3.7 Mutation and Protein Consequences

A mutation is a permanent change in genetic material. A gene mutation can create a new allele, while larger changes can alter chromosome structure or number. Mutations arise spontaneously from replication errors or DNA chemistry and can also be induced by mutagens. A mutation in a somatic cell remains within its descendant cell lineage; one in a germ-line cell can be transmitted to offspring if that cell contributes to fertilization.

The effect of a mutation depends on location and context. A change in a coding region may alter an amino acid, create a stop codon or shift the reading frame. A promoter or enhancer mutation may change transcript abundance. An intronic change may affect splicing. A mutation in a region without a relevant function may have no detectable effect. “Mutation” names a sequence event, not an inevitable disease outcome.

A base substitution replaces one nucleotide pair. It may be silent if the new codon specifies the same amino acid, missense if it specifies a different amino acid, or nonsense if it becomes a stop codon. A missense effect depends on the chemical difference between amino acids and the position in the protein. Replacing a surface residue may have little effect, whereas changing an active-site or structural residue can transform function.

Insertion or deletion of a number of bases not divisible by three shifts the reading frame. Codon boundaries change from the mutation onward, often altering many amino acids and introducing a premature stop. An insertion or deletion of three bases preserves the downstream frame but adds or removes one amino acid. An inversion reverses the orientation of a DNA segment and can disrupt a gene or its regulation depending on breakpoints.

Sickle-cell disease results from a single substitution in the beta-globin gene that replaces glutamic acid with valine in the beta chain. The change introduces a hydrophobic surface that promotes hemoglobin aggregation when oxygen concentration is low. Red blood cells become rigid and sickle-shaped, impairing oxygen transport and obstructing capillaries. One nucleotide affects phenotype through amino-acid chemistry, protein interactions, cell shape and tissue physiology.

Individuals heterozygous for the sickle-cell allele usually have sickle-cell trait rather than severe disease and have increased resistance to severe malaria. In regions with intense malaria selection, this heterozygote advantage can maintain both alleles in a population. The allele is neither universally beneficial nor universally harmful; fitness depends on genotype and environment.

Huntington disease is associated with expansion of a CAG trinucleotide repeat in the HTT gene. Above a threshold, the expanded polyglutamine region makes the protein damaging to neurons; larger repeat numbers tend to be associated with earlier onset. This is an insertion-like expansion that preserves triplet framing but changes the length and behavior of the protein.

A 32-base-pair deletion in both copies of CCR5 can greatly reduce entry of common HIV-1 strains because the cell-surface co-receptor is absent or altered. The example shows that a deletion can be protective in one context, though CCR5 also has normal immune roles. Classification as beneficial, neutral or harmful must refer to particular conditions and effects.

Predictive genetic testing can reveal a mutation before symptoms appear, as with an expanded HTT repeat. The result may guide monitoring and reproductive decisions, but it can also affect relatives who share risk, psychological wellbeing, privacy, insurance or employment. A test therefore needs informed consent and expert interpretation. An uncertain or threshold result must not be presented as a simple binary diagnosis.