Genetic Mutations

Mutations are classified by their scale and their effect on the resulting protein. Small-scale mutations include: 1. Point mutations (substitutions), which can be silent (no change in amino acid), missense (change in one amino acid, e.g., Sickle Cell), or nonsense (creation of a premature stop codon, leading to a truncated protein). 2. Insertions and Deletions (Indels), which, if they are not a multiple of three, cause a 'frameshift' mutation. Frameshifts alter every subsequent amino acid and usually lead to a non-functional protein. Large-scale mutations involve changes in chromosome structure, such as duplications, inversions, and translocations. Mutations can occur in germline cells (passed to offspring) or somatic cells (not inherited, but central to cancer development). Mutations can also occur in non-coding regions, such as splice sites (disrupting intron removal) or promoter regions (altering gene expression levels). The clinical impact of a mutation depends on whether it causes a loss of function (standard for recessive), a gain of function (standard for some dominant), or a dominant-negative effect. Identifying these variants allows for precise diagnosis and, increasingly, 'personalized' or 'stratified' medicine.

DNA stability is maintained by proofreading and repair mechanisms (e.g., mismatch repair, nucleotide excision repair). When these fail or are overwhelmed, a mutation becomes fixed in the genome. A transition is a purine-to-purine or pyrimidine-to-pyrimidine swap; a transversion is a purine-to-pyrimidine swap. Triplet repeat expansions (e.g., CAG in Huntington's) are a special type of dynamic mutation where the number of repeats increases through generations, leading to anticipation. Another key concept is 'loss of heterozygosity' (LOH), frequently seen in oncology. For many tumor suppressor genes (e.g., RB1 in retinoblastoma), an individual may inherit one 'hit' (mutation) in every cell; a somatic mutation (second hit) in a specific cell then leads to cancer. This is Knudson's 'two-hit' hypothesis. High-fidelity DNA polymerases usually keep the error rate low (1 in 10 billion bases), but the sheer size of the genome means some mutations are inevitable.

The type of mutation often correlates with clinical phenotype (genotype-phenotype correlation). For example, in Duchenne Muscular Dystrophy, a frameshift mutation results in no dystrophin (severe), while a non-frameshift deletion in Becker Muscular Dystrophy results in a partially functional protein (milder). In cystic fibrosis, the ΔF508 mutation (a 3-bp deletion) results in a protein that misfolds and is degraded. Knowledge of mutations allows for targeted therapies; for example, 'read-through' drugs are being developed for nonsense mutations, and Ivacaftor is used specifically for the G551D missense mutation in CF.

1. DNA Sequencing (Sanger or NGS) to identify the specific mutation. 2. Allele-specific PCR for known mutations. 3. MLPA (Multiplex Ligation-dependent Probe Amplification) to detect deletions or duplications. 4. Western Blot to see the effect on the resulting protein (e.g., checking for dystrophin in muscle).

Management depends on the disease. However, identifying the mutation allows for: 1. Family screening. 2. Targeted biological therapies. 3. Prognostic information. 4. Reproductive options like PGD. Somatic mutation testing is now standard in oncology (e.g., testing for EGFR mutations in lung cancer) to choose the best chemotherapy or biological agent.