missense mutation

**Definition:** A point mutation in which a single nucleotide substitution alters a codon, resulting in substitution of one amino acid for another in the protein product. Total protein length is unchanged (unlike nonsense or frameshift mutations).

**Point mutation classification (high-yield comparison):** - **Silent:** Codon change, same amino acid (wobble/degenerate code, usually 3rd base). No functional change. - **Missense:** Codon change, different amino acid. - **Nonsense:** Codon change to a stop codon (UAA, UAG, UGA) → truncated protein, often degraded via nonsense-mediated decay. Typically the most severe point mutation. - **Frameshift:** Insertion/deletion not divisible by 3 → altered reading frame downstream. Severe. - **Splice site:** Alters mRNA splicing.

**Conservative vs. nonconservative:** - *Conservative:* replacement with an amino acid of similar chemical properties (e.g., Leu→Ile). Often mild effect. - *Nonconservative:* replacement with a chemically different amino acid (e.g., charged→hydrophobic, as in Glu→Val). More likely to disrupt folding, charge, or active site.

**Transitions vs. transversions:** - Transition: purine↔purine (A↔G) or pyrimidine↔pyrimidine (C↔T). More common. - Transversion: purine↔pyrimidine.

**Mechanisms of effect:** - Loss of function (altered active site, unstable folding, impaired trafficking). - Gain of function (e.g., constitutive activation of a receptor or kinase). - Dominant-negative effect (mutant protein interferes with the normal protein, e.g., collagen triple helix in osteogenesis imperfecta, glycine substitutions).

**High-yield examples:** - **Sickle cell disease:** GAG→GTG at codon 6 of β-globin (Glu→Val); HbS polymerizes when deoxygenated. Autosomal recessive. - **HbC disease:** Glu→Lys at the same position 6 of β-globin. - **Cystic fibrosis:** most common mutation ΔF508 is a 3-bp deletion (in-frame, not missense), but many other CFTR missense mutations exist. Don't confuse these. - **Achondroplasia:** FGFR3 gain-of-function missense (Gly380Arg), autosomal dominant. - **Marfan/osteogenesis imperfecta:** glycine missense mutations can cause dominant-negative effects in collagen. - **BRAF V600E** (Val→Glu): activating mutation in melanoma, hairy cell leukemia, hairy cell, and others; targeted by vemurafenib/dabrafenib. - **JAK2 V617F:** polycythemia vera, essential thrombocythemia, primary myelofibrosis. - **Factor V Leiden:** Arg506Gln missense → resistance to inactivation by activated protein C → hypercoagulability. - **KRAS G12D/G12C:** activating missense in many cancers (G12C targeted by sotorasib). - **Phenylketonuria**, many other inborn errors: numerous missense alleles in PAH.

**Clinical significance:** - Severity is variable and genotype-phenotype correlation depends on site/chemistry. - Missense variants of uncertain significance (VUS) are a common challenge in genetic testing. - Detection: Sanger sequencing, next-generation sequencing, allele-specific PCR. Restriction fragment analysis may detect some (e.g., MstII for HbS historically).

**Exam pearls:** - "Single amino acid change, protein same length" → missense. - Missense mutation can be gain, loss, or dominant-negative. - Nonsense mutations are generally more deleterious than missense; silent mutations are least.