Original article
Genomic, immunophenotypic, and NPM1/FLT3 mutational studies on 17 patients with normal karyotype acute myeloid leukemia (AML) followed by aberrant karyotype AML at relapse

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Abstract

Normal karyotype (NK) is the most common cytogenetic group in acute myeloid leukemia (AML) diagnosis; however, up to 50% of these patients at relapse will have aberrant karyotype (AK) AML. To determine the etiology of relapsed AK AML cells, we evaluated cytogenetic, immunophenotypic, and molecular results of 17 patients with diagnostic NK AML and relapsed AK AML at our institute. AK AML karyotype was diverse, involving no favorable and largely (8 of 17) complex cytogenetics. Despite clear cytogenetic differences, immunophenotype and NPM1/FLT3 gene mutation status did not change between presentation and relapse in 83% (10 of 12) and 94% (15 of 16) cases, respectively. High-resolution array-based comparative genomic hybridization (aCGH) performed via paired aCGH on NK AML and AK AML samples from the same patient confirmed cytogenetic aberrations only in the relapse sample. Analysis of 16 additional diagnostic NK AML samples revealed no evidence of submicroscopic aberrations undetected by conventional cytogenetics in any case. These results favor evolution of NK AML leukemia cells with acquisition of novel genetic changes as the most common etiology of AK AML relapse as opposed to secondary leukemogenesis. Additional studies are needed to confirm whether AK AML cells represent selection of rare preexisting clones below aCGH detection and to further characterize the molecular lesions found at time of AK AML relapse.

Introduction

Acute myeloid leukemia (AML) is a rapidly proliferating neoplasm of immature hematopoietic cells. Diagnostic karyotype is well established as the best independent prognostic factor for AML for complete remission as well as for disease-free and overall survival. Normal karyotype (NK) AML is the most common AML cytogenetic group, comprising 40–49% of adult and 20–25% of pediatric AML diagnoses. NK AML is associated with an intermediate prognosis, defined as a 5-year survival rate of 24–40% [1], [2]. Despite the importance of diagnostic karyotype [3], however, 25–51% of NK AML patients will have AML associated with aberrant karyotype (AK) at relapse [4], [5], [6], [7]. It is unknown how these previously cytogenetically normal AML cells “acquire” additional genetic abnormalities [5]. One possibility is that AK AML cells were present but undetected at time of NK AML diagnosis as a result of low cell frequency and the relative insensitivity of conventional cytogenetic analyses. Recent analyses have shown that most relapsed pediatric acute lymphocytic leukemia (ALL) disease arises from minor tumor subpopulations present but undetected at time of diagnosis [8]. Alternatively, AK AML cells could have arisen from acquired genetic changes to preexisting NK AML cells. Finally, AK AML cells may represent secondary de novo leukemia cells.

To elucidate the etiology of AK AML after NK AML, we identified 17 patients with diagnostic NK AML and relapsed AK AML (designated NK-AK AML) treated at our institute. We compared cytogenetic and immunophenotypic profiles, and NPM1/FLT3 gene mutation status from patient samples at diagnosis and relapse. In addition, we performed high-resolution array-based comparative genomic hybridization (aCGH) on all diagnostic NK AML samples specifically looking for chromosomal aberrations that might have been present but undetected by conventional karyotyping [9], [10], [11].

Section snippets

Patient samples

Under an IRB-approved protocol, we reviewed data on 301 consecutive NK AML patients diagnosed at our institute between 1991 and 2009. Clinical outcomes were defined per Cheson et al. [12]. Relapse was defined as the finding of >5% marrow blasts not attributable to another cause or to extramedullary disease. Event-free survival was defined as time between relapse and death from any cause. Sixty patients had karyotype information at both diagnosis and time of disease progression and/or relapse.

Clinical characteristics

Characteristics of the 17 NK-AK AML patients evaluated here are summarized in Table 1. All patients received induction therapy with cytarabine- and anthracycline-based induction AML regimens before relapse or recurrence. Overall time and time to AK AML development were 470 ± 213 days and 294 ± 198 days, respectively. Event-free survival (or survival after AK AML diagnosis) was only 96 ± 64 days.

Cytogenetics

Abnormal karyotypes at relapse in these 17 patients were complex (defined as three or more

Discussion

We examined immunophenotype profiles and NPM1/FLT3 gene mutation status in 17 paired NK-AK AML samples at diagnosis and relapse. In contrast with prior studies demonstrating a high frequency of immunophenotypic [21] and gene mutation changes (specifically NRAS, FLT3, and TP53) between AML diagnosis and relapse [22], we found that most paired NK-AK AML samples (>83%) did not demonstrate significant antigen or NPM1/FLT3 mutational changes over time. We did not, however, specifically evaluate NRAS

Acknowledgments

This research was supported in part by the Szefel Leukemia Research Fund (RPCI) and a National Cancer Institute Cancer Center support grant (CA016156).

Thanks to Daniel P. Gaile, Lori Shepherd, Song Liu, Jeff Miecznikowski (SUNY-UB and RPCI Biostatistics), Paul Wallace (RPCI Flow Cytometry), AnneMarie W. Block (RPCI Cytogenetics), Maurice Barcos (RPCI Hematopathology), Marc S. Halfon (SUNY-UB COE), and the RPCI Leukemia Service for excellent patient care.

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