Identification of PML-RARA rearrangement by RT-PCR and sequencing in an acute promyelocytic leukemia without t(15;17) on G-banding and FISH. J. Han¹, K.E. Kim¹, K.H. Kim¹, J.S. Kim², J.I. Park³. 1) Department of Laboratory Medicine, Dong-A-University College of Medicine, Busan, Korea; 2) Department of Internal Medicine, Dong-A-University College of Medicine, Busan, Korea; 3) Department of Biochemistry, Dong-A-University College of Medicine, Busan, Korea.
   t(15;17)(q22;q12) is seen in 70-90% of APL cases, leading to the formation of two reciprocal fusion genes, PML-RARA on the der(15) and RARA-PML on the der(17) chromosomes. A number of different variant translocations have been characterized in the remaining cases of APL. There are also APL patients with cytogenetically normal chromosomes 15 and 17. Combining molecular techniques such as RT-PCR, FISH, or sequencing has been a useful tool in identifying those abnormalities. FISH using a commercially available dual color, dual fusion probe could detect almost all PML-RARA fusions including variant and cryptic forms. We report a unique case of de novo APL with a cryptic PML-RARA rearrangement. This patient had a karyotype of 47,XX,+8 and no additional chromosomal abnormalities. Metaphase FISH of the patient did not show PML-RARA fusion signals. However, PML-RARA chimeric transcripts could be identified by RT-PCR and cDNA sequencing. A diagnosis of APL was made and she was treated with combination chemotherapy including idarubicin and all trans retinoic acid (ATRA). The chemotherapy was well tolerated and a complete hematologic remission was achieved four weeks later. Evidences of PML-RARA fusion gene could not be detected any longer and cytogenetic studies showed a normal karyotype. The newer FISH method is significantly superior to the previous FISH probes, accurately detecting cryptic, variants or complex translocations involving chromosomes 15 and 17. However, depending on the size of the insertion, the target could be so small that it does not hybridize with the probe or even hybridization itself might not generate a fluorescent signal large enough to be visualized by FISH. To resolve the presence or absence of a masked PML-RARA fusion, it may be necessary to further evaluate by using of other molecular testing.