Suppression of the alternative lengthening of telomere pathway by the chromatin remodelling factor ATRX.

Fifteen per cent of cancers maintain telomere length independently of telomerase by the homologous recombination (HR)-associated alternative lengthening of telomeres (ALT) pathway. A unifying feature of these tumours are mutations in ATRX. Here we show that expression of ectopic ATRX triggers a suppression of the pathway and telomere shortening. Importantly ATRX-mediated ALT suppression is dependent on the histone chaperone DAXX. Re-expression of ATRX is associated with a reduction in replication fork stalling, a known trigger for HR and loss of MRN from telomeres. A G-quadruplex stabilizer partially reverses the effect of ATRX, inferring ATRX may normally facilitate replication through these sequences that, if they persist, promote ALT. We propose that defective telomere chromatinization through loss of ATRX promotes the persistence of aberrant DNA secondary structures, which in turn present a barrier to DNA replication, leading to replication fork stalling, collapse, HR and subsequent recombination-mediated telomere synthesis in ALT cancers.

ATRX dysfunction induces replication defects in primary mouse cells.

The chromatin remodeling protein ATRX, which targets tandem repetitive DNA, has been shown to be required for expression of the alpha globin genes, for proliferation of a variety of cellular progenitors, for chromosome congression and for the maintenance of telomeres. Mutations in ATRX have recently been identified in tumours which maintain their telomeres by a telomerase independent pathway involving homologous recombination thought to be triggered by DNA damage. It is as yet unknown whether there is a central underlying mechanism associated with ATRX dysfunction which can explain the numerous cellular phenomena observed. There is, however, growing evidence for its role in the replication of various repetitive DNA templates which are thought to have a propensity to form secondary structures. Using a mouse knockout model we demonstrate that ATRX plays a direct role in facilitating DNA replication. Ablation of ATRX alone, although leading to a DNA damage response at telomeres, is not sufficient to trigger the alternative lengthening of telomere pathway in mouse embryonic stem cells.

Functional significance of mutations in the Snf2 domain of ATRX.

ATRX is a member of the Snf2 family of chromatin-remodelling proteins and is mutated in an X-linked mental retardation syndrome associated with alpha-thalassaemia (ATR-X syndrome). We have carried out an analysis of 21 disease-causing mutations within the Snf2 domain of ATRX by quantifying the expression of the ATRX protein and placing all missense mutations in their structural context by homology modelling. While demonstrating the importance of protein dosage to the development of ATR-X syndrome, we also identified three mutations which primarily affect function rather than protein structure. We show that all three of these mutant proteins are defective in translocating along DNA while one mutant, uniquely for a human disease-causing mutation, partially uncouples adenosine triphosphate (ATP) hydrolysis from DNA binding. Our results highlight important mechanistic aspects in the development of ATR-X syndrome and identify crucial functional residues within the Snf2 domain of ATRX. These findings are important for furthering our understanding of how ATP hydrolysis is harnessed as useful work in chromatin remodelling proteins and the wider family of nucleic acid translocating motors.

ATR-X syndrome protein targets tandem repeats and influences allele-specific expression in a size-dependent manner.

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.

Mutations in the chromatin-associated protein ATRX.

ATRX belongs to the SNF2 family of proteins, many of which have been demonstrated to have chromatin remodeling activity. Constitution mutations in the X-encoded gene give rise to alpha thalassemia mental retardation (ATR-X) syndrome and a variety of related conditions that are often associated with profound developmental delay, facial dysmorphism, genital abnormalities, and alpha thalassemia. Acquired mutations in ATRX are observed in the preleukemic condition alpha thalassemia myelodysplastic syndrome (ATMDS). Mutations in ATRX have been shown to perturb gene expression and DNA methylation. This is a comprehensive report of 127 mutations including 32 reported here for the first time. Missense mutations are shown to cluster in the two main functional domains. The truncating mutations appear to be "rescued" to some degree and so it appears likely that most if not all constitutional ATRX mutations are hypomorphs.