The association between FOXO3a rs4946936 gene polymorphism and the levels of FOXO3a among chronic granulocytic leukemia patients treated with imatinib mesylate

Introduction

Chronic granulocytic leukemia (CGL), first recognized in 1845, is a myeloproliferative disease caused by a condition in which a single pluripotential haemopoetic stem cell acquires the Philadelphia chromosome.¹ A global report in 2015 revealed that the incidence of this disease was estimated between 0.7 and 1.0 per 100,000 inhabitants,² while mortality was reported at less than 15%.³ The pathogenesis of CGL is a comprehensive process, and may involve a wide variety of biomarkers including cyclooxgenase 1 (COX1), patched homolog 1 (PTCH1), Forkhead Transcription Factor 3a (FOXO3a), prostaglandin-endoperoxide synthase 1 (PTGS1), and human organic cation transporter 1 (hOCT1).^4–6 Of them, FOXO3a is proposed as one biomarker having a pertinent role in the pathogenesis of CGL, although evidence is limited.⁶

FOXO3a belongs to the FOXO Forkhead transcription factors subfamily. The FOXO transcription factors have a various function in the cell stability, including the activation of target – gene expression and the inhibition of target – gene expression, depending on the type of their subfamily and their interaction to the specific protein.⁷ The subfamily of FOXO includes FOXO1, FOXO3a, FOXO3b, FOXO4 and FOXO6.⁸ Of them, FOXO3a palys a dominant role to activate the expression of target gene in the pathogenesis of CGL, and may contribute to the progression of CGL.^6,7,9 Moreover, the existence of FOXO3a in the circulation is governed by the FOXO3a gene, which has several single nucleotide polymorphisms (SNPs) such as rs2802292, rs2764264, rs4945816, rs9400239, rs4946936, and rs13217795.¹⁰ Of them, the FOXO3a rs4946936 gene variant is suggested to have a crucial role in affecting the levels of FOXO3a in circulation, and therefore may have a potential role in the pathogenesis of CGL. While the gene polymorphism of FOXO3a rs4946936 has been investigated in the case of vitiligo,¹¹ chronic obstructive pulmonary disease (COPD),¹² acute lymphoblastic leukemia,¹³ thyroid cancer,¹⁴ and hepatocellular carcinoma,¹⁵ the role of FOXO3a rs4946936 gene polymorphisms in the development of CGL has never been examined. Therefore, we aimed to assess the correlation between FOXO3a rs4946936 gene polymorphism and the levels of FOXO3a in patients with CGL treated with imatinib mesylate. Our report may serve as an initial insight into the role of the FOXO3a rs4946936 gene polymorphism in the pathogenesis of CGL.

Methods

Genotyping of FOXO3a rs4946936 gene polymorphism

We collected venous blood in plastic vacutainer tubes containing EDTA. We separated the blood samples, and they were stored at −20 °C. We centrifuged the blood samples in EDTA at 1000 ×g for 10 min, and we separated the plasma and stored it at −20 °C until it was used to measure FOXO3a levels. A polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method was applied to genotype the FOXO3a rs4946936 gene polymorphism. The procedures in FOXO3a genotyping including the primer used, amplification, and PCR cycles (initial denaturation, denaturation, annealing, and extension) were adapted from previous studies,^11,12 using the CFX96 TouchTM Real-Time PCR Detection System (Bio-Rad, California, USA). The FOXO3a primer (FOXO3a F: 5′-GGGTCCTGAGAACTTCTGAGT-3′; FOXO3a R: 5′-GACATTCTGTAAGACATTCTGCCT-3′) used in our present study was MyTaq HS Mix, 2X (Bioline, Meridian BioscienceTM, OH, USA). We used SfcI (New England Biolabs, Massachusetts, USA) as the restriction enzyme. PCR was conducted in a 50 μl volume with 20–100 ng DNA, 20 pmol of each primer, 50 μM KCl, 100 μm dNTPs, 20 μM Tris-HCl pH 8.6.

1 mM MgCl₂, and 1 U Taq polymerase (MBI Fermentas, Vilnius, Lithuania). Amplification was carried out using an automated thermal cycler (Techne-Genius, Princeton, NJ). The PCR cycles consisted of initial denaturation at 94 °C for 3 min; 35 cycles of denaturation at 94 °C for 1 min, annealing at 53 °C for 1 min, and extension at 72 °C for 1 min, followed by final extension at 72 °C for 7 min. The amplified products were digested with one unit of Sfcl (rs4946936; C/T). The digested fragments were separated on 2% agarose gel by electrophoresis. The genotypes were identified according to their fragment sizes. The TT genotype was characterized by 224 bp sized bands, the TC genotype was characterized by 224, 152, and 72 bp sized bands, and the size bands of 152 and 72 indicated the CC genotype (Figure 1).

Figure 1. A visualization of the FOXO3a rs4946936 gene polymorphism.

The 224bp sized band indicated TT genotype, the 152bp size band indicated CT genotype, and the 72bp sized band indicated CC genotype.

Results

Main findings

Our findings identified that elevated levels of FOXO3a were observed among CGL patients with CC (CC vs. CT + TT) genotype (MD: 26.56; 95% CI: 0.48, 52.64). While the association between the CT (CT vs. CC + TT) and TT (TT vs. CC + CT) genotypes and the levels of FOXO3a were unverified (Figure 2).

Figure 2. The summary of FOXO3a levels and the gene polymorphism of FOXO3a in chronic granulocytic leukemia patients treated with imatinib mesylate.

A). CC vs. CT+TT, MD: 26.56, 95%CI: 0.48, 52.64, p: 0.0460; B). CT vs. CC+TT, MD: −8.45, 95%CI: −34.09, 17.19, p: 0.5180; C). TT vs. CC+CT, MD: −28.33, 95%CI: −62.06, 5.39, p: 0.1000.

Discussion

Our study identified that the CC genotype of the FOXO3a rs4946936 gene polymorphism was associated with increased levels of FOXO3a among patients with CGL treated with imatinib mesylate. The study in the context of the association between the FOXO3a rs4946936 gene polymorphism and the levels of FOXO3a in patients with CGL treated with imatinib mesylate had, to our knowledge, never been performed previously, therefore, an holistic comparison including the gene-ethnicity or gene-environment interaction could not be discussed. Previous studies in this context had been performed in the case of COPD¹² and vitiligo.¹¹ In the case of vitiligo, a study failed to clarify the correlation between the levels of FOXO3a and the gene polymorphism of FOXO3a rs4946936.¹¹ Moreover, in the case of COPD, no association was observed between the levels of FOXO3a and the gene polymorphism of FOXO3a rs4946936.¹² The different findings between our study in the case of CGL and previous studies in the case of vitiligo and COPD are contradictory and debatable, and a possible reason might be proposed. Briefly, vitiligo and COPD are known as conditions that affect the levels of FOXO3a, and the mechanism of FOXO3a production in the case of CGL might differ to the case of vitiligo and COPD. In the case of vitiligo, FOXO3a may interrupt the oxidative pathway by controlling the target gene expression.¹¹ In the case of COPD, FOXO3a may stimulate the expression of Atrogin-1/MAFbx, a biological marker having a pivotal role in the development of COPD.¹² On the other hand, the role of FOXO3a in the case of CGL occurs through establishing the phosphorylation by p210BCR-ABL tyrosine kinase, and may contribute to the proliferation of leukemic progenitors.^18,19

Theoretically, the association between the levels of FOXO3a and the gene polymorphism of FOXO3a rs4946936 remain conflicting. However, several mechanisms might be proposed. First, the gene-gene interaction or gene-protein interaction might underly our findings. This possible mechanism was supported by a previous study.²⁰ The authors revealed that a dominant homozygote genotype of the FOXO3a rs4946936 gene variant was reported to associate with elevated levels of immunoglobulin (Ig) E in the case of asthma. They proposed that FOXO3a might regulate the production of proinflammatory cytokine, and cause elevated levels of IgE.²⁰ Second, free energy levels between genotypes of the FOXO3a rs4946936 gene polymorphism might affect the levels of FOXO3a in the circulation. In an in silico study, the free energy levels of the CC and TT genotype of the FOXO3a rs4946936 gene variant were −320.80 Kcal/mol and −301.10 Kcal/mol, respectively, indicating that the CC genotype had lower free energy levels.²¹ Low free energy levels were associated with the mRNA structure and translation, and the changes of mRNA translation at the 5′ and 3′ UTR locations were correlated to affect protein synthesis.^21–23 This explanation might describe the possible underlying mechanism that the CC genotype of the FOXO3a rs4946936 gene variant had an association with increased levels of FOXO3a in the case of CGL compared to the CT and TT genotype. Moreover, previous study also supported our findings. They found that elevated levels of FOXO3a was associated with treatment failure of CGL patient treated with imatinib mesylate, suggesting that the levels of FOXO3a might play an important role to contribute the progression of CGL.¹⁶ Furthermore, studies also found that elevated levels of FOXO3a was associated with adverse prognosis of leukemia.^24,25 However, gene – environment interaction should also be considered to affect the disease progression since it was proven that the FOXO transcription factors had various impact in different population, for example: the role of FOXO transcription factors were found inconclusive to affect the human longevity in the population of Japan, Germany, Italia, and Indonesia.^26,27

To the best of our knowledge, our study is the first to report the association between the levels of FOXO3a and the gene polymorphism of FOXO3a rs4946936 in the case of CGL patients. Our findings may serve as an initial investigation regarding the role of FOXO3a rs4946936 gene polymorphism in affecting the levels of FOXO3a, compared to different findings in the case of COPD and vitiligo. In the future, our results may contribute to a better understanding regarding the role of FOXO3a rs4946936 gene polymorphism and the levels of FOXO3a in the pathogenesis of CGL patients. However, understanding the role of genetics in a disease is problematic and further studies involving gene-gene, gene-environment, and gene-disease interaction are warranted to determine the holistic role of FOXO3a gene polymorphism in the pathogenesis of CGL.

Several pertinent limitations to this study should be assessed. First, several factors that might contribute to the severity of CGL in patients including smoking, physical activity, dietary factors, the history of previous medication, and the history of radiation were not analyzed. Second, the small sample size in our study should be interpreted with caution due to the possibility of false positive results. Third, the short time period of the study might not be sufficient to define the real correlation.

Conclusion

Our study indicates that the CC genotype of the FOXO3a rs4946936 gene variant is associated with elevated levels of FOXO3a. Our findings may be used as an initial investigation and may provide a better understanding of the role of the FOXO3a rs4946936 gene variant in the pathogenesis of CGL.

Data availability

Reporting guidelines

Figshare: STROBE checklist for ‘The association between FOXO3a rs4946936 gene polymorphism and the levels of FOXO3a among chronic granulocytic leukemia patients treated with imatinib mesylate’, https://doi.org/10.6084/m9.figshare.16529160.¹⁷

Data are available under the terms of the Creative Commons Attribution 4.0 International license (CC-BY 4.0)

Consent

Written informed consent for publication of the patients’ details was obtained from the patients.