Cystathionine-beta-synthase cDNA transfection alters the sensitivity and metabolism of 1-beta-D-arabinofuranosylcytosine in CCRF-CEM leukemia cells in vitro and in vivo: A model of leukemia in Down syndrome

The significantly higher event-free survival rates of Down syndrome (DS) ch ildren with acute myeloid leukemia compared with non-DS children is linked to increased sensitivity of DS myeloblasts to l-P-D-arabinofuranosylcytosin e (ara-C) and the enhanced metabolism of ara-C to ara-C triphosphate (J. W. Taub et al., Blood, 87: 3395-3403, 1996), The crystathionine-beta -synthas e (CBS) gene (localized to chromosome 21q22.3) may have downstream effects on reduced folate and S-adenosylmethionine pathways; ara-C metabolism and f olate pools are linked by the known synergistic effect of sequential methot rexate and ara-C therapy. We have shown that relative CBS transcripts were significantly higher in DS compared with non-DS myeloblasts, and CBS transc ript levels correlated with in vitro ara-C sensitivity (J, W, Taub et al,, Blood, 94: 1393-1400, 1999), A Leukemia cell line model to study the relati onship of the CBS gene and ara-C metabolism/sensitivity was developed by tr ansfecting CBS-null CCRF-CEM cells with the CBS cDNA, CBS-transfected cells were a median 15-fold more sensitive in vitro to ara-C compared with wild- type cells and generated 8.5-fold higher [H-3]ara-C triphosphate levels aft er in vitro incubation with [H-3]ara-C, Severe combined immunodeficient mic e implanted with CBS-transfected CEM cells demonstrated greater responsiven ess to therapy, reflected in significantly prolonged survivals after ara-C administration compared with mice implanted with wild-type cells and treate d with the same dosage schedule, The transfected cells also demonstrated in creased in vitro and in vivo sensitivity to gemcitabine, Deoxycytidine kina se (dCK) activity was approximately 22-fold higher in transfected CEM cells compared with wild-type cells. However, Levels of dCK transcripts on North ern blots and protein levels on Western blots were nearly identical between CBS-transfected and wild-type cells. Collectively, these results suggest a posttranscriptional regulation of dCK in CBS-overexpressing cells that con tributes to increased ara-C phosphorylation and drug activity. Further eluc idating the mechanisms of increased sensitivity of DS cells to ara-C relate d to the CBS gene may lead to the application of these novel approaches to acute myeloid leukemia therapy for non-DS patients.