Altres ajuts: This work was supported by the Asociación Española Contra el Cáncer (AECC-CI-2015), FERO Foundation. PM/IV also acknowledge financial support from The Obra Social La Caixa-Fundaciò Josep Carreras, The Inocente Inocente Foundation and Fundación Ramón Areces.PM is an investigator of the Spanish Cell Therapy cooperative network (TERCEL).

Bcell acute lymphoblastic leukemia is the commonest childhood cancer. In infants, B-cell acute lymphoblastic leukemia remains fatal, especially in patients with t(4;11), present in ~80% of cases. The pathogenesis of t(4;11)/KMT2A-AFF1 (MLL-AF4) infant B-cell acute lymphoblastic leukemia remains difficult to model, and the pathogenic contribution in cancer of the reciprocal fusions resulting from derivative translocated-chromosomes remains obscure. Here, "multi-layered" genome-wide analyses and validation were performed on a total of 124 de novo cases of infant B-cell acute lymphoblastic leukemia uniformly diagnosed and treated according to the Interfant 99/06 protocol. These patients showed the most silent mutational landscape reported so far for any sequenced pediatric cancer. Recurrent mutations were exclusively found in K-RAS and N-RAS, were subclonal and were frequently lost at relapse, despite a larger number of non-recurrent/non-silent mutations. Unlike non-MLL-rearranged B-cell acute lymphoblastic leukemias, B-cell receptor repertoire analysis revealed minor, non-expanded B-cell clones in t(4;11) infant B-cell acute lymphoblastic leukemia, and RNAsequencing showed transcriptomic similarities between t(4;11) infant B-cell acute lymphoblastic leukemias and the most immature human fetal liver hematopoietic stem and progenitor cells, confirming a "pre-VDJ" fetal cellular origin for both t(4;11) and RAS. The reciprocal fusion AF4-MLL was expressed in only 45% (19/43) of the t(4;11) patients, and HOXA cluster genes are exclusively expressed in AF4-MLL-expressing patients. Importantly, AF4-MLL/HOXA-expressing patients had a significantly better 4-year eventfree survival (62.4% vs. 11.7%, P=0.001), and overall survival (73.7 vs. 25.2%, P=0.016). AF4-MLL expression retained its prognostic significance when analyzed in a Cox model adjusting for risk stratification according to the Interfant-06 protocol based on age at diagnosis, white blood cell count and response to prednisone. This study has clinical implications for disease outcome and diagnostic risk-stratification of t(4;11) infant B-cell acute lymphoblastic leukemia.