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

Mechanisms and novel approaches in overriding tyrosine kinase inhibitor resistance in chronic myeloid leukemia

Chronic myeloid leukemia is a stem cell-initiated but progenitor-driven disease induced by the BCR-ABL oncogene. Tyrosine kinase inhibitors (TKIs) were introduced in the late 1990s and have revolutionized the management of chronic myeloid leukemia in chronic phase. The majority of patients can now expect to live a normal life as long as they continue to comply with TKI treatment. However, in a significant proportion of cases TKI resistance develops over time, requiring a switch of therapy. The most frequent mechanism for drug resistance is the development of kinase domain mutations that reduce or completely ablate drug efficacy. Fortunately, the last 10 years have seen an impressive array of new drugs, some modeled on the mechanism of action of imatinib, others employing more novel approaches, for these patients.

Myeloid neoplasms with isolated isochromosome 17q represent a clinicopathologic entity associated with myelodysplastic/myeloproliferative features, a high risk of leukemic transformation, and wild-type TP53

BACKGROUND

Isolated isochromosome (17q) is a rare cytogenetic abnormality in Philadelphia chromosome-negative myeloid neoplasms, usually myelodysplastic and/or myeloproliferative neoplasms (MDS/MPN). De novo acute myeloid leukemia (AML) with isochromosome 17q has rarely been reported. The frequency of genetic mutations is unknown.

METHODS

The authors assessed clinicopathologic, immunophenotypic, and molecular genetic features of 22 myeloid neoplasms with isolated isochromosome 17q.

RESULTS

Fourteen patients presented as MDS/MPN; 8 as de novo AML. Most presented with leukocytosis, anemia, thrombocytopenia, and splenomegaly. Morphologically, all showed myelodysplastic and myeloproliferative features, including pseudo-Pelger-Huet-like neutrophils, micromegakaryocytic hyperplasia, hypercellularity, fibrosis, and osteosclerosis. Blasts were increased (median, 40% in de novo AML; 9% in MDS/MPN). Immunohistochemical assessment of proliferation and apoptosis rates in AML were similar to a matched group without isochromosome 17q. In most patients, isochromosome 17q occurred at time of blast transformation or disease progression. DNA sequencing revealed no mutation in the uninvolved TP53 allele. Mutational analyses showed rare mutations in NRAS (3 of 10), FLT3 (2 of 16), and JAK2 (1 of 18), and no mutations in NPM1 (0 of 15), KIT (0 of 4), and CEBPA (0 of 4). The median overall survival was 14.5 months for de novo AML, and 11.0 months for MDS/MPN. With a median follow-up of 8.5 months (range, 1.5-107 months), 15 died of disease, 6 had persistent disease, and 1 was in remission after bone marrow transplantation.

CONCLUSIONS

The authors conclude that myeloid neoplasms with isolated isochromosome 17q represent a distinct clinicopathologic entity with myelodysplastic and myeloproliferative features, high risk of leukemic transformation, and wild-type TP53.

The biology of CML blast crisis

Chronic myelogenous leukemia (CML) evolves from a chronic phase characterized by the Philadelphia chromosome as the sole genetic abnormality into blast crisis, which is often associated with additional chromosomal and molecular secondary changes. Although the pathogenic effects of most CML blast crisis secondary changes are still poorly understood, ample evidence suggests that the phenotype of CML blast crisis cells (enhanced proliferation and survival, differentiation arrest) depends on cooperation of BCR/ABL with genes dysregulated during disease progression. Most genetic abnormalities of CML blast crisis have a direct or indirect effect on p53 or Rb (or both) gene activity, which are primarily required for cell proliferation and survival, but not differentiation. Thus, the differentiation arrest of CML blast crisis cells is a secondary consequence of these abnormalities or is caused by dysregulation of differentiation-regulatory genes (ie, C/EBPalpha). Validation of the critical role of certain secondary changes (ie, loss of p53 or C/EBPalpha function) in murine models of CML blast crisis and in in vitro assays of BCR/ABL transformation of human hematopoietic progenitors might lead to the development of novel therapies based on targeting BCR/ABL and inhibiting or restoring the gene activity gained or lost during disease progression (ie, p53 or C/EBPalpha).

Molecular cytogenetics characterization of a novel translocation involving chromosomes 17 and 19 in a Ph+ adult acute lymphoblastic leukaemia

We report a case of positive Philadelphia chromosome adult acute lymphoblastic leukaemia with a novel unbalanced translocation t(17;19), leading to trisomy of 17q21-qter. The patient did not obtain complete haematological response and died a few months after diagnosis. The significance of the 17q21-qter trisomy, resulting from this novel translocation, and its possible role in the progression of the leukaemia is discussed.

17q21-qter trisomy is an indicator of poor prognosis in acute myelogenous leukemia

A reciprocal translocation (9;11) is often found in acute myeloid leukemia (AML), mostly of the M5a type. We report a case of a child with AML, in whom t(9;11) was observed at diagnosis as the sole structural abnormality, together with trisomies 19 and 21. The diagnosis was AML evolving from a myelodysplastic syndrome (MDS), and the blast morphology was undifferentiated. Chemotherapy failed to induce morphological remission and the patient's condition soon worsened. A subclone appeared and expanded during the course of the disease, with an additional unbalanced translocation (1;17) leading to trisomy of the long arm of chromosome 17 (17q). The data available from the literature on acquired anomalies involving 17q and our observation led us to postulate a specific link between the gain of 17q and complete chemoresistance.

Isolated isochromosome 17q: a distinct type of mixed myeloproliferative disorder/myelodysplastic syndrome with an aggressive clinical course

A clinicopathologic study was performed on 15 patients with haematological malignancies in which isochromosome 17q [i(17q)] was the sole structural chromosome abnormality identified in bone marrow. The data indicated that an isolated i(17q) is associated with a distinct type of mixed chronic myeloproliferative/myelodysplastic disorder with an aggressive clinical course. The patients ranged in age from 37 to 83 years (median 60) with a M:F ratio of 3:1. All cases were chronic myeloid disorders with mixed proliferative and dysplastic features, making classification difficult. 11 patients tested for BCR/ABL gene fusion were normal. A low bone marrow blast count (<5%) at presentation was a typical finding. All cases had severe myeloid dysplasia which included non-segmented neutrophils and an increase in the monocyte/macrophage lineage. Fluorescence in situ hybridization (FISH) analysis of one case showed the i(17q) to involve all myeloid lineages, but not the lymphocytes. For cases with complete follow-up (n = 11) the median survival was 2.5 years (range 0.83-5.25) and 64% progressed to AML prior to death. The following features were identified which defined the haematological disorder associated with an isolated i(17q): (1) adult patient, (2) chronic myeloid disorder with clonal involvement of all myeloid lineages, (3) mixed chronic myeloproliferative/myelodysplastic features, (4) severe hyposegmentation of neutrophil nuclei, (5) prominence of the monocyte/macrophage lineage, (6) high risk for progression to AML, and (7) median survival of 2.5 years.

Isochromosome 17q in Blast Crisis of Chronic Myeloid Leukemia and in Other Hematologic Malignancies Is the Result of Clustered Breakpoints in 17p11 and Is Not Associated With Coding TP53 Mutations

An isochromosome of the long arm of chromosome 17, i(17q), is the most frequent genetic abnormality observed during the disease progression of Philadelphia chromosome–positive chronic myeloid leukemia (CML), and has been described as the sole anomaly in various other hematologic malignancies. The i(17q) hence plays a presumably important pathogenetic role both in leukemia development and progression. This notwithstanding, the molecular consequences of this abnormality have not been investigated in detail. We have analyzed 21 hematologic malignancies (8 CML in blast crisis, 8 myelodysplastic syndromes [MDS], 2 acute myeloid leukemias, 2 chronic lymphocytic leukemias, and 1 acute lymphoblastic leukemia) with i(17q) by fluorescence in situ hybridization (FISH). Using a yeast artificial chromosome (YAC) contig, derived from the short arm of chromosome 17, all cases were shown to have a breakpoint in 17p. In 12 cases, the breaks occurred within the Smith-Magenis Syndrome (SMS) common deletion region in 17p11, a gene-rich region which is genetically unstable. In 10 of these 12 cases, we were able to further map the breakpoints to specific markers localized within a single YAC clone. Six other cases showed breakpoints located proximally to the SMS common deletion region, but still within 17p11, and yet another case had a breakpoint distal to this region. Furthermore, using chromosome 17 centromere-specific probes, it could be shown that the majority of the i(17q) chromosomes (11 of 15 investigated cases) were dicentric, ie, they contained two centromeres, strongly suggesting that i(17q) is formed through an intrachromosomal recombination event, and also implicating that the i(17q), in a formal sense, should be designated idic(17)(p11). Because i(17q) formation results in loss of 17p material, potentially uncovering the effect of a tumor suppressor on the remaining 17p, the occurrence of TP53 mutations was studied in 17 cases by sequencing the entire coding region. In 16 cases, noTP53 mutations were found, whereas one MDS displayed a homozygous deletion of TP53. Thus, our data suggest that there is no association between i(17q) and coding TP53 mutations, and that another tumor suppressor gene(s), located in proximity of the SMS common deletion region, or in a more distal location, is of pathogenetic importance in i(17q)-associated leukemia.

P53 gene mutations in chronic myelogenous leukemia medullary and extramedullary blast crisis

Molecular alterations of the P53 gene were investigated in 27 unselected patients with chronic myelogenous leukemia (CML) blast crisis. A rearrangement of the P53 gene was evident by Southern blotting in 3 cases, one of which also showed the same alteration in the chronic phase. Single strand conformation polymorphism and sequencing analysis showed point mutations in 4 blast crisis cases. Of interest, P53 point mutations were evident in all the 3 cases of extramedullary blast crisis examined and the same point mutation was found in the myeloblastoma tissues and in the subsequent peripheral blast cells. These data indicate that: a) P53 gene mutations occur in a significant but not a large number of CML acute phase cases; b) P53 gene point mutations seem to correlate strongly with the infrequent extramedullary presentation of the blast crisis; c) the presence of the same P53 gene point mutation in extramedullary and bone marrow blast cells confirms the common clonal origin of the two cellular populations.

Increasing methylation of the calcitonin gene during disease progression in sequential samples from CML patients

In chronic myeloid leukaemia (CML), disease progression from the initial chronic phase to the acute phase or blast crisis has previously been shown to be correlated with progressive increases in hyper-methylation of the calcitonin gene, located at chromosome 11p15. However, sequential studies of individual patients were not performed in these investigations. We have analysed 44 samples from nine patients with typical Philadelphia chromosome positive CML throughout their disease progression to determine the methylation state of the calcitonin gene at these time points. Densitometry was used to quantitate the ratio of the normal 2.0 kb Hpa II fragments, indicating normal methylation status of the gene, compared to the intensity of the abnormal, hyper-methylated, 2.6-3.1 kb Hpa II fragments. We found a gradual increase in the ratio of methylated:unmethylated calcitonin gene during chronic phase with a dramatic rise at blast crisis. Further, the ratio of the abnormal hypermethylated 3.1 kb fragments to the methylated 2.6 kb fragment resulted in the identification of a clonal expansion of abnormally methylated cells. This expansion of cells with hypermethylation of the calcitonin gene during chronic phase was shown to coincide with the presence of a mutation in the p53 gene. The data presented in this study would suggest that an increased methylation status of the calcitonin gene during disease progression may indicate the expansion of abnormal blast cell populations and subsequent progression to blast crisis.

[Chronic myeloid leukemia, biological aspects]

Chronic myeloid leukemia (CML) is a clonal myeloproliferative disorder of a stem cell, involving myeloid, erythroid, megacaryocyte, lymphoid B-cells and "natural killer" cells. The hallmark of CML is the Philadelphia (Ph) chromosome which is a shortened chromosome 22 (22q-) resulting from a reciprocal translocation involving chromosome 9 and chromosome 22, designed t (9;22) (q34;q11). This translocation juxtaposes parts of two genes; ABL on chromosome 9 and BCR (breakpoint cluster region) on chromosome 22. Transcription of the BCR/ABL fusion gene results in an hybrid mRNA that is translated into a 210 kDa or 190 kDa protein, depending on the location of the breakpoint in the bcr region. This protein plays a key role in CML: its tyrosine-kinase activity, that differs from the normal ABL product, may be involved in leukemic cell growth. Nonetheless, the loss of the negative cell growth regulation by c-ABL, or BCR/ABL fusion protein interaction with other cellular genes (such as RAS or c-MYC) could also be involved in CML pathophysiology. A better understanding of the molecular mecanisms of CML could lead to specific treatment, such as tyrosine-kinase inhibitors, synthetic oligodeoxynucleotides, or site-specific DNA-binding proteins designed against BCR/ABL oncogenic fusion sequence.

A unique spectrum of p53 mutations in B-cell chronic lymphocytic leukemia distinct from that of other lymphoid malignancies

The spectrum and pattern of p53 mutations detected in 42 cases of B-cell chronic lymphocytic leukemia (B-CLL) were analyzed, and several interesting features were noted. Codon 209 in the p53 gene may be a new hot-spot for p53 mutation in B-CLL disease. Four of the 42 (10%) reported B-CLL p53 mutations occurred at codon 209 versus none in 214 cases of other lymphoid malignancies screened for p53 mutations (P = 0.0006). Transversion mutations predominated at codon 273 rather than the transition mutations that are known to occur at this CpG site. Four of six (67%) B-CLL cases had transversions at codon 273 compared with two of 17 (12%) of all other lymphoid tumors examined (P = 0.02). In addition, over 65% of the p53 mutations detected in B-CLL showed a strand bias for p53 mutations on the untranscribed DNA strand. This feature of DNA strand bias is notable in cancers of the lung, esophagus, and head and neck, which may result from high exposure to carcinogens. This spectrum of p53 mutations in B-CLL together with the high frequency of transversion mutations and DNA strand bias may implicate environmental carcinogens associated with p53 gene damage in some B-CLL patients.

Chromosome 17 abnormalities and inactivation of the p53 gene in chronic myeloid leukemia and their prognostic significance

We have reviewed all the relevant studies on the loss of the short arm of chromosome 17 (17p) and inactivation of the p53 gene in chronic myelogenous leukemia (CML) in an attempt to clarify their roles in the progression of CML. Loss of a 17p (hemizygous 17p) and p53 inactivation emerged as the disease progressed and were closely associated with each other. About half of the cases with loss of a 17p, however, did not show p53 inactivation. In these cases loss of a 17p preceded p53 inactivation, which suggested that either reduction of the p53 gene dosage or inactivation of another tumor-suppressor gene on 17p might contribute to the disease progression. Both loss of a 17p and p53 inactivation may serve as poor prognostic factors but the prognostic significance of the former only emerged when metaphase cells with loss of a 17p were dominant amongst the total cell population analyzed.

P53 gene mutations in lymphoid diseases and their possible relevance to drug resistance

Mutations in the p53 tumor suppressor gene occur with a frequency of 12.5% in lymphoid malignancies. The viral-associated diseases, Adult T-cell Leukemia (ATL) and Burkitt's lymphoma, showed higher p53 mutation frequencies of 24% and 41%, respectively. Mutations occurred in the highly conserved regions of the p53 gene. Two new hot spots for mutation were noted in exon 7 at codons 239 and 245. The spectrum of p53 mutations differs among different cancers. Transition mutations occurring in colon and brain tumors also predominated in the majority of the lymphoid malignancies. However, B-cell chronic lymphocytic leukemia (B-CLL) and non-Hodgkin's lymphoma (NHL) had an unusually high frequency of G to T transversions. Among carcinomas of the lung, liver, breast and esophagus there is also a high frequency of G to T transversions. The differences in mutation spectra between different lymphoid diseases may be due to differences in mutagenic factors or differences in the biological properties of the p53 protein in different lymphoid compartments. Mutation of the p53 gene is associated with advanced stage of lymphoid disease and poor prognosis. For B-CLL disease, p53 mutations are associated with drug resistance. Overexpression of the bcl-2 protein is also associated with a block in apoptosis. Resistance to apoptosis could be a general mechanism for drug resistance in B-CLL and other lymphoid diseases.

P53 tumor suppressor gene in chronic myelogenous leukemia: a sequential study

Loss of the p53 gene alleles was investigated in 26 patients with Ph+, BCR/ABL+ chronic myeloid leukemia (CML) by means of the polymerase chain reaction and restriction fragment length polymorphism (PCR-RFLP) analysis using the restriction enzyme AccII. In all cases, peripheral blood and/or bone marrow samples were obtained at different times during the chronic phase of the disease and at blast crisis, and in some of them also at the accelerated phase. Of the 12 cases considered informative, 11 evolved into myeloid type blast crisis and one into a lymphoid blast crisis, whereas only two showed an i(17q) chromosome at cytogenetic study. In four of the 12 informative cases, a loss of one p53 gene allele was observed, in all cases coincident with the development of the accelerated phase or blast crisis. One patient with a deleted p53 gene allele, in whom it was possible to analyze the gene structure in the three CML evolutive phases (chronic and accelerated phases and blast crisis), showed loss of the p53 gene allele in both the accelerated and the blastic phase, but not during the chronic phase. On the other hand, one of the two cases with an i(17q) chromosome exhibited one allelic deletion of the p53 gene. Thus, the relatively frequent monoallelic deletion of the p53 gene coincident with the appearance of the blast crisis registered in the present study would support a possible role of the p53 gene alterations in the evolution of CML to its final stages.

Double P53 point mutation in extramedullary blast crisis of chronic myelogenous leukemia

A patient with Philadelphia-positive chronic myelogenous leukemia (CML) evolved in extramedullary blast crisis, was studied for the presence of alterations of the P53 tumor suppressor gene in the different stages of disease progression. No P53 gene aberrations were detected during the chronic and accelerated phases. Two identical missense point mutations, involving codons 249 and 281 and leading to the amino acid substitutions Arg-Ser and Thy-Asp, were identified in cells of an extramedullary mass and then in peripheral blood blast crisis cells. The data indicate that the medullary and extramedullary blast cells belong to the same cellular clone. They also strongly suggest that in this case, the alteration of P53 gene is strictly related to the progression of the disease, although this mechanism is certainly neither the only nor the most frequent molecular event leading to the acute phase.

Isochromosomes in neoplasia

In order to ascertain the frequency and distribution of isochromosomes in neoplasia, we surveyed the cytogenetic data from 20,007 tumors with clonal chromosome aberrations reported in the literature. Tumor types for which at least 50 cases with acquired aberrations and 10 cases with isochromosomes had been reported were selected, yielding a total of 18,160 neoplasms. Of these, 1,792 cases (9.9%) displayed a total of 2,014 isochromosomes. The 9 most common isochromosomes (detected in at least 50 cases) were, in decreasing order of frequency, i(17q), i(8q), i(1q), i(12p), i(6p), i(7q), i(9q), i(5p), and i(21q). The frequency of isochromosomes varied among the different tumor types, with the highest incidence in germ cell neoplasms (60%) and the lowest in chronic myeloproliferative disorders (2.3%). Also, the spectrum of isochromosomes differed among the neoplasms. The most common isochromosomes in the different tumor types were i(11q), i(17q), and i(21q) in acute myeloid leukemia; i(9q), i(17q), and i(22q) in chronic myeloid leukemia; i(17q) in chronic myeloproliferative disorders; i(X)(q13), i(17q), and i(21q) in myelodysplastic syndromes; i(7q), i(9q), and i(17q) in acute lymphoblastic leukemia; i(1q), i(7q), i(8q), and i(17q) in chronic lymphoproliferative disorders; i(1q), i(6p), i(9p), i(17q), and i(21q) in Hodgkin's disease; i(1q), i(6p), and i(17q) in non-Hodgkin's lymphoma; i(1q), i(8q), and i(17q) in adenocarcinoma; i(1q), i(3q), i(5p), and i(8q) in squamous cell carcinoma; i(5p), i(8q), and i(11q) in transitional cell carcinoma; i(1q), i(7q), and i(17q) in Wilms' tumor; i(1q), i(12p), and i(17q) in germ cell neoplasms; i(1p), i(1q), i(6p), and i(17q) in sarcoma; i(5p), i(6p), i(7p), and i(21q) in mesothelioma; i(1q), i(6p), and i(17q) in malignant neurogenic neoplasms; i(1q), i(6p), and i(17q) in retinoblastoma; and i(1q), i(6p), and i(8q) in malignant melanoma.