Rag1 Co-Expression Signature Identifies Etv6-Runx1-Like B-Cell Precursor Acute Lymphoblastic Leukemia In Children

B-cell precursor acute lymphoblastic leukemia (BCP-ALL) can be classified into subtypes according to the genetic aberrations they display. For instance, the translocation t(12;21)(p13;q22), representing the ETV6-RUNX1 fusion gene (ER), is present in a quarter of BCP-ALL cases. However, around 10% of the cases lack classifying chromosomal abnormalities (B-other). In pediatric ER BCP-ALL, rearrangement mediated by RAG (recombination-activating genes) has been proposed as the predominant driver of oncogenic rearrangement. Herein we analyzed almost 1600 pediatric BCP-ALL samples to determine which subtypes express RAG. We demonstrate that RAG1 mRNA levels are especially high in the ETV6-RUNX1 (ER) subtype and in a subset of B-other samples. We also define 31 genes that are co-expressed with RAG1 (RAG1-signature) in the ER subtype, a signature that also identifies this subset of B-other samples. Moreover, this subset also shares leukemia and pro-B gene expression signatures as well as high levels of the ETV6 target genes (BIRC7, WBP1L, CLIC5, ANGPTL2) with the ER subtype, indicating that these B-other cases are the recently identified ER-like subtype. We validated our results in a cohort where ER-like has been defined, which confirmed expression of the RAG1-signature in this recently described subtype. Taken together, our results demonstrate that the RAG1-signature identifies the ER-like subtype. As there are no definitive genetic markers to identify this novel subtype, the RAG1-signature represents a means to screen for this leukemia in children.