Elsevier

Leukemia Research

Volume 36, Issue 4, April 2012, Pages 418-421
Leukemia Research

Sequential array comparative genomic hybridization analysis identifies copy number changes during blastic transformation of chronic myeloid leukemia

https://doi.org/10.1016/j.leukres.2011.12.021Get rights and content

Abstract

The present study was performed to provide direct evidence on copy number changes during progression from chronic phase (CP) to blastic phase (BP) in chronic myeloid leukemia (CML) through a longitudinal follow-up study. Matched CP and BP samples in three patients were analyzed using high-resolution array comparative genomic hybridization (aCGH) chips. During blastic transformation, loss of large genomic segments including 6q14.1-q22.31, chromosome 7 and 9p13.2-p21.3 were noted. Furthermore, small-sized copy number changes involving cancer-associated genes were observed. In addition, we identified a novel fusion gene consisted of PAX5 and MLLT3 (AF9). It is likely that blastic transformation of CML is a multi-step process associated accumulation of several genomic events which may largely overlap with those found in acute leukemias.

Introduction

Chronic myeloid leukemia (CML) is a myeloproiferative disorder characterized by increased proliferation of the granulocytic cell line without the loss of their capacity to differentiate. The hall mark of CML is the BCR/ABL1 fusion gene resulting from t(9;22)(q34;q11) or its variants, which constitutively enhances ABL1 kinase activity and is thought to be sufficient to drive malignant transformation in chronic phase (CP) [1]. Although recently developed tyrosine kinase inhibitors such as imatinib, dasatinib and nilotinib are shown to be effective for the majorities of CML-CP patients, a small number of patients fail to respond and eventually progress to blastic phase (BP) [2], [3].

The biological basis of CML-BP is largely unknown, but it is repeatedly recognized that additional genetic changes other than BCR/ABL1 translocation may contribute to the transformation from CP to BP. The blasts in CML-BP can be myeloid, lymphoid or mixed, and it is also believed that different genetic mechanisms may underlie the lineage heterogeneity [4], [5]. Traditional cytogenetics found that additional chromosomal abnormalities such as +8, +19, +t(9;22) and i(17q) are frequent in CML-BP [6]. Mutations in cancer genes such as TP53 are also suggested to have a role in CML-BP [4]. More recently, genome-wide scan using array comparative genomic hybridization (aCGH) revealed that small-sized copy number changes such as IKZF1, CDKN2A (p16), IGH and TCR region deletions are common in CML-BP patients [7], [8]. The previous aCGH studies usually performed a cross-sectional analysis on patients who had already progressed to CML-BP, and more longitudinal follow-up studies will be needed.

So the present study was designed to provide direct evidence on copy number changes during progression from CP to BP. The strategy is to analyze BP-CP matched samples using high-resolution aCGH platforms, by which personal copy number variations (CNVs) can be effectively eliminated. With this approach, we could identify several copy number changes and a novel fusion gene which may be potentially important for the pathogenesis of CML-BP.

Section snippets

Subjects

The study subjects were recruited from 18 patients diagnosed with CML-BP at our center from 2001 to 2010. Among them, four patients were previously diagnosed with CML-CP at our center, and we enrolled three patients whose bone marrow samples were available both in CML-CP and CML-BP. Additionally, we could enroll three patients whose bone marrow samples were available only in CML-BP.

Chromosome study and fluorescence in situ hybridization (FISH)

Metaphase chromosome spreads were obtained from cultured bone marrow lymphocytes using the standard methods. The

Clinical data and aCGH analysis results

Table 1 summarizes chromosome and aCGH analysis results of investigated patients. Patient 1 had progressed to acute myeloid leukemia (AML) after 14 months of imatinib therapy. Conventional karyotyping revealed that deletions of 6q14-q22 and chromosome 7 had occurred during blastic transformation. These two aberrations were also confirmed by aCGH analysis. In the 6q region, a total of 145 genes from LCA5 to NT5DC1 (36.3 Mb) were deleted; possible cancer-associated genes in these regions included

Discussion

The biological basis of blastic transformation in CML-CP patients is hardly understood. A useful approach to investigate the pathogenesis may be a direct comparison of CML-BP with CML-CP in an identical individual using genome-wide approaches. Although the present study has certain limitations from the small number of samples, it may give additional information on the biology of CML progression. We found that several small to large-sized copy number changes occur during disease progression. The

Conflict of interest

We declare that we have no conflicts of interest.

Acknowledgments

This study was supported by a grant of the Korea Healthcare Technology R&D Project, Ministry for Health & Welfare Affairs, Republic of Korea. (A092255).

References (20)

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