Prognostic Value of DNA Index By Flow Cytometry for Minimal Residual Disease in Childhood B-Cell Acute Lymphoblastic Leukemia

Background: Prognostic factors in B-ALL represent a critical role for treatment stratification. High hyperdiploids (>50 chromosomes) have been related to better outcomes, and hypodiploids to worse prognosis. DNA Index (DI) quantifies the DNA from leukemic blast by flow cytometry and has been correlated to the number of chromosomes. Our aim was to demonstrate the prognostic value of DI for minimal residual disease in childhood B-ALL.

Methods: 26 blood samples from newly diagnosed childhood B-ALL cases were analyzed between November 2017 and February 2018. DI was evaluated by flow cytometry in samples with Propidium Iodide and leukemic blasts were identified by CD19/CD22/CD10/CD20 antibodies. DI was calculated as the ratio of mean fluorescence of pathologic B blasts and normal cells (T, NK lymphocytes, neutrophils and monocytes) in G0/G1 phase. We stablished three categories: diploid (0.95 - 1.05, 46 chromosomes), hypodiploid (<0.95) and hyperdiploid (>1.05). 8-colors FacsCanto II BD flow cytometer was also used to evaluate minimal residual disease with 0.0025% threshold of detection.

Results: 26 cases were evaluated and 2 died during induction treatment. Median age was 8 years (4mo - 16years) and 54% were males. At diagnosis, 62% showed DI diploid and 38% DI hyperdiploid, no hypodiploids cases were detected. Clinical characteristics were similar between both groups. Median DI in the hyperdiploids cases was 1.25 (R: 1.12 - 1.64). We only detected two hyperdiploids cases by conventional karyotype. All patients received the same BFM-based protocol. After induction, all cases achieved complete remission and 46% had MRD negative at the 28^th day. DI diploid and hyperdiploid cases achieved 47% and 44% MRD negative, respectively. At the end of consolidation (R: 6-8 Mo), 77% cases achieved MRD negative, and between categories, 62% of DI diploid cases had MRD negative and 100% of DI hyperdiploid cases were negative for MRD detection by flow (p=0.02).

Conclusions: The hyperdiploid DNA index by flow cytometry is associated with minimal residual disease negative at the end of consolidation.Flow cytometry offers an alternative over the conventional karyotype to detect good prognosis groups among B-ALL cases.