Is intrachromosomal amplification of chromosome 21 (iAMP21) always intrachromosomal?

Recurrent chromosomal abnormalities in childhood B-cell acute lymphoblastic leukemia (B-ALL) provide prognostic information that is useful in determining treatment stratification. iAMP21 is a more recently recognized cytogenetic entity of B-ALL that was originally described as multiple copies of the RUNX1 gene on a structurally abnormal chromosome 21. Subsequent studies elucidated a common region of highest-level amplification that includes RUNX1. Fluorescence in situ hybridization (FISH) is the most common method used to identify iAMP21, which is defined as the presence of five or more total copies of RUNX1, with three or more extra RUNX1 signals on a single abnormal chromosome 21. More recently, chromosomal microarray (CMA) and next generation sequencing have uncovered a characteristic chromosome 21 copy number profile in cases of iAMP21. We present a case of iAMP21 that does not fit the formal FISH definition. However, CMA uncovered the characteristic chromosome 21 copy number profile that is seen in iAMP21, demonstrating that CMA is helpful for the detection of this entity when FISH results are ambiguous. Furthermore, CMA showed that the highest level of amplification in this case did not include the RUNX1 gene, consistent with current evidence that RUNX1 is not the primary target of amplification.