FOCAL SEGMENTAL GLOMERULOSCLEROSIS AND CHRONIC KIDNEY DISASE IN PEDIATRIC PATIENTS

Abstract

Focal segmental glomerulosclerosis (FSGS) is one of the most common forms of acquired glomerular disease leading to end stage kidney disease (ESKD). Its incidence is rising around the world. There is no proven therapy for those patients who do not respond to corticosteroids and it can recur in 20–25% of patients who receive a kidney transplant. The disease can be primary or secondary to various conditions including vesicoureteral reflux, obesity, medications, and infections. Recent advances have demonstrated the important role of genetic mutations in podocyte proteins as a cause of FSGS. There is an urgent need for randomized clinical trials to develop safe and effective therapy for FSGS that occurs in the native or transplanted kidney.

INTRODUCTION

Focal segmental glomerulosclerosis (FSGS) is one of the most difficult and enigmatic diseases in nephrology. It can occur as a primary disorder without an identifiable cause or as an illness secondary to a variety of problems. Over the last 20–30 years, the incidence of FSGS has been increasing in virtually all ethnic groups, across the entire age spectrum, and around the world. It is one of the most important causes of acquired chronic kidney disease (CKD) in children and adults and there is no proven therapy for steroid-resistant cases. Moreover, patients who progress to end stage kidney disease (ESKD) and who receive a renal transplant, cannot be reassured that the worst is behind them because FSGS can recur in the transplanted kidney in 20–25% of cases. Because of the profound renal morbidity associated with FSGS, it has been the subject of intensive basic science and clinical research since its initial identification in renal biopsy specimens obtained from patients with steroid-resistant nephrotic syndrome over 50 years ago. There has been considerable progress in understanding the pathogenesis of FSGS and renewed efforts have been made to conduct valid randomized clinical trials in this disease. The objectives of this review are: (1) to focus mainly on primary or idiopathic FSGS and on accomplishments in this area during the last 5 years; and (2) to highlight where future efforts should be directed in order to improve the clinical outcomes for patients with all forms of FSGS.

DEFINTION AND HISTOPATHOLOGY

The diagnosis of FSGS is based on the detection of segmental sclerosis and hyalinization of a portion of the glomerular tuft. An effort has been made to create a histological classification in order to determine whether the renal pathology findings can predict response to treatment and long-term outcome. In the most widely used scheme, there are four categories of FSGS – not otherwise specified (NOS), tip, hypercellular, and collapsing (1). The location of the scar can be at the vascular pole (NOS) or adjacent to the onset of the proximal tubule, so-called tip lesion. The degree of mesangial cellularity can vary and there is ongoing controversy whether the severity of this finding has prognostic implications (2,3). Finally, a severe subtype of FSGS, termed collapsing variant, has consistently been shown to be resistant to treatment and to follow a relatively aggressive downhill course. Immunofluorescence studies may show deposition of C3 and IgM in sclerotic segments but this is considered a non-specific finding. In contrast, the implication of isolated C1q staining in the mesangium is less clear with some investigators interpreting this finding as suggestive of an underlying disease process such as systemic lupus erythematosus (4) and others considering it within the spectrum of primary FSGS (5). Electron microscopy shows fusion of the podocyte foot process, segmental scarring, and occasional deposits. Reticular inclusion bodies are seen on certain secondary forms of FSGS, e.g., HIV nephropathy. Foot process effacement is generally more widespread and severe in primary versus secondary FSGS.

While the definition of FSGS appears straightforward, the literature and clinical experience suggest that misclassification of patients as having FSGS is fairly common. For example, over 10% of patients screened for enrollment in the NIDDK-funded FSGS Clinical Trial were found not to have the disease after review of the biopsy material by a central committee of expert renal pathologists (6). This may reflect the inaccurate classification of nonspecific scarring of glomeruli or misinterpretation of findings such as glomerulomegaly, immature glomeruli, or foci of tubulointerstitial fibrosis, which are supportive but not diagnostic of FSGS. In a study of the difficulties in the histological assessment of glomerular sclerotic lesions as FSGS, definite diagnosis required a representative renal biopsy specimen, a comprehensive evaluation with light microscopy, immunofluorescence, electron microscopy and integration of morphological changes with relevant clinical data (7). Histological examination of the renal tissue may not enable reliable differentiation of idiopathic and secondary forms of FSGS (7). Differential immunohistochemical staining of the glomerulus for various podocyte markers such as synaptopodin, dystroglycan, the cyclin kinase inhibitor p27, has been proposed as an alternative method to differentiate FSGS from minimal change nephrotic syndrome in specimens that may not show the classic histopathological lesion on light microscopy (8,9,10). The accurate histopathological diagnosis of FSGS lesions requires collaboration between nephrologists and pathologists and, in the future, is likely to incorporate newer biomarkers in the kidney tissue. In addition, the urinary proteome may provide an additional tool to confirm the diagnosis of FSGS in questionable cases (11)

CLINICAL CLASSIFICATION

In addition to the histopathology categories outlined above, FSGS is divided into two clinical categories – primary versus secondary – based on whether an underlying cause for the abnormality can be identified (Table 1). Primary or idiopathic FSGS represents cases in which there is no demonstrable etiology for the renal histopathological finding. Secondary causes include genetic mutations in podocyte proteins (see below), obesity, medications such as pamidronate, sunitinib, interferon (12,13,14), infections (e.g. HIV), vesicoureteral reflux, reduction in renal mass (e.g., surgical resection of renal tumors, trauma), and systemic illness such as plasma cell dyscrasias (15). An interesting variant of disease related to decreased kidney mass is the observation that very low birth weight due to prematurity or intrauterine growth retardation is associated with reduced nephron number and an increased risk for secondary FSGS (16). In general, patients with secondary causes of FSGS have lower levels of proteinuria and are less likely to present with clinically overt nephrotic syndrome compared to those with primary disease. It is important to maintain vigilance for emerging causes of secondary FSGS linked to changes in environmental factors, life style and recreational practices. For example, a recent report documented the occurrence of FSGS in 10 body builders who were using anabolic steroids at a mean daily intake of 10 g. Most important, in 7 cases the proteinuria decreased with discontinuation of the anabolic steroids (17).

Table 1.

Causes of FSGS

Primary or idiopathic

Secondary

Genetic mutations

Reduction in renal mass

Low birth weight

Vesicoureteral reflux

Obesity

Medications

Infections

Systemic illnesses

INCIDENCE

There has been a steady increase in the incidence of primary FSGS over the last 20 years. In most reports of adult patients that have appeared in recent years, there has been a 2–3 fold rise in the rate of diagnosis of the disease. Although this may reflect more selective reliance on kidney biopsies in patients with proteinuria and nephrotic syndrome, the increase is likely to be real because it is paralleled by an increase in the number of patients reaching ESKD secondary to FSGS. Similar trends have been observed in pediatric nephrology. For example, in an analysis of a hospital computerized database regarding children with primary nephrotic syndrome seen first between the years 1984 and 1995, 23.0% (95% CI 16–29%) of the kidney biopsies (n=62) showed FSGS. A similar percentage was found in a survey of pediatricians to capture cases (n=86) that were outside the database (18). This rate of detection of FSGS was significantly higher than that described by the International Study of Kidney Disease in Children (ISKDC) conducted in the mid-1960s and early 1970s, namely 6.9%. Again, it must be emphasized that the later figure is based on an unselected population of children with new-onset nephrotic syndrome. In contrast, most recent estimates of the incidence of FSGS are derived from single center reports describing renal biopsy findings in patients with a higher likelihood of steroid resistance and with clinical features indicating a non-MCNS lesion. The annual incidence rates of both primary nephrotic syndrome (3.6) and FSGS (1.6) were significantly higher in African-Americans than Caucasians (1.8 and 0.3 cases/105 children per year, respectively) (18). In a similar survey from Children’s Hospital of Eastern Ontario, there has been a three-fold increase in the incidence of FSGS over the 18-year period from 1985–2002 (19). However, although the traditional view is that African Americans have an increased risk of FSGS, it is the leading cause of the nephrotic syndrome in white adults living in Minnesota (20).

ETIOLOGY

Dysfunction of the podocyte is a central feature in current working paradigm for the pathogenesis of FSGS (21). Whether there is an intrinsic abnormality in the podocyte such as a mutation in a structural podocyte or an exogenous factor that initiates podocyte injury, loss of this non-proliferating, terminally differentiated cell is pivotal in the development of FSGS. In experimental models with graded levels of podocyte injury and loss induced by diptheria toxin, the degree of podocytopenia is highly correlated with the histological pattern of damage (22). Thus, if less than 20% of podocytes are lost, there can be regeneration with resident glomerular epithelial stem cells that migrate from a niche adjacent to Bowman’s space into the glomerulus and replace damaged podocytes that are lost by necrosis or apoptosis. If podocyte loss is in the range of 20–40%, then lesions of FSGS ensue, while if podocyte loss exceeds 40%, there is global sclerosis of the glomerulus.

Altered expression of a number of signaling molecules has been associated with podocyte injury and the development of glomerulosclerosis in animal models of FSGS. These include Notch and Sidekick-1, proteins that play a role in normal nephrogenesis and glomerular structure (23,24). Future genome wide association studies may help determine the contribution of these and other candidate molecules to the broad spectrum of disease severity observed in patients with FSGS

One of the most exciting advances that has been made in the last decade in the understanding of FSGS is the linkage between this disorder and genetic mutations in various podocyte proteins. The first report of a genetic basis for FSGS appeared in 2000 and demonstrated occurrence of the glomerulopathy in adults with alterations in the gene for podocin (25). Since that time seven other genes – CD2AP, laminin β2, Wilms’ tumor-1, phospholipase C epsilon 1, α-actininin-4, TRPC6, and IFN2, a member of the forming family of actin regulatory proteins -- have been described in family pedigrees with FSGS (26). In addition, nephrin mutations, which were originally described in association with congenital nephrotic syndrome, can also cause steroid-resistant nephrotic syndrome in children and adults (27). These proteins are usually present in the podocyte cell membrane and interact with the actin cytoskeleton or the slit membrane between adjacent podocytes. They may be involved in the modulation of podocyte movement and contractility in response to the transglomerular pressure. It is worth noting that while the MYH9 gene was originally thought to be associated with the development of FSGS, more refined genetic mapping indicates that the closely linked gene apolipoprotein 1 (APOL1) represents the true risk factor for progressive disease in African Americans with FSGS (28,29,30).

In general, the mutations break down into two categories – autosomal recessive pattern of inheritance with onset of disease in childhood and autosomal dominant disease with onset in late adolescence or adulthood. Patients who are heterozygous for nephrin and podocin mutations, two recessive causes of familial FSGS, are generally responsive to therapy and have a good long-term prognosis (31). The percentage of cases of sporadic FSGS associated with each mutation varies from 3–20% depending on the specific patient cohort being tested. New studies with more detailed genotype-phenotype correlations have attempted to clarify whether specific mutations influence the clinical course of disease in patients with FSGS. For example, specific mutations in podocin are associated with onset of disease earlier in childhood and more rapid progression to ESKD (32). Similarly, mutations in the KTS sequence of the WT-1 gene have a later onset of disease and delayed progression to ESKD compared to children with missense mutations in this gene (33). Most of the patients with genetic mutations in podocyte appear to be resistant to standard therapy, the exception being PLCE1 which may respond to the administration of cyclosporine. Table 2 summarizes the key features of the genetic causes of FSGS.

Table 2.

Podocyte mutations associated with FSGS

Gene	Year of Discovery	Inheritance Recessive (R) or Dominant (D)	Age at presentation Pediatric(P) or Adult (A)
Nephrin	1998	R	P
Laminin β2	1998	D	A
Wilms’ tumor-1	1999	D	P
Podocin	2000	R	P
A-actinin-4	2000	D	A
CD2AP	2000	R	P
TRPC6	2005	D	A
Phospholipase C epsilon-1	2006	R	P
INF2	2010	D	A

While these discoveries have shed new light on the pathogenesis of FSGS, many questions remain to be answered. These include: (1) What is the precise incidence of genetic mutations in sporadic cases of FSGS in different ethnic groups of patients? (2) Does the presence of a genetic mutation rule out the possibility of a favorable response to current immunosuppressive agents? (3) Does FSGS recur post-transplantation in patients with genetic mutations in podocyte proteins?

NATURAL HISTORY

The general teaching about primary FSGS is that approximately 30–60% of patients will inexorably progress to ESKD over a 5–10 year period of observation. This guarded prognosis has been confirmed in patient cohorts from around the world (34). There are reports that the outlook for patients with FSGS may not be so pessimistic. For example, in a group of 34 children with FSGS from Saudi Arabia who were followed for a mean of 8.6±3.3 years, 21% had persistent proteinuria and 21% developed CKD (35). In addition, Somers et al (36) have described a cohort of children with primary FSGS in whom the minority demonstrated a decline in kidney function. Demographic factors such as age, race, and ethnicity may have an impact on prognosis. The higher risk of progressive loss of kidney function in African Americans is linked to mutations in the APOLI gene (29,30). However, determination of the prognosis for individual patients is more subtle and patient outcomes are more heterogeneous than this simple binary division suggests and is directly related to the degree of reduction in protein achieved spontaneously or in response to treatment. Patients with a complete remission in proteinuria do as well as healthy controls and manifest no deterioration in kidney function. In a study of 281 FSGS patients with nephrotic-range proteinuria from the Toronto Glomerulonephritis Registry who were followed for at least 1 year, over a median follow-up of 65 months, 55 experienced a complete remission, 117 had a partial remission (PR), and 109 had no remission in proteinuria. Achievement of a partial remission was independently predictive of the slope defining the decline in kidney function and survival from renal failure by multivariate analysis. The adjusted time-dependent hazard ratio was 0.48; 95% confidence interval, 0.24 to 0.96; P = 0.04). Therapy with high dose corticosteroids and immunosuppressive drugs was associated with a higher likelihood of developing a complete or partial remission (37). Thus, even a PR in proteinuria and its maintenance are important therapeutic targets in FSGS, with implications for both slowing progression rate and improving renal survival. These findings indicate that the prognosis in patients with primary FSGS is a dynamic variable that can be influenced by less than optimal response to medical interventions. It underscores the value of devising safe and well tolerated treatments that can lower proteinuria even if they do not normalize this value in patients with FSGS.

THERAPY

Like nephrology in general and glomerular disease in particular, there is a marked paucity of published works detailing the outcome of randomized clinical trials (RCT) to guide the treatment of FSGS (38). There are numerous questions that remain unanswered including the following: (1) Are there biomarkers that can predict outcome of the disease and guide individualized treatment? (2) What is the role of immunosuppression in secondary forms of FSGS including cases due to genetic mutations in podocyte proteins? (3) What is the optimal second-line treatment in patients who are steroid resistant? (4) What is the appropriate duration of treatment in patients who achieve a reduction in proteinuria in response to therapy?

All patients with FSGS benefit from the use of antiproteinuric agents that antagonize the renin-angiotensin II-aldosterone axis. This includes direct renin antagonists, angiotensin converting enzyme inhibitors, angiotensin receptor blockers, and aldosterone antagonists. There is an emerging recognition that viewing FSGS as a T-cell driven disease and the subsequent focus solely on immunosuppressive agents as potential treatments may be misguided and hindering advances in therapeutics (39). This is supported by recent data indicating that cyclosporine may act to reduce proteinuria in animal models of FSGS by acting directly on the podocyte and stabilizing the actin cytoskeleton rather than targeting an immunological mechanism, namely inhibition of nuclear factor of activated T cell (NFAT) (40). The efficacy of rituximab to induce a remission in 25–40% of patients with steroid-resistant FSGS suggests that B-cells rather than T-cells may be a viable target of immunosuppressive therapy (41). The presence of circulating permeability factors in FSGS (42,43,44) and the availability of treatments that can lower the level of this molecule(s) such as galactose (45) point to the need for new paradigms for the treatment of FSGS (39). This suggests that new insights into the pathogenesis of FSGS are likely to yield unexpected novel therapies to treat patients with this disease. The need to reconsider non-immunological targets is especially important in African Americans who may have genetic factors that enhance the likelihood of progression to ESKD (29,30).

Although corticosteroids have not been evaluated in a systematic manner, a course of this drug is considered the first line therapy in patients with FSGS (46). There is a difference of opinion between pediatric and internal medicine nephrologists about the dose and the duration of steroid therapy that is needed before considering a patient steroid resistant. While children will usually be treated with a combination of daily and alternate day doses of steroids for 8–12 weeks, adults are often treated for up to 6 months with alternate steroids before making this assessment. However, in most studies at most 25% of patients with FSGS achieve a complete or partial remission in proteinuria after a course of corticosteroids (47). One exception to these generally grim results are the reports from a single center indicating that nearly 80% (18 out of 23) of children with FSGS may achieve a complete or partial remission over a mean follow-up period of 46±5 months in response to prolonged treatment with intravenous pulses of methylprednisolone (48). Kopp et al (unpublished data) have tried to replicate this finding in adults using oral dexamethasone pulses but the results have not been as promising as the reports in children. Thus, overall, steroids are not a uniformly effective therapy in patients with FSGS. Moreover, the significant side effects profile substantially limits the use of steroids in patients with hypertension, overweight, or bone disease.

In steroid resistant patients, cyclophosphamide has been used as a second-line agent with mixed results. It is ineffective in children but alkylating agents have a modest rate of achieving a partial or complete remission in adult patients. Cyclosporine is the only agent that is routinely prescribed to patients with steroid resistant FSGS and which has been assessed in a RCT. In children and adults, the calcineurin inhibitor is associated with higher rate of complete or partial remission (49,50). However, it is uncertain if the short term response in proteinuria translates into an improved long-term prognosis. Moreover, the nephrotoxicity of cyclosporine is a major drawback to its use. Tacrolimus, another calcineurin inhibitor with less risk of kidney injury, has been shown to be effective in small studies involving pediatric and adult patients with an overall response rate of approximately 50–75% (51,52). It has its own unique side effects including induction of diabetes (53). The appropriate plasma level of calcineurin inhibitors to achieve a reduction in proteinuria without provoking kidney injury is unknown. All other agents that have been prescribed for patients with FSGS, including mycophenolate mofetil, have only been studied in small patient series and usually without a suitable concurrent control group (54,55).

The newest agent that has generated excitement as a potential therapy is rituximab, the monoclonal antibody to CD20 on B-cells. The initial evidence of efficacy came from individual patients who had post-transplant lymphoproliferative disease and concomitant FSGS and in whom administration of rituximab achieved a complete remission (56). At present, there have been numerous reports involving relatively small numbers of patients with varying treatment regimens and duration of follow-up. In general, the rate of complete and/or partial remission is 25–40% (57). The role of B-cells in the pathogenesis of FSGS remains poorly defined. This underscores the need for a multicenter RCT to evaluate the proper utilization of this costly and potentially toxic agent in patients with FSGS who are resistant to corticosteroids and one other immunosuppressive agent.

Finally, it is worth noting that regardless of the underlying cause of FSGS, patients with disease that is resistant to current therapies will manifest progressive renal fibrosis and steady loss of kidney function. This raises the possibility that, absent a defined treatment target based on an improved understanding of the cause of FSGS, therapies that reduce fibrosis may help halt or slow the progression of FSGS. The Novel therapies in Resistant FSGS (FONT) Clinical Trial is an NIDDK-funded (DK70341) multicenter Phase I/II study (NCT00814255, www.fonttrial.org) designed to identify new agents that will act as antifibrotic agents. Currently two agents – adalimumab (anti-TNF-α antibody and galactose) are undergoing evaluation compared to standard conservative medical therapy (combined treatment with lisinopril, losartan, and atorvastatin) (58).

TRANSPLANTATION

Similar to patients with other glomerular disorders, those with FSGS who progress to ESKD are candidates for kidney transplantation. However, approximately 20–25% will develop recurrent disease in the allograft. Recurrence is more than twice as common in children as in adults and is associated with inferior long-term graft survival (59). The risk of recurrent FSGS post-transplantation approaches 80% in patients who have experienced this complication in a prior allograft. In fact, in many centers, the development of FSGS in a previous allograft is often considered a relative contraindication to repeat transplantation (59). This adverse outcome can occur within hours and as late as 1 year of transplantation. Although patients with mesangial hypercellularity on the original diagnostic biopsy and those who follow a more rapid deteriorating course in their progression to ESKD are art higher risk of recurrence, there are no reliable clinical markers to predict the outcome in individual patients (59). In addition, while initial reports suggested that high levels of a circulating permeability factor were a poor prognostic marker for recurrence FSGS post-transplantation (43,44), subsequent studies indicate that a positive result for this in vitro test is not a consistent finding in those who develop FSGS in the allograft. Thus, the role of plasmapheresis prior to and immediately post-operatively in preventing recurrent FSGS remains a controversial topic (60). Finally, while it was anticipated that patients with a genetic mutation leading to FSGS would not experience recurrent disease, this outcome has been described in at most 5–10% of patients in this subgroup (61,62).

The treatment of recurrent disease in the allograft is generally similar to the management of FSGS in the native kidney. Calcineurin inhibitors, in conjunction with angiotensin converting enzyme inhibitors, are the cornerstone of most regimens. Case reports have suggested that addition low density lipoprotein absorption columns may improve the outcome (61). The only two unique treatments in this context are plasmapheresis and rituximab. The response to plasma therapy varies, with increased efficacy documented in children (~60%) compared to adults (30–40%) (59). In a series of 4 adults with recurrent disease, combined therapy with plasmapheresis and rituximab achieved a complete response in 2 patients and partial response in 2 patients, with stabilization of kidney function (64). Similar findings have been described in two series with 14 and 4 children, respectively, at high risk of recurrent disease post-transplantation (65,66). However, in another case report of 3 patients with recurrent FSGS after a second, third, and fourth transplant, the combination of plasmapheresis and rituximab was ineffective in inducing a remission or reduction in proteinuria (67)

FUTURE DIRECTIONS

Patients with FSGS are searching for better answers to why they got sick and how to get better. Key items that should be on the research agenda to improve the care and outcome for these patients include: (1) defining better histopathological markers of the disease; (2) identifying non-invasive urine and serum biomarkers to improve the delineation of prognosis and response to therapy; (3) establishing an organized infrastructure to conduct long-term observational studies to elucidate pathogenesis (e.g. NEPTUNE, the Nephrotic Syndrome Study Network that is supported by the NIDDK and the Office of Rare Diseases) and to perform RCT (e.g. FONT); and (4) fostering better collaboration between the nephrology community and the pharmaceutical/biotechnology industries to expedite the timely performance of clinical studies.