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POLG
DNA polymerase gamma, catalytic subunit
POLG is located on the long (q) arm of chromosome 15, at band 15q26.1. Arm ratio per GRCh38 - banding schematic.
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Overview
The POLG gene, located at chromosomal position 15q26.1, provides instructions for making the catalytic alpha subunit of DNA polymerase gamma. This enzyme functions exclusively within mitochondria, the cellular structures responsible for energy production through oxidative phosphorylation. Polymerase gamma is the only DNA polymerase capable of replicating mitochondrial DNA, making it essential for maintaining the genetic material that mitochondria require to function properly.
Pathogenic variants in POLG follow an autosomal recessive inheritance pattern and represent one of the most common genetic causes of mitochondrial disease. These variants can produce a wide clinical spectrum ranging from severe infantile-onset conditions to adult-onset neurological disorders. The gene is recognised across multiple NHS Genomic Medicine Service panels for conditions affecting the nervous system, liver, and skeletal muscle.
What the gene does
DNA polymerase gamma operates as a multi-subunit complex, with the POLG-encoded alpha subunit providing the catalytic activity necessary for mitochondrial DNA synthesis and repair. This protein reads existing mitochondrial DNA sequences and uses them as templates to generate new DNA strands during replication. Beyond replication, polymerase gamma plays a crucial role in repairing damaged mitochondrial DNA, helping to maintain genomic stability within these organelles.
The alpha subunit combines with two accessory beta subunits to form the functional polymerase gamma complex. This assembly enhances the enzyme's processivity, allowing it to synthesise longer stretches of DNA without dissociating from the template strand. The enzyme possesses both polymerase activity, which adds nucleotides to growing DNA chains, and exonuclease activity, which provides proofreading capability by removing incorrectly incorporated nucleotides. This dual functionality ensures high fidelity during mitochondrial DNA replication, which is essential because mitochondria lack many of the DNA repair mechanisms present in the cell nucleus.
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Chromosome location
POLG is positioned on the long arm of chromosome 15 at band 26.1, designated cytogenetically as 15q26.1. The gene spans a substantial genomic region and encodes a protein of 1,239 amino acids. Its location on chromosome 15 places it among a cluster of genes involved in various metabolic and developmental processes.
Protein structure
The POLG protein exhibits distinct functional domains organised along its 1,239-amino-acid length. The N-terminal region contains a disordered segment spanning amino acids 1-68, followed by a region from amino acids 43-55 that does not contribute to the protein's polymerase or exonuclease activities. The exonuclease domain, responsible for proofreading function, contains three conserved motifs: Exo I (amino acids 196-200), Exo II (amino acids 267-275), and Exo III (amino acids 395-403). Additional disordered regions appear at amino acids 318-340 and 506-531.
The central portion includes an accessory-interacting determinant spanning amino acids 510-571, which mediates binding to the beta subunits of the polymerase complex. The C-terminal region houses the polymerase domain with its characteristic motifs: the trigger loop (amino acids 858-864), Pol A (amino acids 887-896), Pol B (amino acids 943-958), and Pol C (amino acids 1134-1141). These polymerase motifs coordinate nucleotide binding and catalyse the addition of nucleotides to the growing DNA strand.
Key variants
Pathogenic variants in POLG are distributed throughout the gene and typically result in reduced polymerase gamma activity. Many disease-causing variants involve single amino acid substitutions that impair either the polymerase or exonuclease functions of the enzyme. The severity and age of onset for POLG-related conditions often correlate with the degree of residual enzyme activity, with variants causing near-complete loss of function generally associated with more severe, early-onset presentations.
No pathogenic or likely-pathogenic ClinVar variants recorded yet for this gene.
Associated conditions
Variants in POLG cause a spectrum of autosomal recessive mitochondrial disorders collectively termed POLG-related disorders. These conditions share overlapping features affecting the nervous system, muscles, and liver, though individual presentations vary considerably. Alpers-Huttenlocher syndrome represents the severe infantile end of the spectrum, characterised by seizures, developmental regression, and liver failure. Childhood myocerebrohepatopathy spectrum affects the muscles, brain, and liver in young children. Ataxia neuropathy spectrum, which encompasses conditions previously termed mitochondrial recessive ataxia syndrome and sensory ataxia neuropathy dysarthria and ophthalmoplegia, typically presents in adolescence or adulthood with coordination problems and nerve dysfunction. Myoclonic epilepsy myopathy sensory ataxia combines seizures, muscle weakness, and balance difficulties. POLG variants also contribute to chronic progressive external ophthalmoplegia, a mitochondrial condition affecting eye movement muscles.
Inheritance pattern
Conditions caused by pathogenic POLG variants typically follow autosomal recessive inheritance.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
UK clinical status
POLG holds green (high confidence) status across numerous NHS Genomic Medicine Service gene panels, reflecting its well-established role in human disease. The gene appears on panels for POLG-related disorder (R315), mitochondrial disorders, and mitochondrial DNA maintenance disorder (R352). It is also included in panels addressing specific clinical presentations such as early onset or syndromic epilepsy (R59), hereditary ataxia (R54), hereditary neuropathy (R78), and acute rhabdomyolysis (R419). Additional panel memberships cover cholestasis (R171), mitochondrial liver disease including transient infantile liver failure (R317), intellectual disability (R29), and primary ovarian insufficiency. This extensive panel representation underscores the diverse clinical manifestations that can arise from POLG dysfunction.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What inheritance pattern do POLG variants follow?
POLG-related disorders follow an autosomal recessive inheritance pattern, meaning affected individuals typically inherit one pathogenic variant from each parent. Parents who each carry one variant are generally unaffected but have a 25% chance with each pregnancy of having an affected child.
Why does POLG affect so many different organs?
Because polymerase gamma is essential for maintaining mitochondrial DNA in all cell types, POLG variants can affect any tissue that relies heavily on mitochondrial energy production. Organs with high energy demands, such as the brain, liver, and skeletal muscle, are particularly vulnerable to mitochondrial dysfunction.
Can POLG-related conditions vary in severity within the same family?
Yes, individuals with identical POLG variants can sometimes show different clinical presentations or ages of onset. This variability may reflect differences in mitochondrial DNA background, environmental factors, or other genetic modifiers that influence how severely mitochondrial function is compromised.
References
- Sohl CD, Kasiviswanathan R, Copeland WC. Mutations in human DNA polymerase γ confer unique mechanisms of catalytic deficiency that mirror the disease severity in mitochondrial disorder patients. Human molecular genetics. 2013. PMID: 23208208
- Stumpf JD, Copeland WC. Mitochondrial DNA replication and disease: insights from DNA polymerase γ mutations. Cellular and molecular life sciences : CMLS. 2011. PMID: 20927567
- Milone M, Massie R. Polymerase gamma 1 mutations: clinical correlations. The neurologist. 2010. PMID: 20220442
- Rocher C, Taanman JW, Pierron D. Influence of mitochondrial DNA level on cellular energy metabolism: implications for mitochondrial diseases. Journal of bioenergetics and biomembranes. 2008. PMID: 18415670
- Yu Wai Man CY, Chinnery PF, Griffiths PG. Extraocular muscles have fundamentally distinct properties that make them selectively vulnerable to certain disorders. Neuromuscular disorders : NMD. 2005. PMID: 15639116
- Chan SS, Longley MJ, Copeland WC. The common A467T mutation in the human mitochondrial DNA polymerase (POLG) compromises catalytic efficiency and interaction with the accessory subunit. The Journal of biological chemistry. 2005. PMID: 16024923
- Van Goethem G, Martin JJ, Dermaut B. Recessive POLG mutations presenting with sensory and ataxic neuropathy in compound heterozygote patients with progressive external ophthalmoplegia. Neuromuscular disorders : NMD. 2003. PMID: 12565911
- Van Goethem G, Martin JJ, Van Broeckhoven C. Progressive external ophthalmoplegia characterized by multiple deletions of mitochondrial DNA: unraveling the pathogenesis of human mitochondrial DNA instability and the initiation of a genetic classification. Neuromolecular medicine. 2003. PMID: 12835509
- Van Goethem G, Dermaut B, Löfgren A. Mutation of POLG is associated with progressive external ophthalmoplegia characterized by mtDNA deletions. Nature genetics. 2001. PMID: 11431686
- Moraes CT, Shanske S, Tritschler HJ. mtDNA depletion with variable tissue expression: a novel genetic abnormality in mitochondrial diseases. American journal of human genetics. 1991. PMID: 1998336