In SCID, what type of DNA repair is defective?
**Core Concept**
Severe Combined Immunodeficiency (SCID) is a group of rare genetic disorders characterized by impaired immune function. The underlying cause of SCID is a defect in the body's ability to repair DNA damage, specifically in the context of V(D)J recombination, a process essential for the development and maturation of lymphocytes.
**Why the Correct Answer is Right**
The correct answer is related to the defective DNA repair mechanism in SCID. V(D)J recombination involves the rearrangement of immunoglobulin and T-cell receptor genes, which requires the activation of the enzyme RAG1 and RAG2. This process also involves the use of non-homologous end joining (NHEJ) to repair DNA double-strand breaks. A defect in the NHEJ pathway, specifically in the Ku70/Ku80 complex, is responsible for the impaired DNA repair in SCID.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a particular DNA repair pathway associated with SCID.
**Option B:** This option is incorrect because it relates to a different type of DNA repair, base excision repair, which is not directly related to the pathogenesis of SCID.
**Option C:** This option is incorrect because it refers to a different type of DNA repair, mismatch repair, which is not primarily associated with the development of SCID.
**Clinical Pearl / High-Yield Fact**
It is essential to remember that SCID is a heterogeneous group of disorders, and the defective DNA repair mechanism can vary depending on the underlying genetic mutation.
**Correct Answer:** C. Non-homologous end joining (NHEJ)