CLINICAL OBSERVATIONS, INTERVENTIONS, AND THERAPEUTIC TRIALS Differential mRNA expression of Ara-C–metabolizing enzymes explains Ara-C sensitivity in MLL gene–rearranged infant acute lymphoblastic leukemia

Infant acute lymphoblastic leukemia (ALL) is characterized by a high incidence of mixed lineage leukemia (MLL) gene rearrangements, a poor outcome, and resistance to chemotherapeutic drugs. One exception is cytosine arabinoside (AraC), to which infant ALL cells are highly sensitive. To investigate the mechanism underlying Ara-C sensitivity in infants with ALL, mRNA levels of Ara-C–metabolizing enzymes were measured in infants (n 18) and older children (noninfants) with ALL (n 24). In the present study, infant ALL cells were 3.3-fold more sensitive to Ara-C (P .007) and accumulated 2.3-fold more Ara-CTP (P .011) upon exposure to Ara-C, compared with older children with ALL. Real-time quantitative reverse trancriptase–polymerase chain reaction (RT-PCR) (TaqMan) revealed that infants express 2-fold less of the Ara-C phosphorylating enzyme deoxycytidine kinase (dCK) mRNA (P .026) but 2.5-fold more mRNA of the equilibrative nucleoside transporter1 (hENT1), responsible forAra-Cmembrane transport (P .001). The mRNA expression of pyrimidine nucleotidase I (PN-I), cytidine deaminase (CDA), and deoxycytidylatedeaminase(dCMPD)didnotdiffersignificantly between both groups. hENT1 mRNA expression inversely correlated with in vitro resistance to Ara-C (rs 0.58, P .006). The same differences concerning dCK and hENT1 mRNA expression were observed between MLL gene–rearranged (n 14) and germ line MLL cases (n 25).An oligonucleotide microarray screen (Affymetrix) comparing patients with MLL gene–rearranged ALL with those with nonrearranged ALL also showed a 1.9-fold lower dCK (P .001) anda2.7-foldhigherhENT1 (P .046)mRNA expression in patients with MLL gene– rearranged ALL. We conclude that an elevated expression of hENT1, which transports Ara-C across the cell membrane, contributes toAra-C sensitivity in MLL gene– rearranged infant ALL. (Blood. 2003;101: 1270-1276)