Clinical significance of reverse BCR/ABL gene rearrangement in Ph-negative chronic myelogenous leukemia.

Chronic myelogenous leukemia (CML) is a clonal myeloproliferative disorder resulting from a neoplastic transformation of primitive hematopoietic stem cells of the bone marrow [1]. CML is a multistage disease and is generally asymptomatic in the beginning but progresses through an accelerated phase to blast crisis within 3–5 years and is ultimately fatal [2]. Based on clinical and morphological features, CML, has evolved into a distinct clinical entity from other types of leukemias and myeloproliferative disorders [3]. The genetic basis responsible for the transformation to CML is poorly understood. The presence of the Ph-chromosome in a hyper-cellular bone marrow without increased blasts [5,6] has been associated with the disease [4]. During the past 150 years, this disease has fascinated many. The landmark discovery of the Ph-chromosome in 1960 and its involvement with the long arms of chromosomes 9 and 22 resulting in a balanced translocation [t(9:22)] has provided a novel tool for investigating the chromosomal basis of other hematological malignancies [7]. A decade later the Ph-chromosome was found in a number of non-CML malignancies; this finding intrigued us all. Consequently, this aberration became the subject of molecular scrutiny and debate [8]. During t(9;22) translocation, the most distal band (q34) on chromosome 9, containing the c-abl protooncogene, is translocated onto the 5.8 kb major breakpoint cluster region (M-bcr) on chromosome 22 band q11.2, forming a fusion BCR/ABL gene [9]. The breakpoint on chromosome 9 occurs in a large region of about 200 kb between exon 1b and exon 2 of the c-abl proto-oncogene. The BCR gene on chromosome 22 is 70 kb in length and the breakpoints within the M-bcr generally occur such that either exon b2 or exons b2 and b3 are included in the fusion gene. The two different breakpoints are referred to as b2-a2 and b3-a2 [10]. The frequency of b2-a2 and b3-a2 fusions is equally common in CML, [11]. Nevertheless, a novel 8.5 kb mRNA is transcribed from these two genes producing an aberrant fusion protein of 210 kD [p210] that is phosphorylated on tyrosine and has increased tyrosine kinase activity compared to the normal c-ABL. The BCR-ABL gene has a fundamental role in the pathogenesis of CML [12]. Certain breakpoints within the BCR gene appear to correlate with specific leukemias. The p, p and p fusion proteins are associated with acute lymphoblastic leukemia, CML, and chronic neutrophilic leukemia (CNL), respectively. A classification based on molecular markers may lead to an improved diagnosis of disease for a better therapeutic intervention [13–28]. About 5% of CML patients in whom the Ph-chromosome is not discernible by routine banding techniques are termed Ph-negative [29]. In one-third of such patients, the BCR/ABL gene rearrangement now can be detected by a number of molecular techniques [30–33]. Abbre6iations: CML, chronic myelogenous leukemia; FISH, fluorescence in situ hybridization; M-bcr, major breakpoint cluster region; PCR, polymerase chain reaction.. * Corresponding author. Tel.: +1-718-9636100; fax: +1-7189636759.