Značaj određivanja genotipa Tiopurin-S-metiltransferaze kod dece s akutnom limfoblastnom leukemijom tokom terapije održavanja

Abstract

Uvod. Tiopurin-S-metiltransferaza (TPMT ) je enzim koji katalizuje inaktivaciju merkaptopurina, leka koji se široko primenjuje u lečenju akutne limfoblastne leukemije (ALL) kod dece. Kada se osobe s nedostatkom TPMT leče standardnim dozama merkaptopurina, kod njih se razvija teška i po život opasna mijelotoksičnost. Cilj rada. Cilj rada je bio da se utvrdi da li kod dece s ALL koji su nosioci mutacije u genu za TPMT individualizovanjem doziranja merkaptopurina može da se smanji mijelotoksičnost terapije, te da li broj tandemskih ponovaka (engl. variable number of tandem repeats - VNTR) u promotoru gena za TPMT ima uticaja na efekte terapije merkaptopurinom. Metod rada Metodima lančane reakcije umnožavanja DNK (engl. polymerase chain reaction - PCR) ispitano je 50 nasumično odabrane dece lečene ALL IC-BFM 2002 protokolom na najčešće mutacije u genu za TPMT. Za 20 dece je PCR metodima određen VNTR genotip. Ispitanicima je tokom faze održavanja beležen broj nedelja kada su terapiju dobijali u punim ili smanjenim dozama, kao i broj nedelja bez terapije. Rezultati Među 50 dece bilo je 29 dečaka (58%) i 21 (42%) devojčica, uzrasta od 1,8 do 17,3 godine (medijana 6,2 godine). Utvrđeno je četvoro (8%) heterozigotnih nosilaca mutacija, kod kojih je otkrivena TPMT*3A varijanta. Posle 12, 14, 16 i 19 nedelja lečenja smanjenim dozama merkaptopurina bolesnici su, zbog dobrog podnošenja terapije, postepeno počeli da primaju punu dozu leka. Nije bilo odlaganja terapije. Smanjenje kumulativne doze merkaptopurina za bolesnike sa TPMT mutacijama bilo je 7,8%, 7,4%, 11,2% i 16,6%. Između dece bez TPMT mutacija i heterozigota nije za- beležena statistički značajna razlika u trajanju lečenja punim (53,6 nasuprot 55,7 nedelja) i smanjenim dozama merkaptopurina (19,9 nasuprot 15,2 nedelje). Otkrivenih VNTR bilo je između četiri i sedam. Većina bolesnika imala je različit broj VNTR na homolognim hromozomima. Najčešće uočen polimorfizam bio je VNTR*5. Nije zabeležena korelacija između nasleđivanja TPMT i VNTR genotipa. Zaključak Farmakogenetskim principima u lečenju ALL dece može se postići napredak u podnošenju lečenja merkaptopurinom.

INTRODUCTION Thiopurine S-methyltransferase (TPMT) is an enzyme that catalyses the inactivation of mercaptopurine (MP) which is widely used in the treatment of acute lymphoblastic leukaemia (ALL) of childhood. Potentially fatal myelotoxicity may develop after standard doses of MP in TPMT deficient patients. OBJECTIVES To establish if individually tailored doses of MP can reduce myelotoxicity in ALL patients carrying mutations in the TPMT gene. To establish if variable number of tandem repeats (VNTR) genotype influences the treatment effects of MP. METHOD Fifty randomly selected patients treated according to ALL IC-BFM 2002 protocol were tested for most frequent TPMT gene mutations using PCR based methods. VNTR genotype was determined in 20 children by PCR methods. During the maintenance phase, we recorded the number of weeks when therapy was applied in either full doses, reduced doses or when patients were without any therapy. RESULTS Fifty children were examined, 29 boys (58%) and 21 girls (42%); age ranged from 1.8-17.3 years (median 6.2 years). Four patients (8%) were heterozygous for TPMT mutations, all of them carrying the TPMT*3A variant. After 12, 14, 16 and 19 weeks of therapy with reduced doses of MP, the patients switched to full doses due to good tolerance. There was no therapy omission. Cumulative dose of MP was reduced for 7.8%, 7.4%, 11.2% and 16.6%, respectively, in patients with TPMT mutations. No significant difference was found between children with no mutations and TPMT heterozygotes regarding full dose therapy (53.6 vs. 55.7 weeks, respectively) and reduced dose therapy (19.9 vs. 15.2 weeks respectively). The number of detected VNTRs ranged from four to seven. The majority of patients had different number of VNTRs on homologous chromosomes. Most frequently detected polymorphism was VNTR*5. No correlation was found between TPMT and VNTR genotype inheritance. CONCLUSION Obeying pharmacogenetic principles in the treatment of childhood ALL may improve the tolerance of therapy with MP.