High Density SNP Array Analysis of Tyrosine Kinase Inhibitor (TKI) Resistant Chronic Myeloid Leukemia (CML) Shows Secondary Genomic Alterations

Despite the success story of tyrosine kinase inhibitors (TKIs) for the treatment of Chronic Myeloid Leukemia (CML), patients can develop resistances against the drugs. The main known causes for resistance are mutations or over-expression of the BCR/ABL fusion protein, reduced bioavailability of the drugs and activation of compensatory molecular pathways. It is hypothesized that during disease progression, genomic instability of CML cells increases, which may lead to new genomic lesions harboring additional mechanisms of resistance. In this context, we studied genomic DNA profiles of 32 Imatinib resistant CML patients with high density 250K SNP arrays (Affymetrix). Molecular allelokaryotyping for allele specific copy number and loss of heterozygosity analysis was performed with the CNAG software. Single DNA samples from 27 patients were extracted after they had acquired resistance to Imatinib or alternative TKIs such as Nilotinib or Dasatinib. DNA from 12 patients could be analyzed in sequential samples from the initial diagnosis timepoint and a second timepoint upon the emergence of TKI resistance. All patients were positive for BCR/ABL by PCR and FISH. 10 relapse patient samples had known BCR/ABL mutations of which two were T315I mutations. High density allelokaryotyping confirmed pre-existent data on unbalanced translocations, amplifications and deletions from routine cytogenetics: 5 samples displayed a genomic duplication of the BCR/ABL fusion gene, 4 samples had trisomy 8, 1 sample showed deletion of chromosome 17p, 1 sample had heterozygous deletion of chromosome 9. Apart from this, SNP array analysis revealed numerous new submicroscopic genomic lesions. After exclusion of genomic copy number polymorphisms (CNPs) by comparison to recorded CNPs in the UCSC Genome Browser (http://genome.ucsc.edu/) the following results were obtained: Two patients displayed common heterozygous microdeletions of the reciprocal ABL/BCR fusion product. Furthermore, single samples displayed heterozygous micro-deletions on chromosomes 1, 2, 10, 12, 15, 17, and 22 or microduplications on chromosomes 2,3,6, 8, 9, 11, 12, 14, 15, 22. The affected regions contained potentially interesting genes in respect to resistance to therapy such as tumor suppressor candidate MBP-1, apoptosis related protein RERE, metastasis associated gene MTA3, nuclear body associated gene SP100, alpha-T-catenin (CTNNA3), Cbl-interacting protein Sts-1 and the DNA repair associated gene RAD51. As a new genomic alteration in CML, we detected acquired uniparental disomy (UPD) in 5 samples with a common site of UPD on chromosome 19q in 2 patients. In conclusion, in 14 out of 39 TKI resistant cases, high density SNP arrays enabled us to identify submicroscopic copy number lesions and regions of UPD containing promising candidate genes, which merit further research as sites conferring TKI resistance.