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 9.8×10
/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.