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Review Human Syndromes with Genomic Instability and Multiprotein Histol Histopathol (2003) 18: 225-243 Histology and http://www.hh.um.es Histopathology Cellular and Molecular Biology Review Human syndromes with genomic instability and multiprotein machines that repair DNA double-strand breaks* C. De la Torre1, J. Pincheira2 and J.F. López-Sáez3 1Biological Research Centre, CSIC, Madrid, Spain, 2Human Genetics Program, ICBM, School of Medicine, University of Chile, Santiago, Chile and 3Department of Biology, Autonomous University of Madrid, Madrid, Spain *This review is dedicated to Liles (Dr. Matilde H. Navarrete), a great woman and scientist that shared with us her enthusiasm with cell beauty and left us on May, 2002 after a long and brave fight against uncontrolled proliferation Summary. The present report deals with the functional complex and its activating ATM kinase are integrated in relationships among protein complexes which, when the BRCA1-associated surveillance complex (BASC) mutated, are responsible for four human syndromes that contains, among others, enzymes required for displaying cancer proneness, and whose cells are mismatch excision repair. In short, the proteins missing deficient in DNA double-strand break (DSB) repair. In in these syndromes have in common their BRCA1- some of them, the cells are also unable to activate the mediated assembly into multimeric machines proper checkpoint, while in the others an unduly responsible for the surveillance of DNA replication, override of the checkpoint-induced arrest occurs. As a DSB recombinational repair, and the removal of DNA consequence, all these patients display genome cross-links. instability. In ataxia-telangiectasia, the mutated protein (ATM) is a kinase, which acts as a transducer of DNA Key words: Ionizing radiation, Double-strand breaks, damage signalling. The defective protein in the ataxia- Multimeric repair machines, Ataxia telangiectasia, telangiectasia-like disorder is a DNase (the Mre11 Ataxia telangiectasia-like disorder, Nijmegen breakage nuclease) that in vivo produces single-strand tails at both syndrome, Fanconi anemia, Familial breast and ovary sides of DSBs. Mre11 is always present with the Rad50 cancer, BRCA1-associated surveillance complex ATPase in a protein machine: the nuclease complex. In mammals, this complex also contains nibrin, the protein mutated in the Nijmegen syndrome. Nibrin confers new Introduction abilities to the nuclease complex, and can also bind to BRCA1 (one of the two proteins mutated in familial XXI century biologists should deal with the breast cancer). BRCA1 has a central motif that binds properties of multimeric complexes that constitute with high affinity to cruciform DNA, a structure present protein machines which may display improved or even in places where the DNA loops are anchored to the different capabilities in relation to those shown by any of chromosomal axis or scaffold. The BRCA1•cruciform their components. Changes in a single one of the DNA complex should be released to allow the nuclease components alters the activity of the complex as a complex to work in DNA recombinational repair of whole. Furthermore, some of these complexes are part of DSBs. BRCA1 also acts as a scaffold for the assembly of specific signal transduction pathways that are conserved ATPases such as Rad51, responsible for the somatic by evolution as modules containing some of their homologous recombination. Loss of the BRCA1 gene upstream regulators and downstream effectors (Hartwell prevents cell survival after exposure to cross-linkers. et al., 1999). The BRCA1-RING domain is an E3-ubiquitin ligase. It There are several cancer-prone, autosomal recessive can mono-ubiquitinate the FANCD2 protein, mutated in syndromes, that are hypersensitive to clastogenic agents. one of the Fanconi anemia complementation groups, to The phenotype of their cells is characterized by their regulate it. Finally, during DNA replication, the nuclease inability to remove double-strand breaks and by the presence of genome instability. These syndromes are the Offprint requests to: Prof. Dr. C. De la Torre, CIB-CSIC, Velázquez, 144, following: ataxia-telangiectasia (A-T); the ataxia- E-28006 Madrid, Spain. Fax: +34-91-562 75 18. e-mail: delatorrec@ telangiectasia-like disorder (ATLD); the Nijmegen cib.csic.es breakage syndrome; and the Fanconi anemia (FA). The 226 Human DNA repair machines Table 1. Some syndromes with genomic instability and cancer proneness. SYNDROME SHORTHAND MISSING PROTEIN PROTEIN FUNCTION SENSITIVITY Ataxia-telangiectasia AT ATM kinase IR AT-like disorder ATLD Mre11 nuclease I R Nijmegen Breakage Syndrome NBS Nibrin, NBS1 ? I R Fanconi anemia FA FANCD2 ? cross-linkers + IR IR: ionizing radiation Table 2. Clinical and cellular phenotypes in patients with ataxia- diagnostic features develop gradually with age. Ataxia- telangiectasia (AT), the AT-like disorder (ATLD), the Nijmegen breakage telangiectasia (AT) diagnosis includes ataxia by syndrome (NBS) and Fanconi anemia (FA). progressive loss of Purkinje cells in the cerebellum, with AT ATLD NBS FA an onset from 1 to 3 years of age; ocular apraxia after 2 years of age while telangiectasia may appear many years Clinical phenotype after the ataxia. The patients show elevated α- ataxia + + - - fetoprotein, immunodeficiency, high sensitivity to oculocutaneous telangiectasia + ± - - radiation, and predisposition to cancer, usually elevated serum alpha-protein + - - - radiation hypersensitivity + + + + lymphoma. cancer proneness + ± + + The different clinical features of the four syndromes immunodeficiency + - + ? are displayed in the upper part of Table 2. Proper microcephaly - - + + diagnosis is relevant, as the high radiosensitivity of mental retardation - - ± - patients of any of these syndromes discards the use of premature aging + + ? + radiation therapy for them in case they develop a congenital malformations ± - + + hyperpigmentation spots ± + + + neoplasia. stunted growth ± + + + In spite of the gradual development of symptoms it gonadal dysgenesis + + ± + is still possible to distinguish these syndromes by clinical traits, with the only exception of FA, where Cellular phenotype clinical phenotypes can be different even in monozygotic chromosomal aberrations* + + + + hypersensitivity to radiation + + + + twins. In these patients with disturbances in oxygen hypersensitivity to cross-linkers - - - + metabolism and bone marrow aplasia, the exquisite defective G2 checkpoint + ± ± - sensitivity of their cells to cross-linkers is the best defective intra-S checkpoints** + + + ? diagnostic marker (Auerbach et al., 1998). defective G1 and spindle checkpoints + + + ? A noticeable feature of the phenotype of these *: spontaneously occurring in peripheral lymphocytes. Data on AT, NBS syndromes is that they are either unable to activate cycle and FA from Pincheira et al. (2001; 1998 and 1988, respectively). checkpoint (AT) after being challenged their DNA, or **: formerly known as radio-resistant DNA synthesis. +: present; -: they unduly override the checkpoint induced arrest, absent; ±: contradictory data; ?: unknown enduring a process known as checkpoint “adaptation” (Weinert and Hartwell, 1988), commented on later (ATLD, NBS and FA) (see Table 2). Because of these facts, the cells of all these patients defective in double- cells of this last syndrome are selectively hypersensitive strand break (DSB) repair display also genomic to DNA cross-linkers (Table 1). instability, a landmark of cancer cells (Petrini, 2000). The incidence of these four syndromes in the population is quite different. Thus, AT is present in Molecular motifs and domains in the proteins about 1 in 40,000 live births (Gatti, 1998), while there mutated in the syndromes with inability to repair are less than 100 individuals registered in the Nijmegen double-strand breaks syndrome record, most of them of Slavic origin. The frequency of the Ataxia-telangiectasia-like disorder In the present review, the relationships among the remains to be elucidated, as the two first families with proteins mutated in these selected four syndromes are ATLD were only distinguished from true AT patients in considered. The number of aminoacids of each protein is December 1999 (Stewart et al., 1999). The incidence of shown at the right part of the bar representing it (Fig. 1). Fanconi anemia is the highest of all the four syndromes. This figure also shows the main motifs and the known Actually, up to 0.5 % of the world population is probably interacting domains of each of these proteins. carrying one of the mutated FA genes (Auerbach et al., ATM (Ataxia-Telangiectasia Mutated) is a member 1998). of the Phosphatydil Inositol 3 kinase (PI3K) family of The phenotypes of the patients affected by these four lipid kinases (Savitsky et al., 1995). They have lost their syndromes are pleitropic in nature and many of their capability to phosphorylate lipid substrates. Instead, they 227 Human DNA repair machines are S/T-Q directed kinases, i.e. they phosphorylate serine dependent phosphorylation of Mre11 has not been yet or threonine residues, followed by glutamine at the +1 reported. position. The three main members of this family are the As the Mre11 nuclease is always interacting with the ATM kinase, the ATR (ATM and Rad3-related) kinase, Rad50 in the nuclease complex, both in mammals and and the DNA-PKcs (DNA-damage-dependent Protein yeasts (Hofner et al., 2001), Rad50 has also been Kinase catalytic subunit). Their targets are proteins depicted in Fig. 1. The Rad50 ATPase is a member of mostly involved in the negative
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