• Using whole genome sequencing and RNA-seq, we characterized the genomic landscape of 295 DS-ALL cases and identified 15 distinct subtypes.

  • DS-ALL exhibits marked enrichment of RAG-mediated and C/EBP gene alterations, and inferior outcomes in BCR::ABL1-like CRLF2-r cases.

Trisomy 21, the genetic cause of Down syndrome (DS), is the most common congenital chromosomal anomaly. It is associated with a 20-fold increased risk of acute lymphoblastic leukemia (ALL) during childhood and results in distinctive leukemia biology. To comprehensively define the genomic landscape of DS-ALL, we performed whole genome sequencing and whole-transcriptome sequencing (RNA-Seq) on 295 cases. Our integrated genomic analyses identified 15 molecular subtypes of DS-ALL, with marked enrichment of CRLF2-r, IGH::IGF2BP1, and C/EBP altered (C/EBPalt) subtypes compared to 2257 non-DS-ALL cases. We observed abnormal activation of the CEBPD, CEBPA, and CEBPE genes in 10.5% of DS-ALL cases, via a variety of genomic mechanisms, including chromosomal rearrangements and noncoding mutations leading to enhancer hijacking. 42.3% of C/EBP-activated DS-ALL also have concomitant FLT3 point mutation or indel, relative to 4.1% in other subtypes (P=7.2×10-6). CEBPD overexpression enhanced the differentiation of mouse hematopoietic progenitor cells into pro-B cells in vitro, particularly in a DS genetic background. Notably, RAG-mediated somatic genomic abnormalities were common in DS-ALL, accounting for a median of 27.5% of structural alterations, compared to 7.7% in non-DS-ALL (P=2.1×10-12). Unsupervised hierarchical clustering analyses of CRLF2-rearranged DS-ALL identified substantial heterogeneity within this group, with the BCR::ABL1-like subset linked to an inferior event-free survival (hazard ratio=5.27, P=9.3×10-8), even after adjusting for known clinical risk factors (hazard ratio=4.32; P=0.0020). These results provide important insights into the biology of DS-ALL and point to opportunities for targeted therapy and treatment individualization.

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