Development of a Gal-free Calcification Resistant Porcine Pericardial Heart Valve.

Lead Research Organisation: University College London
Department Name: Institute of Cardiovascular Science

Abstract

There are valves in the heart which work to control the normal flow of blood. These valves may be damaged because of birth defects, old age, or after repeated infection with rheumatic fever. If left untreated this is a life threatening condition. There are about 500,000 surgeries to repair or replace heart valves each year. Replacement valves are either mechanical, made of plastic and metal, or biological, made of non-living tissue generally obtained from pigs or cows. Patients and doctors prefer biological heart valves (BHVs) because they work for a long time in older patients, the most frequent group, and do not require daily use of blood thinners which must be used with mechanical valves. In younger adults (30 - 50 years) and in children BHVs wear out rapidly, often within 5 years. These BHVs fail because they build up bone-like deposits of calcium which weaken the valve, leading to tears, or obstruct blood flow because they block the opening of the valve. Scientist and commercial valve companies have long sought to produce BHVs which do not calcify because these could be used in younger patients without the need for blood thinners. So far the calcification blocking treatments which have been developed have been able to greatly reduce valve calcification, when they are tested in animals, but have not been able stop calcification or broaden the use of BHV in younger adults. We have identified an immune driven inflammation which accelerates calcification of biological heart valve materials. This inflammation is unique to humans because a portion of their immune system reacts with a substance, called Gal, not made by people but commonly made in animals and present on the BHV. To block this immune inflammation we have genetically altered pigs so they no longer make Gal and have shown that this Gal-free tissue has a reduced rate of calcification. These Gal knockout pigs are healthy and normal. This project is designed to compare the mechanical function of BHVs made from normal pig tissue with the function of BHVs made from Gal-free tissue. Successfully performing these test which are required by British standards, is the first forward to making a new BHV which resists calcification and may be useful in younger patients

Technical Summary

Our final objective is to produce a new more durable bioprosthetic heart valve. Heart valve disease occurs due to birth defect, infection and age related degeneration. About 500,000 operations to fix or replace heart valves are performed annually worldwide. There are two types of replacement valves, mechanical heart valves (MHVs) which require lifetime anticoagulation and bioprosthetic heart valves (BHVs) made from biological tissues, typically human or porcine heart valve leaflets or animal pericardium. BHVs are preferred in older patients where they are more durable. Patients under 60 generally receive MHVs due to more rapid age-dependent BHV calcification and degeneration. Significant efforts have been devoted to develop calcification resistant BHVs which could be used in younger patients without chronic anticoagulation.
BHV calcification is attributed to mechanical stress, passive calcium binding to phospholipids, and immune injury. Current anti-calcification treatments rely on phospholipid depletion. These processes are effective in animal models but have not improved BHV durability or permitted their routine use in younger patients. We have identified an antibody-mediated inflammatory process involving the dominant xenogeneic antigen galactose alpha 1,3 galactose (Gal) binding to universally present human anti-Gal antibody which enhances calcification of fixed Gal-positive (WT), but not Gal-free bioprosthetic materials. Growing evidence indicates that this antibody mediated inflammatory mechanism may significantly contribute to age-dependent calcification. Genetically modified pigs which do not make the Gal antigen (GTKO) are a potential new source for BHVs which resists anti-Gal induced calcification and may be more durable in younger patients. This study is designed to produce prototype BHVs made from WT and GTKO porcine pericardium and determine by in vitro testing their equivalence. This is an essential first step in commercial GTKO BHV development.

Planned Impact

There are two main beneficiaries of this technology, patients affected by heart valve disease and manufacturers of replacement heart valves. Approximately 250,000 heart valve replacement operations are performed annually worldwide. The Society of Thoracic Surgeons National Database for aortic valve replacement (AVR) in North America shows an increase in the use of BHVs compared to MHVs. In 1997 43% of patients undergoing AVRs received BHVs which increased to 78% by 2006. The Society for Cardiothoracic Surgery in Great Britain and Ireland database shows a similar trend which extends even to patients under age 55 where BHV usage increased from 18 to 25% between 2004 and 2009. [3, 4] This shift represents the growing favour by patients and physicians for BHVs. Because of rheumatic heart disease (RHD), patients in developing nations likely represent an even larger more significantly affected end user patient population. The WHO estimates 15.6 million cases of RHD occur worldwide with an estimated 230,000 deaths annually. In developing countries these patients are often subject to suboptimal valve repair as the facilities to manage MHV anticoagulation are frequently not available.
Manufacturers of clinical heart valves (5 - 7 major corporations) are a second end user.
Successful demonstration in this proposal of equivalency between WT and GTKO BHVs is expected to enhance interest in this technology from industry manufacturers. Clinical development of a GTKO BHV is expected to broaden the patient population and improve the quality of life of recipients who receive this improved therapy.

Publications

10 25 50
 
Description UCL Confidence in Concept Scheme
Amount £39,356 (GBP)
Funding ID MC_PC_14118 
Organisation Medical Research Council (MRC) 
Department MRC Confidence in Concept Scheme
Sector Charity/Non Profit
Country United Kingdom
Start 06/2015 
End 08/2016
 
Description Breeding and Maintenance of Gal-deficient Swine 
Organisation Royal Veterinary College (RVC)
Country United Kingdom 
Sector Academic/University 
PI Contribution Our research group and the RVC have collaborated since 2009 on the importation, breeding and maintenance of Gal-free GTKO GMO pigs. This collaboration was instrumental in bringing these animals to the United Kingdom, without which our current funding would have been impossible.
Collaborator Contribution This grant is based on a close partnership with the Royal Veterinarian College and the large animal facilities at the Bolton Park farm. The RVC holds the project license for breeding and maintaining these animals. They provide all the necessary animal husbandry expertise.
Impact This collaboration was instrumental in obtaining our current MRC funding (MR/13193/1) and will be essential for any future funding.
Start Year 2009
 
Description Genetically Modified Bovine Pericardium in Valve Design: Rational and Initial Outcome 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Health professionals
Results and Impact I presented the scientific underpinnings of why genetically modified Gal-free bovine source animals would be preferred for making bioprosthetic heart valves. This initiated group discussion.

My presentation stimulated discussion about the future role of this technology in the Yale-UCL collaborative.
Year(s) Of Engagement Activity 2014
 
Description Glycosylation and non-Gal Xenoantigens 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact This was an invited state-of-the-art presentation at the International Association of Xenotransplantation meeting.
Year(s) Of Engagement Activity 2015
 
Description Human antibody reactivity and their impact on xenograft rejection 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact This was a poster presentation at the International Society of Heart Lung Transplantation describing the specificities of human antibody and how they impact on xenograft rejection and on the function of animal based biomedical devices such as heart valves.
Year(s) Of Engagement Activity 2015
 
Description Large Animal Models in Cardiovascular Medicine 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Health professionals
Results and Impact I presented a summary of our research, specifically highlighting our work developing new Gal-free bioprosthetic heart valves, at the RVC symposium on "Use of Large Animal Models in Cardiovascular Research" in June 2014. This was a multi disciplinary meeting of scientists, clinicians and representatives of funding agencies designed to discuss animal models for cardiovascular research and to explore how the RVC might expand its role in this field. There was extensive discussion about the broader utility of our technology in other medical applications.

As a direct consequence of this presentation we were approached to explore a collaborative potential for our technology in a new field and we were encouraged by a funding agency to submit new research applications based on this technology.
Year(s) Of Engagement Activity 2014