Dual role of BCL11B in T-cell malignancies

The zinc finger transcription factor B-cell CLL/lymphoma 11B gene (BCL11B, CTIP2) plays a crucial role in T-cell development, but its role in T-cell malignancies has not yet been definitively clarified. In the literature, 2 contradictory hypotheses on the function of BCL11B exist. One suggests that BCL11B functions as tumor suppressor gene, and the other suggests that BCL11B functions as oncogene. The aim of this review is to revise the current knowledge about the function of BCL11B in T-cell malignancies, confront these 2 hypotheses and present a new model of dual role of BCL11B in T-cell malignancies and potential new therapeutic approach, based on recent findings of the function of BCL11B in DNA damage repair. Decreased BCL11B expression, resulting in deficient DNA repair, may facilitate DNA mutations in rapidly proliferating T-cell progenitors that undergo gene rearrangements, thereby leading to malignant transformation. On the other hand, decreased BCL11B expression and inefficient DNA repair may result in accumulation of DNA damages in genes crucial for the cell survival and in apoptosis of malignant T cells. We hypothesize that T-cell malignancies expressing high levels of BCL11B might be dependent on it. In those cases, targeted inhibition of BCL11B expression may have a therapeutic effect. The antitumor effect of BCL11B suppression might be strengthened by generation of induced T to NK cells (ITNK). Therefore, there is an urgent need to develop a specific BCL11B inhibitor.


INTRODUCTION
The C(2)H(2) zinc finger transcription factors B-cell CLL/ lymphoma 11A (Bcl11a), encoded by the BCL11A (CTIP1) gene, and its paralog Bcl11b encoded by the BCL11B (CTIP2) gene, were first identified in mice in 2000, as binding partners of the chicken ovalbumin upstream promoter-transcription factor (COUP-TF) family of nuclear hormone receptors. 1One year later, human homologs of BCL11A and BCL11B were identified, and localized to chr2p13 and chr14q32.1,respectively. 2espite substantial similarities in their structure and binding sites, [2][3][4] the 2 factors have distinct biological functions and roles in lymphocyte development.Bcl11a has a broader function in hematopoiesis.5][6][7] In contrast to BCL11A, and to its misleading name, BCL11B is neither expressed in B-cell CLL nor in B-cell lymphoma.In the hematopoietic system, BCL11B is expressed almost exclusively in the T-cell lineage.4][15] It is also responsible for proper function of mature T lymphocytes, and removal of Bcl11b at the double-positive stage of T-cell development or in T(reg) cells causes autoimmune diseases. 16,17Besides T cells, BCL111B is implicated in the development of other nonhematopoietic tissues, including neurogenesis, skin development, adipogenesis, tooth formation, and cranial suture ossification, 18,19 but this is beyond the scope of this article.
Although in the last 2 decades many studies have been published on BCL11B, its role in T-cell malignancies has not yet been definitively clarified.The aim of this review is to discuss the 2 contradictory hypotheses on the function of BCL11B in T-cell neoplasms; tumor suppressor; or oncogene.

BCL11B ALTERATIONS IN T-CELL MALIGNANCIES
First recurrent chromosomal alteration involving BCL11B, t(5;14)(q35;q32), associated with expression of the TLX3 oncogene, due its juxtaposition to BCL11B, were reported by Bernard et al. 20 First rearrangement affecting directly the BCL11B gene was reported by Przybylski et al. 21This rearrangement resulted in the expression of the 5ʹ part of BCL11B, including exons 1 to 3, fused to the constant region of the T-cell receptor delta gene (TRDC).Since then, many other genetic alterations have been described in T-cell and T/myeloid-mixed phenotype acute leukemia. 223][24][25] The effect of chromosomal rearrangements activating known oncogenes is quite evident, but the effect of heterogenous rearrangements affecting BCL11B gene itself is less obvious, and has not yet been sufficiently studied.

BCL11B AS TUMOR SUPPRESSOR GENE
Originally, BCL11B was reported by the team of Kominami, as radiation-induced tumor suppressor gene (Rit1). 26Using genome-wide allelic loss analysis, it has been shown that in murine γ-ray-induced thymic lymphomas a region of chromosome 12, containing, at that time, unknown BCL11B gene, was frequently deleted.In subsequent studies, they have found biallelic changes of BCL11B in p53-proficient lymphomas, what suggested an association between the presence of functional p53 and inactivation of BCL11B in the lymphoma development. 27urthermore, introduction of BCL11B into HeLa cells lacking BCL11B expression suppressed cell growth.The authors concluded that loss-of-function mutations of BCL11B contribute to cancer development.On the other hand, the same group showed that mice born with homozygous BCL111B knockout show block of α/β T-cell differentiation and die but do not develop T-cell malignancies. 12In a large study on T-ALL, monoallelic BCL11B deletions or missense mutations were found in 9% (10 of 117) of cases. 28Some of them disrupted the structure of zinc finger domains required for DNA binding.The authors postulated that BCL11B is a haploinsufficient tumor suppressor that collaborates with all major T-ALL oncogenic lesions in human thymocyte transformation, although this has not been confirmed in functional studies.

BCL11B AS ONCOGENE
In ATM −/− mice heterozygous loss of BCL11B reduced lethal thymic lymphoma by suppressing lymphoma progression but not initiation.The suppression was associated with a T cellmediated immune response, revealing a haploid insufficient function of Bcl11b in immune modulation against lymphoma and offering an explanation for the complex relationship between Bcl11b status with T-ALL prognosis. 29In our recent study, we created mice with heterozygous BCL11B deletion.These mice have a normal life span and do not develop malignancies. 30The hypothesis of the oncogene function of BCL11B is further supported by research of our group, showing that BCL11B is overexpressed in the majority of T-cell acute lymphoblastic leukemia (T-ALL), 21 and inhibition of BCL11B using siRNA leads to apoptosis of malignant but not normal T cells. 31his indicates that malignant T cells need BCL11B for their survival.Subsequently, we showed that forced overexpression of BCL11B resulted in markedly increased resistance to radiomimetic drugs, whereas no influence on death-receptor apoptotic pathway was observed. 32Apoptosis resistance triggered by BCL11B overexpression was accompanied by a cell cycle delay caused by accumulation of cells at G1.This cell cycle restriction was associated with upregulation of cyclin-dependent kinase inhibitors.Moreover, the SKP2 gene encoding a protein of the ubiquitin-binding complex responsible for their degradation was repressed and the expression of the MYCN oncogene was silenced.Furthermore, it was shown that enhancer hijacking drives oncogenic BCL11B expression in lineage-ambiguous stem cell leukemia with expression of myeloid and T lymphoid markers.This upregulation was driven by chromosomal rearrangements that juxtapose BCL11B to superenhancers active in hematopoietic progenitors, or amplifications that generate a superenhancer from a noncoding elements distal to BCL11B.These data support the role of BCL11B overexpression as an oncogenic event in leukemia with T-cell markers. 33,34Our studies in human naïve T cells showed increased proliferation upon BCL11B overexpression and reduced proliferation upon its downregulation. 35The data suggest a potential role of BCL11B in tumor survival and encourage developing Bcl11b-inhibitory approaches as a potential tool to specifically target chemoresistant tumor cells.Very recently, we have showed that BCL11B promotes T-ALL cell survival via the XRCC5/C11ORF21 axis. 36hese results are in line with our recent data showing better prognosis for patients with T-cell leukemia and lymphoma with BCL11B mutations. 37,38However, this research included a small number of cases with BCL11B mutations, and has to be confirmed in a larger study.

ROLE OF BCL11B IN DNA REPAIR
Very recent study on BCL11B involvement in DNA repair provided new important information on role in tumor development.Vickridge et al 39 had shown that BCL11B increases the enzymatic activity of NTHL1 glycosylase and Pol β polymerase, responsible for base excision repair (BER) of DNA, by stimulating binding to their substrate.Furthermore, they showed that BCL11B knockdown increases DNA damage, delays the repair of oxidized bases and abasic sites, and increases the spontaneous and radiation-induced mutation rates, thereby leading to apoptosis of malignant cells.Ectopic overexpression of a small, lacking transcription regulation potential, fragment of BCL11B (BCL11B  ) accelerated DNA repair and increased resistance to oxidative DNA damage. Most inerestingly, they showed that overexpressed BCL11B  cooperates with RAS oncogene in primary cell transformation, by repairing DNA damage caused by RAS induced production of reactive oxygen species, and thereby avoiding cellular senescence.The evidence for tumor suppressor or oncogene role of BCL11B has been summarized in Table 1.

THERAPEUTIC IMPLICATIONS OF THE DUAL ROLE OF BCL11B IN T-CELL MALIGNANCIES
Although BCL11B inactivation plays a role in accumulation of mutations and may be responsible for malignant transformation of T-cell progenitors, in already developed T-cell malignancy it is BCL11B expression that is necessary for the survival of malignant T cells and progression of the disease.This applies only to T-cell leukemia and lymphoma, and probably also to some nonhematopoietic tumors, that express high levels of BCL11B.Those malignant cells are dependent on DNA repair provided by BCL11B, and deprived of this would eventually undergo cell senescence and die.This makes inhibition of BCL11B a new, promising therapeutic approach, in patients with T-cell malignancies expressing high levels of BCL11B.Furthermore, Li et al 9 had shown that BCL11B suppression in normal T lymphocytes leads to their transition to induced T to NK cells (ITNK) exhibiting a strong antitumor effect.Therefore, simultaneous inhibiting of BCL11B in malignant and normal T cells could have a synergistic antitumor effect (Fig. 1).To test the usefulness of BCL11B targeted therapy, a specific BCL11B inhibitor has to be developed.

SUMMARY
Early studies on BCL11B role in T-cell malignancies, mostly based on the deletions and mutations, suggested its tumor suppressor function.Later, more and more evidence was accumulated indicating that BCL11B might act as oncogene.Currently, based on recent findings of the function of BCL11B in base excision repair, it seems that BCL11B has a dual role in BCL11B malignancies.On the one hand, decreased BCL11B expression resulting in inefficient DNA repair, especially in rapidly proliferating and undergoing T-cell receptor genes rearrangements T-cell progenitors, may facilitate DNA lesions and lead to malignant transformation.On the other hand, decreased BCL11B expression and inefficient DNA repair may result in further accumulation of DNA damages in genes crucial for the cell survival, and result in apoptosis of malignant T cells.
It can be hypothesized that a fraction of T-cell malignancies, most likely those expressing high levels of BCL11B, might be dependent on it.Additionally, suppression of BCL11B in normal T cells leads to their transition to induced T to NK cells (ITNK) with cytotoxic antitumor activity. 9Both mechanisms synergistically contribute to the death of tumor cells.

Figure 1 .
Figure 1.Potential therapeutic effect of BCL11B inhibition in T-cell malignancies.Suppression of BCL11B in malignant T cells results in insufficient DNA repair and thereby to cell senescence and apoptosis.31,35Additionally, suppression of BCL11B in normal T cells leads to their transition to induced T to NK cells (ITNK) with cytotoxic antitumor activity.9Both mechanisms synergistically contribute to the death of tumor cells.

Table 1 .
Evidence for tumor suppressor or oncogene role of BCL11B.