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Correspondence

Indian Pediatr 2014;51: 1020-1021

Identification of PML/RARá Fusion Gene by RT-PCR Acute Promyelocytic Leukaemia

 

Radheshyam Purkait and *Tuphan Kanti Dolai

Department of Paediatric Medicine and *Haematology, NRS Medical College & Hospital, Kolkata, WB, India.
Email: radheshyampurkait@gmail.com 

 
 


The genetic hallmark of acute promyelocytic leukemia (APL) is the balanced reciprocal chromosomal translocation of (15;17) leading to a fusion of the promyelocytic leukemia (PML) gene on chromosome 15 and the retinoic acid receptor-á (RARá) gene on chromosome 17 [1]. Identification of PML/RARá fusion gene by reverse transcriptase-polymerase chain reaction (RT-PCR) without evidence of t(15;17) both on conventional karyotype and fluorescence in situ hybridization (FISH) is rare [2,3].

An 8-year-old girl was admitted with the complaints of fever and generalized weakness for last 1˝ month, and epistaxis and bleeding from gums for last 20 days. She had moderate pallor and hepatosplenomegaly. Routine blood examination revealed hemoglobin 7.4 g/dL, total white cell count 9.8×109/L and platelet count was 40×109/L. Peripheral blood smear showed 35% neoplastic promyelocytes with cytoplasmic hypergranulation. Coagulation profile revealed normal prothrombin time and activated partial thromboplastin time, but serum fibrinogen level was low. Bone marrow aspiration showed a hypercellular marrow with 66% neoplastic promyelocytes and presence of multiple Auer rods (faggot cells).

Cytochemical staining with myeloperoxidase was strongly positive. Child was diagnosed morphologically as AML-M3 (hypergranular variant). Immunophenotyping analysis of the bone marrow cells was consistent with APL. Conventional cytogenetic analysis showed 46, XY, isochromosome (17q). FISH analysis was negative for t(15;17). RT-PCR was positive for PML–RARá fusion transcripts. She had completed both induction and consolidation phase of all-trans retinoic acid (ATRA) based chemotherapy regimen. Child responded dramatically and went into molecular remission.

Conventional cytogenetic analysis can identify reciprocal chromosomal translocation t(15;17) in upto 90% of cases with APL. The remaining 10% cases lacking t(15;17) stay associated with the cryptic insertion of the PML/RARá fusion gene [4,5]. FISH analysis and RT- PCR are the valuable tools to identify the PML/RARá hybrid transcript in a cytogenetically negative APL patient. The routinely used dual-colored break apart probes that are used in FISH are not sensitive enough to hybridize with such small cryptic insertions and therefore do not produce a signal as in our case. However, these small cryptic insertions can be amplified and detected by RT-PCR. These RT-PCR positive cases for hybrid PML/RARá transcript classify a new cytogenetic subgroup of APL.

We suggest that RT-PCR should be performed at baseline to detect this small subset of t(15;17) negative APL cases, with cryptic or masked insertions.

References

1. Duan Y, Nie J, Zhang Z, Zhou L, Zhu F, Zhang H, et al. A rare case with typical acute promyelocytic leukemia morphology associated with isolated isochromosome 17q without RARá rearrangement. Hematol Oncol Stem Cell Ther. 2013;6:42-5.

2. Huh J, Moon H, Chi H, Chung W. Acute promyelocytic leukemia with i(17)(q10) on G-banding and PML/RARA rearrangement by RT-PCR without evidence of PML/RARA rearrangement on FISH. Int J Lab Hematol. 2009;31:372-4.

3. Choughule A, Polampalli S, Amre P, Shinde S, Banavali S, Prabhash K, et al. Identification of PML/RARalpha fusion gene transcripts that showed no t(15;17) with conventional karyotyping and fluorescent in situ hybridization. Genet Mol Res. 2009;8:1-7.

4. Zaccaria A, Valenti A, Toschi M, Salvucci M, Cipriani R, Ottaviani E, et al. Cryptic translocation of PML/RARA on 17q. A rare event in acute promyelocytic leukemia. Cancer Genet Cytogenet. 2002;138:169-73.

5. Kim M, Lim J, Kim Y, Han K, Lee DH, Chung NG, et al. The genetic characterization of acute promyelocytic leukemia with cryptic t(15;17) including a new recurrent additional cytogenetic abnormality i(17)(q10). Leukemia. 2008;22:881-3.

 

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