Breakpoints in Philadelphia chromosome (Ph1)-positive leukemias

We studied chromosome 22 breakpoints in 24 Philadelphia (Ph1)-positive leukemias. Nineteen of 21 patients with chronic myelogenous leukemia (CML) possessed a chromosomal break within the 5.8 kilobase (kb) breakpoint cluster region (bcr). Furthermore, in one of three cases of acute lymphocytic leukemia (ALL), we found a chromosomal rearrangement in the bcr locus. No chromosomal rearrangements were found in two cases of CML and two cases of ALL using several restriction enzymes. The bcr rearrangement is highly specific for CML. Further analyses will be necessary to determine whether some cases without bcr rearrangement have another specific locus of rearrangement. Two of three cases of CML with blastic crisis had rearrangement of the immunoglobulin gene and T-cell receptor gene. One CML patient with blastic crisis, who achieved complete remission but relapsed thereafter, was found to have the same immunoglobulin gene rearrangement in the blastic crisis and relapse phases, suggesting that the same clone was involved in both crisis and relapse.     

Acute T-lymphocytic leukemia with Ph1 and 5q-chromosome abnormalities and rearrangements of bcr and TCR-delta genes

Almost all cases of Ph1-positive acute lymphocytic leukemia (ALL) have an immature B-cell phenotype and are CD10-positive. A very rare case of Ph1-positive ALL with T-cell features (T-ALL) is presented. Cytogenetic analyses revealed a clone with a Ph1 chromosome and 5q- at diagnosis, and mosaic clones with an additional complex abnormal karyotype at relapse. DNA analysis revealed rearrangement of the breakpoint cluster region (bcr) gene with deletion of the 5' side and of the T-cell receptor (TCR) delta gene, without any rearrangement of other immune-associated genes. From the results of immunophenotypic and genetic analyses, the origin of leukemic cells seemed to be an immature T-cell at a very early stage on T-cell ontogeny.     

Philadelphia chromosome-positive chronic myelogenous leukemia with deleted fusion of BCR and ABL genes

In the great majority of patients with chronic myelogenous leukemia (CML) the reciprocal translocation between chromosomes 9 and 22, t(9;22)(q34;q11), resulting in the Philadelphia (Ph) chromosome produces fusion DNA sequences consisting of the 5' part of the major breakpoint cluster region-1 (M-BCR-1) and the ABL protooncogene which encodes for the P210BCR-ABL phosphoprotein with tyrosine kinase activity implicated in the pathogenesis of CML. Molecular analysis was performed on 25 patients with Ph-positive CML using 2 breakpoint cluster region (bcr) probes within the M-BCR-1 DNA sequences, and two of them did not contain either detectable rearranged DNA homologous to the 5' side bcr probe or ABL-related fusion mRNA. The chromosomal in situ hybridization technique revealed that these two Ph-positive CML cases did not carry DNAs homologous to the 5' bcr or ABL probes on the Ph chromosome. Furthermore, one of the two Ph-positive CML cases did not show either rearranged DNA or regions homologous to the 3' bcr probe on a 9q+ chromosome, while the other CML case showed a rearrangement detected by the 3' bcr probe and transposition of the 3' bcr homologous to the 9q+ chromosome. Thus, the possibility is raised that the BCR/ABL fusion DNA has been deleted in rare CML cases, and that the deletion possibly occurred in a stepwise manner following the formation of the Ph chromosome at any stage of the disease.     

Leukemia cell lines: in vitro models for the study of Philadelphia chromosome-positive leukemia

The Philadelphia (Ph) chromosome, the main product of the (9;22)(q34;q11) translocation, is the cytogenetic hallmark of chronic myeloid leukemia (CML), a clonal myeloproliferative disorder of the hematopoietic stem cell; the Ph chromosome is also found in a sizeable portion of acute lymphoblastic leukemia (ALL) patients and in a small number of acute myeloid leukemia (AML) cases. At the molecular level, the t(9;22) leads to the fusion of the BCR gene (on chromosome 22) to the ABL gene (translocated from chromosome 9); this fusion gene BCR-ABL with its elevated tyrosine kinase activity must to be central to the pathogenesis of these disorders. Three different breakpoint cluster regions are discerned within the BCR gene on chromosome 22: M-bcr, m-bcr, and mu-bcr. Ph + leukemia cell lines are important tools in this research area. More than 20 ALL-and more than 40 CML-derived Ph + leukemia cell lines have been described. Furthermore, three Ph + B-lymphoblastoid cell lines, established from patients with Ph + ALL or CML, are available. Molecular analysis has documented BCR-ABL fusion genes in three apparently Ph chromosome-negative cell lines, all three derived from CML. Nearly all Ph + ALL cell lines have the m-bcr e1-a2 fusion gene (only two ALL cell lines have a b3-a2 fusion) whereas all CML cell lines, but one carry the M-bcr b2-a2, b3-a2 or both hybrids. The mu-bcr e19-a2 has been detected in one CML cell line. Four cell lines display a three-way translocation involving chromosomes 9, 22 and a third chromosome. Additional Ph chromosomes (up to five) have been found in four Ph + ALL cell lines and in 18 CML cell lines; though in some cell lines the extra Ph chromosome(s) might be caused by the polyploidy (tri- and tetraploidy) of the cells. Another modus to acquire additional copies of the BCR-ABL fusion gene is the formation of tandem repeats of the BCR-ABL hybrid as seen in CML cell line K-562. Both mechanisms, selective multiplication of the der(22) chromosome and tandem replication of the fusion gene BCR-ABL, presumably lead to enhanced levels of the fusion protein and its tyrosine kinase activity (genetic dosage effect). The availability of a panel of Ph + cell lines as highly informative leukemia models offers the unique opportunity to analyze the pathobiology of these malignancies and the role of the Ph chromosome in leukemogenesis.     

Cytogenetic study of a new Ph1-positive cell line (NALM-1).

The chromosomes of a cell line (NALM-1) derived from the leukocytes of a patient with chronic myelocytic leukemia (CML) were examined with several banding techniques. The modal chromosome number was 46 and the cells contained a Philadelphia chromosome (Ph1), due to the standard translocation of the missing segment of the long arm of chromosome No. 22 onto the distal end of the long arm of chromosome No. 9, i.e., t(9;22) (q34;q11). The Ph1-positive modal cells of the NALM-1 line also had two common marker chromosomes, an extra X-chromosome, and missing chromosomes in groups No. 7, 9, and 15. Immunologic examination of the NALM-1 cells revealed them to have non-T-non-B (null) surface characteristics. An antigen specific for cells of acute leukemia and a human la-like antigen were detected. These facts suggested that the NALM-1 cell line originated from CML cells and maintained the cytogenetic and Immunologic characteristics of such cells.     

Characterization of the translocation breakpoint in a patient with Philadelphia positive, bcr negative acute lymphoblastic leukaemia

Approximately 5% of children and 10-20% of adults with acute lymphoblastic leukaemia (ALL) have a chromosome translocation t(9;22) which at the cytogenetic level appears identical to that in chronic myeloid leukaemia (CML). The t(9;22) translocation was first recognised in CML patients by its 22q- or Philadelphia (Ph) chromosome. While all Ph positive CML patients so far described have a chromosome 22 breakpoint within the breakpoint cluster region (bcr) located in the 3' part of the phl gene, only some Ph positive ALL patients have breakpoints in bcr. We have cloned the breakpoint of the 9q+ chromosome from the DNA of a Ph positive ALL patient in whom there is no breakpoint in the bcr. The non-chromosome 9 sequences of the breakpoint region are shown to be derived from chromosome 22. The breakpoint in chromosome 22 is shown to be the first intron of the phl gene about 66kb upstream of the bcr. Using probes from this intron, rearrangements were detected in the DNA of two out of twelve additional Ph positive, bcr negative ALL patients.     

Philadelphia chromosome-positive acute lymphoblastic leukemia cell lines without classical breakpoint cluster region rearrangement

The Philadelphia (Ph) chromosome translocation which is classically observed in chronic myeloid leukemia (CML) is sporadically found in acute lymphoblastic leukemia (ALL). In CML the translocation breakpoint on chromosome 22 is within the breakpoint cluster region, while in childhood ALL, no detectable change in breakpoint cluster region is routinely observed. In order to investigate the nature of this difference, we have established and characterized two cell lines from a child with Ph positive ALL. The cell lines have retained the cytochemical staining pattern, enzyme activity, monoclonal antibody profile, and immunoglobulin gene rearrangements of the child's malignant cells. The cell lines had the same Ph translocation t(9;22) (q34;q11) as the child's malignant cells along with additional chromosome changes. Southern blot analysis showed that the Ph translocation did not involve the 5.8-kilobase breakpoint cluster region segment characteristically seen in CML. The cell lines reported here will be a valuable resource in ascertaining the biological significance of the Ph translocation seen in ALL.     

Clinical evaluation of a DNA probe assay for the Philadelphia (Ph1) translocation in chronic myelogenous leukemia

We report the clinical evaluation of an improved DNA probe assay for the characteristic genetic marker of human CML, observed by cytogenetics and designated the Philadelphia chromosome (Ph1). The Ph1 chromosome results from the fusion of c-abl proto-oncogene sequences from chromosome 9 to phl gene sequence on chromosome 22. (The phl gene is often referred to as bcr. However, for clarity we prefer to reserve the designation "bcr" for the region within the phl gene in which translocation breakpoints have been found to occur. We also find it useful to distinguish between two such regions in phl, bcr-210 and bcr-190, named after the 210- and 190-kDa phl/abl fusion proteins resulting from translocations with breakpoints in the respective regions. We refer to the corresponding chromosomal translocations as Ph1(bcr-210) and Ph1(bcr-190).) DNA, extracted from peripheral blood (PB) or bone marrow (BM) and digested with restriction endonuclease BglII, is hybridized with a probe (phl/bcr-3) spanning a breakpoint cluster region within phl. Rearrangements are revealed by the presence of one or two novel junction fragments. Clinical specimens from leukemic patients with active disease were compared by cytogenetic and DNA probe analysis at seven centers in the United States and Europe. The probe assay identified the phl rearrangement in 190 of 191 cases of Ph1-positive CML, as well as in 12 of 27 clinically diagnosed CML specimens lacking a typical Ph1 chromosome. DNA rearrangements also were seen in two of six cases of Ph1-positive ALL. No false positive results were obtained among 93 non-leukemic controls. Mixing experiments showed that the DNA probe assay can detect as few as 1% leukemic cells in a specimen. A preliminary study of CML patients in remission after allogeneic BM transplantation revealed a small fraction of residual Ph1-positive leukemic cells in a significant number of such patients.     

No correlation between locations of bcr breakpoints and clinical states in Ph1-positive CML patients

The majority of patients with chronic myelogenous leukemia (CML) have a characteristic reciprocal translocation between chromosome 9 and 22, resulting in the Philadelphia (Ph1) chromosome. During this translocation, the c-abl oncogene on chromosome 9 is transferred to the Ph1 chromosome and linked to a breakpoint cluster region (bcr), which is part of a large bcr gene. This phenomenon results in the formation of a bcr-c-abl fusion gene, which is transcribed into an 8.5 kb chimeric mRNA encoding a 210 kd bcr-c-abl fusion protein. The fusion protein has tyrosine kinase activity implicated in the pathogenesis of CML. The breakpoint near the c-abl locus on chromosome 9 can occur within a large area. In contrast, the breakpoints on chromosome 22 cluster within the bcr region of 5.8 kb. A chronic phase lasts for an average of 2 to 3 years; and, subsequently, most patients enter blast crisis. In the present study, we examined 15 Ph1-positive CML patients (eight in chronic phase, one in accelerated phase, and six in blast crises) as to whether the identifiable difference in the locations of the bcr breakpoints exist between CML patients in chronic phase and those in blast crisis. In seven of eight CML patients in chronic phase, in one in accelerated phase, and in four out of six CML patients in blast crisis, the bcr breakpoints clustered in the 3' portion of the bcr. Thus, we could not find out the correlation between the locations of the bcr breakpoints and the clinical stage of the CML patients. This might imply that blastic transformation in Ph1-positive CML was caused by other mechanisms than the transition of the bcr breakpoints.     

Molecular cloning of a 5' segment of the genomic phl gene defines a new breakpoint cluster region (bcr2) in Philadelphia-positive acute leukemias

Molecular rearrangements of Ph1 chromosome, the hallmark of CML, are clustered in a 5.8-kb DNA segment, the so-called breakpoint cluster region (bcr) of the phl gene that is localized to chromosome 22q11. In Ph1-positive (Ph1+) ALLs, the rearrangements have been shown to involve either the 5.8-kb bcr (called bcr+) or a region upstream of bcr in the 5' end of the phl gene (bcr-). To gain insight into the rearrangements occurring in Ph1+ acute leukemias, a 64-kb DNA fragment from the 5' end of phl was analyzed in order to generate molecular probes covering 40 kb of the phl gene first intron. A panel of seven cases of bcr-Ph1+ acute leukemia (three nonlymphocytic and four lymphocytic) was investigated with these intron 1-derived probes. Strikingly, in six of the seven leukemias, the breakpoints were located in a 10.8-kb DNA segment, defining a new bcr which appears to be specific for Ph1+ acute leukemias. By analogy with the CML bcr region located in the 3' part of the phl gene, we propose to designate this 10.8-kb fragment bcr2.     

P190-type bcr/abl expressed in myeloid colonies in a patient with Ph1-positive acute lymphoblastic leukemia.

In Philadelphia chromosome (Ph1)-positive acute lymphoblastic leukemia (ALL), some cytogenetic studies have suggested clonal derivation from a multipotential stem cell. The role of the product of the chimeric gene, P190, is not, however, well understood. We examined the expression of P190-type bcr/abl in single hematopoietic colonies obtained at various clinical stages of a patient with Ph1-positive ALL, using the polymerase chain reaction (PCR). Seven out of 58 colonies examined expressed P190-type bcr/abl. Five out of seven colonies were granulocyte/macrophage (GM) colonies and two were erythroid colonies. The cell lineages of these colonies were confirmed by testing for the expressions of the myeloperoxidase (MPO) gene in the GM colonies and the beta-globin gene in the erythroid colonies. These results suggest transformation of multipotential stem cell in this patient and confirm that expression of the P190-type bcr/abl fusion gene permits stem cell differentiation leading to Ph1-positive ALL.     

Molecular construction of Philadelphia chromosome and its relation to the clinical features

In 1960, Nowell and Hungerford found, for the first time, a minute chromosome at the metaphase in chronic myelocytic leukemia (CML) cells, which was called Philadelphia chromosome (Ph1 chromosome) later. Ph1 chromosome was considered to be specific for the disease and was frequently used as an important marker for the definite diagnosis. However, in mid-1970s, some cases with acute lymphocytic leukemia (ALL) were also found to have Ph1 chromosome in the leukemic cells. Therefore, Ph1 chromosome seemed to be non-specific for the diagnosis of CML. In 1980s, molecular-biology techniques were applied in the fields of leukemia research. As a result, clear differences were demonstrated between the two diseases (CML and ALL with Ph1 chromosome, respectively) at the molecular level using oncogene concept. In this review, molecular-genetic constructions of ABL, BCR and BCR-ABL hybrid genes in CML, as well as m-BCR-ABL hybrid gene in Ph1 positive ALL are focused in detail. Relationship between these molecular-genetic changes with the clinical features and the mechanism of cell growth in these cells with BCR-ABL or m-BCR-ABL hybrid genes are also discussed.     

bcr-abl oncogene activation in Philadelphia chromosome-positive acute lymphoblastic leukemia

Tumor-specific alterations in oncogenes are thought to play a central role in the development of cancer. An example is the consistent fusion of the bcr gene to the c-abl oncogene on the Ph chromosome in CML. The Ph chromosome can also be observed in ALL. About 50% of Ph+ ALL cases, in contrast to CML, do not exhibit chromosomal breakpoints in the major cluster region or mcr (Ph+ mcr- ALL). These cases may have a novel bcr-abl fusion gene instead. We tested this hypothesis in eight Ph+ mcr- ALL patients by amplifying the putative hybrid part of the bcr-abl cDNA, using the polymerase chain reaction method. All cases examined showed the same joining of the first exon of the bcr gene to the c-abl oncogene. Thus, the novel bcr-abl fusion in Ph+ mcr- ALL is the result of a molecularly distinct Ph chromosome. This allows the definition of Ph+ leukemias by their respective bcr-abl oncogene activation. Moreover, the cDNA amplification method we use is a clinically useful tool to screen for bcr-abl oncogene activations in leukemia patients.