CO51 | Pediatric PAX5-rearranged acute lymphoblastic leukemia is a high-risk molecular subgroup of B-ALL with a poor MRD-dependent prognostic profile

Introduction. PAX5-altered acute lymphoblastic leukemia (PAX5-alt ALL) is a recently defined molecular subtype of BCP-ALL, which displays a specific gene expression and is frequently associated with PAX5 aberrations, including rearrangements (PAX5r). Poor/intermediate outcomes were reported for PAX5-alt ALL. However, evidence on clinical features and prognosis of pediatric PAX5r ALL remains limited.Methods: Clinical data of all newly diagnosed PAX5r ALL patients (pts) (100x109/L) (22%) was found. According to NCI criteria, 53.3% had high-risk (HR) status. IKZF1plus profile was found in 15.1% of the pts. A tendency to slow treatment response was observed: 13% of pts had prednisone poor response (PPR), 74.7% were PCR-MRD+ at the end of induction (EOI), with 28.6% showing MRD≥5x10-4, and 31.5% were still PCR-MRD+ at the end of consolidation. Per protocol final risk stratification: 25.8%, 42.9% and 31.3% of the pts were standard-risk (SR), medium-risk (MR) and HR, respectively. Further, we restricted the analyses solely to the consecutive PAX5r pts (n=99) enrolled in the AIEOP-BFM ALL 2017 study and compared their clinical features with all non-PAX5r pts (n=1945) enrolled in the same protocol. ETV6::RUNX1 pts were excluded from the analysis. Lower median age at diagnosis (3.4 vs 4.7 yrs), higher hyperleukocytosis rate (23.2% vs 8%, p