The Werner syndrome. A model for the study of human aging.

Human aging is a complex process that leads to the gradual deterioration of body functions with time. Various models to approach the study of aging have been launched over the years such as the genetic analysis of life span in the yeast S. cerevisiae, the worm C. elegans, the fruitfly, and mouse, among others. In human models, there have been extensive efforts using replicative senescence, the study of centenerians, comparisons of young versus old at the organismal, cellular, and molecular levels, and the study of premature aging syndromes to understand the mechanisms leading to aging. One good model for studying human aging is a rare autosomal recessive disorder known as the Werner syndrome (WS), which is characterized by accelerated aging in vivo and in vitro. A genetic defect implicated in WS was mapped to the WRN locus. Mutations in this gene are believed to be associated, early in adulthood, with clinical symptoms normally found in old individuals. WRN functions as a DNA helicase, and recent evidence, summarized in this review, suggests specific biochemical roles for this multifaceted protein. The interaction of WRN protein with RPA (replication protein A) and p53 will undoubtedly direct efforts to further dissect the genetic pathway(s) in which WRN protein functions in DNA metabolism and will help to unravel its contribution to the human aging process.

The Werner syndrome protein is involved in RNA polymerase II transcription.

Werner syndrome (WS) is a human progeroid syndrome characterized by the early onset of a large number of clinical features associated with the normal aging process. The complex molecular and cellular phenotypes of WS involve characteristic features of genomic instability and accelerated replicative senescence. The gene involved (WRN) was recently cloned, and its gene product (WRNp) was biochemically characterized as a helicase. Helicases play important roles in a variety of DNA transactions, including DNA replication, transcription, repair, and recombination. We have assessed the role of the WRN gene in transcription by analyzing the efficiency of basal transcription in WS lymphoblastoid cell lines that carry homozygous WRN mutations. Transcription was measured in permeabilized cells by [3H]UTP incorporation and in vitro by using a plasmid template containing the RNA polymerase II (RNA pol II)-dependent adenovirus major late promoter. With both of these approaches, we find that the transcription efficiency in different WS cell lines is reduced to 40-60% of the transcription in cells from normal individuals. This defect can be complemented by the addition of normal cell extracts to the chromatin of WS cells. Addition of purified wild-type WRNp but not mutated WRNp to the in vitro transcription assay markedly stimulates RNA pol II-dependent transcription carried out by nuclear extracts. A nonhelicase domain (a direct repeat of 27 amino acids) also appears to have a role in transcription enhancement, as revealed by a yeast hybrid-protein reporter assay. This is further supported by the lack of stimulation of transcription when mutant WRNp lacking this domain was added to the in vitro assay. We have thus used several approaches to show a role for WRNp in RNA pol II transcription, possibly as a transcriptional activator. A deficit in either global or regional transcription in WS cells may be a primary molecular defect responsible for the WS clinical phenotype.

Enzymatic and DNA binding properties of purified WRN protein: high affinity binding to single-stranded DNA but not to DNA damage induced by 4NQO.

Mutations in the WRN gene result in Werner syndrome, an autosomal recessive disease in which many characteristics of aging are accelerated. A probable role in some aspect of DNA metabolism is suggested by the primary sequence of the WRN gene product. A recombinant His-tagged WRN protein (WRNp) was overproduced in insect cells using the baculovirus system and purified to near homogeneity by several chromatographic steps. This purification scheme removes both nuclease and topoisomerase contaminants that persist following a single Ni(2+)affinity chromatography step and allows for unambiguous interpretation of WRNp enzymatic activities on DNA substrates. Purified WRNp has DNA-dependent ATPase and helicase activities consistent with its homology to the RecQ subfamily of proteins. The protein also binds with higher affinity to single-stranded DNA than to double-stranded DNA. However, WRNp has no higher affinity for various types of DNA damage, including adducts formed during 4NQO treatment, than for undamaged DNA. Our results confirm that WRNp has a role in DNA metabolism, although this role does not appear to be the specific recognition of damage in DNA.

Genomic organization, sequence, and chromosomal localization of the human helix-loop-helix Id1 gene.

The helix-loop-helix protein Id-1 regulates growth and differentiation in many mammalian cells. In human fibroblasts, Id1 and Id1', a putative splicing variant, are cell cycle regulated, essential for proliferation, repressed by senescence, and overexpressed by some tumor cells. To better understand Id1, we determined the complete sequence, transcriptional start, and localization of the human Id1 gene. Human Id1 has two exons (426 bp and 42 bp), separated by an intron (239 bp). Id1' results from failure to splice the intron, which encodes 7 amino acids prior to a stop codon. Thus, Id1 and Id1' proteins differ only at the extreme C-terminus. Id1 transcription initiated 96 bp upstream of the initiation AUG; 2 kb of upstream sequence stimulated transcription of a reporter gene. Human Id1 maps to chromosome 20 at q11, very close to the centromere but outside the amplicons frequently found in human cancers.

The helix-loop-helix protein Id-1 and a retinoblastoma protein binding mutant of SV40 T antigen synergize to reactivate DNA synthesis in senescent human fibroblasts.

Normal somatic cells of higher organisms do not divide indefinitely. After a finite number of divisions, normal cells irreversibly cease proliferation by a process termed replicative or cellular senescence. Replicative senescence is controlled by multiple, dominant-acting genes about which very little is known. The only genes known to reactivate DNA synthesis in senescent cells are viral oncogenes encoding proteins that bind and inactivate the p53 and retinoblastoma (pRb) tumor suppressor proteins. SV40 T antigen is the best studied of these viral oncoproteins. T[K1] is a T antigen point mutant that selectively is defective in binding pRb and the pRb-related proteins p107 and p130. We show that T[K1] stimulated quiescent human fibroblasts to synthesize DNA nearly as well as wild-type T but was incapable of stimulating senescent cells. We tested several growth regulatory genes that are repressed in senescent cells for ability to restore activity to T[K1]. These included c-fos, c-jun, Id-1, Id-2, E2F-1, and cdc2. Only the helix-loop-helix (HLH) protein, Id-1, restored the ability of T[K1] to reactivate DNA synthesis in senescent cells. This activity of Id-1 was not shared by Id-2, a related protein, and depended on an intact HLH domain. It did not appear that Id-1 interacted directly with pRb or p107. Constitutive Id-1 expression failed to rescue proliferating cells from growth inhibition by pRb, p107, or p130, and failed to interact with pRb in the yeast two hybrid system. Because Id proteins negatively regulate basic-HLH (bHLH) transcription factors, we suggest that senescent cells express one or more bHLH factor that cooperates with pRb, or pRb-related proteins, to suppress proliferation.

Coming of age in culture.

Replicative senescence is a fundamental feature of most, if not all, somatic higher eukaryotic cells. The phenomenon has been studied for more than three decades, during which time the genetics and cell biology of senescent cells were characterized. In recent years, progress has been made on understanding the molecular basis for replicative senescence. At present, we now have a good, albeit still incomplete, understanding of some of the immediate causes for the growth arrest of senescent cells. The challenges for the future will be to understand the molecular bases for the prime causes of senescent phenotype, including the growth arrest and the altered differentiation.

Isolation and characterization of the mouse PDGF beta-receptor promoter.

The PDGF beta-receptor expression is tightly regulated during embryonic development and in several physiological and pathological situations. To determine the regulatory mechanism of the receptor, a 1.9 kb 5' flanking genomic fragment of the mouse PDGF beta-receptor gene was cloned and analyzed by functional promoter assays. The fragment was shown to exert promoter activity in the luciferase expression vector system in mouse NIH 3T3 fibroblast and NB41 neuroblastoma cell lines as well as rat ST15A cerebellar cell lines. Functional studies on deletion mutants revealed several putative regulatory sequences. The deletion mutants acted similarly in NB41 cells and in ST15A cells, both of neuronal origin, but differently in the NIH 3T3 fibroblasts. No TATA box was found in the analyzed promoter region, however, site directed mutagenesis of a CCAAT motif, located 60 basepair upstream of the transcriptional start site, almost completely abolished the promoter activity in all cell types.

Importance of a flanking AT-rich region in target site recognition by the GC box-binding zinc finger protein MIG1.

MIG1 is a zinc finger protein that mediates glucose repression in the yeast Saccharomyces cerevisiae. MIG1 is related to the mammalian Krox/Egr, Wilms' tumor, and Sp1 finger proteins. It has two fingers and binds to a GCGGGG motif that resembles the GC boxes recognized by these mammalian proteins. We have performed a complete saturation mutagenesis of a natural MIG1 site in order to elucidate its binding specificity. We found that only three mutations within the GC box retain the ability to bind MIG1: G1 to C, C2 to T, and G5 to A. This result is consistent with current models for zinc finger-DNA binding, which assume that the sequence specificity is determined by base triplet recognition within the GC box. Surprisingly, we found that an AT-rich region 5' to the GC box also is important for MIG1 binding. This AT box is present in all natural MIG1 sites, and it is protected by MIG1 in DNase I footprints. However, the AT box differs from the GC box in that no single base within it is essential for binding. Instead, the AT-rich nature of this sequence seems to be crucial. The fact that AT-rich sequences are known to increase DNA flexibility prompted us to test whether MIG1 bends DNA. We found that binding of MIG1 is associated with bending within the AT box. We conclude that DNA binding by a simple zinc finger protein such as MIG1 can involve both recognition of the GC box and flanking sequence preferences that may reflect local DNA bendability.

Yeast MIG1 repressor is related to the mammalian early growth response and Wilms' tumour finger proteins.

We have cloned a yeast gene, MIG1, which encodes a C2H2 zinc finger protein involved in glucose repression. The fingers of MIG1 are very similar to those present in the mammalian Egr finger proteins, which are induced during the early growth response, and also to the finger protein encoded by a human gene that is deleted in Wilms' tumour cells. MIG1 protein binds to two sites in the upstream region of SUC2, a yeast gene that is repressed by glucose. The MIG1 sites closely resemble the sequence recognized by the Egr proteins. Thus, finger proteins that are similar in both amino acid sequence and DNA specificity are involved in the response of yeast to glucose, and in the mammalian early growth response.

Yeast galactose permease is related to yeast and mammalian glucose transporters.

We have cloned and sequenced the GAL2 gene of Saccharomyces cerevisiae, which encodes galactose permease. The GAL2 protein is related to the yeast glucose transporter encoded by the SNF3 gene, and also to mammalian and bacterial sugar permeases. Like the other members of this protein family, GAL2 has twelve hydrophobic segments that are separated by loops of charged amino acids. A comparison of different members of this protein family shows that those parts of the polypeptides thought to be on the cytoplasmic side of the cell membrane, are more conserved than other parts of the molecules.