RESUMO
ATRX is an X-linked gene of the SWI/SNF family, mutations in which cause syndromal mental retardation and downregulation of α-globin expression. Here we show that ATRX binds to tandem repeat (TR) sequences in both telomeres and euchromatin. Genes associated with these TRs can be dysregulated when ATRX is mutated, and the change in expression is determined by the size of the TR, producing skewed allelic expression. This reveals the characteristics of the affected genes, explains the variable phenotypes seen with identical ATRX mutations, and illustrates a new mechanism underlying variable penetrance. Many of the TRs are G rich and predicted to form non-B DNA structures (including G-quadruplex) in vivo. We show that ATRX binds G-quadruplex structures in vitro, suggesting a mechanism by which ATRX may play a role in various nuclear processes and how this is perturbed when ATRX is mutated.
Assuntos
DNA Helicases/metabolismo , Proteínas Nucleares/metabolismo , Animais , Células Cultivadas , Imunoprecipitação da Cromatina , Cromossomos de Mamíferos/metabolismo , Ilhas de CpG , DNA Helicases/genética , DNA Ribossômico/metabolismo , Quadruplex G , Expressão Gênica , Estudo de Associação Genômica Ampla , Histonas/metabolismo , Humanos , Camundongos , Repetições Minissatélites , Mutação , Proteínas Nucleares/genética , Telômero/metabolismo , Proteína Nuclear Ligada ao XRESUMO
Accurate read-out of chromatin modifications is essential for eukaryotic life. Mutations in the gene encoding X-linked ATRX protein cause a mental-retardation syndrome, whereas wild-type ATRX protein targets pericentric and telomeric heterochromatin for deposition of the histone variant H3.3 by means of a largely unknown mechanism. Here we show that the ADD domain of ATRX, in which most syndrome-causing mutations occur, engages the N-terminal tail of histone H3 through two rigidly oriented binding pockets, one for unmodified Lys4 and the other for di- or trimethylated Lys9. In vivo experiments show this combinatorial readout is required for ATRX localization, with recruitment enhanced by a third interaction through heterochromatin protein-1 (HP1) that also recognizes trimethylated Lys9. The cooperation of ATRX ADD domain and HP1 in chromatin recruitment results in a tripartite interaction that may span neighboring nucleosomes and illustrates how the 'histone-code' is interpreted by a combination of multivalent effector-chromatin interactions.