"PPP2R5D encodes B56δ, one of the PP2A regulatory B subunits. PP2A-B56 is a key cellular serine-threonine phosphatase with roles in multiple pathways including negative regulation of the PI3K/AKT growth regulatory cascade … " (Loveday et al.)
Lets try to simplify things...
Let’s think of our brain as a chocolate factory and our brain cells as chocolates being produced by the factory
PPP2R5D (a gene) is part of a larger system directing a cells growth cycle. Think of it as an on/off switch regulating when a cell grows, divides, lives or dies. Cellular death is a necessary and important part of our biology and ensures our bodies develop and regulate themselves correctly. Due to the misspelling in the PPP2R5D gene, the on/off switch is partially broken resulting in our cells not knowing when to turn themselves off. The cells now continue to multiply and grow resulting in more than we need. This tends to be more true in the brain than any other region. While having more brain cells may seem harmless or even positive, it interferes with regular neural activity causing the general delays we see in our loved ones. We don't yet quite understand why having more or larger brain cells interferes with normal brain function but it's only a matter of time before we do. To get a little more technical, this biological on/off switch works by different enzymes (proteins that speed things up) exchanging Phosphate molecules. In our case, you can think of adding a Phosphate molecule (phosphorylation) as turning the growth on and removing a phosphate molecule as turning it off (dephosphorylation). PPP2R5D is primarily responsible for turning growth off and hence a misspelling in the gene results in the creation of a protein that doesn't quite fit our cell's on/off switch. Though this gene is also involved in a number of other cellular functions besides growth, its impact on the cell's growth cycle is thought to be the biggest contributor to development delays.
While individuals diagnosed with the variation seem to span across multiple abilities levels. These traits seem to be common:
1- Macrocephaly or head overgrowth
2- Muscle tone anomalies including Hypotonia
3- Global development delays affecting gross and fine motors as well as speech development
B56δ-related protein phosphatase 2A dysfunction identified in patients with intellectual disability