P404: immunoglobulin gene rearrangement in korean patients with b-lymphoblastic leukemia – an immunophenotype and repertoire analysis

Background: Next-generation sequencing for the IGH gene is helpful for minimal residual disease (MRD) monitoring in patients with B-cell acute lymphoblastic leukemia (B-ALL); however, the benefit of IGK rearrangement test has rarely been investigated as well as that of TRG and TRB rearrangement tests. Also, IGH/IGK gene usage has never been reported in Korean patients with B-ALL. Aims: We analyzed the relationship between clonal IGH/IGK rearrangement and immunophenotype and the additional benefit of IGK, TRB, and TRG tests. Repertoire analysis was performed for the first time in Korean patients. Methods: The study included 70 B-ALL patients, including 23 adults (2 pro-B ALL and 25 common ALL immunophenotypes) and 47 pediatric patients (5 pro-B-ALL, 37 common ALL, and 5 pre-B-ALL). NGS for clonal IGH/IGK was performed in all the patients. In 8 patients without clonal IGH/IGK rearrangements, TRB/TRG genes were tested. Results: In adults, clonal IGH rearrangements were observed in 18 (78.3%), and all showed more than 60% of clonal reads. Clonal IGK rearrangements were observed in 14 (60.9%), including 2 (8.7%) without clonal IGH rearrangement. Overall, 20 (87.0%) had clonal IGH and/or IGK rearrangements. The 3 with no rearrangement were common B-ALL (12% of such immunophenotype). All the 3 showed both clonal TRB and TRG rearrangements (3/23, 13.0%). In total, 28 and 20 clonal IGH and IGK rearrangements were observed. IGHV3 (46.4%) was the most used, followed by IGHV4 (25.0%). IGHV4-34 (21.4%) was the most used subtype. IGHJ4 (46.4%) was the most used, followed by IGHJ6 (21.4%). IGKV2 and IGKV3 (25.0% each) were the most used, followed by IGKV1 (15.0%) and IGKV4 (15.0%). In pediatric patients, clonal IGH rearrangements were observed in 44 patients (93.6%), among which 5 showed less than 40% of clonal reads (particularly including 2 with less than 20% of clonal reads), which might lead to underestimation of disease burden. Clonal IGK rearrangements were observed in 26 (55.3%), including 1 (2.1%) without clonal IGH rearrangement. None of the 5 with clonal IGH rearrangements with less than 40% of clonal reads showed clonal IGK rearrangements. Overall, 45 (95.7%) had clonal IGH and/or IGK rearrangement, but only 39 (82.9%) had suitable clonal IGH and/or IGK rearrangement for MRD monitoring. The 8 who did not (5 with too low clonal read and 3 with the absence of clonality) include 3 pro-B-ALL, 4 common B-ALL, and 1 with pre-B-ALL (60%, 10.8%, and 20.0% of such phenotypes, respectively). TRB/TRG analysis in 3 with no clonal IGH/IGK rearrangement showed 2 (2/44, 4.5%) with clonal TRB and/or TRG rearrangement. In total, 70 and 48 clonal IGH and IGK rearrangements were observed. IGHV3 (52.9%) was the most used, followed by IGHV6 (14.3%), IGHV4 (12.9%), and IGHV2 (5.7%). IGHV6-1 (14.3%) was the most used subtype. IGHJ4 was dominant (40.0%), followed by IGHJ6 (21.0%) and IGHJ5 (13.0%). IGKV2 (27.1%) was the most used, followed by IGKV3 (20.8%), IGKV1 (10.4%), IGKV4 (10.4%), IGKV7 (10.4%), IGKV5 (2.1%), and IGKJ-C-intron-KDE rearrangement (18.8%). Summary/Conclusion: Clonal IGH and/or IGK rearrangements were observed in 95.7% of pediatric patients and 87.0% of adult patients; however, only 82.9% of pediatric patients had clonal rearrangements, which is suitable for MRD monitoring. Absence or low clonal IGH/IGK read was observed in 60% of pro-B-ALL in pediatric patients. TRB and TRG tests were helpful for finding clonal sequences. IGHV3 was the most used gene in Korean patients, whereas IGHV6 was the most used one in Western patients. Keywords: Clonality, Immunoglobulin gene, B cell acute lymphoblastic leukemia