Homeobox genes can promote oncogenesis

Homeobox genes can promote oncogenesis through multiple mechanisms, including gene translocation, loss of heterozygosity, gene amplification, DNA methylation, T7 High Yield RNA Synthesis remodeling, etc. For instance, translocation-mediated fusion of HOXA9 or HOXA13 on chromosome 7p15 with the nucleoporin gene NUP98 on chromosome 11p15 in human myeloid leukemia suggested their roles in regulating hematologic malignancy [[3], [4], [5]]. Many homeobox genes are differentially regulated by polycomb and trithorax proteins, suggesting their involvement in aberrant epigenetics of cancer development and progression [6]. HOXC10 belongs to the HOXC cluster of HOX genes. Accumulating evidence has shown that HOXC10 overexpression contributes to various malignant transformations. In preinvasive and invasive cervical squamous cell carcinomas, HOXC10 plays a key role in promoting cancer progression [7]. In human thyroid cancer, elevated HOXC10 expression correlated to poor prognosis of the patients [8]. In human lung adenocarcinoma, HOXC10 could promote cancer metastasis and implicate poor survival outcomes [9]. The promoter of the HOXC10 gene contains several estrogen response elements and thus its expression can be regulated by estrogen receptor-mediated histone methylation in breast cancer [10]. Another report also demonstrated that various epigenetic events, including histone modifications and DNA methylation, contributed to aberrant expression of HOXC10 in endocrine-resistant breast cancer [11]. G-quadruplexes are stable four-chain structures of DNA or RNA formed by guanines, stacked as three or more planar quartets and stabilized by Hoogsteen hydrogen bonding [12]. The stability of G-quadruplex structures can be further enhanced by monovalent cations in an order of K+ > Na+ > NH4+ > Li+ [13]. Different algorithms have been proposed to predict the potential of candidate sequences in forming G-quadruplex structures [[14], [15], [16], [17]]. Numerous studies using synthetic oligonucleotides derived from sequences of endogenous promoters with G-runs demonstrated G-quadruplex structure formation in vitro with the help of various biochemical techniques, such as circular dichroism, NMR spectroscopies, native polyacrylamide gel electrophoresis, etc. [12]. Recent studies indicate G-quadruplex structures play an important role in regulating many cancer-related genes. Telomerase contributes to cell immortality and oncogenesis through maintaining or extending telomere lengths [18]. Several studies proved that G-rich telomere DNA overhang can form G-quadruplex structures that blocks telomerase activity [19,20]. G-quadruplex structures have been discovered in the promoters of many oncogenes or proliferative genes, such as MYC, KRAS, BCL2 and YY1 [[21], [22], [23], [24]]. Typically, G-quadruplex structures in a promoter exert inhibitory effects on expression of the gene; thus, they mostly repress oncogenes and possess antiproliferative functions. As a result, chemicals that stabilize G-quadruplex structures have potential as anticancer agents in cancer therapies [[25], [26], [27]]. However, recent studies revealed that the presence of G-quadruplex structures in promoters may positively regulate gene expression. In the promoter of the relaxin gene, G-quadruplex formation could significantly reduce the binding of STAT3 and increase the recruitment of RNA polymerase II that improves relaxin gene expression [28]. A recent report also demonstrated that presence of G-quadruplex structure in the Oct4 promoter facilitates its gene expression [29].
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Results
Discussion The regulatory role of homeotic proteins in the development and differentiation of mammals has been well documented. Meanwhile, many HOX genes are overexpressed in various cancers and also demonstrated to promote cancer cell proliferation and metastasis [42,43]. Several epigenetic mechanisms were proposed to mediate HOX gene expression in cancers, including lncRNA-recruited chromatin remodeling complexes, DNA methylation and histone modifications [44,45]. However, to our best knowledge, no study regarding G-quadruplex-regulated HOX gene expression was previously reported. In this study, we first discovered a G/C-rich region with multiple consecutive G-tracts and high potential of forming G-quadruplex structures in the negative strand of the HOXC10 promoter, and then demonstrated the presence of G-quadruplex structures using biochemical approaches and cell-based assays.