Location of the BCR/ABL Fusion Genes on Both Chromosomes 9 in Ph Negative Young CML Patients: An Indian Experience

Abstract

The BCR/ABL gene rearrangement is cytogenetically visualized in most chronic myeloid leukemia (CML) cases. About 5-10 % of CML patients lack its cytogenetic evidence, however, shows BCR/ABL fusion by molecular methods. We describe two CML patients with Philadelphia (Ph) negative (-ve) and BCR/ABL positive by fluorescence in situ hybridization (FISH). Both the cases were in chronic phase at diagnosis. Conventional cytogenetics and different FISH assays were adopted using BCR/ABL probes. Home-brew FISH assay using bacterial artificial clone (BAC) for BAC-CTA/bk 299D3 for chromosomal region 22q13.31-q13.32 was performed in case 1. Both the patients were Ph-ve. In first case, dual color dual fusion (DCDF)-FISH studies revealed 1 Red (R) 2 Green (G) 1 Fusion (F) signal pattern in 80 % of cells indicating BCR/ABL fusion signals on chromosomes 9 instead of Ph and 2G2F signal pattern in 20 % of cells indicating two BCR/ABL fusions on both chromosomes 9q34 on presentation. In second case, FISH studies revealed the 1R1G1F signal pattern indicating BCR/ABL fusion signals on chromosomes 9 instead of Ph in 100 % of cells at presentation. During follow-up, both the patients exhibited 2G2F signal pattern indicating two BCR/ABL fusions on both chromosomes 9q34, which indicated a clonal evolution in 100 % cells. Both the patients did not achieve therapeutic response. Relocation of BCR/ABL fusion sequence on sites other than 22q11 represents a rare type of variant Ph, the present study highlights the hot spots involved in CML pathogenesis and signifies their implications in Ph-ve BCR/ABL positive CML. This study demonstrated the genetic heterogeneity of this subgroup of CML and strongly emphasized the role of metaphase FISH, especially in Ph-ve CML cases, as it detects variations of the classical t(9;22).

Cryptic BCR-ABL fusion gene as variant rearrangement in chronic myeloid leukemia: molecular cytogenetic characterization and influence on TKIs therapy

“…The duplication of rearrangement appeared as 2 fusion genes on both the homologous chromosomes 9, like in 2 other cases previously reported [10]. Similarly to our case, the 2 patients did not achieve CgR.…”

“…During imatinib treatment, a not responder patient showed duplication of the BCR/ABL rearrangement, confirming that this change is an event of clonal evolution. The duplication of rearrangement appeared as 2 fusion genes on both the homologous chromosomes 9, like in 2 other cases previously reported [ 10 ]. Similarly to our case, the 2 patients did not achieve CgR.…”

“…In Ph-neg CML cases, the BCR/ABL fusion gene is often located on der(22) and rarely on der(9), indeed its presence on der(9) has been reported only in 24 Ph-neg CML patients [ 4 – 10 ]. Conversely, in our series 4/6 (66,7%) Ph-neg CML cases showed the rearrangement on der(9) and one displayed 2 fusion signals on both derivative chromosomes 9 and 22, as in the classic translocation.…”

“…The outcome of 2 patients has not been clearly reported. Two patients achieved at least Major Cytogenetic Response (CgR) after imatinib treatment [ 9 , 11 – 12 ], meanwhile 5 patients failed the treatment, suggesting that they were more resistant to imatinib therapy [ 5 , 10 ]. Among the latter, a recent report [ 10 ] described two cases with fusion genes on both the homologous chromosomes 9 (Table 1 ).…”

Normal ABL1 is a tumor suppressor and therapeutic target in human and mouse leukemias expressing oncogenic ABL1 kinases

“…[1][2][3] This possibility is supported by previous observations that loss of normal ABL1 expression resulting from interstitial deletion in the normal chromosome 9 [del(9q34)] and/or transcriptional silencing of the alternative ABL1 promoter within BCR-ABL1 translocation occurs during progression of CML-CP to CML-BP. 12,13 Of note, in the absence of ABL1, BCR-ABL1 cells displayed reduced sensitivity to tyrosine kinase inhibitors (TKIs) such as imatinib. 14 Therefore, we hypothesized that normal ABL1 is a tumor suppressor in CML-CP and therapeutic target in leukemias induced by oncogenic forms of ABL1 kinase.…”