## Investigation of Choice for NER Deficiency ### Why Unscheduled DNA Synthesis (UDS) Assay is Correct **Key Point:** The UDS assay is the gold-standard functional test for nucleotide excision repair capacity. It measures the incorporation of radioactive nucleotides (typically ^3^H-thymidine) into DNA during the repair phase following UV irradiation — a process that occurs outside S phase (hence "unscheduled"). **High-Yield:** In normal cells, UV damage triggers rapid NER, resulting in significant UDS activity. In xeroderma pigmentosum (XP) and other NER-deficient syndromes, UDS is markedly reduced or absent, making this assay highly sensitive and specific for functional NER defects. ### How the Test Works 1. Cultured patient fibroblasts are exposed to controlled UV-B or UV-C irradiation 2. Cells are then incubated in medium containing radioactive thymidine 3. DNA is extracted and radioactivity quantified by scintillation counting or autoradiography 4. **Normal result:** 5–10% of total DNA synthesis occurs as UDS 5. **Abnormal result:** <1% UDS indicates severe NER deficiency **Clinical Pearl:** This test directly assesses the functional capacity of the entire NER pathway, not just individual gene mutations, making it superior for diagnosis of suspected repair syndromes. ### Why Other Investigations Fall Short | Investigation | Limitation | |---|---| | Gene sequencing | Identifies mutations but does not prove functional impairment; variants of uncertain significance are common | | Thymine dimer measurement | Detects damage but not repair capacity; elevated levels reflect accumulated damage, not the repair mechanism itself | | Histone acetylation | Measures chromatin remodeling, a component of NER, but is not specific for NER deficiency and does not assess overall repair function | **Mnemonic:** **UDS = Unscheduled DNA Synthesis** — the hallmark of active nucleotide excision repair outside the normal S phase.
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