[Show abstract][Hide abstract] ABSTRACT: Three human RecQ DNA helicases, WRN, BLM and RTS, are involved in the genetic disorders associated with genomic instability and a high incidence of cancer. RecQL1 and RecQL5 also belong to the human RecQ helicase family, but their correlation with genetic disorders, if any, is unknown. We report here that in human B cells transformed by Epstein-Barr virus (EBV), human fibroblasts and umbilical endothelial cells transformed by simian virus 40, the expression of WRN, BLM, RTS and RecQL1 was sharply up-regulated. In B cells this expression was stimulated within 5-40 h by the tumor promoting agent phorbol myristic acetate (PMA). Interestingly, RecQL5beta, an alternative splicing product of RecQL5 with a nuclear localization signal, is expressed in resting B cells without significant modulation of its synthesis by EBV or PMA, suggesting it has a role in resting cells. We also roughly determined the number of copies per cell for the five RecQ helicase in B cells. In addition, levels of the different RecQ helicases are modulated in different ways during the cell cycle of actively proliferating fibroblasts and umbilical endothelial cells. Our results support the view that the levels of WRN, BLM, RTS and RecQL1 are differentially up-regulated to guarantee genomic stability in cells that are transformed or actively proliferating.
[Show abstract][Hide abstract] ABSTRACT: The RecQ helicase superfamily has been implicated in DNA repair and recombination. At least five human RecQ-related genes exist: RecQ1, BLM, WRN, RecQ4 and RecQ5. Mutations in BLM, WRN and RecQ4 are associated with Bloom, Werner and Rothmund-Thomson syndromes, respectively, involving a predisposition to malignancies and a cellular phenotype that includes increased chromosome instability. RecQ5 is small, containing only a core part of the RecQ helicase, but three isomer transcripts code for small RecQ5alpha (corresponding to the original RecQ5 with 410 amino acids), new large RecQ5beta (991 amino acids) and small RecQ5gamma (435 amino acids) proteins that contain the core helicase motifs. By determining the genomic structure, we found that the three isoforms are generated by differential splicing from the RecQ5 gene that contains at least 19 exons. Northern blot analysis using a RecQ5beta-specific probe indicates that RecQ5beta mRNA is expressed strongly in the testis. Immunocytochemical staining of three N-terminally tagged RecQ5 isomers expressed in 293EBNA cells showed that RecQ5beta migrates to the nucleus and exists exclusively in the nucleoplasm, while the small RecQ5alpha and RecQ5gamma proteins stay in the cytoplasm. Immunoprecipitation and an extended cytochemical experiment suggested that the nucleoplasmic RecQ5beta, like yeast Sgs1 DNA helicase, binds to topoisomerases 3alpha and 3beta, but not to topoisomerase 1. These results predict that RecQ5beta may have an important role in DNA metabolism and may also be related to a distinct genetic disease.
Full-text · Article · May 2000 · Nucleic Acids Research
[Show abstract][Hide abstract] ABSTRACT: Rothmund-Thomson syndrome (RTS), an autosomal recessive disorder, comprises poikiloderma, growth deficiency, some aspects of premature aging, and a predisposition to malignancy, especially osteogenic sarcomas. Two kindreds with RTS were recently shown to segregate for mutations in the human RECQL4 helicase gene. We report identification of a new RTS kindred in which both brothers developed osteosarcomas. Mutation analysis of the RECQL4 gene was performed on both brothers and both parents. The brothers were shown to be compound heterozygotes for mutations in the RECQL4 gene, including a single basepair deletion in exon 9 resulting in a frameshift and early termination codon and a base substitution in the 3-prime splice site in the intron-exon boundary of exon 8, which would be predicted to cause a deletion of at least part of a consensus helicase domain. Each parent was shown to be a heterozygote carrier for one mutation. This report strengthens the association between mutations in RECQL4 helicase gene and RTS. Two other recessive disorders, Bloom syndrome and Werner syndrome, are known to be due to other human RECQ helicase gene mutations. These three disorders all manifest abnormal growth, premature aging, and predisposition to site-specific malignancies. The clinical and molecular aspects of RTS, Bloom syndrome, and Werner syndrome are compared and contrasted.
No preview · Article · Feb 2000 · American Journal of Medical Genetics
[Show abstract][Hide abstract] ABSTRACT: RECQL4 is the fourth gene identified as a member of the human DNA helicase RecQ gene family including the genes for Werner syndrome (WRN) and Bloom syndrome, both of which are characterized by genomic instability. Recently, RECQL4 was identified as the gene responsible for some cases of Rothmund-Thomson syndrome (RTS), a rare autosomal recessive genetic disorder that shows chromosomal instability, premature aging, and a high risk of mesenchymal tumors. In this study, we show the genomic organization of the RECQL4 gene, including the exon-intron boundaries, the transcription initiation sites, and the potential promoter sequences, which facilitates further mutation analysis of the RECQL4 gene and studies to elucidate the pathogenesis behind RTS. The RECQL4 gene is in a small genome of 6.5 kb and consists of 21 exons. In the 5' upstream region, one Sp1 site and several AP 2 sites exist near the capping site, suggesting that the expression of RECQL4 is regulated by a housekeeping-type promoter similar to WRN. By comparative Northern blot analysis, we show that the RECQL4 transcripts are severely down-regulated in the cells from RTS patients, similar to our previous observation for WRN transcripts in cells from Werner patients. Immunocytochemical analysis indicated that the RECQL4 protein expressed in HeLa cells is in the nucleus and appears to be localized mainly in the nucleoplasm similar to WRN helicase.
[Show abstract][Hide abstract] ABSTRACT: Rothmund-Thomson syndrome (RTS; also known as poikiloderma congenitale) is a rare, autosomal recessive genetic disorder characterized by abnormalities in skin and skeleton, juvenile cataracts, premature ageing and a predisposition to neoplasia. Cytogenetic studies indicate that cells from affected patients show genomic instability often associated with chromosomal rearrangements causing an acquired somatic mosaicism. The gene(s) responsible for RTS remains unknown. The genes responsible for Werner and Bloom syndromes (WRN and BLM, respectively) have been identified as homologues of Escherichia coli RecQ, which encodes a DNA helicase that unwinds double-stranded DNA into single-stranded DNAs. Other eukaryotic homologues thus far identified are human RECQL, Saccharomyces cerevisiae SGS1 and Schizosaccharomyces pombe rqh1. We recently cloned two new human helicase genes, RECQL4 at 8q24.3 and RECQL5 at 17q25, which encode members of the RecQ helicase family. Here, we report that three RTS patients carried two types of compound heterozygous mutations in RECQL4. The fact that the mutated alleles were inherited from the parents in one affected family and were not found in ethnically matched controls suggests that mutation of RECQL4 at human chromosome 8q24.3 is responsible for at least some cases of RTS.
[Show abstract][Hide abstract] ABSTRACT: We have recently described an automated approach for homology modeling using restrained molecular dynamics and simulated annealing procedures (Li et al, Protein Sci., 6:956-970,1997). We have employed this approach for constructing a homology model of the putative RNA-binding domain of the human RNA-binding protein with multiple splice sites (RBP-MS). The regions of RBP-MS which are homologous to the template protein snRNP U1A were constrained by "homology distance constraints," while the conformation of the non-homologous regions were defined only by a potential energy function. A full energy function without explicit solvent was employed to ensure that the calculated structures have good conformational energies and are physically reasonable. The effects of mis-alignment of the unknown and the template sequences were also explored in order to determine the feasibility of this homology modeling method for distinguishing possible sequence alignments based on considerations of the resulting conformational energies of modeled structures. Differences in the alignments of the unknown and the template sequences result in significant differences in the conformational energies of the calculated homology models. These results suggest that conformational energies and residual constraint violations in these homology-constrained simulated annealing calculations can be used as criteria to distinguish between correct and incorrect sequence alignments and chain folds.
No preview · Article · Jan 1999 · Proteins Structure Function and Bioinformatics
[Show abstract][Hide abstract] ABSTRACT: Two new human DNA helicase genes, RecQ4 and RecQ5, that belong to the RecQ helicase family were cloned and characterized. The addition of these genes increases the total to five helicase genes in the human RecQ family, which includes helicases involved in Bloom and Werner syndromes, the genetic diseases manifesting the distinctive but overlapping clinical phenotypes of immunodeficiency, premature aging, and an enhanced risk of cancer. The RecQ4 helicase is as large as the Bloom (BLM) and Werner (WRN) helicases, and its gene expression profile is organ-specific, resembling that of BLM helicase. In contrast, the RecQ5 helicase has a low molecular weight, similar to the human progenitor RecQ1 helicase, and is expressed in all the organs examined. All five human helicase genes are expressed in cultured K562 leukemia and fibroblast cells. Synchronized K562 cell cultures showed that the genes RecQ4 and BLM, and RecQ1 and WRN, seem to be upregulated at the G1/S and G2/M phases, respectively, of the cell cycle. The biological significance of multiple species of human RecQ helicases, which are apparently nonessential for life but may be related to distinct diseases, is discussed in light of the fact that unicellular organisms, like Escherichia coli and yeast, contain only one species of helicase of this particular family.
[Show abstract][Hide abstract] ABSTRACT: Detailed physical maps of the human genome are important resources for identification and isolation of genes responsible for diseases and for the study of their structure and function. We constructed a 2.0-Mb high-resolution physical map within the human chromosome 8p12-p21 region extending from marker D8S131 to D8S283. The map comprises a series of contigs mostly P1/PAC clones, which span the loci of potential tumor suppressor genes and the Werner's syndrome gene. Each P1/PAC DNA was defined by its size, restriction sites, terminal sequences, intermarker distances and location relative to major genes and markers. The genes on these P1/PAC DNAs were analyzed by an exon amplification method to determine their locations. The genes newly found by the exon amplification method together with other known genes, including those of glutathion reductase, a general transcription factor, protein phosphatase 2A beta subunit and Werner's syndrome, were precisely mapped within the contigs. These P1/PAC DNAs are useful reagents for the generation of new microsatellite markers to narrow the candidate region of the tumor suppressor gene(s) and/or genes responsible for other diseases, which are believed to exist in this region by linkage analysis.
[Show abstract][Hide abstract] ABSTRACT: The gene responsible for Werner's syndrome (WRN) contains a region homologous to the Escherichia coli RecQ type DNA helicase and was thought to code for a DNA helicase belonging to this helicase family. However, no evidence has been shown before to substantiate this prediction. Here, we show data that the product of the WRN gene is indeed a DNA helicase. The gene product, a polypeptide with a relative molecular mass of 170 kDa, expressed in the insect Spodoptera frugiperda (Sf21) cell and purified by affinity column chromatography contained both the ATPase and DNA unwinding activities characteristic of DNA helicase. Expressions in Sf21, as well as in HeLa cells, showed that the WRN DNA helicase is exclusively transported to the nucleoplasm, which is consistent with its function in DNA metabolism. Our studies on strand displacement suggest that WRN helicase can unwind not only a duplex DNA, but also an RNA-DNA heteroduplex, while the latter reaction seems less efficient. Enzymological features learned from the purified WRN helicase are discussed with respect to the biological function, which remains to be clarified.
Full-text · Article · Sep 1997 · Nucleic Acids Research
[Show abstract][Hide abstract] ABSTRACT: The correlation between mutations in the Werner's syndrome (WRN) gene and the haplotypes of surrounding markers was studied in Japanese patients. We have elucidated the genomic structure of WRN helicase, and found five additional mutations, designated mutations 6-10. Mutations 4 and 6 were found to be the two major mutations in this population; these mutations comprised 50.8% and 17.5%, respectively, of the total in a sample of 126 apparently unrelated chromosomes. Almost all the patients homozygous for mutation 4 shared a haplotype around the WRN gene, consistent with the view that they are derived from a single ancestor. This important advantage demonstrated in the identification of the WRN gene suggests that the Japanese present a unique population for the cloning of other disease genes. The conserved haplotype was observed across 19 loci, extending a distance estimated to be more than 1.4 Mbp around the WRN gene. This haplotype is rare among random Japanese individuals. Unexpectedly, all the nine patients homozygous for mutation 6 shared a haplotype that was identical to this haplotype at 18 of these 19 markers. These results suggest that mutations 4 and 6 arose independently in almost identical rare haplotypes. The remaining mutations (1, 5, 7, 8, 9, and 10) occurred rarely, and were each associated with different haplotypes.
[Show abstract][Hide abstract] ABSTRACT: A novel human gene referred to as the WS-3 gene, in the short arm of human chromosome 8, was cloned by a combination of exon trapping, thermal asymmetric interlaced-PCR (TAIL-PCR) and the Marathon-Ready cDNA amplification method. The gene consists of 7 exons separated by 6 introns, and is at the telomere side of the STS marker, D8S1055. The full-length WS-3 gene contains 1052 nucleotides and codes for a protein of 190 amino acids with a calculated mol. wt. of 20,747. Southern blot experiments showed that the WS-3 gene exists as a single copy in the human genome. A protein encoded by the WS-3 gene has an R-G-D (Arg-Gly-Asp) motif in the N-terminal region, which seems to confer adhesive properties to macromolecular proteins like fibronectin. Although WS-3 is a small gene with unknown biological function, its ubiquitous expression in various tissues and organs suggests that the encoded protein is one of the essential components of all organs and tissues.
[Show abstract][Hide abstract] ABSTRACT: A unique gene, RBP-MS, spanning over 230 kb in the human chromosome 8p11-12 near the Werner syndrome gene locus is described. The single-copy RBP-MS gene is alternatively spliced, resulting in a family of at least 12 transcripts (average length of 1.5 kb). Nine different types of cDNAs that encode an RNa-binding motif at the N terminus and helix-rich sequences at the C terminus have been identified thus far. Among the 16 exons identified, four 5'-proximal exons contained sequences homologous to the RNA-binding domain of Drosophila couch potato gene. Northern blot analysis showed that the RBP-MS gene was expressed strongly in the heart, prostate, intestine, and ovary, and poorly in the skeletal muscle, spleen, thymus, brain, and peripheral leukocytes. The possible role of this gene in RNA metabolism is discussed.
Full-text · Article · Nov 1996 · Proceedings of the National Academy of Sciences