Nielsen, K. B., Sorensen, S., Cartegni, L., Corydon, T. J., Doktor, T. K., Schroeder, L. D., Reinert, L. S., Elpeleg, O., Krainer, A. R., Gregersen, N., Kjems, J., Andresen, B. S. (March 2007) Seemingly neutral polymorphic variants may confer immunity to splicing-inactivating mutations: A synonymous SNP in exon 5 of MCAD protects from deleterious mutations in a flanking exonic splicing enhancer. American Journal of Human Genetics, 80 (3). pp. 416-432. ISSN 0002-9297
Abstract
The idea that point mutations in exons may affect splicing is intriguing and adds an additional layer of complexity when evaluating their possible effects. Even in the best-studied examples, the molecular mechanisms are not fully understood. Here, we use patient cells, model minigenes, and in vitro assays to show that a missense mutation in exon 5 of the medium-chain acyl-CoA dehydrogenase (MCAD) gene primarily causes exon skipping by inactivating a crucial exonic splicing enhancer (ESE), thus leading to loss of a functional protein and to MCAD deficiency. This ESE functions by antagonizing a juxtaposed exonic splicing silencer (ESS) and is necessary to define a suboptimal 3' splice site. Remarkably, a synonymous polymorphic variation in MCAD exon 5 inactivates the ESS, and, although this has no effect on splicing by itself, it makes splicing immune to deleterious mutations in the ESE. Furthermore, the region of MCAD exon 5 that harbors these elements is nearly identical to the exon 7 region of the survival of motor neuron (SMN) genes that contains the deleterious silent mutation in SMN2, indicating a very similar and finely tuned interplay between regulatory elements in these two genes. Our findings illustrate a mechanism for dramatic context-dependent effects of single-nucleotide polymorphisms on gene-expression regulation and show that it is essential that potential deleterious effects of mutations on splicing be evaluated in the context of the relevant haplotype.
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