A WW domain binding region in methyl-CpG-binding protein MeCP2: impact on Rett syndrome.

Rett syndrome is a dominant neurological disorder caused by loss-of-function mutations of methyl-CpG-binding protein 2 (MeCP2). MeCP2 is an abundant chromatin-associated protein that contains two well characterized domains. Through an N-terminal domain it recognizes methyl-CpGs and binds to nonmethylated DNA. A domain in the middle of the protein can act as a transcriptional repressor in transient transfection studies. The C-terminal region of the protein is equally essential for the function of MeCP2, as documented by recurrently found frameshift mutations. However, little is known about its functional role. Here we mapped a domain within MeCP2 capable of binding specifically to Group II WW domains of splicing factors formin-binding protein (FBP) 11 and HYPC. Binding was assessed by glutathione S-transferase pull-down assays and coimmunoprecipitation assays. The Group II WW domain binding region was localized from residue 325 to the C-terminus, with the interacting proline-rich sequence at its center. We then used comparison with genotype-phenotype studies in Rett syndrome patients to evaluate the relevance of Group II WW domain interactions of MeCP2 for pathogenesis. Truncation of the WW domain binding region by 48 C-terminal amino acids (to residue 438), causing Rett syndrome, resulted in reduced or loss of WW domain binding activity. Truncation to residue 400, representing a large group of frameshift mutations accounting for approx. 10% of Rett syndrome cases, abolished WW domain binding activity completely. On the other hand, two benign missense mutations did not affect binding. Furthermore, a short C-terminal truncation and an internal deletion, both causing mild to moderate mental retardation in males, were associated with weak or loss of WW domain binding activity.