81. Discrepancies in the detection of PML::RARA gene rearrangement by FISH using commonly used dual-color dual-fusion probes

Abstract

Acute promyelocytic leukemia (APL) is a medical emergency with serious complications (e.g., DIC), requires prompt treatment and rapid test turnaround time (TAT) for diagnosis confirmation. APL results from a fusion of the PML gene (15q24) with the RARA gene (17q21) creating a fusion gene on the derivative chromosome 15. There are multiple breakpoints that can occur. PML::RARA is the most common (98%) rearrangement. Fluorescent in situ Hybridization (FISH) study helps in the diagnosis of APL with rapid TAT. PML::RARA dcdf FISH probes from either Abbott Molecular or Cytocell are commonly used in the Cytogenetics laboratories. Although they both detect PML::RARA gene rearrangements they differ in their probe design, resulting in the limitation of breakpoint coverage in atypical PML-RARA insertion rearrangements. Our lab has identified two unique cases of APL which were tested at diagnosis using PML::RARA FISH probe set. Both cases showed an atypical rearrangement with single fusion signal. First case was positive by Abbott probe and negative by Cytocell, metaphase FISH confirmed insertion of PML region near RARA on Chromosome 17. Other case was positive by Cytocell and negative by Abbott, metaphase FISH confirmed insertion of RARA region near PML on chromosome 15. Both cases were positive for PML::RARA transcript presence by qRT-PCR. This data suggests potential limitation of widely used PML::RARA FISH probes from different FISH probe vendors. In cases with high suspicion of APL with a negative FISH result, reflex FISH test using probes having coverage different from initial tested probe and molecular PCR