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Use of Minigene Systems to Dissect Alternative Splicing Elements Methods 37 (2005) 331–340 www.elsevier.com/locate/ymeth Use of minigene systems to dissect alternative splicing elements Thomas A. Cooper * Departments of Pathology and Molecular and Cellular Biology, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030, USA Accepted 5 July 2005 Abstract Pre-mRNA splicing is an essential step for gene expression in higher eukaryotes. The splicing efficiency of individual exons is deter- mined by multiple features involving gene architecture, a variety of cis-acting elements within the exons and flanking introns, and interactions with components of the basal splicing machinery (called the spliceosome) and auxiliary regulatory factors which transiently co-assemble with the spliceosome. Both alternative and constitutive exons are recognized by multiple weak protein:RNA interactions and different exons differ in the interactions which are determinative for exon usage. Alternative exons are often regulated according to cell-specific patterns and regulation is mediated by specific sets of cis-acting elements and trans-acting factors. Transient expression of minigenes is a commonly used in vivo assay to identify the intrinsic features of a gene that control exon usage, identify specific cis-acting elements that control usage of constitutive and alternative exons, identify cis-acting elements that control cell-specific usage of alternative exons, and once regulatory elements have been identified, to identify the trans-acting factors that bind to these elements and modulate splicing. This chapter describes approaches and strategies for using minigenes to define the cis-acting elements that determine splice site usage and to identify and characterize the trans-acting factors that bind to these elements and regulate alternative splicing. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Minigene; Deletion analysis; RNA binding; Exon splicing enhancer; Exon splicing silencer; Intronic splicing enhancer; Intronic splicing silencer 1. Introduction In addition, multiple negative and positive cis-acting ele- ments contribute to both the basal level of exon recognition The majority of human genes express multiple mRNAs and the modulation of exon use in response to specific sig- via alternative splicing. These mRNAs can encode func- nals (Fig. 1). Alternative exons are typically recognized less tionally diverse protein isoforms or can promote the on:off efficiently than constitutive exons allowing for their modu- control of gene expression by introducing premature trans- lated use. The consensus 50 and 30 splice sites flanking alter- lation stop codons [1]. A large fraction of these alternative native exons are typically non-canonical and are less splicing events are regulated according to cell-specific pat- efficient binding sites for the components of the basal splic- terns or in response to acute stimulation [2]. Whether or ing machinery (the spliceosome). Exons contain exonic not a splice site is used is determined by a large number splicing enhancers (ESEs) and exonic splicing silencers of features. Some features establish a basal level of exon (ESSs) that enhance or repress exon usage, respectively recognition and others are determinative for the modula- (Fig. 1). The first ESEs to be identified in vivo based on tion of alternative splicing in response to specific cues. functional analyses using minigenes were in alternative The features that establish a basal level of exon recognition exons [3–5]. The best characterized ESEs are purine-rich include splice site strength, the size of the exon, the sizes of and their enhancer activities are mediated though binding flanking introns, and the presence of secondary structures. to members of the SR protein family [6]. A lesser character- ized A/C-rich family of ESEs mediate their effects by bind- ing to the cold box protein, YB-1 [7]. ESSs bind to hnRNP * Fax: +1 713 798 5838. proteins and functional analysis using minigenes have E-mail address: [email protected]. demonstrated that this binding of hnRNP proteins 1046-2023/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.ymeth.2005.07.015 332 T.A. Cooper / Methods 37 (2005) 331–340 Fig. 1. Exonic and intronic elements. mediates repression [8]. Recent analyses that combine bio- 2. Initial considerations computational identification and functional analysis in minigenes have determined that ESEs and ESSs are com- Minigenes can be used to address a number of different mon to both alternative and constitutive exons [8–11]. questions with regard to alternative splicing regulation. While some of the motifs identified by biocomputational The first step of an investigation to dissect the features that analysis match previously identified ESE and ESS motifs, regulate alternative splicing is to establish the specific the majority do not, indicating that a great deal of informa- issue(s) to be addressed. The common uses of minigenes tion remains to be discovered regarding how exons are rec- include: (i) determine the role of the splice sites in establish- ognized. Intronic splicing enhancers (ISEs) and intronic ing a basal level of exon recognition, (ii) identify exonic and splicing silencers (ISSs) within the upstream and down- intronic elements that enhance or repress splicing, (iii) stream introns (Fig. 1) also serve to enhance or repress, determine whether an allelic variant has an effect on splic- respectively, use of associated splice sites [12]. ing efficiency, (iv) identify elements required for cell-specific Minigene constructs are an important tool for the iden- regulation, and (v) identify elements required for regulation tification and in vivo analysis of the cis-acting regulatory by a specific trans-acting factor. The approaches for these elements and trans-acting factors that establish splicing effi- goals differ and will be addressed separately in the section ciency and that regulate alternative splicing. Expression of entitled, ‘‘Analysis of results.’’ minigene pre-mRNAs by transient transfection provides a rapid assay for loss-of-function and gain-or-function anal- 3. Description of the method yses for cis-elements and trans-acting factors that affect splicing regulation. This chapter will present approaches 3.1. Minigene design to define the intronic and exonic cis-acting elements that establish basal splicing efficiency, identify elements respon- Minigenes contain a genomic segment from the gene of sible for cell-specific regulation, and identify specific interest that includes the alternatively spliced region and sequence motifs that are required for regulation by individ- flanking genomic regions. The genomic segment is generated ual trans-acting factors. Some considerations for the iden- by PCR amplification either directly from genomic DNA or tification of elements that regulate cell-specific alternative a cloned genomic DNA fragment as template. The lengths of splicing have been previously described [13]. the flanking genomic segments depend on the goals of the T.A. Cooper / Methods 37 (2005) 331–340 333 analysis and can range from tens to thousands of nucleo- depend on the available cloning sites within the minigene tides. To determine whether an allelic variant is responsible (RS#1 and RS#2, Fig. 2). It is most convenient to select for altered splicing or to test whether a specific motif is the two restriction enzymes that cut efficiently in the same buff- response element for a putative regulatory factor, the mini- er. The cloning sites within the priming oligonucleotides gene will include the cis-element(s) of interest. To identify need to be unique to the PCR product. Usually the avail- the cis-acting elements required for cell-specific regulation able minigene plasmid has only one or two restriction of an alternative exon or in response to overexpression and enzyme sites that can be used for insertion of the genomic knockdown studies of specific proteins, one must choose a segment. If the cloning site within the vector is also found fragment that is likely to contain the primary regulatory ele- in the genomic segment to be cloned, different restriction ments. Based on current knowledge, the primary elements enzyme sites with identical overhangs can be incorporated regulating alternative splicing are typically within 200–300 into the PCR primers to incorporate a site unique to the nucleotides upstream and/or downstream of the regulated PCR fragment that can be used for cloning. For example, exon. While there are cases in which cell-specific regulatory if the cloning sites within the minigene are BamHI (50 site) elements have been found more than one kilobase from the and XhoI(30 site) but there are BamHI and XhoI sites with- regulated splice site [14–16], a genomic fragment containing in the genomic segment to be amplified, BglII and SalI up to 300 nucleotides of flanking intronic sequences restriction sites can be incorporated into the upstream upstream and downstream from the exon is a reasonable and downstream PCR primers, respectively. The BamHI/ starting point (Fig. 2). Once a genomic segment has been BglII and SalI/XhoI pairs have identical overhangs and shown to contain elements that are sufficient for regulated can be ligated allowing for use of the limited number of splicing, sequential deletions from the 30 and 50 ends can be sites within the minigene for cloning. Note that ligation used to define the smallest genomic fragment that is neces- of different sites with the same ‘‘sticky ends’’ results in a sary and sufficient for regulation. It is advantageous to define loss of both sites. A full list of restriction
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