FLT3 internal tandem duplication in acute promyelocytic leukemia: central nervous system relapse.

Abstract

Dear Editor, Lucena-Araujo et al. recently reported that internal tandem duplication (ITD) of the FLT3 gene in acute promyelocytic leukemia (APL) treated with all-trans retinoic acid (ATRA) and chemotherapy led to inferior overall survival, although there was apparently no significant effect on disease-free survival (DFS) [1]. A 55-year-old man was diagnosed with APL in July 2009. Karyotyping showed t(15;17)(q22;q12), and molecular analysis showed PML-RARA of the BCR3 isoform. Induction treatment with ATRA and idarubicin resulted in first complete remission (CR1). This was followed by consolidation with idarubicin, mitoxantrone and ATRA; and maintenance with ATRA, oral methotrexate and 6-mercaptopurine. First relapse (R1) developed 1 year afterwards. Karyotype showed t(15;17)(q22;q12) without additional aberrations. Reinduction with oral arsenic trioxide (As2O3, 10 mg daily), ATRA (45 mg/m/day), and ascorbic acid (1 g daily) for 6 weeks led to CR2, which was consolidated with two monthly cycles of daunorubicin (50 mg/m/day×2) and cytarabine (100 mg/m/day×5). Four doses of intrathecal methotrexate (12 mg) and cytarabine (50 mg) were given as central nervous system (CNS) prophylaxis. That was followed by maintenance with oral-As2O3, ATRA, and ascorbic acid, given 2 weeks every 2 months for 2 years [2]. Molecular monitoring of peripheral blood for PML-RARA had been persistently negative. Three months after completion of maintenance, progressive headache developed. Cerebrospinal fluid (CSF) showed a pleocytosis of 220×10/L, consisting almost exclusively of abnormal promyelocytes (Fig. 1a), indicating frank CNS infiltration and therefore second relapse (R2). However, bone marrow examination only showed 6 % blasts (Fig. 1b), indicating minimal leukemia infiltration. He was treated with intrathecal methotrexate and cytarabine; and re-induced with oral-As2O3, ATRA, and ascorbic acid. On confirmation of CR3 and negativity for PML-RARA, he received craniospinal irradiation and then an autologous hematopoietic stem cell transplantation (HSCT), as he was judged unsuitable for allogeneic HSCT in view of his age and advanced leukemia. Polymerase chain reaction (PCR) [3] showed wild type FLT3 in the peripheral blood and marrow at R2. However, FLT3 ITD was detectable in the R2 CSF blasts (Fig. 1c). To increase the sensitivity for detecting the FLT3 ITD subclone, the FLT3 ITD band was gel purified and sequenced (Fig. 1c). Based on the FLT3 ITD sequence, a clonospecific PCR primer was designed. With the more sensitive clonospecific PCR, FLT3 ITD was still undetectable in the peripheral blood and bone marrow, indicating that at R2, the subclone causing CNS relapse was distinct from the subclone causing marrow relapse. To trace the origin of the FLT3-ITD subclone, the marrow and peripheral blood blasts at R1 were studied. Interestingly, FLT3 ITD was found in the R1 marrow blasts but not in the R1 peripheral blood blasts. On DNA sequencing, this ITD was identical to that in the R2 CSF blasts, showing that the FLT3-ITD subclone in the R1 marrow was related to the FLT3-ITD subclone leading to the CNS infiltration at R2. With clonospecific PCR, R1 peripheral blood blasts was still negative for FLT3 ITD. Therefore, the FLT3-ITD subclone at R1 was a minor subclone that was present only in the marrow. Materials on initial presentation were not available for analysis. In APL, mutations of FLT3, most frequently ITD, are associated with high-presenting WBC count, microgranular variant and the BCR3 isoform of PML-RARA [4, 5]. The impact of FLT3 ITD on outcome has been contentious. H. Gill :A.W. K. Pang : J. Sum :A. Y. H. leung :Y.<L. Kwong (*) Department of Medicine, Queen Mary Hospital, Hong Kong, China e-mail: ylkwong@hkucc.hku.hk