Reduced folate carrier expression in acute lymphoblastic leukemia: A mechanism for ploidy but not lineage differences in methotrexate accumulation

Methotrexate (MTX) is one of the most active and widely used agents for the treatment of acute lymphoblastic leukemia (ALL). To elucidate the mechanis m for higher accumulation of MTX polyglutamates (MTX-PG) in hyperdiploid AL L and lower accumulation in T-lineage ALL, expression of the reduced folate carrier (RFC) was assessed by reverse transcription-polymerase chain react ion in ALL blasts isolated from newly diagnosed patients. RFC expression ex hibited a go-fold range among 29 children, with significantly higher expres sion in hyperdiploid B-lineage ALL (median, 11.3) compared with nonhyperdip loid ALL (median, 2.1; P <.0006), but no significant difference between non hyperdiploid B-lineage and T-lineage ALL. Furthermore, mRNA levels of RFC ( mapped by FISH to chromosome 21) were significantly related to chromosome 2 1 copy number (P =.0013), with the highest expression in hyperdiploid ALL b lasts with 4 copies of chromosome 21. To assess the functional significance of gene copy number, MTX-PG accumulation was compared in ALL blasts isolat ed from 121 patients treated with either low-dose MTX (LDMTX; n = 60) or hi gh-dose MTX (HDMTX; n = 61). After LDMTX, MTX-PG accumulation was highest i n hyperdiploid B-lineage ALL with 4 copies of chromosome 21 (P = .011), but MTX-PG accumulation was not significantly related to chromosome 21 copy nu mber after HDMTX (P = .24). These data show higher RFC expression as a mech anism for greater MTX accumulation in hyperdiploid B-lineage ALL and indica te that lineage differences in MTX-PG accumulation are not due to lower RFC expression in T-lineage ALL. (C) 1999 by The American Society of Hematolog y.