nonsense mutation

**Definition:** A single-nucleotide substitution that converts a sense codon into a stop codon. The stop codons are UAA ("U Are Annoying"), UAG ("U Are Gone"), and UGA ("U Go Away"). Translation ends early, so the protein is truncated.

**Pathophysiology:** - The ribosome terminates at the premature termination codon (PTC). The truncated polypeptide usually lacks key functional domains and is unstable. - If the PTC lies more than about 50-55 nucleotides upstream of the last exon-exon junction, **nonsense-mediated mRNA decay (NMD)** degrades the transcript. This prevents production of potentially harmful truncated proteins. The result is effectively a null allele (loss of function). - A PTC in the last exon or close to the 3' end can escape NMD. The truncated protein may then be made and can sometimes act in a dominant-negative way. - The effect is typically more severe the earlier (more 5') the stop codon appears.

**Comparison with other point mutations (high-yield):** - **Silent:** a codon change with no amino acid change (wobble position). No effect. - **Missense:** one amino acid is replaced by another. The effect varies. Example: sickle cell disease (Glu→Val at position 6 of β-globin). - **Nonsense:** an amino acid codon becomes a stop codon. This gives a truncated protein and is usually the most severe point mutation. - **Frameshift:** insertion or deletion of a number of nucleotides not divisible by 3. The reading frame shifts and usually a premature stop appears downstream. Examples: Tay-Sachs (4-bp insertion in *HEXA*), many cases of cystic fibrosis, and Duchenne muscular dystrophy. - **Splice-site:** a mutation at the splice site that alters mRNA processing.

**Clinical examples:** - **Duchenne muscular dystrophy:** *DMD* (dystrophin) mutations are usually frameshift or nonsense, causing absent dystrophin. Becker muscular dystrophy has in-frame deletions that leave partially functional dystrophin. - **β⁰-thalassemia:** nonsense mutations (such as codon 39 C→T) in the β-globin gene cause absent β-chain production. Splice-site and promoter mutations cause β⁺ or β⁰ disease. - **Cystic fibrosis:** the class I *CFTR* mutations (such as G542X and W1282X) are nonsense mutations that cause absent protein production. - **Others:** some cases of familial hypercholesterolemia (*LDLR*), *BRCA1/2* hereditary breast and ovarian cancer, and tumor suppressor loss in *APC* (familial adenomatous polyposis) and *TP53*.

**Therapeutic relevance:** - **Read-through agents** (such as ataluren, and aminoglycosides like gentamicin) promote ribosomal read-through of premature stop codons by allowing insertion of a near-cognate tRNA. They have been studied in nonsense-mutation Duchenne muscular dystrophy and cystic fibrosis, with limited or variable efficacy. Ataluren has not received FDA approval. - **Exon-skipping antisense oligonucleotides** (such as eteplirsen) are used in DMD to restore the reading frame, mostly for frameshift deletions.

**Exam pearls:** - A nonsense mutation causes a **shortened protein**. It does not change the protein length in the way an in-frame deletion does. - Mutations in the middle of a gene give the most truncated proteins. Mutations near the 3' end may leave partial function. - Remember the trio: **silent (same), missense (different), nonsense (stop)**. - Mutation severity ranking, generally: frameshift and nonsense > missense > silent. - Aminoglycosides normally cause misreading of mRNA by binding the 30S subunit in bacteria. At high doses they can also cause read-through in eukaryotes, which is the basis for their use in nonsense mutation research.