Somatic mutation in Primary Sjögren's Syndrome

Autoimmune diseases affect 5% of the population and cause a wide range of painful disabilities that affect quality of life and may lead to life-threatening complications and early death. The most common autoimmune diseases in order of prevalence are rheumatoid arthritis, primary Sjögren's syndrome (PSS) and SLE or 'lupus'. In all these conditions, the immune system becomes hyperactive and damages tissues of the body including the joints, skin, mucous membranes, glands and vital organs. In health, immune responses are closely regulated so that auto-immunity dose not occur. Although research has identified many new ways to suppress auto-immunity, the 'aetiology' or fundamental reason that the immune system escapes control, is completely unknown.

Through this research we are testing a new hypothesis that could explain why immune responses escape control in autoimmune disease. Throughout life, tissues of the body accumulate genetic mutations due to random errors in copying the DNA when cells divide. These mutations are known as 'somatic' mutations to indicate that they occur within an individual's lifetime and are not inherited from one generation to the next. Somatic mutations are the cause of cancer and may play an important role in many chronic and age-related diseases. Our research aims to test whether somatic mutations also cause autoimmunity. The idea is that genes that control immune responses may become defective so that immune cells escape from normal regulatory signals. There is evidence to support this idea from rare patients who are born with mutations that develop autoimmunity at a young age. We also know that patients with autoimmunity, especially those with PSS, are at risk of developing lymphoma and that some patients with lymphoma-related diseases get autoimmune problems. Our hypothesis suggests that somatic mutation is the factor linking both autoimmunity and lymphoma

The evidence we are looking for is whether patients with PSS have somatic DNA mutations in the immune cells that are causing their disease. Patients with PSS suffer dryness and severe irritation because their tear glands, salivary glands and other glands are attacked by immune cells. They are invited to join a UK Register and to allow their tissue biopsies to be used for research. We will use the latest tissue dissection methods to purify small populations of immune cells from the salivary glands of PSS patients and then ultra-sensitive DNA sequencing to detect somatic mutations in the DNA of the cells. If we find mutations, we will then determine which particular cell or cells of the immune system have the mutations and what problems the mutations cause in order to understand exactly how a mutation leads to the disease.

This research promises to change our understanding of autoimmunity. If the hypothesis is correct then we anticipate that DNA mutation testing will become a new way to diagnose autoimmune disease, to predict how serious the condition will be and to monitor treatment. It will also provide specific information that will enable drugs to be targeted to individual somatic mutations. Ultimately this will improve the treatment of patients with autoimmune disease, enhancing their health and economic welfare and reducing the burden of autoimmune disease upon society.

Primary Sjögren's Syndrome (PSS) is a common autoimmune disease associated with significant morbidity due to sicca syndrome and a risk of death from systemic involvement or progression to lymphoma. Although a complex array of immune phenomenology has been documented, including multiple genetic risk loci and a suite of biomarkers associated with diagnosis and progression, the primary aetiology of PSS and of human autoimmune disease in general, remains obscure.

Hypothesis: PSS is caused by somatic mutation of immune regulatory or checkpoint genes in lymphoid cells.

Many lines of evidence are consistent with this hypothesis: a) immune tolerance is dependent upon multiple regulatory genes that when mutated in germline DNA cause autoimmunity; b) lymphocytes have a high intrinsic rate of somatic mutation and somatic mutation is a common age-dependent process in healthy human tissues; c) somatic mutation causes diverse inflammatory manifestations in histiocytosis and autoimmune syndromes in lymphoproliferative disorders.

To test this hypothesis we bring together 1) the MRC-funded UK PSS Registry to provide tissue; 2) high resolution cell sorting of tissue lymphocytes; 3) state of the art sequencing and cancer genome biology. Aim 1 is to dissect the salivary glands of patients with PSS into multiple lymphoid populations. Aim 2 is use ultra-deep sequencing to identify rare somatic mutations and to map antigen receptor usage within specific lymphoid populations. Aim 3 is to link theses findings to existing models of PSS pathogenesis, to survey the range mutations causing PSS in a larger cohort and to investigate the presence of somatic mutation in peripheral blood.

The demonstration of an acquired genetic aetiology of autoimmunity is anticipated to have a major impact on medicine, forming the basis of new diagnostic and prognostic tests and identifying the definitive molecular targets for therapy.

In this research we propose to test a new hypothesis about the cause of Primary Sjögren's Syndrome (PSS) and other human autoimmune diseases. At the moment, autoimmune disease is thought of as 'idiopathic' or occurring without a specific cause. We propose that DNA mutations acquired during the lifetime of an individual ('somatic' mutations) could break down control systems in the immune system that then leads to autoimmunity. It is known that variation in the inherited genetic make up of individuals can influence the risk of developing autoimmunity but no previous research has tested whether acquired DNA mutations could be the cause of autoimmune disease. If correct, this idea will have a far-reaching impact on biology and medicine and improve the lives of patients through the provision of better tests and treatments for PSS.

Other research scientists in rheumatology, immunology and cancer medicine
The identification of acquired genetic defects in autoimmunity promises to change the way that autoimmune disease is conceived from an idiopathic disease, to one with a known cause. Through the research, a new level of detail will be revealed about how the immune system becomes deranged in autoimmunity. If the study is successful, it will also link autoimmunity with cancer in a completely new way, showing that both share a common genetic cause. Scientists will also benefit from the new methods of analysis developed during the research.

Patients with PSS and autoimmune disease, charities and policy makers
The proposal involves Prof Fai Ng who is Chief Investigator of the UK PSS Registry, a founder of the North East Sjögren's Syndrome Association and Trustee of the British Sjögren's Syndrome Association (BSSA). Through these and other connections, the research findings will be communicated with patients with PSS, charities and policy makers. An annual newsletter will be delivered in person and through email and websites to reach patients with PSS and their doctors.

Industry and commercial partners
One of the co-investigators, Dr Peter Campbell is a geneticist and Head of the cancer genetics and genomics programme at the Wellcome Trust Sanger Institute, a renowned genetics institute in Cambridge. He is also a founder of 14M Genomics, a spin-out company of the Sanger Institute created to translate discoveries in cancer genome medicine for the benefit of patients and health care services. This places him in an ideal position to bring the research to commercial partners so that they may help to develop new findings into tests and treatments for patients. An annual technical summary will be published for consultation with potential commercial partners

The NHS and wider public
The principal investigator, Professor M Collin, is also a clinician who sees patients with lymphoma and works closely with NHS diagnostic labs and a local charity dealing with blood cancer (Bright Red). He has previously brought genetic tests from the research lab to routine clinical use and will use this experience to bring the research into medicine and to the wider public. He will be assisted by Dr Chris Bacon who is a pathologist researching the cause of lymphoma and bringing new molecular tests to clinical use through an MRC-funded 'Molecular Pathology Node' at Newcastle. The investigators will meet monthly to plan and implement the translation of new research findings for the benefit of all these groups. A final video summary will be produced for dissemination of the findings via presentation and websites.