Treatment Outcomes of Childhood Picalm:MLLT10⁺ Acute Leukemias: An International Retrospective Study

Introduction: Acute leukemia with translocation t(10;11)(p12-13;q14-21) results in generation of the PICALM:MLLT10 fusion gene, and is distinct from t(10;11)(p11-13;q23) which gives rise to KMT2A (formerly MLL) gene rearrangement. Defective PICALM function leads to aberrant hematopoiesis, while defective MLLT10 function impairs chromatin-mediated gene regulation. The PICALM:MLLT10 gene has been reported in patients with Acute Myeloid Leukemia (AML), Acute Lymphoblastic Leukemia (ALL) predominantly T-cell, Acute Undifferentiated Leukemia (AUL) and Mixed Phenotype Acute Leukemia (MPAL). MLLT10 interacts directly with DOT1L in a complex that regulates the methylation of H3K79. MLLT10 fusions are thought to misdirect DOT1L to the promoters of HOXA genes, leading to hypermethylation of H3K79. This hypermethylation causes constitutive activation of HOXA activity and prevents cell maturation and differentiation. The clinico-biological characteristics, response to conventional treatments, and survival outcomes of childhood PICALM:MLLT10⁺ leukemias are poorly understood.

Methods: This was an international, multicenter, retrospective cohort study. Data were submitted by ten International-BFM (I-BFM) study Group-AML and ALL committees and by the Children`s Oncology Group (COG)-AML Study committee. For study inclusions, patients were required to be ≤ 21 years of age at diagnosis, diagnosed between January 1995 and December 2019 with de novo ALL, AML, AUL or MPAL and PICALM:MLLT10 fusion-positive. Patients with KMT2A-rearranged leukemia or acute promyelocytic leukemia were excluded. Kaplan Meier curves were employed for survival analysis with Cox proportional hazard modelling to compare effect of AML vs ALL therapy, and receipt of hematopoietic stem cell transplant (HSCT) vs no HSCT, on survival. Pearson's Chi Squared was employed as a non-parametric measure to determine the effect of end of induction-1 (EIO-1) MRD positive vs negative status on incidence of relapse. P value cut-off for data to reach statistical significance was <0.05.

Results: Ninety-eight children with acute leukemia were included, 60 were males. Median age at diagnosis was 12 years (IQR:9-14.8 years) and median initial white cell count was 40.5 x 10⁹/L (IQR:13.8-168.9). Fifty-five (55.6%) patients were diagnosed with T-ALL, 39 (39.4%) with AML, while precursor-B ALL, AUL and MPAL were each diagnosed in single patients; 1 subtype diagnosis was unknown. Ten (10.1%) patients presented with CNS disease and 13 (13.1%) presented with non-CNS extramedullary manifestations. Of 66 with cytogenetic/molecular data available, 33 (50%) had additional cytogenetic-molecular aberrations; NOTCH1, WT1 and RAS pathway mutations predominated. Most patients received an initial chemotherapy regimen guided by their disease immunophenotype: 58/98 (58.6%) received an ALL-like chemotherapy regimen upfront, 38 (38.4%) received an AML-like regimen, 1 (1.0%) a hybrid regimen, 1 initial chemotherapy regimen was unknown. Five-year event-free survival (EFS) was 50.4% (CI 39.1-64.9). For AML patients, 5-year EFS was 22.5% (CI 9.75-52.0) and for ALL 67.7% (CI 54.2-84.7), p=<0.001. Five-year overall survival (OS) was 56.3% (CI 46.3-68.5). For AML patients, 5-year OS was 26.7% (CI 13.3-53.6) and for ALL 75.7% (CI 64.6-88.8), p=<0.001. Thirty-eight patients died, 22 (57.9%) deaths were disease-related, 4 (10.5%) treatment-related. Patients who received HSCT in CR1, demonstrated a trend towards an inferior 5-year EFS (23.7% vs 56.5%) and OS (42.6% vs 64.5%) than those who did not; however, analysis was limited by small sample size. Four (22.2%) HSCT recipients had ALL but most, 14 (77.8%), had AML, which may account for the unfavourable outcomes associated with HSCT. There was no statistically significant difference in rate of relapse in the EOI-1 MRD-positive compared with MRD-negative patients (p=0.49). Of those who relapsed, 25 (75.8%) had an isolated bone marrow relapse, 3 (9.1%) isolated CNS, 3 (9.1%) other sites and, for 2 patients, site of relapse was unknown.

Conclusions: Children with PICALM:MLLT10⁺ AML have inferior 5-year EFS and OS outcomes compared to their T-ALL counterparts. MRD status at EOI-1 was not predictive of relapse. Targeted therapies with DOT1L inhibitors or DNA demethylating agents should be explored, particularly for patients with AML.

Tomizawa:Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Schmiegelow:Medscape: Speakers Bureau; Servier: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Amgen: Speakers Bureau; Novo Nordisk: Research Funding; Illumina: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau.