An increase in CRABP-II has frequently been invoked as a cause of resistance to ATRA in APL due to cytoplasmic sequestration and catabolism of ATRA. However, recent evidence indicates that CRABP-II has a positive rather than a negative role in ATRA activity by facilitating delivery of ATRA to retinoic acid receptor-alpha (RARα) associated with ATRA target genes in the cell nucleus or/and by serving as a co-activator of RARα-regulated transcription. This implies that if CRABP-II has a role in the development of ATRA resistance in APL, this would more likely occur through a deficiency rather than from an increase in CRABP-II. We previously reported that CRABP-II is constitutively expressed at similar levels in pretreatment and relapse APL cells (
Zhou, et al, Cancer Res 58, 5770, 1998
), suggesting that putative CRABP-II deficiency is not due to the loss of CRABP-II expression. To investigate the alternative possibility that CRABP-II deficiency might arise through inactivating mutations, we sequenced the entire coding region of CRABP-II from 18 cases of APL who had relapsed from prior ATRA-containing treatment regimens. In 8 cases RNA was transcribed by reverse transcriptase to cDNA and amplified by polymerase chain reaction (PCR), using primers anchored in the 5′ and 3′ untranslated region of mRNA; in 10 cases, genomic DNA was PCR amplified for sequence analysis, using primers anchored in the introns between the 4 exons of the gene. No CRABP-II mutations were identified. The samples tested included 11 first-relapse cases, 2 of whom were refractory to reinduction therapy with intravenous liposomal-ATRA, and 7 multiple relapse cases, all of whom were clinically refractory to ATRA and had, additionally, relapsed from arsenic trioxide therapy. Also, no mutations were found in 3 APL patients who had relapsed from chemotherapy-induced remissions or in 3 APL cell lines (NB4, UF-1 and AP-1060). Two heterozygous base substitutions were incidentally identified in CRABP-II intron 2 in a chemotherapy-only treated patient. These results indicate that CRABP-II mutations rarely, if ever, contribute to ATRA-resistance or disease relapse in APL.
