mutations and types
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Based on Harrison's Principles of Internal Medicine and foundational genetics knowledge, here is a comprehensive overview of mutations and their types:
Mutations: Definition and Types
What is a Mutation?
A mutation (or variant) is any change in the primary nucleotide sequence of DNA, regardless of its functional consequences (Harrison's, p. 13225). The more neutral term "variation" is now increasingly preferred by professional organizations. Mutations can be lethal, benign, or even confer an evolutionary advantage.
Classification of Mutations
1. By Scale (Size)
| Type | Description | Example |
|---|---|---|
| Point mutation | Single nucleotide change | Sickle cell disease (GAG→GTG) |
| Small insertions/deletions (indels) | Addition or removal of a few bases | Frameshift mutations |
| Structural variants | Large-scale rearrangements | Inversions, translocations |
| Copy Number Variants (CNVs) | Duplication or deletion of large segments | DiGeorge syndrome (22q11 deletion) |
| Aneuploidies | Whole chromosome gain/loss | Trisomy 21 (Down syndrome) |
2. By Molecular Mechanism (Point Mutations)
| Type | Definition | Example |
|---|---|---|
| Substitution | One base replaced by another | Most SNPs |
| — Transition | Purine↔Purine or Pyrimidine↔Pyrimidine (A↔G, C↔T) | Common in aging |
| — Transversion | Purine↔Pyrimidine (A/G↔C/T) | UV-induced mutations |
| Insertion | Addition of one or more bases | Causes frameshifts |
| Deletion | Removal of one or more bases | Causes frameshifts |
| Inversion | Segment reversed within chromosome | Hemophilia A |
| Translocation | Segment moved to another chromosome | CML (Philadelphia chromosome) |
| Duplication | Segment duplicated | Charcot-Marie-Tooth disease |
3. By Functional Effect
| Type | Effect | Example |
|---|---|---|
| Silent (synonymous) | No amino acid change (due to codon degeneracy) | — |
| Missense | Single amino acid change | HbS in sickle cell disease |
| Nonsense | Creates a premature stop codon → truncated protein | Duchenne muscular dystrophy |
| Frameshift | Shifts reading frame → garbled protein | Tay-Sachs disease |
| Splice-site | Disrupts mRNA splicing → exon skipping/intron retention | β-Thalassemia |
| Regulatory/promoter | Alters gene expression level | β-Thalassemia (some forms) |
| Trinucleotide repeat expansion | Expansion of repetitive sequences | Huntington's disease, Fragile X |
4. By Origin
| Type | Definition |
|---|---|
| Germline mutation | Inherited; present in every cell; passed to offspring |
| Somatic mutation | Acquired in non-germ cells; affects only certain tissues; not heritable |
| De novo mutation | New mutation not present in parents; accounts for many dominant diseases |
5. By Effect on Protein Function
| Type | Mechanism | Example |
|---|---|---|
| Loss-of-function (LOF) | Reduces/eliminates protein activity; often recessive | CFTR mutations in cystic fibrosis |
| Gain-of-function (GOF) | Confers new/enhanced activity; often dominant | RAS mutations in cancer |
| Dominant negative | Mutant protein interferes with wild-type | Some collagen mutations in OI |
| Haploinsufficiency | Single functional copy insufficient | BRCA1 mutations |
6. By Effect on Fitness
- Pathogenic – causes disease
- Likely pathogenic – strong evidence of disease association
- Variant of uncertain significance (VUS) – unknown clinical impact
- Likely benign / Benign – no disease association
- Polymorphism – common variant (≥1% population frequency); usually benign
Key Concepts
- SNP (Single Nucleotide Polymorphism): The most common type of genetic variation in humans (~10 million in the genome)
- INDEL: Insertion or deletion of bases; if not a multiple of 3, causes a frameshift
- Microsatellite instability (MSI): Accumulation of mutations in repetitive sequences, seen in Lynch syndrome-associated cancers
- Mutational hotspots: Regions of DNA particularly prone to mutation (e.g., CpG dinucleotides for methylation-related deamination)
Source: Harrison's Principles of Internal Medicine, 21st Edition, p. 13225, supplemented with foundational molecular genetics.
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