Significance of chromosomal alterations and mutations of the N-RAS and TP53 genes in relation to leukemogenesis of acute myeloid leukemia

Rational Combinations of Targeted Agents in AML

Despite modest improvements in survival over the last several decades, the treatment of AML continues to present a formidable challenge. Most patients are elderly, and these individuals, as well as those with secondary, therapy-related, or relapsed/refractory AML, are particularly difficult to treat, owing to both aggressive disease biology and the high toxicity of current chemotherapeutic regimens. It has become increasingly apparent in recent years that coordinated interruption of cooperative survival signaling pathways in malignant cells is necessary for optimal therapeutic results. The modest efficacy of monotherapy with both cytotoxic and targeted agents in AML testifies to this. As the complex biology of AML continues to be elucidated, many “synthetic lethal” strategies involving rational combinations of targeted agents have been developed. Unfortunately, relatively few of these have been tested clinically, although there is growing interest in this area. In this article, the preclinical and, where available, clinical data on some of the most promising rational combinations of targeted agents in AML are summarized. While new molecules should continue to be combined with conventional genotoxic drugs of proven efficacy, there is perhaps a need to rethink traditional philosophies of clinical trial development and regulatory approval with a focus on mechanism-based, synergistic strategies.

Frequency of KRAS mutations in adult Korean patients with acute myeloid leukemia

Mutation of KRAS genes occurs with a frequency of 0.5-32 % in AML. In the present study, mutations of KRAS codon 12, 13, and 61 were detected by pyrosequencing and direct sequencing in AML. Seven KRAS mutations (7/123, 5.7 %) were detected. The most common mutation was a G-to-A transition in the second base of KRAS codon 13. No mutations were detected in KRAS codon 61. Combinations of KRAS and FLT3 mutation were not found in the same patient. There was no statistically significant difference between patients with KRAS mutations and patients with wild-type KRAS in terms of sex, age, CBC at diagnosis, CD34 positivity, MPO positivity, FLT3 mutation, karyotype, progression-free survival, and overall survival, although this may be attributable to the small sample size. To our knowledge, this is the first report of the detection of KRAS mutation in Asian AML patients using pyrosequencing and direct sequencing. These two methods showed identical efficiencies in their ability to detect KRAS mutations in 84 patients.

Frequency of RAS gene mutation and its cooperative genetic events in Southeast Asian adult acute myeloid leukemia

RAS gene as one of the most frequently mutated genes in acute myeloid leukemia (AML) has become an attractive target for molecular therapy. The role of oncogenic RAS and its associated genetic events in AML are not yet defined. We examined the frequency of RAS mutation in 239 Thai de novo adult AML patients using polymerase chain reaction-single-strand conformational polymorphism analysis. Thirty-five RAS mutations were found in 32 cases (13%) predominantly classified as M1/M2 (53%) followed by M4/M5 subtype (38%). Ten cases were positive for N-RAS codon 12, 11 cases for N-RAS codon 61, 13 cases for N-RAS codon 13, and one case for K-RAS codon 13. No mutation was found in K-RAS exon 2 or H-RAS. The most common base substitution was the G to A transition at codon 13. Most M1/M2 cases had mutations at codon 12 or 13, whereas M4/M5 cases preferentially affected codon 61. Half of the patients with RAS mutations had abnormal karyotypes with the majority involving chromosomes 21, 11 and 7. Four patients had core-binding factor leukemia and four additional patients had coexisting FLT3 or AML1 mutation. One patient had RAS, FLT3 and t(8;21) and the other had RAS, AML1 point mutation and del(9q). In conclusion, mutation of RAS gene was not as common in the Thais as in the western population. Several additional genetic abnormalities occurred in RAS-mutated patients. Future molecular-targeting approaches should take into account the multiple genetic events that coexist with RAS mutations in AML patients.

A pilot study of high-throughput, sequence-based mutational profiling of primary human acute myeloid leukemia cell genomes

In this pilot study, we used primary human acute myeloid leukemia (AML) cell genomes as templates for exonic PCR amplification, followed by high-throughput resequencing, analyzing approximately 7 million base pairs of DNA from 140 AML samples and 48 controls. We identified six previously described, and seven previously undescribed sequence changes that may be relevant for AML pathogenesis. Because the sequencing templates were generated from primary AML cells, the technique favors the detection of mutations from the most dominant clones within the tumor cell mixture. This strategy represents a viable approach for the detection of potentially relevant, nonrandom mutations in primary human cancer cell genomes.

Molecular, cytogenetic and genetic abnormalities in MDS and secondary AML

Myelodysplasia (MDS) is a clonal disease, which increases with age, suggesting that multiple steps are required for the evolution of the condition. Approximately 30% of MDS evolve into acute myelogenous leukemia (AML). In this review, we intend to delineate the genetic events, which may drive this sequence and therefore we will focus primarily on cytogenetic abnormalities where the genes have been identified and oncogenes and suppressor genes that have been implicated. In terms of the biological mechanisms, which characterise this process, it is generally thought that the MDS cell has impaired differentiation, and has increased apoptosis. As the disease progresses in addition, the cells have increased proliferation. As the disease evolves, the population of cells, which predominate remain immature, have decreased apoptosis and in many cases, upregulate anti-apoptotic genes and have deregulated proliferation as the number of blast cells increase. Etiological factors, which contribute to the development of leukemia, include therapeutic agents administered for a primary malignancy. The cytogenetic abnormalities, predisposition factors and genes involved in secondary leukemia will also be reviewed.

Mutations in the p53 gene in acute myeloid leukemia patients correlate with poor prognosis

Inactivation of tumor suppressor genes, whose products exert an inhibitory influence on cell cycle progression, can lead to neoplastic transformation. In acute myeloid leukemia (AML), the frequency of p53 gene mutations ranges from 4 to 15% in populations from USA and Europe. In an attempt to investigate the frequency of point mutations in the p53 gene in AML Brazilian patients, DNA samples of 35 patients were studied using PCR-SSCP techniques, screening exons 4-10. Mutations were identified in bone marrow DNA in 5 of the 35 AML patients (14.3%), a frequency similar to those reported for Northern American and European populations. The overall survival of patients with mutations in the p53 gene was significantly shorter than for patients without mutations.

Mutations with loss of heterozygosity of p53 are common in therapy-related myelodysplasia and acute myeloid leukemia after exposure to alkylating agents and significantly associated with deletion or loss of 5q, a complex karyotype, and a poor prognosis

PURPOSE

To study mutations and loss of heterozygosity (LOH) of p53 in therapy-related myelodysplasia (t-MDS) and acute myeloid leukemia (t-AML).

PATIENTS AND METHODS

Fifty-two unselected patients with t-MDS and 25 patients with t-AML were studied by polymerase chain reaction (PCR)-single-strand conformational polymorphism (SSCP) at the DNA level and by reverse transcriptase (RT)-PCR-SSCP at the mRNA level, and cases with aberrant SSCP patterns were sequenced.

RESULTS

Somatically acquired mutations of p53 were observed in 21 of 77 cases of t-MDS or t-AML, and 19 of these 21 patients had received alkylating agents. Single-base substitutions at A:T pairs were more common in t-MDS and t-AML, whereas single-base substitutions at G:C pairs are most common in MDS and AML de novo and in solid tumors. Six patients demonstrated a cytogenetic loss of 17p13, and these six and an additional nine patients with p53 mutations demonstrated LOH of p53 at the DNA or mRNA level. This suggests a cytogenetic loss of the normal p53 allele in these nine cases combined with duplication of the homologous chromosome 17 carrying the mutated p53 allele. Mutations of p53 were significantly associated with deletion or loss of 5q (P <.0001) and a complex karyotype (P =.0001), but surprisingly were not associated with deletion or loss of 7q (P =.73), and were infrequent in patients with balanced chromosome translocations (P =.03). Mutations of p53 were more common in older patients (P =.036) and were associated with an extremely poor prognosis (P =.014), apparently restricted to the 15 cases with LOH of p53 ( P =.046).

CONCLUSION

Mutations with loss of function of p53 are significantly associated with deletion or loss of 5q in t-MDS and t-AML after previous treatment with alkylating agents and are associated with genetic instability.