Characterization of 14 NUP214-ABL1 Fusions in T-Cell Acute Lymphoblastic Leukaemia (T-ALL) Exhibits a Genomic Heterogeneity.

The recently described episomal amplification of the NUP214-ABL1 fusion in 6% of T-ALL leads to the expression of a constitutively activated chimeric tyrosine kinase sensitive to imatinib. We collected additional cases in order to better characterize this new entity with respect to genetic presentations and clinical course.

We collected 14 new NUP214-ABL1 positive cases by FISH (LSI-BCR/ABL1 ES, Vysis and ABL1 break-apart home-made probes) or RT-PCR (fusion transcript) screening.

FISH analysis detected episomal amplification of NUP214-ABL1 in 11 patients with a highly variable number of nuclei with amplification (<1% to 90%). Interestingly, one case showed a higher percentage of nuclei with amplification when using frozen non cultured cells rather than cultured fixed cells (10% vs 1%) suggesting loss of episomes during culture. FISH showed also intrachromosomal amplification of NUP214-ABL1 in two cases: one presented a HSR at the original 9q34 site without detectable episomes, the other associated HSR (probably on a chromosome 10), episomes (<1% of nuclei) and 9q34 chromosomal insertions including NUP214 and ABL3′ that encode the tyrosine kinase domain but not ABL5′, on variable chromosomes including 14p (33%). One NUP214-ABL1 RT-PCR positive case did not show any FISH aberration. Median age: 16 y (3–45) with a male predominance (10:4). There were no T-cell lymphoblastic lymphoma. Immunophenotype (EGIL): mature (n=2), cortical (n=6) or pre-T (n=4). Karyotype: structural chromosomal alterations in 8 patients (including 4 with 10q24/HOX11 rearrangements), only numerical chromosomal alterations in 4 (including 2 with + 8), normal in 1, failed in 1. All samples with available information (10/14) showed a HOX11 or HOX11L2 abnormality. Among the 13 cases with available outcome data, we observed 5 early relapses, including both patients with NUP214-ABL1 HSR, and 1 refractory ALL.

These observations

1. emphasize the interest of combining both (quantitative) RT-PCR and FISH for the screening and characterisation of NUP214-ABL1 fusion and amplification,

2. demonstrate the coexistence of different NUP214-ABL1 genomic presentations in one patient (episomal amplification, 9q34 insertions, HSR) compatible with the model in which genomic amplification start with episome formation in order to create the NUP214-ABL1 fusion followed by their amplification and optional secondary reintegration,

3. confirm occurrence of NUP214-ABL1 in T-ALL with HOX11 and HOX11L2 involvement,

4. raise the question of the rather worse prognosis for cases with intrachromosomal amplification as previously suggested,

5. raise the signification of minor NUP214-ABL1 clones and variable genomic presentations in the leukemogenesis of this subgroup of T-ALL that could potentially benefit from imatimib.

° on behalf of the GFCH (Groupe Francophone de Cytogénétique Hématologique) and the BCGHO (Belgian Cytogenetic Group for Hematology and Oncology).