Which of the following is the most common site of spontaneous point mutations in the human genome?

Explanation

## Spontaneous Point Mutations: CpG Hotspots ### CpG Dinucleotides as Mutation Hotspots **Key Point:** CpG dinucleotides are the most common sites of spontaneous point mutations in the human genome, accounting for ~30% of all point mutations despite comprising only ~1% of the genome. ### Why CpG Sites Are Hypermutable #### 1. **Methylation of Cytosine** In vertebrate genomes, cytosine in CpG dinucleotides is frequently methylated to **5-methylcytosine (5mC)**: - Occurs at ~70–80% of CpG sites in the human genome - 5mC serves regulatory functions (gene silencing, X-inactivation, imprinting) - Creates a "molecular time bomb" for mutagenesis #### 2. **Spontaneous Deamination of 5-Methylcytosine** The critical mutagenic mechanism: | Process | Normal Cytosine | 5-Methylcytosine | |---------|---|---| | **Spontaneous deamination product** | Uracil | **Thymine** | | **Repair mechanism** | Uracil-DNA glycosylase (BER) | No specific repair enzyme | | **Outcome** | Recognized as abnormal; removed | **Recognized as normal base; persists** | | **Mutation result** | C→T transition prevented | C→T transition fixed | **High-Yield:** Uracil in DNA is recognized as abnormal and excised by uracil-DNA glycosylase. However, thymine (the deamination product of 5mC) is a normal DNA base and escapes repair, leading to a permanent C→T transition mutation [cite:Lehninger Principles of Biochemistry Ch 26]. #### 3. **Evolutionary Evidence** - CpG dinucleotides are **depleted** in the human genome (~25% of expected frequency) - This depletion reflects millions of years of C→T mutations at CpG sites - CpG islands (unmethylated CpG-rich regions) near gene promoters are preserved from this depletion ### Mechanism Flowchart ```mermaid flowchart TD A[CpG dinucleotide]:::outcome --> B[Cytosine methylated to 5mC]:::action B --> C[Spontaneous deamination]:::action C --> D[5mC converts to Thymine]:::outcome D --> E{Repair recognition?}:::decision E -->|Normal cytosine → uracil| F[Uracil-DNA glycosylase removes uracil]:::action E -->|5mC → thymine| G[Thymine recognized as normal base]:::outcome F --> H[Lesion repaired]:::outcome G --> I[C→T transition mutation fixed]:::urgent ``` ### Mutation Spectrum at CpG Sites **Mnemonic: CpG Transitions — "CpG to TpA"** - **C→T transitions** (on one strand) = **G→A transitions** (on complementary strand) - These are the most common spontaneous mutations in humans - Represent ~50% of all point mutations in some genes (e.g., p53, BRCA1) ### Clinical Examples **Clinical Pearl:** Many inherited genetic diseases show CpG hotspot mutations: - **Hemophilia B Leyden:** CpG mutations in Factor IX gene - **Osteogenesis imperfecta:** CpG mutations in COL1A1/COL1A2 - **Familial hypercholesterolemia:** CpG mutations in LDLR gene - **p53 mutations in cancer:** ~50% occur at CpG sites ### Why Other Options Are Less Common **Comparison Table:** | Site Type | Mutation Frequency | Reason | |---|---|---| | **CpG dinucleotides** | Very high (~30% of all mutations) | 5mC deamination escapes repair | | AT-rich regions | Moderate | No special mutagenic mechanism | | Homopolymeric tracts | Moderate | Slippage during replication; repaired by mismatch repair | | Intergenic sequences | Low | Not enriched for spontaneous mutations | **Key Point:** While homopolymeric tracts (e.g., AAAA, GGGG) are prone to slippage mutations, they are repaired more efficiently than CpG deamination products. CpG sites remain the single most common hotspot.