New types of unusual and complex Philadelphia chromosome (Ph1) translocations in chronic myeloid leukemia

Cytogenetic and molecular genetic evolution of chronic myeloid leukemia

Chronic myeloid leukemia (CML) is genetically characterized by the presence of the reciprocal translocation t(9;22)(q34;q11), resulting in a BCR/ABL gene fusion on the derivative chromosome 22 called the Philadelphia (Ph) chromosome. In 2-10% of the cases, this chimeric gene is generated by variant rearrangements, involving 9q34, 22q11, and one or several other genomic regions. All chromosomes have been described as participating in these variants, but there is a marked breakpoint clustering to chromosome bands 1p36, 3p21, 5q13, 6p21, 9q22, 11q13, 12p13, 17p13, 17q21, 17q25, 19q13, 21q22, 22q12, and 22q13. Despite their genetically complex nature, available data indicate that variant rearrangements do not confer any specific phenotypic or prognostic impact as compared to CML with a standard Ph chromosome. In most instances, the t(9;22), or a variant thereof, is the sole chromosomal anomaly during the chronic phase (CP) of the disease, whereas additional genetic changes are demonstrable in 60-80% of cases in blast crisis (BC). The secondary chromosomal aberrations are clearly nonrandom, with the most common chromosomal abnormalities being +8 (34% of cases with additional changes), +Ph (30%), i(17q) (20%), +19 (13%), -Y (8% of males), +21 (7%), +17 (5%), and monosomy 7 (5%). We suggest that all these aberrations, occurring in >5% of CML with secondary changes, should be denoted major route abnormalities. Chromosome segments often involved in structural rearrangements include 1q, 3q21, 3q26, 7p, 9p, 11q23, 12p13, 13q11-14, 17p11, 17q10, 21q22, and 22q10. No clear-cut differences as regards type and prevalence of additional aberrations seem to exist between CML with standard t(9;22) and CML with variants, except for slightly lower frequencies of the most common changes in the latter group. The temporal order of the secondary changes varies, but the preferred pathway appears to start with i(17q), followed by +8 and +Ph, and then +19. Molecular genetic abnormalities preceding, or occurring during, BC include overexpression of the BCR/ABL transcript, upregulation of the EVI1 gene, increased telomerase activity, and mutations of the tumor suppressor genes RB1, TP53, and CDKN2A. The cytogenetic evolution patterns vary significantly in relation to treatment given during CP. For example, +8 is more common after busulfan than hydroxyurea therapy, and the secondary changes seen after interferon-alpha treatment or bone marrow transplantation are often unusual, seemingly random, and occasionally transient. Apart from the strong phenotypic impact of addition of acute myeloid leukemia/myelodysplasia-associated translocations and inversions, such as inv(3)(q21q26), t(3;21)(q26;q22), and t(15;17)(q22;q12-21), in CML BC, only a few significant differences between myeloid and lymphoid BC are discerned, with i(17q) and TP53 mutations being more common in myeloid BC and monosomy 7, hypodiploidy, and CDKN2A deletions being more frequent in lymphoid BC. The prognostic significance of the secondary genetic changes is not uniform, although abnormalities involving chromosome 17, e.g., i(17q), have repeatedly been shown to be ominous. However, the clinical impact of additional cytogenetic and molecular genetic aberrations is most likely modified by the treatment modalities used.

Chromosomal in situ hybridization and Southern blot analyses using c-abl, c-sis, or bcr probe in chronic myelogenous leukemia cells with variant Philadelphia translocations

The Philadelphia (Ph) chromosome is a cytogenetic hallmark of chronic myelogenous leukemia (CML). Whereas the majority of Ph-positive CML patients show the standard Ph translocation involving chromosomes 9 and 22, t(9;22)(q34;q11), the minority of cases exhibit a variant type of Ph translocation involving these two and other chromosomes (complex type) or those involving #22 and chromosomes other than #9 (simple type). To get an insight into the nature of variant Ph translocations and the process of their formation, we examined the localization of the c-abl and c-sis oncogenes and the breakpoint cluster region (bcr) gene by chromosomal in situ hybridization in ten variant Ph translocations of CML including five simple and five complex ones as initially interpreted. In situ hybridization showed that c-abl localized to band 9q34 and c-sis localized to band 22q12-q13 were translocated on the Ph and on one of the rearranged chromosomes other than #9, respectively, in all the variant translocations examined. On the other hand, bcr localized to band 22q11 was translocated on various chromosomes but mostly on chromosome 9. Parallel Southern blot analyses on DNA from leukemic cells of five patients including two with simple translocations and three with complex ones revealed rearrangements of bcr with breakpoints occurring mostly in a 5' portion of 5.8-kb BamHI/BglII sequences, which are quite similar to those detected so far in CML cases with the standard Ph translocation. The present findings strongly suggest that variant Ph translocations of CML are all complex, and some of them are formed stepwisely from the standard translocation.

Complex translocation involving chromosomes #1, #9, and #22 in a patient with chronic myelogenous leukemia

A case of Philadelphia chromosome positive chronic myelogenous leukemia with a complex translocation involving chromosomes #1, #9, and #22 is described. All cells in the bone marrow showed this rearrangement, and Q-banding analysis showed the predominant karyotype to be 46,XY, t(1;9;22)(p22;q34;q11).

The Philadelphia chromosome. Considerations based on studies of variant Ph translocations

The nature of the Philadelphia (Ph) translocation and the process of its formation were studied by attempting various chromosome banding analyses of variant Ph translocations among 210 patients with Ph-positive chronic myelocytic leukemia examined at the National Institute of Radiological Sciences, Chiba. The following assumptions could be drawn from the results of the analyses: 1) The involvement of specific regions of chromosomes #9 and #22, q34 and q11, respectively, is an indispensable condition of the Ph translocation. 2) The so-called variant Ph translocations are all complex and are derived from a standard Ph translocation. 3) The Ph translocations, both standard and complex ones, are not always stable. The complex translocations are subject to further chromosome evolution, as is the conversion of the standard translocation to complex translocations. There seems to be no fundamental difference between the standard and complex Ph translocations, with the latter being merely a more progressed form of the former. Analyses at the molecular level of the same cases employed in this study are yielding results that support the above assumptions.

Activation of the abl oncogene and its involvement in chromosomal translocations in human leukemia

Activation of the abl gene and its involvement in human leukemia is one of the most thoroughly characterized examples of the structural alterations of chromosomes associated with the conversion of a normal cell into a cancer cell. The abl oncogene as first identified on the Abelson murine leukemia virus (A-MuLV). Activation of the viral oncogene is associated, in part, with the truncation of the gene at its 5' end. As in studies with other retroviruses, results with A-MuLV presaged the mechanism of activation by abl in naturally occurring human malignancies. Thus, chronic myelogenous leukemia (CML) is consistently associated with a translocation of a piece of chromosome 9 onto chromosome 22 creating what is known as the Philadelphia chromosome (Ph1). The result of this translocation is the truncation of the 5' end of the cellular abl gene, which is located at the breakpoint of chromosome 9. The function of the abl gene product is poorly understood but is thought to participate in an, as yet, undefined pathway of growth control signals, which originate outside the cell, and traverse through the cell into its nucleus. The loss of the gene product's N-terminal amino acid sequences brought about by the truncation of the 5' portion of the gene is consistent with the hypothesis that the protein's growth-controlling activity is deregulated by the structural alterations which occur in the cancer cells. The abl gene and CML serve as a paradigm of the mechanism of activation of proto-oncogenes by chromosomal alterations. The case of CML and the Ph1 chromosome illustrates the findings we might expect as other chromosomal abnormalities are characterized at the molecular level.

Frequency of constitutional chromosome alterations in patients with hematologic neoplasias

From 1978 to 1985 cytogenetic studies were performed on 718 patients with different hematologic diseases. Nine (1.25%) had a constitutional chromosome alteration. One patient had trisomy 21, four had balanced translocations and four had sex chromosome anomalies. Although the frequency of constitutional alterations was twice that seen in the newborn population, an analysis of these data and also from the literature shows a random association between constitutional chromosome alterations and hematologic neoplasias, except for patients with Down's syndrome.

Genomic diversity of Philadelphia-positive chronic myelogenous leukemia

The incidence of breakpoints in CML patients with variant translocations was investigated. There was no relationship between the length of various chromosomes with breakpoint frequency. However, a significantly higher (p less than 0.05) incidence of breaks were seen on the long arms as compared to the short arms due mainly to the involvement of 9q and 22q in these translocations. Chromosome 17 showed a significantly (p less than 0.005) higher involvement in these translocations, however only when 9q34-qter was not cytogenetically involved. A total of 683 breaks were found in 225 cases. 362 of these were located at c-abl and c-sis, while 110 were at other oncogenetic sites. The prognostic and hematologic features of patients with variant translocations are not significantly different from those of CML cases with the typical 9q;22q translocation. Some of these complex translocation, where the breakpoints are correlated with oncogenetic sites, are further discussed in molecular terms.

High resolution banding of chronic myeloid leukemia chromosomes

Bone marrow aspirates from 29 patients with chronic myeloid leukemia were studied using the methotrexate synchronization culture method. Successful cytogenetic preparations exhibiting long and well banded chromosomes were obtained from all of them. The standard t(9;22) was seen in 23 patients, four had variant translocations, and two were Ph-negative. Of the four patients with variant translocations, one had a simple translocation in which the missing segment of chromosome #22 was translocated onto the short arm of chromosome #9. The remaining three patients had complex translocations. The first involved chromosomes #11, #19, and #22, the second involved chromosomes #9, #11, and #22, and the third involved chromosomes #9, #14, and #22. Karyotypic abnormalities in addition to the Ph chromosome were seen in four patients: in three these changes developed during the chronic phase and in one during the blastic phase. Using Q-, R-, and G-banding techniques, we found that the breakpoint on chromosome #22 is just below the centromere, namely in band 22q11.2 and on chromosome #9 in band 9q34.1. The standard translocation, therefore, can be written as t(9;22)(q34.1;q11.2). Furthermore, the breakpoint on 22q appeared to be identical in all cases with standard as well as the variant translocations. Our results show that the methotrexate synchronization method permits consistent high resolution banding of CML chromosomes, and support the concept that there is no difference in the amount of material translocated from 22q in different patients.

Unusual translocations involving chromosomes 12;22 and 9;12 in a case of chronic myelogenous leukemia

A case of chronic myelogenous leukemia (CML) with highly unusual translocations involving both chromosomes #12 is reported. The origin of the Ph1 chromosome was due to a 12p/22q translocation. Chromosome #9 was involved in a translocation with the other chromosome #12. By critical examination of the "size" of the Philadelphia chromosome, it was noted that the breakpoints on 22q were different when compared with a previous case (see Verma and Dosik [16]), although the short arm of chromosome #12 (12p) was involved in both instances. So far, no apparent differences in the course of the disease have been attributed to the types of translocation observed in these cases.

Translocations (4p+; 6q-) and (12q-; Xp+) in blastic phase of a Ph1-positive chronic myeloid leukemia

This paper reports a 28-year-old woman with Philadelphia chromosome (Ph1)-positive chronic myeloid leukemia (CML) who developed two marrow cell lines during the blastic phase: one with the translocation (12;X) (q 11;p22) and the other with the translocation (4;6) (p 16;q 25). The literature on involvement of chromosomes 12 and X in translocations and the appearance of 6 q- aberration in CML is summarized. The relationship between the 6 q-aberration and the blast cells with lymphoid appearance in the plastic phase of CML is discussed.

Is the chromosomal region 9q34 always involved in variants of the Ph1 translocation?

Six variants of the Ph1 translocation are described. The clinical diagnoses were chronic myeloid leukemia (CML) in 5 cases (patients 1-5) and acute lymphocytic leukemia (ALL) in patient 6. Three Ph1 variants were clear complex translocations, involving chromosomes #9, #22, and a third chromosome, i.e., #16, #11, or #14. The other three Ph1 variants appeared as "simple" translocations between chromosome #22 and chromosome #19, #4, or #12 when G- or Q-banding were used. When studied with high resolution R-banding, a small deletion of the terminal part of one chromosome #9 was visible, strongly suggesting that these variants were also complex translocations, i.e., t(9;19;22)(q34;p13;q11),t(4;9;22) (p16;q34;q11), and t(9;12;22)(q34;p13;q11). In the latter two cases, using in situ hybridization techniques, we demonstrated the presence of c-abl sequences on the Ph1 chromosome. This proved the involvement of 9q34 in these two variants. Our proposal is that most, and probably all, variants of Ph1 are complex translocations involving part of 9q34 and that the conjunction of a specific region of 22q11 with a specific segment of 9q34 (carrying the c-abl protooncogene) is essential for the development of Ph1 + CML.

"Masked" Ph1 chromosome in a complex three-way translocation

A patient with myelofibrosis was found to have a 46,XX,del(1)(q24),del(11)(p11),-22,+mar karyotype in unstimulated peripheral blood (PB) and spleen cells. On detailed cytogenetic examination it was determined that this patient had an apparently "masked" Ph1 chromosome contained in a complex three-way translocation. Since phytohemagglutinin (PHA)-stimulated PB and spleen cells were essentially normal, the masked Ph1 chromosome was assumed to be an acquired cytogenetic abnormality. The portion missing from the masked Ph1 chromosome was apparently translocated onto del(1). Thus, the detailed karyotype was 46,XX,t(1;11;22)(q24;p11;q11 or q12),t(1;22)(q24;q11 or q12). This complex rearrangement was present primarily in cells belonging to the granulocyte-macrophage cell lineage, whereas E-rosetting cells, and presumably T lymphocytes, had normal karyotypes.

Abnormalities of chromosome No. 17 in myeloproliferative disorders

In routine analyses, abnormalities of chromosome No. 17 were found in the bone marrow cells of 28 patients with Ph1-positive and three patients with Ph1-negative chronic myeloid leukemia (CML), 4 patients with acute nonlymphocytic leukemia (ANLL), and 4 patients with preleukemia. With three exceptions, all patients were in the blastic (CML) or the terminal phase. In 28 patients, the aberrant chromosome No. 17 arose by clonal evolution from the karyotype found at diagnosis or before the terminal phase. The abnormalities encountered were an isochromosome for the long arm, i(17q), (26 cases), translocations involving No. 17 (12 cases), trisomy 17 (three cases), and ring 17 (one case). In 35 patients, there was an unbalanced structural aberration of at least one of the No. 17 chromosomes. In every case (35/35), detailed analysis of the structurally abnormal No. 17 revealed loss of the distal part of the short arm (or possibly most of the short arm). Gain of the long arm (or at least its proximal part) was also common, but not invariably present (26/35). It is suggested that loss of 17p is a highly nonrandom event related to blastic crisis in CML and the terminal phase in other myeloid leukemias.

"Masked" philadelphia chromosome (Ph1) due to an unusual translocation

The case of a 58-year-old white female with Philadelphia (Ph1)-positive chronic myelocytic leukemia (CML) and a "masked" Ph1 chromosome is described. The Ph1 was due to a translocation involving chromosomes #12 and #22, in which most of the long arm of #12 had been translocated to the deleted #22 (Ph1). The necessity of performing banding analysis in those cases of CML where a Ph1 is not readily apparent is stressed and the literature on"masked" Ph1 chromosomes is briefly summarized.