Cryptic Insertional PML-RARA Translocations.
Jeffery M Klco, John S Welch, Michael Evenson, Jacqueline E Payton, Peter Westervelt, John L Frater, Timothy J Ley, Shashikant Kulkarni. Washington University School of Medicine, St. Louis, MO
Background: Acute myeloid leukemia (AML) with t(15;17)/acute promyelocytic leukemia (APL) is characterized by a reciprocal translocation involving the PML and RARA genes on chromosomes 15 and 17, respectively. Rare cases with cryptic PML-RARA translocations have been identified, including a case that we recently characterized by whole genome sequencing in which we identified an insertional fusion of 77 Kb of chromosome 15, including PML through exon 3, into the 2nd intron of RARA.
Design: We explored if other cases of cytogenetically cryptic APL at our institution could be explained by insertional PML-RARA translocations by searching for cases of AML with morphologic features of APL that lacked the classic t(15;17) and the normal dual fusion pattern by fluorescence in situ hybridization (FISH). Cases with variant RARA translocations were excluded. To screen for cases we developed 3 novel fosmid-based FISH probes, one of which is complimentary to the upstream regulatory sequences and first 3 exons of PML and the other two flank the invariant breakpoint in RARA.
Results: The FISH probes were validated on normal bone marrow and cases of APL with t(15;17). The classic t(15;17) cases demonstrated the expected pattern of one fusion, two RARA signals (one normal and one derivative) and one PML signal. We identified 12 possible cases that had frozen tissue for ancillary studies and found 2 additional cases with abnormal interphase and metaphase FISH patterns suggestive of insertional events : 1. Fusion on chromosome 17; 2. Fusion on chromosome 15 with an extra copy of PML. Including the original case, 2 of the 3 had RT-PCR confirmation of a PML-RARA fusion (bcr1 and bcr3 isoforms). The 3 cases had the following clinical features: Age: 30-41(range), WBC: 0.4-1.2 K/cumm (range), and morphologically had features compatible with the typical (hypergranular) variant of APL. The promyelocytes had the following immunophenotype by flow cytometry: CD34 (0/3), HLA-DR (0/2), CD33 (3/3), CD117 (2/2), CD13 (2/3) and CD56 (1/2).
Conclusions: In total we identified 3 cases of APL with cryptic insertional translocations involving PML and RARA, which we confirmed by a novel set of FISH probes that more specifically target the regions of PML and RARA involved in the translocation characteristic of APL. These cases have clinical and pathologic features similar to classic APL, yet are difficult to decipher using the standard FISH-based diagnostics.
Monday, February 28, 2011 1:00 PM
Poster Session II # 184, Monday Afternoon