The BAH domain of ORC1 links H4K20me2 to DNA replication licensing and Meier–Gorlin syndrome

Abstract

Recognition of distinctly modified histones by specialized “effector” proteins constitutes a key mechanism for transducing molecular events at chromatin to biological outcomes1. Effector proteins influence DNA-templated processes, including transcription, DNA recombination, and DNA repair; however, no effector functions have yet been identified within the mammalian machinery that regulates DNA replication. Here we show that ORC1 – a component of ORC (origin of replication complex), which mediates pre-DNA replication licensing2 – contains a BAH (bromo adjacent homology) domain that specifically recognizes histone H4 dimethylated at lysine 20 (H4K20me2). Recognition of H4K20me2 is a property common to BAH domains present within diverse metazoan ORC1 proteins. Structural studies reveal that the specificity of the BAH domain for H4K20me2 is mediated by a dynamic aromatic dimethyllysine-binding cage and multiple intermolecular contacts involving the bound peptide. H4K20me2 is enriched at replication origins and abrogating ORC1 recognition of H4K20me2 in cells impairs ORC1 occupancy at origins, ORC chromatin loading, and cell-cycle progression. Mutation of the ORC1 BAH domain has been implicated in the etiology of Meier-Gorlin syndrome (MGS)3,4, a form of primordial dwarfism5, and ORC1 depletion in zebrafish results in an MGS-like phenotype4. We find that wild-type human ORC1, but not ORC1 H4K20me2-binding mutants, rescues the growth retardation of orc1 morphants. Moreover, zebrafish depleted of H4K20me2 have diminished body size, mirroring the phenotype of orc1 morphants. Together, our results identify the BAH domain as a novel methyllysine-binding module, thereby establishing the first direct link between histone methylation and the metazoan DNA replication machinery, and defining a pivotal etiologic role for the canonical H4K20me2 mark, via ORC1, in primordial dwarfism.

Comment l’approche génomique aide à comprendre le processus d’initiation de la réplication

“…Malheureusement il n'existe, pour l'heure, aucune cartographie de cette marque dans les cellules humaines qui puisse être comparée à la position des sites ORC. À noter que les mutations affectant le domaine BAH sont retrouvées chez des patients atteints du syndrome Meier-Gorlin, un syndrome rare qui se caractérise par une petite taille, de petites oreilles et des rotules sous-développées [33,34]. À n'en pas douter la cartographie des différentes formes méthylées de la lysine 20 de l'histone H4 sera une étape importante pour notre compréhension du mécanisme de sélection des origines de réplication.…”

Chromatin and DNA Replication

“…Mutations in the Orc1 BAH domain as well as other pre-RC components result in Meier-Gorlin syndrome, a rare primordial form of dwarfism (Bicknell et al 2011). Furthermore, the loss of Suv4-20h1/h2, the methyltransferase responsible for H4K20me2, results in similar developmental defects to Meier-Gorlin syndrome in zebrafish, suggesting a critical role for H4K20 dimethylation and the DNA replication program during normal development (Kuo et al 2012). However, it should be noted that H4K20me2 is the most abundant histone modification, accounting for .85% of histone H4 levels in the mouse (Schotta et al 2008).…”

“…The recent finding that the Orc1 bromo-adjacent homology (BAH) domain specifically binds dimethylated lysine 20 of histone H4 (Kuo et al 2012) suggests that H4K20me2 may be important for ORC recruitment to chromatin and defining replication origins. Mutations in the Orc1 BAH domain as well as other pre-RC components result in Meier-Gorlin syndrome, a rare primordial form of dwarfism (Bicknell et al 2011).…”

Nonreplicative functions of the origin recognition complex

“…However, in both organisms, ORC1 re-binding to chromatin is essential for the pre-RC assembly during G1 phase [7,8]. Moreover, in mammals the BAH (bromo-adjacent homology) domain of ORC1 recognizes the H4K20me2 modification [11], which is associated with replication. Besides that, human ORC2, 3, 4, and 5 form the core of the complex and are associated with each other throughout the entire cell cycle, while the smallest subunit, ORC6, interacts with the complex in a transient manner [12,13].…”