LIG4

LIG4 — DNA Ligase 4

Introduction

Lig4 is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

<div class="infobox infobox-gene"> [@dna2019]
<table> [@nhej2020]
<tr><th colspan="2" style="background:#e8f4f8; text-align:center; font-size:1.1em;">DNA Ligase 4 (LIG4)</th></tr> [@dna2021]
<tr><td><strong>Gene Symbol</strong></td><td>LIG4</td></tr> [@lig2022]
<tr><td><strong>Full Name</strong></td><td>DNA Ligase 4</td></tr>
<tr><td><strong>Chromosome</strong></td><td>13q33.3</td></tr>
<tr><td><strong>NCBI Gene ID</strong></td><td>[3981](https://www.ncbi.nlm.nih.gov/gene/3981)</td></tr>
<tr><td><strong>OMIM</strong></td><td>[601837](https://www.omim.org/entry/601837)</td></tr>
<tr><td><strong>Ensembl ID</strong></td><td>ENSG00000174444</td></tr>
<tr><td><strong>UniProt ID</strong></td><td>[P18859](https://www.uniprot.org/uniprot/P18859)</td></tr>
<tr><td><strong>Associated Diseases</strong></td><td>LIG4 syndrome, Cancer, Neurodegeneration</td></tr>
</table>
</div>

Overview

...

LIG4 — DNA Ligase 4

Introduction

Lig4 is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

<div class="infobox infobox-gene"> [@dna2019]
<table> [@nhej2020]
<tr><th colspan="2" style="background:#e8f4f8; text-align:center; font-size:1.1em;">DNA Ligase 4 (LIG4)</th></tr> [@dna2021]
<tr><td><strong>Gene Symbol</strong></td><td>LIG4</td></tr> [@lig2022]
<tr><td><strong>Full Name</strong></td><td>DNA Ligase 4</td></tr>
<tr><td><strong>Chromosome</strong></td><td>13q33.3</td></tr>
<tr><td><strong>NCBI Gene ID</strong></td><td>[3981](https://www.ncbi.nlm.nih.gov/gene/3981)</td></tr>
<tr><td><strong>OMIM</strong></td><td>[601837](https://www.omim.org/entry/601837)</td></tr>
<tr><td><strong>Ensembl ID</strong></td><td>ENSG00000174444</td></tr>
<tr><td><strong>UniProt ID</strong></td><td>[P18859](https://www.uniprot.org/uniprot/P18859)</td></tr>
<tr><td><strong>Associated Diseases</strong></td><td>LIG4 syndrome, Cancer, Neurodegeneration</td></tr>
</table>
</div>

Overview

DNA Ligase 4 (LIG4) is a critical enzyme in the non-homologous end joining (NHEJ) pathway of DNA double-strand break repair. It plays an essential role in maintaining genomic stability, particularly in [neurons](/entities/neurons) which are highly metabolically active cells with long lifespans and limited regenerative capacity. LIG4 deficiency leads to LIG4 syndrome, a rare autosomal recessive disorder characterized by immunodeficiency, microcephaly, growth retardation, and predisposition to malignancies. Research has increasingly implicated DNA repair defects, including NHEJ dysfunction, in the pathogenesis of neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS).

Function

DNA Double-Strand Break Repair via NHEJ

LIG4 is the ATP-dependent DNA ligase that catalyzes the final ligation step in the classical NHEJ (c-NHEJ) pathway, which is the predominant mechanism for repairing DNA double-strand breaks (DSBs) in mammalian cells. The NHEJ pathway operates throughout the cell cycle and does not require sequence homology, making it essential for genomic stability.

The LIG4 protein forms a stable heterodimer with XRCC4, which is essential for its stability and function. This LIG4-XRCC4 complex is recruited to DNA break sites by the Ku70/Ku80 heterodimer, which binds to DNA ends and recruits the DNA-dependent protein kinase catalytic subunit (DNA-PKcs). Additional processing enzymes, including Artemis and polymerase mu/lambda, prepare the DNA ends for ligation. LIG4-XRCC4 then catalyzes the formation of phosphodiester bonds to reconnect the broken DNA strands.

V(D)J Recombination

LIG4 is absolutely essential for V(D)J recombination, the process that generates antibody and T-cell receptor diversity in the developing immune system. During V(D)J recombination, RAG1/2 endonuclease creates DNA double-strand breaks at recombination signal sequences, and the NHEJ machinery, including LIG4, joins these broken ends to form functional immune receptor genes. Mutations in LIG4 cause severe combined immunodeficiency (SCID) due to defective V(D)J recombination.

Role in Neuronal Development and Survival

Neurons are post-mitotic cells that must survive for decades while maintaining genomic integrity. The NHEJ pathway, with LIG4 as a core component, is particularly important for:

• Repair of endogenous DNA damage from oxidative metabolism and environmental exposures
• Transcriptional processes that involve transient DNA breaks
• Maintenance of telomere integrity
• Neuronal development, including proliferation and differentiation

Disease Associations

LIG4 Syndrome

LIG4 syndrome (OMIM #606593) is a rare autosomal recessive disorder caused by biallelic mutations in the LIG4 gene. Clinical features include:

• Severe combined immunodeficiency (SCID) with T and B cell lymphopenia
• Microcephaly and developmental delay
• Growth retardation and short stature
• Characteristic facial dysmorphism
• Increased predisposition to lymphoid malignancies
• Radioresensitivity

The phenotype resembles ataxia-telangiectasia but with milder neurological features. Fibroblasts from LIG4 syndrome patients show profound defects in NHEJ repair and extreme sensitivity to ionizing radiation.

Neurodegenerative Diseases

Accumulating evidence links NHEJ dysfunction to neurodegenerative diseases:

Alzheimer's Disease (AD):

• Neurons in AD brains show evidence of increased DNA damage accumulation
• The NHEJ pathway is impaired in AD patient-derived neurons and animal models
• LIG4 expression and activity are altered in AD brain tissue
• DNA damage accumulation may contribute to neuronal dysfunction and death

Parkinson's Disease (PD):

• Mitochondrial dysfunction in PD leads to increased oxidative stress and DNA damage
• NHEJ efficiency declines with age, potentially accelerating neurodegeneration
• DNA damage response pathways are activated in PD substantia nigra neurons

Amyotrophic Lateral Sclerosis (ALS):

• RNA processing defects in ALS may lead to increased DNA damage
• Defective DNA repair contributes to motor neuron vulnerability
• [TDP-43](/mechanisms/tdp-43-proteinopathy) pathology, a hallmark of ALS, may impair DNA repair gene expression

Cancer

LIG4 polymorphisms and expression changes have been associated with various cancers. While complete LIG4 deficiency causes immunodeficiency, partial defects may allow survival of damaged cells and tumor progression. LIG4 expression is often upregulated in cancers as a survival mechanism to cope with genotoxic stress from rapid proliferation.

Expression

LIG4 is ubiquitously expressed in all tissues, including high expression in:

• Brain (neurons, glia)
• Lymphoid tissues (thymus, spleen)
• Bone marrow
• Testis

In the brain, LIG4 is expressed in:

• Cerebral [cortex](/brain-regions/cortex) neurons
• Hippocampal pyramidal neurons
• Cerebellar Purkinje cells
• Substantia nigra dopamine neurons

Expression is generally higher in proliferating cells but remains essential in post-mitotic neurons for DNA repair.

Therapeutic Implications

DNA Repair Enhancement

Given the role of DNA damage in neurodegeneration, enhancing NHEJ activity is being explored as a therapeutic strategy:

• Small molecules that stabilize the LIG4-XRCC4 complex
• Gene therapy approaches to increase LIG4 expression
• Antioxidants to reduce oxidative DNA damage burden

Cancer Therapy

LIG4 inhibition is being explored as a therapeutic target in cancer:

• Synthetic lethality approaches combining LIG4 inhibition with DNA-damaging chemotherapy
• Radiation sensitization strategies

Cross-links

• [DNA Repair Mechanisms](/mechanisms/dna-repair-neurodegeneration)
• [Genomic Stability](/mechanisms/genomic-stability)
• [Alzheimer's Disease Pathogenesis](/diseases/alzheimers-disease)
• [Parkinson's Disease Mechanisms](/diseases/parkinsons-disease)
• [XRCC4](/genes/xrcc4)
• [DNA-PKcs](/proteins/dna-pkcs-protein)

See Also

• [Genes Index](/genes)
• [DNA Repair Genes](/entities/dna-repair-genes)

Background

The study of Lig4 has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.

Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.

External Links

• [PubMed](https://pubmed.ncbi.nlm.nih.gov/) - Biomedical literature
• [Alzheimer's Disease Neuroimaging Initiative](https://adni.loni.usc.edu/) - Research data
• [Allen Brain Atlas](https://brain-map.org/) - Brain gene expression data

References

[Unknown, LIG4 syndrome: clinical features and molecular basis (2018) (2018)](https://pubmed.ncbi.nlm.nih.gov/29372668/)

[Unknown, DNA ligase IV deficiency causes immunodeficiency with microcephaly (2019) (2019)](https://pubmed.ncbi.nlm.nih.gov/31150123/)

[Unknown, NHEJ deficiency and neurodegeneration (2020) (2020)](https://pubmed.ncbi.nlm.nih.gov/32890125/)

[Unknown, DNA damage in Alzheimer's disease (2021) (2021)](https://pubmed.ncbi.nlm.nih.gov/34421076/)

[Unknown, LIG4 in neuronal development and survival (2022) (2022)](https://pubmed.ncbi.nlm.nih.gov/35653892/)