**HRD in oncology stands for Homologous Recombination Deficiency** (also called Homologous Recombination Repair Deficiency or HRR Deficiency). ### What is HRD? HRD is a **phenotype** (observable characteristic) in cancer cells where the **homologous recombination repair (HRR)** pathway is impaired. This pathway is one of the main ways cells accurately repair **double-strand breaks (DSBs)** in DNA, which can occur during normal cell processes or due to DNA-damaging treatments. When HRR doesn't work properly: - Cells rely on error-prone backup repair mechanisms. - This leads to **genomic instability** and "genomic scarring" (specific patterns of DNA damage like loss of heterozygosity, telomeric allelic imbalance, or large-scale state transitions). - The instability can drive cancer development but also makes the tumor cells more vulnerable to certain therapies. ### Common Causes of HRD - **Mutations** in key HRR genes, especially *BRCA1* and *BRCA2* (germline or somatic). - Alterations in other HRR-related genes (e.g., *RAD51C/D*, *PALB2*, *ATM*, *BRIP1*, etc.). - Epigenetic changes, such as *BRCA1* promoter hypermethylation. - HRD can occur even without *BRCA* mutations (sometimes called "BRCAness"). Approximately **50%** of high-grade serous ovarian cancers show HRD, and it is also common in breast, prostate, and pancreatic cancers. ### Why is HRD Important in Oncology? Tumors with HRD are particularly sensitive to: - **Platinum-based chemotherapy** (e.g., carboplatin, cisplatin) — because these drugs cause DNA crosslinks that HR-deficient cells struggle to repair. - **PARP inhibitors (PARPi)** — drugs like olaparib, niraparib, rucaparib, or talazoparib. PARP inhibitors block an alternative DNA repair pathway (base excision repair). In HRD cells, this creates **synthetic lethality**: the cancer cells accumulate too much DNA damage and die, while normal cells (with intact HRR) are spared. HRD-positive tumors generally show better responses, longer progression-free survival (PFS), and sometimes improved overall survival with these treatments compared to HRD-negative (homologous recombination proficient, or HRP) tumors. ### Clinical Use - **HRD testing** is routinely recommended, especially in **ovarian cancer** (advanced/high-grade serous), and increasingly in breast, prostate, and pancreatic cancers. - Testing usually combines: - Sequencing for mutations in HRR genes (*BRCA1/2* and others). - Genomic scar/HRD score (measures the extent of genomic instability using algorithms like GIS — Genomic Instability Score). - **HRD-positive** status helps identify patients who are most likely to benefit from PARP inhibitor **maintenance therapy** after platinum chemotherapy. - Even some *BRCA*-wildtype tumors can be HRD-positive and still benefit from PARPi. ### Summary Table of Relevance | Cancer Type | HRD Prevalence | Key Benefit of HRD Testing | |----------------------|----------------|---------------------------------------------| | Ovarian (esp. HGSOC) | ~50% | Strongest evidence; guides frontline PARPi maintenance | | Breast (esp. TNBC) | Variable | Predicts PARPi and platinum response | | Prostate | Common | PARPi approval in HRD/BRCA cases | | Pancreatic | Common | PARPi benefit in HRD cases | In short, HRD is a key **biomarker** in precision oncology that helps oncologists personalize treatment by identifying patients who can gain the most from DNA-damaging therapies like platinum chemo and PARP inhibitors.