WRN's tenth anniversary

Werner syndrome: Clinical features, pathogenesis and potential therapeutic interventions

Werner syndrome (WS) is a prototypical segmental progeroid syndrome characterized by multiple features consistent with accelerated aging. It is caused by null mutations of the WRN gene, which encodes a member of the RECQ family of DNA helicases. A unique feature of the WRN helicase is the presence of an exonuclease domain in its N-terminal region. Biochemical and cell biological studies during the past decade have demonstrated involvements of the WRN protein in multiple DNA transactions, including DNA repair, recombination, replication and transcription. A role of the WRN protein in telomere maintenance could explain many of the WS phenotypes. Recent discoveries of new progeroid loci found in atypical Werner cases continue to support the concept of genomic instability as a major mechanism of biological aging. Based on these biological insights, efforts are underway to develop therapeutic interventions for WS and related progeroid syndromes.

Search and insights into novel genetic alterations leading to classical and atypical Werner syndrome

Segmental progeroid syndromes are a group of disorders with multiple features resembling accelerated aging. Adult-onset Werner syndrome (WS) and childhood-onset Hutchinson-Gilford progeria syndrome are the best known examples. The discovery of genes responsible for such syndromes has facilitated our understanding of the basic mechanisms of aging as well as the pathogenesis of other common, age-related diseases. Our International Registry of Werner Syndrome accesses progeroid pedigrees from all over the world, including those for whom we have ruled out a mutation at the WRN locus. Cases without WRN mutations are operationally categorized as 'atypical WS' (AWS). In 2003, we identified LMNA mutations among a subset of AWS cases using a candidate gene approach. As of 2013, the Registry has 142 WS patients with WRN mutations, 11 AWS patients with LMNA mutations, and 49 AWS patients that have neither WRN nor LMNA mutations. Efforts are underway to identify the responsible genes for AWS with unknown genetic causes. While WS and AWS are rare disorders, the causative genes have been shown to have much wider implications for cancer, cardiovascular disease and the biology of aging. Remarkably, centenarian studies revealed WRN and LMNA polymorphic variants among those who have escaped various geriatric disorders.

[Werner syndrome. A prototypical form of segmental progeria.]

Werner syndrome is a segmental progeroid disorder with onset in adolescence or early adulthood. Typical symptoms contributing to patients' prematurely aged appearance include postpubertal development of short stature, cataracts, premature greying/thinning of scalp hair, scleroderma-like skin changes and regional atrophy of subcutaneous fat tissue. In addition, an increased rate and early onset of typical age-related diseases such as type 2 diabetes mellitus, osteoporosis, atherosclerosis, and various malignancies is observed. Werner syndrome is autosomal recessively inherited and caused by mutations in the Werner gene (WRN). To date, more than 70 WRN mutations have been identified. These are spread over the entire gene and typically represent loss of function mutations. WRN encodes a RecQ type helicase involved in DNA repair and the maintenance of DNA integrity, which is reflected by an increased genetic instability in patient cells. Despite the relative rarity of Werner syndrome, its analysis provides important general insights into the roles of DNA stability and integrity for the ageing process and the development of age-associated diseases.

The clinical characteristics of Werner syndrome: molecular and biochemical diagnosis

Werner syndrome (WS) is an adult onset segmental progeroid syndrome caused by mutations in the WRN gene. The WRN gene encodes a 180 kDa nuclear protein that possesses helicase and exonuclease activities. The absence of WRN protein leads to abnormalities in various DNA metabolic pathways such as DNA repair, replication and telomere maintenance. Individuals with WS generally develop normally until the third decade of life, when premature aging phenotypes and a series of age-related disorders begin to manifest. In Japan, where a founder effect has been described, the frequency of Werner heterozygotes appears to be as high as 1/180 in the general population. Due to the relatively non-specific nature of the symptoms and the lack of awareness of the condition, this disease may be under-diagnosed in other parts of the world. Genetic counseling of WS patients follows the path of other autosomal recessive disorders, with special attention needed for cancer surveillance in relatives. Molecular diagnosis of WS is made by nucleotide sequencing and, in some cases, protein analysis. It is also of potential interest to measure WRN activities in WS patients. More than 50 different disease-causing mutations in the WRN gene have been identified in WS patients from all over the world. All but one of these cases has mutations that result in the premature termination of the protein. Here we describe the clinical, molecular and biochemical characteristics of WS for use by medical professionals in a health care setting. Additional information is available through the International Registry of WS (http://www.wernersyndrome.org).

Biochemical characterization of the WRN-1 RecQ helicase of Caenorhabditis elegans

The highly conserved RecQ helicases are essential for the maintenance of genomic stability. Werner syndrome protein, WRN, is one of five human RecQ helicase homologues, and a deficiency of the protein causes a hereditary premature aging disorder that is characterized by genomic instability. A WRN orthologue, wrn-1 lacking the exonuclease domain, has been identified in the nematode Caenorhabditis elegans. wrn-1(RNAi) in C. elegans has a shortened life span, increased sensitivity to DNA damage, and accelerated aging phenotypes. However, little is known about its enzymatic activity. We purified the recombinant C. elegans WRN-1 protein (CeWRN-1) and then investigated its substrate specificity in vitro to improve our understanding of its function in vivo. We found that CeWRN-1 is an ATP-dependent 3'-5' helicase capable of unwinding a variety of DNA structures such as forked duplexes, Holliday junctions, bubble substrates, D-loops, and flap duplexes, and 3'-tailed duplex substrates. Distinctly, CeWRN-1 is able to unwind a long forked duplex compared to human WRN. Furthermore, CeWRN-1 helicase activity on a long DNA duplex is stimulated by C. elegans replication protein A (CeRPA) that is shown to interact with CeWRN-1 by a dot blot. The ability of CeWRN-1 to unwind these DNA structures may improve the access for DNA repair and replication proteins that are important for preventing the accumulation of abnormal structures, contributing to genomic stability.

Human premature aging, DNA repair and RecQ helicases

Genomic instability leads to mutations, cellular dysfunction and aberrant phenotypes at the tissue and organism levels. A number of mechanisms have evolved to cope with endogenous or exogenous stress to prevent chromosomal instability and maintain cellular homeostasis. DNA helicases play important roles in the DNA damage response. The RecQ family of DNA helicases is of particular interest since several human RecQ helicases are defective in diseases associated with premature aging and cancer. In this review, we will provide an update on our understanding of the specific roles of human RecQ helicases in the maintenance of genomic stability through their catalytic activities and protein interactions in various pathways of cellular nucleic acid metabolism with an emphasis on DNA replication and repair. We will also discuss the clinical features of the premature aging disorders associated with RecQ helicase deficiencies and how they relate to the molecular defects.