Intron splice sites

Inactivating mutations (AG 1226, AT 1597, C/T 1831 stop, A A intron splice site) create receptors that lead to dominant forms of pseudohypoaldosteronism (PHA1) (45)... 

Figure 11.5. Mutation of intron splice sites involved in formation of a mutant allele of CYP2D6. (A) Normal intron consensus sequences of mammalian gene. (B) Mutation of the CYP2D6 splice site that moves the splice site downstream to the next 3 consensus sequence, altering the reading frame of the protein and resulting in a premature stop codon (CYP2D6 4) and a truncated, inactive protein.

In addition to affecting the efficiency of promoter utilization, eukaryotic cells employ alternative RNA processing to control gene expression. This can result when alternative promoters, intron-exon splice sites, or polyadenylation sites are used. Occasionally, heterogeneity within a cell results, but more commonly the same primary transcript is processed differendy in different tissues. A few examples of each of these types of regulation are presented below. 

Nielsen, D. A., Jenkins, G. L., Stefanisko, K. M., Jefferson, K. K. Goldman, D. (1997). Sequence, splice site and population frequency distribution analyses of the polymorphic human tryptophan hydroxylase intron 7. Brain Res. Mol. Brain Res., 45, 145-8. 

To make mRNA, the primary transcript must be spliced to bring the protein-coding sequences (exons) together and to remove the intervening sequences (introns). The splice signals consist of a 5 and a 3 set of sequences that are always found at splice junctions. However, this is generally believed to provide too little information to recognize a splice site specifically and correctly. Some sequences in the intron are also important. 

Frank deletions of the gene are not common. A frequent mutation among northern Europeans (=40%) is a G to A transition at the 5 donor splice site in intron 12, resulting in absence of the C terminus. Another relatively common (=20%) mutation in northern Europeans involves a C to T transition in exon 12, resulting in substitution of a tryptophan for an arginine residue [21]. Over 70 different mutations have been described to date in the American population [22]. 

Introns are removed from hnRNA by splicing, accomplished by spliceosomes (also known as an snRNP, or snurp), which are complexes of snRNA and protein. The hnRNA molecule is cut at splice sites at the 5 (donor) and 3 (acceptor) ends of the intron. The intron is excised in the form of a lariat structuie and degraded. Neighboring exons are joined together to assemble the coding region of the mature mRNA. 

Mutations in splice sites affect the accuracy of intron removal from hnRNA during posttran-scriptionai processing. As illustrated in Figure 1-4-4, if a splice site is lost through mutation, spliceosomes may ... 

Splice-site mutations (choice E) occur at intron-exon boundaries and typically result in the loss of an exon or the inclusion of part of an intron in the coding sequence. Thus, more than a single amino acid -would he altered in a typical splice-site mutation. 

The removal of introns from pre-messenger RNAs in eukaryotes is catalyzed by the spliceosome, which is a large ribonucleoprotein consisting of at least 70 proteins and five small nuclear RNAs (snRNA) [144]. This splicing pathway involves two phosphotransfer reactions. In the first step, the 5 splice site is attacked by a 2 hydroxy group of an adenosine nucleotide within the intron [indicated by A in Fig. 12] that corresponds to the branch point in the lariat intermediate (Fig. 12,middle). In the second step, the 3 -OH group of the free 5 exon attacks the phosphodiester bond between the intron and... 

Fig. 12 The spliceosome splicing reaction. In the first step, the 2 -OH of an adenosine residue that is conserved in the intron attacks the phosphorus at the 5 splice site and generates an intron-3 -exon 2 intermediate and a free 5 exon 1. In the second step, the free 3 -OH of the 5 exon attacks the phosphorus at the 3 splice site to produce ligated exons and an excised intron...

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