Candidates for tumor-specific alternative splicing

Detection of Alternatively Spliced or Processed RNAs in Cancer Using Oligonucleotide Microarray

Deregulation of gene expression plays a pivotal role in tumorigenesis, so the ability to detect RNA alterations is of great value in cancer diagnosis and management. DNA microarrays have been used to measure changes in mRNA or microRNA level, but less often the change of RNA isoforms. Here we appraise the utilization of microarray in detecting alternatively processed RNAs, which have alternative splice forms, retained introns, or altered 3' untranslated regions. We cover the methodology and focus on cancer studies. Recent development in parallel or deep sequencing used in transcriptome analysis is also discussed.

Clinical perspective on chemo-resistance and the role of RNA processing

Pre-messenger RNA splicing is significantly changed in cancer cells leading to the expression of cancer-specific transcripts. These transcripts have the potential to be used as cancer biomarkers and also as targets for new therapeutic approaches. In addition, the cancer-specific transcripts have the potential to alter the drug response of the cancer cells creating a chemo-resistant state. This later property of alternative splicing presents a challenge to clinicians in the design of effective therapeutic regimens. When a patient's cancer relapses it is frequently refractory to standard chemotherapies resulting in a poor clinical outcome. Therefore, understanding the mechanisms of how alternative splicing can lead to chemo-resistance is critical to the effective delivery of treatment. Here, we will discuss the impact of alternative splicing variants on drug metabolism and activation; on drug interactions with cell signaling pathways; and on cell death pathways in cancer therapeutics. In addition to the initial characterization of splicing variants, the mechanisms leading to alterations in splicing are being studied in the setting of chemo-resistance and will be discussed here. The promise of therapeutic intervention to obviate the impact of these splicing variants will significantly enhance treatment options for cancer patients.

Mammary gland selective excision of c-jun identifies its role in mRNA splicing

The c-jun gene regulates cellular proliferation and apoptosis via direct regulation of cellular gene expression. Alternative splicing of pre-mRNA increases the diversity of protein functions, and alternate splicing events occur in tumors. Here, by targeting the excision of the endogenous c-jun gene within the mouse mammary epithelium, we have identified its selective role as an inhibitor of RNA splicing. Microarray-based assessment of gene expression, on laser capture microdissected c-jun(-/-) mammary epithelium, showed that endogenous c-jun regulates the expression of approximately 50 genes governing RNA splicing. In addition, genome-wide splicing arrays showed that endogenous c-jun regulated the alternate exon of approximately 147 genes, and 18% of these were either alternatively spliced in human tumors or involved in apoptosis. Endogenous c-jun also was shown to reduce splicing activity, which required the c-jun dimerization domain. Together, our findings suggest that c-jun directly attenuates RNA splicing efficiency, which may be of broad biologic importance as alternative splicing plays an important role in both cancer development and therapy resistance.

Alternative splicing and tumor progression

Alternative splicing is a key molecular mechanism for increasing the functional diversity of the eukaryotic proteomes. A large body of experimental data implicates aberrant splicing in various human diseases, including cancer. Both mutations in cis-acting splicing elements and alterations in the expression and/or activity of splicing regulatory factors drastically affect the splicing profile of many cancer-associated genes. In addition, the splicing profile of several cancer-associated genes is altered in particular types of cancer arguing for a direct role of specific splicing isoforms in tumor progression. Deciphering the mechanisms underlying aberrant splicing in cancer may prove crucial to understand how splicing machinery is controlled and integrated with other cellular processes, in particular transcription and signaling pathways. Moreover, the characterization of splicing deregulation in cancer will lead to a better comprehension of malignant transformation. Cancer-associated alternative splicing variants may be new tools for the diagnosis and classification of cancers and could be the targets for innovative therapeutical interventions based on highly selective splicing correction approaches.

Exon array analysis using re-defined probe sets results in reliable identification of alternatively spliced genes in non-small cell lung cancer

Background

Treatment of non-small cell lung cancer with novel targeted therapies is a major unmet clinical need. Alternative splicing is a mechanism which generates diverse protein products and is of functional relevance in cancer.

Results

In this study, a genome-wide analysis of the alteration of splicing patterns between lung cancer and normal lung tissue was performed. We generated an exon array data set derived from matched pairs of lung cancer and normal lung tissue including both the adenocarcinoma and the squamous cell carcinoma subtypes. An enhanced workflow was developed to reliably detect differential splicing in an exon array data set. In total, 330 genes were found to be differentially spliced in non-small cell lung cancer compared to normal lung tissue. Microarray findings were validated with independent laboratory methods for CLSTN1, FN1, KIAA1217, MYO18A, NCOR2, NUMB, SLK, SYNE2, TPM1, (in total, 10 events) and ADD3, which was analysed in depth. We achieved a high validation rate of 69%. Evidence was found that the activity of FOX2, the splicing factor shown to cause cancer-specific splicing patterns in breast and ovarian cancer, is not altered at the transcript level in several cancer types including lung cancer.

Conclusions

This study demonstrates how alternatively spliced genes can reliably be identified in a cancer data set. Our findings underline that key processes of cancer progression in NSCLC are affected by alternative splicing, which can be exploited in the search for novel targeted therapies.

Identification of novel alternatively splicedBRCA1-associated RING domain (BARD1) messenger RNAs in human peripheral blood lymphocytes and in sporadic breast cancer tissues

BARD1 (BRCA1-associated RING domain) is the dominant binding partner of BRCA1 in vivo. The BARD1 gene has been reported to be mutated in a subset of breast and ovarian cancer patients and BARD1 germ-line mutations have been identified in breast cancer patients negative for BRCA1 or BRCA2 gene alterations. In the present study, we show by RT-PCR and direct sequencing analysis the occurrence of seven novel and one previously identified BARD1 splicing variants in human lymphocytes and breast cancers. Two of the eight variants (BARD1delta and BARD1 DeltaRIN) preserve a correct open reading frame and could encode BARD1 internally deleted proteins, while the remaining six variants display premature stop codons. Characterization of the relative expression of BARD1 FL, BARD1delta, and BARD1 DeltaRIN using quantitative PCR analysis indicated that the mean expression levels of BARD1 FL, BARD1delta, and BARD1 DeltaRIN were significantly higher in tumors than in morphologically normal tissues and lymphocytes. However, we were unable to identify either qualitatively or quantitatively tumor-specific expression patterns of the identified BARD1 splicing variants.

Transposable elements in human cancers by genome-wide EST alignment

Transposable elements may affect coding sequences, splicing patterns, and transcriptional regulation of human genes. Particles of the transposable elements have been detected in several tissues and tumors. Here, we report genome-wide analysis of gene expression regulated by transposable elements in human cancers. We adopted an analysis pipeline for screening methods to detect cancer-specific expression from expressed human sequences. We developed a database (TECESdb) for understanding the mechanism of cancer development in relation to transposable elements. A total of 999 genes fused with transposable elements were found to be cancer-related in our analysis of the EST database. According to GO (Gene Ontology) analysis, the majority of the 999 cancer-specific genes have functional association with gene receptor, DNA binding, and kinase activity. Our data could contribute greatly to our understanding of human cancers in relation to transposable elements.

Alternative splicing and differential gene expression in colon cancer detected by a whole genome exon array

Background

Alternative splicing is a mechanism for increasing protein diversity by excluding or including exons during post-transcriptional processing. Alternatively spliced proteins are particularly relevant in oncology since they may contribute to the etiology of cancer, provide selective drug targets, or serve as a marker set for cancer diagnosis. While conventional identification of splice variants generally targets individual genes, we present here a new exon-centric array (GeneChip Human Exon 1.0 ST) that allows genome-wide identification of differential splice variation, and concurrently provides a flexible and inclusive analysis of gene expression.

Results

We analyzed 20 paired tumor-normal colon cancer samples using a microarray designed to detect over one million putative exons that can be virtually assembled into potential gene-level transcripts according to various levels of prior supporting evidence. Analysis of high confidence (empirically supported) transcripts identified 160 differentially expressed genes, with 42 genes occupying a network impacting cell proliferation and another twenty nine genes with unknown functions. A more speculative analysis, including transcripts based solely on computational prediction, produced another 160 differentially expressed genes, three-fourths of which have no previous annotation. We also present a comparison of gene signal estimations from the Exon 1.0 ST and the U133 Plus 2.0 arrays.

Novel splicing events were predicted by experimental algorithms that compare the relative contribution of each exon to the cognate transcript intensity in each tissue. The resulting candidate splice variants were validated with RT-PCR. We found nine genes that were differentially spliced between colon tumors and normal colon tissues, several of which have not been previously implicated in cancer. Top scoring candidates from our analysis were also found to substantially overlap with EST-based bioinformatic predictions of alternative splicing in cancer.

Conclusion

Differential expression of high confidence transcripts correlated extremely well with known cancer genes and pathways, suggesting that the more speculative transcripts, largely based solely on computational prediction and mostly with no previous annotation, might be novel targets in colon cancer. Five of the identified splicing events affect mediators of cytoskeletal organization (ACTN1, VCL, CALD1, CTTN, TPM1), two affect extracellular matrix proteins (FN1, COL6A3) and another participates in integrin signaling (SLC3A2). Altogether they form a pattern of colon-cancer specific alterations that may particularly impact cell motility.

Data analysis of alternative splicing microarrays

The importance of alternative splicing in drug and biomarker discovery is best understood through several example genes. For most genes, the identification, detection and particularly quantification of isoforms in different tissues and conditions remain to be carried out. As a result, the focus in drug and biomarker development is increasingly on high-throughput studies of alternative splicing. Initial strategies for the parallel analysis of alternative splicing by microarrays have been recently published. The design specificities and goals of alternative splicing microarrays, in terms of identification and quantification of multiple mRNAs from one gene, are promoting the development of novel methods of analysis.

Microarray Analysis of Alternative Splicing

Alternative splicing, defined as the generation of multiple RNA transcript species from a common mRNA precursor, is one of the mechanisms for the diversification and expansion of cellular proteins from a smaller set of genes. Current estimates indicate that at least 60% of genes in the human genome exhibit alternative splicing. Over the past decade, alternative splicing has increasingly been recognized as a major regulatory process with a critical role in normal development. Furthermore, the importance of alternative splicing in disease development and treatment is starting to be appreciated. Therefore, an increasing number of high-throughput genomics and proteomics studies are being performed in order to delineate (a) the changes in alternative splicing under various conditions; (b) the properties and functions of protein isoforms; and (c) the splicing and alternative splicing regulation process. Strategies for the parallel analysis of alternative splice forms by microarray experiments have been conceived, and examples have been published. In addition to the differences in microarray probe design, the analysis of microarrays with probes for exons, exon/exon junctions as well as specific splice forms is significantly different from the standard experiment. Several methods are being developed in order to address the particular needs of alternative splicing microarrays. Many reviews have already dealt with alternative splicing. However, high-throughput analysis methods that are becoming increasingly popular have not received much attention. Here, we will provide an overview of the tools and analysis methods that were developed specifically for alternative splicing microarrays described in terms of specific experiments.

Isomers of the TCF1 gene encoding hepatocyte nuclear factor-1 alpha show differential expression in the pancreas and define the relationship between mutation position and clinical phenotype in monogenic diabetes

The generation of multiple transcripts by mRNA processing has the potential to moderate differences in gene expression both between tissues and at different stages of development. Where gene function is compromised by mutation, the presence of multiple isoforms may influence the resulting phenotype. Heterozygous mutations in the transcription factor hepatocyte nuclear factor-1 alpha (HNF1A or TCF1 gene) result in early-onset diabetes as a result of pancreatic beta-cell dysfunction. We investigated the expression of the three alternatively processed isoforms of the HNF1A gene and their impact on the phenotype associated with mutations. Real-time PCR demonstrated variation in tissue expression of HNF1A isomers: HNF1A(A), with the lowest transactivation activity compared with the truncated isoforms HNF1A(B) and HNF1A(C), is the major isomer in liver (54%) and kidney (67%) but not in adult pancreas (24%) and islets (26%). However, in fetal pancreas HNF1A(A) is the major transcript (84%), which supports developmental regulation of isomer expression. We examined whether the isomers affected by the mutation altered the diabetes phenotype in 564 subjects with 123 mutations in HNF1A. Mutations that affected only isomer HNF1A(A) (exons 8-10) were diagnosed later (25.5 years) than mutations affecting all three isomers (exons 1-6) (18.0 years) (P=0.006). This first genotype/phenotype relationship described for patients with HNF1A mutations is explained by isomer structure and not by either mutation type or functional domain. We conclude that all three isomers may be critical for beta-cell function and could play a role in both the developing and mature beta cell.

Unbalanced alternative splicing and its significance in cancer

Alternative pre-mRNA splicing leads to distinct products of gene expression in development and disease. Antagonistic splice variants of genes involved in differentiation, apoptosis, invasion and metastasis often exist in a delicate equilibrium that is found to be perturbed in tumours. In several recent examples, splice variants that are overexpressed in cancer are expressed as hyper-oncogenic proteins, which often correlate with poor prognosis, thus suggesting improved diagnosis and follow up treatment. Global gene expression technologies are just beginning to decipher the interplay between alternatively spliced isoforms and protein-splicing factors that will lead to identification of the mutations in these trans-acting factors responsible for pathogenic alternative splicing in cancer.

Expression analysis and prognostic significance of the SRA1 gene, in ovarian cancer

The SR-related-CTD-associated-factors (SCAFs) have the ability to interact with the C-terminal domain of the RNA polymerase II, linking this way transcription to splicing. SRA1 (SR-A1) gene, encoding for a human high-molecular weight SCAF protein, is located on chromosome 19, between the IRF3 and the R-RAS oncogene and it has been demonstrated from members of our group that SRA1 is constitutively expressed in most of the human tissues, while it is overexpressed in a subset of ovarian tumors. In this study, we examine the expression of SRA1 gene in 111 ovarian malignant tissues and in the human ovarian carcinoma cell lines OVCAR-3, TOV21-G, and ES-2, using a semi-quantitative RT-PCR method. SRA1 gene was overexpressed in 61/111 (55%) of ovarian carcinomas. This higher expression was positively associated to the size of the tumor (p<0.001), the grade and the stage of the disease (p=0.003 and p=0.006, respectively), and the debulking success (p<0.001). Kaplan-Meier survival analysis revealed that lower SRA1 expression increases the probability of both the longer overall and the progression free survival of the patients. Multivariate Cox regression analysis revealed that SRA1 may be used as an independent prognostic biomarker in ovarian cancer. Our results suggest that SRA1 is associated with cancer progression and may possibly be characterized as a new marker of unfavorable prognosis for ovarian cancer.

Molecular models for the tissue specificity of DNA mismatch repair-deficient carcinogenesis

A common feature of all the known cancer genetic syndromes is that they predispose only to selective types of malignancy. However, many of the genes mutated in these syndromes are ubiquitously expressed, and influence seemingly universal processes such as DNA repair or cell cycle control. The tissue specificity of cancers that arise from malfunction of these apparently universal traits remains a key puzzle in cancer genetics. Mutations in DNA mismatch repair (MMR) genes cause the most common known cancer genetic syndrome, hereditary non-polyposis colorectal cancer, and the fundamental biology of MMR is one of the most intensively studied processes in laboratories all around the world. This review uses MMR as a model system to understand mechanisms that may explain the selective development of tumors in particular cell types despite the universal nature of this process. We evaluate recent data giving insights into the specific tumor types that are attributable to defective MMR in humans and mice under different modes of inheritance, and propose models that may explain the spectrum of cancer types observed.