PDF
Citation
Print
TO THE EDITOR: NUP214-ABL1 translocation is probably the most common tyrosine kinase inhibitor (TKI)-targetable cytogenetic abnormality observed in T-cell acute lymphoblastic leukemia (T-ALL), accounting for 5–6% of the T-ALL cases [1]. However, there are only few case reports and small series in the literature, possibly indicating the underdiagnosis of this entity. Unlike in B-cell acute lymphoblastic leukemia, testing for gene fusions using reverse transcriptase polymerase chain reaction (RT-PCR) or fluorescent in-situ hybridization (FISH) is often avoided in T-ALL because of the absence of distinct associations with prognosis or targeted therapy, especially in resource-limited settings. However, TKI-targetable abnormalities have also been described in T-ALL, including NUP214-ABL1, BCR-ABL1, and mutations in STAT5b [2, 3]. Among these, FISH testing using BCR-ABL1 dual-color dual-translocation probes can help to identify not only BCR:ABL1 but also NUP214-ABL1 based on the characteristic patterns in FISH testing. This is an illustrative case highlighting an easy and cost-effective approach for routine detection of cytogenetic abnormalities.
A developmentally normal male child, appropriately immunized for his age, presented with continuous moderate-grade fever, progressive pallor, and mild hepatosplenomegaly since 10 days. Peripheral blood examination revealed that the hemoglobin level was 71 g/L; total leukocyte count, 45.3×109/L; and platelet count, 65×109/L. The peripheral blood film revealed 72% blasts, 9% neutrophils, 16% lymphocytes, and 3% monocytes. The blasts were negative for myeloperoxidase. Flow cytometry confirmed T-ALL [positive for cluster of differentiation (CD) 7, CD4, CD5, CD2, cytoCD3, and terminal deoxynucleotidyl transferase (TdT); negative for surface CD3, CD1a, CD8, T-cell receptor (TCR) ab, TCRgd, B cell, and myeloid antigens]. FISH testing using BCR-ABL1 dual-color dual-fusion probe (Metasystems, Germany) showed amplification of ABL1 (3–50 copies) using the protocol as previously described [4]. Further testing using ABL1 (Metasystems, Germany) and NUP214 (Zytolight, Germany) dual-color break-apart probes showed a characteristic signal pattern confirming NUP214-ABL1 translocation (Fig. 1). Details of immunophenotyping and FISH cytogenetics are summarized in Table 1.
Bone marrow examination was not performed, as a confirmatory diagnosis could be made from the peripheral blood investigation, and the child was not willing to undergo bone marrow examination. Conventional cytogenetics of the peripheral blood did not show metaphase. Augmented Berlin-Frankfurt-Munich protocol plus imatinib was administered. Post-induction bone marrow was hypocellular with 2% blasts, and no measurable residual disease was detected using 10-color flow-cytometric immuno-phenotyping. The delayed intensification phase 2 was completed uneventfully, and the child is now in the maintenance phase.
Both ABL1 and NUP214 genes are located at 9q34.1, with the latter on the telomeric side. The fusion of these genes results from extrachromosomal episome formation and amplification of both genes. The episome containing the fused gene exists autonomously and freely replicates in the cytoplasm or integrates with the chromosome and replicates with it. This episomal amplification, varying between 5–50 copies/cell, can be visualized using FISH, multiplex ligation-dependent probe amplification, or chromosomal microarray; however, it is undetectable with conventional cytogenetics. Amplification of ABL1 does not appear to be the only mechanism involved in the pathogenesis of T-ALL; there have been reports of associated alterations of other genes, such as CDKN2A, TLX1, TLX3, and NOTCH1. These observations indicate a multigene contribution to the pathogenesis of T-ALL with NUP214-ABL1 fusion. NUP214-ABL1 fusion is found predominantly in men, and these patients usually present with high-risk factors, including elevated leukocyte count, mediastinal mass, or extramedullary involvement, often with early relapse and dismal outcomes [5]. While an occasional patient has survived for more than 194 months, the median overall survival reported in previous series is only 18 months. These patients are reported to benefit from TKI, especially dasatinib; hence, it is imperative to diagnose this entity in the clinics [6]. However, the long-term benefit of adding TKI in the treatment remains unclear owing to the lack of randomized controlled trials. A summary of the cases reported until now is presented in Table 2.
The limited number of reports and absence of definite treatment guidelines may indicate underdiagnosis of this entity. Amplification of ABL1 (9q34) is an indirect indicator of NUP214-ABL1 fusion, which can easily be detected by routine testing of BCR-ABL1 using FISH in T-ALL cases. NUP214-ABL1 fusion can be further confirmed using NUP214 break-apart FISH testing or RT-PCR. In addition to BCR-ABL1 translocation, this dual-color probe FISH helps to detect other ABL1-related and possibly TKI-responsive cytogenetic abnormalities associated with T-ALL, such as EML1-ABL1 and ETV6-ABL1 fusions or 9q34 duplication associated with therapeutic resistance [7]. Although BCR-ABL1-positive T-ALL is rare, the frequency of its detection has increased, with approximately 30 cases reported in the literature and pediatric cases accounting for more than 40% of the total cases. In most reported cases, the prognosis was poor with a median survival of only 7 months (range, 0.1–60 mo), and nearly 50% of the patients died by the time of the last follow-up [8-15].
To summarize, this illustrative report highlights the utility of incorporating routine FISH testing using a BCR/ABL1 dual-color probe in the workup for T-ALL in resource-constrained settings, especially in the absence of advanced testing, such as ribonucleic acid -sequencing.
XML Download