The AKT3 gene provides instructions for making a protein that is most active in the nervous system. The AKT3 protein is a key regulator of a chemical signaling pathway called the PI3K-AKT-mTOR pathway. This signaling influences many critical cell functions, including the creation (synthesis) of new proteins, cell growth and division (proliferation), and the survival of cells. The PI3K-AKT-mTOR pathway is essential for the normal development of many parts of the body, including the brain. Studies suggest that the AKT3 protein plays a critical role in determining brain size.
Health Conditions Related to Genetic Changes
Several mutations in the AKT3 gene have been found to cause megalencephaly-polymicrogyria-polydactyly-hydrocephalus (MPPH) syndrome. This rare condition affects the development of the brain, causing an unusually large brain and head size (megalencephaly) and other abnormalities of the brain's structure.
Each of the known mutations changes a single protein building block (amino acid) in the AKT3 protein. These changes are described as "gain-of-function" because they increase the activity of the protein. This enhanced activity increases chemical signaling through the PI3K-AKT-mTOR pathway, which causes excessive cell growth and division. The increased number of cells leads to rapid and abnormal brain growth starting before birth.More About This Health Condition
Changes involving the AKT3 gene are also involved in other disorders of brain growth. Megalencephaly without the other features of MPPH syndrome (described above) has been associated with gain-of-function AKT3 gene mutations or extra copies (duplication) of the region of chromosome 1 containing the AKT3 gene. These genetic changes increase the amount or activity of the AKT3 protein, which enhances chemical signaling through the PI3K-AKT-mTOR pathway and causes excessive cell growth and division, particularly in the brain.
Other genetic changes involving the AKT3 gene are associated with an unusually small brain and head size (microcephaly). These changes include a deletion of the AKT3 gene or a loss of the region of chromosome 1 containing the AKT3 gene. The resulting reduction in AKT3 protein activity likely decreases signaling through the PI3K-AKT-mTOR pathway and restricts cell growth and division in the developing brain.
Changes involving the AKT3 gene can also cause a brain malformation called isolated hemimegalencephaly. This brain abnormality is an enlargement of one of the two major halves (hemispheres) of the cerebrum, which is the large part of the brain that controls most voluntary activity, language, sensory perception, learning, and memory. Like the genetic changes that cause MPPH syndrome and megalencephaly (described above), the AKT3 gene changes that result in isolated hemimegalencephaly are gain-of-function, ultimately leading to increased cell growth and division in the developing brain. However, unlike the mutations that cause those other abnormalities of brain growth, the genetic changes related to isolated hemimegalencephaly are somatic, meaning they occur at some point during embryonic development. As brain cells continue to grow and divide, some of these cells will have the genetic change, and others will not (a situation known as mosaicism). The mosaic nature of these genetic changes helps explain why they cause overgrowth in only one of the two cerebral hemispheres.
Other Names for This Gene
- PKB gamma
- RAC-gamma serine/threonine protein kinase
- v-akt murine thymoma viral oncogene homolog 3 (protein kinase B, gamma)
Additional Information & Resources
Tests Listed in the Genetic Testing Registry
Scientific Articles on PubMed
Catalog of Genes and Diseases from OMIM
- Ballif BC, Rosenfeld JA, Traylor R, Theisen A, Bader PI, Ladda RL, Sell SL, Steinraths M, Surti U, McGuire M, Williams S, Farrell SA, Filiano J, Schnur RE, Coffey LB, Tervo RC, Stroud T, Marble M, Netzloff M, Hanson K, Aylsworth AS, Bamforth JS, Babu D, Niyazov DM, Ravnan JB, Schultz RA, Lamb AN, Torchia BS, Bejjani BA, Shaffer LG. High-resolution array CGH defines critical regions and candidate genes for microcephaly, abnormalities of the corpus callosum, and seizure phenotypes in patients with microdeletions of 1q43q44. Hum Genet. 2012 Jan;131(1):145-56. doi: 10.1007/s00439-011-1073-y. Epub 2011 Jul 29. Citation on PubMed
- Cohen MM Jr. The AKT genes and their roles in various disorders. Am J Med Genet A. 2013 Dec;161A(12):2931-7. doi: 10.1002/ajmg.a.36101. Epub 2013 Sep 3. Citation on PubMed
- Lee JH, Huynh M, Silhavy JL, Kim S, Dixon-Salazar T, Heiberg A, Scott E, Bafna V, Hill KJ, Collazo A, Funari V, Russ C, Gabriel SB, Mathern GW, Gleeson JG. De novo somatic mutations in components of the PI3K-AKT3-mTOR pathway cause hemimegalencephaly. Nat Genet. 2012 Jun 24;44(8):941-5. doi: 10.1038/ng.2329. Citation on PubMed or Free article on PubMed Central
- Mirzaa G. MPPH Syndrome. 2016 Nov 17 [updated 2022 Jul 28]. In: Adam MP, Mirzaa GM, Pagon RA, Wallace SE, Bean LJH, Gripp KW, Amemiya A, editors. GeneReviews(R) [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2023. Available from http://www.ncbi.nlm.nih.gov/books/NBK396098/ Citation on PubMed
- Mirzaa GM, Riviere JB, Dobyns WB. Megalencephaly syndromes and activating mutations in the PI3K-AKT pathway: MPPH and MCAP. Am J Med Genet C Semin Med Genet. 2013 May;163C(2):122-30. doi: 10.1002/ajmg.c.31361. Epub 2013 Apr 16. Citation on PubMed
- Nakamura K, Kato M, Tohyama J, Shiohama T, Hayasaka K, Nishiyama K, Kodera H, Nakashima M, Tsurusaki Y, Miyake N, Matsumoto N, Saitsu H. AKT3 and PIK3R2 mutations in two patients with megalencephaly-related syndromes: MCAP and MPPH. Clin Genet. 2014 Apr;85(4):396-8. doi: 10.1111/cge.12188. Epub 2013 Jun 10. No abstract available. Citation on PubMed
- Nellist M, Schot R, Hoogeveen-Westerveld M, Neuteboom RF, van der Louw EJ, Lequin MH, Bindels-de Heus K, Sibbles BJ, de Coo R, Brooks A, Mancini GM. Germline activating AKT3 mutation associated with megalencephaly, polymicrogyria, epilepsy and hypoglycemia. Mol Genet Metab. 2015 Mar;114(3):467-73. doi: 10.1016/j.ymgme.2014.11.018. Epub 2014 Dec 5. Citation on PubMed
- Poduri A, Evrony GD, Cai X, Elhosary PC, Beroukhim R, Lehtinen MK, Hills LB, Heinzen EL, Hill A, Hill RS, Barry BJ, Bourgeois BF, Riviello JJ, Barkovich AJ, Black PM, Ligon KL, Walsh CA. Somatic activation of AKT3 causes hemispheric developmental brain malformations. Neuron. 2012 Apr 12;74(1):41-8. doi: 10.1016/j.neuron.2012.03.010. Citation on PubMed or Free article on PubMed Central
- Riviere JB, Mirzaa GM, O'Roak BJ, Beddaoui M, Alcantara D, Conway RL, St-Onge J, Schwartzentruber JA, Gripp KW, Nikkel SM, Worthylake T, Sullivan CT, Ward TR, Butler HE, Kramer NA, Albrecht B, Armour CM, Armstrong L, Caluseriu O, Cytrynbaum C, Drolet BA, Innes AM, Lauzon JL, Lin AE, Mancini GM, Meschino WS, Reggin JD, Saggar AK, Lerman-Sagie T, Uyanik G, Weksberg R, Zirn B, Beaulieu CL; Finding of Rare Disease Genes (FORGE) Canada Consortium; Majewski J, Bulman DE, O'Driscoll M, Shendure J, Graham JM Jr, Boycott KM, Dobyns WB. De novo germline and postzygotic mutations in AKT3, PIK3R2 and PIK3CA cause a spectrum of related megalencephaly syndromes. Nat Genet. 2012 Jun 24;44(8):934-40. doi: 10.1038/ng.2331. Citation on PubMed or Free article on PubMed Central
- Wang D, Zeesman S, Tarnopolsky MA, Nowaczyk MJ. Duplication of AKT3 as a cause of macrocephaly in duplication 1q43q44. Am J Med Genet A. 2013 Aug;161A(8):2016-9. doi: 10.1002/ajmg.a.35999. Epub 2013 Jun 21. Citation on PubMed