S24 S-III: Mismatch repair. replication fidelity and cancer

10 III c.21 Early and late clonal genomic alterations In cancercells

Bernard Dutrillaux. Institut Curie and CEA/DSV/DRR. France

Genetic predrsposmons to human cancers provide infonnative data. Mostcorrespond to gene mutations resulting m loss of function, transmitted asdommant traits, but with a recessiveexpressionat the cellular level.The firstlesion being known, it is particularly interesting to study additionalgenomicalterations occurring further in tumor cells to understand the pathwaysleading to a given tumor type.

As regards colorectal cancer. two major genetic predispositionsexist. Oneis famihal adenomatous polyposis, caused by the inactivation of the APCgene, located on Sq arm. Tumor progression is characterized by multiplechromosome rearrangements and losses Including that of the Sq arm, whichcorresponds to a major pathway, that we called the monosomic type (MT).By contrast, cancers developed in predisposed patients without polyposiscold (HNPCC) are characterized by either normal karyotypes, or minimalchromosome rearrangements and trisomies. a minor pathway called thetnsomic type (TT).

In other tumors caused by genetic predispositions, the few karyotypesestablished up to now can be classified as Mr, as is the case for almost allepithelial cancers. By contrast, for many leukemias and sarcomas, there isa specific translocation activating an oncogene or forming a fusion gene.which is regarded as a dominant alteration. In these malignancies, thekaryotypeeither does not acquire furtheralterations,or followsa TT pathway.Thus, the karyotypic pattern acquired during tumor progression appears todepend on the dominant or recessive expression of the initial oncogenicevent. Looking at chromosome alterations. TPS3 mutations. microsatelhteinstability, it appears that these alterations arc not associated at random intumor cells.

Another set of informationprovidedby genetic predispositionsis the delaynecessary between the first mutation and the tumor onset: it may vary frommonths in embryonal tumors to several decades. Considering all oncogenicmutations in non predisposedpatients, it is proposed that cancers initiated bythe dominant mutation of an oncogene are not age dependent. By contrast,those initiated by the recessive mutation of a tumor suppressor gene arestrongly age dependent, simply because there is the need for the alterationsof the two alleles of a same gene in a given cell. Thus, cancers developing inyoung patients arc caused by dominant. and those occurring with ageing arealmost all caused by recessive mutations. The long delay between exposureto mutagens and cancer development is thus a strong argument to considerthat recessive mutations are in cause.

Kcyword(s): chromosomes; genes; alterations; ageing; cancer

10 III c.31 Evolullon, cancer and mlsgulded fidelity

Donald MacPhee. School ofMicrobiology, La Trobe University. Bundoora.Victoria , Australia

The genome must not be too rigid, or else there will be insufficientscopefor the kind of lIexible change that is the sine qua non of evolution.Equally. the genome must not be too readily altered, or else there willbe no persistence of the new forms of organisation that come into beingthroughout the evolutionary process. Surprisingly,we know a lot less aboutthe various factors which are responsible for lIexibility of the genome (i.e,about those which act to ensure that spontaneous mutationsarise) than we doabout those whose task it is to reduce genomic variability by reducing errorrates in DNA synthesis, by ensunng that errors which escape detection inthe replication process are immediately corrected, or by correctly repairingthe damage to DNA which results from radiation or chemical exposures.[ have examined various ways in which these very diverse factors interactin the production and prevention of spontaneous mutations, and will pointout that certain of the well-known post-replicativemismatch repair systemswhich we usually think of exclusively as mutation avoidance systems mayalso generate mutations when circumstances dictate that they can no longerhandle mismatches(or mismatch-likestructures)correctly on everyoccasion.

Indeed, it may be precisely because cells are forced to shift from a strand­directed form of MMR to a randomly-templated fonn of MMR with thepassage of time that they are able to strike the balance between Variabilityand stabihty which enables them to cope with both their present environmentand whatever the future holds In store for them. I WIll also suggest that oneof the major factors predisposing us to certain forms of cancer i. a lifespanwlnch is sufficientlylong (45-50 years) for some of our stem cells to haveaccumulated 3. 4 or 5 spontaneous mutations in key genes. This may wellplace us at greatly-increased risk of developing tumourse especially solidtumours6when and If these multIply-mutated stem cells are stImulated todivide.

POSTERS

Ip III c.tl Geeetle predlsposltlon.lnducllon of chromosome aber_rations and development of cancer

William W. Au. Department ofPreventive Medicine and Community Health,The University of Texas Med ical Branch . Galveston, Tuas, USA

Wehave investigatedthe hypothesizedsequenceofevents that inheritance of"unfavorable" chemical metabolizinggenes can cause increased body burdenof reactive metabolites from exposure to environmental toxicants, inductionof chromosome damage and elevated risk for cancer. In our study of 54lung cancer patients and SO matched normal controls. the inheritance of"unfavorable" CYP2D6. CYP2EI and GSTTI genes is strongly associatedwith the cigarette smoking-related lung cancer (relative risk z 2.4. 2.4 and1.6. respectrvely), The relativensk IS increased further WIth the inheritance oftwo or more of the "unfavorable" genes. These genes predisposed moderateto light smokers«30 pack-year) for lung cancer with a relative risk of 3.46(95% CL = 0.46-402). For patients with the "unfavorable" genotypes, theirmean age at the time of diagnosis of lung cancer was 48.8 ± 11.1 (verses60.2 ± 11.3 for all lung cancer patients) and their smoking history was 45.0± 28.9 pack-years (verses 67.1 ± 31.6 pack-years). Using a subgroup ofthe patients (LCP). with 22 normal cigarette smokers (NCS) and with 13non-smokingcontrols (NC). we have used the tandem-probe lIuorescence inSitU hybndizanon assay to show that the aberration frequencies were 1.4 :i:0.1, 1.25 ± 0 I and 0.4 ± 0.1, respectively (p < 0.001). In addition, LCPmissing one or both GST genes (GSTMI and GSTT1) had significantlyhigher chromosome breaks than LCP with both genes present (p < 0.05) .From our study of 37 Egyptian bladder cancer patients and 27 matchedcontrols. inheritance of "unfavorable genes. particularly detoxifying genes,was significantly associated with bladder cancer outcome. The relativerisk was enhanced by the patient's history of schistosomal infection andcigarette smoking habits. In conclusion, our study indicates that predisposedindividuals have increased chromosome aberration leading to increased riskfor environmenUl cancer. In addition. inheritance of "unfavorable" genesalso inlluenced the clinical course of the disease, e.g, histological outcome.

Ip III c.21 DNA mismatch bindIng defects In human lunl: tumorcell lines

Shanbeh Zienolddiny", David Ryberg' . Adi F. Gazdar', Aage HaugenI .

1Department of Toxicology, National Institute ofOccupational Health. P.O.BOJ: 8149 Dep., N-0033 Oslo. Norway; 1Sou thwestern Medical Center atDallas. Unloersity of Texas, Dallas. Texas. USA

Defects in mismatch repair (MMR) genes have been involved in severaltypes of sporadic and hereditary cancers, In order to elucidate the role ofMMR in human lung carcinogenesiswe examined cell-free extracts of tumorand Iympboblastoid cell lines from lung cancer patients using the techniqueof delayedoligonucleotidemigration through polyacrylamide gels. We havedemoostratedthat some of the tumor lines are deficient in binding to specificDNA mismatches. The majority of these extracts were deficient in bindingto orr mispair whereas one showed deficiency in binding to all types ofmismatches. Survival experimentswith mismatchbinding deficient cell linesindicated a higher resistance 10 the alkylalmg agent MNNO as comparedto corresponding Iymphoblastoid cell lines and HeLa S3 cells which are