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 (Ara-C), 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 transporter I (hENT1), responsible for Ara-C membrane transport (P = .001). The mRNA expression of pyrimidine nucleotidase I (PN-I), cytidine deaminase (CDA), and deoxycytidylate deaminase (dCMPD) did not differ significantly 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) and a 2.7-fold higher hENT1 (P = .046) mRNA expression in patients with MLL generearranged ALL. We conclude that an elevated expression of hENT1, which transports Ara-C across the cell membrane, contributes to Ara-C sensitivity in MLL generearranged infant ALL.,
Department of Pediatrics

Stam, R., den Boer, M., Meijerink, J., Ebus, M. E., Peters, G., Noordhuis, P., … Pieters, R. (2003). Differential mRNA expression of Ara-C-metabolizing enzymes explains Ara-C sensitivity in MLL gene-rearranged infant acute lymphoblastic leukemia. Blood, 101(4), 1270–1276. doi:10.1182/blood-2002-05-1600