Identification of the Novel Fusion Gene, SPAG9-JAK2, in Adolescent Acute Lymphoblastic Leukemia (ALL) with t(9;17)(p24;q21), and Its Association with Gene Alterations Involved in Ph1/BCR-ABL1-like ALL

Introduction: JAK2, which is located on chromosome 9p24, encodes a cytoplasmic tyrosine kinase involved in cytokine-mediated signal transduction, and is fused to 10 partner genes in hematological malignancies, such as acute lymphoblastic leukemia (ALL), acute myeloid leukemia, atypical chronic myelogenous leukemia, myelodysplastic syndrome/ myeloproliferative neoplasms (MPN), and Hodgkin lymphoma with 9p24 translocations. Furthermore, a somatic acquired activating mutation of JAK2 (V617F) was noted in a majority of patients with MPN, and mutations at R683 of JAK2 was detected not only in ALL with Down syndrome, but also in high-risk pediatric B-cell progenitor (BCP)-ALL.

Case presentation: A 14-year-old boy presenting with a persistent fever was admitted to our hospital because of hyperleukocytosis and thrombocytopenia. Cytogenetic analysis demonstrated the 46,XY,t(9;17)(p24;q21) in 17 of 20 bone marrow cells. He was classified as extremely high risk BCP-ALL with myeloid marker and treated on TCCSG L95-14 HEX/SCT protocol. He obtained a complete remission after the induction therapy, relapsed after 7 months. Although he underwent an auto bone marrow transplant because he had no matched donor, 19 months after the initial diagnosis, he died due to progressive disease.

Materials&Methods: To characterize the breakpoints, FISH analysis was performed on metaphase chromosome using BAC clones closely flanking JAK2. Genomic DNA and total RNA were isolated from bone marrow cells at diagnosis and relapse. For the mRNA-Seq sample preparation, sequencing libraries were generated according to the standard Illumina protocol for high-throughput sequencing. Using a HiSeq 2000 sequencer, 100 bp-paired end reads were obtained. Mapping reads to genes were performed by using a Bowtie software and fusion transcript discovery was done by a deFuse data analysis software. We evaluated DNA copy number changes by multiplex ligation-dependent probe amplification (MLPA) analysis and Single nucleotide polymorphism (SNP) array analysis.

Results: FISH showed a fusion signal on normal chromosome 9, and split signals on der(9) and der(17). We identified a novel JAK2 fusion gene by paired-end mRNA-seq. This fusion transcript was identified. RT-PCR followed by direct sequencing confirmed the in-frame fusion of SPAG9 exon 26 and JAK2 exon 19. Moreover, the homozygous deletion of CDKN2A, TRG, TRA/D, and IGH and hemizygous deletion of CDKN2B, PAX5, BTG1, CDK6, TRB, ADARB2, IGL and IKZF1were identified by MLPA and SNP array.

Discussion: This ALL having both rearrangement activating tyrosine kinase and genomic lesions affecting lymphoid transcription factors is similar to the Philadelphia chromosome (Ph1)/BCR-ABL1 like ALL subgroup. In conclusion, we have identified SPAG9 as a novel fusion partner of JAK2 gene in adolescent BCP-ALL as a consequence of a t(9;17)(p24;q21). The further examination of this fusion gene may be worthwhile to consider that JAK2 would be a good target for treatment in refractory ALL patients with rearrangements in JAK2.