Abstract
Cystinuria is a genetic disease leading a defective dibasic amino acid transporter in the renal proximal tubules resulting in an accumulation of urinary cystine. Urinary cystine precipitates into crystals which is believed to be a necessary step to stone formation. There is a wide variation in disease presentation that is not well understood and cannot be explained by either compliance with medical and dietary interventions, or differences in patient management. Predicting disease severity and managing patients expectantly is confounded by a paucity of validated methods to monitor disease activity and heterogeneity in how urinary dibasic amino acid levels are measured and reported in literature.Management of this disease is largely preventative and based on historical data. At Guy’s and St Thomas’, a clinic was set up to allow for a multidisciplinary approach to the management of these patients. It also provided a premise for research into patients with this disease.
The objective of my research was threefold; to understand the factors that lead to severe disease, to investigate clinical markers of disease activity, and to understand the genetic mutations that cause the disease.
This thesis is divided into seven chapters incorporating five supporting publications. Chapter one details what is already known on the subject. The evidence for dietary recommendations, current medical therapy and treatments are discussed. Chapter two describes the challenges to the clinician in the management of this disease. The basis of the specialist multidisciplinary clinic is outlined, the roles of each team member and the geographic distribution of our patient cohort.
Chapter three investigates the utility of dibasic amino acids in the management of cystinuria in particular, the association between urinary dibasic amino acids levels and stone formation. Dr Caroline Pardy and Dr Erin Mozley contributed significantly to this chapter. Spot urine samples were collected at the time of each clinic visit. The levels of the urinary dibasic amino acids and the association with stone formation as evaluated by ultrasound were analysed. There was a statistically significant association between the levels of urinary ornithine and the presence of stones seen on ultrasound scan. However, as current cystinuria medications aim at reducing cystine levels only with no known effects on levels of urinary ornithine, the prognostic value of this remains uncertain.
Chapter four describes original research on the diagnostic value of crystalluria. Separate early morning and clinic urine were collected at each clinic visit and the association between presence of crystals and presence of stones and new stone growth were analysed. Malassez counting chamber and conventional cytospin methods were also compared. The results demonstrated that the presence of crystals in patients with cystinuria is associated with stone formation and new stone growth when based on clinic urine using cytospin method. This may serve as a useful adjunct rather than as a single diagnostic tool.
For the first time, the genetic mutations found in a UK population are characterized and described in chapter five. Dr Rachael Mein contributed significantly to this work. We found 23 new mutations in our UK population. We have found that in patients with mutations in SLC3A1, the presence of a missense mutation leads to lower levels of urinary lysine, ornithine and arginine. This is the first time such a genotype-phenotype association has been found and has the potential to improve our risk stratification of patients at the time of diagnosis and tailor their subsequent follow up. This association was not seen for cystine levels however there are limitations in current cystine assays that may account for this and is further discussed in the chapter.
In chapter six, the use of protein modelling to model the two proteins encoded by SLC3A1 and SLC7A9 and the mutations that lead to protein dysfunction is described. The severity of the mutations in SLC7A9 as determined by the proximity of the mutation to the ligand binding sites and size of conformational change is shown to lead to a more severe biochemical phenotype as evidenced by raised levels of urinary dibasic amino acids. Further work in this area has the potential to lead to a personalized approach to the management of this disease.
Finally, chapter seven summarises all the research findings, discusses the implications to current practice and future challenges in the management of these patients.
| Date of Award | 2018 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Prokar Dasgupta (Supervisor) |