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CNGA3
cyclic nucleotide gated channel subunit alpha 3
The CNGA3 gene provides instructions for a protein that is a crucial part of cyclic nucleotide-gated (CNG) channels found in cone photoreceptor cells in the retina, which are vital for colour vision and seeing in bright light. The CNGA3 gene encodes the alpha subunit of a cyclic nucleotide-gated channel, primarily active in cone cells of the retina.
CNGA3 is located on the long (q) arm of chromosome 2, at band 2q11.2. Arm ratio per GRCh38 - banding schematic.
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Overview
The CNGA3 gene, or cyclic nucleotide gated channel subunit alpha 3, is essential for normal vision, particularly colour vision and vision in bright light conditions. It provides the genetic blueprint for a component of cyclic nucleotide-gated (CNG) channels, which are specialised ion channels predominantly found in the cone photoreceptor cells of the eye's retina. These channels are critical for initiating the visual signal pathway, translating light into electrical signals that the brain can process.
Disruptions to the CNGA3 gene can impair the function of these channels, leading to various inherited retinal disorders. Among these, achromatopsia is a notable condition linked to CNGA3 variants, characterised by severely impaired or absent colour vision.
What the gene does
The CNGA3 gene is responsible for producing the alpha subunit of the cyclic nucleotide-gated (CNG) channel. These channels are integral membrane proteins located exclusively in the cone photoreceptor cells within the retina. Cone cells are specialised to detect bright light and differentiate colours, making them vital for daytime vision.
CNG channels function as pores in the cell membrane, allowing positively charged ions to flow into cone cells. In darkness or low light, these channels remain open, facilitating ion influx. When light strikes the eye, a cascade of events leads to the closure of these channels, halting the ion flow. This change in ion transport alters the electrical charge across the cone cell membrane, generating an electrical signal. This signal, part of the phototransduction process, is then transmitted to the brain for visual interpretation.
The CNGA3 protein is thus a key player in converting light stimuli into neural signals, enabling the perception of colour and detailed vision in illuminated environments.
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Chromosome location
The CNGA3 gene is located on chromosome 2, specifically at position 2q11.2. The full length of the protein encoded by this gene consists of 694 amino acids. This genomic location positions CNGA3 in a region of the genome often studied in relation to inherited ocular conditions.
Protein structure
The CNGA3 protein, comprising 694 amino acids, exhibits a complex domain architecture crucial for its channel function. It features several disordered regions, including one at the N-terminus (amino acids 1-24) and another within the main body (amino acids 109-152). The critical Ion conduction pathway spans amino acids 298-406, containing the highly selective Selectivity filter (amino acids 365-368) which dictates ion permeability. A C-linker region (amino acids 408-485) connects to the Cyclic nucleotide-binding domain (amino acids 488-608), which is essential for regulating channel opening and closing. Towards the C-terminus, a Coiled coil motif (amino acids 626-669) is present, followed by another disordered region (amino acids 662-694).
Key variants
Variants within the CNGA3 gene can lead to alterations in the structure or function of the encoded protein, impacting the cyclic nucleotide-gated channels in cone photoreceptor cells. These changes can disrupt the phototransduction process, ultimately affecting vision. The severity and specific visual impairments experienced can vary depending on the nature and location of the variant.
Sample of pathogenic variants
10 pathogenic / likely-pathogenic variants from ClinVar, ranked by review status (expert-panel-reviewed first). This is a sample; recurrent founder variants in a specific population may not appear here - see the full ClinVar listing via the link above.
| Variant (HGVS) | Protein change | Classification | Evidence | Associated condition |
|---|---|---|---|---|
c.1039C>T | p.Arg347Cys | Pathogenic/Likely pathogenic | ★★☆☆ | not provided |
c.1070A>G | p.Tyr357Cys | Pathogenic | ★★☆☆ | CNGA3-related disorders |
c.1071C>G | p.Tyr357Ter | Pathogenic/Likely pathogenic | ★★☆☆ | CNGA3-related disorders |
c.1088T>C | p.Leu363Pro | Pathogenic | ★★☆☆ | Achromatopsia 2 |
c.1231del | p.Ala411fs | Pathogenic | ★★☆☆ | Achromatopsia 2 |
c.1443dup | p.Ile482fs | Pathogenic | ★★☆☆ | Achromatopsia 2 |
c.1537G>T | p.Gly513Ter | Pathogenic/Likely pathogenic | ★★☆☆ | CNGA3-related disorders |
c.162_163insT | p.Arg55Ter | Pathogenic | ★★☆☆ | not provided |
c.271C>T | p.Gln91Ter | Pathogenic | ★★☆☆ | Retinal dystrophy |
c.385A>T | p.Arg129Ter | Pathogenic | ★★☆☆ | not provided |
Evidence stars indicate ClinVar review status. Individual variant interpretation should always be performed by a qualified clinical laboratory - many variants remain classified as Variants of Uncertain Significance (VUS) pending more research.
Associated conditions
Variants in the CNGA3 gene are primarily associated with inherited retinal disorders. The most commonly linked condition is Achromatopsia (AR), a severe form of colour blindness characterised by a significant reduction or complete absence of colour perception. Individuals with Achromatopsia also typically experience reduced visual acuity and increased sensitivity to light. The gene has also been implicated in other conditions such as progressive cone dystrophy.
UK clinical status
The CNGA3 gene is included in the UK's National Genomic Test Directory. It is listed on the 'Retinal disorders' panel (R32) with a green status, indicating that testing for this gene is considered of high clinical utility and is routinely offered within the NHS Genomic Medicine Service.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What is the role of the CNGA3 gene in vision?
The CNGA3 gene provides instructions for a key component of cyclic nucleotide-gated channels in the cone photoreceptor cells of the retina. These channels are crucial for converting light into electrical signals, enabling colour vision and vision in bright light.
Which conditions are associated with CNGA3 gene variants?
Variants in the CNGA3 gene are primarily associated with inherited retinal disorders, most notably Achromatopsia (AR), which causes severe colour blindness and other visual impairments. It has also been linked to progressive cone dystrophy.
How common are CNGA3 variants in individuals with achromatopsia?
Variants in the CNGA3 gene are recognised as a significant genetic cause of achromatopsia.
References
- Thiadens AA, Roosing S, Collin RW. Comprehensive analysis of the achromatopsia genes CNGA3 and CNGB3 in progressive cone dystrophy. Ophthalmology. 2010. PMID: 20079539
- Koeppen K, Reuter P, Ladewig T. Dissecting the pathogenic mechanisms of mutations in the pore region of the human cone photoreceptor cyclic nucleotide-gated channel. Human mutation. 2010. PMID: 20506298
- Reuter P, Koeppen K, Ladewig T. Mutations in CNGA3 impair trafficking or function of cone cyclic nucleotide-gated channels, resulting in achromatopsia. Human mutation. 2008. PMID: 18521937
- Patel KA, Bartoli KM, Fandino RA. Transmembrane S1 mutations in CNGA3 from achromatopsia 2 patients cause loss of function and impaired cellular trafficking of the cone CNG channel. Investigative ophthalmology & visual science. 2005. PMID: 15980212
- Tränkner D, Jägle H, Kohl S. Molecular basis of an inherited form of incomplete achromatopsia. The Journal of neuroscience : the official journal of the Society for Neuroscience. 2004. PMID: 14715947
- Johnson S, Michaelides M, Aligianis IA. Achromatopsia caused by novel mutations in both CNGA3 and CNGB3. Journal of medical genetics. 2004. PMID: 14757870
- Wissinger B, Gamer D, Jägle H. CNGA3 mutations in hereditary cone photoreceptor disorders. American journal of human genetics. 2001. PMID: 11536077
- Kohl S, Marx T, Giddings I. Total colourblindness is caused by mutations in the gene encoding the alpha-subunit of the cone photoreceptor cGMP-gated cation channel. Nature genetics. 1998. PMID: 9662398
- Adam MP, Bick S, Mirzaa GM. Achromatopsia. 1993. PMID: 20301591