Summary
Lupus nephritis (LN) is one of the most serious manifestations of systemic lupus erythematosus. Invasive renal biopsy remains the gold standard for the diagnosis and management of LN. The objective of this study is to validate serum insulin‐like growth factor binding protein‐2 (IGFBP‐2) as a novel biomarker for clinical disease and renal pathology in LN. Eighty‐five biopsy‐proven lupus nephritis patients, 18 chronic kidney disease (CKD) patients and 20 healthy controls were recruited for enzyme‐linked immunosorbent assay (ELISA) testing of serum IGFBP‐2 levels. Compared to CKD patients of origins other than lupus or healthy controls, serum IGFBP‐2 levels were elevated significantly in LN patients. Serum IGFBP‐2 was able to discriminate LN patients from the other two groups of patients [area under the curve (AUC) = 0·65, 95% confidence interval (CI) = 0·52–0·78; P = 0·043 for LN versus CKD; 0·97, 95% CI = 0·93–1·00; P < 0·0001 for LN versus healthy controls]. Serum IGFBP‐2 was a potential indicator of both global disease activity and renal disease activity in LN patients, correlated with serum creatinine levels (r = 0·658, P < 0·001, n = 85) and urine protein‐to‐creatinine levels (r = 0·397, P < 0·001, n = 85). More importantly, in 19 concurrent patient samples, serum IGFBP‐2 correlated with the chronicity index of renal pathology (r = 0·576, P = 0·01, n = 19) but not renal pathological classification. In conclusion, serum IGFBP‐2 is a promising biomarker for lupus nephritis, reflective of disease activity and chronicity changes in renal pathology.
Keywords: biomarkers, IGFBP2, lupus nephritis, pathology
Introduction
Systemic lupus erythematosus (SLE) is a chronic autoimmune disease with multi‐organ involvement, among which kidney is one of the most commonly affected organs. Approximately 35% of adults show signs of lupus nephritis at the time of SLE diagnosis and 50–60% will develop lupus nephritis (LN) during the first 10 years of disease 1, 2. LN remains the major cause of morbidity and mortality in SLE patients, either as a result of renal failure or secondary to the side effect of aggressive immunosuppressive therapies 1, 3, 4, 5, 6. Hence, early diagnosis and management of LN is of tremendous importance.
Current guidelines for LN diagnosis and management depend largely upon renal pathology, which requires renal biopsy 7, 8. Although renal biopsy remains the gold standard for the diagnosis and management of LN, it has several disadvantages. Renal biopsy is invasive, with complications such as bleeding and infection. It is also not feasible to perform renal biopsies repeatedly or serially. Last, but not least, renal biopsy reflects only existing pathology, but cannot predict imminent renal flare in LN patients. Given that LN has an unpredictable disease course, the lack of reliable markers that can predict renal flares precludes the development of preventive strategies for disease relapses 9. Conventional biomarkers for LN, including anti‐double‐stranded DNA antibodies (dsDNA) and complement components 3 and 4 (C3, C4), are neither sensitive nor specific in reflecting concurrent renal activity or predicting impending renal flare 10, 11, 12. Therefore, it is important to identify biomarkers that have high specificity for the early diagnosis of LN, can reflect renal activity in follow‐up monitoring and are predictive of renal pathology and impending renal flare.
High‐throughput proteomics‐based approaches have provided an efficient screening approach for protein biomarker candidates worthy of further validation. Using multiplexed‐targeted proteomic slide array based screens and enzyme‐linked immunosorbent assays (ELISA), several novel serum and urine biomarkers have been discovered and validated in small groups of SLE patients 13, 14, 15, 16, 17. However, most of the studies have focused on identifying biomarkers that can reflect disease activity. Fewer studies have pursued biomarkers predicting renal pathology. Our preliminary proteomic study using a commercially available antibody‐coated microarray screen revealed increased levels of insulin‐like growth factor binding protein‐2 (IGFBP‐2) in African American or Hispanic LN patients (Wu et al., manuscript submitted). In this study, we validate serum IGFBP‐2 further as a new candidate biomarker for predicting concurrent disease activity and renal pathology in a large cohort of LN patients.
IGFBP‐2 belongs to the IGFBPs family, which binds insulin‐like growth factors (IGFs) with high affinity. It is the second most abundant IGFBP found in serum, and has been found to be a robust diagnostic and prognostic biomarker for several malignant tumours 18. IGFBP‐2 has also been reported to be increased in nephrotic syndrome 19 and to be a predictor of longitudinal deterioration of renal function in type 2 diabetes 20.
Patients and methods
Study subjects
This is a cross‐sectional study, from 2007 to 2011, into which a total of 85 biopsy‐proven lupus nephritis patients were enrolled from renal clinics at Parkland Hospital and St Paul University Hospital. Both were affiliated with the University of Texas Southwestern Medical Center at Dallas. Inclusion criteria were as follows: (a) age 18 years and older; (b) fulfilling the 1982 revised American College of Rheumatology (ACR) classification criteria for SLE 21; and (c) LN confirmed by renal biopsy. Patients with renal failure were excluded. Age‐ and gender‐matched healthy volunteers were enrolled as healthy controls (n = 20). Eighteen patients with chronic kidney diseases (CKD) due to causes other than lupus nephritis were also enrolled as disease controls. LN was diagnosed at the time of kidney biopsy (duration: 2·3 ± 0·3 years). At the time of diagnosis, the diagnosis of SLE in many patients was of lupus nephritis and in others several years/months preceding the diagnosis of lupus nephritis (duration: 5·9 ± 0·8 years). Diabetes was diagnosed at the time of kidney biopsy (duration: 2·8 ± 0·8 years). Informed consent was obtained from all participants before entering the study. This study was reviewed and approved by the ethics committee of the University of Texas Southwestern Medical Center and conducted in accordance with good clinical practice.
Disease activity and laboratory tests
Global disease activity was assessed by SLE Disease Activity Index (SLEDAI), as described by Bombardier et al. 22. The calculation of SLEDAI was based upon patients’ notes review as well as laboratory tests at the time of sample collection. The definition for disease activity was as described elsewhere 23. In this study, we divided the patients into two groups: low disease activity group (SLEDAI <= 5) and high disease activity group (SLEDAI > 5). Renal SLEDAI (rSLEDAI), which refers to the total score of the four kidney‐related parameters (i.e. haematuria, pyuria, proteinuria and urinary casts), was used to assess renal disease activity. In this study, we considered rSLEDAI = 0 as inactive LN and rSLEDAI > 0 as active LN.
Laboratory tests recorded in this study included urinalysis, serum creatinine level, urine protein‐to‐creatinine ratio, serum C3 and C4 levels (categorized as low or normal) and the levels of serum anti‐nuclear antibodies (ANA) and anti‐dsDNA) antibodies (categorized as positive or negative). A random urine protein‐to‐creatinine ratio was used to estimate 24‐h urine protein excretion rate 24.
Renal pathology
Renal biopsies were reviewed and classified by an experienced renal pathologist, using the 2004 International Society of Nephrology/Renal Pathological Society (ISN/RPS) classification 25: class I (minimal mesangial LN), class II (mesangial proliferative LN), class III (focal LN), class IV (diffuse LN), class V (membranous LN) and class VI (advanced sclerosing LN).
In patients with proliferative LN (i.e. ISN/RPS classes III or IV), the activity index and chronicity index of renal pathology were calculated as described 26. The activity index is based on scoring of glomerular endocapillary proliferation, glomerular neutrophilic infiltration, wire‐loop/hyaline thrombi, fibrinoid necrosis/karyorrhexis, cellular crescents and interstitial inflammation. Each component was scored on a scale of 0–3, except for fibrinoid necrosis and crescents, which were weighted twice (maximum 24). The chronicity index is based on scoring (0–3) of glomerular sclerosis, fibrous crescents, tubular atrophy and interstitial fibrosis (maximum score of 12).
In this study, we regarded serum samples that were obtained at the time of or within 3 months of the kidney biopsy as concurrent samples.
Measurement of serum IGFBP‐2 levels
Whole blood was collected from each patient into BD Vacutainer tubes without any additives (Cat. no.: 367812). After 20 min of incubation at room temperature, the tubes were centrifuged for 10 min at 1000 g. The supernatant was separated carefully and aliquoted into 200 μl and stored at −80°C until use. In order to avoid protein degradation from multiple freeze–thaw cycles, each aliquot was retrieved and thawed only once for assays in this study.
Serum levels of IGFBP‐2 were measured using the human IGFBP‐2 ELISA kit (Cat. no.: DY674) from R&D Systems (Minneapolis, MN, USA), according to the manufacturer's manual. All serum samples were diluted 1 : 500. The optical density at 450 nm wavelength was measured using a microplate reader ELX808 from BioTek Instruments (Winooski, VT, USA) and IGFBP‐2 concentration was then calculated according to the standard curve.
All measurements were made in duplicate in a blinded manner. Serum IGFBP‐2 levels were expressed as ng/ml.
Statistical analysis
Continuous variables are expressed as mean ± standard error of the mean (s.e.m.). Kolmogorov–Smirnov and Shapiro–Wilk tests were used for establishing normality of data. Comparison between two groups was performed using a Student's t‐test where the data were distributed normally. Otherwise, log‐transformed data or the non‐parametric Mann–Whitney U‐test were used. One‐way analysis of variance (anova) was used for comparison of three or more groups. Pearson's method was used for correlation analysis in continuous and normally distributed data. Otherwise, the non‐parametric Spearman's method was used. A two‐tailed value of P < 0·05 was considered statistically significant. All statistical analysis was performed using spss version 20·0 (IBM Corporation, Armonk, NY, USA) and data were plotted using GraphPad Prism version 5·0 (GraphPad, San Diego, CA, USA).
Results
Increased serum IGFBP‐2 levels in lupus nephritis patients
A total of 85 LN patients, 18 CKD controls and 20 healthy controls were enrolled into this cross‐sectional study. In LN patients, 73 (86%) were female and the average age was 34·9 ± 1·2 years. Most LN patients were African American (39%) or Hispanic (49%). Detailed demographic and clinical characteristics are described in Table 1, Table 2 and Table 3. Among the CKD controls, the most common cause for CKD was diabetic nephropathy (11 of 18). Other causes included membranous nephropathy (three of 18), anti‐neutrophil cytoplasmic antibodies (ANCA)‐associated glomerulonephritis (two of 18), focal segmental glomerulosclerosis (one of 18) and minimal change disease (one of 18).
Table 1.
Demographics and clinical characteristics of lupus nephritis patients.
| n (%) | |
|---|---|
| (N = 85) | |
| Demographic features | |
| Age* (years) | 34·9 ± 1·2 |
| Female | 73 (86) |
| Asian | 3 (4) |
| African American | 33 (39) |
| Hispanic | 42 (49) |
| Caucasian | 7 (8) |
| Disease activity | |
| SLEDAI median (interquartile) | 9 (4–14) |
| rSLEDAI median (interquartile) | 4 (4–8) |
| Renal pathology (ISN/RPS classification) | |
| Type I | 0 (0) |
| Type II | 11 (13) |
| Type III/III+V | 22 (26) |
| Type IV/IV+V | 37 (44) |
| Type V | 15 (17) |
| Type VI | 0 (0) |
| Laboratory tests | |
| Protein‐to‐creatinine ratio* (mg/mg) | 1·51 ± 0·21 |
| Serum creatinine* (mg/dl) | 1·52 ± 0·15 |
| Positive ANA/total tested | 39/46 (85) |
| Positive anti‐dsDNA/total tested | 33/70 (47) |
| Hypocomplementaemia/total tested | 46/67 (69) |
| BMI | 29·5 ± 1·2 |
| Total cholesterol (mg/dl) | 183·1 ± 1·2 |
| Triglycerides (mg/dl) | 179·9 ± 15·6 |
| LDL (mg/dl) | 105·3 ± 6·6 |
| Comorbidity Hypertension | 17 (20) |
| Diabetes mellitus | 2 (2) |
| Hyperlipidaemia | 13 (15) |
| Hypothyroidism | 2 (2) |
| Pulmonary embolism | 2 (2) |
| Current medications | |
| Prednisone | 61 (72) |
| Cyclophosphamide | 8 (9) |
| Mycophenolate mofetil | 27 (32) |
| Azathioprine | 9 (11) |
| Methotrexate | 2 (2) |
| Cyclosporin/tacrolimus | 2 (2) |
| Hydrochloroquine | 39 (46) |
| ACE inhibitors/ARB | 41 (48) |
Mean ± standard error of the mean. SLEDAI = Systemic Lupus Erythematosus Disease Activity Index; ANA = anti‐nuclear antibodies; dsDNA = double‐stranded DNA; ACE = angiotensin converting enzyme; ARB = angiotensin receptor blocker; LDL = low‐density lipoprotein; BMI = body mass index; ISN/RPS = International Society of Nephrology/Renal Pathological Society.
Table 2.
Demographics and clinical characteristics of chronic kidney disease patients
| n (%) | |
|---|---|
| (N = 18) | |
| Demographic features | |
| Age* (years) | 51·3 ± 3·7 |
| Female | 5 (28) |
| Asian | 1 (6) |
| African American | 4 (22) |
| Hispanic | 7 (39) |
| Caucasian | 6 (33) |
| Renal pathology | |
| Diabetic nephropathy | 11 (61) |
| Focal segmental glomerulosclerosis | 1 (5) |
| ANCA‐associated GN | 2 (11) |
| Membranous nephropathy | 2 (11) |
| MN+FSGS | 1 (6) |
| Minimal change disease | 1 (6) |
| Laboratory tests | |
| Protein‐to‐creatinine ratio* (mg/mg) | 2·92 ± 1·13 |
| Serum creatinine* (mg/dl) | 1·68 ± 0·22 |
Mean ± standard error of the mean. ANCA‐associated GN = anti‐neutrophil cytoplasmic antibody‐associated glomerulonephritis; MN = membranous nephropathy; FSGS = focal segmental glomerulosclerosis.
Table 3.
Demographics and clinical characteristics of healthy controls
| n (%) | |
|---|---|
| (N = 20) | |
| Demographic features | |
| Age* (years) | 35·3 ± 1·9 |
| Female | 14 (70) |
| Asian | 1 (5) |
| African American | 7 (35) |
| Hispanic | 10 (50) |
| Caucasian | 2 (10) |
Mean ± standard error of the mean.
In this cohort of LN patients, serum IGFBP‐2 levels (380·8 ± 37·4 ng/ml) were increased significantly when compared to those of CKD disease controls (213·6 ± 43·9 ng/ml, P = 0·013) and healthy controls (42·1 ± 6·8 ng/ml, P < 0·0001) (Fig. 1a). A significant difference between CKD patients and healthy controls was also noted (P < 0·0001) (Fig. 1a), indicating that serum IGFBP‐2 levels could be indicative of chronic kidney disease arising from other causes.
Figure 1.
Serum insulin‐like growth factor binding protein‐2 (IGFBP‐2) levels were elevated in patients with lupus nephritis (LN). (a) Serum IGFBP‐2 levels in LN patients (380·8 ± 37·4 ng/ml) were increased significantly when compared to those of chronic kidney disease (CKD) controls (213·6 ± 43·9 ng/ml, P = 0·013) and healthy controls (HC) (42·1 ± 6·8 ng/ml, P < 0·0001); (b) receiver operating characteristic (ROC) curve analysis. The area under the curve (AUC) value was 0·65 [95% confidence interval (CI) = 0·52–0·78; P = 0·043] for LN versus CKD and 0·97 (95% CI = 0·93–1·00; P < 0·0001) for LN versus HC.
In order to evaluate the diagnostic value of IGFBP‐2 in discriminating LN patients from other CKD patients or healthy controls, we further performed receiver operating characteristic (ROC) curve analysis. The area under the curve (AUC) value was 0·65 [95% confidence interval (CI) = 0·52–0·78; P = 0·043] for LN versus CKD and 0·97 (95% CI = 0·93–1·00; P < 0·0001) for LN versus healthy controls (Fig. 1b). In the ROC analysis, we found the maximum AUC values for LN versus healthy controls were 0·742 and 0·841 for anti‐dsDNA and complement C3, respectively (assuming all negative for healthy controls). However, when we combined IGFBP2, anti‐dsDNA and complement C3 together as a composite marker using logistic regression analysis, the AUC value was increased to 0·986. All these data indicate that IGFBP2 could indeed add diagnostic values significantly to current yardsticks.
Relationship of serum IGFBP‐2 to disease activity
After determining that serum IGFBP‐2 levels were increased significantly in LN patients, we explored whether serum IGFBP‐2 levels can reflect SLE disease activity and immunological characteristics. Patients in two disease activity groups were examined: low disease activity (SLEDAI <= 5, n = 31) and high disease activity (SLEDAI > 5, n = 54). A significant difference in serum IGFBP‐2 levels was observed between the low disease and high disease activity groups (263·7 ± 41·7 ng/ml versus 448·1 ± 51·9 ng/ml; P = 0·009). Serum IGFBP‐2 levels in both groups were significantly higher than those of healthy controls (Fig. 2a). To assess further the relationship between serum IGFBP‐2 levels and renal disease activity in LN patients, we subdivided the LN patients into inactive LN (renal SLEDAI = 0, n = 19) and active LN (renal SLEDAI > 0, n = 66) groups according to renal SLEDAI score. Serum IGFBP‐2 levels were increased significantly in active LN patients compared to inactive LN patients (429·1 ± 44·5 ng/ml versus 213·3 ± 49·1 ng/ml; P = 0·007). However, we did not find any significant difference in IGFBP‐2 levels between inactive LN patients and CKD controls (P = 0·81) (Fig. 2b).
Figure 2.
Correlation between serum insulin‐like growth factor binding protein‐2 (IGFBP‐2) with disease activity. (a) Serum IGFBP‐2 levels were elevated significantly in the high global disease activity group (448·1 ± 51·9 ng/ml) compared to the low global disease activity group (263·7 ± 41·7 ng/ml). Both were increased significantly when compared to healthy controls (HC). (b) Serum IGFBP‐2 levels were increased significantly in active lupus nephritis (LN) patients compared to inactive LN patients (429·1 ± 44·5 ng/ml versus 213·3 ± 49·1 ng/ml; P = 0·007). However, there was no significant difference in IGFBP‐2 levels between inactive LN patients and chronic kidney disease (CKD) controls (P = 0·81). (c) Correlation analysis showed a significant correlation between serum IGFBP‐2 levels and Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) score (r = 0·379, P < 0·0001, n = 85); (d) correlation analysis showed a significant correlation between serum IGFBP‐2 levels and rSLEDAI score (r = 0·409, P < 0·0001, n = 85).
As could have been predicted, correlation analysis showed a significant correlation between serum IGFBP‐2 levels and SLEDAI score (r = 0·379, P < 0·0001, n = 85) (Fig· 2c) as well as rSLEDAI score (r = 0·409, P < 0·0001, n = 85) (Fig. 2d).
Relationship of serum IGFBP‐2 levels to clinical parameters
We next analysed the relationship between serum IGFBP‐2 levels and clinical and laboratory parameters. There was a significant difference in serum IGFBP‐2 levels between anti‐dsDNA‐positive and anti‐dsDNA‐negative patients (526·7 ± 76·7 ng/ml versus 262·6 ± 38·4 ng/ml; P = 0·012) (Fig. 3a). Serum IGFBP‐2 levels also showed a difference in patients with different complement status. Patients with low complement levels had higher serum IGFBP‐2 concentrations than patients with normal complement levels (465·1 ± 61·0 ng/ml versus 215·1 ± 32·8 ng/ml; P = 0·027) (Fig. 3b). Low complement levels were defined as decreased serum C3 or C4 concentrations. However, no significant differences in serum IGFBP‐2 levels were observed in patients with different ANA status (Fig. 3c).
Figure 3.
Correlation between serum insulin‐like growth factor binding protein‐2 (IGFBP‐2) with clinical parameters. (a) A significant increase in serum IGFBP‐2 levels was observed between anti‐double‐stranded DNA (dsDNA)‐positive and anti‐dsDNA‐negative patients (526·7 ± 76·7 ng/ml versus 262·6 ± 38·4 ng/ml; P = 0·012); (b) significant difference in serum IGFBP‐2 levels between patients with low and normal complement levels (465·1 ± 61·0 ng/ml versus 215·1 ± 32·8 ng/ml; P = 0·027); (c) no significant difference in serum IGFBP‐2 levels in patients with different anti‐nuclear antibodies (ANA) status. (c–e) Serum IGFBP‐2 levels were correlated positively with serum creatinine levels (r = 0·658, P < 0·001, n = 85), urine protein‐to‐creatinine levels (r = 0·397, P < 0·001, n = 85).
Serum IGFBP‐2 levels were correlated positively with serum creatinine levels (r = 0·658, P < 0·001, n = 85) (Fig. 3d) and urine protein‐to‐creatinine ratios (r = 0·397, P < 0·001, n = 85) (Fig. 3e), indicating a relationship between serum IGFBP‐2 and renal function.
Relationship of serum IGFBP‐2 to concurrent renal pathology
In this study, we had access to a unique set of a concurrent serum/renal biopsy cohort in which the patients’ serum sampling time was within 3 months of the renal biopsy date. A total of 36 patient samples met the criteria of being concurrent sample. Among the 36 patient samples, three had type II LN, seven had types III/III+V LN, 19 had types IV/IV+V LN and seven had type V LN. We used this set of samples to explore whether IGFBP‐2 reflected concurrent renal pathology classification and activity index/chronicity index in lupus patients. Within the 36 renal biopsy samples, 19 samples had also been scored for their activity index and chronicity index (activity index range = 2–18; chronicity index range = 0–9).
We first compared the differences of serum IGFBP‐2 levels in patients with different renal pathology GN classes, which demonstrated no significant differences between the GN classes (data not shown). However, in the 19 renal biopsy samples assessed for activity index and chronicity index in renal pathology, serum IGFBP‐2 levels did not correlate with activity index (r = 0·273, P = 0·25, n = 19, Fig· 4a) but correlated with chronicity index (r = 0·576, P = 0·01, n = 19, Fig. 4b).
Figure 4.
Correlation between serum insulin‐like growth factor binding protein‐2 (IGFBP‐2) levels with renal pathology in patients with lupus nephritis. (a) Serum IGFBP‐2 levels did not correlate with renal pathology activity index (r = 0·273, P = 0·25, n = 19); (b) Serum IGFBP‐2 levels correlated with renal pathology chronicity index (r = 0·576, P = 0·01, n = 19). For this analysis, the serum was obtained concurrent to the renal biopsy.
Discussion
Despite intensive immunosuppressive therapy, current management of LN remains unsatisfactory in terms of both remission rates and immunosuppressant tolerance 3, 10. In order to improve the outcome of LN, it is crucial to identify biomarkers that can reflect early renal involvement in SLE. During the past few years, several novel biomarkers have been identified as promising candidates for LN 13, 14, 15, 16, 17, 27, 28, 29, 30, 31, 32, 33.
In this study, we have demonstrated that the levels of serum IGFBP‐2 were elevated in LN patients compared to CKD patients and healthy controls. IGFBP2 had high sensitivity and specificity in discriminating LN patients from other CKD patients and the healthy population. Our data also showed that serum IGFBP‐2 was a good indicator of both global disease activity and renal disease activity in LN patients, with correlations with serological (anti‐dsDNA antibody titres and complement levels) and renal function parameters (serum creatinine levels, urine protein‐to‐creatinine levels).
One important finding in this study arises from our unique set of concurrent serum/biopsy samples. This set of samples provided us with the opportunity to explore candidate biomarkers that were associated with concurrent renal pathology and chronicity index, which are important risk factors for poor renal prognosis 34, 35. Our study showed that serum IGFBP‐2 correlated with the chronicity index in renal pathology but did not correlate with the GN class. Two other studies have also reported protein biomarkers correlating with chronicity index in renal pathology, including angiostatin 16, neutrophil gelatinase‐associated lipocalin (NGAL), monocyte chemotactic protein 1 (MCP‐1) 36, tumour necrosis factor‐like weak inducer of apoptosis (TWEAK) 37 and periostin 38. Taken together, one could develop a panel of biomarkers that can be used to predict chronicity changes in renal pathology. As mentioned above, serum IGFBP‐2 also correlated with serum creatinine levels, which is indicative of renal function. Thus, increased serum IGFBP‐2 might be a predictor of longitudinal deterioration of renal function as well as a risk factor for poor prognosis in LN.
IGFBP‐2 is a member of the insulin‐like growth factor binding protein (IGFBP) family and regulates IGF's biological activities through IGF receptors 18. IGFBP‐2 is the second most abundant IGFBP found in serum 39, which has both regulatory activities of IGFs and IGF‐independent activities on metabolism and malignancy 18, 40. IGFBP‐2 is expressed in a wide range of normal tissues, including the glomerulus in both humans and animals 41, 42. Interestingly, animal model studies have demonstrated the increased expression of IGFBP‐2 in the glomerulus in anti‐glomerular basement membrane (GBM) glomerulonephritis in the rat 42 and Murphy Roths Large lymphoproliferation (MRL/lpr) lupus mice [46]. In human immunoglobulin (Ig)A nephropathy, glomerular expression levels of IGFBP‐2 mRNA were increased significantly compared to normal glomeruli 42. Of relevance to our study, IGFBP‐2 has also been reported to be increased in nephrotic syndrome in paediatric patients 19 and as a predictor of longitudinal deterioration of renal function in type 2 diabetes 20. Although the above reports suggest that IGFBP‐2 is a reliable biomarker of renal deterioration, our results still indicate that it had high sensitivity and specificity in discriminating kidney disease caused by SLE from other origins.
Several follow‐up studies are warranted. Because we included only LN patients in this study, we do not know whether the elevated levels of serum IGFBP‐2 is due to end organ involvement or due to systemic autoimmunity. To address this, we need to study systematically SLE patients with or without LN. Another limitation of this study is the limited number of concurrent samples. Ongoing recruitment efforts are aimed at boosting these patient numbers. As it has already been demonstrated that expression levels of IGFBP‐2 mRNA were increased significantly in human IgA nephropathy glomeruli, it is reasonable to hypothesize that urine and renal IGFBP‐2 levels might also be increased in LN patients. This will be examined in the future. Because this is a cross‐sectional study, future longitudinal studies are needed to assess the performance of IGFBP‐2 as biomarkers for predicting disease flare. Finally, mechanistic studies are also warranted to dissect out the pathological roles of IGFBP2 in LN.
Disclosure
The authors have no disclosures to report.
Acknowledgements
We are grateful to Professor Dr Chandra Mohan for his critical reading of this manuscript. This work was supported by grants from the National Institutes of Health, Lupus Research Institute and start‐up funds from partly University of Houston (TW).
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