剪接变异体的肿瘤靶向性研究进展

剪接变异体是mRNA前体选择性剪接过程中形成的不同功能或活性的转录本。随着越来越多的肿瘤特异性剪接变异体的发现, 剪接变异体在肿瘤发生发展过程中所发挥的作用也逐渐受到学者的关注。由于肿瘤特异性剪接变异体的表达模式和活性都极具特征性, 提示针对靶向肿瘤细胞的剪接体的基因治疗将更具特异性, 并对正常组织产生较小的影响。临床试验应用反义寡核苷酸下调了影响肿瘤细胞存活率的mRNA并改变其剪接模式, 为肿瘤基因治疗提供了新的思路。随着新的肿瘤特异性剪接变异体的识别以及治疗策略的不断改进, 剪接变异体及剪接模式的改变将成为研究肿瘤安全、有效治疗方法的一个重要领域。      

正常肝和肝癌细胞中DHRS4L2基因选择性剪接亚型的克隆及其表达调控研究

目的：鉴定正常肝细胞HL-7702和肝癌细胞Hep-G2中DHRS4L2基因的选择性剪接新亚型,并探讨其表达调控机制。方法：应用RT-PCR,3′RACE和生物信息学方法发现并鉴定DHRS4L2基因的选择性剪接新亚型;去甲基化药物5-aza-dC处理HL-7702和Hep-G2细胞,通过RT-PCR和real-time PCR方法检测DHRS4L2的表达情况。结果：发现了2个新的DHRS4L2剪接亚型DHRS4L2A和DHRS4L2A3,2个新亚型均出现了新的第1个外显子,命名为Ea,前者缺失外显子1,后者缺失外显子1和3;在正常肝细胞中去甲基化后含DHRS4L2外显子1的转录产物比处理前明显增加,在肝癌细胞中则比处理前减少。结论：获得2个新的DHRS4L2剪接亚型,并发现DHRS4L2的表达和甲基化调控相关,且甲基化的调控作用在正常肝与肝癌细胞之间存在差异,其机制尚待进一步探讨。     

Human osteopontin splicing isoforms: Known roles,potential clinical applications and activated signaling pathways

Human osteopontin is subject to alternative splicing, which generates three isoforms, termed OPNa, OPNb and OPNc. These variants show specific expression and roles in different cell contexts. We present an overview of current knowledge of the expression profile of human OPN splicing isoforms (OPN-SIs), their tissue-specific roles, and the pathways mediating their functional properties in different pathophysiological conditions. We also describe their putative application as biomarkers, and their potential use as therapeutic targets by using antibodies, oligonucleotides or siRNA molecules. This synthesis provides new clues for a better understanding of human OPN splice variants, their roles in normal and pathological conditions, and their possible clinical applications.     

Implication of alternative splice transcripts of caspase-3 and survivin in chemoresistance

Recent studies have shown that resistance to apoptosis may contribute to chemoresistance. Alteration of caspases, such as caspase-3, results on decreased apoptosis. Genes of IAP (inhibitor of apoptosis proteins) family, such as survivin, were also implicated in tumor development where they are mutated or have deregulated expression. Initial studies revealed strong survivin expression in several fetal tissues and some proliferating adult tissues, whereas no survivin expression was detected in a range of adult tissues. Although the factors at the origins on survivin re-expression in tumors are still unknown, the anti-apoptotic function of survivin is mediated in part by inhibiting caspase-3 activity. Recently, functionally divergent splice variants resulting from alternative splicing, with apoptotic (for caspase-3) or anti-apoptotic (for survivin) opposite activities have been described. The alternative splice variant, caspase-3s results from exon 6 deletion and shows antagonist of apoptotic property of caspase-3. Three alternative splice variants of survivin (survivin-DeltaEx3, survivin-2B and survivin-3B) differing in their anti-apoptotic properties were reported. While the anti-apoptotic effect of survivin-DeltaEx3 is preserved, survivin-2B has lost its anti-apoptotic potential and may act as a naturally occurring antagonist of survivin and survivin-DeltaEx3. At present, little is known about properties of survivin-3B. Several evidences indicate that in several cancers, the ratio of splice variants is significantly altered, and modifications of splicing pathways have been developed for cancer treatment. Recent investigations have shown that expression of alternative splice variants of caspase-3 and of survivin were also altered in many human cancers, and that variations in their expression were associated with tumor progression and chemoresistance. In this article, we describe recent data concerning alternative splice variants of these two proteins.     

Regulation of apoptosis and caspase-8 expression in neuroblastoma cells by isoforms of the IG20 gene

The IG20 gene undergoes alternative splicing resulting in the differential expression of six putative splice variants. Four of these (IG20pa, MADD, IG20-SV2, and DENN-SV) are expressed in virtually all human tissues. However, investigations examining alternative splicing of the IG20 gene to date have been largely limited to nonneural malignant and nonmalignant cells. In this study, we investigated the expression of alternative splice isoforms of the IG20 gene in human neuroblastoma cells. We found that six IG20 splice variants (IG20-SVs) were expressed in two human neuroblastoma cell lines (SK-N-SH and SH-SY5Y), highlighted by the expression of two unique splice isoforms (i.e., KIAA0358 and IG20-SV4). Similarly, we found enriched expression of these two IG20-SVs in human neural tissues derived from cerebral cortex, hippocampus, and, to a lesser extent, spinal cord. Using gain-of-function studies and siRNA technology, we determined that these "neural-enriched isoforms" exerted significant and contrasting effects on vulnerability to apoptosis in neuroblastoma cells. Specifically, expression of KIAA0358 exerted a potent antiapoptotic effect in both the SK-N-SH and SH-SY5Y neuroblastoma cell lines, whereas expression of IG20-SV4 had proapoptotic effects directly related to the activation of caspase-8 in these cells, which have minimal or absent constitutive caspase-8 expression. These data indicate that the pattern of expression of these neural-enriched IG20-SVs regulates the expression and activation of caspase-8 in certain neuroblastoma cells, and that manipulation of IG20-SV expression pattern may represent a potent therapeutic strategy in the therapy of neuroblastoma and perhaps other cancers.     

TRAIL-beta and TRAIL-gamma: two novel splice variants of the human TNF-related apoptosis-inducing ligand (TRAIL) without apoptotic potential

Tumour necrosis factor (TNF) related apoptosis-inducing ligand (TRAIL/APO2L) is a recently identified member of the TNF family, which induces programmed cell death in a variety of neoplastic cell types, but not in most nonneoplastic cells. In this study, we report on the identification of two novel alternative splice variants of TRAIL in neoplastic and non-neoplastic human cells lacking either exon 3 (TRAIL-beta) or exons 2 and 3 (TRAIL-gamma). In both splice variants, loss of exon 3 resulted in a frame shift generating a stop codon with consecutive extensive truncation in the extracellular domain. Ectopic expression revealed a loss of proapoptotic potential for both alternative splice variants. In contrast to the predominantly cytoplasmatic localisation of GFP-tagged TRAIL-alpha and TRAIL-beta, TRAIL-gamma showed an additional association with the cell surface and nuclear membrane. In conclusion, alternative splicing might be involved in fine tuning of TRAIL-induced apoptosis and underlines the complexity of the TRAIL system.     

Splice variants but not mutations of DNA polymerase beta are common in bladder cancer

DNA polymerase beta (POLbeta) is a highly conserved protein that functions in base excision repair. Loss of the POLbeta locus on chromosome 8p is a frequent event in bladder cancer, and loss of POLbeta function could hinder DNA repair leading to a mutator phenotype. Both point mutations and large intragenic deletions of POLbeta have been reported from analysis of various tumor cDNAs but not from genomic DNA. We noticed that the breakpoints of the presumed rearrangements were delineated by exon-exon junctions, which could instead be consistent with alternative splicing of POLbeta mRNA. We tested the hypothesis that the reported intragenic deletion were splice variants by screening genomic DNA of human bladder tumors, bladder cancer cell lines, and normal bladder tissues for mutations or deletions in exons 1-14, exon alpha, and the promoter region of POLbeta. We found no evidence of somatic mutations or deletions in our sample set, although two polymorphisms were identified. Examination of cDNA from a subset of the original sample set revealed that truncated forms of POLbeta were surprisingly common. Forty-eight of 89 (54%) sequenced cDNA clones had large deletions, each beginning and/or ending exactly at exon-exon junctions. Because these deletions occur at exon-exon junctions and are seen in cDNA but not genomic DNA, they are consistent with alternative mRNA splicing. We describe 12 different splicing events occurring in 18 different combinations. Loss of exon 2 was the most frequent, being found in 42 of 49 (86%) of the variant sequenced clones. The splice variants appear to be somewhat more common and variable in bladder cancer cell lines and tumor tissues but occur at a high frequency in normal bladder tissues as well. We examine alternative splicing in terms of the information content of splice donor and acceptor site sequences, and discuss possible explanations for the predominant splicing event, the loss of exon 2.     

Increased alternative splicing of the KLF6 tumour suppressor gene correlates with prognosis and tumour grade in patients with pancreatic cancer

The aim of this study was to correlate the status of the KLF6 tumour suppressor gene including loss of heterozygosity (LOH), mutation and alternative splicing in human pancreatic cancer with tumour grade and survival. Whereas neither KLF6 loss nor mutation was identified, expression of the KLF6 alternative splice forms was significantly increased in pancreatic tumour samples and cell lines. These cancers demonstrated marked cytoplasmic KLF6 expression, consistent with over-expression and accumulation of KLF6 splice form(s), which lack a nuclear localisation signal. In addition, KLF6 splicing correlated significantly with tumour stage and survival. In summary, pancreatic cancer displays a novel pattern of KLF6 dysregulation through selectively increased expression of KLF6 splice variants. Therefore, determination of KLF6 mRNA splicing levels may represent a novel biomarker predicting prognosis.