Balanced X chromosome inactivation patterns in the Rett syndrome brain

Abstract

In Rett syndrome (RTT), an X-linked disorder essentially limited to females, neurological development goes awry. Causing this disarray in neuronal function is a mutated form of a protein known as methyl-CpG-binding protein 2 (MeCP2). Because the MECP2 gene is subject to X chromosome inactivation (XCI) in females, a number of studies have addressed whether the percentage of cells inactivating the normal vs. mutant chromosome in heterozygous females influences the phenotypic outcome of MECP2 mutations. Because most of these studies measured XCI in peripheral blood, however, interpretation of the results requires the assumption that XCI patterns in blood are representative of those in the brain, the primarily affected tissue. Here, we have analyzed the MECP2 sequence and XCI status in 13 brains of RTT patients. Mutations were identified in nine of the cases, with eight of these representing C to T transitions at CpG dinucleotides, and one being a novel frameshift mutation (765delA). Patterns of XCI were balanced in 10 of 10 cases for which the assay was informative. As previous studies have shown that a majority of RTT patients have balanced XCI patterns in peripheral blood, our results suggest that the pattern in blood is an accurate indicator of XCI patterns in the brain for a majority of cases, but there are some notable exceptions that this study may help explain. Given the correlation between balanced XCI and classic RTT, these results suggest that a certain percentage of neurons expressing the mutant MECP2 gene may be required for RTT to become manifest.

Increased skewing of X chromosome inactivation in Rett syndrome patients and their mothers

“…A few studies, employing small patient sample sizes, have shown random X inactivation in brain tissue. 22,23 This is in contrast with results using Mecp2 knockout mouse models, where an association between skewed X inactivation in brain cells and phenotypic severity was found. 24,25 The difference in reported frequencies of skewed X inactivation may partly be attributed to different definitions of the 'skewed' state (ranging from 465 to 485%) and different patient groups.…”

Methyl CpG-binding Protein Isoform MeCP2_e2 Is Dispensable for Rett Syndrome Phenotypes but Essential for Embryo Viability and Placenta Development

“…A num- ber of studies have addressed the contribution of XCI to the pathogenesis of MeCP2 mutations (38,39). It is suggested that XCI patterns may partly explain phenotypic variability in human RTT with MeCP2 mutations (38) and in mouse RTT models (39). Our findings indicate that this is not the full picture and that paternal X chromosome inactivation in the extraembryonic lineage also contributes to the deleterious consequences of MeCP2 mutations and, most likely, other X-linked gene mutations.…”

“…A num- ber of studies have addressed the contribution of XCI to the pathogenesis of MeCP2 mutations (38,39). It is suggested that XCI patterns may partly explain phenotypic variability in human RTT with MeCP2 mutations (38) and in mouse RTT models (39).…”

Mutations and polymorphisms in the human methyl CpG-binding protein MECP2

“…Several reports agree on a balanced XCI of peripheral leukocytes DNA as well as DNA from the brains of deceased patients in the majority of RTT cases so that the theoretically plausible hypothesis that the XCI is one of the major phenotype influencing factors cannot be definitively confirmed on an experimental level [Anvret and Wahlstrom, 1994;Auranen et al, 2001;Hoffbuhr et al, 2001;Nielsen et al, 2001;Shahbazian et al, 2002]. Besides, almost all investigations of the XCI have been performed on DNA from peripheral tissues, mostly leukocytes.…”