Key Points• A novel RARa fusion gene, TBLR1-RARa, was found in rare cases of APL with t(3;17) chromosomal translocation.• TBLR1-RARa exhibited diminished transcriptional activity by recruiting more corepressors compared with RARa.The majority of acute promyelocytic leukemia (APL) cases are characterized by the PMLRARa fusion gene. Although the PML-RARa fusion gene can be detected in >98% of APL cases, RARa is also found to be fused with other partner genes, which are also related to all-trans retinoic acid (ATRA)-dependent transcriptional activity and cell differentiation.In this study, we identified a novel RARa fusion gene, TBLR1-RARa (GenBank KF589333), in a rare case of APL with a t(3;17)(q26;q21),t(7;17)(q11.2;q21) complex chromosomal rearrangement. To our knowledge, TBLR1-RARa is the 10th RARa chimeric gene that has been reported up to now. TBLR1-RARa contained the B-F domains of RARa and exhibited a distinct subcellular localization. It could form homodimers and also heterodimers with retinoid X receptor a. As a result, TBLR1-RARa exhibited diminished transcriptional activity by recruitment of more transcriptional corepressors compared with RARa. In the presence of pharmacologic doses of ATRA, TBLR1-RARa could be degraded, and its homodimerization was abrogated. Moreover, when treated with ATRA, TBLR1-RARa could mediate the dissociation and degradation of transcriptional corepressors, consequent transactivation of RARa target genes, and cell differentiation induction in a dose-and time-dependent manner. (Blood. 2014;124(6):936-945) IntroductionAcute promyelocytic leukemia (APL) is a special disease entity of acute myeloid leukemia. With the clinical use of all-trans retinoic acid (ATRA), APL turns into the most curable form of acute myeloid leukemia. The majority of APL cases are characterized by the fusion between the promyelocytic leukemia (PML) gene and the retinoic acid receptor a (RARa) gene, which is the consequence of t(15;17)(q22;q21) chromosomal translocation.1 Although the PML-RARa fusion gene can be detected in .98% of APL patients, 2 RARa is also found to be fused with other partner genes, 3-10 such as PML zinc finger (PLZF), nucleophosmin (NPM), and so forth. The RARa portion within fusion proteins is conserved, containing B-F domains of RARa, which cover the DNA-binding and ligand-binding motifs, so that RARa can be combined with retinoid X receptor a (RXRa) to bind the retinoic acid responsive element (RARE). 1,11 As a result, the fusion proteins usually form heterodimers with RXRa.9,12 A critical property that all RARa fusion proteins have in common is the ability to self-associate, forming homodimers. 13 The chimeric proteins, such as PML-RARa, PLZF-RARa, and NPMRARa, can robustly recruit transcriptional corepressors, including nuclear receptor corepressor/silencing mediator of retinoid and thyroid receptors (N-CoR/SMRT) and histone deacetylases (HDACs), to RARE and consequently result in ectopic repression of RARa target genes. 1,13,14 Transducin b-like 1 X-linked receptor 1 (TBLR1) ...

RUNX1 is a key transcription factor in hematopoiesis and its disruption is one of the most common aberrations in acute myeloid leukemia. RUNX1 alterations affect its DNA binding capacity and transcriptional activities, leading to the deregulation of transcriptional targets, and abnormal proliferation and differentiation of myeloid cells. Identification of RUNX1 target genes and clarification of their biological functions are of great importance in the search for new therapeutic strategies for RUNX1-altered leukemia. In this study, we identified and confirmed that KLF4 , a known tumor suppressor gene, as a direct target of RUNX1, was down-regulated in RUNX1-ETO leukemia. RUNX1 bound to KLF4 promoter in chromatin to activate its transcription, while the leukemogenic RUNX1-ETO fusion protein had little effect on this transactivation. KLF4 was also identified as a novel binding partner of RUNX1. RUNX1 interacted with KLF4 through Runt domain and further co-activated its target genes. However, RUNX1-ETO competed with RUNX1 to bind KLF4 through Runt and ETO domains, and abrogated transcription of KLF4. Finally, overexpression experiments indicated that RUNX1 inhibited proliferation and induced apoptosis of t(8;21) leukemia cells via KLF4-mediated upregulation of P57. These data suggest KLF4 dysregulation mediated by RUNX1-ETO enhances proliferation and retards apoptosis, and provides a potential target for therapy of t(8;21) acute myeloid leukemia.