The spectrum of molecular alterations in the evolution of chronic myelocytic leukemia

Expression of p210 bcr/abl increases hematopoietic progenitor cell radiosensitivity

PURPOSE

The cytogenetic finding of the Ph1+ chromosome and its molecular biologic marker bcr/abl gene rearrangement in cells from patients with chronic myeloid leukemia are associated with a proliferative advantage of the Ph1+ clone in vivo. Although the transition to the acute terminal phase or blastic crisis is often associated with additional cytogenetic abnormalities, the molecular events which correlate the initial cytogenetic lesion with the terminal phase are poorly understood. Defective cellular DNA repair capacity is often associated with chromosomal instability, increased mutation frequency, and biologic alterations.

METHODS AND MATERIALS

We, therefore, tested whether the protein product of the bcr/abl translocation (p210) could alter DNA repair after gamma-irradiation of murine cell lines expressing the bcr/abl cDNA.

RESULTS

The 32D cl 3 parent, 32D cl 3 pYN (containing the control vector plasmid) and each of two sources of 32D cl 3 cells expressing p210 bcr/abl cDNA (32D-PC1 cell line and 32D-LG7 subclone) showed a D0 of 1.62, 1.57, 1.16, and 1.27 Gy, respectively. Thus, expression of the p210 bcr/abl product induced a significant (p < 0.05) increase in radiosensitivity at the clinically relevant radiation therapy dose-rate (1.16 Gy/min). The increased radiosensitivity of p210 bcr/abl expressing cells persisted if cells were held before plating in a density-inhibited state for 8 hr after gamma-irradiation, indicating little effect on the repair of potentially lethal gamma-irradiation damage. The IL-3 dependent parent 32D cl 3 cells demonstrated programmed cell death in the absence of growth factor or following gamma-irradiation to 200 cGy. Expression of bcr/abl cDNA in the 32D-PC1 and 32D-LG7 sub clones abrogated IL-3 requirement of these cell lines and inhibited gamma-irradiation induced programmed cell death.

CONCLUSION

These data suggest a role for bcr/abl p210 in amplifying gamma-irradiation DNA damage or broadly inhibiting DNA repair, conditions that may stimulate further cytogenetic alterations in hematopoietic cells.

Occurrence of point mutations in p53 gene is not increased in patients with acute myeloid leukaemia carrying an activating N-ras mutation

The frequency of simultaneously detecting N-ras and p53 gene mutations was studied in leukaemia cells of patients with acute myeloid leukaemia (AML) or with myelodysplastic syndrome (MDS). Using in vitro DNA amplification followed by oligonucleotide hybridization analysis, 45 AML and six MDS patients were screened for activating mutations in codons 12, 13 and 61 of N-ras. Ten of them (eight AML and two MDS) were found positive. These 10 patients and 10 others without activating N-ras mutation were further analysed by direct sequencing of the amplified exons for p53 mutations and for atypical N-ras mutations. Beside the activating mutations in the N-ras gene, no additional transforming or nontransforming mutations could be detected in the N-ras. However, exon 7 of p53 was mutated in two AML patients without activating N-ras mutation. These data show that p53 mutations occurred with half the frequency of N-ras mutations in AML and that no positive correlation could be found between the onset of mutations in N-ras and p53 genes.

Analysis of the p53 gene in patients with isochromosome 17q and Ph1-positive or -negative myeloid leukemia

Increased incidence of p53 gene aberrations or chromosome 17p monosomy resulting from an isochromosome 17q [i(17q)] has been observed with transition of chronic myelogenous leukemia (CML) to myeloid blast crisis (BC), and in some patients with poor risk acute myeloid leukemia (AML) progressing from myelodysplastic syndrome (MDS). These data suggested that disease progression may be linked to bi-allelic inactivation of p53. Here, we report on p53 gene analyses of nine patients with CML-BC and AML who showed an i(17q) as characteristic cytogenetic anomaly. Using Southern blots, agarose gel electrophoresis and single-strand conformation polymorphism analyses of PCR products from genomic DNA and cDNA, spanning exons 4 through 9, we did not detect any structural abnormalities of the remaining p53 allele. These findings question the hypothesis that p53 gene alterations are the principal molecular event responsible for progression of CML chronic phase or MDS to i(17q)-positive CML-BC or AML, respectively.

Phenotypic and molecular analysis of Ph1-chromosome-positive acute lymphoblastic leukemia cell lines

We have established 2 Philadelphia chromosome (Ph1)-positive acute lymphoblastic leukemia (ALL) cell lines, designated PALL-1 and PALL-2, from distinct adult Ph1-positive ALL patients. PALL-1 was established in nude mice, and PALL-2 was established in culture. Both retained the Ph1 chromosome and expressed the ALL type bcr/abl chimeric mRNA containing the junction of the first exon of BCR gene (e1) and second exon of c-abl gene (a2). PALL-1 and PALL-2 expressed CD34 surface antigen which is characteristic of early hematopoietic progenitor cells. PALL-2 expressed antigens for both pre-B and early myeloid cells and had rearrangements of both the heavy chain of immunoglobulin gene and the beta chain of T-cell-receptor gene. Both PALL-1 and PALL-2 expressed detectable levels of p53 gene RNA. Polymerase-chain-reaction-single-strand conformation polymorphism (PCR-SSCP) analysis of the p53 gene showed a normal pattern of mobility in both cell lines. Taken together, the 2 cell lines had features of Ph1-positive ALL: (i) hematopoietic progenitor cells with pre-B-cell phenotype and, (ii) activation of e1-a2 type bcr/abl oncogene without alterations of p53 gene. These unique lines should provide a valuable tool for studying the pathogenesis of Ph1-positive ALL.

p53 gene rearrangements in chronic myelocytic leukemia

We performed Southern-blot analysis of the p53 gene in 41 consecutive patients with typical chronic myelocytic leukemia (CML). In two of them, we were able to study cells during both the chronic and the accelerated phases. Only one of the 29 chronic-phase samples had rearrangement of the p53 gene, whereas three of the nine accelerated-phase samples and one of the five patients in blast crisis exhibited rearrangements. Gene deletion was observed in two patients, one in accelerated phase and the other in blast crisis. One patient with a nonrearranged p53 gene in chronic phase showed rearrangement after progression to the accelerated phase. On the other hand, one patient in accelerated phase exhibited rearrangements which disappeared after reversion to chronic phase with successful treatment. Our findings support the opinion that alterations of the p53 gene may play an important role in CML evolution.

Structural organization of BCR-ABL gene in chronic phase and blast transformation in chronic myeloid leukemia patients

We studied 36 DNA samples of 18 patients affected with chronic myeloid leukemia (CML) for the presence of mutations in the first exon of the BCR gene was divided into four regions amplified by polymerase chain reaction (PCR). By single strand conformation polymorphism analysis (SSCP) and direct sequencing of amplified fragments, we found different banding profiles in 9 out of 18 patients in the PCR fragment spanning nucleotide 506-826. In one patient, sequence analysis revealed the presence of a point mutation at nucleotide 669 (A-T; Gln-Leu). No difference was found between DNA samples collected during the chronic phase and the blastic transformation. No different mobility shifts of single stranded PCR products were found in the other amplified fragments. The activation of BCR-ABL involves direct interaction between BCR first exon sequences and the tyrosine kinase regulatory domains of ABL. In the first BCR exon, and around the mutated sequences two SH-2-binding sites, are retained. These domains are essential for BCR-ABL-mediated transformation. Our results demonstrate the presence of point mutation in this regulatory region, which may suggest a role for the altered BCR sequence in activation of the BCR-ABL oncogene.

Loss of constitutional heterozygosity in human astrocytomas

Inactivation of tumour suppressor genes or anti-oncogenes as well as activation of dominant acting oncogenes seem to be important mechanisms in the pathogenesis of gliomas. We compared constitutional and tumoural genotypes at different restriction fragment length polymorphism loci (RFLP) on chromosomes 10 and 17 in 15 unrelated individuals. Loss of heterozygosity (LOH) pointing to chromosomal loss or deletions was detected for at least one chromosome 17 marker in 11 gliomas (astrocytomas grades I-III and glioblastoma multiforme), whereas LOH for chromosome 10 loci was only detected in 3 out of 9 cases of glioblastoma multiforme and was not detected in low grade gliomas. Since LOH for chromosome 10 loci seems to be restricted only to glioblastoma multiforme, it is possible that recessive mutations on chromosome 10 are engaged in tumour progression from astrocytomas to glioblastoma multiforme. As LOH of chromosome 17 markers occurs in astrocytomas as in glioblastoma multiforme, chromosome 17 loci probably are involved in early tumour development.

Rare occurrence of P53 gene mutations in multiple myeloma

We looked for mutations of exons 5-8 of the P53 gene in bone marrow cell from 37 cases of multiple myeloma, using polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) analysis and DNA sequencing. 25 patients also had cytogenetic analysis. A point mutation, leading to an amino acid change in the P53 protein was found in only one case, involving exon 5. These findings suggest that P53 mutations are very rare in multiple myeloma, and that this disease may be categorized among the few neoplasms where P53 abnormalities have very limited role, if any.

Molecular mechanisms in the evolution of chronic myelocytic leukemia

Chronic myelocytic or Ph1-positive acute lymphoblastic leukemias have been analyzed for alterations in a variety of proto-oncogenes and anti-oncogenes implicated in the progression of chronic myeloid leukemia (CML) from its chronic phase to blast crisis. The most frequent genetic change found in disease evolution is an alteration of the p53 gene involving a point mutation, a rearrangement or a deletion. These gene changes are common in myeloid and undifferentiated variants of blast crisis but are usually undetectable in lymphoid leukemic transformants. Other molecular changes also occur in the clonal evolution of CML. The retinoblastoma-susceptibility (Rb) gene is an anti-oncogene. Structural abnormalities of Rb are frequent in all types of human acute leukemia, but are particularly common in Ph1-positive leukemia of lymphoid phenotype including both Ph1-positive ALL and lymphoid blast crisis of CML. Changes in Rb occur early in the transition to blast crisis with loss of Rb protein being the common factor. Mutations in the N-RAS gene also occur, but are rare in typical blast crisis. They are sometimes seen in Ph1-negative myeloid blast crisis. Since changes in the p53 gene are generally associated with progression of disease of a myeloid phenotype and changes in the Rb gene occur more often with a lymphoid phenotype, a particular molecular alteration may influence the character of disease evolution in CML.

Mutations of the p53 tumour suppressor gene in haematologic neoplasms

Mutations of the p53 tumour suppressor gene have frequently been observed in several types of solid tumours and are believed to be implicated in the development of these tumours. To determine the relevance of p53 mutations in haematologic neoplasms, we performed polymerase chain reaction-single strand conformation polymorphism analysis on the p53 gene in 45 patients with various types of haematologic neoplasms. In exons 5-8 containing highly conserved regions, mobility shifts indicating sequence alterations were detected in four of the 45 patients, and subsequent sequencing was performed. A point mutation resulting in a novel stop codon was detected at codon 213 in one of 23 cases of chronic myelogenous leukaemia (one of five cases of blast crisis). Point mutations causing amino acid substitutions were detected in one of four cases of myelodysplastic syndrome at codon 195, one of three cases of adult T-cell leukaemia at codon 281, and one of eight cases of acute lymphoblastic leukaemia at codon 281, and these missense mutations were accompanied by loss of the wild type allele. Patients harbouring these nonsense and missense mutations were in advanced disease stages. These findings suggest that mutational inactivation of the p53 gene is infrequent but is involved in the tumorigenesis of several types of haematologic neoplasms at least in some cases.

TP53 tumor suppressor gene: a model for investigating human mutagenesis

More than 350 independent point mutations of the TP53 gene, found in a wide variety of human cancers, were compiled and analysed. From this study, we confirm the presence of four hot-spot regions which colocalize with some highly conserved domains of the protein. We also define a new hot-spot region which is observed predominantly in lung tumors. Analysis of the mutational events suggests the direct involvement of environmental carcinogens in lung tumors and hepatocarcinomas, and spontaneous mutagenesis generating essentially CpG transitions in most of the remaining ones. Furthermore, we demonstrate in this work that the TP53 gene is an informative model with which to study the molecular mechanisms of mutagenesis in the human genome.