NG2 Molecule Expression in Acute Lymphoblastic Leukemia B Cells: A FlowCytometric Marker for the Rapid Identification of KMT2A Gene Rearrangements

Background: B-lineage acute lymphoblastic leukemias (B -ALL) harboring rearrangements of the histone lysine [K]-Methyltransferase 2A (KMT2A) gene on chromosome 11q23 (KMT2A-r) represent a category with dismal prognosis. The prompt identification of these cases represents an urgent clinical need. Considering the correlation between rat neuron glialantigen 2 (NG2) chondroitin-sulfate-proteoglycan molecule expression and KMT2A-r , we aimed to identify an optimized cytofluorimetric diagnostic panel to predict the presence of KMT2A-r. Materials and Methods: We evaluated 88 NG2+ B -ALL cases identified with an NG2 positivity threshold >10% from a cohort of 1382 newly diagnosed B-ALLs referred to the Division of Hematology of 'Sapienza' University of Rome. Results: Eighty-five of 88 (96.6%) NG2+ B-ALLs harbored KMT2A-r and were mainly pro -B ALL (77/85; 91%). Only 2 B-ALLs with KMT2A-r showed NG2 expression below 10%, probably due to the steroid therapy administered prior to cytofluorimetric analysis. Compared to KMT2A-r- cases, KMT2A r+ B-ALLs showed a higher blast percentage, significantly higher mean fluorescence intensity (MFI) of CD45, CD38, and CD58, and significantly lower MFI of CD34, CD22, TdT, and CD123. The study confirmed differences in CD45, CD34, CD22, and TdT MFI within the same immunologic EGIL group (European Group for the immunological classification of leukemias), indicating no influence of the B-ALLs EGIL subtype on the KMT2A-r+ B-ALLs immunophenotype. Conclusions: Our data demonstrate the association between NG2 and KMT2A-r in B-ALLs identify a distinctive immunophenotypic pattern, useful for rapid identification in diagnostic routines of these subtypes of B-ALLs with a poor prognosis that benefits from a specific therapeutic approach.