XRCC7 (DNA-PKcs) - DNA-Dependent Protein Kinase Catalytic Subunit

XRCC7 (DNA-PKcs) — DNA-Dependent Protein Kinase Catalytic Subunit

Introduction

XRCC7, also known as DNA-PKcs (DNA-dependent protein kinase catalytic subunit), encodes the catalytic subunit of the DNA-dependent protein kinase (DNA-PK) complex. DNA-PK is a central component of the non-homologous end joining (NHEJ) pathway, the predominant mechanism for repairing DNA double-strand breaks (DSBs) in mammalian cells. This gene is essential for maintaining genomic integrity in all cells, with particular importance in [neurons](/entities/neurons) where DNA repair defects have been implicated in [Alzheimer's Disease](/diseases/alzheimers-disease), [Parkinson's Disease](/diseases/parkinsons-disease), and other neurodegenerative conditions.

Gene Overview

| Attribute | Value |
|-----------|-------|
| Gene Symbol | XRCC7 (DNA-PKcs) |
| Full Name | X-Ray Repair Cross-Complementing 7 / DNA-PK Catalytic Subunit |
| Chromosomal Location | 8q24.21 |
| NCBI Gene ID | 5591 |
| OMIM | 604743 |
| Ensembl ID | ENSG00000137413 |
| UniProt | P78527 |
| Protein Length | 4128 amino acids |
| Protein Class | Serine/threonine protein kinase |
| Molecular Weight | ~469 kDa |

XRCC7 (DNA-PKcs) — DNA-Dependent Protein Kinase Catalytic Subunit

Introduction

XRCC7, also known as DNA-PKcs (DNA-dependent protein kinase catalytic subunit), encodes the catalytic subunit of the DNA-dependent protein kinase (DNA-PK) complex. DNA-PK is a central component of the non-homologous end joining (NHEJ) pathway, the predominant mechanism for repairing DNA double-strand breaks (DSBs) in mammalian cells. This gene is essential for maintaining genomic integrity in all cells, with particular importance in [neurons](/entities/neurons) where DNA repair defects have been implicated in [Alzheimer's Disease](/diseases/alzheimers-disease), [Parkinson's Disease](/diseases/parkinsons-disease), and other neurodegenerative conditions.

Gene Overview

| Attribute | Value |
|-----------|-------|
| Gene Symbol | XRCC7 (DNA-PKcs) |
| Full Name | X-Ray Repair Cross-Complementing 7 / DNA-PK Catalytic Subunit |
| Chromosomal Location | 8q24.21 |
| NCBI Gene ID | 5591 |
| OMIM | 604743 |
| Ensembl ID | ENSG00000137413 |
| UniProt | P78527 |
| Protein Length | 4128 amino acids |
| Protein Class | Serine/threonine protein kinase |
| Molecular Weight | ~469 kDa |

<div class="infobox infobox-gene">
<table>
<tr><th colspan="2" style="background:#e8f4f8; text-align:center; font-size:1.1em;">XRCC7 (DNA-PKcs) Gene Information</th></tr>
<tr><td><strong>Gene Symbol</strong></td><td>XRCC7 (DNA-PKcs)</td></tr>
<tr><td><strong>Full Name</strong></td><td>X-Ray Repair Cross-Complementing 7 / DNA-PK Catalytic Subunit</td></tr>
<tr><td><strong>Chromosome</strong></td><td>8q24.21</td></tr>
<tr><td><strong>NCBI Gene ID</strong></td><td>5591</td></tr>
<tr><td><strong>OMIM</strong></td><td>604743</td></tr>
<tr><td><strong>Ensembl ID</strong></td><td>ENSG00000137413</td></tr>
<tr><td><strong>UniProt</strong></td><td>P78527</td></tr>
<tr><td><strong>Associated Diseases</strong></td><td>[Alzheimer's Disease](/diseases/alzheimers-disease), [Parkinson's Disease](/diseases/parkinsons-disease), Ataxia-telangiectasia, SCID, Cancer</td></tr>
</table>
</div>

Discovery and Nomenclature

XRCC7 was identified through complementation studies of X-ray-sensitive Chinese hamster ovary (CHO) cell lines. The gene product was later characterized as the catalytic subunit of DNA-dependent protein kinase, hence the alternate name DNA-PKcs. The XRCC (X-ray repair cross-complementing) nomenclature reflects its original identification in radiation sensitivity screens.

Protein Structure

DNA-PKcs is one of the largest known proteins and belongs to the phosphatidylinositol 3-kinase-related kinase (PIKK) family[@goodwin2019]. Its structure includes:

Structural Domains

• HEAT repeats (Hungarian for "spiral")
• Flexibly linked arm segments
• Contains the DNA-binding channel

• Catalytic core with typical kinase motifs
• Phosphorylation sites for activation
• ATP-binding pocket

• FAT (FRAP-ATM-TRRAP) domain
• PRD (PIKK regulatory domain)
• FATC (C-terminal) domain

• DNA-binding channel
• Ku70/80 interaction surfaces

Post-Translational Modifications

• Phosphorylation: Multiple serine/threonine sites
• Acetylation: Lysine modifications
• Sumoylation: Regulation of kinase activity
• Ubiquitination: Degradation and signaling

The DNA-PK Complex

DNA-PK functions as a holoenzyme composed of:

DNA-PKcs (XRCC7)

Ku70/Ku80 Heterodimer

• Binds DNA ends with high affinity
• Recruits DNA-PKcs to DNA damage sites
• Acts as the regulatory component
• Forms the "DNA-PK" complex when bound to DNA-PKcs

Complex Assembly

Biological Functions

DNA Double-Strand Break Repair

The primary function of DNA-PK is in the NHEJ pathway[@meeks2020]:

Step 1: DNA end recognition

• Ku70/80 binds to DNA double-strand breaks
• Rapid diffusion along DNA

• Ku recruits DNA-PKcs to the break site
• DNA-PKcs positioning on DNA ends

• DNA binding stimulates DNA-PKcs autophosphorylation
• Kinase activity increases dramatically

• Recruitment of Artemis nuclease
• Processing of incompatible DNA ends

• XRCC4/LIG4 complex joins DNA ends
• DNA-PKcs dissociates from repaired DNA

V(D)J Recombination

DNA-PKcs is essential for V(D)J recombination in developing B and T lymphocytes[@alt2021]:

• Generates antigen receptor diversity
• Required for proper lymphocyte development
• DNA-PKcs deficiency causes severe combined immunodeficiency (SCID)

Transcription Regulation

DNA-PKcs has transcription-related functions:

• Modulates RNA polymerase II activity
• Regulates transcription factor function
• Involved in chromatin remodeling

Cell Cycle Regulation

• DNA-PKcs contributes to checkpoint signaling
• Affects cell cycle progression
• Regulates p53 activity

Expression Pattern

DNA-PKcs is expressed in virtually all cell types:

Tissue Distribution

• Brain: High expression in neurons
• Lymphoid tissues: T cells, B cells
• Testis: Germ cell development
• Proliferating cells: High in S phase

Cellular Localization

• Predominantly nuclear: Diffuse nuclear staining
• Associated with chromatin: Enrichment at DNA damage sites
• Cytoplasmic pool: Minor fraction

Brain Expression

In the central nervous system:

• Neurons: High expression in cortex, hippocampus
• Astrocytes: Moderate expression
• Microglia: Lower expression

Disease Associations

Alzheimer's Disease

DNA-PKcs has emerged as an important player in [Alzheimer's Disease](/diseases/alzheimers-disease)[@anderson2022]:

Pathological involvement:

• Increased DSBs in AD brain
• Impaired repair capacity
• DNA-PKcs dysregulation

• DNA-PKcs phosphorylates tau
• Interacts with neurofibrillary tangles
• Contributes to tau aggregation

• Aβ induces DNA damage
• DNA-PKcs activation in response
• Contributes to neuronal stress

• DNA repair defects affect neurons
• Cognitive decline correlation

• DNA-PKcs inhibitors under investigation
• Modulation of DNA repair capacity

Parkinson's Disease

In [Parkinson's Disease](/diseases/parkinsons-disease)[@choi2020]:

Dopaminergic neuron vulnerability:

• Increased DNA damage in substantia nigra
• DNA-PKcs activity alterations
• Impaired NHEJ efficiency

• Mitochondrial DNA damage
• Oxidative stress-induced DSBs
• Age-related decline in repair

• DNA-PKcs modulators for neuroprotection
• Enhancement of DNA repair

Ataxia-Telangiectasia

DNA-PKcs interacts with ATM in AT pathogenesis[@vasquez2021]:

• Synergistic effects with ATM deficiency
• Contributes to neurodegeneration
• DNA damage hypersensitivity

Cancer

DNA-PKcs is frequently overexpressed in cancers:

Oncogenic functions:

• Promotes tumor cell survival
• Confers radiation resistance
• Supports genomic instability

• DNA-PKcs inhibitors sensitize to radiation[@liu2021]
• Combination with PARP inhibitors
• Synthetic lethality approaches

Immunodeficiency

Biallelic DNA-PKcs mutations cause[@chen2023]:

• Severe combined immunodeficiency (SCID)
• V(D)J recombination defects
• Radiosensitivity

DNA-PKcs in Brain Aging and Neurodegeneration

Aging Brain

DNA-PKcs activity changes with age[@zhang2019]:

• Declining NHEJ efficiency
• Accumulation of DNA damage
• Contributing to cognitive decline

Synaptic Function

DNA-PKcs has additional roles in synaptic biology[@santiago2022]:

• Regulates synaptic plasticity
• Affects learning and memory
• Neuronal function beyond DNA repair

Circadian Regulation

DNA-PKcs connects DNA repair to circadian rhythm[@kim2020]:

• Links DNA damage response to clock
• Affects daily rhythms of repair
• Implications for neurodegeneration

Therapeutic Implications

Cancer Therapy

DNA-PKcs is a validated cancer target[@kelley2019]:

• DNA-PKcs inhibitors enhance RT effect
• Lower doses effective
• Reduced normal tissue toxicity

• With PARP inhibitors
• With chemotherapy
• With immunotherapy

• Multiple compounds in development
• Clinical trials ongoing

Neurodegeneration

Modulating DNA-PKcs in the brain is being explored[@park2023]:

• May reduce pathological activation
• Protect neurons from damage

• Enhance DNA repair capacity
• Prevent damage accumulation

• Viral vector delivery
• Increase expression

Challenges

• Blood-brain barrier penetration
• Balancing DNA repair in different contexts
• Tissue-specific effects

Research Methods

Genetic Studies

• Knockout/knockin mice
• Conditional deletions
• Patient mutation analysis

Biochemical Studies

• Kinase activity assays
• Phosphorylation analysis
• Protein-protein interactions

Cellular Models

• Neuronal cultures
• Patient-derived cells
• iPSC models

Animal Models

• DNA-PKcs-deficient mice
• Transgenic overexpression
• Disease models

Key Publications

See Also

• [NHEJ Pathway](/mechanisms/nhej-pathway)
• [DNA Damage Response](/mechanisms/dna-damage-response)
• [Alzheimer's Disease](/diseases/alzheimers-disease)
• [Parkinson's Disease](/diseases/parkinsons-disease)
• [Ataxia-telangiectasia](/diseases/ataxia-telangiectasia)
• [V(D)J Recombination](/mechanisms/vdj-recombination)

External Links

• [NCBI Gene: XRCC7](https://www.ncbi.nlm.nih.gov/gene/5591)
• [UniProt: P78527](https://www.uniprot.org/uniprot/P78527)
• [Ensembl: ENSG00000137413](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000137413)
• [OMIM: 604743](https://www.omim.org/entry/604743)
• [PubMed](https://pubmed.ncbi.nlm.nih.gov/?term=XRCC7+DNA-PKcs)

References