Introduction
Nephrotic syndrome is one of the most common glomerular disorders in children and affects 1–7 per 100,000 children per year (Dutch data 1.52/100,000) with a male predominance (2:1) [
1]. The disease is characterized by the triad of severe proteinuria, hypoalbuminemia, and edema. Glucocorticoids have been the cornerstone of the treatment of childhood nephrotic syndrome for over 60 years, as over 80–90% of the patients achieve complete remission with prednisone or prednisolone treatment [
2]. Unfortunately, 80% of these patients will have one or several relapses and will need additional courses of glucocorticoid therapy. Furthermore, approximately 10% of children with nephrotic syndrome are steroid resistant and do not respond to the standard steroid treatment regimen. Treatment guidelines for the first manifestation and a relapse of steroid-sensitive nephrotic syndrome are mostly standardized and based on practice guidelines rather than clinical trials [
3]. As the optimal glucocorticoid dosing regimens for childhood nephrotic syndrome are still under debate and large-scale clinical trials are lacking, current clinical practice among physicians is variable [
4], especially in the treatment of subsequent relapses and the choice of second-line immunosuppressive drugs. Unfortunately, the variability in the treatment of nephrotic syndrome is mostly based on the protocol preference of the physician, rather than the individual characteristics of the patient. Therefore, clinical trials are needed to develop international treatment guidelines with recommendations for several aspects of the treatment of nephrotic syndrome. Recently, a nationwide study in the Netherlands showed that duration of corticosteroids for the initial presentation had no impact on subsequent relapses [
5]. Furthermore, a recently published abstract of the PREDNOS study indicated no clinical benefit associated with an extended steroid course for the initial presentation in UK children [
6]. Currently, a few clinical trials are underway to further investigate the optimal dosing regimens for both the first manifestation (
https://clinicaltrials.gov/ct2/show/NCT02649413?recrs=abdf&cond=Nephrotic+Syndrome&age=0&draw=2&rank=13) as well as relapses of nephrotic syndrome [
7].
Large inter-individual variation is present in children with nephrotic syndrome regarding both the clinical course of disease and the intensity and spectrum of side effects of its treatment. Nephrotic syndrome is characterized by podocyte foot process effacement; however, the exact mechanism of disease is still largely unknown and often debated. Several etiologies have been investigated over the years, and different subgroups of the disease are likely to have a different pathogenesis [
8,
9]. Damage to the filtration barrier can be caused by genetic defects primarily affecting podocytes. Patients with an underlying genetic defect are often primary steroid resistant, and to date, 53 genes associated with steroid-resistant nephrotic syndrome have been identified [
10]. Furthermore, a substantial proportion of the patients is likely to have an immune-mediated circulating factor disease. These patients are also often steroid resistant, but screen negative for the known steroid-resistant nephrotic syndrome genes. The existence of a circulating permeability factor would explain the rapid recurrence of proteinuria after kidney transplantation in some patients with nephrotic syndrome [
11,
12]. Many candidates have been identified over the years; however, the definitive factor remains to be discovered [
13]. Lastly, involvement of the immune system in the pathogenesis of nephrotic syndrome is highly suspected as relapses often occur after the immune system is triggered by an infection, allergy, or vaccination, and glucocorticoid treatment is effective in most patients. Nephrotic syndrome has been considered to be a T cell disorder based on several observations, including remission following measles infection, the association with Hodgkin disease, and the response to immunosuppressive drugs [
9]. Furthermore, in the last few years, a potential role for B cells has been proposed as well due to the effectiveness of B cell depletion with rituximab in patients with nephrotic syndrome [
9,
14].
All in all, most children with nephrotic syndrome have a minimal change disease [
15] and, therefore, the large inter-patient variability cannot fully be attributed to the disease histology. Previous research has indicated that pharmacogenetics can have an influence on both pharmacokinetics (PK) and pharmacodynamics (PD) of the individual patient [
16]. Genetic factors influencing the individual pharmacokinetic and pharmacodynamic profile may account for 20–95% of the variability in the efficacy and side effects of medication [
17]. Each of the processes involved in PK and PD can potentially be influenced by a clinical significant genetic variation [
18]. Therefore, pharmacogenetics may have a promising role in personalized medicine. By implementing pharmacogenetics in the clinical work-up of the patients, this may ultimately lead to individualized drug therapy to maximize drug efficacy and minimize drug toxicity. In the era of precision medicine, however, current knowledge on the influence of pharmacogenetics on the steroid response in nephrotic syndrome is limited [
19].
This review describes the mechanisms of glucocorticoid action and clinical PK and PD of prednisone and prednisolone in nephrotic syndrome patients. Furthermore, the current data available on pharmacogenetics of prednisone and prednisolone in patients with nephrotic syndrome is summarized and areas for future research to improve individualization of glucocorticoid therapy in children with nephrotic syndrome are identified.
Pharmacogenetics
It is well known that different patients respond in different ways to the same medication. Many non-genetic factors influence the individual differences in drug response, including age, sex, disease, organ function, concomitant therapy, drug adherence, and drug interactions (for a review on such factors, see [
20,
48]). In addition, genetic factors may also have a major influence on the efficacy of a drug and risk of side effects [
18,
64]. Pharmacogenetics is the study of the role of inheritance in inter-individual variation in drug response. Genetic factors influencing the patient pharmacokinetic or pharmacodynamic profiles may account for 20–95% of variability in the efficacy and side effects of therapeutic agents [
17]. For example, polymorphisms in the CYP3A5 gene account for 40–50% of the variability in tacrolimus dose requirement in Caucasians [
65]. After administration, the drug is absorbed and distributed to the site of action. It interacts with targets (such as receptors and enzymes), undergoes metabolism, and is then excreted. Each of these processes could potentially involve a clinical significant genetic variation [
18]. Understanding the basis of such variations, i.e., pharmacogenetics, is vital to come to personalized medicine, which ultimately may lead to individualized drug therapy to maximize drug efficacy and minimize drug toxicity.
Clinical practice
In children with nephrotic syndrome, large inter-individual variability is present in the course of disease, and efficacy and side effects of glucocorticoids. As the variable response to glucocorticoids in patients with nephrotic syndrome cannot completely be attributed to the disease histology, it is difficult to predict the response based on clinical observations alone. For nephrotic syndrome, research on the impact of genetic polymorphisms on steroid response and susceptibility to steroid-related toxicities is limited [
19]. For a few other diseases, however, pharmacogenetics has already been implemented in clinical practice [
66,
67]. Furthermore, in the field of pediatric nephrology, new guidelines on tacrolimus dosing recommend involvement of CYP3A5 genotyping to optimize the immunosuppressive treatment of the individual transplant patients [
68].
In line with the aforementioned examples, we believe that the involvement of pharmacogenetics in the work-up of nephrotic syndrome patients as well might be beneficial, preventing exposure to ineffective drug courses and minimizing drug toxicity. As the mechanism of action of glucocorticoids involves numerous receptors, enzymes, and proteins, a variety of potential targets of genetic polymorphisms may be present. Although limited evidence is available, an overview of previously conducted studies on pharmacogenetics of prednisone and prednisolone in patients with nephrotic syndrome is provided below. Table
1 (glucocorticoid targets) and Table
2 (glucocorticoid PK) provide an overview and brief description of the most important studies conducted in pediatric patients with nephrotic syndrome in which a positive association between the polymorphism and steroid response was described. A summary of the mechanisms and most important results is provided for the targets involved.
Table 1
Effects of gene polymorphisms affecting glucocorticoid targets in nephrotic syndrome patients
GR | NR3C1 | Alteration of GR/GC complex formation | N = 108 Age 4.0 (3.1–6.5) | GR-9β + TthIII-1 rs6198 + rs10052957 | Higher incidence of glucocorticoid dependence | |
N = 118 Age 5.1 (± 3.2) | GTA haplotype rs33388 rs33389 Bcl-1 | Higher glucocorticoid sensitivity | |
N = 100 Children | rs41423247 | Higher incidence of frequent relapsing nephrotic syndrome | |
N = 154 Unknown | rs6196 rs10052957 rs258751 | Decreased risk of glucocorticoid resistance | |
GR heterocomplex | FKBP5 | Alteration of GR activity | N = 66 Children | rs4713916 | Higher incidence of glucocorticoid dependence | |
Anti-inflammatory factors | IL-4 promoter | IL-4 production is upregulated in patients with nephrotic syndrome | N = 58 Children | rs2243250 | Higher frequency in patients with nephrotic syndrome | |
N = 150 Age 11.0 (± 6.6) | rs2243250 | Association with glucocorticoid resistance | |
N = 150 Children | rs2243250 | Association with glucocorticoid resistance | |
IL-4Rα | | N = 85 Children | rs1805010 | Lower frequency in patients with frequent relapsing nephrotic syndrome | |
IL-6 | IL-6 production is increased in patients with steroid-resistant nephrotic syndrome | N = 150 Age 11.0 (± 6.6) | rs1800795 | Association with glucocorticoid resistance | |
N = 150 Children | rs1800795 | Association with glucocorticoid resistance | |
Pro-inflammatory factors | IL-12Bpro1 | Decrease in IL-12 production | N = 79 Age 10.7 (± 4.5) | rs17860508 | Association with glucocorticoid dependence | |
TNF-α | Increased TNF transcription, leading to an increase in TNF-α synthesis | N = 150 Age 11.0 (± 6.6) | rs1800629 | Association with glucocorticoid resistance | |
N = 150 Children | rs1800629 | Association with glucocorticoid resistance | |
MIF | Increase of MIF level in serum causes a pro-inflammatory response | N = 214 Age 3.5 (± 2.9) | rs755622 | Association with glucocorticoid resistance | |
N = 257 Age 5.8 (± 4.2) | rs755622 | Association with glucocorticoid resistance | |
N = 80 Children | rs755622 | Association with glucocorticoid resistance | |
Table 2
Effects of gene polymorphisms affecting glucocorticoid pharmacokinetics in nephrotic syndrome patients
P-glycoprotein | MDR-1 | Enhanced P-glycoprotein function | N = 108 Age 11.13 (± 4.83) | rs1128503 rs2032582 rs1045642 | Association with late response to glucocorticoids | |
N = 74 Children | rs1128503 | Association with glucocorticoid resistance | |
N = 138 Age 4.2 (± 1.6) | rs2032582 | Association with glucocorticoid resistance | |
N = 216 Children | rs2032582 | Association with glucocorticoid resistance | |
N = 120 Children | rs1128503 | Association with glucocorticoid resistance | |
N = 100 Children | rs2032582 rs1128503 rs1045642 | Association with different medication regimes | |
PXR | NR1I2 | Decreased expression of PXR, leading to underexpression of GRs | N = 66 Age 4.9 (± 3.7) | rs3842689 | Association with glucocorticoid resistance | |
Targets
Glucocorticoid receptor
Polymorphisms in the GR gene (NR3C1) are known to be associated with variations in the GR function, because they may alter the formation of the GR/GC complex. Therefore, the hypothesis is that genetic alterations in the gene encoding for the GR receptor may account for some degree of inter-individual variability in the glucocorticoid response and steroid-related toxicity in individuals [
88]. Three polymorphisms are known to be associated with reduced sensitivity in both endogenous and exogenous glucocorticoids: TthIIII (rs10052957), ER22/23K (rs6189/rs6190), and GR-9β (rs6198). In contrast, the polymorphisms N363S (rs6195) and BC1I (rs41423247) are associated with an increased sensitivity to glucocorticoids [
88,
89]. Increased glucocorticoid sensitivity due to a genetic polymorphism might also be associated with increased susceptibility to steroid-related toxicities. Previously, Eipel et al. showed that pediatric patients with acute lymphoblastic leukemia (ALL) carrying the N363S polymorphism were more prone to steroid-related toxicities [
90,
91]. In contrast, children with the ER22/23EK polymorphism were less susceptible [
90]. To our knowledge, the role of genetic polymorphisms in the GR gene in susceptibility of steroid-related toxicities has only been investigated in patients with nephrotic syndrome in one study. Teeninga et al. found no association between the GR-9β polymorphism and side effects [
69]. To date, a few studies investigated the role of NR3C1 polymorphisms on the glucocorticoid response in pediatric patients with nephrotic syndrome [
69‐
72,
92,
93]. Four studies found a potential influence of genetic polymorphisms in the GR gene on the steroid response in patients with nephrotic syndrome (Table
1).
Glucocorticoid receptor heterocomplex
Components of the glucocorticoid heterocomplex are essential to keep the GR in the correct folding for hormone binding and prevent nuclear localization of unoccupied GRs. Abnormalities in the chaperones and co-chaperones that make up the heterocomplex may contribute to decreased glucocorticoid responsiveness, as the integrity of the GR heterocomplex is required for optimal ligand binding and subsequent activation of the transcriptional response. For several diseases, which are treated with glucocorticoids, including nephrotic syndrome, altered levels of chaperone protein hsp90 were found in peripheral blood mononuclear cells from individuals with a steroid-resistant course of disease [
94‐
96]. Although limited, some evidence exists about the association of polymorphisms in the gene encoding for one of the co-chaperones, FKBP5, with steroid resistance in Crohn’s disease [
97]. Interestingly, this could also hold true for nephrotic syndrome patients as well. Recently, one study was published on the potential role of FKBP5 polymorphism (rs4713916) in a small group of pediatric nephrotic syndrome patients showing a higher frequency in patients with a steroid-dependent nephrotic syndrome [
73].
Nuclear translocation receptors
Nuclear translocation receptors, known as importins, play a significant role in the mechanism of glucocorticoid action. These receptors are responsible for the effective transport of the GR/GC complex to the cell nucleus. IPO13 is a primary regulator to facilitate the transfer of the GR/GC complex across the nuclear membrane. In children with asthma, polymorphisms encoding the IPO13 gene resulted in increased sensitivity for glucocorticoids, which was most likely due to the increased availability of glucocorticoids in the nucleus [
98]. The role of genetic polymorphisms in the gene encoding for IPO13 in patients with nephrotic syndrome is unknown [
19].
Pro- and anti-inflammatory factors
To date, the exact underlying pathophysiological mechanisms of nephrotic syndrome are still unknown. One of the hypotheses is that nephrotic syndrome is associated with an immunoregulatory imbalance between T helper subtype 1 (Th1) and T helper subtype 2 (Th2) cells. Cytokines produced by the T helper cells play a role as mediators of inflammation. Several studies have been conducted in patients with various diseases to evaluate the association with genetic polymorphisms in the IL-1, IL-4, IL-6, IL-13, and TNF-α genes. The evidence for genetic polymorphisms in the cytokine genes in patients with nephrotic syndrome is, however, limited.
Minimal change nephrotic syndrome is associated with atopy and IgE production [
99]. T helper subtype 2 cytokines, such as IL-4 and IL-13, are known to be involved in the development of atopy. Previously genetic variations of IL-4 and IL-13 and their receptors have been shown to be associated with predisposition to atopy and/or elevated serum IgE levels [
100]. Several studies have been conducted to investigate the role of polymorphisms in the genes coding for IL-4, IL-6, and IL-13 in pediatric patients with nephrotic syndrome [
74‐
76,
101‐
103]. The IL-4 polymorphism rs2243250 was associated with nephrotic syndrome and an increased risk of steroid resistance [
74‐
76]. Furthermore, previous research conducted by Jafar et al. and Tripathi et al. indicate that a genetic polymorphism in the IL-6 gene is associated with decreased responsiveness to steroids [
75,
76]. No significant association was found between the IL-13 gene polymorphisms and disease susceptibility or steroid responsiveness [
71,
101,
103].
An important pro-inflammatory cytokine is macrophage migration inhibitory factor (MIF). MIF has the unique ability to override the inhibitory effects of glucocorticoids on the immune system. Due to its regulatory properties, MIF is considered a critical mediator in various immune and inflammatory diseases. The allele MIF-173*C (rs755622) is associated with higher serum MIF levels. Several studies have been conducted to investigate the potential role of this genetic polymorphism in the gene encoding for MIF in patients with nephrotic syndrome [
71,
79‐
81,
104,
105]. A meta-analysis conducted by Tong et al. showed that the gene polymorphism rs755622 plays an important role in the risk of glucocorticoid resistance in patients with nephrotic syndrome [
106]. The hypothesis is that the G/C substitution at 173 bp of the MIF gene increases the MIF level in serum and could therefore cause a pro-inflammatory response, induce injury to podocytes, and accelerate the progression of glomerulosclerosis [
107]. TNF-α is also an important pro-inflammatory cytokine involved in the inflammatory process. Elevation of TNF-α has been found in the plasma and urine of patients with nephrotic syndrome [
108,
109]. Conflicting results have been published for the role polymorphisms in the gene encoding for TNF-α in patients with nephrotic syndrome [
75,
76,
110,
111]. Lastly, Müller-Berghaus et al. investigated the role of polymorphisms in gene encoding for pro-inflammatory mediator IL-12B and found an association of the IL12Bpro-1.1 genotype with a steroid-dependent course of disease [
78].
GLCCI1 (glucocorticoid-induced transcript 1 gene)
Little is known about the exact function of GLCCI1. GLCCI1 was initially described as a thymocyte-specific transcript that is rapidly upregulated in response to dexamethasone treatment [
112]. In addition, GLCCI1 is expressed in the kidney and, in particular, in the glomeruli. Knockdown of the GLCCI1 gene resulted in disruption of the glomerular permeability filter and podocyte foot process effacement. A genome-wide association study in patients with asthma showed a significant association between the genetic polymorphism rs37972 of the GLCCI1 gene and a decreased response to glucocorticoid inhalation therapy [
113]. In contrast, two studies in pediatric nephrotic syndrome patients could not confirm the association between this specific polymorphism and steroid responsiveness in patients with nephrotic syndrome [
71,
114].
P-glycoprotein
P-glycoprotein is an efflux pump encoded by the multidrug resistance protein 1 gene (MDR1). Glucocorticoids are known substrates for P-glycoprotein and may also induce P-glycoprotein expression [
52,
115]. In the kidney, P-glycoprotein is expressed in the brush border membrane of proximal tubular epithelial cells. Increased expression of P-glycoprotein results in decreased intracellular drug concentrations and may consequently decrease treatment response. Previous research has shown higher expression of MDR1 and increased P-glycoprotein activity in children with steroid-resistant nephrotic syndrome [
116,
117]. To date, approximately 50 genetic polymorphisms have been reported in the MDR1 gene. Among the genetic polymorphisms, C1236T (rs1128503), G2677T/A (rs2032582), and C3435T (rs1045642) are the most common variants in the coding region of MDR1. The interpretation of the influence of the genetic polymorphisms on P-glycoprotein expression, however, is unresolved and may vary depending on tissue type, pathological status, and ethnicity [
118]. A recent systematic review on pharmacogenetics and adverse drug reactions in pediatric oncology patients indicated protective effects from two genetic polymorphisms of the MDR1 gene in methotrexate- and vincristine-related neurotoxicity in pediatric ALL patients [
119]. In nephrotic syndrome patients, however, no studies have been conducted to investigate the potential role of genetic polymorphisms in the MDR1 gene in steroid-related toxicities. Several studies have been conducted to evaluate the association of P-glycoprotein polymorphisms with the responsiveness to glucocorticoids in patients with nephrotic syndrome. The results of these studies on the significance of the genetic polymorphisms are contradictory [
71,
82‐
84,
86,
104,
120,
121]. A recent meta-analysis concluded that there is evidence of an association between rs1128503 and increased risk of steroid resistance in children with nephrotic syndrome [
122].
Pregnane X receptor
Pregnane X receptor (PXR) gene (NR1I2) encodes an intracellular receptor that, upon binding with glucocorticoids or xenobiotic substances, activates a set of genes involved in the metabolism of drugs. Turolo et al. described an association of the presence of a PXR deletion polymorphism (rs3842689) with steroid resistance. The hypothesis is that a reduced expression of PXR leads to an underexpression of GRs, which may be the explanation for the development of steroid resistance [
87].
Summary
The results of the aforementioned reported papers are generally inconclusive and contradictory. However, some genetic polymorphisms appear to be promising in the prediction of steroid response or steroid-related toxicities in children with nephrotic syndrome. Especially, polymorphisms in the genes encoding for the GR and GR heterocomplex seem to have an association with steroid responsiveness. Nevertheless, most studies are hampered by small patient cohorts. Therefore, studies in larger cohorts with nephrotic syndrome patients are necessary to draw conclusions about the influence of genetic polymorphisms on the glucocorticoid response. Furthermore, as mentioned above, pharmacogenetics may also play a role in the intensity and spectrum of side effects. Currently, little is known about the influence of pharmacogenetics on steroid-related toxicities in patients with nephrotic syndrome. However, as previous research in mostly cancer patients has shown a potential role of genetic polymorphisms in the susceptibility on steroid-related toxicities, this area is an important opportunity for future research as well.
Conclusion
Glucocorticoids are essential in the treatment of childhood nephrotic syndrome. Currently, standardized treatment guidelines with high doses of prednisone or prednisolone are proposed worldwide. As current treatment guidelines are largely based on empiric recommendations rather than clinical trials, large variability in the treatment of nephrotic syndrome is present among physicians [
4], especially regarding the treatment of subsequent relapses and the choice of second-line immunosuppressive drugs. As large-scale clinical trials are lacking, treatment decisions are frequently based on either the preference or common practice of the treating physician or guidelines of the country, rather than the individual characteristics of the patient. Therefore, effort should be made to first provide international guidelines based on clinical trials to uniformly treat patients with nephrotic syndrome. Subsequently, effort should be made to identify specific markers to individualize treatment, as large inter-individual differences are present in both the clinical course of disease and adverse effects of glucocorticoids in children with nephrotic syndrome. Pharmacogenetics has a promising role in working towards personalized medicine. Despite the fact that the evidence about the role of pharmacogenetics in children with nephrotic syndrome is limited, we feel that available data do show a potential role for pharmacogenetics in clinical practice to maximize drug efficacy, minimize drug toxicity, and avoid exposure to ineffective drug courses. Nowadays, the evidence to implement these genetic markers in clinical practice is too little and, therefore, clinical implementation of pharmacogenetics in nephrotic syndrome patients is not possible yet. Therefore, we feel that further research is highly important to identify specific and sensitive markers for steroid resistance in patients without genetic podocyte mutations as well as for patients more at risk for steroid-related toxicities. As nephrotic syndrome is a rare kidney disease in childhood and large patient cohorts are needed to ultimately implement pharmacogenetics in the clinical work-up, we believe that this research preferably should be conducted in international collaborative studies.
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Answers
1. a; 2. c; 3. b; 4. b; 5. c