Megalencephaly-polymicrogyria-polydactyly-hydrocephalus (MPPH) is a developmental brain disorder characterized by megalencephaly and bilateral perisylvian polymicrogyria due to defects in genes of the PI3K-AKT pathway.
The increasing use of high throughput sequencing methods, including high depth sequencing to more accurately detect and quantify mosaic mutations, has allowed us to identify the molecular etiologies of many MEG syndromes, including most notably the PI3K-AKT-MTOR related MEG disorders.
Patients with megalencephaly and a cortical malformation may be considered at risk of hypoglycaemia and monitored accordingly, at least until a PI3K-AKT-mTOR pathway variant has been excluded.
Mutations of genes within the phosphatidylinositol-3-kinase (PI3K)-AKT-MTOR pathway are well known causes of brain overgrowth (megalencephaly) as well as segmental cortical dysplasia (such as hemimegalencephaly, focal cortical dysplasia and polymicrogyria).
Yet, prevalence of the involvement of the PI3K-AKT-mTOR pathway in patients with megalencephaly remains to be elucidated, and molecular diagnosis is challenging.
This is best exemplified by the recent identification of mutations within components of the PI3K-AKT-mTOR pathway in hemimegalencephaly and megalencephaly syndromes, and the rapidly increased identification of mutations within the tubulin family in a broad range of cortical and non-cortical brain malformations.
These observations suggest that cyclin D2 stabilization, caused by CCND2 mutation or PI3K-AKT activation, is a unifying mechanism in PI3K-AKT-related megalencephaly syndromes.