Cryptic chromosomal aberrations leading to an AML1/ETO rearrangement are frequently caused by small insertions

Abstract

The translocation t(8;21)(q22;q22), which results in the fusion of the AML1 (RUNX1) and ETO (CBFA2T1) genes, is a recurrent aberration in acute myeloid leukemia (AML), preferentially correlated with FAB M2, and has the highest incidence in childhood AML. Because of the favorable prognosis, the evidence of the t(8;21) or the AML1/ETO fusion gene is mandatory in most of the therapy trials, allowing the stratification of the patients to the correct risk group in terms of treatment. Here we present six out of 59 children with AML who were positive for AML1/ETO by RT-PCR, but showed no evidence of the classical t(8;21)(q22;q22) by conventional cytogenetics. Because of the discrepancies between molecular and cytogenetic analyses, these six patients were further investigated by fluorescence in situ hybridization analysis. Small hidden interstitial insertions resulting in an AML1/ETO rearrangement were detected in five (8.5%) of the 59 patients, whereas the sixth patient showed a cryptic three-way translocation. The insertions could be characterized as ins(21;8) in three patients and ins(8;21) in the remaining two. Additionally, three of the patients showed secondary chromosome aberrations leading to a higher complexity of the karyotype. In conclusion, the combination of more than one standard technique in the analysis of AML1/ETO is useful to reveal the overall frequency of cryptic chromosome rearrangements and permits a better understanding of the mechanisms involved in the generation of this fusion gene.

Disruption of ETV6 in intron 2 results in upregulatory and insertional events in childhood acute lymphoblastic leukaemia

“…[50][51][52] Very large insertions associated with the RUNX1-ETO fusion occurred in B8% of positive cases and varied between 2 and 44 megabases in size. 38,39 A case that was positive for ETV6-RUNX1 fusion had an insertion of most of the short arm of chromosome 12 into 21q. Thus the interphase FISH pattern of 1R1G1F/2R1G1F did not suggest anything out of the ordinary.…”

“…36,37 As for the RUNX1 gene, its involvement in insertions which result in a t(8;21)(q22;q22) translocation, is well established. 38,39 The five cases which are presented here were identified during routine diagnostic interphase FISH screening for the ETV6-RUNX1 fusion. 40 Each had a different number of ETV6 signals to that expected.…”

Clinical relevance of mutations and gene-expression changes in adult acute myeloid leukemia with normal cytogenetics: are we ready for a prognostically prioritized molecular classification?

“…In most instances, these fusion genes are created by cryptic insertions of very small chromosome segments that do not alter the chromosome morphology. 27,29,30 Patients with these cryptic rearrangements are usually categorized with and treated as patients with typical t(15;17), t (8;21), and inv (16). Both reverse transcriptase-polymerase chain reaction (RT-PCR) and fluorescence in situ hybridization (FISH) can be used to detect the presence of the aforementioned hidden rearrangements.…”

Insertion (21;8)(q22;q22q22): a masked t(8;21) in a patient with acute myelocytic leukemia

“…Results of dual-color FISH with AML1 and ETO probes are shown in Figure 3 [5,16]. Only molecular analysis revealed the presence of AML1/ETO fusion gene in these cases that could be thought to have otherwise normal karyotype.…”

“…In 6 reports of masked t(8;21), the precise translocations were determined by FISH analysis [9,11,13,14,15,16]. …”