DNA repair defect is associated with –
**Core Concept**
DNA repair defects are a group of genetic disorders characterized by impaired ability to repair DNA damage, leading to increased genetic mutations and cancer susceptibility. This involves defects in mechanisms such as nucleotide excision repair (NER), base excision repair (BER), mismatch repair (MMR), and double-strand break repair (DSBR).
**Why the Correct Answer is Right**
The most common DNA repair defect associated with increased cancer risk is hereditary nonpolyposis colorectal cancer (HNPCC), also known as Lynch syndrome. This condition is caused by mutations in the genes MLH1, MSH2, MSH6, and PMS2, which are crucial for mismatch repair (MMR) during DNA replication. As a result, genetic mutations accumulate, particularly in the tumor suppressor gene TP53, leading to colorectal cancer and other neoplasms. The defective MMR system fails to correct mismatched bases, causing genetic instability and cancer.
**Why Each Wrong Option is Incorrect**
**Option A:** While BRCA1 and BRCA2 mutations are associated with increased breast and ovarian cancer risk due to defects in homologous recombination repair (HRR), they are not classified as DNA repair defects in the same category as HNPCC.
**Option B:** Fanconi anemia is a genetic disorder associated with defects in homologous recombination repair (HRR), but it primarily presents with developmental abnormalities, bone marrow failure, and increased cancer risk, rather than primarily as a DNA repair defect.
**Option C:** Ataxia-telangiectasia is a genetic disorder caused by mutations in the ATM gene, which is involved in double-strand break repair (DSBR). However, it is primarily characterized by ataxia, telangiectasias, and immunodeficiency, rather than increased cancer risk due to a DNA repair defect.
**Option D:** Xeroderma pigmentosum is a genetic disorder caused by mutations in genes involved in nucleotide excision repair (NER), but it primarily presents with skin photosensitivity and increased skin cancer risk, rather than the broader spectrum of cancers seen in HNPCC.
**Clinical Pearl / High-Yield Fact**
Lynch syndrome is the most common hereditary cancer syndrome, accounting for approximately 3% of all colorectal cancer cases. It is essential to screen individuals with a family history of colorectal cancer for mutations in MMR genes.
**Correct Answer: D. Xeroderma pigmentosum**