Team:SCU-WestChina/Background
Uric acid is a product of purine nucleotide catabolism in nucleic acid constituent units. Most mammals and poultries are able to produce uricase, which decomposes uric acid into allantoin, then into NH3, CO2 and H2O. However, humans and apes lack the ability to produce uricase, so in humans uric acid is the end product of purine metabolism. For humans, increased production or decreased excretion of uric acid can lead to its accumulation in the body, causing hyperuricemia. Research shows that hyperuricemia is an independent risk factor of arteriosclerosis, hypertension, heart failure and metabolic syndrome[1].
As the concentration of uric acid in blood increases, if sodium urate crystals deposit from super-saturated extracellular fluids to joints, synovium or other tissues and organs, it will cause a syndrome clinically known as "gout", including features like arthritis, tophi, uric acid kidney stones and gouty nephropathy. At this stage the patient suffer from acute gouty arthritis, having acute pains in the joint from time to time.
Refractory gout is caused by recurrent episodes of acute gouty arthritis for several years, manifested as chronic, multiple, destructive arthritis with pain and tophi formation and / or uric acid kidney stones. The number of such patients grows with the increasing prevalence of gout and the younger onset of the incidence, and these patients have severe illness and low quality of life. 3% of the 3 million gout patients in the United States suffer from refractory gout. Some patients treated by existing medications get poor result or have intolerance, and the treatments are time-consuming and difficult, so how to treat the disease effectively is a major clinical issue faced by physicians.
The biochemical basis of refractory gout is hyperuricemia, so the key to the treatment is to control the level of serum uric acid effectively, the target value of which is <356μmol / L (<6.0 mg / dl). The daily production of uric acid by a normal adult is 750mg, 80% of which is endogenous, and 20% of which exogenous. The uric acid gets into the metabolic pool of urate, the daily metabolic rate of which is about 60%.Of the uric acid metabolized, 1/3 is catabolized in the intestine, and 2/3 is excreted through the kidneys. This process can maintain a stable level of uric acid in the body, and any problem within the process can lead to hyperuricemia.
The mechanism of traditional uric acid–lowering agents is mainly to inhibit xanthine oxidase, reducing uric acid production and to inhibit renal tubular reabsorption of uric acid, promoting uric acid excretion[4].
Controversy still exists on whether to use medications on ordinary patients with hyperuricemia: specialists of rheumatic and immunologic fields state that hyperuricemia without symptoms of gout does not require drug treatment; however, in the Cardiology field hyperuricemia is considered as an independent risk factor of atherosclerosis and need to be controlled by medications[5,6]. And for patients with refractory gout, traditional drug treatment is slow, taking several years to complete, and for some of them the uric acid level still cannot get low enough for tophi to dissolve. At the same time, for some patients, age factors and complications like hypertension, renal insufficiency, diabetes and vascular disease have also limited drug use on them [7].
In recent years, Pegloticase, a new type of uric acid–lowering agent, has brought new solutions for patients with refractory gout and tophi. Pegloticase is a kind of PEG-modified uric acid oxidase which can transform uric acid into allantoin, which can be easily excreted. It can reduce the level of serum uric acid effectively, so as to quickly dissolve tophi and treat chronic gout. The PEG modification helps to improve its antigenicity, and FDA has approved the marketing of the drug (trade name Krystexxa®) in 2010. In the 2012 American College of Rheumatology Guidelines for Management of Gout, the drugs was recommended drug for treatment of refractory gout: “Pegloticase is appropriate for patients with severe gout disease burden and refractoriness to, or intolerance of, conventional and appropriately dosed urate-lowering therapy”[8]. However, in clinical practice, it is hard to ignore the immune response caused by injections (Infusion Reactions). Peter E Lipsky et al. found in 2014 that among the 169 patients treated with the drug, 41% of them had high-titer anti-uricase antibodies, 40% had high-titer anti-PEG antibodies, and one patient had high-titer anti-uricase antibodies even after treatment with mere placebo[9]. Because of the presence of high-titer antibodies, the efficacy of Pegloticase is low for one-half of the patients treated, which means that for the small number of patients, refractory gout is incurable[10]. Furthermore, the price of the medication is high, costing around 3,500 USD per month, which also limits its clinical application. Moreover, the strong uric acid–lowering effect of the drug can cause very fast dissolve of tophi, leading to acute gout.
Based on the reasons above, our project will put forward a new method to control hyperuricemia and treat refractory gout from the following two aspects:
1) Control of ordinary hyperuricemia (in the intestinal tract);
2) Treatment of refractory gout when Pegloticase treatment is of no avail (with methods of dialysis)
1. Control of ordinary hyperuricemia/gout (in the intestinal tract)
As mentioned above, current drug targets mainly focus on inhibition of uric acid synthesis and inhibition of uric acid reabsorption protein, with more attention paid to the latter. The intestinal tract itself, however, as a place for uric acid production, is a potential new target. The mechanism of uric acid reabsorption in the intestine is not yet fully elucidated. It is found that the purine metabolism in the intestine may be an important source of uric acid, and the urate transporter—solute carrier protein 2 family member 9 (SLC2A9) expressed by the intestinal epithelial cells plays an important role in the transport of uric acid to the blood to maintain the homeostasis[11,12]. An analysis of intestinal microbes in patients with gout carried out in 2016 showed that the compared to heathy adults, the intestinal microbes in gout patients produce more uric acid and have fewer enzymes to catabolize uric acid, resulting in its accumulation in the body[13]. In a study of the effect of montmorillonite on adsorbing uric acid and lowering blood uric acid levels carried out by Juntao Li et al., the uric acid concentration in the blood and urinary tract of the mice (administrated uric acid intragastrically) decreased with the increase of the amount of montmorillonite (Figure 5); for the hyperuricemia model conducted by intraperitoneal injection of uric acid, the mice of montmorillonite group (administrated intragastrically) had a significantly lower blood uric acid level than those in the model group. The conclusion is that montmorillonite can adsorb uric acid significantly and can reduce the serum uric acid level in mice with hyperuricemia[14].
Furthermore, Ming Li et al. fed mice with hyperuricemia with lactobacilli which utilizes uridine and inosine (important components in purine metabolism) well, and found their serum uric acid concentration significantly lowered (Figure 6)[15].
Based on the information above, we speculated that the reduction of intestinal uric acid concentration can mean a reduced serum uric acid concentration, thus treating hyperuricemia. Meanwhile, the use of probiotics offers a perfect solution for the dispute about whether to use medications to treat hyperuricemia: the blood uric acid level is lowered through proper application of intestinal microbes without the administration medications.
We hope to construct a metabolic pathway of uric acid in E. coli Nissle 1917 expressing urate permease (transporter) on its surface. We try to introduce engineered bacteria into the intestine to consume intestinal uric acid, thus reducing blood uric acid concentration, so as to treat gout.
The clinical treatment by using intestinal microbinology has become a hot research topic in recent years. Our intestinal therapy provides a long-time therapy in a harmless way. This project provides a perfect solution for the controversy that whether the hyperuricemia patients without gout symptoms should use drugs. The prevalence rate of hyperuricemia in mainland China has reached 8.4%[16], which means that 100 million of Chinese people are suffering from the risks the disease brings. The engineered bacteria constructed in our project provide a new treatment for the hyperuricemia patients all around the world to avoid the use of medications and also help the gout patients to reduce their blood urate concentration. It is of far-reaching influence to the global prevention of gout and the metabolic diseases hyperuricemia cause.
2. Treatment of refractory gout when Pegloticase treatment is of no avail (with methods like dialysis)
The key to the treatment of refractory gout is to lower the blood uric acid concentration, but direct protein contact may be blocked immediately by the IgG antibodies (of a long half-life) of the uricase-resistant patient, so it is necessary to build a relatively independent immunologically privileged sites. Through the establishment of a dialysis system, we try to solve the problem of refractory gout. At the same time, if the efficiency of the device is acceptable, it can be used as an inexpensive treatment regimen to accelerate the dissolution of gout and may be able to change the principles of treatment for chronic tophi.
The treatment of refractory gout is a difficult clinical problem. FDA-approved Pegloticase is effective for some patients. However, if the patient is not sensitive to Pegloticase, it means that there is no proper treatment all over the world. However, we should not give up on every single patient. For rare diseases, we still need to focus on their health and quality of life. Our dialysis device therapy method provides an ultimate treatment for those who suffer from refractory gout and are insensitive to Pegloticase. If the device works well, it can be applied to all patients with chronic gout and facilitates the elimination of tophi in a relatively quicker and less expensive way. At the same time, we have put forward a completely new approach for the treatment of metabolic diseases: with the guidance of synthetic biology and biomedical engineering, the bacteria strains can be applied into the blood to treat any metabolic disease. It provides a brand new idea for the clinical application of microorganisms, to improve the application of immunoadsorption, for example
Immunoadsorption is a newly developed technique ultilizing the chemical ligand or anitibody to "catch" the substrate that should be excluded from the blood. It's used in clinics now, which is pretty like our dialysis method. However, according to our human practice, it's expensive and for one kind of immunoadsorption tube, it can only absorb one kind of substrate. However, if we can utilize our dialysis method, we can use the membrane surface display technology or inner metabolic pathway to utilize multiple substrates, which is quick and cheap, revealing the potential of our dialysis method in the future.
References
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