Molecular and functional characterization of the newly identified interaction between the Rett syndrome-associated factor MeCP2 and the pro-apoptotic factor HIPK2.

RTT Syndrome is an X-linked neuronal progressive disorder causing mental retardation mostly in females. MeCP2, a methyl-DNA-binding protein that represses transcription by modulating chromatin structure, is mutated in about 80% of classical RTT-patients. The rather large number of patients with a normal MeCP2 gene and the lack of an apparent genotype-phenotype correlation, suggests that other genes may be involved in the onset of this neurological disorder. Based on these considerations, we reasoned that the identification of novel MeCP2 interacting factors might help to refine the comprehension of the molecular mechanism(s) involved in RTT syndrome. Thus, we performed a yeast two hybrid screening using MeCP2 as a bait. We identified the homeodomaininteracting kinase 2 (HIPK2) as a new MeCP2 interactor. HIPK2 belongs to a family of nuclear serine/threonine kinases originally cloned due to their ability to interact with homeodomain transcription factors. HIPK2, the most studied of this family, binds also to a series of proteins involved in the regulation of cell survival and proliferation, including the oncosoppressor p53. In the present study, we have confirmed the physical interaction between MeCP2 and HIPK2 and, by in vitro phosphorylation assays, we have demonstrated that HIPK2 is able to phosphorylate MeCP2. Furthermore, we have mapped the specific MeCP2 site targeted by HIPK2. At the functional level, we have demonstrated that the ectopic expression of MeCP2 leads to apoptosis in different cell lines and that this effect is increased by the presence of HIPK2. Importantly, the serine residue on MeCP2 which is phosphorylated by HIPK2 in vitro, is required for the cooperation between the two proteins in inducing apoptosis, strongly suggesting that this residue is also an in vivo target of HIPK2. In conclusion our findings indicate that MeCP2 belongs to the HIPK2 apoptotic pathway and clearly suggest a novel role for MeCP2 in programmed cell death that might provide novel valuable insights into Rett syndrome.