NATURE MEDICINE • VOLUME 4 • NUMBER 10 • OCTOBER 1998 1119

NEWS & VIEWS

MOST IF NOT all cancers result from theinteraction between genes and the

environment1. Acting in concert withindividual susceptibility, environmentalfactors are thought to explain the major-ity of sporadic cancers. Leading the groupis tobacco smoke: no other dietary con-stituent, pollutant or lifestyle factor hasbeen shown to produce such a dramaticeffect on cancer development. Despitecessation and prevention strategies, smok-ing remains a major medical, social andethical problem. Furthermore, scientistscontinue to face the challenge of docu-menting the interactions between tobaccocarcinogens and genes in order to rapidlytranslate this knowledge into publichealth interventions.

Molecular genetic studies use select bio-markers that are powerful indicators ofcancer risk, such as carcinogen–DNAadducts and characteristic mutationalspectra in ‘cancer’ genes and in reportergenes. In a paper in this issue of NatureMedicine, Finette and colleagues2 examinethe spectrum of mutations in a reporter

gene (hypoxanthine-guanine phosphori-bosyltransferase, HPRT) in cord blood Tlymphoctyes of neonates born to mothersexposed to passive cigarette smoke. In thefirst demonstration to our knowledge ofsmoke-induced mutations in utero, theauthors report that T cells from newbornswith mothers exposed to passive smokehad a characteristic mutational spectrumthat differed from that of newborns unex-posed to cigarette smoke. They convinc-ingly show that smoke-exposed neonateshave a much higher frequency of agenomic deletion event that is commonlyfound in leukemias and lymphomas ofearly childhood.

That smoking is the causative factor inthe acquisition of genetic lesions can beproven by the demonstration of specificgenetic damage not only in transformedcells of cancer patients but also in normaltarget cells from asymptomatic individu-

als. In such individuals, the direct causaleffect of the interaction between carcino-genic tobacco constituents and genes canbe more convincingly demonstratedbecause these genetic lesions are earlyevents in cancer development. Differentmutational spectra in the p53 gene havebeen found in lung cancers from smokers(where base-pair transversions predomi-nate) compared with lung cancers fromnon-smokers (where base-pair transitionsare more frequent)3. The coincident loca-tion of these p53 mutational 'hotspots'and of DNA adducts induced by thetobacco carcinogen benzo[α]pyrene diolepoxide provides a direct link between asmoke carcinogen and cancer mutations4.In addition, an increased rate of p53mutations has been observed in head andneck squamous cell carcinomas frompatients with a history of tobacco andalcohol use compared with that in can-cers from unexposed subjects5. Finally,loss of heterozygosity and exon deletionswithin the FHIT (fragile histidine triad)gene6—a tumor suppressor gene that is

When smoke gets in your genesEvidence accumulates that transplacental exposure to cigarette smoke causes

genetic damage in utero (pages 1144–1151).

GABRIELLA SOZZI& MARCO A. PIEROTTI

Zanjani (U. Nevada), Dick (U. Toronto)and Nakauchi (U. Tsukuba) showed thatcells negative for CD34 (CD34–/low) fromadult human bone marrow, cord blood,and mouse bone marrow, respectively, allhad the capacity for self renewal anddifferentiation, hallmarks of HSC func-tion (see Nature Med. 4, 1009–1010,1038–1045, 1998). Although thousands ofclinical transplants using CD34-positivestem cells have achieved engraftment, theCD34–/low HSC may be even more primitivethan HSC expressing the CD34 marker.On the other hand, Adamson (New YorkBlood Center), reporting on 493 human

cord bloodclinical trans-plants (both

matched and unmatched), provided sta-tistical data indicating that the mostimportant prognostic factor for clinicaloutcome was the number of CD34-posi-tive cells in the graft. Finally, both Reisner(Weizmann Institute) and Handgretinger(U. Tübingen) presented work in progressthat seeks to establish whether grafts con-taining very high numbers of CD34-posi-tive HSC can render host T lymphocytestolerant and overcome histocompatibilitybarriers in haplo-identical transplants.Handgretinger's clinical data are particu-larly interesting because engraftment wasachieved in 22 of 23 patients.

What this workshop makes clear is thatthese many diverse approaches will helpanswer questions about the biology ofHSC including their origin, self renewal,phenotypic expression patterns, and dif-ferentiation. We hope that theseadvances will translate into betterclinical outcomes for HSC transplantpatients.

1Johns Hopkins University School of MedicineDepartment of OncologyBaltimore, Maryland 21287-8967, USA2Medizinische Klinik und PoliklinikUniversitätsklinikum TübingenTübingen, Germany3Hematopoiesis Section, NHGRI, NIHBethesda, Maryland 20892-4442, USA

Fig. 2 Pluripotent hematopoietic stem cells (HSC) are idealtargets for retrovirus-mediated gene transfer. They have thecapacity for self renewal and can differentiate into all blood celllineages. When a retrovirus carrying a therapeutic gene binds toits specific receptor on the surface of a HSC, the viral genome en-ters the cytoplasm and is reverse transcribed into double-strandedDNA. In a dividing HSC the viral genome can be integrated intothe DNA of the HSC. All progenitor cells and mature cells derivedfrom transduced HSC will carry the transferred gene.

1120 NATURE MEDICINE • VOLUME 4 • NUMBER 10 • OCTOBER 1998

NEWS & VIEWS

altered in cancer cells of many tissues,including lung, and overlaps the mostcommon fragile site of the humangenome, FRA3B—were associated withsmoking habits in lung cancer patients7,8.

Insight into the possible relationshipbetween environmental mutagen expo-sure and genetic damage in normal,somatic cells of healthy people stemsfrom earlier studies indicating anincreased incidence of fragile sites (pre-dominantly FRA3B) at chromosomalbreakpoints in peripheral blood lympho-cytes9. Moreover, detailed analyses ofmolecular changes in normal bronchialcells of smokers showed extensive allelicdeletions, which decrease after smokingcessation and are absent in the respira-tory epithelia of lifetime non-smokers10.But all of these studies have been per-formed on cancerous and non-cancerouscells from adults, who have accumulatedbackground mutations from a lifetime ofexposure to environmental mutagens. In

contrast, newborns and youngchildren have far fewer back-ground mutations and so theircells might provide a suitablesystem for studying the initialstages of smoking-induced car-cinogenesis.

This is the approach that hasbeen taken by Finette and co-workers2. Of particular interest istheir finding that even thoughthe frequency of HPRT mutationsin T cells is not significantlyincreased in smoke-exposed com-pared with unexposed neonates,the spectrum of mutations is dif-ferent in exposed compared withunexposed newborns. Of crucialinterest is the observation that aparticular type of mutation—illegitimate V(D)J-recombinase-mediated genomic deletions—appears at an increased frequencyin the smoking-exposed new-borns. Similar deletions havebeen observed in hematologicalmalignancies and include thewell-characterized t(4;11) chro-mosomal translocation in child-hood acute lymphoblasticleukemia. Acquired geneticchanges in the HPRT locus mayaugur similar changes in criticalregions of the genome that affectspecific disease genes.

Although epidemiologic stud-ies have suggested that maternaland paternal passive smoke

exposure increases cancer risk in chil-dren, the Finette study is the first demon-stration of smoking-induced geneticdamage in utero. It is noteworthy that arecent study by Hecht and colleagues(presented in August at the AmericanChemical Society meeting) found thaturine from 19 of 31 neonates born tomothers that smoked during pregnancycontained metabolites of NNK (4-methylnitrosamino-1-(-3-pyridyl)-1-butanone), a carcinogen found only intobacco smoke. Metabolites were notfound in urine samples from any infantsborn to non-smoking mothers.

Given the small sample size of theFinette study, additional investigationsof the transplacental effects of passivesmoke in newborns are required. Thesestudies should include analysis oftransplacental exposure of preterminfants and newborns to ‘active’ as wellas passive cigarette smoke. (In adults asimilar spectrum of p53 mutations in

lung tumors from passive and activesmokers has been found12.) In addition,measurement of the ‘rate’ of tobaccoconsumption in actively smoking moth-ers as well as a more precise quantitationof passive smoke exposure in non-smok-ing mothers should be obtained. V(D)J-recombinase-mediated HPRT deletionsalso occur spontaneously, thus the com-parison of these changes in exposed andin unexposed groups is critical.

This study provides incontrovertiblegenetic evidence of the devastatingeffects of tobacco smoke particularlyamong the young, who suffer a greaterrisk from environmental toxicants, suchas tobacco smoke, not only because oftheir smaller size but also because of theirphysiological immaturity. The time hascome to proclaim an end to the exposureof preterm infants, newborns and chil-dren of all ages to tobacco smoke.

1. Perera, F.P. Environment and cancer: Who aresusceptible? Science 278, 1068–1073 (1997).

2. Finette, B.A., O’Neill, J.P., Vacek, P.M. &Albertini, R.J. Gene mutations with characteristicdeletions in cord blood T lymphocytes associatedwith passive maternal exposure to tobaccosmoke. Nature Med. 4, 1144–1151 (1998).

3. Hollstein, M., Sidransky, D., Vogelstein, B. &Harris,C.C. p53 mutations in human cancers.Science 253, 49–53 (1991).

4. Denissenko, M.F., Pao, A., Tang, M. & Pfeifer,G.P. Preferential formation ofbenzo[alpha]pyrene adducts at lung cancer mu-tational hotspots in p53. Science 274, 430(1996).

5. Brennan, J.A. et al. Association between cigarettesmoking and mutation of the p53 gene in squa-mous cell carcinoma of the head and neck. N.Engl. J. Med. 332, 712–717 (1995).

6. Ohta, M. et al. The FHIT gene, spanning thechromosome 3p14.2 fragile site and renal carci-noma-associated t(3;8) breakpoint, is abnormalin digestive tract cancer. Cell 84, 587–597(1996).

7. Sozzi, G. et al. Association between cigarettesmoking and FHIT gene alterations in lung can-cer. Cancer Res. 57, 2121–2123 (1997).

8. Nelson, H.H. et al. Chromosome 3p14 alterationsin lung cancer: Evidence that FHIT exon deletionis a target of tobacco carcinogens and asbestos.Cancer Res. 58, 1804–1807 (1998).

9. Yunis, J.J., Soreng, A.L. & Bowe, A.E. Fragile sitesare targets of diverse mutagens and carcinogens.Oncogene 1, 59–69 (1987).

10. Mao, L. et al. Clonal genetic alterations in thelungs of current and former smokers. J. Natl.Cancer Inst. 89, 857–862 (1997).

11. Gu, Y. et al. The (4;11)(q21;q23) chromosometranslocations in acute leukemias involve theV(D)J recombinase. Proc. Natl. Acad. Sci. USA 89,10464–10468 (1992).

12. Greenblatt, M.S., Bennett, W.P., Hollstein, M. &Harris, C.C. Mutations in the p53 tumor suppres-sor gene: Clues to cancer etiology and molecularpathogenesis. Cancer Res. 54, 4855–4878(1994).

Division of Experimental Oncology A,Istituto Nazionale Tumori,Milan, Italyemail: sozzi@istitutotumori.mi.it

Mutations induced by exposure to tobacco smoke. Inlung tissue (right), the principal tobacco smoke-inducedmutations are DNA adducts and base-pair transversionsin the p53 gene and exon deletions and loss ofheterozygosity (LOH) in the FHIT gene (which overlapsthe fragile site, FRA3B). Lung cancers from smokers andnon-smokers have different mutational spectra. Similarly,a recent study2 reports that cord blood T lymphocytesfrom newborns exposed transplacentally to cigarettesmoke (because their mothers were exposed to passivesmoke during pregnancy) showed a much higherfrequency of genomic deletions in a HPRT reporter gene(induced by an illegitimate V(D)J recombinase event)than unexposed infants.

HPRT p53

IllegitimateV(D)J recombinase-

mediated genomic deletions

Targeted DNA adductsformation of basepair transversions

FHIT/FRA3Bexon deletions LOH