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Normal Function
The GNAT2 gene provides instructions for making one part (called the cone-specific alpha subunit) of a protein called transducin. This protein is found in light-detecting (photoreceptor) cells called cones, which are located in a specialized tissue at the back of the eye known as the retina. Cones provide vision in bright light (daylight vision), including color vision. Other photoreceptor cells, called rods, provide vision in low light (night vision).
Transducin plays an essential role in transmitting visual signals from photoreceptor cells in the retina to the brain through a process called phototransduction. Photoreceptors contain special pigments (called photopigments) that absorb light. The photopigments activate transducin, which triggers a series of chemical reactions within the cell. These reactions alter the cell's electrical charge, ultimately generating a signal that is interpreted by the brain as vision.
Health Conditions Related to Genetic Changes
Achromatopsia
At least 10 mutations in the GNAT2 gene have been found to cause the vision disorder achromatopsia. These mutations are a relatively uncommon cause of complete achromatopsia, a form of the disorder characterized by a total lack of color vision and other vision problems that are present from early infancy. GNAT2 gene mutations have also been identified in a few individuals with incomplete achromatopsia, a milder form of the disorder associated with limited color vision.
The GNAT2 gene mutations that underlie complete achromatopsia lead to an abnormally small, nonfunctional version of the cone-specific alpha subunit of transducin. Without this subunit, cones have no functional transducin, and
they are unable to carry out phototransduction. (The subunit produced from the GNAT2 gene is specific to cones, so rods are typically unaffected by this disorder.) A loss of cone function underlies the lack of color vision and other vision problems in people with complete achromatopsia.
At least one known GNAT2 gene mutation causes incomplete achromatopsia. The mutation, which is written as c.461+2G>A, affects the way the gene's instructions are pieced together to form the subunit protein. This mutation allows the production of some functional cone-specific alpha subunit, although the amount of the subunit is greatly reduced. As a result, a small amount of functional transducin is available to play its role in phototransduction, and the partially functioning cones can transmit some visual information to the brain.
More About This Health ConditionOther Names for This Gene
- ACHM4
- cone-type transducin alpha subunit
- GNAT2_HUMAN
- GNATC
- guanine nucleotide binding protein (G protein), alpha transducing activity polypeptide 2
- guanine nucleotide binding protein, alpha transducing activity polypeptide 2
- transducin alpha-2 chain
- transducin, cone-specific, alpha polypeptide
Additional Information & Resources
Tests Listed in the Genetic Testing Registry
Scientific Articles on PubMed
Catalog of Genes and Diseases from OMIM
References
- Aligianis IA, Forshew T, Johnson S, Michaelides M, Johnson CA, Trembath RC, Hunt DM, Moore AT, Maher ER. Mapping of a novel locus for achromatopsia (ACHM4) to 1p and identification of a germline mutation in the alpha subunit of cone transducin (GNAT2). J Med Genet. 2002 Sep;39(9):656-60. doi: 10.1136/jmg.39.9.656. Citation on PubMed or Free article on PubMed Central
- Kohl S, Baumann B, Rosenberg T, Kellner U, Lorenz B, Vadala M, Jacobson SG, Wissinger B. Mutations in the cone photoreceptor G-protein alpha-subunit gene GNAT2 in patients with achromatopsia. Am J Hum Genet. 2002 Aug;71(2):422-5. doi: 10.1086/341835. Epub 2002 Jun 20. Citation on PubMed or Free article on PubMed Central
- Kohl S, Jagle H, Wissinger B, Zobor D. Achromatopsia. 2004 Jun 24 [updated 2018 Sep 20]. In: Adam MP, Feldman J, Mirzaa GM, Pagon RA, Wallace SE, Amemiya A, editors. GeneReviews(R) [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2025. Available from http://www.ncbi.nlm.nih.gov/books/NBK1418/ Citation on PubMed
- Michaelides M, Aligianis IA, Holder GE, Simunovic M, Mollon JD, Maher ER, Hunt DM, Moore AT. Cone dystrophy phenotype associated with a frameshift mutation (M280fsX291) in the alpha-subunit of cone specific transducin (GNAT2). Br J Ophthalmol. 2003 Nov;87(11):1317-20. doi: 10.1136/bjo.87.11.1317. Citation on PubMed or Free article on PubMed Central
- Ouechtati F, Merdassi A, Bouyacoub Y, Largueche L, Derouiche K, Ouragini H, Nouira S, Tiab L, Baklouti K, Rebai A, Schorderet DF, Munier FL, Zografos L, Abdelhak S, El Matri L. Clinical and genetic investigation of a large Tunisian family with complete achromatopsia: identification of a new nonsense mutation in GNAT2 gene. J Hum Genet. 2011 Jan;56(1):22-8. doi: 10.1038/jhg.2010.128. Epub 2010 Nov 25. Citation on PubMed
- Pina AL, Baumert U, Loyer M, Koenekoop RK. A three base pair deletion encoding the amino acid (lysine-270) in the alpha-cone transducin gene. Mol Vis. 2004 Apr 8;10:265-71. Citation on PubMed
- Rosenberg T, Baumann B, Kohl S, Zrenner E, Jorgensen AL, Wissinger B. Variant phenotypes of incomplete achromatopsia in two cousins with GNAT2 gene mutations. Invest Ophthalmol Vis Sci. 2004 Dec;45(12):4256-62. doi: 10.1167/iovs.04-0317. Citation on PubMed
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