BCR/ABL Fusion Gene On Constitutional Der(14;22) Roberstsonian Translocation in a Case of Chronic Myeloid Leukemia with Masked Philadelphia Chromosome

Abstract

4822 Chronic myeloid leukemia (CML) is characterized by the Philadelphia chromosome (Ph) observed in more than 90% of patients with CML as a result of t(9;22)(q34;q11), leading to the formation of the BCR/ABL chimeric gene. The remaining 5–10% of CML cases exhibit a variant Ph translocation generally involving a third or even a fourth chromosome in addition to chromosome 9 and 22, potentially leading to masked Ph chromosome or reveal cryptic translocations that remains undetected under conventional cytogenetic analysis. These chromosome rearrangements can be disclosed by means of fluorescence in situhybridization (FISH) or polymerase chain reaction (PCR) procedures. A very few Ph positive CML cases were reported with constitutional robertsonian translocations, i.e. translocation between two acrocentric chromomosomes (13–15, 21–22), with breakpoints in the short arms, leading to a dicentric chromosome and thus to 45 instead of 46 chromosomes Case Report. 42 year-old woman presenting with asthenia. Physical examination: Grade 1 splenomegaly. Peripheral blood count showed: hemoglobin concentration 117g/L, platelet count: 329×109/L and white blood cell count (WBC): 199×109/L. Peripheral blood smear: myelemia exhibiting 3% of myeloid blasts. Cytogenetic analysis by G-banding performed on bone marrow metaphase cells afforded the following karyotype: 45, XX, der(14;22)(q10;q10)c?, t(9;22;11)(q34;q11;q13) [20]. The analysis of the BCR-ABLfusion gene according to standard protocols detected the presence of the b3a2 isoform. FISH studies using dual color dual fusion probes in metaphases showed a 1F2G2R signal pattern. We detect a normal ABL signal on chromosome 9 and BCR signal on chromosome 22; the fusion signal was present on the der(14;22);extra-signals BCR and ABL with reduced intensities were present on der(11) and der(9) respectively: ish der(9)(ABLdim+), der(11)(BCRdim+), der(14;22)(BCR+,ABL+) [10]. FISH analysis on interphase nuclei (n=200) presented the same signal pattern. Nuc ish (ABL, BCRx3)(BCR con ABL x1) [200]. Chromosome analysis of bone marrow cells after six months of Imatinib therapy showed the following karyotype: 45, XX, der(14;22)(q10;q10)c [20] thus demonstrating complete cytogenetic remission and that der(14;22) is a robertsonian constitutional abnormality that could be inherited and thus necessitate a familial genetic councelling to inform about the familial risk of congenital malformations and miscarriage. Discussion. To explain the formation of variant chromosome Ph translocations one-step, two-step and multi-step mechanisms have been proposed. In our case complex translocations involving four chromosomes and the participation of two acrocentric chromosomes, led to the hypothesis of the presence of a constitutional or acquired Robertsonian translocation. Karyotype analysis six months after treatment confirmed the presence of a constitutional Robertsonian translocation. According to the FISH pattern, this variant Ph chromosome was formed in one step. The occurrence of Philadelphia positive CML in a patient with a constitutional Robertsonian translocation is probably coincidental. The role of constitutional chromosomes abnormalities in hematologic malignancies is well known in Down syndrome patients and in chromosome breakage syndromes such as Fanconi anemia. In the literature, only one case of CML patients with Robertsonian t(14;22) have been described. To our knowledge this is the first report showing a Robertsonian t(14;22) in a variant Ph involving four chromosomes and exhibiting the fusion FISH signal in a derivative chromosome 14, with masked Ph. Disclosures: No relevant conflicts of interest to declare.