The first goal of this study was to develop a noninvasive method for diagnosis and prognosis of native and posttransplant kidney disease. The identification of components associated specifically with normal kidney function, and several components associated with kidney disease, provided proof of concept for prognosis of kidney deterioration or failure based on the presence or absence of specific protein biomarkers. A less obvious goal was to determine the nature of variation and change in the urinary proteome across the spectrum from healthy individuals to kidney allograft recipients. A relatively small number of components offered insight into these several issues. We limited our analysis to peaks that appear to best differentiate the different states of kidney health. These components appeared in a large number of samples, making them useful in group comparisons. Other peaks were found in only a few samples and were therefore of low utility.
To optimize experimental descriptions and accessibility of the method, this study used an inexpensive, readily accessible method and evaluated only the most intense peaks of the profiles. The method shared some properties with an automated commercial system (SELDI). The peak identified as B2M (m/z
= 9073 and 9746) correlated with SELDI peaks reported at m/z
= 9.0, 9.7, and 9.8 kDa (6
). Loss of saposin B was associated with kidney disease.
Two properties of protein profiles of healthy individuals were identified. First, this method and analysis detected a small number of major components among healthy persons, making every new peak a potential biomarker. Second, the ratio of saposin B isoforms was stable in each individual over time. This correlated with the suggestion that longitudinal change in a protein ratio of an individual may detect a change in health status even though the actual value (peak ratio in this case) remained within the range found among healthy individuals (21
Proteins or peptides associated with pathologic conditions appeared in a sequence that correlated with value in prognosis. Components at m/z = 4303 and 10,350 appeared frequently but showed little value for diagnosis or prognosis over the time of this study. B2M had some value for prognosis but much less than the component at m/z = 9480. Optimum prognosis of kidney failure was provided by components found in advanced native kidney disease (m/z = 4272, 4337, 4860, 5007, 13,350, and 15,835). These findings might be rationalized by the suggestion that early, reversible markers occur far from the endpoint, and are subject to intervening, unpredictable events that have impact on the outcome. Markers associated with advancing disease seemed more tightly correlated with final outcome and, therefore, had better prognostic ability. If this general property is applicable to protein biomarkers, it follows that optimum diagnosis/prognosis will use a complex pattern of proteins that are evaluated in sequential samples from each individual. For example, although a single detection of B2M had relatively modest value, its continual or repeat presence may provide an early indication of developing disease. Although the findings did not seem to improve on creatinine, comparable outcomes served to validate the method and provide proof of principle that proteome analysis can work. This new technology can be improved by a nearly unlimited potential for additional discovery of additional peaks to increase the sensitivity and specificity for this approach.
For the current study, limited data at longer times suggested that kidneys producing pathologic components may survive for 2 years or longer but with impaired function. It is possible that the markers of advanced disease described in this study will be most useful for management of transplanted kidneys with impaired but useful function. It is possible that future combination of protein biomarkers with laboratory and biopsy results will improve diagnostic and prognostic value in kidney disease and transplant. Identification of a profile that reduces the need for a biopsy would be a valuable outcome even if applicable to a subpopulation of healthy transplant patients.