Hyper-Recornbination and Bloom's Syndrome: Microbes Again Provide Clues About Cancer Rodney Rothstein and Serge Gangloff
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Downloaded from genome.cshlp.org on October 6, 2021 - Published by Cold Spring Harbor Laboratory Press REVIEW Hyper-recornbination and Bloom's Syndrome: Microbes Again Provide Clues about Cancer Rodney Rothstein and Serge Gangloff Department of Genetics and Development, Columbia University College of Physicians and Surgeons, New York, New York 10032 Once again, a gene family first identified in mi- microbes and man that specifically increase re- croorganisms has been linked to human cancer combination. predisposition. The first in a series of intriguing links between microbial genetics and tumorigen- esis in man emerged when the genes responsible Bloom's Syndrome for xeroderma pigmentosum were identified as Bloom's syndrome, originally identified as a dis- homologs of yeast DNA repair genes (for review, tinct erythematous facial rash (Bloom 1954), is see Cleaver 1994; Tanaka and Wood 1994). An- inherited as an autosomal recessive trait. In addi- other link surfaced 2 years ago with the identifi- tion, individuals affected with Bloom's syndrome cation of human colorectal cancer susceptibility are characterized by growth retardation, abnor- and mutations in a homolog of a bacterial and mal spermatogenesis, and an elevated risk of de- yeast mismatch repair gene (Fishel et al. 1993; veloping cancer (German 1993). Besides these Parsons et al. 1993). Earlier this year, the gene clinical manifestations, cells from individuals responsible for ataxia-telangiectasia was identi- with Bloom's syndrome exhibit elevated levels of fied as a member of a kinase gene family that may chromosome breaks and gaps, somatic cross- play a role in DNA damage recognition (Green- overs, and sister chromatid exchange (German well et al. 1995; Hari et al. 1995; Morrow et al. 1995; Savitsky et al. 1995). The latest link is the 1993). Genetic experiments with Bloom's syndrome identification of the Bloom's syndrome gene, cell lines showed that transfer of chromosome 15 BLM (Ellis et a]. 1995a). BLM encodes a protein could complement increased sister chromatid ex- homologous to the RecQ/Sgsl DNA helicase sub- change. Further refinements in the genetic map family first identified in bacteria and yeast localized the defective gene to a region near (Umezu et al. 1990; Gangloff et al. 1994). Muta- 15q26.1 (Ellis et al. 1994). An important break- tions in BLM and in the yeast homolog SGS1 con- through came when German's group found that fer the common phenotype of hyper-recom- they could exploit an early observation. They no- bination to cells. ticed that although most cells of individuals with Hyper-recombination collectively refers to Bloom's syndrome exhibit high sister chromatid any cellular state in which the frequency of ge- exchange, there exists a minor population of netic exchange is elevated. The exchanges can lymphocytes with low sister chromatid ex- occur, for example, between sister chromatids, al- change. Cells with low sister chromatid exchange lelic genes, or repetitive DNA sequences scattered were predominantly found in individuals whose throughout the genome. The frequency of re- parental alleles were not identical by descent and combination can be measured genetically be- therefore contained different mutations in BLM. tween marked intervals and also cytologically as An intragenic crossover between the paternal and deletions, inversions, translocations, sister chro- maternal alleles generated a wild-type gene on matid exchanges, and somatic crossovers. In this one chromosome and a gene containing both pa- review we limit our discussion to mutations in rental mutations on the homologous chromo- some. After segregation, half of the recombined cell lines became homozygous for polymorphic ~Corresponding author. markers distal to the BLM locus, whereas they E-MAIL [email protected]; FAX (212) 923- 2090. remained heterozygous for polymorphic markers 5:421-426 @1995 by Cold Spring Harbor Laboratory Press ISSN 1054-9803/95 $5.00 GENOME RESEARCH ~ 421 Downloaded from genome.cshlp.org on October 6, 2021 - Published by Cold Spring Harbor Laboratory Press ROTHSTEIN AND GANGLOFF proximal to the locus (Ellis et al. 1995b). Thus, that its protein sequence shared significant ho- somatic crossover point mapping helped demar- mology with bacterial RecQ. In addition, another cate the boundaries of the gene and narrow down RecQ homolog has been identified as a result of the chromosomal region. It is interesting to note the Caenorhabditis elegans sequencing project that the hyper-recombinogenic nature of the bhn (GenBank no. U00052, K02F3.1). To date, no dis- mutation probably led to the high incidence of ease mapping to the human RECQL gene ho- this somatic event. Then, using direct cDNA se- molog has been reported. For C. elegans, the phe- lection, a clone was isolated that mapped to the notype of mutants is also unknown. In both of candidate region. Translation of the DNA se- these cases, the homology shared with RecQ is quence of this clone revealed motifs homologous restricted to the seven conserved helicase motifs. to the RecQ/Sgsl helicase subfamily (Ellis et al. BLM is also homologous to Sgsl, a yeast pro- 1995a). tein (Gangloff et al. 1994). On the basis of a com- parison of their primary structure, four lines of BLM and Sgsl May Be Homologs evidence suggest that BLM may be the human homolog of Sgsl (see Fig. 1). First, unlike what is RecQ, the founding member of a subfamily of observed for RecQ and RECQL, the sequence sim- DNA helicases, is a DNA-dependent ATPase that ilarity between BLM and Sgsl extends beyond the possesses a 3' --4 5' DNA translocation activity. It helicase domains. Second, the size of the two pro- was first identified as a protein involved in the teins are nearly identical, more than twice the RecF recombination pathway in Escherichia coli size of the other RecQ family members. Third, the (Umezu et al. 1990). The absence of RecQ in E. conserved helicase domain is in the same posi- coli does not have any discernible phenotype un- tion in the two proteins, situated -700 amino less a major recombination pathway (RecBCD, acids from the amino terminus. Lastly, the SbcBC) is mutated (Clark 1991). In this situation, amino-terminal domain of BLM contains two loss of RecQ function leads to UV sensitivity and clusters of charged amino acid residues parallel- to a decrease in recombination. Recently, the ing the Sgsl organization. In addition to the RecQ subfamily of helicases has grown. A human structural similarities of BLM and Sgsl, blm and helicase, RECQL, was identified biochemically sgsl mutant cells both exhibit increased levels and was shown to possess the same in vitro prop- of mitotic recombination. In yeast cells, hyper- erties as bacterial RecQ (Puranam and Blackshear recombination is manifested by an increased 1994). Cloning and sequence analysis revealed frequency of marker loss. In Bloom's cells, it is revealed by increased sis- ter chromatid exchange RecQ and symmetrical quadr- iradials, which probably RECQL represent exchange fig- ures between homolo- Sgsl gous chromosomes in metaphase (German BLM 1964). On the other hand, recQ mutants ex- hibit decreased levels of Helicase domain recombination. Acidic amino acid cluster Extended homology Sgsl May Provide Clues to BLM Function Protein backbone with no homologies 200 AA First identified as an ex- Figure 1 Alignment representing the primary amino acid sequence of four mem- tragenic suppressor of the bers of the RecQ/Sgsl DNA helicase subfamily. The four proteins share strong sequence similarities within the region corresponding to the helicase domain. BLM slow-growth phenotype and Sgsl exhibit regions of homology directly surrounding the helicase domain, of topo Ill-deficient mu- which are not found within the other members of the subfamily. BLM and Sgsl also tants, the SGS1 locus was contain two clusters of acidic residues in their amino-terminal moieties that, in shown to interact physi- Sgsl, have been proposed to mediate transcriptional activation. cally with both topo II 422 ~ GENOME RESEARCH Downloaded from genome.cshlp.org on October 6, 2021 - Published by Cold Spring Harbor Laboratory Press MICROBIAL CLUES ABOUT CANCER PREDISPOSITION and topo III (Gangloff et al. 1994; Watt et al. crossover can become homozygous, resulting in 1995). As both of these DNA topoisomerases are loss of heterozygosity. potentially involved in the resolution of replica- tion intermediates (Wang 1991; Fig. 2), it is Hyper-recombinafion and Cancer tempting to speculate that Sgsl differentially modulates their activity. For example, in the ab- A few human disorders have been associated with sence of Sgsl, the failure to resolve the inter- increased levels of chromosomal rearrangements twined strands generated at the point where rep- (see Table 1). A common facet of these "hyper- lication forks merge would give rise to recombi- recombination" syndromes is their high cancer nogenic lesions (e.g., breaks) in a newly risk. Table 1 also lists selected genes from bacteria replicated chromatid. Repair of such lesions, us- and yeast that show increased recombination ing information from the sister chromatid, could (Hartwell and Smith 1985; Barnes et al. 1992; lead to a sister chromatid exchange event (Fig. Gangloff et al. 1994; Kato and Ogawa 1994; Mor- 3A). Alternatively, a cell could repair the dam- row et al. 1995; Savitsky et al. 1995; Smith and aged chromatid using information on the homol- Rothstein 1995). In bacteria, many of these genes ogous chromosome resulting in a mitotic ex- were identified using a simple assay in which re- change (Fig. 3B). In the mitotic division follow- combination between direct repeats leads to a se- ing such an exchange, DNA distal to the lectable phenotype (Konrad 1977). The fre- A topo II topo llI - Sgsl l Figure 2 Resolution of merging replicons. Two pathways have been proposed for the resolution of intertwined strands found at the site of merging replication forks (Wang 1991 ). As the two replicons approach one another, topological constraints block the merging forks leaving a small stretch of unreplicated duplex.