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Bloom's Syndrome and the Ataxia-Telangiectasia- Mutated Protein, ATM

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Bloom's Syndrome and the Ataxia-Telangiectasia- Mutated Protein, ATM Bloom’s syndrome Author: Doctor Mounira Amor-Guéret1 Creation date: February 2004 Scientific Editor: Professor Gilbert Tchernia 1Previous address: Interactions moléculaires et cancer, CNRS UMR 8126, Institut Gustave Roussy, 39 Rue Camille Desmoulins, 94805 VILLEJUIF cedex, France. Present address: Institut Curie - Section de Recherche, CNRS UMR 2027, Bâtiment 110, Centre Universitaire, F-91405 ORSAY cedex, France. [email protected] Abstract Key-words Disease name/synonyms Definition Differential diagnosis Etiology Clinical description Diagnostic methods Epidemiology Genetic counseling Antenatal diagnosis Management including treatment Unresolved questions References Abstract Bloom’s syndrome (BS) is a rare human autosomal recessive disorder belonging to a group of “chromosomal breakage syndromes”. BS is characterized by marked genetic instability, including a high level of sister chromatid exchanges, associated with a greatly increased predisposition to a wide range of cancers commonly affecting the general population. The constant clinical features of BS are proportionate pre- and postnatal growth retardation and cancer predisposition. Additional clinical features include dolichocephaly, facial sun-sensitive telangiectatic erythema, patchy areas of hyper- and hypopigmentation of the skin and moderate to severe immunodeficiency manifested by recurrent respiratory tract and gastrointestinal infections. A 10-fold increase in the rate of sister chromatid exchanges (SCEs) in BS cells compared to normal cells is the only objective criteria for BS diagnosis. Clinical diagnosis is confirmed cytogenetically by demonstrating characteristic chromosome instability. BS arises through mutations in both copies of the BLM gene which encodes a 3’-5’ DNA helicase, a member of the RecQ family. The function of the BLM protein remains unclear, but several lines of evidence support a major role in maintaining genomic stability during DNA replication, recombination and repair. BS frequency in the general population is unknown, probably because this disease is very rare. In Askenazic Jewish population, the frequency of BS is approximately 1 in 48 000. This is due to a founder effect, approximately 1% of the Ashkenasi Jewish population being heterozygous carriers for the blmAsh mutation. There is no curative treatment for BS. However, a physician should carefully follow BS patients in order to ensure early diagnosis of cancer. Key-words Bloom’s syndrome, cancer predisposition, genetic instability, sister chromatid exchanges, RecQ helicase. Amor-Guéret M. Bloom’s syndrome. Orphanet Encyclopedia. February 2004. http://www.orpha.net/data/patho/GB/uk-Bloomsyndrome.pdf 1 Disease name/synonyms on chromosome 15 at 15q26.1. Currently, there Bloom syndrome (Bloom’s syndrome) is no argument for a possible genetic Synonyms: Bloom-Torre-Mackacek syndrome, heterogeneity in BS. Nonsense or frameshift Congenital Telangiectatic Erythema. mutations leading to a premature termination codon as well as missense mutations have been Definition found in BLM gene from BS patients (Ellis et al., Bloom’s syndrome (BS) is a rare human 1995b; Foucault et al., 1997; Barakat et al., autosomal recessive disorder characterized by 2000). One particular BLM gene mutation marked genetic instability associated with a corresponding to a 6-bp deletion and a 7-bp greatly increased predisposition to a wide range insertion at nucleotide position 2281, referred as of cancers commonly affecting the general blmAsh mutation, is homozygous in nearly all BS population. The predominant and constant patients with Ashkenasi Jewish ancestry (Ellis et clinical feature of BS is proportionate pre- and al., 1995b) due to a founder effect (Ellis et al., postnatal growth retardation. Additional clinical 1994). features are dolichocephaly, narrow facies with BLM gene codes for the 1417 amino acids BLM nasal prominence and malar and mandibular protein with a predicted molecular mass of 159 hypoplasia, facial sun-sensitive telangiectatic kDa, and which belongs to the DExH box- erythema in the butterfly area, patchy areas of containing RecQ helicase subfamily (Ellis et al., hyper- and hypopigmentation of the skin (café- 1995b). Recombinant (Karow et al., 1997) and au-lait spots), and moderate to severe endogenous (Dutertre et al., 2002) BLM display immunodeficiency manifested by recurrent an ATP- and Mg2+ dependent 3’-5’-DNA respiratory tract and gastrointestinal infections helicase activity that separate the (German, 1993). BS was first described in 1954 complementary strands of DNA in a 3’-5’ as “congenital telangiectatic erythema direction. But BLM function is still unclear. BLM resembling lupus erythematosus in dwarfs” protein has been shown to accumulate in S and (Bloom, 1954). G2 phases of the cell cycle (Dutertre et al., 2000; Sanz et al., 2000; Bischof et al., 2001a) and to Differential diagnosis localize in two distinct nuclear structures, PML Bloom’s syndrome belongs to a group of nuclear bodies (also called ND10) (Ishov et al., “chromosomal breakage syndromes” which are 1999) and the nucleolus (Yankiwski et al., 2000). genetic disorders that are typically transmitted in The preferred substrates for BLM are G- an autosomal recessive mode. Cells from quadruplex DNA (Sun et al., 1998; Mohaghegh affected individuals are characterized by an et al., 2001), D-loops structures (van Brabant et increased frequency of breaks and interchanges al., 2000) and X-junctions (Karow et al., 2000). occurring either spontaneously or following BLM also promotes branch migration of RecA- exposure to various DNA-damaging agents. generated Hollyday junctions (Karow et al., Patients with these disorders show increased 2000). BLM interacts with several proteins predisposition to cancer. The commonly involved in the maintenance of genome integrity. acknowledged chromosomal breakage It participates in a super complex of BRCA1- syndromes are Fanconi anemia, ataxia associated proteins named BASC (BRCA1- telangiectasia, Xeroderma Pigmentosum and Associated genome Surveillance Complex) that Bloom’s syndrome. However, the hallmark of BS includes BRCA1, mutated in some familial breast cells is an approximately 10-fold increase in the cancers, ATM, NBS1 and MRE11 proteins, rate of sister chromatid exchanges (SCEs) defective in Ataxia Telangectasia (AT), Nijmegen compared to normal cells (Chaganti et al., 1974). syndrome and ataxia-telangiectasia-like The increased level of SCEs is the only objective disorder, respectively, MLH1, MSH2 and MSH6, criteria for BS diagnosis. It should be noted that involved in Human non-polyposis colorecal in about 20% of BS patients, normal levels of cancer (HNPCC syndrome), and several other SCE are observed in a subpopulation of B and T proteins known to be involved in replicational lymphocytes (Ellis et al., 1995a; Weksberg, and/or post-replicational repair process (Wang et 1995). These low-SCE revertant BS cells result al., 2000). BLM also participates in a complex from an intragenic crossing over between the containing RPA and topoisomerase IIIa, known paternal and maternal BLM alleles, generating a to interact independently with BLM (Brosh et al., wild type allele in compound heterozygote 2000; Wu et al., 2000; Hu et al., 2001), and five patients (Ellis et al., 1995a; Foucault et al., of the Fanconi anemia (FA) complementation 1997). group proteins (FANCA, FANCG, FANCC, FANCE and FANCF): this complex has been Etiology termed BRAFT (BLM, RPA, FA, Topoisomerase Bloom’s syndrome arises through mutations in IIIa) (Meetei AR et al., 2003). The other proteins both copies of the BLM gene, which is located Amor-Guéret M. Bloom’s syndrome. Orphanet Encyclopedia. February 2004. http://www.orpha.net/data/patho/GB/uk-Bloomsyndrome.pdf 2 known to interact physically and/or functionally Clinical description with BLM are: A program of surveillance referred to as Bloom’s • the tumor suppressor protein p53 Syndrome Registry has been established in (Garkavtsev et al., 2001; Wang et al., 1960 in which the follow-up of 168 BS patients 2001; Yang et al., 2002; Sengupta et al., (93 males, 75 females) has been reported until 2003) 1991 (German et al., 1977; 1979; 1984; German • the WRN protein, a RecQ helicase and Passarge, 1989). As reported in details by defective in the Werner syndrome, a German (1995) on the basis of the data collected premature aging disease (von Kobbe et in the Bloom’s Syndrome Registry, the two al., 2002), constant clinical features associated with BS are • MLH1, which participates in the growth retardation starting in utero and mismatch repair pathway (Langland et persisting throughout life with normal al., 2001; Pedrazzi et al., 2001), proportioning and accompanied by • RAD51, the key protein of the dolichocephaly, and predisposition to all types of homologous recombination (Wu et al., cancers. The mean adult height for men is 147,5 2001), cm (range 130 to 162), and for women is 138,6 • TRF2, a double-stranded telomeric DNA cm (range 122 to 151). German (1995) has also binding protein likely involved in the noted eleven additional clinical features that are regulation of telomeric length (Opresko not constant and that vary in severity among BS et al., 2002; Stavropoulos et al., 2002). patients: (1) a “bird-like” facies with a narrow The BLM protein has also been shown to be: face and prominent nose, and malar and • cleaved into 40-47 kDa N-terminal and mandibular hypoplasia; (2) sun-sensitive 110-120 kDa C-terminal major erythema affecting the butterfly area of the face fragments
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