Functional studies of mutant nucleases in a genetic model of inflammatory and viral disease.

Aicardi Goutihres Syndrome (AGS) closely resembles viral illnesses such as congenital rubella and cytomegalovirus infections. However it is caused by a faulty gene instead of viral infection. We have recently identified several genes that cause this condition. These genes encode enzymes called nucleases.

Nucleases are found in all cells in the body and their job is to breakdown DNA and RNA into their component parts. The discovery of a relationship of these particular nucleases to the body's immune response was unexpected. It may be that these nucleases clean up naturally produced 'waste' DNA and RNA. Mutation in these genes could result in failure of this process, leading to the body mounting an immune reaction against itself, and so mimicking the effects of a viral infection. This immune response mechanism is also relevant to other common autoimmune diseases, such as lupus.

We propose to study these enzymes to understand their normal roles in cells and to establish what happens when these enzymes fail due to gene mutations in AGS patient cells. These studies will provide us with a more detailed understanding of autoimmune disease processes and how the human body responds to viral infections.

Innate immunity is important in human health as the first line of defence against pathogens. It is also implicated in the pathogenesis of systemic autoimmune disease. Nucleic acids are potent pathogen-associated molecular patterns (PAMPs) activating innate immunity, also important in the aetiology of autoimmune diseases.

Aicardi Goutihres syndrome (AGS) is an autodestructive disorder mimicking in utero viral infection of the brain. It is inherited as an autosomal recessive trait and manifests as severe neurological dysfunction in early childhood, with intracranial calcification and white matter destruction evident on neuroimaging. This disorder has an inflammatory basis, with raised levels of lymphocytes in the cerebrospinal fluid, as well as significant intrathecal and systemic production of Interferon alpha, a marker of innate immune activation. Patients can also exhibits systemic autoimmune-like phenomena, including vasculitic skin lesions, thrombocytopenia, haemolytic anemia, sterile pyrexias, hypergammaglobinemia and autoantibodies.

We have recently cloned four genes for AGS and demonstrated that mutations in these genes impair enzymatic function in their respective nucleases:- the Ribonuclease (RNase) H2 complex that degrades DNA:RNA hybrids, and TREX1 (DNase III) a 3'-5' DNA exonuclease. Notably, nucleases have also been implicated in activating innate immunity in mouse knockout models of lupus and rheumatoid arthritis; and DNase I mutations have been found in patients with systemic lupus erythematosis (SLE). The phenotypic and immunological overlap between SLE and AGS suggests common pathogenic mechanisms involving as yet unidentified nucleic acid substrates. In particular it is hypothesised that in AGS and SLE, nucleic acid by-products stimulate a chronic and inappropriate innate immune response.

I propose to use AGS, as a monogenic disorder with a defined molecular basis, to explore the cellular pathogenesis and molecular pathways implicated in nucleic acid triggered inflammatory responses, with the intention of providing new insights into the aetiology of systemic autoimmune disease and immune-mediated damage of host by inappropriate response to viral infection.