining features of TCS with DBA is genetically heterogeneous. Each of the pathogenic variants identified is predicted to impede ribosome biogenesis, which in turn could result in altered cell growth and proliferation, causing abnormal embryologic development, defective erythropoiesis and reduced growth. The phenotype combining TCS/MFD and DBA is highly variable, overlaps with DBA and lies within the phenotypic spectrum of ribosomopathies. © 2014 Wiley Periodicals, Inc.

Human diseases of the SSU processome

Ribosomes are the cellular machines responsible for protein synthesis. Ribosome biogenesis, the production of ribosomes, is a complex process involving pre-ribosomal RNA (rRNA) cleavages and modifications as well as ribosomal protein assembly around the rRNAs to create the functional ribosome. The small subunit (SSU) processome is a large ribonucleoprotein (RNP) in eukaryotes required for the assembly of the SSU of the ribosome as well as for the maturation of the 18S rRNA. Despite the fundamental nature of the SSU processome to the survival of any eukaryotic cell, mutations in SSU processome components have been implicated in human diseases. Three SSU processome components and their related human diseases will be explored in this review: hUTP4/Cirhin, implicated in North American Indian childhood cirrhosis (NAIC); UTP14, implicated in infertility, ovarian cancer, and scleroderma; and EMG1, implicated in Bowen-Conradi syndrome (BCS). Diseases with suggestive, though inconclusive, evidence for the involvement of the SSU processome in their pathogenesis are also discussed, including a novel putative ribosomopathy. This article is part of a Special Issue entitled: Role of the Nucleolus in Human Disease.

Assembly and nuclear export of pre-ribosomal particles in budding yeast

The ribosome is responsible for the final step of decoding genetic information into proteins. Therefore, correct assembly of ribosomes is a fundamental task for all living cells. In eukaryotes, the construction of the ribosome which begins in the nucleolus requires coordinated efforts of >350 specialized factors that associate with pre-ribosomal particles at distinct stages to perform specific assembly steps. On their way through the nucleus, diverse energy-consuming enzymes are thought to release assembly factors from maturing pre-ribosomal particles after accomplishing their task(s). Subsequently, recruitment of export factors prepares pre-ribosomal particles for transport through nuclear pore complexes. Pre-ribosomes are exported into the cytoplasm in a functionally inactive state, where they undergo final maturation before initiating translation. Accumulating evidence indicates a tight coupling between nuclear export, cytoplasmic maturation, and final proofreading of the ribosome. In this review, we summarize our current understanding of nuclear export of pre-ribosomal subunits and cytoplasmic maturation steps that render pre-ribosomal subunits translation-competent.

The roles of SSU processome components and surveillance factors in the initial processing of human ribosomal RNA

During eukaryotic ribosome biogenesis, three of the mature ribosomal (r)RNAs are released from a single precursor transcript (pre-rRNA) by an ordered series of endonucleolytic cleavages and exonucleolytic processing steps. Production of the 18S rRNA requires the removal of the 5' external transcribed spacer (5'ETS) by endonucleolytic cleavages at sites A0 and A1/site 1. In metazoans, an additional cleavage in the 5'ETS, at site A', upstream of A0, has also been reported. Here, we have investigated how A' processing is coordinated with assembly of the early preribosomal complex. We find that only the tUTP (UTP-A) complex is critical for A' cleavage, while components of the bUTP (UTP-B) and U3 snoRNP are important, but not essential, for efficient processing at this site. All other factors involved in the early stages of 18S rRNA processing that were tested here function downstream from this processing step. Interestingly, we show that the RNA surveillance factors XRN2 and MTR4 are also involved in A' cleavage in humans. A' cleavage is largely bypassed when XRN2 is depleted, and we also discover that A' cleavage is not always the initial processing event in all cell types. Together, our data suggest that A' cleavage is not a prerequisite for downstream pre-rRNA processing steps and may, in fact, represent a quality control step for initial pre-rRNA transcripts. Furthermore, we show that components of the RNA surveillance machinery, including the exosome and TRAMP complexes, also play key roles in the recycling of excised spacer fragments and degradation of aberrant pre-rRNAs in human cells.

Diverse diseases from a ubiquitous process: the ribosomopathy paradox

Collectively, the ribosomopathies are caused by defects in ribosome biogenesis. Although these disorders encompass deficiencies in a ubiquitous and fundamental process, the clinical manifestations are extremely variable and typically display tissue specificity. Research into this paradox has offered fascinating new insights into the role of the ribosome in the regulation of mRNA translation, cell cycle control, and signaling pathways involving TP53, MYC and mTOR. Several common features of ribosomopathies such as small stature, cancer predisposition, and hematological defects, point to how these diverse diseases may be related at a molecular level.

Tissue specific roles for the ribosome biogenesis factor Wdr43 in zebrafish development

During vertebrate craniofacial development, neural crest cells (NCCs) contribute to most of the craniofacial pharyngeal skeleton. Defects in NCC specification, migration and differentiation resulting in malformations in the craniofacial complex are associated with human craniofacial disorders including Treacher-Collins Syndrome, caused by mutations in TCOF1. It has been hypothesized that perturbed ribosome biogenesis and resulting p53 mediated neuroepithelial apoptosis results in NCC hypoplasia in mouse Tcof1 mutants. However, the underlying mechanisms linking ribosome biogenesis and NCC development remain poorly understood. Here we report a new zebrafish mutant, fantome (fan), which harbors a point mutation and predicted premature stop codon in zebrafish wdr43, the ortholog to yeast UTP5. Although wdr43 mRNA is widely expressed during early zebrafish development, and its deficiency triggers early neural, eye, heart and pharyngeal arch defects, later defects appear fairly restricted to NCC derived craniofacial cartilages. Here we show that the C-terminus of Wdr43, which is absent in fan mutant protein, is both necessary and sufficient to mediate its nucleolar localization and protein interactions in metazoans. We demonstrate that Wdr43 functions in ribosome biogenesis, and that defects observed in fan mutants are mediated by a p53 dependent pathway. Finally, we show that proper localization of a variety of nucleolar proteins, including TCOF1, is dependent on that of WDR43. Together, our findings provide new insight into roles for Wdr43 in development, ribosome biogenesis, and also ribosomopathy-induced craniofacial phenotypes including Treacher-Collins Syndrome.

Nucleolar stress in Diamond Blackfan anemia pathophysiology

Diamond Blackfan anemia is a red cell hypoplasia that typically presents within the first year of life. Most cases of Diamond Blackfan anemia are caused by ribosome assembly defects linked to haploinsufficiency for structural proteins of either ribosomal subunit. Nucleolar stress associated with abortive ribosome assembly leads to p53 activation via the interaction of free ribosomal proteins with HDM2, a negative regulator of p53. Significant challenges remain in linking this nucleolar stress signaling pathway to the clinical features of Diamond Blackfan anemia. Defining aspects of disease presentation may relate to developmental and physiological triggers that work in conjunction with nucleolar stress signaling to heighten the p53 response in the developing erythron after birth. The growing number of ribosomopathies provides additional challenges for linking molecular mechanisms with clinical phenotypes. This article is part of a Special Issue entitled: Role of the Nucleolus in Human Disease.

Interaction, mobility, and phosphorylation of human orthologues of WD repeat-containing components of the yeast SSU processome t-UTP sub-complex

We previously proposed a dynamic scaffold model for inner nuclear structure formation. In this model, structures in inter-chromatin regions are maintained through dynamic interaction of protein complex modules, and WD repeat- and disordered region-rich proteins and others act as scaffolds for these protein complexes. In this study, three WD-repeat proteins, i.e., CIRH1A, UTP15, and WDR43, were found in the nuclear matrix fraction and speculated to be present in the human t-UTP sub-complex of SSU processomes. The results obtained as to their subnuclear localization, binding with each other, mobilities, and phosphorylation were: (i) the majority of these proteins fused with GFP are localized to the fibrillar center region in nucleoli. (ii) these 3 proteins bind directly with each other in vitro. (iii) the movement of these proteins is very slow in living cells and independent of rDNA transcription. (iv) His-CIRH1A is phosphorylated at Thr131 by a mitotic Xenopus egg extract, and binding with GST-UTP15 and GST-WDR43 is suppressed. These findings and others suggest that these 3 WD proteins found in the matrix fraction bind directly with each other, bind tightly to fibrillar center regions, and comprise a part of the nucleolar structure. These results are also consistent with our dynamic scaffold model.

Ribosome biogenesis in the yeast Saccharomyces cerevisiae

Ribosomes are highly conserved ribonucleoprotein nanomachines that translate information in the genome to create the proteome in all cells. In yeast these complex particles contain four RNAs (>5400 nucleotides) and 79 different proteins. During the past 25 years, studies in yeast have led the way to understanding how these molecules are assembled into ribosomes in vivo. Assembly begins with transcription of ribosomal RNA in the nucleolus, where the RNA then undergoes complex pathways of folding, coupled with nucleotide modification, removal of spacer sequences, and binding to ribosomal proteins. More than 200 assembly factors and 76 small nucleolar RNAs transiently associate with assembling ribosomes, to enable their accurate and efficient construction. Following export of preribosomes from the nucleus to the cytoplasm, they undergo final stages of maturation before entering the pool of functioning ribosomes. Elaborate mechanisms exist to monitor the formation of correct structural and functional neighborhoods within ribosomes and to destroy preribosomes that fail to assemble properly. Studies of yeast ribosome biogenesis provide useful models for ribosomopathies, diseases in humans that result from failure to properly assemble ribosomes.

p53-mediated biliary defects caused by knockdown of cirh1a, the zebrafish homolog of the gene responsible for North American Indian Childhood Cirrhosis

North American Indian Childhood Cirrhosis (NAIC) is a rare, autosomal recessive, progressive cholestatic disease of infancy affecting the Cree-Ojibway first Nations of Quebec. All NAIC patients are homozygous for a missense mutation (R565W) in CIRH1A, the human homolog of the yeast nucleolar protein Utp4. Utp4 is part of the t-Utp subcomplex of the small subunit (SSU) processome, a ribonucleoprotein complex required for ribosomal RNA processing and small subunit assembly. NAIC has thus been proposed to be a primary ribosomal disorder (ribosomopathy); however, investigation of the pathophysiologic mechanism of this disease has been hindered by lack of an animal model. Here, using a morpholino oligonucleotide (MO)-based loss-of-function strategy, we have generated a model of NAIC in the zebrafish, Danio rerio. Zebrafish Cirhin shows substantial homology to the human homolog, and cirh1a mRNA is expressed in developing hepatocytes and biliary epithelial cells. Injection of two independent MOs directed against cirh1a at the one-cell stage causes defects in canalicular and biliary morphology in 5 dpf larvae. In addition, 5 dpf Cirhin-deficient larvae have dose-dependent defects in hepatobiliary function, as assayed by the metabolism of an ingested fluorescent lipid reporter. Previous yeast and in vitro studies have shown that defects in ribosome biogenesis cause stabilization and nuclear accumulation of p53, which in turn causes p53-mediated cell cycle arrest and/or apoptosis. Thus, the nucleolus appears to function as a cellular stress sensor in some cell types. In accordance with this hypothesis, transcriptional targets of p53 are upregulated in Cirhin-deficient zebrafish embryos, and defects in biliary function seen in Cirhin-deficient larvae are completely abrogated by mutation of tp53. Our data provide the first in vivo evidence of a role for Cirhin in biliary development, and support the hypothesis that congenital defects affecting ribosome biogenesis can activate a cellular stress response mediated by p53.

Genetics. Mysterious ribosomopathies

Ribosomes are essential in all cell types, yet mutations to ribosomal proteins or assembly factors cause tissue-specific disease.

Alagille syndrome and other hereditary causes of cholestasis

Neonatal conjugated jaundice is a common presentation of hereditary liver diseases, which, although rare, are important to recognize early. Developments in molecular genetic techniques have enabled the identification of causative genes, which has improved diagnostic accuracy for patients and has led to a greater understanding of the molecular pathways involved in liver biology and pathogenesis of liver diseases. This review provides an update of the current understanding of clinical and molecular features of the inherited liver diseases that cause neonatal conjugated jaundice.

Dysregulation of RNA polymerase I transcription during disease

Transcription of the ribosomal RNA genes by the dedicated RNA polymerase I enzyme and subsequent processing of the ribosomal RNA are fundamental control steps in the synthesis of functional ribosomes. Dysregulation of Pol I transcription and ribosome biogenesis is linked to the etiology of a broad range of human diseases. Diseases caused by loss of function mutations in the molecular constituents of the ribosome, or factors intimately associated with RNA polymerase I transcription and processing are collectively termed ribosomopathies. Ribosomopathies are generally rare and treatment options are extremely limited tending to be more palliative than curative. Other more common diseases are associated with profound changes in cellular growth such as cardiac hypertrophy, atrophy or cancer. In contrast to ribosomopathies, altered RNA polymerase I transcriptional activity in these diseases largely results from dysregulated upstream oncogenic pathways or by direct modulation by oncogenes or tumor suppressors at the level of the RNA polymerase I transcription apparatus itself. Ribosomopathies associated with mutations in ribosomal proteins and ribosomal RNA processing or assembly factors have been covered by recent excellent reviews. In contrast, here we review our current knowledge of human diseases specifically associated with dysregulation of RNA polymerase I transcription and its associated regulatory apparatus, including some cases where this dysregulation is directly causative in disease. We will also provide insight into and discussion of possible therapeutic approaches to treat patients with dysregulated RNA polymerase I transcription. This article is part of a Special Issue entitled: Transcription by Odd Pols.