LEF1 Is a Tumor Suppressor in T Cell Acute Lymphoblastic Leukemia

In an effort to further unravel the molecular pathogenesis of T cell acute lymphoblastic leukemia (T-ALL), we performed array CGH on diagnostic specimens from 47 pediatric patients with T-ALL. The LEF1 transcription factor is best known as a positive mediator of oncogenic β-catenin signaling, and it is required for the expression of MYC, Cyclin D1 and Survivin in some contexts, including some normal hematopoietic progenitors. Additionally, Lef1 has been shown to be required for the survival of murine Notch-dependent T cell lymphoma cells. We were thus surprised to find highly focal deletions of LEF1 in 10.6 % (n = 5 of 47) of primary T-ALL patient samples, with 3 cases harboring homozygous deletions and 2 harboring heterozygous deletions of this locus. These deletions involved no other known genes. We then sequenced this gene in 45 of these cases and found heterozygous mutations in 3 additional T-ALL samples, including 2 cases with frameshift mutations predicted to truncate the protein prior to its context-dependent activation and HMG box domains, and one case with an Asp85Asn substitution. Seven of the eight samples harboring LEF1 abnormalities also had mutations in the heterodimerization or PEST domains of NOTCH1. In an effort to characterize the biologic consequences of LEF1 loss in T-ALL, we took advantage of expression microarrays that were previously performed on most of these samples. LEF1 loss defines a novel T-ALL subtype characterized by arrest at the CD1-positive early cortical stage, and whose gene expression profile resembles that of HOX11-positive cases, although expression of HOX11 was generally low in these samples. Loss of LEF1 was mutually exclusive to overexpression of TAL1 and of the HOXA/MEIS1 cluster. Interestingly, LEF1-negative T-ALL cases were associated with increased expression of MYC, and gene set enrichment analysis identified a significant association between LEF1 loss and upregulation of MYC target genes. In addition to its role as a transcriptional activator in the setting of active WNT/β-catenin signaling, LEF1 can also act as a transcriptional repressor in some cellular contexts. Experiments are currently underway to establish the mechanism mediating the tumor suppressor activity of LEF1 in T-ALL.