30. Characterization of atypical iAMP21 observed in B-Lymphoblastic Leukemia (B-ALL): a 2.5-year retrospective study from the Mayo Clinic

      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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