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Background Following the advances in genetic tests, including next-generation sequencing, there have been new insights into hereditary hematopoietic malignancies. The germline mutation in DDX41 was included in a new category, myeloid neoplasms with germline predisposition, of the updated 2016 WHO classification. Based on the reported data to date, there seem to be racial differences in the mutation variants of DDX41 gene, which were found in patients with myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML). Idiopathic cytopenia of undetermined significance (ICUS) is known to be a precursor lesion of MDS, but the DDX41 mutations have not been evaluated in patients with ICUS. In this study, we aimed to reveal the incidence, genetic characteristics, and clinical features of the DDX41 mutations in patients with ICUS, MDS, and AML. Methods We performed targeted deep sequencing of 141 genes with a MiSeqDx sequencer (Illumina) using bone marrow (BM) samples obtained from the patients with ICUS (n=77), MDS (n=175), and AML (n=148) between May 2009 and June 2019. ICUS was defined by the proposed criteria of 2007 Consensus Group. The cut-off level of variant allele frequency (VAF) was set to 2.0% of mutant allele reads. We divided ICUS into clonal cytopenia of undetermined significance (CCUS), which was defined as ICUS with ≥ 2% VAF of somatic mutations of myeloid malignancy-associated genes and non-CCUS. Results Overall, DDX41 mutations were detected in 6 (7.8%) of 77 ICUS, 19 (10.9%) of 175 MDS, and 8 (5.4%) of 148 AML patients. Thirty-eight (49.4%) of 77 ICUS patients had CCUS. Of 6 DDX41 mutated patients with CCUS, 5 showed biallelic mutations with the median VAF of 44.7% (range, 29.3−50.0) and 10.2% (range, 3.3−25.4), indicating that one germline and one somatic mutation exists. Of 175 MDS patients, 78 were categorized into lower-risk MDS (revised international prognostic scoring system [IPSS-R] < 3.5) and 97 into higher-risk MDS (IPSS-R ≥ 3.5), and DDX41 mutations were identified in 6 (7.7%) of 78 lower-risk MDS and 13 (13.4%) of 97 higher-risk MDS patients. Interestingly, biallelic mutations were found in 16 of 18 DDX41-mutated MDS patients with the median VAF of 47.75% (range, 43.4−55.6) and 13.8% (range, 2.7−35.8). In contrast, only one of 8 DDX41-mutated AML patients had biallelic mutation. Patients with DDX41 mutations typically showed hypocellular marrow (median BM cellularity, 30%; range, 5−95) with significant neutropenia (median neutrophil counts, 607/μL; range, 142−1675), male predominance (29/33, 87.9%), and relatively older age (median age, 64 years; range, 41−79) at diagnosis. In addition, we found novel mutation locations, which were different between presumed germline and somatic variants: V152G in germline, and T227M in somatic (Table 2). During a median follow-up duration of 2.9 years, 1 of 6 ICUS patients progressed to MDS-EB-1 after 17.3 months and 1 to non-severe aplastic anemia after 51.3 months. Conclusion Our data show that a significant proportion of ICUS, MDS, and AML patients had DDX41 mutations, many of which are presumably germline. These findings suggest that careful consideration of the predisposing germline mutation is important when selecting a familial donor for allogeneic HCT. We also found novel mutation locations of DDX41 gene which were different between somatic and germline variants. Further studies are warranted to define the clinical and molecular characteristics of DDX41 mutations and therapeutic implications in myeloid neoplasms. Disclosures No relevant conflicts of interest to declare.

Background: Unexplained cytopenia comprises a spectrum of hematological diseases from idiopathic cytopenia of undetermined significance (ICUS) to myelodysplastic syndrome (MDS). Revised International Prognostic Scoring System (IPSS-R) is the standard tool to assess risk in MDS. Here, we investigated the occurrence, characteristics, and changing pattern of mutations in patients with ICUS and MDS stratified by IPSS-R score. Methods: A total of 211 patients were enrolled: 73 with ICUS and 138 with MDS. We analyzed the sequencing data of a targeted gene panel assay covering 141 genes using the MiSeqDx platform (Illumina). The lower limit of variant allele frequency (VAF) was set to 2.0% of mutant allele reads. Bone marrow components were assessed for the revised diagnosis according to the 2016 WHO classification. Lower-risk (LR) MDS was defined as those cases with very low- or low-risk MDS according to the IPSS-R. Higher-risk (HR) MDS was defined as those cases with high- or very high-risk MDS according to the IPSS-R. Results: Patients with ICUS were classified as very low-risk (39.7%), low-risk (54.8%), and intermediate-risk (5.5%) according to the IPSS-R. Patients with MDS were classified as LR (35.5%), intermediate-risk (30.4%), and HR (34.1%). In the ICUS, 28 (38.4%) patients carried at least one mutation in the recurrently mutated genes in MDS (MDS mutation). The most commonly mutated genes were DNMT3A (11.0%), followed by TET2 (9.6%), BCOR (4.1%), and U2AF1, SRSF2, IDH1 and ETV6 (2.7% for each). IPSS-R classification was not associated with mutational VAF and the number of mutations in ICUS. In the 49 LR MDS, 28 (57.1%) patients carried at least one MDS mutation. The most commonly mutated genes were SF3B1 (20.4%), followed by TET2 (12.2%), U2AF1 (10.2%), DNMT3A (10.2%), ASXL1 (10.2%), and BCOR (6.1%). Higher VAF and number of mutations were observed in LR MDS compared to ICUS patients. In the 42 intermediate-risk MDS, 27 (64.3%) patients carried at least one MDS mutation. The most commonly mutated genes were ASXL1 (23.8%), followed by TET2 (21.4%), RUNX1 (16.7%), U2AF1 (14.3%), DNMT3A (14.3%), SF3B1 (9.5%), and SRSF2, BCOR, STAG2 and CBL (7.1% for each). In the 47 HR MDS, 36 (76.6%) patients carried at least one MDS mutation. The most commonly mutated genes were TET2 (25.5%), followed by DNMT3A (14.9%), TP53 (14.9%), RUNX1 (12.8%), U2AF1 (10.6%), ASXL1 (10.6%), and SRSF2 and KRAS (6.4% for each). As the disease progressed, VAF and number of the MDS mutations gradually increased, and mutations involving RNA splicing, histone modification, transcription factor or p53 pathway had a trend for increasing frequency. Specifically, ASXL1, TP53, and RUNX1 mutations were the most striking features in patients with advanced stage of the disease. Cohesin mutations were not detected in ICUS, whereas these mutations were detected at a relatively high frequency in HR MDS. Our data were summarized in Table 1. Conclusions: We demonstrate that on disease progression, MDS mutations are increased in number as well as are expanded in size. Furthermore, a subset of mutations tends to be enriched for intermediate- to HR MDS. The results of this study can aid both diagnostic and prognostic stratification in patients with unexpected cytopenia. In particular, characterization of MDS mutations can be useful in refining bone marrow diagnosis in challenging situations such as distinguishing LR MDS from ICUS. Disclosures No relevant conflicts of interest to declare.

Some of the recurrently occurring somatic mutations are known to be diagnostic or prognostic in myelodysplastic syndrome (MDS). Targeted gene capture and next-generation sequencing (NGS) has rapidly become routine clinical tools to detect the somatic mutations in patients with MDS. Copy number variants (CNVs) may have additional clinical significance in MDS. Chromosomal microarray analysis is a standard technique for genome-wide CNV detection, but multiple testing strategies require high costs and time. Recent advancements in NGS technologies have developed more cost-effective and rapid methods to allow simultaneous identification of targeted CNVs as well as somatic mutations using the same panel-based NGS data. In this study, we investigated whether the detection of CNVs using the targeted NGS data provided an additional value other than the clinical implications of somatic mutations. We performed targeted deep sequencing analysis on bone marrow samples obtained from 266 patients with MDS using an MDS panel targeting 28 well-known MDS-related genes (NRAS, DNMT3A, SF3B1, IDH1, TET2, NPM1, LAMB4, EZH2, JAK2, CBL, ETV6, KRAS, FLT3, IDH2, PRPF8, TP53, NF1, SRSF2, SETBP1, DNMT1, ASXL1, RUNX1, U2AF1, ZRSR2, ATRX, STAG2, MMP8, and ARID2). Sequencing libraries were generated using the AmpliSeq Library Kit 2.0 (Life Technologies, Carlsbad, CA) and the MDS panel was then sequenced using the Ion Torrent Proton system (Life Technologies) according to the manufacturer's instructions. The multiscale reference module and Rank Segmentation statistical algorithm in NEXUS software v9.0 (Biodiscovery) were used to define the CNVs for each sample. Overall survival (OS) and acute myeloid leukemia (AML)-free survival (AFS) were estimated from the date of MDS diagnosis to death or AML progression using the Kaplan-Meier method, and the differences in survival were compared using the log-rank test (for univariate analysis) and the Cox proportional hazards model (for multivariate analysis). Overall, 215 patients (80.8%) carried at least one somatic mutations, and 67 (25.2%) had one or more CNVs. The number of mutated genes per patient ranged from 0 to 6, and the number of genes with CNVs per patient ranged from 0 to 10. Of 51 patients who did not have somatic mutations, 12 (23.5%) had the targeted CNVs. The mutated genes in more than 10% of patients were 8: U2AF1 (21.8%), TET2 (17.7%), ASXL1 (13.5%), TP53 (13.2%), SETBP1 (12.8%), NF1 (10.9%), SF3B1 (10.5%), and RUNX1 (10.5%). The genes with CNVs detected in 10 or more patients were 5: EZH2 (loss in 7q, 6.8%), KRAS (gain and loss in 12p, 5.3%), ASXL1 (gain and loss in 20q, 4.5%), LAMB4 (loss in 7q, 3.8%), and RUNX1 (gain and loss in 21q, 3.8%). Interestingly, all five patients with TP53 deletion exhibited TP53 mutation as well, suggesting a bi-allelic alteration (mutation + copy loss). The higher number of genes with CNVs per patient were significantly associated with inferior OS (P Our study suggests that simultaneous detection of targeted CNVs as well as somatic mutations using the same panel-based NGS data add clinically useful information on the prognosis of MDS patients. Disclosures No relevant conflicts of interest to declare.

Background: The JL1 antigen is a novel epitope of CD43, a cell surface glycoprotein of mucin family. JL1 is a differentiation antigen expressed on stage II double positive (CD4+CD8+) human cortical thymocytes. The antigen is not expressed on mature peripheral blood cells or other normal tissues. The anti-JL1 monoclonal antibody binds to human leukemia MOLT-4 cells with 5,100-9,600 binding sites per cell. Preclinical studies have shown the cytotoxic effects of anti-JL1-based immunotoxin against JL1-positive leukemic cells, sparing most normal tissues other than thymocytes and some bone marrow mononuclear cells. Phase I clinical trial of new anti-leukemic agent with an anti-JL1 antibody (Leukotuximab; DiNonA, Korea) is now underway. In this study, we prospectively investigated the JL1 expression in patients with acute leukemia and myelodysplastic syndrome (MDS). Patients & methods: Flow cytometric analysis for the JL1 expression on leukemic blasts was performed using a FACSCanto II (Becton-Dickinson, Sunnyvale, CA, USA). The percent expression of JL1 antigen among leukemic blasts was recorded. Positive JL1 expression was defined if 20% or more leukemic blasts expressed the antigen. Association of JL1 expression with clinical, pathologic, and genetic characteristics was analyzed. Influence of JL1 expression on clinical outcomes of patients was also explored. Results: Between March 2014 and June 2015, a total of 245 adult patients with acute myeloid leukemia (AML, n=170), acute lymphoblastic leukemia (ALL, n=52), and MDS (n=23) were enrolled in this study. Positive JL1 expression was observed in 96 (57.1%) patients with AML, 28 (51.9%) with ALL, and 5 (21.7%) with MDS (P =0.006), while three normal controls showed negative JL1 antigen expression. Interestingly, JL1 expression was positive in all 14 patients with AML M3 with a median expression of 94.3% (range, 60.3-97.8%). In contrast, only 13 (39.4%) of 33 patients with AML with myelodysplasia-related changes (MRC) had positive JL1 expression. In AML patients, positive JL1 expression was significantly associated with CD34- (P =0.003), HLA-DR- (P =0.019), PML-RARA + (P =0.001), FLT3-ITD + (P =0.026), mutated NPM1 (P =0.003), and complex karyotype (3 or more clonal chromosomal abnormalities) (P =0.020). Cytarabine plus anthracycline based chemotherapy was given to 117 patients with AML, and the complete remission (CR) rate was significantly different between 63 JL1 expression positive patients and 54 negative patients (84.1% vs. 59.3%, P =0.003). Positivity of JL1 expression was not significantly associated with overall survival in all patients with AML (median survival, JL1 positive vs. negative, 20.6 vs. 18.2 months, P =0.489). In ALL patients, positive JL1 expression was significantly associated with CD13- (P =0.032) and the CR rate was not significantly different by JL1 expression. JL1 expression was measured twice or more in 85 patients during their clinical courses and positivity of JL1 expression was not changed in 61 (71.8%) (P =0.307). Five MDS patients progressed to AML and JL1 expression was changed in only one patient (JL1 positive to JL1 negative). Conclusion: JL1 was expressed in around 50% of patients with AML or ALL while less frequent expression of JL1 was observed in MDS and AML with MRC. JL1 expression was significantly associated with some immunophenotypic and genetic features, especially PML-RARA +. JL1 expression was significantly associated with the CR rate of AML patients. Expression of JL1 seems to be stable during clinical courses. Our data suggest that immunotherapeutic approach targeting JL1 antigen may be feasible in significant proportion of patients with acute leukemia and MDS. Disclosures Kim: Dinona Institute, Dinona Inc.: Employment. Yoon:Dinona Institute, Dinona Inc.: Employment. Jung:Dinona Institute, Dinona Inc.: Employment.

Abstract 4852 Introduction: Acquired mutations of the WT1 gene have been reported in 5–10% of normal karyotype AML (NK-AML) patients. Poor prognostic impact of the mutations has been reported, but some studied found no such correlation. Recent report from Germany showed the association of single nucleotide polymorphism (SNP) rs16754 status with clinical outcomes in NK-AML (J Clin Oncol 2009; 28:578). We assessed clinical implication of WT1 gene alterations in Korean patients with NK-AML. Patients and Methods: This study included a total of 75 patients with NK-AML. All patients received standard induction chemotherapy (‘7+3’ regimen) at the Asan Medical Center, Seoul, Korea between May 1999 and Dec 2006. WT1 exons 7 and 9 were amplified using polymerase chain reaction (PCR) and purified PCR fragments were directly sequenced. The clinico-laboratory data were retrieved from the AMC Leukemia Registry. Results: WT1 mutations were found in four patients (5.3%): two duplication mutations and two insertion mutations. All mutations were in exon 7 and none in exon 9. Two of four patients harboring WT1 mutation failed to achieve complete remission (CR) and died at 3.9 and 7.0 months after diagnosis of AML. One of two patients attaining CR was alive without relapse at 44.0 month and another died in CR at 4.7 months. WT1 SNP rs16754 was AA genotype (WT1AA) in 5 (3.7%), AG (WT1AG) in 29 (38.2%), and GG (WT1GG) in 41 (54.7%). The findings are significantly different from those of German report (WT1AA in 74.3%, WT1AG in 24.1%, and WT1GG in 1.6%) (P Conclusion: The prognostic impact of WT1 mutation in NK-AML could not be assessed in this study due to low number of patients harboring the mutation, but three of four patients died with relatively short survival duration, suggesting poor prognosis of the patients with WT1 mutation. The frequency of WT1 SNPs rs16754 in our patients was different from that of German report and survivals were not significantly different according to the SNP genotypes. Disclosures: No relevant conflicts of interest to declare.

Abstract 1179 Poster Board I-201 Introduction: Intravenous (iv) busulfan can yield a more consistent dosing and pharmacokinetic profile than oral formulation, but there is still inter-patient variability in systemic exposure with iv busulfan. GST gene polymorphisms may explain the variability because busulfan is metabolized in liver through conjugation with GST family. Thus, we investigated the influence of polymorphisms of 3 GST genes, GSTA1, GSTM1, and GSTT1 on the clearance of iv busulfan in adult patients undergoing hematopoietic cell transplantation (HCT). Patients and Methods: We analyzed the PK data from 60 patients, who were included in a randomized trial of 4-times-daily (0.8 mg/kg q 6 h) versus once-daily (3.2 mg/kg once a day) iv busulfan in a conditioning therapy for HCT (Biol Blood Marrow Transplant 2007;13:1095). Limited sampling strategy was used for PK studies, which were performed for the first busulfan dosing using a validated LC with tandem MS. Busulfan plasma clearance (CL) was derived from 1 compartment model. GSTA1 was genotyped by PCR followed by RFLP, and GSTM1- and GSTT1-null genotypes were identified by PCR procedure. Results: GSTA1 genotyping revealed GSTA1*A/*A in 46 patients (77%) and GSTA1*A/*B in 14 (23%). GSTM1-null genotype was found in 25 patients (43%) and GSTT1-null genotype in 34 (59%). Each polymorphism of GST genes was not associated with sex or age of the patients. In univariate analysis, clearance (CL, mL/min/kg) of iv busulfan was significantly associated with GSTA1 polymorphisms (*A/*A vs. *A/*B, 2.036 ± 0.340 vs. 1.789 ± 0.295, P=0.017), but not with GSTM1 (present vs. null, 2.012 ± 0.390 vs. 1.915 ± 0.279, P=0.274) or GSTT1 (present vs. null, 2.047 ± 0.286 vs. 1.917 ± 0.380, P=0.064) polymorphisms. Actual body weight in kilogram was also significantly associated with CL of iv busulfan (Pearson's correlation coefficient, -0.420; P=0.001). Linear regression analysis demonstrated that GSTA1 gene polymorphism (regression coefficient, -0.255; 95% CI, -0.440 to -0.071; P=0.008) and actual body weight (regression coefficient, -0.012; 95% CI, -0.091 to -0.005; P=0.001) were independently significant factors for CL of iv busulfan. Null genotype of GSTM1 and/or GSTT1, although each polymorphism was not a significant factor, showed decreased clearance of iv busulfan both in patients with GSTA1 *A/*A and those with GSTA1 *A/*B (Figure 1). The CL was similar between in patients with GSTA1 *A/*A with null genotype of both GSTM1 and GSTT1 and in those with GSTA1 *A/*B with present genotype of both GSTM1 and GSTT1. Conclusion: GSTA1 gene polymorphism was an important determining factor for the clearance of iv busulfan. Polymorphisms of GSTM1 and GSTT1 genes also appeared to have a supplementary role in the pharmacokinetics of iv busulfan. Disclosures: No relevant conflicts of interest to declare.

P-glycoprotein (P-gp) function of leukemic blasts is associated with chemotherapy resistance in acute myeloid leukemia (AML). The C3435T polymorphism in the human MDR1 gene has been studied in AML and contradictory results were reported regarding clinical implications of the polymorphism. We investigated the association of C3435T polymorphism of MDR1 gene with P-gp function of leukemic blasts and clinical outcomes in patients with AML excluding M3 subtype. A total of 200 patients, 127 males and 73 females, were included in this study. Median age was 44 years (range, 14–75). All patients were newly diagnosed and were treated with standard ‘7+3’ induction chemotherapy regimens at the Asan Medical Center, Seoul, Korea between May 1999 and November 2006. The C3435T polymorphism was analyzed with PCR/RFLP method. P-gp function of leukemic blasts was measured at diagnosis by the rhodamine-123 efflux assay. The clinico-laboratory data at diagnosis and data on clinical outcomes were obtained from the AMC Leukemia Registry. Genotype frequency of C3435T polymorphism was 71 (35.5%) for CC, 93 (46.5%) for CT, and 36 (18.0%) for TT. There were no significant differences between different genotypes of C3435T polymorphism regarding age, sex, FAB subtypes, initial leukocyte counts, percentages of blasts in bone marrow or blood at diagnosis, and cytogenetic risk groups. P-gp function of leukemic blasts was not significantly different according to the genotype of C3435T polymorphism: 32.2 ± 24.0% for CC, 35.8 ± 21.4% for CT, and 29.1 ± 24.0% for TT (P=0.370). Complete remission was induced in 79.2% for CC, 79.8% for CT, and 77.8% for TT (P=0.969). Overall, relapse-free and event-free survival probabilities at 5 years were 47.6%, 55.4% and 51.0% for CC, 51.3%, 51.0% and 42.5% for CT, and 53.1%, 60.0% and 55.0% for TT (P= 0.594, 0.932 and 0.764, respectively). In conclusion, C3435T polymorphism does not have significant clinical implications in AML regarding P-gp function of leukemic blasts and clinical outcomes.

Several single-arm studies have shown that once-daily or twice-daily intravenous busulfan (iBU) may be equally safe and effective compared to traditional 4-times-daily iBU. We performed a prospective randomized trial of 4-times-daily vs. once-daily dosing of iBU in a conditioning therapy for HCT, and compared pharmacokinetic (PK) characteristics of iBU and clinical outcomes in terms of engraftment, post-HCT toxicities, non-relapse mortality (NRM), and overall survival. All of 60 patients, who received iBU on the first day of conditioning therapy for HCT, were randomized to receive iBU either as 0.8 mg/kg over 2 hr 4 times a day (BU4 arm) or 3.2 mg/kg over 3 hr once a day (BU1 arm) between May 2004 and August 2005. Limited sampling strategy was used for PK studies, which were done for the first busulfan dosing. Baseline patient and donor characteristics were well balanced between 2 arms regarding sex, age, diagnosis of underlying disease, time from diagnosis to HCT, disease status at HCT, GVHD prophylaxis regimen, conditioning therapy regimen, ABO mismatch between donor and recipient, donor-recipient sex pair, donor age, type of graft, donor type, and cell doses of graft. The complete PK data were obtained from all 60 patients and those were comparable between 2 arms: elimination half-life (mean ± SD) was 2.75 ± 0.22 vs. 2.83 ± 0.21 hr in BU4 arm vs. BU1 arm, respectively (P=0.304), estimated AUC was 7933.43 ± 2177.71 vs. 8556.86 ± 2452.07 μM × min per day (P=0.367), and clearance was 2.05 ± 0.36 vs. 1.91 ± 0.31 ml/min/kg (P=0.133). Median times to engraftment of neutrophils (BU4 arm vs. BU1 arm, 14 vs. 14 days, P=872) and platelets (26.5 vs. 25.5 days, P=0.946) after HCT were similar between 2 arms. There were no significant differences in cumulative incidence of acute GVHD (BU4 arm vs. BU1 arm, 31.0% vs. 13.8%, P=0.145) and occurrence of CMV infection (40.0% vs. 23.3%, P=0.165) or hepatic veno-occlusive disease (10.0% vs. 16.7%, P=0.448). Other toxicities within 100 days after HCT were not significantly different between two arms. Cumulative incidence of NRM was 25.5% vs. 17.3% (P=0.488) and overall survival probability at 1 year was 69.0% vs. 70.0% (P=0.758). In conclusion, our randomized study demonstrates that once-daily iBU has similar PK profiles and results in at least not inferior clinical outcomes compared to traditional 4-times-daily iBU.