CCND2

Megalencephaly-polymicrogyria-polydactyly-hydrocephalus syndrome

At least seven mutations in the CCND2 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 a brain abnormality called bilateral perisylvian polymicrogyria (BPP). Some affected individuals also have an extra finger or toe on one or more of their hands or feet (polydactyly).

Each of the known mutations changes a single protein building block (amino acid) in the cyclin D2 protein. These changes prevent the protein from being broken down (degraded) when it is no longer needed. The resulting buildup of cyclin D2 in cells triggers them to continue dividing when they otherwise would not have, leading to abnormal cell proliferation. In the brain, the increased number of cells leads to rapid and abnormal brain growth starting before birth. It is less clear how a buildup of cyclin D2 contributes to polydactyly, although the extra digits are probably related to abnormal cell proliferation in the developing hands and feet.