1801

DPH1

DPH2L|DPH2L1|OVCA1;DPH1 homolog (S. cerevisiae);DPH1;DPH1 homolog (S. cerevisiae)

DPH-like 1|DPH2-like 1|candidate tumor suppressor in ovarian cancer 1|diphthamide biosynthesis protein 1|diphthamide biosynthesis protein 2 homolog-like 1|diptheria toxin resistance protein required for diphthamide biosynthesis (Saccharomyces)-like 1|dipt

17p13.3

protein-coding

Count: 3; Pubmed_search,Generif,UniProt

A second tumor suppressor locus on 17p that is distinct from TP53 has been identified in brain, breast, lung, and ovarian tumors. Using allelic loss mapping and positional cloning methods, we have recently identified two novel genes, which we refer to as OVCA1 and OVCA2, that map to 17p13.3. The two genes are ubiquitously expressed and encode proteins of 443 and 227 amino acids, respectively, with no known functional motifs. Sequence comparison of OVCA1 and OVCA2 revealed extensive sequence identity and similarity to hypothetical proteins from Saccharomyces cerevisiae, Caenorhabditis elegans, and Rattus species. Northern blot analysis reveals that OVCA1 and OVCA2 mRNA were expressed in normal surface epithelial cells of the ovary, but the level of this transcript is significantly reduced or is undetectable in 92% (11/12) of the ovarian tumors and tumor cell lines analyzed. The location, high degree of amino acid conservation, and reduced expression in ovarian tumors and tumor cell lines suggest that decreased expression of these two genes contributes to ovarian tumorigenesis and should be considered candidate tumor suppressor genes.

OVCA1, also known as DPH2L1, is a tumor suppressor gene associated with ovarian carcinoma and other tumors. Ovca1 homozygous mutant mice die at birth with developmental delay and cell-autonomous proliferation defects. Ovca1 heterozygous mutant mice are tumor-prone but rarely develop ovarian tumors. OVCA1 appears to be the homolog of yeast DPH2, which participates in the first biosynthetic step of diphthamide, by modification of histidine on translation elongation factor 2 (EF-2). Yeast dph2 mutants are resistant to diphtheria toxin, which catalyses ADP ribosylation of EF-2 at diphthamide. Thus, there appears to be growing evidence implicating alterations in protein translation with tumorigenesis.

OVCA1 is a tumor suppressor identified by positional cloning from chromosome 17p13.3, a hot spot for chromosomal aberration in breast and ovarian cancers. It has been shown that expression of OVCA1 is reduced in some tumors and that it regulates cell proliferation, embryonic development, and tumorigenesis. However, the biochemical function of OVCA1 has remained unknown. Recently, we isolated a novel mutant resistant to diphtheria toxin and Pseudomonas exotoxin A from the gene trap insertional mutants library of Chinese hamster ovary cells. In this mutant, the Ovca1 gene was disrupted by gene trap mutagenesis, and this disruption well correlated with the toxin-resistant phenotype. We demonstrated direct evidence that the tumor suppressor OVCA1 is a component of the biosynthetic pathway of diphthamide on elongation factor 2, the target of bacterial ADP-ribosylating toxins. A functional genetic approach utilizing the random gene trap mutants library of mammalian cells should become a useful strategy to identify the genes responsible for specific phenotypes.

We previously identified and characterized a novel p53-regulated gene in mouse prostate cancer cells that was homologous to a human gene that had been identified in brain cancers and termed RTVP-1 or GLIPR. In this report, we document that the human RTVP-1 gene is also regulated by p53 and induces apoptosis in human prostate cancer cell lines. We show that the expression of the human RTVP-1 gene is down-regulated in human prostate cancer specimens compared with normal human prostate tissue at the mRNA and protein levels. We further document epigenetic changes consistent with RTVP-1 being a tumor suppressor in human prostate cancer.

The OVCA1 gene is a candidate for the breast and ovarian tumor suppressor gene at chromosome 17p13.3. To help determine the function(s) of OVCA1, we used a yeast two-hybrid screening approach to identify OVCA1-associating proteins. One such protein, which we initially referred to as BOV-1 (binder of OVCA1-1) is 173 or 174 amino acids in length and appears to be a new member of a highly conserved RNA-binding motif (RBM) protein family that is highly conserved evolutionarily. Northern blot analysis revealed that BOV-1 is ubiquitously expressed and that three distinct messenger RNA species are expressed, 1-, 3.2-, and 5.8-kb transcripts. The 1-kb transcript is the most abundant and is expressed at high levels in the testis, heart, placenta, spleen, thymus, and lymphocytes. Using fluorescence in situ hybridization and the 5.8-kb complementary DNA probe, we determined that BOV-1 maps to both chromosome 5q13-q14 and chromosome 14q22-q23. Further sequence analysis determined that the gene coding the 1- and the 3.2-kb transcripts (HGMW-approved gene symbol RBM8A) maps to 14q22-q23, whereas a second highly related gene coding for the 5.8-kb transcript resides at chromosome 5q13-q14 (HGMW-approved gene symbol RBM8B). The predicted proteins encoded by RBM8A and RBM8B are identical except that RBM8B is 16 amino acids shorter at its N-terminus. Molecular modeling of the RNA-binding domain of RBM8A and RBM8B, based on homology to the sex-lethal protein of Drosophila, identifies conserved residues in the RBM8 protein family that are likely to contact RNA in a protein-RNA complex. The conservation of sequence and structure through such an evolutionarily divergent group of organisms suggests an important function for the RBM8 family of proteins.CI - Copyright 2000 Academic Press.

Loss of all or part of one copy of chromosome 17p is very common in ovarian and breast tumors. OVCA1 is a candidate tumor suppressor gene mapping to a highly conserved region on chromosome 17p13.3 that shows frequent loss of heterozygosity in breast and ovarian carcinomas. Western blot analysis of extracts prepared from breast and ovarian carcinomas revealed reduced expression of OVCA1 compared with extracts from normal epithelial cells from these tissues. Subcellular localization studies indicate that OVCA1 is localized to punctate bodies scattered throughout the cell but is primarily clustered around the nucleus. Attempts to create cell lines that stably expressed OVCA1 from the cytomegalovirus promoter were generally unsuccessful in a variety of different cell lines. This reduction of colony formation was quantified in the ovarian cancer cell line A2780, where it was demonstrated that cells transfected with plasmids expressing OVCA1 had a 50-60% reduction in colony number as compared with appropriate controls, and only a few of these clones expressed OVCA1, albeit at low levels. The clones that expressed exogenous OVCA1 were found to have dramatically reduced rates of proliferation. Reduced growth rates correlated with an increased proportion of the cells in the G1 fraction of the cell cycle compared with the parental cell line and decreased levels of cyclin D1. The low levels of cyclin D1 appeared to be caused by an accelerated rate of cyclin D1 degradation. Overexpression of cyclin D1 was able to override OVCA1's suppression of clonal outgrowth. These results suggest that slight alterations in the level of OVCA1, such as would occur after reduction of chromosome 17p13.13 to hemizygosity, may result in cell cycle deregulation and promote tumorigenesis.

Loss of OVCA1/DPH2L1 correlates with ovarian and breast cancer. To study its in vivo role, we generated Ovca1 mutant alleles in mice. Ovca1 heterozygotes spontaneously develop cancer. Ovca1 mutant mice die during embryonic development and at birth with developmental delay and defects in multiple organ systems. Cell proliferation defects were observed in Ovca1 mutant mouse embryonic fibroblasts (MEFs). p53 deficiency can rescue these Ovca1 mutant MEF proliferation defects and partially rescue Ovca1 mutant embryonic phenotypes. Furthermore, Ovca1; p53 double heterozygotes developed tumors quicker than p53 heterozygotes and with an increased carcinoma incidence. Multiple tumor burden in Ovca1 heterozygotes that were also p53 deficient was significantly higher than in p53 homozygous mutants. These in vivo findings demonstrate that Ovca1 is a tumor suppressor that can modify p53-induced tumorigenesis and suggest that it acts as a positive regulator for cell cycle progression. The close linkage of OVCA1 and p53 on human Chromosome 17 suggests that coordinated loss may be an important mechanism for the evolution of ovarian, breast, and other tumor phenotypes.