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Werner’s syndrome (WS) and Cockayne syndrome (CS) are rare human autosomal recessive disorders classified as segmental progeroid disorders. WS is marked by premature onset of age-related phenotypic changes (such as cataract and greying of hair etc) and genome instability. Cells derived from CS patients are defective in DNA repair, and CS patients display severe neurological abnormalities and certain features of premature aging. The accelerated aging features observed in these two genetically distinct syndromes make them ideal model systems to understand some of the basic mechanism(s) for aging process. WS is caused by mutations in the gene (wrn) encoding the Werner syndrome protein (WRN), a member of the RecQ family of DNA helicases with helicase and exonudease domains. Biochemical characterization of WRN enzymatic activities, identification of WRN interacting proteins and the cellular localization studies all suggest WRN’s participation in multiple DNA metabolic pathways for maintaining genomic integrity. CS arises from mutations in the CSA and CSB genes. While CSA belongs to “WD” repeat containing protein family, CSB is a member of SNF2-like family with only a DNA stimulated ATPase activity. CSB is involved in different DNA transactions, notably transcription and DNA repair. Our recent understanding of the mechanism of action(s) of WRN and CSB proteins are expected to provide new insights into the aging process.

RecQ family DNA helicases are defined by amino acid sequence similarities to RecQ which has been known to act in homologous recombination and to suppress illegitimate recombination, particularly during the repair of DNA double strand breaks. Five family genes have been identified in humans, and three (, and ) have been identified as defective in the human genetic disorders; Bloom syndrome, Werner syndrome, and a subset of Rothmund-Thomson syndrome. Despite strong homology in the helicase domains, human family genes differ markedly outside these domains. Indeed, each syndrome presents different phenotypes. however, all are characterized by an increased predisposition to cancer, which is consistent with increased chromosomal aberrations and hypermutability observed in cultured cells. These data suggest that each of these helicases contributes to maintaining genomic stability and that an important function of these helicases appears to be the resolution of recombination intermediates.

Trichothiodystrophy (TTD) is a rare autosomal recessive multisystem disorder characterized by sulfur-deficient brittle hair, mental and physical retardation, ichthyosis, and, in many patients, cutaneous photosensitivity but no cancer incidence. All sun-sensitive TTD cases appear to be defective in nudeotide excision repair (NER) as a consequence of alterations in one of three genes, namely and . Intriguingly, in view of the very marked differences in the clinical phenotypes, defects in two of the genes altered in TTD ( and ) can also cause the cancer-prone disorder xeroderma pigmentosum (XP) or, in rare cases, the combined symptoms of XP and Cockayne syndrome (XP/CS). A breakthrough in understanding the perplexing features of this complex triad of hereditary disorders came from the discovery that the genes mutated in TTD are all related to TFIIH, a multiprotein complex involved in both transcription and NER. and encode two subunits of TFIIH, and also TTDA has been recendy identified as a new component of the TFIIH complex. The discovery of this unexpected link between DNA repair and transcription was crucial to rationalize the TTD pathological phenotype as well as the puzzling genotype-phenotype relationships related to defects in and . The last few years have witnessed significant progress in this field. It was shown that the mutations associated with the three disorders are located at different sites in the gene and that all the genetic and molecular alterations responsible for the NER-defective form of TTD cause a decrease by up to 70% in the cellular concentration of TFIIH. This implies that a limited availability of TFIIH interferes with NER but is compatible with life and therefore its impact on transcription must be selective only under certain conditions or in selective cellular compartments. We are also beginning to understand how different mutations in the gene result in different clinical entities: all the mutations found in patients, independent of the associated phenotype, are detrimental for the XPD helicase activity, thus explaining the NER defect, but only those responsible for the TTD phenotype affect basal transcription. Emerging evidence indicates that the involvement of TFIIH in transcription is multifaceted, ranging from transcription by RNA poly-merase I and II to regulation of gene expression. Although the multiple roles of TFIIH in transcription remain to be fully explored, these discoveries represent major advances in our understanding of fundamental cellular processes. They have important impacts on clinical medicine with implications for cancer prevention, aging, differentiation and development.

Since the cloning of the and genes less than 10 years ago, a great deal of effort has been expended in attempting to uncover the functions of the encoded proteins. BRCA1 and BRCA2 have now been linked to a wide variety of cellular functions through binding or colocalization with other proteins. In this chapter we focus on the best characterized of these potential functions, DNA repair.

Down’s syndrome (DS) is an autosomal recessive human disorder caused by an extra copy of chromosome 21. DS patients are characterized by dwarfism and mental retardation accompanied by an increased incidence of cancer development in various tissues and organs. DS patients also show signs of premature aging phenotypes. Many of the phenotypic features of DS patients are presumably due to the excess of genetic material of chromosome 21. Cells derived from DS patients show abnormal response to ionizing radiation-induced DNA damage, and this review deals with some aspects of the radiosensitive phenotype in DS.

Studies on cancer-prone and rare human genetic disorders often lead to significant advances in our understanding of the complex network of genome stability and DNA repair pathways that have evolved in the human genome to prevent the harmful effects of exposure to DNA damaging agents. One such disorder is Fanconi Anemia, an autosomal recessive disease characterized by an increased spontaneous and DNA cross-linkers induced chromosome instability, progressive pancytopenia and cancer susceptibility. At least eleven genes are involved in Fanconi anemia, including the breast cancer susceptibility gene BRCA2. Six of the Fanconi anemia proteins (FANCA, C, E, F, G and L) assemble in a complex that is required for FANCD2 activation by monoubiquitination in response to DNA damage or during S-phase progression. Active FANCD2 then colocalizes with the product of the breast cancer susceptibility gene BRCA1 in discrete nuclear . FANCD2 is also independently phosphorylated by ATM in response to ionising radiation and interacts with the MRE1 l/Rad50/NBSl complex, which is directly involved in homologous recombination DNA repair pathway and in cell cycle checkpoint response to DNA damage. Available data indicate that FANCD2 is involved in cell cycle regulation and DNA repair. Our current knowledge on the functional significance of FA pathway and more specifically FANCD2 and its interacting proteins in pathways of genomic surveillance and maintenance will be discussed in this chapter.

Ataxia telangiectasia (AT) is an autosomal recessive multisystem human disorder and patients are characterized by cerebellar ataxia, oculocutaneous telangiectasia, immunodeficiency, chromosomal instability and radio sensitivity with an increased predisposition to lymphoid cancer in childhood. The gene responsible for AT, ataxia telangiectasia mutated (ATM), has been cloned and its protein product has been biochemically characterized as a serine/threonine kinase belonging to the family of phosphatidylinositol (PI-3) like kinases. Subsequent biochemical studies by several laboratories have identified a number of DNA repair and cell cycle proteins that are phosphorylated by kinase in response to different DNA damaging agents. One intriguing question that comes to mind is whether the phenotypic features of AT stem from a DNA repair defect or a cell cycle defect or both. The scope of this review is focused on the potential functions of in both DNA repair and cell cycle checkpoint regulation and how deficiencies in these overlapping functions can lead to some of the phenotypic features of AT patients.

As highlighted above, the role of oxidative stress in the pathogenesis of AD is a burgeoning field. However, while much is known, much remains unknown as to the impact of therapeutic intervention in patients with disease. Translation of basic scientific findings into efficacious treatment strategies remains to be determined.

Loss of telomere homeostasis via chromosome-genomic instability might effectively promote tumour progression. Telomere function may have contrasting roles: inducing replicative senescence and promoting tumourigenesis and these roles may vary between cell types depending on the expression of telomerase enzyme, the level of mutations induced, and deficiency of related DNA repair pathways. Earlier studies in yeast and their recent extension to mammalian systems have convincingly indicated a role for DNA repair proteins in telomere maintenance. An alternative telomere maintenance mechanism has been identified in mouse embryonic stem cells lacking the telomerase RNA unit (mTERC) in which nontelomeric sequences adjacent to existing short stretches of telomere repeats are amplified. Our quest for identifying telomerase-independent or alternative mechanisms for telomere maintenance in mammalian cells has identified the involvement of potential DNA repair factors in such pathways. Studies by us and others have shown the association between the DNA repair factors and telomere function in mammalian cells. Mice deficient in a DNA-break sensing molecule, PARP-1 (poly [ADP]-ribopolymerase), have increased levels of chromosomal instability associated with extensive telomere shortening. Ku80 null cells showed telomere shortening associated with extensive chromosome end fusions whereas Ku80 cells exhibited an intermediate level of telomere shortening. This overview will focus mainly on the role of DNA repair/recombination and DNA damage signalling molecules such as PARP-1, DNA-PKcs, Ku70/80, XRCC4 and ATM, which we have been studying for quite sometime. As the maintenance of telomere function is crucial for genomic stability, our results are likely to provide new insights into the telomere regulatory mechanisms and their impact on chromosome instability, ageing and tumour formation.

It is clear that UV sensitivity and high cancer susceptibility can be explained by defective DNA repair. However, many mutations in DNA repair genes result in multisystem disorders. Mutations in the same gene can result in different clinical outcomes and severity depending on the site of a mutation and the gene dosage. This is further complicated by the possibility of pleiotropic effects caused by disturbances in other cellular processes. While not discussed here, attempts to recapitulate the human genotype-phenotype relationships using transgenic mice are providing valuable insights into these disorders. It may soon be possible to determine a patient’s clinical prognosis by analyzing the site of the mutation in the affected gene.