Multiple molecular mechanisms contribute to radiation sensitivity in mantle cell lymphoma.

Mantlecelllymphomas(MCL)arecharacterizedbytheiraggressivebehaviorandpoorresponsetochemotherapyregimens.Wereporthereevidenceofincreasedin vitroradiation sensitivity in two cell lines that we havegeneratedfromtwoMCLpatients(UPN1andUPN2).However, despite their increased radiation sensitivity,UPN2cellsweretotallyresistanttoapoptoticcelldeath,whereasUPN1cellsunderwentmassiveapoptosis6hafterirradiation. The frequency of induced chromosomalabnormalities was higher in UPN1 as compared toUPN2. Distinct mechanisms have been found to con-tributetothisphenotype:amajortelomereshortening(UPN1andUPN2),deletionofoneATMalleleandapointmutationintheremainingalleleinUPN2,mutationofp53gene(UPN1andUPN2)withabsenceoffunctionalp53 as revealed by functional yeast assays. Afterirradiation,Ku70levelsinUPN1increasedanddecreasedinUPN2,whereasinthesameconditions,DNA-PKcsprotein levels decreased in UPN1 and remained un-changedinUPN2.Thus,irradiation-inducedapoptoticcelldeathcanoccurdespitethenonfunctionalstatusofp53(UPN1),suggestingactivationofauniquepathwayinMCLcellsfortheinductionofthisevent.Overall,ourstudy demonstrates that MCL cells show increasedradiationsensitivity,whichcanbetheresultofdistinctmolecular events. These findings could clinically beexploitedtoincreasethedismalresponseratesofMCLpatientstothecurrentchemotherapyregimens.Oncogene (2003)22,7905–7912.doi:10.1038/sj.onc.1206826Keywords:DNArepair;ATM;P53;mantlecelllym-phoma;irradiationIntroductionMantle cell lymphomas (MCL) represent a specificsubtypeofnon-Hodgkin’slymphomas(NHL)derivedfrom naive CD5þ B cells residing in the primaryfolliclesorinthemantlezonesofsecondaryfollicles(Tolksdorfet al.,1980;WeisenburgerandArmitage,1996).MCLareclinicallycharacterizedbytheirpoorprognosis,duetoeitherresistancetochemotherapyregimensusedinothertypesofaggressiveNHLortorapidrelapseafteraninitialresponse.OneofthemajorcharacteristicsofMCListhepresenceofaderegulatedcyclinD1overexpression.Thet(11;14)(q13;q32),whichleadstojuxtapositionofCCDN1geneandimmunoglo-bulinheavy-chainpromoter,isthoughttobeattheoriginofthederegulationoftheCCDN1genecodingforcyclinD1expression,agenewhichisinfact120kbawayfromthemajortranslocationregion(Boschet al.,1994;Dreylinget al.,1997).Themechanismsofthislong-distanceactivationremainundeterminedatthepresenttime.Itis,however,thoughtthatthecyclinD1geneoverexpressionaloneisnotsufficientforhemato-poietictransformationastransgenicmiceforcyclinD1donotdeveloplymphoma(Bodruget al.,1994;Lovecet al.,1994).TheunregulatedcyclinD1expressionseemstobe,however, a major event in the pathogenesis andprogressionofMCL,especiallyinthepresenceofaninefficient p53 expression due to a deletion or amutation(Campoet al.,1999).Deletionsinvolvingbands11q22–q23arealsocom-monlyobservedinMCLaswellasinotherB-celllymphoproliferativedisorderssuchaschroniclympho-cyticleukemia(Stankovicet al.,1999;Schaffneret al.,2000).ThesedeletionsaffectthegenomicregionoftheATM gene, which is known to play a role inchromosomal instability, radiation sensitivity anddefectivecell cyclecheckpoint activation.ATM hasbeenshowntophosphorylatedirectlythec-Ablandp53inresponsetoDNAdamage(Baskaranet al.,1997; Shafman et al., 1997; Banin et al., 1998;Canman et al., 1998). ATM inactivation has alsobeen found to be a major molecular event inMCL,eithermutationordeletionatthegeneticlevel(Schaffneret al.,2000).Recently, high-dose myeloablative radioimmuno-