Double translocation or gene insertion? A BCR/ABL1 fusion in Philadelphia-negative CML

A 75-year-old man presented for the investigation of marked leukocytosis, neutrophilia, mild normocytic anemia, and thrombocytosis found on routine complete blood count. Bone marrow (BM) aspirate revealed myeloid hyperplasia and 2% basophils without dysplasia, suggestive of chronic myeloid leukemia (CML) (panels A-C; original magnification ×250 [A], ×500 [B], and ×1000 [C]; Wright-Giemsa stain). BM karyotype presented no abnormalities (panel D; R-banding karyotype, 46,XY). Real-time polymerase chain reaction revealed the presence of the typical e14a2 (b3a2, M-BCR) fusion transcript. However, fluorescence in situ hybridization (FISH) analysis using dual-color, dual-fusion probes for BCR/ABL1 showed only 1 BCR/ABL1 fusion, present on a cytogenetically normal chromosome 9, associated with diminished ABL1 signal. BCR signal was reduced on 1 chromosome 22 (panel E; D-FISH BCR/ABL1 probes; ABL1[9q34.12], red; BCR[22q11], green). Triple-color, dual-fusion probes, targeting ASS1 (212 kb from the centromeric end of ABL1) revealed a loss in the q region of the fusion chromosome (panel F; T-FISH BCR/ABL1/ASS1 probes; ABL1[9q34.12], red; BCR[22q11], green; ASS1[9q34.11], aqua; F, fusion; G, normal green signal; g, dim green signal; R, normal red signal). The probable rearrangement mechanism thus involves the partial insertion of BCR into chromosome 9 along with an interstitial deletion upstream the 5′ region of ABL. The resultant karyotype is 46,XY[20].ish der(9)del(9)(q34q34)ins(9;22)(q34;q11.2)(ASS1-,ABL1 dim,BCR+)[5].nuc ish(ASS1x1,ABL1x1,ABL1 dimx1,BCRx3)(ABL1 con BCR1x1)[96/100].

This is a rare case of CML harboring the BCR/ABL1 fusion in the absence of a Philadelphia chromosome. It highlights the importance of FISH as an initial tool in the elucidation of complex rearrangements allowing targeted confirmation by molecular diagnostic techniques.