Background
Idiopathic membranous nephropathy (iMN) is a common cause of adult-onset nephrotic syndrome and increased by more than twice over the past decade in China [
1]. Most iMN patients maintained renal function for prolonged periods and likely to have spontaneous remission (SR). Only about 30% of the iMN patients with nephrotic proteinuria will develop end stage renal disease (ESRD) without appropriate treatment [
2]. Even with restrictive treatment, iMN patients with nephrotic proteinuria had quite favorable long-term outcomes. They had an overall renal survival of 86% after 10 years [
3]. As immunosuppressive treatment will predispose an iMN patient to opportunistic infections and other severe side effects, the Kidney Disease Improving Global Outcomes (KDIGO) guidelines recommend using immunosuppressive agents only in patients at high risk for developing kidney failure [
4].
It is of interest to many researchers to distinguish these high-risk iMN patients. Cattran group created the Toronto Risk Score which accurately predicted the progression of iMN and was based on the change in creatinine clearance and the maximum of proteinuria during a 6-month period [
5]. Prolonged observation is necessary for this model, and the patients were kept in uncertainty during this period. Alternatively, several urinary markers have been proposed to predict progression in iMN [
6,
7], especially low-molecular-weight proteins including α1 and β2 microglobulins [
2].
The intrarenal renin-angiotensin system (RAS) activation has several functions not only regulating blood pressure but also developing renal cell growth, and affecting renal function eventually [
8,
9]. Angiotensinogen (AGT) is the sole precursor of all angiotensin peptides. Human AGT has 485 amino acids and can be present in non-glycosylation or glycosylation forms in serum with a molecular weight of 53 kDa or 75 kDa respectively [
10]. Serum AGT cannot be freely filtered through the glomerular basement membrane in normal conditions. Urinary AGT (UAGT) was mainly generated locally and accurately reflect intrarenal RAS activity [
11,
12]. The correlations between elevated UAGT levels and the severity of kidney disease had been reported in immunoglobulin A nephropathy and diabetic nephropathy [
13‐
16].
It had been shown that intrarenal RAS was activated in progressive iMN and elevated levels of in situ generation of angiotensin II and angiotensin-converting enzyme were associated with the disease severity [
17]. However, the correlation between UAGT and disease severity of iMN had not been extensively studied. In the present study, we attempted to explore the relationship between UAGT and severity of iMN patients at the onset. We aimed to find another urine marker which will add to the distinguishing ability of the above-mentioned risk markers.
Discussion
In the present study, we found that UAGT levels were higher in glomerulonephritis patients than normal subjects; meanwhile, no differences of UAGT levels were found between iMN patients and MCD patients. Further analysis indicated that the UAGT levels were correlated with several parameters relevant to disease severity of iMN patients, including serum albumin, eGFR, and proteinuria. However, these correlations were found not statistically significant in MCD patients. Multivariate regression analysis showed that proteinuria was the independent determinant of UAGT levels in iMN patients.
Defining factors relating to the disease severity is crucial to iMN patients and of interest to many researchers. The natural clinical course of iMN is that one-third tend to have spontaneous remission, one-third have stable renal function, and one-third progress to renal failure. If treated properly, the iMN patients will have overall renal survival of about 86% after 10 years [
3]. However, immunosuppressive treatments for iMN, including corticosteroids, alkylating agents, and calcineurin inhibitors, will predispose the patients to opportunistic infections. In addition, alkylating agents, such as cyclophosphamide, increased cancer risk about threefold [
20]. As the renal survival of iMN is quite good and the immunosuppressive treatment is toxic, KDIGO guidelines recommend that immunosuppressive agents be used only in patients at high risk of developing renal failure [
4]. Some authors suggested that conservative treatment may be extended beyond 6 months of follow-up [
3]. However, the prolonged observation period required to identify a high-risk patient would expose the patient to severe complications of nephrotic syndromes, such as edema, thrombosis, and infections. Many researchers have attempted to identify early predictors for prognosis which would allow early treatment and rapid disappearance of nephrotic syndrome in high-risk patients and avoid unnecessary exposure to toxic therapy in low-risk patients. Dr. van den Brand JA et al. had shown that low-molecular-weight proteins including α1 and β2 microglobulins were associated with the disease severity and prognosis of iMN patients [
2,
21]. We aimed to find another marker that was relevant to iMN severity.
In the present study, UAGT levels were shown to be associated with severity and predictive markers of iMN patients, including low serum albumin, low eGFR, and high proteinuria. These risk factors had been proven by several studies. Low serum albumin levels at the onset were found to be associated with poor renal prognosis of iMN patients by Huh et al. [
22]. High level of serum creatinine at the diagnosis was shown to be the main predictor for progression to ESRD in membranous nephropathy patients [
23]. Also, severe proteinuria was used as a major predictor for renal prognosis of iMN in a conventional predictive model, Toronto Risk Score [
5]. Based on this, our data supported that the levels of UAGT were associated with the severity of iMN patients.
It is doubted whether the elevated levels of UAGT were nonspecific consequences of proteinuria as both iMN and MCD patients presented heavy proteinuria in our study. Our analysis further showed that UAGT was correlated with the above parameters only in iMN patients, but not in MCD patients. This difference supported the hypothesis that UAGT was not a non-specific consequence of proteinuria as both iMN and MCD patients had severe proteinuria. Actually, several studies supported this hypothesis. Kobori and coworkers found low UAGT levels in MCD patients [
24]. Jang’s work showed that UAGT was predominantly composed of filtered AGT from the systemic circulation in patients with nephrotic-range proteinuria. However, this phenomenon only presented in MCD or diabetic nephropathy patients but not in iMN patient [
25]. A few authors suggested that UAGT levels reflected the intrarenal RAS activity and severity of chronic kidney diseases rather than a non-specific consequence of proteinuria [
14‐
16,
24,
26].
One of the interesting findings of the present study was that UAGT was significantly correlated with several parameters in iMN patients but not in MCD patients. We supposed that this may be due to several mechanisms. Firstly, as had been shown by Dr. Jang et al., the origin of UAGT was different in different subtypes of glomerulonephritis. UAGT was correlated with serum AGT in MCD patients but not in iMN patients. They suggested that UAGT was mainly filtered from the serum in nephrotic MCD but produced locally in iMN [
25]. Secondly, the different pathogenesis of MCD and iMN may cause the different behavior of UAGT. The activation of the RAS system is likely consequence of the primary lesion in the kidney. The most recent finding for the pathogenesis of iMN suggested that it was a humoral autoimmune disease caused by circulating auto-antibodies against receptors on the podocyte, notably anti-phospholipase A2 receptor (PLA2R) [
27]. However, MCD is dominantly believed to be caused by dysfunctions of T cells [
28‐
30]. Furthermore, we noticed that, clinically, RAS inhibitors may reduce the activity of iMN but not MCD. The mechanisms why UAGT behaves differently in iMN and MCD need to be further elucidated.
A few limitations remained in our study. First, the number of iMN patients in our study is relatively small. As it is a pilot study to explore the utility of UAGT as a biomarker for the severity of iMN patients, we will expand the observation number inspired by these promising results. Second, the cross-sectional character of the present study cannot provide proof of the predictive ability of UAGT on kidney outcomes. We can only suggest that the UAGT levels were associated with the severity markers of iMN patients. A prospective cohort study is needed to test the predictive value of UAGT in iMN patients. Another limitation of our study is the lack of anti-PLA2R antibody which had been suggested highly correlated with the disease severity of iMN in recent years. This was due to the cross-sectional nature of the study and measurement of anti-PLA2R antibody was not practicable at that time. We are now measuring anti-PLA2R antibody titer of all iMN patients in our clinic.
Acknowledgements
The authors thank all the staff in the Department of Nephrology, Peking University Third Hospital.