The Role of Membrane Attack Complex in Atypical Haemolytic Uraemic Syndrome

Lead Research Organisation: Newcastle University
Department Name: Institute of Cellular Medicine

Abstract

Atypical haemolytic uraemic syndrome (aHUS) is a kidney disorder which without treatment rapidly progresses to kidney failure. Until recently, renal transplantation has been contraindicated due to the high recurrence rate. Over the last 20 years research into aHUS has found that it results from over activation of the complement system due to loss of its integral regulation. The complement system is an ancient defence mechanism which protects the body from foreign cells and removes our own damaged ones. In certain conditions, people lack the ability to regulate the complement system, this in turn allows the complement system to attack healthy tissue which subsequently causes disease. In aHUS people have genetic mutations for key complement proteins, this results in a defective protein being produced, which is unable to function normally, this in turn then causes them to develop disease.

After research had shown the role of the complement system in aHUS, it led to the use of the drug 'Eculizumab' which has transformed patients' lives. This drugs works by 'switching' off the final pathway of the complement system, preventing the direct attack on healthy cells and preventing the creation of an inflammatory environment. However, the drug is very costly and as it silences one of the body's defences against foreign cells it can make people vulnerable to serious, life threatening infections.

In light of the above we have developed a comparative model of aHUS which will allow us to investigate the 'terminal pathway' of the complement system. My experiments will determine which protein of the terminal pathway is responsible for the development of disease. Once we have defined the proteins role in disease we will be able to inhibit its function by using a targeted drug therapy. This will confirm our understanding of the basic mechanisms of the disease whilst simultaneously testing a targeted therapy in a pre-clinical model. This will enable me to develop and deliver a targeted treatment for patients; reducing their risks of increased infection and improve health economics for all. My experiments will provide detailed data regarding the efficacy of the current front line anti-complement therapy in this model, and will heavily inform and dictate whether new complement therapeutics in pre-clinical evaluation can provide better protection for patients in the future. The terminal pathway of the complement system is critical to the development of many more diseases; stemming from sepsis to spinal cord injury. Understanding the roles of the terminal pathway proteins in a model of complement dysregulation (my comparative model of aHUS) will significantly contribute to an improved overall understanding; producing translatable benefits to a wide variety of disease and begin to provide an understanding of anti-complement therapies in these conditions.

Technical Summary

Atypical haemolytic uraemic syndrome (aHUS) encompasses the triad of; haemolytic anaemia, thrombocytopenia, and renal failure. Tremendous advances led by the Newcastle group has shown aHUS arises due to dysregulation of the alternative pathway of the complement system. The complement system is a complex series of proteins that results in an enzymatic cascade culminating in activation of the terminal pathway, producing C5a and Membrane attack complex (MAC). These two components whilst protecting self can injure cells and local tissues through their pro-inflammatory and lytic roles; thus are implicated in numerous diseases.

We know that aHUS is driven by complement dysregulation of the terminal pathway evidenced by the successful treatment of aHUS patients with a C5 convertase inhibitor. However, this treatment silences the terminal pathway in its entirety, predisposing vulnerable patients to infections and the cost of therapy prevents access to all. We do not currently know which protein plays the critical role in the development of aHUS. I have developed a novel murine model of aHUS engineered around a mutation in complement C3 from our aHUS patient cohort. My data clearly shows that my murine model absolutely mirrors aHUS in man, and ongoing experiments confirm the critical nature of C5. I therefore will determine the role of MAC and C5a in aHUS. The increased understanding of MAC and C5a in complement mediated renal disease will enable me to deliver targeted therapies for patients.
I seek to investigate the roles of C5a and MAC in aHUS by rescuing the phenotype of the C3 gain of function D1115N mouse model of aHUS by blocking the terminal pathway:
a.) by breeding onto the C5-/- strain
b.) through the use of a murine C5 inhibitor
c.) by breeding onto a C6 deficient mouse (no MAC).

I will assess the role C5a in the model:
a) through breeding the D1115N mouse with the C5AR1-/- & C5ALR-/- mice
b) therapeutic inhibition using the C5aR1 antagonist.

Planned Impact

Who will benefit?
Complement system malfunction is linked to clinical conditions affecting nearly every organ in the body. Hence my first outputs and second outputs - the full pathomechanistic characterisation of aHUS, allowing targeted testing of therapeutic agents against the current gold standard could impact indirectly on the work of physicians, and potentially on clinical practice and disease management in all renal units. Beyond this a C3 gain-of-function mouse model maybe of use in the study of any inflammatory condition. The ability to perform reliable, insightful pre-clinical tests on complement-suppressing molecules, will provide significant steps towards an optimal complement therapeutic strategy for treating individuals with complement dysregulation. This will expedite the development of anti- complement therapeutics and will directly benefit pharmaceutical companies and of course patients and their relatives and carers.

How will they benefit?

1. A deeper understanding of the relationship between the complement system and disease; aHUS routinely progress to end-stage renal failure. While Eculizumab can treat aHUS in most cases, it is extremely expensive, and carries a significant immunosuppressive burden for patients.

2. The generation of the mouse models in this proposal will allow pre-clinical screening of potentially therapeutic agents designed to correct defects in the complement system in man. This will benefit a range of big and small pharma companies currently competing to get anti-complement therapies to market.

3. Improving quality of life for patients inflicted by deregulated complement activation is a major strategic goal. Complement maintains homeostasis as well as fighting infections. Complement malfunctions increase with age, for example the largest single risk factor of susceptibility to age-related macular degeneration are linked to polymorphisms in complement proteins that control C3 activation; there is the link between Alzhemier's disease and variants in the genes for clusterin and complement receptor-type 1. The mouse models we are developing are based on 'subtle' individual residue substitutions (rather than gene deletions) affecting specific biological functions of multifunctional proteins. These will allow the chronic effects of tiny genetic variations and long-term therapeutic interventions - for example elevating FH or FI levels or administering Mini-FH or TT30 - to be assessed over a lifespan

Publications

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Smith-Jackson K (2019) Hyperfunctional complement C3 promotes C5-dependent atypical hemolytic uremic syndrome in mice. in The Journal of clinical investigation