08 - 05 - 2006: Inhibitors of Differentiation (ID1, ID2, ID3 and ID4) genes are neuronal targets of MeCP2 that are elevated in Rett syndrome

Abstract
Rett syndrome (RTT) is an X-linked dominant neurodevelopmental disorder caused by mutations in MECP2, encoding methyl-CpG binding protein 2. MeCP2 is a transcriptional repressor elevated in mature neurons and is predicted to be required for neuronal maturation by regulating multiple target genes. Identifying primary gene targets in either Mecp2-deficient mice or human RTT brain has proven to be difficult, perhaps because of the transient requirement for MeCP2 during neuronal maturation. In order to experimentally control the timing of MeCP2 expression and deficiency during neuronal maturation, human SH-SY5Y cells undergoing mature neuronal differentiation were transfected with methylated MeCP2 oligonucleotide decoy to disrupt the binding of MeCP2 to endogenous targets. Genome-wide expression microarray analysis identified all four known members of the inhibitors of differentiation or inhibitors of DNA binding (ID1, ID2, ID3 and ID4) subfamily of helix-loop-helix (HLH) genes as novel neuronal targets of MeCP2. Chromatin immunoprecipitation analysis confirmed binding of MeCP2 near or within the promoters of ID1, ID2 and ID3, and quantitative RT-PCR confirmed increased expression of all four Id genes in Mecp2-deficient mouse brain. All four ID proteins were significantly increased in Mecp2-deficient mouse and human RTT brain using immunofluorescence and laser scanning cytometric analyses. Because of their involvement in cell differentiation and neural development, ID genes are ideal primary targets for MeCP2 regulation of neuronal maturation that may explain the molecular pathogenesis of RTT.

Lay Summary
We know MeCP2 is mutated in Rett syndrome. But what effect does this MeCP2 mutation have on other genes? Various approaches have been used to identify so-called "gene targets" of MeCP2. However, identifying these gene targets in either the Rett mouse model or the Rett brain tissue has proven to be difficult. Encouragingly, each of the previous attempts to identify such gene targets has found several whose levels are altered. However, none of these individual gene targets can completely explain the mechanisms of the neuronal developmental defects observed in Rett girls. In this paper, the authors use a novel approach to identify further potential gene targets. They narrowed their results to four genes that are from the same family; namely, 'inhibitors of differentiation' or 'inhibitors of DNA binding' (ID) genes, called ID1, ID2, ID3 and ID4. As several studies have shown that MeCP2 might play an important role during brain cell development and maturation, these are attractive gene targets, as the functions of the ID family of genes fit well with the mechanism of disease in Rett syndrome. The authors suggest that by through the further unveiling of the normal role of these ID proteins during brain cell maturation, ID genes might become therapeutic targets to reverse the arrest in brain cell maturation observed in Rett syndrome. This is a very encouraging result, and potentially interesting pathway of research.