Cellular drivers of type I interferon-mediated neuropathology

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The immune system has evolved primarily to protect us against infection. Yet if not properly controlled, the potent responses that it unleashes can lead to inadvertent damage to normal cells and tissues. An example is a family of immune substances known as 'interferons'. Interferons are chemical messengers released by virally exposed cells, signalling neighbouring cells to adopt a state that blocks viral infection and spread. Interferons also regulate other immune functions and slow the growth of cancer cells. As such, they have been developed clinically to treat a range of diseases. Whilst these properties are beneficial, exposure to too much interferon, for too long, can be harmful. The brain appears especially vulnerable to the damaging effects of interferon. This has been suggested by the careful study of patients with a group of severe genetic diseases known as 'type I interferonopathies', which are largely untreatable, leading to neurological illness and disability and premature death. The type I interferonopathies are associated with rare 'spelling errors' in DNA (mutations) that result in the interferon system being turned on inappropriately. We have recently discovered a new form of type I interferonopathy associated with mutation of a gene called STAT2 that plays a critical role in signalling immediately downstream of the type I interferon receptor. The mutated STAT2 can't deliver negative feedback on the receptor that activates it, so generates an abnormally strong and long inflammatory response to interferon. This new disease provides evidence to support the notion that interferon is neurotoxic but also raises questions about precisely how interferon leads to brain damage. Answering these questions should help us to identify better treatments for patients with this devastating diagnosis. It will also likely help us to understand how interferon might contribute to various more common disease states, such as dementia or stroke, in which it has been implicated. In unpublished studies we have developed a rodent model of STAT2-associated type I interferonopathy. The animals show similar clinical features to patients. This is an important advance, providing an experimental model in which to investigate how interferon leads to the abnormalities seen in patients and how to intervene with treatment(s). In this model, we have identified problems affecting a range of brain cell types, suggesting that interferon causes disease through complex actions in multiple cell types. In this project we will use cutting-edge methods to express the abnormal STAT2 gene in different brain cell types and then establish the consequences for brain function using clinical tests and detailed study of tissues. We will investigate the underlying molecular processes using techniques to measure gene expression of individual cells. This will help us to develop theories about the way that interferons operate to produce disease. To test these, we will make use of human stem cells that we can turn into different brain cell types in a dish. We have produced stem cells bearing the mutant STAT2 gene for use in these experiments. By comparing the behaviour of cell types bearing the mutant STAT2 with identical cells lacking the mutant STAT2, we will learn ways in which interferons perturb the normal function of different cell types in the brain. Together, these results will explain how interferons lead to brain damage and give insight into the generation of type I interferonopathy. It is possible that this occurs through the direct action of interferons on these brain cell types, or toxic effects may be indirect. This information is relevant to the development of treatments, especially as we move toward the next generation of treatments for genetic diseases such as gene therapy or cell transplantation. Our findings may even help to inform the safer clinical use of interferons.