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Accounting for ∼2% of B-ALL, iAMP21 is a recurrent genetic abnormality defined by
the presence of ≥ 5 copies of the RUNX1 gene or ≥ 3 extra copies of RUNX1 on a single abnormal chromosome 21 by fluorescence in situ hybridization (FISH) using
the ETV6/RUNX1 probe. Our goal was to evaluate the performance of the ETV6/RUNX1 probe in identifying iAMP21. We performed a retrospective study from January 2018
to August 2020 of all pediatric B-ALL that had FISH, chromosome and/or chromosomal
microarray studies. Of 704 total cases, 20 (2.8%) were classified as iAMP21.We expanded
our cohort to include any iAMP21 case from the same study period (but not requiring
clinical trial enrollment). From 32 iAMP21 cases, the median age was 12 years and
59.4% were female. In 27 cases (84.4%), the median number of RUNX1 signals was 5 (range 3-10). In 5 cases (15.6%), the total number of RUNX1 signals was innumerable and indicated as “amplification.” Of these 32 cases, 25 (78.1%)
were considered “typical iAMP21”, while 7 (21.9%) did not meet the standard criteria
and were classified as “atypical iAMP21”. Chromosomal microarray studies confirmed
the presence of iAMP21 (uneven amplification of chromosome 21) in all atypical cases.
The median age for the “atypical iAMP21” group was 11 years and 85.7% were females.
In both groups, the median number of RUNX1 copies was 5, but the range was greater in the typical group (5-10) compared to atypical
group (3-8). All atypical iAMP21 cases had < 3 additional copies of RUNX1 located on a derivative chromosome 21. Interestingly, 3 cases had ≥ 5 total copies
of RUNX1 per cell, but the additional copies of RUNX1 were located on chromosomes other than chromosome 21. Future studies should clarify
the true incidence of atypical iAMP21 and determine its clinical significance.
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