-GlcNAc is a regulatory post-translational modification of nucleocytoplasmic proteins that has been implicated in multiple biological processes, including transcription. In humans, single genes encode enzymes for its attachment (-GlcNAc transferase (OGT)) and removal (-GlcNAcase (OGA)). An X-chromosome exome screen identified a missense mutation, which encodes an amino acid in the tetratricopeptide repeat, in (759G>T (p.L254F)) that segregates with X-linked intellectual disability (XLID) in an affected family. A decrease in steady-state OGT protein levels was observed in isolated lymphoblastoid cell lines from affected individuals, consistent with molecular modeling experiments. Recombinant expression of L254F-OGT demonstrated that the enzyme is active as both a glycosyltransferase and an HCF-1 protease. Despite the reduction in OGT levels seen in the L254F-OGT individual cells, we observed that steady-state global-GlcNAc levels remained grossly unaltered. Surprisingly, lymphoblastoids from affected individuals displayed a marked decrease in steady-state OGA protein and mRNA levels. We observed an enrichment of the OGT-containing transcriptional repressor complex mSin3A-HDAC1 at the proximal promoter region of and correspondingly decreased promoter activity in affected cells. Global transcriptome analysis of L254F-OGT lymphoblastoids compared with controls revealed a small subset of genes that are differentially expressed. Thus, we have begun to unravel the molecular consequences of the 759G>T (p.L254F) mutation in that uncovered a compensation mechanism, albeit imperfect, given the phenotype of affected individuals, to maintain steady-state-GlcNAc levels. Thus, a single amino acid substitution in the regulatory domain (the tetratricopeptide repeat domain) of OGT, which catalyzes the -GlcNAc post-translational modification of nuclear and cytosolic proteins, appears causal for XLID.

Little is known about the roles of histone tails in modulating nucleosomal DNA accessibility and its recognition by other macromolecules. Here we generate extensive atomic level conformational ensembles of histone tails in the context of the full nucleosome, totaling 65 microseconds of molecular dynamics simulations. We observe rapid conformational transitions between tail bound and unbound states, and characterize kinetic and thermodynamic properties of histone tail-DNA interactions. Different histone types exhibit distinct binding modes to specific DNA regions. Using a comprehensive set of experimental nucleosome complexes, we find that the majority of them target mutually exclusive regions with histone tails on nucleosomal/linker DNA around the super-helical locations ± 1, ± 2, and ± 7, and histone tails H3 and H4 contribute most to this process. These findings are explained within competitive binding and tail displacement models. Finally, we demonstrate the crosstalk between different histone tail post-translational modifications and mutations; those which change charge, suppress tail-DNA interactions and enhance histone tail dynamics and DNA accessibility.

Supplementary data are available at Bioinformatics online.

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