Hematological Oncology Hematol Oncol 2006; 24: 86–88 Published online 6 April 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hon.775 Case Report Generation of the BCR/ABL fusion gene in a Philadelphia chromosome-negative chronic myeloid leukaemia: insertion of 5.6 Mb of 9q34 into the BCR region of chromosome 22 y Laura Valle 1 *, Victoria Ferna ´ndez 2 , Concepcio ´n Pe ´rez-Pons 3 , Fe ´lix Garcı ´a Sa ´nchez 4 , Javier Benı ´tez 2 and Miguel Urioste 1 1 Familial Cancer Unit, Human Cancer Genetics Program, Centro Nacional de Investigaciones Oncolo´gicas (CNIO), Madrid, Spain 2 Human Genetics Group, Human Cancer Genetics Program, Centro Nacional de Investigaciones Oncolo´gicas (CNIO), Madrid, Spain 3 Hematology Service, Severo Ochoa Hospital, Madrid, Spain 4 Unit of Histocompatibility and Molecular Biology. Centro de Transfusio´n de Madrid, Spain Abstract This report describes a chronic myelogenous leukaemia patient with an apparently nor- mal bone marrow karyotype but BCR/ABL fusion-gene-positive. Commercial FISH probes showed an atypical pattern and the BCR/ABL fusion transcript was detected by RT-PCR, but not the reciprocal ABL/BCR. Consecutive FISH assays clarified the mechanism of the masked Ph. The ABL gene and the following 5.6–5.7 Mb of 9q are inserted into the BCR region of chromosome 22. There is no transference of 22q material to chromosome 9 or to any other chromosomes. Clinical features and evolution of the patient are similar to those cases with classic Ph chromosome. Copyright # 2006 John Wiley & Sons, Ltd. Keywords: Chronic Myeloid Leukaemia; CML; BCR/ABL; Masked Ph Introduction Chronic Myeloid Leukaemia (CML) is characterized by the occurrence of the Philadelphia (Ph) chromosome, origi- nated from the reciprocal translocation t(9;22)(q34;q11), which results in the chimerical fusion gene BCR/ABL [1]. In 5–10% of CML cases, there is a variant Ph translocation with, generally, a third or more chromosomes involved with chromosomes 9 and 22 [2]. In rare cases, a cryptic rearrangement is present in a karyotype interpreted as nor- mal by conventional cytogenetics, however, FISH and RT- PCR studies reveal the formation of BCR/ABL [3]. In most cases, these ‘masked Ph’ cases are caused by complex translocations [4,5]. In this report we describe a CML case of Ph chromo- some negative but BCR/ABL fusion gene positive detected by FISH with commercial probe. We have clarified the mechanism explaining the BCR/ABL generation by means of FISH and RT-PCR methods. Case report A bone marrow sample belonging to a 74-year-old man was referred to cytogenetic analysis due to leukocytosis and thrombocytosis. Examination revealed hepatosplenome- galy. His white blood cell count was 260  10 9 /L with a dif- ferential of 3% myeloblasts, 5% promyelocytes, 17% myelocytes, 23% metamyelocytes, 5% eosinophils, 40% neutrophils, 2% basophils, 3% monocytes, and 1% lympho- cytes. The patient’s haemoglobin level was 10.7 g/dL, and his platelet count was 589  10 9 /L. The bone marrow examination showed findings typical of chronic myelopro- liferative syndrome, CML. Abdominal ultrasound showed 160 mm splenomegaly, and cardiomegaly was detected by thoracic Rx. The karyotype on G-banded metaphases at diagnosis was apparently normal, with a 300 band resolution level. FISH with ‘BCR/ABL Two Colour, Two Fusion Translocation Detection Probe’ (Cancer Genetics, River Vale, NJ) showed an atypical two red, one green, one fusion pattern in 80% among 300 analysed cells. Metaphases revealed that the fusion was on chromosome 22, which corresponds to BCR/ABL, while fusion signal corresponding to ABL/BCR on chromosome 9 was not present. Only the red signal cor- responding to the ABL gene and surrounding regions, appeared in both chromosomes 9 (Figure 1). FISH with ‘LSI BCR/ABL Dual Colour, Single Fusion Translocation Probe’ (Vysis, Downers Grove, IL.), showed the classic Ph pattern. The patient was treated with Imatinib at a well-tolerated 300 mg/day dose and after 7 days, at a 400 mg/day dose. Five months later, the dose was again reduced to 300 mg/ day, due to the apparition of a pruriginous and descamative exanthema. Twenty-six months after the diagnosis, the patient’s peripheral blood and bone marrow show remission criteria, which is confirmed by quantitative RT-PCR and FISH. Copyright ß 2006 John Wiley & Sons, Ltd. *Correspondence to: Laura Valle, Familial Cancer Unit, Centro Nacional de Investigaciones Oncolo´gicas (CNIO), Melchor Ferna´ndez Almagro, 3, 28029, Madrid, Spain. E-mail: lvalle@cnio.es y No potential conflicts of interest. Received: 21 December 2005 Revised: 8 February 2006 Accepted: 17 February 2006