The regulation of splice-site selection, and its role in human disease.

The pre-mRNA splicing machinery recognizes exons ternative splicing events (Takagaki et al. 1996), this mechanism is unlikely to account for all regulated splicand joins them together with remarkable precision to form mRNAs with intact translational reading frames. ing observed in vertebrates. There are many examples in which different regulatory programs run concurrently Splicing requires canonical sequences at the intron/exon border, and mutation of these sequences may cause abwithin the same cell, suggesting that different alternatively spliced pre-mRNAs are regulated by distinct pronormal splicing patterns that affect gene expression and cause disease. Recent studies indicate that distinct segrams that use different sets of cis elements and transacting factors. Strong evidence that cell-specific factors quence elements that are distant from the splice sites are also needed for normal splicing. These elements can are responsible for alternative splicing comes from studies on intronic elements that mediate cell-specific splicing affect splice-site recognition during constitutive splicing and also play important roles in directing alternative (Guo et al. 1991; Tacke and Goridis 1991; Black 1992; Gooding et al. 1994; Huh and Hynes 1994; Ryan and splicing, a common phenomenon in which multiple mRNAs, encoding functionally distinct proteins, are Cooper 1996). One model system in which such elements have been identified is the cardiac troponin T generated by use of different combinations of splice junctions, according to developmentally regulated or tis(cTNT) gene (Ryan and Cooper 1996). Figure 1A shows a diagram of cTNT exons 4–6, in which the alternative sue-specific programs. A number of auxiliary splicing elements required for cell-specific modulation of alternaexon 5 is included in embryonic striated muscle and is skipped in the adult. Exon inclusion in embryonic mustive splicing have been found within introns that flank alternative exons. A second set of splicing elements, excle requires intronic elements, referred to as ‘‘musclespecific splicing enhancers’’ (MSEs), located a short disonic splicing enhancers (ESEs), are found within both coding and noncoding exons. These enhancers direct the tance upstream and downstream of the exon (shown as small boxes in fig. 1A). Evidence from transient transfecspecific recognition of splice sites during constitutive and alternative splicing. The prevalence of alternative spliction into embryonic muscle and nonmuscle cell cultures suggests that these elements regulate splicing via regulaing as a mechanism for regulation of gene expression makes it a likely target for alterations leading to human tory factors specific to embryonic muscle that promote disease. Below we summarize what is known about variinclusion of the exon (Ryan and Cooper 1996). ous sequences that affect splice-site selection and illusFew potential regulators of vertebrate alternative trate how changes in alternative splicing may lead either splicing have been found, but recent studies have identito disease or, conversely, to an amelioration of the effied one candidate, called ‘‘SWAP,’’ a homologue of a fects of certain genetic lesions. Drosophila splicing regulator, suppressor of white apricot (Zachar et al. 1987; Denhez and Lafyatis 1994; Intronic Splicing Elements and Splicing Regulators Spikes et al. 1994). In both humans and Drosophila, the SWAP splicing factor negatively regulates its own Although modulation of the nuclear concentrations expression at the posttranscriptional level. SWAP inhibof constitutive RNA processing factors causes some alits splicing of its own pre-mRNA, which, in its unspliced form, encodes a nonfunctional truncated protein. Recent evidence from transient-transfection experiments demReceived May 9, 1997; accepted for publication June 6, 1997. onstrates that overexpressed SWAP protein in mammaAddress for correspondence and reprints: Dr. Thomas A. Cooper, lian cells regulates splicing of several alternatively Department of Pathology, Room S201, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030. E-mail: tcooper@bcm.tmc. edu spliced pre-mRNAs (Sarkissian et al. 1996). SignifiThis article represents the opinion of the authors and has not been cantly, in transient-transfection experiments, overexpeer reviewed. pression of the SWAP protein affects splicing patterns 1997 by The American Society of Human Genetics. All rights reserved. 0002-9297/97/6102-0002$02.00 differently than does overexpression of a general splicing