Proteins Predict Progression and Mortality of Kidney Disease

Researchers develop way to measure abundance of 12 different complement proteins in the urine.

Diabetic kidney disease (DKD) is a leading cause of chronic kidney disease and end-stage renal disease. Tools to diagnose and treat DKD are not up to pace with the rising rates of diabetes. Diabetes rates are also growing among ethnic minorities. This article studies ways to identify DKD for early diagnosis in the Mexican-American population.

The complement cascade has been shown to mediate the tubulointerstitial injury that can lead to kidney disease. This area has not been studied due to the fact that there is no quantitative way to assess complement protein assembly. The researchers of this article developed a way to measure the abundance of 12 different complement proteins in the urine.

Researchers used a cross-sectional subset of participants from the University of Washington (UW) DKD repository and participants from the Golden State cohort. Patients in the UW group with type 1 diabetes were used. DKD was defined as an estimated glomerular filtration rate of 60mL/min/1.73m2 and a urine albumin-to-creatinine ratio greater than or equal to 30 mg/g or an albumin-to-creatinine ratio over 300 mg/g. Control participants had type 1 diabetes for over 30 years, an eGFR >90 mL/min/1.73 m2, and albumin-to-creatinine ratio <300 mg/g. Participants from the Golden State cohort all had type 2 diabetes. DKD was defined as a urine protein-to-creatinine ratio >0.5 g/g and one or more of the following: typical histologic changes on kidney biopsy, diabetes duration >5 years and diabetic retinopathy, or diabetes duration >10 years without diabetic retinopathy.

Urine samples were thawed and incubated overnight, and precipitated proteins were reconstituted. After several steps, mass spectrometry was used to identify the urine proteins and peptides. The targeted proteins were log2 transformed and converted to Z scores. Pearson correlation was used to calculate the association between the abundance of each protein and the eGFR and the urine protein.

Out of 141 participants who were of Mexican-American descent, the median estimated glomerular filtration rate was 54 mL/min/1.73m2 and had a urine protein-to-creatinine ratio of 2.6g/g. End-stage renal disease and/or death occurred in 67 participants. A higher level of CD59, an inhibitor of terminal complement complex formation, was linked with a lower ESRD risk and death. It was also found that higher C4 and C8 complement proteins were associated with lower risk for death. Higher urine factor H-related protein 2 was associated with greater risk of death. These findings were still significant after adjustment for demographic, eGFR, proteinuria, and other clinical covariates.

DKD pathogenesis is still being studied and timing of the complement pathway cascade and progression of kidney damage is unknown. CD59 is a major regulator of complement activity and is found in most cells in the body. This means the source of CD59 in the urine samples could be renal or from other cells. Further investigation into CD59 origin would help determine if the increase in CD59 is actually due to kidney injury or if it is from a systemic process that affects DKD progression. C4 and C8 were associated with lower risk of death. These proteins are activated in the lectin or classical pathway cascade. FHR2 is in the complement pathway and block activation of the alternative pathway. This suggests greater activation of the classical pathway is associated with longer survival and the alternative pathway is associated with shorter survival. This is a very generalized assumption and more research needs to be done to make any claims.

Practice Pearls:

• A higher level of CD59 in urine was linked with a lower ESRD risk and death.
• Higher C4 and C8 complement proteins were associated with lower risk for death.
• Higher urine factor H-related protein 2 was associated with greater risk of death.

References:

Tomas Vaisar, Blythe Durbin-Johnson, Kathryn Whitlock, Ilona Babenko, Rajnish Mehrotra, David M. Rocke, and Maryam Afkarian. Urine Complement Proteins and the Risk of Kidney Disease Progression and Mortality in Type 2 Diabetes. Diabetes Care. (August 2018). doi.org/10.2337/dc18-0699