Genetic alterations of PTEN in human melanoma.

The PTEN gene is one of the most frequently inactivated tumor suppressor genes in sporadic cancers. Inactivating mutations and deletions of the PTEN gene are found in many types of cancers, including melanoma. However, the exact frequency of PTEN alteration in melanoma is unknown. In this study, we comprehensively reviewed 16 studies on PTEN genetic changes in melanoma cell lines and tumor biopsies. To date, 76 PTEN alterations have been reported in melanoma cell lines and 38 PTEN alterations in melanoma biopsies. The rate of PTEN alterations in melanoma cell lines, primary melanoma, and metastatic melanoma is 27.6, 7.3, and 15.2%, respectively. Three mutations were found in both melanoma cell lines and biopsies. These mutations are scattered throughout the gene, with the exception of exon 9. A mutational hot spot is found in exon 5, which encodes the phosphatase activity domain. Evidence is also presented to suggest that numerous homozygous deletions and missense variants exist in the PTEN transcript. Studying PTEN functions and implications of its mutations and other genes could provide insights into the precise nature of PTEN function in melanoma and additional targets for new therapeutic approaches.

Elevated expression of Rad18 regulates melanoma cell proliferation.

The E3 ligase Rad18 is a key regulator for the lesion bypass pathway, which plays an important role in genomic stability. However, the status of Rad18 expression in melanoma is not known. Using melanoma tissue microarray (TMA), we showed that nuclear Rad18 expression was upregulated in primary and metastatic melanoma compared to dysplastic nevi. Rad18 expression was significantly reduced in sun-exposed sites compared to the sun-protected sites. Strong Rad18 expression correlated with worse 5-year patient survival and was an independent prognostic factor for melanoma found in the sun-protected sites. Furthermore, we showed that melanoma cell proliferation and the expression of pAkt and cyclin D1 were reduced upon Rad18 knockdown. We, for the first time, showed that Rad18 is significantly increased in melanoma and predicts the poor outcome for melanoma in the sun-protected sites. Rad18 is involved in the regulation of melanoma cell proliferation, which can be exploited in designing new strategy for melanoma treatment.

The ING family tumor suppressors: from structure to function.

The Inhibitor of Growth (ING) proteins belong to a well-conserved family which presents in diverse organisms with several structural and functional domains for each protein. The ING family members are found in association with many cellular processes. Thus, the ING family proteins are involved in regulation of gene transcription, DNA repair, tumorigenesis, apoptosis, cellular senescence and cell cycle arrest. The ING proteins have multiple domains that are potentially capable of binding to many partners. It is conceivable, therefore, that such proteins could function similarly within protein complexes. In this case, within this family, each function could be attributed to a specific domain. However, the role of ING domains is not definitively clear. In this review, we summarize recent advances in structure-function relationships in ING proteins. For each domain, we describe the known biological functions and the approaches utilized to identify the functions associated with ING proteins.

The Noc-domain containing C-terminus of Noc4p mediates both formation of the Noc4p-Nop14p submodule and its incorporation into the SSU processome.

Noc1p, Noc3p and Noc4p are eukaryotic proteins which play essential roles in yeast ribosome biogenesis and contain a homologous stretch of about 45 aminoacids (Noc-domain) of unknown function. Yeast Noc4p is a component of the small ribosomal subunit (SSU) processome, can be isolated as a stable Noc4p-Nop14p SSU-processome submodule from yeast cells, and is required for nuclear steps of small ribosomal subunit rRNA maturation. We expressed a series of mutated alleles of NOC4 in yeast cells and analysed whether the corresponding protein variants support vegetative growth, interact with Nop14p, and are incorporated into the SSU-processome. The data reveal that the essential C-terminus of Noc4p which contains 237 aminoacids including the Noc-domain represents a protein-protein interaction module. It is required and sufficient for its association with Nop14p and several nuclear precursors of the small ribosomal subunit. The N-terminal Noc4-part seems to be targeted to pre-ribosomes via the C-terminus of Noc4p and plays there an essential role in SSU-processome function. Replacement of the Noc4p-Noc-domain by its homologues Noc1p-counterpart results in a hybrid Noc4p variant which fails to associate with Nop14p and pre-ribosomes. On the other hand, exchange of 6 amino acids in the Noc1-Noc-domain of this hybrid Noc4p protein is sufficient to restore its essential in vivo functions. These data suggest that Noc-domains of Noc1p and Noc4p share a common structural backbone in which diverging amino acids play crucial roles in mediating specific regulated interactions. Our analysis allows us to distinguish between different functions of certain domains within Noc4p and contribute to the understanding of how incorporation of Noc4p into ribosomal precursors is coupled to rRNA processing and maturation of the small ribosomal subunit.

[Diamond-Blackfan anemia reveals the dark side of ribosome biogenesis]. / Anémie de Diamond-Blackfan: le côté obscur de la biogenèse des ribosomes.

Diamond-Blackfan anemia (DBA), a rare congenital erythroblastopenia, has recently become a paradigm for a growing set of genetic diseases linked to mutations in genes encoding ribosomal proteins or factors involved in ribosome biogenesis. Recent studies of the structure and the function of ribosomal proteins affected in DBA indicate that their mutation in DBA primarily impacts ribosome biogenesis. Accordingly, cells from DBA patients display anomalies in the maturation of ribosomal RNAs. The explanation of this unexpected link between ribosome biogenesis, a ubiquitous process, and a disease mostly affecting erythroid differentiation may stem in part from the emerging concept of ribosomal stress response, a signaling pathway triggering cell cycle arrest in response to a defect in ribosome synthesis. Future studies of DBA and other diseases related to defects in ribosome biogenesis are likely to rapidly provide important insights into the regulatory mechanisms linking cell cycle progression to this major metabolic pathway.

Mutation of ribosomal protein RPS24 in Diamond-Blackfan anemia results in a ribosome biogenesis disorder.

Diamond-Blackfan anemia (DBA) is a rare congenital disease affecting erythroid precursor differentiation. DBA is emerging as a paradigm for a new class of pathologies potentially linked to disorders in ribosome biogenesis. Three genes encoding ribosomal proteins have been associated to DBA: after RPS19, mutations in genes RPS24 and RPS17 were recently identified in a fraction of the patients. Here, we show that cells from patients carrying mutations in RPS24 have defective pre-rRNA maturation, as in the case of RPS19 mutations. However, in contrast to RPS19 involvement in the maturation of the internal transcribed spacer 1, RPS24 is required for processing of the 5' external transcribed spacer. Remarkably, epistasis experiments with small interfering RNAs indicate that the functions of RPS19 and RPS24 in pre-rRNA processing are connected. Resolution of the crystal structure of RPS24e from the archeon Pyroccocus abyssi reveals domains of RPS24 potentially involved in interactions with pre-ribosomes. Based on these data, we discuss the impact of RPS24 mutations and speculate that RPS19 and RPS24 cooperate at a particular stage of ribosome biogenesis connected to a cell cycle checkpoint, thus affecting differentiation of erythroid precursors as well as developmental processes.

Molecular basis of Diamond-Blackfan anemia: structure and function analysis of RPS19.

Diamond-Blackfan anemia (DBA) is a rare congenital disease linked to mutations in the ribosomal protein genes rps19, rps24 and rps17. It belongs to the emerging class of ribosomal disorders. To understand the impact of DBA mutations on RPS19 function, we have solved the crystal structure of RPS19 from Pyrococcus abyssi. The protein forms a five alpha-helix bundle organized around a central amphipathic alpha-helix, which corresponds to the DBA mutation hot spot. From the structure, we classify DBA mutations relative to their respective impact on protein folding (class I) or on surface properties (class II). Class II mutations cluster into two conserved basic patches. In vivo analysis in yeast demonstrates an essential role for class II residues in the incorporation into pre-40S ribosomal particles. This data indicate that missense mutations in DBA primarily affect the capacity of the protein to be incorporated into pre-ribosomes, thus blocking maturation of the pre-40S particles.