Overexpression of BP1, a homeobox gene, is associated with resistance to all-trans retinoic acid in acute promyelocytic leukemia cells

The success and the challenge of all-trans retinoic acid in the treatment of cancer

All-trans retinoic acid (ATRA), an active metabolite of vitamin A, plays important roles in cell proliferation, cell differentiation, apoptosis, and embryonic development. The effects of ATRA are mediated by nuclear retinoid receptors as well as non-genomic signal pathway, such as MAPK and PKA. The great success of differentiation therapy with ATRA in acute promyelocytic leukemia (APL) not only improved the prognosis of APL but also spurred the studies of ATRA in the treatment of other tumors. Since the genetic and physiopathological simplicity of APL is not common in human malignancies, the combination of ATRA with other agents (chemotherapy, epigenetic modifiers, and arsenic trioxide, etc) had been extensively investigated in a variety of tumors. In this review, we will discuss in details about ATRA and its role in cancer treatment.

BP1, a potential biomarker for breast cancer prognosis

Homeobox genes are critical in tumor development. An isoform protein of DLX4 called BP1 is expressed in 80% of invasive ductal breast carcinomas. BP1 overexpression is implicated in an aggressive phenotype and poor prognosis. BP1 upregulation is associated with estrogen receptor negativity so those tumors do not respond to antiestrogens. Breast cancer is the second leading cause of death in women. BP1 could serve as both a novel prognostic biomarker for breast cancer and a therapeutic target. In this review, we address the role of BP1 protein in tumorigenesis of breast cancer and four other malignancies. A number of functions of BP1 in cancer are also discussed.

Dasatinib synergizes with ATRA to trigger granulocytic differentiation in ATRA resistant acute promyelocytic leukemia cell lines via Lyn inhibition-mediated activation of RAF-1/MEK/ERK

All-trans retinoic acid (ATRA) resistance has been a critical problem in acute promyelocytic leukemia (APL) relapsed patients. In this study, dasatinib synergized with ATRA to trigger differentiation in ATRA-resistant APL cell lines. The combined treatment activated RAF-1, MEK and ERK as well as enhanced ATRA-promoted up-regulation of the protein level of PU.1, C/EBPβ and C/EBPε. U0126 (MEK specific inhibitor) and sorafenib tosylate (RAF-1 specific inhibitor) suppressed the combined treatment-induced differentiation, ERK phosphorylation and the up-regulation of C/EBPs and PU.1. Sorafenib tosylate also attenuated the MEK activity. However, the combined treatment did not enhance Ras activity and Ras inhibitor neither blocked MEK activation nor inhibited differentiation. Therefore, the combined treatment induced differentiation via Ras independent RAF-1/MEK/ERK. Earlier than RAF-1 activation, dasatinib suppressed Lyn activity, the predominant activated Src family kinase (SFK) and dephosphorylated RAF-1 at S259. Furthermore, SFK inhibitor, PP2 did suppress Lyn activity and mimicked the effect of dasatinib on ATRA-induced differentiation as well as decreased phosphorylation of RAF-1 at S259. Thus, it was suggested that Lyn inhibition might activate RAF-1 by the dephosphorylation of RAF at S259 and lead to differentiation. In conclusion, the combination of dasatinib and ATRA could overcome ATRA resistance through Lyn inhibition-mediated activation of RAF-1/MEK/ERK.

Hypermethylation of DLX4 predicts poor clinical outcome in patients with myelodysplastic syndrome

BACKGROUND

Hypermethylation of DLX4 (distal-less homeobox 4) has been disclosed in a variety of cancers. Our work was aimed to examine the pattern of DLX4 methylation and further investigate its clinical relevance in patients with myelodysplastic syndrome (MDS).

METHODS

Real-time quantitative methylation-specific PCR and bisulfite sequencing PCR were carried out to detect the level of DLX4 methylation. Clinical significance of DLX4 methylation was analyzed between the DLX4 hypermethylated and non-hypermethylated patients.

RESULTS

DLX4 was significantly hypermethylated in MDS patients than controls (p<0.001). No significant differences were observed between the hypermethylated and non-hypermethylated MDS patients in white blood cells, platelets, age, WHO classifications, FAB classifications, IPSS risks, and common gene mutations (p>0.05). However, DLX4 hypermethylated patients tended to have higher hemoglobin (HB) than DLX4 non-hypermethylated patients (p=0.079). Moreover, there was a trend that male patients, poor karyotype patients, and IPSS Int-2/High patients had a higher frequency of DLX4 hypermethylation (p=0.067, 0.065, and 0.068). DLX4 hypermethylated patients had significantly shorter overall survival than DLX4 non-hypermethylated patients (p=0.004). Multivariate analysis confirmed the prognostic value of DLX4 methylation in MDS patients (p<0.001).

CONCLUSIONS

Our study indicated that DLX4 hypermethylation was a frequent event and acted as an independent prognostic biomarker in de novo MDS patients.

BP1 overexpression is associated with adverse prognosis in de novo acute myeloid leukemia

To investigate DLX4 isoforms expression and their clinical significance in acute myeloid leukemia (AML). DLX4 transcript variant 1 (BP1) expression was significantly up-regulated in AML patients compared with normal controls. However, DLX4 transcript variant 2 (DLX7) was significantly down-regulated in AML patients. Both in the overall AML and the non-M3 AML cohorts, those patients with high BP1 expression (BP1(high)) showed significantly lower rates of complete remission than those with low BP1 expression (BP1(low)). BP1(high) cases had significantly shorter overall survival than BP1(low) cases in the overall AML cohort, non-M3 AML, and cytogenetically normal AML (CN-AML). Multivariate analysis confirmed the independent prognostic value of BP1 expression among both the overall AML cohort and non-M3 AML as well as CN-AML patients. However, we did not observe the impact of DLX7 expression on prognosis in AML patients. Our study reveals that BP1 overexpression serves as an independent risk factor in de novo AML patients.

DLX4 is associated with orofacial clefting and abnormal jaw development

Cleft lip and/or palate (CL/P) are common structural birth defects in humans. We used exome sequencing to study a patient with bilateral CL/P and identified a single nucleotide deletion in the patient and her similarly affected son—c.546_546delG, predicting p.Gln183Argfs*57 in the Distal-less 4 (DLX4) gene. The sequence variant was absent from databases, predicted to be deleterious and was verified by Sanger sequencing. In mammals, there are three Dlx homeobox clusters with closely located gene pairs (Dlx1/Dlx2, Dlx3/Dlx4, Dlx5/Dlx6). In situ hybridization showed that Dlx4 was expressed in the mesenchyme of the murine palatal shelves at E12.5, prior to palate closure. Wild-type human DLX4, but not mutant DLX4_c.546delG, could activate two murine Dlx conserved regulatory elements, implying that the mutation caused haploinsufficiency. We showed that reduced DLX4 expression after short interfering RNA treatment in a human cell line resulted in significant up-regulation of DLX3, DLX5 and DLX6, with reduced expression of DLX2 and significant up-regulation of BMP4, although the increased BMP4 expression was demonstrated only in HeLa cells. We used antisense morpholino oligonucleotides to target the orthologous Danio rerio gene, dlx4b, and found reduced cranial size and abnormal cartilaginous elements. We sequenced DLX4 in 155 patients with non-syndromic CL/P and CP, but observed no sequence variants. From the published literature, Dlx1/Dlx2 double homozygous null mice and Dlx5 homozygous null mice both have clefts of the secondary palate. This first finding of a DLX4 mutation in a family with CL/P establishes DLX4 as a potential cause of human clefts.

The homeobox gene DLX4 promotes generation of human induced pluripotent stem cells

The reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) by defined transcription factors has been a well-established technique and will provide an invaluable resource for regenerative medicine. However, the low reprogramming efficiency of human iPSC is still a limitation for clinical application. Here we showed that the reprogramming potential of human dental pulp cells (DPCs) obtained from immature teeth is much higher than those of mature teeth DPCs. Furthermore, immature teeth DPCs can be reprogrammed by OCT3/4 and SOX2, conversely these two factors are insufficient to convert mature teeth DPCs to pluripotent states. Using a gene expression profiles between these two DPC groups, we identified a new transcript factor, distal-less homeobox 4 (DLX4), which was highly expressed in immature teeth DPCs and significantly promoted human iPSC generation in combination with OCT3/4, SOX2, and KLF4. We further show that activation of TGF-β signaling suppresses the expression of DLX4 in DPCs and impairs the iPSC generation of DPCs. Our findings indicate that DLX4 can functionally replace c-MYC and supports efficient reprogramming of immature teeth DPCs.

Dynamics of leukemia stem-like cell extinction in acute promyelocytic leukemia

Many tumors are believed to be maintained by a small number of cancer stem-like cells, where cure is thought to require eradication of this cell population. In this study, we investigated the dynamics of acute promyelocytic leukemia (APL) before and during therapy with regard to disease initiation, progression, and therapeutic response. This investigation used a mathematical model of hematopoiesis and a dataset derived from the North American Intergroup Study INT0129. The known phenotypic constraints of APL could be explained by a combination of differentiation blockade of PML-RARα-positive cells and suppression of normal hematopoiesis. All-trans retinoic acid (ATRA) neutralizes the differentiation block and decreases the proliferation rate of leukemic stem cells in vivo. Prolonged ATRA treatment after chemotherapy can cure patients with APL by eliminating the stem-like cell population over the course of approximately one year. To our knowledge, this study offers the first estimate of the average duration of therapy that is required to eliminate stem-like cancer cells from a human tumor, with the potential for the refinement of treatment strategies to better manage human malignancy.

NKL homeobox genes in leukemia

NK-like (NKL) homeobox genes code for transcription factors, which can act as key regulators in fundamental cellular processes. NKL genes have been implicated in divergent types of cancer. In this review, we summarize the involvement of NKL genes in cancer and leukemia in particular. NKL genes can act as tumor-suppressor genes and as oncogenes, depending on tissue type. Aberrant expression of NKL genes is especially common in T-cell acute lymphoblastic leukemia (T-ALL). In T-ALL, 8 NKL genes have been reported to be highly expressed in specific T-ALL subgroups, and in ~30% of cases, high expression is caused by chromosomal rearrangement of 1 of 5 NKL genes. Most of these NKL genes are normally not expressed in T-cell development. We hypothesize that the NKL genes might share a similar downstream effect that promotes leukemogenesis, possibly due to mimicking a NKL gene that has a physiological role in early hematopoietic development, such as HHEX. All eight NKL genes posses a conserved Eh1 repressor motif, which has an important role in regulating downstream targets in hematopoiesis and possibly in leukemogenesis as well. Identification of a potential common leukemogenic NKL downstream pathway will provide a promising subject for future studies.

All-trans retinoic acid induces Thrombospondin-1 expression in acute promyelocytic leukemia cells though down-regulation of its transcription repressor, c-MYC oncoprotein

Thrombospondin-1 (TSP-1) was found to mediate the therapeutic effects of all-trans retinoic acid (ATRA) for leukemia. The aim of the present study was to evaluate the role of c-MYC, a key transcription factor that contributes to the genesis of many human tumors, in TSP-1 induction by ATRA in acute promyelocytic leukemia (APL). ATRA treatment markedly increased TSP-1 level and inhibited c-MYC expression in NB4 APL leukemic cells compared with controls. Promoter assays indicated that c-MYC responsive element is functional relevant to the induction of TSP-1 promoter activity by ATRA. c-MYC recruitment to TSP-1 promoter was dramatically decreased in NB4 cells following ATRA treatment. shRNA-mediated inhibition of c-MYC resulted in a marked up-regulation of endogenous TSP-1 expression. Moreover, transient over-expression of c-MYC totally abolished TSP-1 induction by ATRA in NB4 cells. Collectively, our results indicate that ATRA induces TSP-1 expression in APL cells though down-regulation of its transcription repressor, c-MYC oncoprotein.

Prognostic significance of BP1 mRNA expression level in patients with non-small cell lung cancer

OBJECTIVES

To examine the association of BP1 mRNA level with tumor characteristics and clinical prognosis in non-small cell lung cancer (NSCLC) patients.

DESIGN AND METHODS

Tumor specimens from 98 NSCLC patients who underwent surgical resection were quantitatively determined for BP1 mRNA expression by real-time RT-PCR.

RESULTS

BP1 mRNA was expressed at significantly higher levels in tumors than in adjacent nontumorous tissues and normal lung samples. The level of BP1 transcript was significantly associated with tumor histological type and cell differentiation grade, but not related with other clinicopathological factors and p53 mutations. Patients with high BP1 mRNA expression had a poorer prognosis in terms of both disease-free survival (DFS) and overall survival (OS) rates. Additionally, BP1 mRNA expression level was an independent prognostic factor for DFS.

CONCLUSIONS

BP1 may be part of a pathway contributing to NSCLC development and/or progression. BP1 mRNA level could be a novel prognostic marker for NSCLC.