Abstract
Objective
The aim of this study is to analyze the 5 year prognosis of IgA nephropathy (IgAN) patients with varying degrees of foot process effacement (FPE).
Method
We retrospectively analyzed the clinical and pathological data of 863 patients with primary IgAN confirmed by renal biopsy at Hangzhou Traditional Chinese Medicine Hospital from 1 January 2015 to 31 December 2018. According to the degree of FPE, the group was divided into extensive FPE group and segmental FPE group. After propensity score matching (PSM), the effect of different levels of FPE on the prognosis of IgAN was analyzed using Log-rank tests.
Result
Notably, 61(7.1%) of the 863 patients reached renal composite endpoint. 53 patients in the foot process extensive effacement group were matched with 100 patients in the segmental effacement group after propensity score matching. There were no significant differences in any of the baseline characteristics between the two groups (P > 0.05). Additionally, Kaplan–Meier survival analysis revealed that patients with extensive FPE (≥ 50%) exhibited a significantly poorer prognosis compared with those with segmental FPE (< 50%) (Log-rank test P = 0.030).In the Pearson correlation analysis, FPE was moderately positively associated with albuminuria and mildly negatively associated with eGFR.
Conclusion
Extensive foot process effacement is an independent risk factor for IgAN prognosis.
Keywords: Podocyte injury, Foot process effacement, IgA nephropathy, Prognosis, Electron microscopy
Introduction
Immunoglobulin (Ig)A nephropathy (IgAN) is the most common primary glomerular disease in China and one of the main causes of end-stage renal disease (ESRD) [1]. Approximately 25–40% of patients with IgAN in clinical practice progress to ESRD within 10–20 years, thereafter requiring renal replacement therapy [2]. Various factors and intricate mechanisms can influence IgAN progression, with recent studies highlighting the role of podocyte pathology [3]. Podocytes, the epithelial cells found in the visceral and outermost layers of the glomerulus and glomerular basement membrane (GBM), respectively, play a crucial role in the formation of the glomerular filtration barrier. Additionally, podocytes contribute considerably to maintaining the structural integrity and proper functioning of glomerular filtration, thereby preventing the incidence of proteinuria [4]. Damage to podocytes, particularly the loss of foot processes, can lead to a reduction in the number of foot cells [5]. Foot process effacement (FPE) is characterized by the disappearance of neighboring normal toe-like structures of the foot cells under an electron microscope. FPE can lead to the fusion and notable enlargement of foot processes, covering the GBM and ultimately increasing the permeability of the glomerulus to macromolecules [6]. Previous studies have predominantly examined the relationship between foot process lesions and proteinuria in IgAN; however, studies on the association between FPE and IgAN disease prognosis are limited. Recent findings indicate that FPE may contribute to the pathogenesis of proteinuria and glomerulosclerosis, thereby increasing IgAN progression [7].
This study aimed to investigate the prognostic implications of varying degrees of FPE in patients with IgAN over 5 years. Additionally, the correlation between FPE observed through electron microscopy and IgAN prognosis was determined. The findings of this study may contribute to advancing knowledge on IgAN and enhancing its prognostic outcomes.
Materials and methods
Research object
Patients with IgAN, who were diagnosed through renal biopsy at the Hangzhou Traditional Chinese Medicine Hospital from 1 January 2015 to 31 December 2018, were included in this study. The inclusion criteria were as follows: (1) complete case data; (2) follow-up duration of at least 6 months; and (3) age between 18 and 75 years. Exclusion criteria included (1)lupus nephritis and diabetic nephropathy and other kidney diseases; (2) secondary IgAN, such as IgA vasculitis and hepatitis B virus-associated nephritis; (3) acute kidney injury; (4) ESRD at the time of biopsy; (5) follow-up less than 6 months and no composite endpoint event; and (6) lack of MEST-C score or FPE data. This retrospective study was approved by the Ethics Committee of the Hangzhou Traditional Chinese Medicine Hospital, and the requirement for signed informed consent was waived (Ethics Number: 2021KY045).
Clinical and laboratory data
All the clinical and laboratory data, including that of the renal biopsy, were extracted from the medical record system of the Hangzhou Traditional Chinese Medicine Hospital. The data included detailed information such as renal biopsy time, gender, age, systolic blood pressure (SBP), diastolic blood pressure (DBP), body–mass index (BMI), hemoglobin (HB), serum albumin (Alb), cholesterol (TC), triglycerides (TG), low-density lipoprotein (LDL), high-density lipoprotein (HDL), blood uric acid (UA), serum creatinine (Scr), 24 h urine protein quantification (24h-UTP), microscopic examination of red blood cells, estimated glomerular filtration rate (eGFR), and serum humoral immune indicators (such as IgA, IgG, IgM, complementary component C3, and C4). Hypertension was defined as SBP ≥ 140 and/or DBP ≥ 90 mmHg, with three measurements taken on different dates, and/or having a history of hypertension. The eGFR was calculated using the chronic kidney disease epidemiological collaboration creatinine equation. All indicators were defined using traditional cutoff values, following the Kidney Disease Improving Global Outcomes guidelines [8].
Organizational pathology data
The histopathological data were collected by the Nephrology Department of the hospital. Renal puncture tissue employ routine hematoxylin and eosin, periodic acid-Schiff, hexamine silver, and Masson staining. Direct immunofluorescence was used to determine the expression intensities and deposition sites of IgA, IgG, IgM, C3, and C4. For every patient, the scores of pathological indicators (MEST-C score) and the electron microscopy images of FPE were observed and recorded. The Oxford IgAN score (MEST-C score) was used to describe the tissue lesions as follows: M, increased mesangial cells; E, increased intracellular cells in capillaries; S segmental sclerosis, T renal tubular atrophy or interstitial fibrosis; and C crescent formation. Based on the proportion of mesangial cells, M0 and M1 were used to represent the mesangial lesions. E and S lesions were classified into two levels based on their presence or absence: E0/E1 and S0/S1, respectively. The crescent is defined as the extravascular proliferation of capillaries of any size exceeding two cell layers. The severity of renal tubular atrophy/interstitial fibrosis and crescent lesions was expressed as T0 (0–25%), T1 (26–50%), or T2 (> 50%) and C0 (none), C1 (0–25%), or C2 (≥ 25%), respectively. The FPE is defined as the disappearance of the normal fissure diaphragm between adjacent foot processes or the complete FPE. The degree of foot process effacement was estimated by measuring the mean foot process width by the method of Gundersen, Seefeldt, and Osterby [9].The results have been expressed as the percentage of the length of the FPE to the total length of each capillary loop. Approximately 1–2 intact glomeruli will be visualized electron microscopically per patient, with approximately 5–8 capillary loops in an intact glomerulus. We will assess the average percentage of FPE for all capillary loops in each glomerulus. The results have been expressed as the percentage of the length of the FPE to the total length of each capillary loop. We used a semi-quantitative method according to the percentage of FPE and classified it into 5 classes: 0––negative, 1–– ≤ 25%, 2–– < 50%, 3–– ≥ 50%, and 4–– ≥ 75%.The FPE of ≥ 50% was defined as the extensive FPE and the FPE of < 50% was defined as the segmental FPE [10, 11]. All biopsy specimens were reviewed by two renal pathologists who were unaware of the clinical data.
Treatment, endpoint, and follow-up
The information on whether glucocorticoids, immunosuppressants, or renin-angiotensin system (RAS) blockers have been used throughout the entire course of the disease was recorded. This study defined a composite endpoint, namely, ESRD (eGFR < 15 mL/min/1.73 m2 or dialysis treatment) and permanent reduction of eGFR by ≥ 50%. The information regarding the Scr, eGFR, total follow-up duration, and if the patient has entered the renal composite endpoint at each follow-up within 5 years after the renal biopsy was recorded. If unable to obtain information, make a phone inquiry.
Statistical analyses
The statistical package for social sciences 26.0 software (international business machines Corporation, Chicago, IL, USA) was used to analyze the data. The GraphPad Prism version 9.5.1 software (GraphPad Software, San Diego, CA, USA) was used to plot the graphs. The Kolmogorov–Smirnov test was performed for the normality test of the econometric data, and the homogeneity of variance was determined through the Levene test. The measurement data that conform to the normal distribution are represented in the _ ± s form, and intergroup comparisons were performed using the t test. The measurement data that do not follow a normal distribution are presented asx M (P25, P75), and intergroup comparisons were performed using the Mann–Whitney U test. Count or categorical variables have been expressed as frequency (%), and the X2 test or Fisher’s exact probability test was used for intergroup comparisons, and the Mann–Whitney U test was used for intergroup comparison of the rank data. Multiple-factor Cox regression analysis was performed to analyze the risk factors for composite endpoints in patients with IgAN. The Kaplan–Meier survival curve and Log-rank test were performed to compare the impact of different degrees of FPE on renal survival. Pearson correlation analysis was performed to analyze the influencing factors of FPE. Propensity score matching (PSM) analysis was performed in this study with the matching package in R software (version 4.4.1 for Windows, Bell Laboratories) and conducted with the 1:2 nearest neighbor matching method. The covariates included sex, age, BMI, SBP, DBP, HB, Alb, TC, TG, LDL, HDL, UA, Scr, 24 h-UTP, URBC, eGFR, serum humoral immune indicators, and the use of glucocorticoids, immunosuppressant, and RAS inhibitor. All P values are for bilateral tests, and P < 0.05 represents statistically significant differences.
Result
Baseline characteristics of patients before and after propensity score matching
The enrollment process of the study subjects is shown in Fig. 1. In total, 863 patients with primary IgAN were enrolled. Patients were divided into extensive effacement and segmental effacement groups according to the degree of FPE of patients. Baseline characteristics of patients included gender, age, body mass index, SBP, DBP, HB, Alb, TC, TG, LDL, HDL, UA, Scr, 24 h-UTP, URBC, eGFR, serum humoral immunity markers, and the use of glucocorticoid, immunosuppressant, and RAS inhibitor. Baseline characteristics of patients before and after propensity score matching are shown in Table 1. Before propensity score matching, age, proportion of males, BMI, blood pressure, TC, TG, LDL, Scr, UA, 24 h-UTP, IgG, serum C3 and C4 levels, and proportion of glucocorticoid use were significantly higher in the extensive effacement group than in the segmental effacement group(P < 0.05). This suggests that patients in the foot-process extensive effacement group had a higher severity of disease at enrollment than those in the segmental effacement group. We used all baseline factors as covariates and after 1:2 propensity score matching, a total of 53 patient groups were successfully matched. Endpoint events occurred in 14 (26.4%) of 53 cases in the group with extensive FPE and in 12 (12%) of 100 cases in the group with segmental FPE. Photographs of typical TEM images of extensive and segmental effacement of foot processes are detailed in Fig. 2. There were no significant differences in any of the baseline characteristics between the two groups (P > 0.05).
Fig. 1.
Process flow chart for enrollment of research subject
Table 1.
Baseline characteristics of patients before and after propensity score matching
| Variables | Before PSM | After PSM | ||||||
|---|---|---|---|---|---|---|---|---|
| Segmental effacement (n = 789) | Extensive effacement (n = 74) | Total (n = 863) | P value | Segmental effacement (n = 100) | Extensive effacement (n = 53) | Total (n = 153) | P value | |
| Age (years) | 36 [29,45] | 40 [30,50.3] | 36 [29,45] | 0.034 | 37.0 [30.5,47.0] | 40.5 [30.8,51.2] | 39 [30.5,49] | 0.332 |
| Male, n(%) | 307 (38.9%) | 38 (51.3%) | 345 (40%) | 0.037 | 45 (44.6%) | 25 (46.3%) | 0.837 | |
| Time (months) | 48 [31.5, 60] | 42 [24.8, 59.3] | 48 [31, 60] | 0.102 | 48[30, 60] | 43 [31.5, 59] | 48 [30, 60] | 0.763 |
| BMI (kg/m2) | 22.5 [20.4, 24.6] | 23.6 [21.5, 26] | 22.6 [20.4, 24.8] | 0.001 | 23.5 [20.8, 26.1] | 23.6 [21.4, 26.0] | 23.5 [21.1, 26] | 0.329 |
| SBP (mmHg) | 121 [110.8, 133] | 128 [114, 143.3] | 122 [111, 134] | 0.007 | 128.5 [117, 143] | 128 [118, 143.5] | 128 [117, 143] | 0.835 |
| DBP (mmHg) | 77 [68, 85] | 81 [71.8, 93] | 77 [68, 86] | 0.003 | 83 [73, 90] | 81.5 [72, 93.3] | 82 [73, 90] | 0.791 |
| HB (g/L) | 120 [110, 133] | 122.5 [109, 136] | 120 [110, 133] | 0.236 | 120.17 ± 18.36 | 119.66 ± 18.5 | 119.99 ± 18.35 | 0.871 |
| TC (mmol/L) | 4.4 [3.8, 5] | 5.3 [4.4, 7.2] | 4.4 [3.8, 5.1] | < 0.001 | 4.8[3.9,5.9] | 4.7[4.2,5.7] | 4.7[4,5.8] | 0.759 |
| TG (mmol/L) | 1.2 [0.8, 1.7] | 1.8 [1.3, 2.4] | 1.2 [0.9, 1.8] | < 0.001 | 1.47 [1, 2.1] | 1.8 [1.3, 2.4] | 1.6 [1.1, 2.2] | 0.085 |
| LDL (mmol/L) | 2.6 [2.2, 3] | 3.4 [2.6, 4.4] | 2.6 [2.2, 3.2] | < 0.001 | 2.9 [2.3, 3.7] | 2.9[2.4,3.7] | 2.9[2.3,3.7] | 0.630 |
| HDL (mmol/L) | 1.1 [0.9, 1.3] | 1.1 [0.9, 1.4] | 1.1 [0.9, 1.3] | 0.079 | 1.1 [0.9, 1.4] | 1.1[0.9,1.3] | 1.1[0.9,1.3] | 0.503 |
| Alb (g/L) | 38.3 [35.8, 40.7] | 32.6 [22.9, 37] | 37.9 [35.3, 40.5] | 0.236 | 35.1 [31.6, 38.8] | 34.7[30.2,38.4] | 35.2[31.6,38.5] | 0.168 |
| Scr (umol/L) | 73[57,102] | 102[72,115] | 75[58,103] | < 0.001 | 96.5 [65.3, 125.8] | 103 [77, 113.5] | 102 [70.5, 122] | 0.247 |
| eGFR (ml· min-1· 1.73m−2) | 91.7 [68.2, 111.1] | 63.9 [49.7, 79.2] | 88.7 [64.2, 109.5] | < 0.001 | 73.6 [51.4, 91.7] | 64[49.7, 76.3] | 69.1 [50, 87] | 0.081 |
| UA(umol/L) | 301 [247, 363] | 321 [282, 400] | 304 [249, 365] | 0.003 | 340.28 ± 79.81 | 336.41 ± 78.89 | 339.58 ± 79.51 | 0.773 |
| 24h-UTP (g/day) | 0.8 [0.5, 1.5] | 2.9 [1.3, 6.5] | 0.9 [0.5, 1.7] | < 0.001 | 2.0 [1,3.4] | 1.8 [1.1, 3.1] | 1.9 [1, 3.1] | 0.968 |
| URBC | 1 [0, 3] | 0.5 [0, 2] | 1 [0, 3] | 0.073 | 1 [0, 2] | 1 [0, 2.5] | 1 [0, 2] | 0.866 |
| IgG (g/L) | 1080 [919, 1270] | 872.5 [554, 1080] | 1062 [899, 1250] | < 0.001 | 1010.34 ± 278.11 | 951 ± 286.58 | 989.78 ± 281.56 | 0.216 |
| IgA (g/L) | 295 [239, 356] | 291 [216.8, 374.5] | 295 [236, 357] | 0.715 | 293.5 [232, 369.3] | 293 [224, 367] | 293 [229, 367.5] | 0.697 |
| IgM (g/L) | 106 [78, 140] | 112.5 [84.5, 146.3] | 107 [79, 140] | 0.350 | 105.5 [82.3, 145] | 108 [79.5, 136] | 108 [81, 142.5] | 0.771 |
| C3 (g/L) | 96[85, 108] | 104 [93.8, 116.5] | 97 [86, 109] | < 0.001 | 103.92 ± 18.53 | 102.59 ± 18.05 | 103.73 ± 18.28 | 0.669 |
| C4 (g/L) | 23[18, 26] | 26.5[22,32.3] | 23[19, 27] | < 0.001 | 23[19, 28] | 25 [20, 29.5] | 23 [19, 28.5] | 0.295 |
| Treatments and outcomes | ||||||||
| Treatment, n (%) | ||||||||
| Glucocorticoids, n (%) | 630 (79.8%) | 67 (90.5%) | 697 (80.8%) | 0.026 | 86 (86%) | 47 (88.7%) | 133 (86.9%) | 0.640 |
| Immunosuppressant, n (%) | 619 (78.5%) | 53 (71.6%) | 672 (77.9%) | 0.176 | 84 (84%) | 39 (73.6%) | 123 (80.4%) | 0.123 |
| RAS inhibitor, n (%) | 706 (89.5%) | 62 (83.8%) | 768 (89%) | 0.135 | 91 (91%) | 48 (90.6%) | 139 (90.8%) | 1.000 |
| Number of final-entry events, n (%) | 43 (5.4%) | 18 (24.3%) | 61 (7.1%) | < 0.001 | 12 (12%) | 14 (26.4%) | 26 (17%) | 0.024 |
All data are expressed in terms of frequency (ratio) or mean ± standard deviation or median (interquartile range). URBC microscopic examination of red blood cells (0 is less than 3/HP, 1 is 4–9/HP, 2 is 10–30/HP, 3 is greater than 30/HP); eGFR estimates glomerular filtration rate; IgA, IgG, IgM, C3, C4 blood, and fluid immune indicators
SBP systolic blood pressure, DBP diastolic blood pressure, BMI body mass index, HB hemoglobin, Alb serum albumin, TC cholesterol, TG triglycerides, LDL low-density lipoprotein, HDL high-density lipoprotein, UA blood uric acid, Scr serum creatinine, 24h-UTP 24-h urine protein quantification
*P < 0.05 is considered statistically significant
Fig. 2.
Electron microscopic images of FPE lesions of different degrees of severity. A Segmental FPE (Magnification × 3000 to 8000). B Extensive FPE (Magnification × 3000 to 8000). Red arrows show FPE
Baseline histopathological characterization of the two groups of patients before and after propensity score matching
As shown in Table 2, before PSM, the foot-process extensive effacement group had more severe M lesions and T lesions (P < 0.05). After PSM, there was no significant difference in MEST-C scores between the two groups (P > 0.05).
Table 2.
Baseline histopathological characteristics of renal biopsies in the two groups of patients before and after matching
| Variables | Before PSM | After PSM | ||||||
|---|---|---|---|---|---|---|---|---|
| Segmental effacement (n = 789) | Extensive effacement (n = 74) | Total (n = 863) | P value | Segmental effacement (n = 100) | Extensive effacement (n = 53) | Total (n = 153) | P value | |
| M, n (%) | 0.030 | |||||||
| 0 | 4 (0.5%) | 2 (2.7%) | 6 (0.7%) | 0 | 0 | 0 (0%) | ||
| 1 | 785 (99.5%) | 72 (97.3%) | 857 (99.3%) | 100 (100%) | 53 (100%) | 153 (100%) | ||
| E, n (%) | 0.686 | 0.735 | ||||||
| 0 | 580 (73.5%) | 56 (75.7%) | 636 (73.7%) | 71 (71%) | 39 (73.6%) | 110 (71.9%) | ||
| 1 | 209 (26.5%) | 18 (24.3%) | 227 (26.3%) | 29 (29%) | 14 (26.4%) | 43 (28.1%) | ||
| S, n (%) | 0.100 | 0.901 | ||||||
| 0 | 123 (15.6%) | 17 (23%) | 140 (16.2%) | 12 (12%) | 6 (11.3%) | 18 (11.8%) | ||
| 1 | 666 (84.4%) | 57 (77%) | 723 (83.8%) | 88 (88%) | 47 (88.7%) | 135 (88.2%) | ||
| T, n (%) | < 0.001 | 0.083 | ||||||
| 0 | 583 (73.9%) | 36 (48.6%) | 619 (71.7%) | 52 (52%) | 18 (34%) | 70 (45.8%) | ||
| 1 | 173 (21.9%) | 30 (40.5%) | 203 (23.5%) | 37 (37%) | 29 (54.7%) | 66 (43.1%) | ||
| 2 | 33 (4.2%) | 8 (10.8%) | 41 (4.8%) | 11 (11%) | 6 (11.3%) | 17 (11.1%) | ||
| C, n (%) | 0.094 | 0.152 | ||||||
| 0 | 265 (33.6%) | 37 (50%) | 302 (35%) | 34 (34%) | 24 (45.3%) | 58 (37.9%) | ||
| 1 | 464 (58.8%) | 26 (35.1%) | 490 (56.8%) | 56 (56%) | 21 (39.6%) | 77 (50.3%) | ||
| 2 | 60 (7.6%) | 11 (14.9%) | 71 (8.2%) | 10 (10%) | 8 (15.1%) | 18 (11.8%) | ||
All data are expressed in frequency (ratio) or median (interquartile range)
M increased mesangial cells, E increased intracellular cells in capillaries, S segmental sclerosis, T renal tubular atrophy or interstitial fibrosis, C crescent formation, FPE foot process effacement
P < 0.05 is considered statistically significant
Results of Cox regression analysis in pre-PSM patients
The univariate Cox analysis showed that age, blood pressure (systolic and diastolic), HB, Alb, TC, TG, LDL, UA, Scr, 24 h-UTP, eGFR, blood IgG, complement C4, FPE, E-lesion, and T-lesion were significant influence factors associated with reaching the renal composite endpoint (Table 3). All factors that had a meaningful univariate Cox analysis were included in the multivariate Cox analysis. The results showed that FPE (HR 2.223, 95% CI 1.161–4.258, P = 0.016) and T lesions (T1 HR: 2.807, 95% CI: 1.089–7.233, P = 0.033; T2 HR: 7.009, 95% CI: 2.221–22.114, P = 0.001) were significant independent risk factors for IgAN. The Cox regression equation for the final prognostic factors of IgAN was as follows: h (t) = h0 (t) × exp (− 0.03 × eGFR + 0.799 × FPE + 1.032 × T1 + 1.947 × T2).
Table 3.
Univariate and multivariate Cox regression models for renal composite endpoints before PSM (n = 863)
| Variable | Univariate | Multivariable | ||
|---|---|---|---|---|
| HR (95%Cl) | P value | HR (95%Cl) | P value | |
| Age (years) | 1.031 (1.010–1.053) | 0.004 | ||
| SBP (mmHg) | 1.029 (1.017–1.041) | < 0.001 | ||
| DBP (mmHg) | 1.024 (1.012–1.036) | < 0.001 | ||
| HB (g/L) | 0.975 (0.960–0.990) | 0.001 | ||
| Alb (g/L) | 0.911 (0.881–0.942) | < 0.001 | ||
| TC (mmol/L) | 1.261 (1.117–1.424) | < 0.001 | ||
| TG (mmol/L) | 1.238 (1.082–1.416) | 0.002 | ||
| LDL (mmol/L) | 1.424 (1.681–1.665) | < 0.001 | ||
| UA (umol/L) | 1.009 (1.006–1.012) | < 0.001 | ||
| Scr (umol/L) | 1.019 (1.016–1.022) | < 0.001 | ||
| 24 h-UTP (g/day) | 1.348 (1.230–1.477) | < 0.001 | ||
| eGFR (ml· min−1·1.73m−2) | 0.947 (0.937–0.958) | < 0.001 | 0.970 (0.948–0.993) | 0.009 |
| IgG (g/L) | 0.999 (0.998–1.000) | 0.017 | ||
| C4 (g/L) | 1.039 (1.008–1.070) | 0.013 | ||
| FPE | ||||
| Segmental | Reference | |||
| Extensive | 4.991 (2.877–8.657) | < 0.001 | 2.223 (1.161–4.258) | 0.016 |
| E | ||||
| E0 | Reference | |||
| E1 | 0.398 (0.181–0.857) | 0.022 | ||
| T | ||||
| T0 | Reference | |||
| T1 | 8.113 (3.927–16.763) | < 0.001 | 2.807 (1.089–7.233) | 0.033 |
| T2 | 43.612 (20.846–91.242) | < 0.001 | 7.009 (2.221–22.114) | 0.001 |
SBP systolic blood pressure, DBP diastolic blood pressure, HB hemoglobin, Alb serum albumin, TC cholesterol, TG triglycerides, LDL low-density lipoprotein, UA blood uric acid, Scr serum creatinine, 24h-UTP 24 h urine protein quantification, eGFR estimated glomerular filtration rate, IgG immunoglobulin G, C4 complement C4; E increased number of cells in capillaries, T renal tubular atrophy or interstitial fibrosis, FPE foot process effacement
Pearson’s correlation analysis of FPE in pre-PSM patients
The results of Pearson correlation between FPE and significant factors identified in univariate COX univariate analysis are shown in Table 4. Among these factors, 24h-UTP was moderately and positively correlated with FPE (r = 0.478), whereas Alb was moderately and negatively correlated (r = − 0.432). TC, LDL, IgG, eGFR, and TG were weakly correlated with FPE. The Pearson correlation analysis heatmap showing the relationships between various risk factors and FPE is depicted in Fig. 3.
Table 4.
Pearson correlation analysis between various risk factors and FPE before PSM
| Correlation | Variable | Correlation coefficient |
|---|---|---|
| Moderately correlated | 24h-UTP | 0.478 |
| Alb | − 0.432 | |
| TC | 0.323 | |
| LDL | 0.311 | |
| Weak correlation | IgG | − 0.233 |
| eGFR | − 0.220 | |
| TG | 0.211 | |
| C4 | 0.167 | |
| T | 0.159 | |
| Scr | 0.153 | |
| Extremely weak correlation | DBP | 0.116 |
| UA | 0.099 | |
| SBP | 0.083 | |
| age | 0.070 | |
| HB | 0.037 | |
| E | − 0.014 |
SBP, systolic blood pressure; DBP, diastolic blood pressure; HB, hemoglobin; Alb, serum albumin; TC, cholesterol; TG, triglycerides; LDL, low-density lipoprotein; UA, blood uric acid; Scr, serum creatinine; 24h-UTP, 24-h urine protein quantification; eGFR, estimated glomerular filtration rate; IgG, C4, blood and fluid immune indicators; E. Increased number of cells in capillaries; T. Renal tubular atrophy or interstitial fibrosis
Fig. 3.
Heatmap of Pearson correlation analysis between various risk factors and foot process effacement. Note: Red represents positive correlation, blue represents negative correlation. SBP systolic blood pressure, DBP diastolic blood pressure, HB hemoglobin, Alb serum albumin, TC cholesterol, TG triglycerides, LDL low-density lipoprotein, UA blood uric acid, Scr serum creatinine, 24h-UTP 24 h urine protein quantification, eGFR estimated glomerular filtration rate, IgG C4, blood and fluid immune indicators, E Increased number of cells in capillaries, T Renal tubular atrophy or interstitial fibrosis, FPE foot process effacement
Renal survival analysis
As shown in Fig. 4, the Kaplan–Meier renal survival curves before PSM showed that the prognosis of patients with extensive FPE was significantly worse than that of patients with segmental FPE (Log-rank test P < 0.001).The Kaplan–Meier renal survival curves after PSM showed that the prognosis of patients with extensive FPE was still worse than that of patients with segmental FPE (Log-rank test P = 0.030).
Fig. 4.
Kaplan–Meier analysis of cumulative renal survival rate for segmental and extensive foot process effacement. A Pre-PSM; B Post-PSM
Discussion
There is a group of patients with nephrotic syndrome (NS) in IgAN, and the incidence of IgAN combined with NS is only 5 ~ 14.7% according to previous studies [12]. In Jiang et al.’s study [13], patients with IgAN combined with NS clinically presented with massive proteinuria, hypoproteinemia, etc., and pathologically, in addition to IgA deposition, they had more severe FPE on electron microscopy than other patients with IgAN without NS, and the prognosis of these patients was often poor.
In the present study, 863 patients diagnosed with primary IgAN were analyzed. Men account for about 40% and women account for about 60%. A total of about 7.1% of the patients entered the endpoint events, aligning with the long-term follow-up results reported by Chang et al. [14].We found that 74 renal biopsy electron microscopes showed extensive FPE, whereas the remaining 789 cases showed segmental FPE. These findings suggest a high incidence of foot cell damage in patients with IgAN. This deserves further study.
Podocytes are considered terminally differentiated cells. They have a central cell body that extends long primary protrusions that are terminally divided into foot process that are tightly connected to the glomerular basement membrane (GBM) [15]. The structure of podocytes supports their major physiological functions, the foot process of podocytes interlock with each other to form a special intercellular connection and the slit diaphragm maintains the size and charge selectivity of the glomerular filtration barrier and prevents protein leakage [15]. Characteristic electron microscopy findings of podocyte injury reveal microcystic and pseudocystic changes, vacuolization, the presence of cytoplasmic inclusion bodies, and detachment from GBM [16]. Furthermore, FPE is considered the most distinctive alteration in podocyte damage.
The degree of FPE affects renal function in patients with IgAN via the following possible mechanisms: (1) Extensive FPE leads to a large amount of proteinuria [7]. The progression of proteinuria can cause glomerular sclerosis [17], further decreasing renal functions. (2) The FPE is directly related to glomerulosclerosis, which affects renal function. Previous studies suggest that FPE is an early event of glomerulosclerosis [16, 18]. (3) Kidney disease progresses via the mesangial podocyte axis [19] IgA immune complexes deposit in the mesangium and activate mesangial cell damage. Through a series of cascade reactions, they secrete cells, and inflammatory factors, and activate the complement system [20]. They cause podocyte apoptosis and renal interstitial fibrosis through the mesangial podocyte renal tubular epithelial cell signaling pathway.
Therefore, in our retrospective cohort study, we divided the group into extensive effacement group (FPE ≥ 50%) and segmental effacement group (FPE < 50%) according to the degree of FPE, as suggested by previous literature [10, 11]. Before propensity score matching, all baseline statistical factors were entered into a one-way COX regression for analysis, taking into account the effect of confounders, and then all factors that were significant in the one-way COX regression analysis were included in the multifactorial COX regression analysis. The results showed that in addition to eGFR and T lesions, extensive foot process effacement was an independent risk factor for IgAN prognosis. The effect of FPE on prognosis was observed using the Log-rank test and K-M survival curves, and it was found that the prognostic outcome was worse in the extensive FPE group. To have a robust result and reduce bias, we then performed a 1:2 matching between the two groups using propensity scores for demographics, clinical test results, pathological results, and medication use. After matching, there was no statistical difference in all baseline demographics, clinical test results, pathological results, and medication use between the two groups. And the effect of FPE on prognosis was again observed using the Log-rank test and the K-M survival curve. It was found that the extensive effacement group had a significantly worse prognosis than the segmental effacement group. Both statistical methods suggest that FPE is an independent influence on IgAN prognosis.
Terinte et al. [21] performed a retrospective study at a single center involving 107 patients with IgAN. Through multivariate Cox proportional hazards regression analysis, they reported that foot process lesions are a significant independent risk factor for ESRD in patients with IgAN. They found that the most common podocyte lesion in IgAN was foot process effacement (88%), followed by activation (46%), microvillous transformation (45%) and intracellular droplets (24%). In the patients with IgAN, patients who progressed to ESRD exhibited more frequent podocyte activation and effacement, as well as microvilli in the urinary space. As mentioned by Farzamikia et al. [22], IgAN-associated podocytopathy appears to result from abnormal crosstalk between mesangial cells and podocytes. For example, mesangial cell-derived inflammatory and pro-apoptotic signals, including tumor necrosis factor α (TNFα), interleukin 6 (IL-6) and transforming growth factor-β (TGF-β), induce podocyte dedifferentiation and disruption of intercellular junctions. FPE is primarily attributed to impaired formation of the slit diaphragm complex [23], abnormalities in the GBM or adhesion between podocytes and GBM, abnormalities in actin cytoskeleton and associated proteins [24], and changes in the apical membrane domain of podocytes [25].
Previous studies reported that FPE can be divided into two stages [24, 26]. The first stage is characterized by the retraction of the foot processes and the loss or displacement of special adhesion between the foot processes of the foot cells, known as the slit diaphragm, which is replaced by a connection similar to an occlusion. In the second stage, the cytoskeletons are rearranged, and the foot processes completely retract, fuse, and detach, exposing space for GBM. If not repaired properly, this can lead to the formation of glomerular scars. Rangel et al [27]. showed that early FPE allows partial repair by kidney-derived c-kit + -activated autophagy and α -actin-4 regulation. However, severe FPE is usually associated with chronic pathological changes such as glomerulosclerosis in IgAN [28]. This indicates that the disease is difficult to reverse. Additionally, after the effacement of several foot processes, the local shear stress increases, which aggravates the loss of foot cells and increases the mechanical stress of the remaining foot cells [12]. The large amount of proteinuria due to mechanical stress and FPE can prevent the differentiation of resident foot progenitor cells into foot cells, leading to ineffective foot cell regeneration and glomerular scar formation. Based on the results of this study, we speculate that when widespread FPE occurs, the repair ability of foot cells is limited, and FPE is difficult to reverse, leading to a poorer IgAN prognosis.
Unnersjö-Jess et al. [29] found that IgAN, FSGS, and Finnish-type nephrotic syndrome can all have significant FPE. They also found a different morphological pattern of pedunculopathies between a genetic model of FSGS and a model of inflammatory glomerulonephritis, suggesting that the view of FPE as a morphologically homogeneous process may need to be modified. Effacement patterns may vary depending on the underlying pathological mechanisms and treatment modalities. These findings will not only have implications for basic science research, but may also help improve clinical diagnosis.
We did a correlation analysis between FPE and each index. The results suggest a positive correlation between FPE and proteinuria. Proteinuria is recognized as an independent risk factor for chronic kidney disease. And a very important cause of proteinuria is podocytosis [12]. In addition, FPE was negatively correlated with eGFR, suggesting that extensive foot process effacement is more prominent in the IgAN population with poorer renal function. As to whether FPE is a natural consequence of chronic renal failure or whether FPE leads to the progression of renal function, such a causal relationship cannot be conclusively determined in this study. However, it is at least certain in this study that FPE can be used as a prognostic indicator of chronic renal failure [21]. As for the mechanism, further studies are needed.
In terms of podocyte observation, transmission electron microscope (TEM) can capture the ultrastructure of podocyte foot process. It is also the main clinical way of looking at FPE today [21]. However, we cannot completely avoid glomerular distortion during the production of tissue sections [30]. Therefore, each patient in this study had at least 1 intact glomerulus under TEM. Approximately 5–8 capillary rings in 1 intact glomerulus. We will evaluate the mean percentage of FPE for all capillary rings in each glomerulus to mitigate data bias due to ultrathin sections. Other observation methods of podocytes also have their advantages and disadvantages. For example, the advantages of WT-1 staining include WT-1 high expression in podocytes, high specificity, sensitivity, ability to distinguish podocytes from cells in other glomeruli, and WT-1 can identify diseased podocytes at the level of gene expression, which can be used to detect the early progression of podocyte injury [9, 31]. The disadvantage is that WT-1 staining only marks the number and distribution of podocytes, but cannot help distinguish the functional status of podocyte morphology, such as the FPE situation, and FPE observation using WT-1 staining should also be combined with other detection techniques [31].
The present study has some limitations that should be addressed. (1) Retrospective and observational studies cannot completely evaluate residual confounding factors. (2) This study was single-center and the sample size was not large enough, reducing the applicability of the findings to the wider IgAN population. (3) The follow-up time of this study was not sufficiently long, which may affect the assessment of long-term prognosis in patients with IgAN. (4) Due to limitations in renal biopsy techniques, pathological changes and biomarkers of other foot cell injuries were not observed and analyzed, except for the degree of FPE. Therefore, subsequent multicenter studies with larger samples and longer follow-up times are needed to further confirm the research findings.
To summarize, we confirmed that IgAN is often accompanied by varying degrees of FPE. Furthermore, extensive FPE (FPE ≥ 50%) is an independent risk factor for adverse outcomes of IgAN.
Acknowledgements
This study was funded by the National Natural Science Foundation of China (82205008); Hangzhou Municipal Health Commission Project (A20210083); and Project of Zhejiang Provincial Administration of Traditional Chinese Medicine (2023ZF137); Key Research Project of Zhejiang University of Traditional Chinese Medicine Affiliated Hospital (2022FSYZZ14), and supported by the Zhejiang Provincial Department of Health Project (2023RC242). The author would like to thank the Nephrology Laboratory of Hangzhou Hospital of Traditional Chinese Medicine, affiliated with Zhejiang University of Chinese Medicine, for their support in this study.
Author contributions
Li Gao independently wrote the article and wrote the data analysis, Qin Zhu led the study design and proofread all drafts, Xuan Zhang assisted in data collection, Dongrong Yu gave the first guidance for the paper, and Hong Zhu conducted the objective review of the article. All authors have contributed to the further revision of this article.
Data availability
The experimental data that support the findings of this study are available in Figshare with the identifier https://doi.org/10.6084/m9.figshare.25801555.
Declarations
Conflict of interest
The authors have no conflicting financial interests.
Footnotes
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
References
- 1.Jia Q, Ma F, Yang X et al (2022) Long-term outcomes of IgA nephropathy patients with less than 25% crescents and mild proteinuria. Clin Exp Nephrol 26(3):257–265. 10.1007/s10157-021-02154-0 [DOI] [PubMed] [Google Scholar]
- 2.Suzuki H, Novak J (2021) IgA glycosylation and immune complex formation in IgAN. Semin Immunopathol 43(5):669–678. 10.1007/s00281-021-00883-8 [DOI] [PubMed] [Google Scholar]
- 3.Menon MC, Chuang PY, He JC (2013) Role of podocyte injury in IgA nephropathy. Contrib Nephrol 181:41–51. 10.1159/000348461 [DOI] [PubMed] [Google Scholar]
- 4.Suh KS, Choi SY, Bae GE et al (2019) Concurrent anti-glomerular basement membrane nephritis and IgA nephropathy. J Pathol Transl Med 53(6):399–402. 10.4132/jptm.2019.08.05 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 5.Matsusaka T, Sandgren E, Shintani A et al (2011) Podocyte injury damages other podocytes. J Am Soc Nephrol JASN. 22(7):1275–1285. 10.1681/ASN.2010090963 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Sachs M, Wetzel S, Reichelt J et al (2021) ADAM10-mediated ectodomain shedding is an essential driver of podocyte damage. J Am Soc Nephrol 32(6):1389–1408. 10.1681/ASN.2020081213 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Lemley KV, Lafayette RA, Safai M et al (2002) Podocytopenia and disease severity in IgA nephropathy. Kidney Int 61(4):1475–1485. 10.1046/j.1523-1755.2002.00269.x [DOI] [PubMed] [Google Scholar]
- 8.Levey AS, de Jong PE, Coresh J et al (2011) The definition, classification, and prognosis of chronic kidney disease: a KDIGO Controversies Conference report [published correction appears in Kidney Int. 2011 Nov; 80(9):1000] [published correction appears in Kidney Int. 2011 Nov 1;80(9):1000]. Kidney Int 80(1):17–28. 10.1038/ki.2010.483 [DOI] [PubMed] [Google Scholar]
- 9.Gundersen HJ, Seefeldt T, Osterby R (1980) Glomerular epithelial foot processes in normal man and rats. Distribution of true width and its intra- and inter-individual variation. Cell Tissue Res 205(1):147–155. 10.1007/BF00234450 [DOI] [PubMed] [Google Scholar]
- 10.Gaughan WJ, Sheth VR, Francos GC, Michael HJ, Burke JF (1993) Ranitidine-induced acute interstitial nephritis with epithelial cell foot process fusion. Am J Kidney Dis 22(2):337–340. 10.1016/s0272-6386(12)70328-1 [DOI] [PubMed] [Google Scholar]
- 11.Zhao Y, Liu L, Huang J et al (2015) Plasma Soluble Urokinase Receptor Level Is Correlated with Podocytes Damage in Patients with IgA Nephropathy. PLoS One. 10(7):e0132869. 10.1371/journal.pone.0132869 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 12.Kopp JB, Anders HJ, Susztak K, Podestà MA, Remuzzi G, Hildebrandt F, Romagnani P (2020) Podocytopathies nature reviews. Disease Primers 6(1):68. 10.1038/s41572-020-0196-7 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 13.Kim JK, Kim JH, Lee SC, Kang EW, Chang TI, Moon SJ, Yoon SY, Yoo TH, Kang SW, Choi KH, Han DS, Kie JH, Lim BJ, Jeong HJ, Han SH (2012) Clinical features and outcomes of IgA nephropathy with nephrotic syndrome. Clin J Am Soc Nephrol CJASN 7(3):427–436. 10.2215/CJN.04820511 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 14.Chang D, Cheng Y, Luo R, Zhang C, Zuo M, Xu Y, Dai W, Li Y, Han M, He X, Ge S, Xu G (2021) The prognostic value of platelet-to-lymphocyte ratio on the long-term renal survival in patients with IgA nephropathy. Int Urol Nephrol 53(3):523–530. 10.1007/s11255-020-02651-3 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 15.Shankland SJ (2006) The podocyte’s response to injury: role in proteinuria and glomerulosclerosis. Kidney Int 69(12):2131–2147. 10.1038/sj.ki.5000410 [DOI] [PubMed] [Google Scholar]
- 16.Lu CC, Wang GH, Lu J et al (2019) Role of podocyte injury in glomerulosclerosis. Adv Exp Med Biol 1165:195–232. 10.1007/978-981-13-8871-2_10 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 17.Xu L, Yang HC, Hao CM et al (2010) Podocyte number predicts progression of proteinuria in IgA nephropathy. Mod Pathol 23(9):1241–1250. 10.1038/modpathol.2010.110 [DOI] [PubMed] [Google Scholar]
- 18.Kim JS, Han BG, Choi SO et al (2016) Secondary focal segmental glomerulosclerosis: from podocyte injury to glomerulosclerosis. Biomed Res Int 2016:1630365. 10.1155/2016/1630365 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 19.Lai KN, Tang SC, Leung JC (2011) Recent advances in IgA nephropathy–the glomerulopodocytic-tubular communication. Adv Otorhinolaryngol 72:40–44. 10.1159/000324593 [DOI] [PubMed] [Google Scholar]
- 20.Maillard N, Wyatt RJ, Julian BA et al (2015) Current understanding of the role of complement in IgA nephropathy. J Am Soc Nephrol 26(7):1503–1512. 10.1681/ASN.2014101000 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 21.Terinte-Balcan G, Stancu S, Zugravu A et al (2023) Prognostic role of glomerular electron microscopy lesions in IgA nephropathy: “the devil is in the details.” J Nephrol 36(8):2233–2243. 10.1007/s40620-023-01744-3 [DOI] [PubMed] [Google Scholar]
- 22.Farzamikia N, Baradaran B, Mostafavi S, Ahmadian E, Hosseiniyan Khatibi SM, Zununi Vahed S, Ardalan M (2021) Podocyte-derived microparticles in IgA nephropathy. Biom Pharmacother Biomed Pharmacother 141:111891. 10.1016/j.biopha.2021.111891 [DOI] [PubMed] [Google Scholar]
- 23.Barisoni L, Mundel P (2003) Podocyte biology and the emerging understanding of podocyte diseases. Am J Nephrol 23(5):353–360. 10.1159/000072917 [DOI] [PubMed] [Google Scholar]
- 24.Kriz W, Shirato I, Nagata M et al (2013) The podocyte’s response to stress: the enigma of foot process effacement. Am J Physiol Renal Physiol 304(4):F333–F347. 10.1152/ajprenal.00478.2012 [DOI] [PubMed] [Google Scholar]
- 25.Nagata M (2016) Podocyte injury and its consequences. Kidney Int 89(6):1221–1230. 10.1016/j.kint.2016.01.012 [DOI] [PubMed] [Google Scholar]
- 26.Cellesi F, Li M, Rastaldi MP (2015) Podocyte injury and repair mechanisms. Curr Opin Nephrol Hypertens 24(3):239–244. 10.1097/MNH.0000000000000124 [DOI] [PubMed] [Google Scholar]
- 27.Rangel EB, Gomes SA, Kanashiro-Takeuchi R et al (2018) Kidney-derived c-kit+ progenitor/stem cells contribute to podocyte recovery in a model of acute proteinuria. Sci Rep 8(1):14723. 10.1038/s41598-018-33082-x [DOI] [PMC free article] [PubMed] [Google Scholar]
- 28.Bosquetti B, Santana AA, Gregório PC et al (2023) The role of α3β1 integrin modulation on fabry disease podocyte injury and kidney impairment. Toxins (Basel) 15(12):700. 10.3390/toxins15120700 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 29.Unnersjö-Jess D, Butt L, Höhne M, Sergei G, Fatehi A, Witasp A, Wernerson A, Patrakka J, Hoyer PF, Blom H, Schermer B, Bozek K, Benzing T (2023) Deep learning-based segmentation and quantification of podocyte foot process morphology suggests differential patterns of foot process effacement across kidney pathologies. Kidney Int 103(6):1120–1130. 10.1016/j.kint.2023.03.013 [DOI] [PubMed] [Google Scholar]
- 30.Liu XJ, Zhang YM, Wang SX, Liu G (2014) Ultrastructural changes of podocyte foot processes during the remission phase of minimal change disease of human kidney. Nephrology (Carlton, Vic.) 19(7):392–397. 10.1111/nep.12256 [DOI] [PubMed] [Google Scholar]
- 31.Terinte-Balcan G, Stancu S, Zugravu A et al (2023) Prognostic role of glomerular electron microscopy lesions in IgA nephropathy: “the devil is in the details.” J Nephrol 36(8):2233–2243. 10.1007/s40620-023-01744-3 [DOI] [PubMed] [Google Scholar]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The experimental data that support the findings of this study are available in Figshare with the identifier https://doi.org/10.6084/m9.figshare.25801555.