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A novel TRB@/NOTCH1 fusion gene in T‐cell lymphoblastic lymphoma with t(7;9)(q34;q34)

Background: In T‐cell acute lymphoblastic leukemia/lymphoma (T‐ALL/LBL), activating mutations of NOTCH1 are observed in more than 50% of cases, whereas the t(7;9)(q34;q34) involving NOTCH1 at 9q34 and TRB@ at 7q34 is an extremely rare but recurrent translocation. Patient: A 41‐year‐old male with a large mediastinal mass, pleural effusion, and lymphadenopathy was diagnosed as having T‐LBL. Lymphoma cells were positive for CD4, CD8, CD2, CD3, CD5, CD7, CD10, and TdT. Results: G‐banding and spectral karyotyping of pleural effusion cells showed 47,XY,dup(1)(q21q32),t(7;9)(q34;q34),+20. Genomic polymerase chain reaction (PCR) revealed that the 5′ end of TRB@ J1‐5 was connected with the middle of NOTCH1 exon 25 (434 bp downstream from its 5′ end) in a ‘head‐to‐head’ configuration on the der(9)t(7;9), although nine extra bases were inserted between the two genes. Reverse transcription‐PCR confirmed expression of the TRB@/NOTCH1 fusion transcripts. Similarly, the 5′ end of J1‐5 was fused to the shortened exon 25 with nine extra bases. The NOTCH1 breakpoint in exon 25 was very close to transcription start sites of deleted Notch1 in murine T‐ALL. Conclusions: The TRB@/NOTCH1 fusion gene with a NOTCH1 breakpoint in exon 25, which has not previously been detected in four other reported cases with t(7;9), could lead to aberrant expression of the truncated NOTCH1 by TRB@ enhancer elements. The resultant NOTCH1 receptor deleting most of the extracellular domain may be implicated in the pathogenesis of T‐LBL by ligand‐independent, constitutive activation of the NOTCH1 pathway, suggesting avenues for future therapy with γ‐secretase inhibitors.