Houge-Janssens syndrome.

Houge-Janssens syndrome (HJS) is caused by protein phosphatase type 2A (PP2A) dysfunction. The core features are neurodevelopmental delay, especially concerning language, prolonged hypotonia, high risk of seizures, and behavior problems. PP2A oppose the activity of serine/threonine protein kinases, including growth promoting kinases of the PIK3CA/AKT/mTOR and RAS/MAPK pathways. Decreased PP2A activity can thus be growth promoting, as evidenced by recurrent pathogenic de novo missense variants in several PP2A subunits, some of which are associated with macrocephaly if congenital, or cancer if somatic. The current review gives an overview of both the clinical spectrum and known or potential pathogenic mechanisms in Houge-Janssens syndrome. For the latter, a basic insight in PP2A-mediated serine/threonine dephosphorylation is needed, although many fundamental questions regarding PP2A substrate specificity and activity determinants remain currently insufficiently resolved to provide a fully satisfactory molecular explanation of the effect of some mutations. So far, dominant pathogenic variants in at least two B subunits (PPP2R5D in HJS type 1 and PPP2R5C in HJS type 4), the major scaffolding subunit (PPP2R1A in HJS type 2) and the major catalytic subunit (PPP2CA in HJS type 3) can cause Houge-Janssens syndrome. The main aim is to explain why and how the different Houge-Janssens syndrome subtypes biochemically and clinically overlap, providing a framework for understanding new variants and new subtypes that will be found in the future. Hypothetically, small molecules that alleviate substrate blockade by affected B subunits or correct misfolding of affected A subunit, could represent treatment options, but these remain to be found.