TBLR1 fuses to retinoid acid receptor α in a variant t(3;17)(q26;q21) translocation of acute promyelocytic leukemia

Abstract

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) ...

NPM-RAR binding to TRADD selectively inhibits caspase activation, while allowing activation of NFκB and JNK

“…To date nine variant translocations have been characterized, all of which share the same C-terminal domains of RARA as PML-RAR. The N-terminal sequences of the fusions involve PLZF in t(11;17)q(23;q21) [2]; NPM in t(5;17)(q35;q21) [3]; NUMA in t(11;17)(q13;q21) [4]; STAT5b in der17[5]; PRKAR1A in t(17;17)(q24;q24) [6]; BCOR in t(X;17)(p11;q21) [7]; FIP1L1 in t(4;17)(q12;q21) [8]; OBFC2A in der (2)t(2;17)(q32;q21) [9]; and TBLR1 in t(3;17)(q26;q21) [10]. We have focused our studies on t(5;17), which stands out as the second most common variant after t(11;17)q(23;q21) and which shares a similar phenotype with t(15;17), including ATRA responsiveness[11].…”

Molecular Characteristics and Clinical Significance of 12 Fusion Genes in Acute Promyelocytic Leukemia: A Systematic Review

“…Corresponding reciprocal fusion genes, RARα-PML , RARα-ZBTB16 , RARα-NPM , and RARα-FIP1L1 , are expressed and exist with the rest of the reciprocal fusion genes that have yet to be identified [7,8,9,30,65,72,76,85,98]. The reciprocal product potentially contains the N-terminal transactivation domain of the RARα protein that is linked to the C-terminal domains of the fusion partner.…”

“…Chen et al [7] provided a comprehensive and detailed description of this novel fusion gene. TBLR1-RARα was shown to share some common features with other RARα fusion proteins.…”

“…To date, 11 variant fusion genes in APL, all involving the RARα gene, have been reported in a small percentage of APL cases. The partner genes include zinc finger and BTB domain-containing 16 ( ZBTB16 ; 11q23), nucleophosmin ( NPM ; 5q35), nuclear mitotic apparatus ( NuMA ; 11q13), signal transducer and activator of transcription (STAT) 5β ( STAT5b ; 17q21), protein kinase A (PKA) regulatory subunit type ( PRKAR1A ; 17q24), FIP1-like 1 ( FIP1L1 ; 4q12), BCL6 corepressor ( BCoR ; Xp11), oligonucleotide/oligosaccharide-binding fold-containing 2A ( OBFC2A ; 2q32) [6], transducin β-like 1 X-linked receptor 1 ( TBLR1 ; 3q26) [7], general transcription factor II-I ( GTF2I ; 7q11.23) [8], and interferon regulatory factor 2 binding protein 2 ( IRF2BP2 ; 1q42.3) [9]. The X-RARα (X:partner product) fusion pattern highlights the central role of RARα in the pathogenesis of APL.…”

“…Compared to PML-RARα, TBLR1-RARα acts more as a functional transcriptional activator, although it is less effective than wild-type RARα. When treated with pharmacological doses of ATRA, the fusion protein is degraded and prevented from undergoing homodimerization as well as mediating in the dissociation and degradation of transcriptional corepressors [7]. …”