Isodicentric Philadelphia chromosomes in imatinib mesylate (Gleevec)-resistant patients

Genomic amplification of BCR-ABL1 fusion gene and its impact on the disease progression mechanism in patients with chronic myelogenous leukemia

Identification of BCR-ABL1 fusion gene amplification status is critically important in the effective management of chronic myelogenous leukemia (CML) patients. Earlier reports suggested that overexpression of BCR-ABL1 either through amplification of BCR-ABL1 fusion gene or by the up regulation of BCR-ABL1 transcript level might be an early phenomenon in the establishment of IM resistance and disease evolution in CML. In the current study, we performed dual color dual fusion locus specific BCR/ABL1 FISH analysis along with karyotype analysis using GTG banding (G-banding using trypsin and Giemsa) technique in 489 patients with different clinical stages of CML at diagnosis or during the course of the disease to unravel the spectrum of BCR-ABL1 fusion gene amplification status. Among the study group analyzed, it was found that prevalence of occurrence of BCR-ABL1 fusion gene amplification was significantly higher in advanced stages of disease and in IM resistant CML-CP patients when compared to initial stage of disease, de novo CML-CP. Cytogenetic and metaphase FISH characterization on our study samples revealed that BCR-ABL1 fusion gene amplification was occurred through the formation of extra copies Ph chromosomes and isoderived Ph chromosomes. Current study suggests that unrestrained activity of BCR-ABL1 played a vital role in resistance to targeted therapy and disease evolution in CML. In our study population, patients in progressive stage CML and in IM resistant CP with multiple copies of BCR-ABL1 fusion gene displayed a poor response to targeted treatment with IM. Hence, the early identification of BCR-ABL1 fusion gene amplification using FISH technique will lead to improved interventions and outcome in future CML patients.

Co-existence of isodicentric Ph chromosomes and the three-way Ph chromosome variant t(3;9;22)(p21;q34;q11) in a rare case of chronic myeloid leukemia

More than 90% of patients with chronic myeloid leukemia (CML) have the chromosomal translocation t(9;22)(q34;q11), while 5-8% of patients have complex variant translocations that have previously been thought not to affect the efficacy of imatinib therapy. The present study reports a patient with CML in B-lymphoid blast crisis who had a rare three-way Philadelphia (Ph) variant t(3;9;22)(p21;q34;q11), in addition to isodicentric Ph chromosomes. The patient was initially treated with imatinib for >2 months with a very poor response. When no T315I or F317L mutations in the ABL proto-oncogene 1 region were detected, the patient received dasatinib treatment (140 mg daily) and achieved a complete hematologic response. Following allo-hematopoietic stem cell transplantation, the patient displayed clinical, hematological and cytogenetic remission, with complete molecular response and complete donor chimerism, and stopped taking dasatinib at the last follow-up. The present data suggest that BCR-ABL gene amplification may be associated with imatinib resistance, which can be overcome with dasatinib. The present analysis suggests an alternative therapy strategy for CML involving isodicentric Ph chromosomes.

Isodicentric Philadelphia chromosome: an uncommon chromosomal abnormality in the chronic phase of chronic myeloid leukemia (CML)

An isodicentric Philadelphia chromosome is an uncommon finding previously described as a secondary chromosomal abnormality in accelerated- or blast-phase of chronic myeloid leukemia (CML) with resistance to imatinib mesylate or dasatinib. Here, we present a case with idic(Ph) chromosome identified at initial diagnosis in a patient with chronic-phase CML.

Multiple copies of BCR-ABL fusion gene on two isodicentric Philadelphia chromosomes in an imatinib mesylate-resistant chronic myeloid leukemia patient

The so-called Philadelphia (Ph) chromosome is present in more than 90% of chronic myeloid leukemia (CML) cases. Amplification or duplication of the BCR-ABL gene has been found to be one of the key factors leading to drug resistance to imatinib mesylate (IM). In the present study, we identified the presence of isodicentric Ph chromosomes [idic(Ph)] in an IM-resistant patient. Fluorescence in situ hybridization (FISH) analysis on metaphase chromosomes confirmed the heterogeneity and amplification of the fused BCR-ABL gene. FISH analysis superimposed on G-banding confirmed the presence of idic(Ph) chromosomes. Reverse transcription-polymerase chain reaction (RT-PCR) products revealed the presence of the BCR-ABL fusion transcript b3a2. The idic(Ph) chromosomes in CML were shown to be fused at the satellite regions of the short arms. The patient did not respond to IM chemotherapy and did not achieve remission. In this study, the impact of the idic(Ph) chromosomes on genomic instability, heterogeneity and amplification of the BCR-ABL gene in IM-resistant patients is discussed.

Genomic amplification of BCR/ABL1 and a region downstream of ABL1 in chronic myeloid leukaemia: a FISH mapping study of CML patients and cell lines

Background

Chronic myeloid leukaemia (CML) is characterized by the expression of the BCR/ABL1 fusion gene, a constitutively activated tyrosine kinase that commonly results from the formation of the Philadelphia (Ph) chromosome after a t(9;22)(q34;q11) or variant rearrangement. The duplication of the Ph chromosome is a recurring abnormality acquired during disease progression, whereas intrachromosomal amplification of BCR/ABL1 is a rare phenomenon and has been associated with imatinib therapy resistance. Archival bone marrow chromosome suspensions from 19 CML patients known to carry more than 1 copy of BCR/ABL1 and 10 CML cell lines were analyzed by fluorescent in situ hybridization with a panel of probes from 9q34.1-qter to investigate whether they carried two identical copies of the Ph chromosome or, instead, one or both Ph contained cryptic imbalances of some regions.

Results

A duplication of the entire Ph chromosome with no further events involving the derivative 22 was found in 12 patients. In contrast, a sideline with either 1 or 2 isochromosomes of the Ph chromosome was identified in 6 patients but none of the cell lines. In one of the patients a translocation between the distal end of one arm of the isoderivative chromosome 22 and a third chromosome was revealed. 2 patients were found to carry marker structures harbouring high copy number gains of BCR/ABL1 fusion along with a variable part of 9q34 region downstream of ABL1 breakpoint, similarly to the markers present in the imatinib resistant cell line K562. We identified the following regions of amplification: 9q34.1 → q34.2 and 9q34.1 → qter, with a common minimum amplified region of 682 Kb. One of the patients had 5 BCR/ABL1 positive clones with variable level of 9q34 amplifications on a variety of structures, from an isoderivative 22 to tandem duplications.

Conclusions

These data confirm that the intrachromosomal genomic amplification of BCR/ABL1 that occurs in some CML patients during disease progression also involves amplification of 9q34 gene-rich sequences downstream of ABL1 breakpoint. The variety of rearrangements identified in this relatively small cohort demonstrates that the Ph chromosome is not a stable structure but prone to further rearrangements during disease progression.