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Mona D Shahbazian

Publications and source records attributed to Mona D Shahbazian.

6 recordsLinked to original sources
PubMed2005

Histone H2B ubiquitylation controls processive methylation but not monomethylation by Dot1 and Set1.

Methylation is a relatively stable histone modification, yet regulation of the transition between mono-, di-, and trimethylation of lysine (K) residues may control dynamic processes such as transcription and DNA repair. Identifying factors that regulate the ability of methyltransferases to perform successive rounds of methylation on the same lysine residue is important for understanding the functions of histone methylation. Previous reports have indicated that ubiquitylation of histone H2B K123 is required for methylation of lysines 4 and 79 of histone H3 by the methyltransferases Set1 and Dot1, respectively. In contrast, by using chromatin immunoprecipitation and mass spectrometry, we find that ubiquitylation of H2B-K123 is dispensable for monomethylation of H3-K4 and H3-K79 but is required for the transition from monomethylation to subsequent methylation states. Dot1 binding to chromatin occurs normally in the absence of H2B-K123 ubiquitylation, suggesting that ubiquitylation does not regulate enzyme recruitment but does regulate the processive activity of the histone methyltransferase.

Chromatin Immunoprecipitation↗
PubMed2005

Reduced proportion of Purkinje cells expressing paternally derived mutant Mecp2308 allele in female mouse cerebellum is not due to a skewed primary pattern of X-chromosome inactivation.

Rett syndrome (RTT) is an X-linked disorder caused by mutations in the methyl CpG binding protein 2 (MECP2) gene. The pattern of X-chromosome inactivation (XCI) is thought to play a role in phenotypic severity. In the present study, patterns of XCI were assessed by lacZ staining of embryos and adult brains of mice heterozygous for a X-linked Hmgcr-nls-lacZ transgene on a mutant mouse model of RTT. We found that there was no difference between the lacZ staining patterns in the brain of wild-type and heterozygous mutant embryos at embryonic day 9.5 (E9.5) suggesting that Mecp2 has no effect on the primary pattern of XCI. At 20 weeks of age, there was no significant difference between XCI patterns in the Purkinje cells in the cerebellum of heterozygous mutant and wild-type mice when the mutant allele was inherited from the mother. However, when the mutant allele was paternally inherited, a significant difference was detected. Thus, parental origin of the mutation may have a bearing on phenotype through XCI patterns. An estimation of the Purkinje cell precursor number based on XCI mosaicism revealed that, when the mutation was paternally inherited, the precursor number was less than that in the wild-type mice. Therefore, it is likely that the number of precursor cells allocated to the Purkinje cell lineage is affected by a paternally inherited mutation in Mecp2. We also observed that the pattern of XCI in cultured fibroblasts was significantly correlated with patterns in the Purkinje cells in mutant animals but not in wild-type mice.

Animals↗
PubMed2002

Infantile hypotonia as a presentation of Rett syndrome.

Rett syndrome (RTT) is classically defined by meeting certain clinical diagnostic criteria. It affects mostly females, and one possible pathogenic mechanism was considered to involve mitochondrial function. This was based on the finding of ultrastructural alterations in the mitochondria and decreased respiratory chain enzyme activity. However, the principal etiology of RTT has since been found to be mutations in the MECP2 gene, which is located on the X chromosome. Molecular analysis has allowed the phenotype of MECP2 mutations to be broadened beyond RTT to include girls who have mild mental retardation, autism, and an Angelman syndrome phenotype, as well as males with severe encephalopathy. We present a girl with a previously described mutation in the MECP2 gene whose phenotype is of atypical RTT. She presented with hypotonia and developmental delay in infancy without a clear period of normal development. As part of her evaluation for hypotonia, a muscle biopsy and respiratory chain enzyme analysis showed a slight decrease in respiratory chain enzyme activity consistent with previous reports. This report supports broadening the phenotype of patients who should be considered for MECP2 mutation analysis to include cases of developmental delay and hypotonia without evidence of an initial period of normal development. Furthermore, it supports the hypothesis of an underlying secondary defect in energy metabolism contributing to the pathogenesis of RTT.

Amino Acid Substitution↗
PubMed2002

Balanced X chromosome inactivation patterns in the Rett syndrome brain.

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.

Adolescent↗
PubMed2002

Insight into Rett syndrome: MeCP2 levels display tissue- and cell-specific differences and correlate with neuronal maturation.

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the methyl-CpG-binding protein 2 (MECP2) gene. Previous data have shown that MECP2 RNA is present in all mouse and human tissues tested, but the timing of expression and regional distribution have not been explored. We investigated the spatial and temporal distribution of the MeCP2 protein during mouse and human development. We found that in the adult mouse, MeCP2 is high in the brain, lung and spleen, lower in heart and kidney, and barely detectable in liver, stomach and small intestine. There was no obvious correlation between protein levels and RNA levels, suggesting that translation may be post-transcriptionally regulated by tissue-specific factors. The timing of MeCP2 expression in mouse and human correlated with the maturation of the central nervous system, with the ontogenetically older structures such as the spinal cord and brainstem becoming positive before newer structures such as the hippocampus and cerebral cortex. In the cortex, MeCP2 first appeared in the Cajal-Retzius cells, then in the neurons of the deeper, more mature cortical layers, and finally in the neurons of the more superficial layers. The MeCP2 protein was eventually present in a majority of neurons but was absent from glial cells. Our data suggest that MeCP2 may become abundant only once a neuron has reached a certain degree of maturity, and that this may explain some aspects of the RTT phenotype.

Animals↗