insertion mutation

**Definition:** Addition of one or more nucleotide base pairs into a DNA sequence. It is the counterpart of a deletion mutation. Together they are called "indels."

**Types and consequences:** - **Frameshift insertion (1, 2, 4, 5... bases; not a multiple of 3):** Shifts the reading frame downstream of the insertion. This changes the amino acid sequence from that point on and usually produces a premature stop codon. The result is a truncated, nonfunctional protein, or nonsense-mediated mRNA decay. The effect is usually severe (loss of function), and it is typically more damaging than a missense mutation. - **In-frame insertion (multiple of 3):** Adds one or more amino acids but preserves the downstream reading frame. The effect is variable and is often milder than a frameshift. It depends on where the insertion falls and whether it disrupts a functional domain. - **Insertion in a noncoding region:** May disrupt splice sites, promoters, or regulatory elements. This can cause aberrant splicing or altered expression.

**Mechanisms:** - **Replication slippage** at repetitive sequences (e.g., microsatellites) causes insertions or deletions of repeat units. This is the classic link to **mismatch repair defects** (Lynch syndrome/HNPCC, with MLH1, MSH2, MSH6, and PMS2 mutations), which cause microsatellite instability. - **Unequal crossing over** during meiosis. - **Transposable elements** (e.g., LINE-1/Alu insertion) and **viral integration** (insertional mutagenesis). Examples are retroviral integration activating proto-oncogenes, and gene therapy vector–induced leukemia in early SCID-X1 trials. - **Intercalating agents** (e.g., acridine dyes, ethidium bromide) cause frameshift insertions and deletions.

**Clinical examples (high-yield):** - **Cystic fibrosis:** The most common mutation, ΔF508, is a 3-bp deletion (in-frame). Some other CFTR mutations are frameshifts. - **Tay-Sachs disease:** The common Ashkenazi mutation is a 4-bp insertion (TATC) in *HEXA*, which causes a frameshift. - **Duchenne muscular dystrophy:** Frameshift (and nonsense) mutations in *DMD* disrupt the reading frame and cause severe disease. In-frame mutations cause the milder Becker form (the "frameshift rule"). - **Trinucleotide repeat expansion disorders** (Huntington disease, fragile X, myotonic dystrophy, Friedreich ataxia) are essentially insertions of repeated units. They show anticipation. - **Neurofibromatosis type 1, BRCA1/2, and familial hypercholesterolemia:** Frameshift insertions and deletions can cause loss of function. - **Hemoglobin:** Hb Constant Spring is caused by a stop codon mutation. Frameshift mutations in β-globin can cause β-thalassemia.

**Comparison with other point mutations (frequently tested):** - **Silent:** Same amino acid. The change is often in the third (wobble) base. - **Missense:** Different amino acid (e.g., sickle cell, Glu→Val). - **Nonsense:** Premature stop codon. - **Frameshift:** Insertion or deletion of a non-multiple of 3 bases, which causes the most severe protein disruption. - **Splice site:** Abnormal intron/exon processing (e.g., β-thalassemia).

**Test-taking pearls:** - Frameshift mutations (insertion or deletion, not a multiple of 3) are more severe than missense mutations. - Mismatch repair deficiency leads to microsatellite instability, which leads to frameshift mutations in repeat-containing genes. - Duchenne vs. Becker muscular dystrophy is explained by out-of-frame vs. in-frame mutations. - Insertions don't need to disrupt the whole gene to be harmful. Because the frame shifts, downstream codons are all misread.