Pre-mRNA splicing and human disease

Abstract

The precision and complexity of intron removal during pre-mRNA splicing still amazes even 26 years after the discovery that the coding information of metazoan genes is interrupted by introns (Berget et al. 1977;Chow et al. 1977). Adding to this amazement is the recent realization that most human genes express more than one mRNA by alternative splicing, a process by which functionally diverse protein isoforms can be expressed according to different regulatory programs. Given that the vast majority of human genes contain introns and that most pre-mRNAs undergo alternative splicing, it is not surprising that disruption of normal splicing patterns can cause or modify human disease. The purpose of this review is to highlight the different mechanisms by which disruption of pre-mRNA splicing play a role in human disease. Several excellent reviews provide detailed information on splicing and the regulation of splicing (Burge et al. 1999;Hastings and Krainer 2001; Black 2003). The potential role of splicing as a modifier of human disease has also recently been reviewed (NissimRafinia and Kerem 2002). Constitutive splicing and the basal splicing machineryThe typical human gene contains an average of 8 exons. Internal exons average 145 nucleotides (nt) in length, and introns average more than 10 times this size and can be much larger (Lander et al. 2001). Exons are defined by rather short and degenerate classical splice-site sequences at the intron/exon borders (5Ј splice site, 3Ј splice site, and branch site; Fig. 1A). Components of the basal splicing machinery bind to the classical splice-site sequences and promote assembly of the multicomponent splicing complex known as the spliceosome. The spliceosome performs the two primary functions of splicing: recognition of the intron/exon boundaries and catalysis of the cut-and-paste reactions that remove introns and join exons. The spliceosome is made up of five small nuclear ribonucleoproteins (snRNPs) and >100 proteins. Each snRNP is composed of a single uridinerich small nuclear RNA (snRNA) and multiple proteins. The U1 snRNP binds the 5Ј splice site, and the U2 snRNP binds the branch site via RNA:RNA interactions between the snRNA and the pre-mRNA (Fig. 1B). Spliceosome assembly is highly dynamic in that complex rearrangements of RNA:RNA, RNA:protein, and protein:protein interactions take place within the spliceosome. Coinciding with these internal rearrangements, both splice sites are recognized multiple times by interactions with different components during the course of spliceosome assembly (for example, see Burge et al. 1999;Du and Rosbash 2002;Lallena et al. 2002;Liu 2002). The catalytic component is likely to be U6 snRNP, which joins the spliceosome as a U4/U6 · U5 tri-snRNP (Villa et al. 2002).A splicing error that adds or removes even 1 nt will disrupt the open reading frame of an mRNA; yet exons are correctly spliced from within tens of thousands of intronic nucleotides. This remarkable precision is, in part, built into the mechanism of intron removal because once...

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