Clonal Evolution Underlying the Progression from Myelodysplastic Syndromes to Ph-Positive Acute Lymphoblastic Leukemia

Introduction:

Myelodysplastic syndromes (MDS) are considered as a "stem cell disorders", in which hematopoietic stem cells and lineage-committed progenitor cells acquire genetic and epigenetic alterations and provide aberrant, clonal hematopoiesis, sometimes resulted in the progression to acute myeloid leukemia (Elias HK et al, Oncogene 2014). We previously reported a rare case of which the patient developed Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph+ALL) 2.5 years after being diagnosed with MDS (Kohno T et al, Br J Haematol 1996). p190 BCR-ABL1 mRNA was detected in the Ph+ALL cells. Metaphase cytogenetics showed the karyotypes: 46, XY, 20q- in MDS phase and 46, XY, t(9;22)(q34;q11), 20q- in ALL phase, indicating that MDS and Ph+ALL in this patient were of the same clonal origin.

To uncover the detail of the clonal evolution, we analyzed bone marrow samples of MDS and Ph+ALL in this patient by targeted massively parallel sequencing with a panel of 154 genes including known driver genes of hematologic malignancies.

Methods:

Genomic DNAs (gDNAs) were extracted from the bone marrow mononuclear cells of MDS and Ph+ALL in this patient. Targeted sequencing was performed on the Illumina HiSeq2500 platform. Single nucleotide variants (SNVs) and small insertions and deletions (INDELs) were called using HaplotypeCaller of Genome Analysis Toolkit (GATK) version 3.4-46. We also attempted to detect the breakpoint of BCR-ABL1 translocation from the targeted sequencing data using the computational method, BreaKmer (Abo RP et al, Nucleic Acids Research 2015). The candidates of the mutations and structural variations were validated by amplicon-based deep sequencing and Sanger sequencing. Copy number variations were analyzed using Affymetrix CytoScan HD Array.

Results:

The mutations in ASXL1 and U2AF1 genes were identified in the MDS sample with variant allele frequencies (VAFs) of about 45%. At the progression of Ph+ALL, the mutations in SETBP1, SMC1A, and SLC5A8 genes were newly acquired while the ASXL1 and U2AF1 mutations were also identified with the same level of VAFs (about 50%) as the other mutations. VAFs of all of the mutations were decreased to about 20% after the chemotherapy for Ph+ALL, and then increased to about 40% at the recurrence of the disease. Furthermore, we identified the breakpoint of BCR-ABL1 translocation at intron 1 of ABL1 genes and intron 1 of BCR genes, that is the well-known cluster region, m-bcr, only among the samples of Ph+ALL. Copy number analysis confirmed that both MDS and Ph+ALL samples harbored the deletion of chromosome 20q. And the deletion of IKZF1 gene, which is frequently identified in Ph+ALL cases (Mullighan CG et al, Nature 2008), was not identified during the progression from MDS to Ph+ALL. These results demonstrated that the MDS clone harboring 20q- and ASXL1 and U2AF1 mutations acquired the mutations in SETBP1, SMC1A, and SLC5A8 genes and the p190 BCR-ABL1, resulted in the development of Ph+ALL in this patient.

Conclusion:

The alterations of SETBP1, SMC1A, and SLC5A8 genes are usually reported in myeloid malignancies (Makishima H et al, Nat Genet 2013, Kon A et al, Nat Genet 2013, Whitman SP et al, Blood 2008). Previous study in transgenic mouse demonstrated the distinct role of p190 BCR-ABL1 in the development of an ALL (Voncken JW et al, Blood 1995). Recapitulating this scenario, p190 BCR-ABL1 may play a critical role in the development of Ph+ALL from the MDS stem cells in this patient. This study may provide a new insight into the stem cell origin of MDS and the role of p190 BCR-ABL1 in the development of Ph+ALL.