Characterisation of ATRX, a chromatin remodelling protein

Despite the revolution in genetics in the last decade and in particular the sequencing of the human genome, the regulation of gene expression largely remains a mystery. Every cell of the body contains a full complement of genes but only expresses a small repertoire required for that tissues function, such as the oxygen carrying protein, haemoglobin, in red blood cells. A complex hierarchy of regulation is emerging involving tissue-specific signals, chemical modifications to the DNA molecule and the proteins about which DNA is wrapped, enzymes that package and unpackage DNA and different cellular compartments that either favour or repress gene expression. Human genetic diseases are now being identified in which the components of this complex apparatus are disrupted. In one such condition, ATR-X syndrome, the affected children have profound learning difficulties, a characteristic facial appearance, abnormalities of genital development and a form of anaemia. It arises because of mutations in a protein involved in the regulation of gene expression. The diverse problems probably reflect the many different genes whose expression is perturbed. The overall aim of this programme is to determine the role of such proteins in the regulation of gene expression and their involvement in human disease.

In the next phase of our research we wish to build on the success of the last five years to understand the role of ATRX in the gene expression and in particular how mutations in ATRX lead to down regulation of ? globin gene expression. The long term aim of this work is to see if this pathway is open to manipulation and whether it might be exploited therapeutically to modify the severity of ? thalassaemia by reducing the pathological accumulation of ? globin chains. It is apparent from our recent work that ATRX binds to tandem repeats throughout the genome. Many of these repeats are predicted to fold into non-B forms of DNA including G4 structures and we have shown that ATRX can bind G4 in vitro. When ATRX is mutated the expression of genes close to these ATRX targets is perturbed and the magnitude of the effect is related to the size of the repeat. It seems likely that ATRX modulates the effects of these repeats on gene expression. The current conundrum is how this might occur. Preliminary evidence suggests that ATRX plays a role in DNA replication and our working hypothesis is that these repeats and the structures they form inhibit the progress of the replication fork, a DNA damage response is triggered and as a consequence of this the expression of nearby genes is perturbed. A central focus of this programme will be to test this hypothesis and understand the mechanism underlying the abnormality in gene expression seen in individuals with ATR-X syndrome.