ALBERT

Description: The Philadelphia translocation, encoding the BCR-ABL1 (BCR-ABL) fusion gene, is typically found in chronic myeloid leukemia (CML) and precursor B-cell acute lymphoblastic leukemia (B-ALL), but is exceptionally rare in T-cell acute lymphoblastic leukemia (T-ALL). To study the potential involvement of ABL1 gene rearrangements in T-cell malignancies, we screened 90 T-ALL cases by fluorescence in situ hybridization (FISH), using BCR and ABL1 probes. No BCR-ABL1 fusion signals were observed, confirming the low frequency of this rearrangement in T-ALL, but we did observe marked amplification (〉 10 signals per nucleus) ABL1 of in 5 of 90 (5.5 %) T-ALL patients. Amplification of ABL1 occurred on small extrachromosomal elements that were not detectable by conventional cytogenetics. and hence are referred to as episomes. FISH, and array-CGH analyses delineated the amplicon as a 500 kb region from chromosome band 9q34, containing the oncogenes ABL1 and NUP214 (CAN). Molecular analysis led to the identification of a NUP214-ABL1 fusion gene, which is generated as result of the circularization of the genomic region between ABL1 and NUP214 to form the episomes. This is the first example of an oncogenic fusion gene generated by extrachromosomal amplification. The NUP214-ABL1 transcript was detected in 5 patients with ABL1 amplification, in 5 of 85 (5.8 %) additional T-ALL patients, and in 3 of 22 T-ALL cell lines. The constitutively phosphorylated tyrosine kinase NUP214-ABL1 is sensitive to the tyrosine kinase inhibitor imatinib mesylate (STI-571). The recurrent cryptic NUP214-ABL1 rearrangement is associated with increased expression TLX1 of (HOX11) or TLX3 (HOX11L2), and with deletion of CDKN2A (p16), consistent with a multi-step pathogenesis of T-ALL. Our results identify a novel mechanism for the generation of a fusion gene on extrachromosomal elements, and indicate the importance of activated tyrosine kinase signaling in the pathogenesis of T-ALL. NUP214-ABL1 expression defines a new subgroup of T-ALL patients that could benefit from imatinib treatment.