A Novel t(3;17) Variant of Acute Promyelocytic Leukemia with Rearrangement of the RARA Locus.
Authors: Redner Robert L. , Contis Lydia C. , Evans Carol , Sherer Maureen E. , Shekhter-Levin Sofia 
Published: April 2020

The vast majority of patients with Acute Promyelocytic Leukemia (APL, FAB M3) have the t(15;17)(q12;q21) chromosomal translocation. This introduces the gene for PML into the retinoic acid receptor alpha (RARA) locus, which leads to expression of a PML-RARA fusion. There is convincing evidence that expression of PML-RARA underlies the APL phenotype. Yet, there have been identified rare cases of APL that do not manifest t(15;17). Many of these cases exhibit cryptic rearrangements of PML and RARA. However, in a number of cases it has clearly been shown that a fusion protein different than PML-RARA is expressed. These include the t(11;17)(q23;q21) that expresses a PLZF-RARA fusion; t(5;17)(q35;q21) that encodes NPM-RARA; t(11;17)(q13;q21) that encodes NUMA-RARA; and der(17) with duplication of 17q21.3-q23 that fuses STAT5b to RARA. We report here a novel case of APL with t(3;17) with rearrangement of RARA, but not PML. A 72 year old man presented with leukocytosis, anemia, and thrombocytopenia: wbc 20.4 X10E+9/L; hgb 10.3 g/L; PLT 22 x10E+9/L. The wbc differential showed 20% polys, 4% bands, 15% lymphocytes, 19% monocytes, 34% blasts, 1% promyelocyte, 6% myelocyte, 1% metamyelocytes. Auer rods were seen. The bone marrow was hypercellular (approximately 80%), with 88% blasts, 1.7% promyelocytes, 0.3% myelocyte, 0.3% polys, 0.3% eosinophile, 3% monocytes, 0.3% pronormoblasts, 3.7% normoblasts, and 2.3% lymphocytes. The blasts demonstrated prominent cytoplasmic granulation, Flow cytometric analysis showed the blasts to be CD117 positive, myeloperoxidase positive, CD13/33 positive, but lacking CD34 or HLA-DR expression, consistent with a diagnosis of APL. Cytogenetic studies indicated a mosaic abnormal analysis with an apparent normal cell line and one that demonstrated a 47,XY,t(3;17)(p25;q12-21), +8 karyotype. Analysis for PML-RARA expression by RT-PCR was indeterminate, owing to poor quality of the extracted RNA. Fluorescence In Situ Hybridization (FISH) was therefore performed on two hundred unstimulated cells, primarily in interphase, using the Vysis t(15;17) dual color DNA probe. 98.5% of the cells were negative for PML-RARA rearrangement (the value of 1.5% positivity is within the laboratory’s control range for false positives). To confirm that the t(3;17) rearrangement involved the RARA locus, we scored 203 unstimulated cells using the LSI RARA dual color DNA probe. 100% were positive for the RARA gene rearrangement (split signal). Four metaphase cells each showed one fused red/green signal on the normal chromosome 17, one red signal on der (17), and one green signal on the distal arm of chromsome 3. The FISH analysis therefore indicated rearrangement of the RARA, but not the PML locus. The patient expired before treatment could begin. To determine whether the t(3;17) blasts could differentiate (a hallmark of t(15;17) APL), we cultured the bone marrow cells in RPMI 1640 with 10% FCS and 10E-6 M ATRA. At 10 days 58% of the cells resembled metamyelocytes, bands, or mature polys, compared with none in the control culture. This indicates that t(3;17) retains its ability to differentiate in the presence of ATRA, consistent with its classification as a novel variant of APL.