Abstract
Point mutations can generate defective and sometimes harmful proteins.
The nonsense-mediated mRNA decay (NMD) pathway minimizes the potential damage
caused by nonsense mutations. In-frame nonsense codons
located at a minimum distance upstream of the last exon-exon junction are
recognized as premature termination codons (PTCs), targeting the mRNA for
degradation. Some nonsense mutations cause skipping of one or more exons,
presumably during pre-mRNA splicing in the nucleus; this phenomenon is termed
nonsense-mediated altered splicing (NAS), and its underlying mechanism is
unclear. By analyzing NAS in BRCA1, we show
here that inappropriate exon skipping can be reproduced in vitro, and
results from disruption of a splicing enhancer in the coding sequence. Enhancers
can be disrupted by single nonsense, missense and translationally silent point
mutations, without recognition of an open reading frame as such. These results
argue against a nuclear reading-frame scanning mechanism for NAS. Coding-region
single-nucleotide polymorphisms (cSNPs) within exonic splicing
enhancers or silencers may affect the patterns or efficiency of mRNA splicing,
which may in turn cause phenotypic variability and variable penetrance of
mutations elsewhere in a gene.