Assessment of Renal Pathology and Dysfunction in Pediatric Patients with Fabry Disease

Abstract

Overt renal disease often first presents in males with Fabry disease in early-to-mid adulthood, but proteinuria and reduced glomerular filtration rate may occur in adolescents and in young children. More recently, kidney biopsy data have shown early renal histological changes in pediatric patients. Renal investigations and their timing in children remain poorly defined. A consensus on renal investigations is necessary to understand the natural progression of the disease and to evaluate the efficacy of treatments such as enzyme replacement therapies. This manuscript addresses three main categories, including the use of glomerular filtration rates, measuring albuminuria and renal biopsies in children.

Introduction

Progressive Fabry nephropathy is one of the main features of Fabry disease (1-3) and is marked by an insidious development. By adulthood, renal failure frequently becomes a major complication of Fabry disease, with over half of males and more than 20% of female patients eventually developing advanced or end stage renal disease (ESRD) (4). Although typically occurring by the third to fifth decade of life in males with Fabry disease (1), ESRD, can occur as early as 16 years of age (5). Renal pathology can arise in pediatric Fabry patients but indicators are difficult to assess.

Microalbuminuria is one of the first signs of impairment of renal function (6, 7) and overt proteinuria may start as early as 10 years of age (5), however, chronic kidney lesions may already be present (8). In young patients, glomerular hyperfiltration (9-11) can mask the detection of early decline in glomerular filtration rate (GFR) to the extent that a critical number of nephrons are damaged and cannot maintain adequate glomerular filtration. The decline in GFR typically commences once proteinuria is established (1), but may precede it (4).

In adults, there is compelling evidence that proteinuria is an indicator of renal dysfunction in Fabry disease (2, 3) requiring immediate intervention with enzyme replacement therapy (ERT) and/or antiproteinuric medications (12). However, there are no clear guidelines for initiation of ERT in Fabry children and these decisions are usually based on clinical judgment, for indications such as improving quality of life, and patient or family requests. There are currently no early biomarkers or predictors of disease progression in practice. However, microalbuminuria is a non-invasive measure which may be an early indicator of renal disease and is recommended and important to monitor. Although renal biopsies could have significant prognostic value, few studies have focused on the early renal pathologic lesions of Fabry (13, 8) and thus further evaluation is needed before biopsy is recommended as a standard tool to assess renal disease in young Fabry patients. A further consideration is that diagnostic tests such as GFR can be overestimated, particularly in the early stages of chronic kidney disease (CKD) (14). The objective of the present article is to summarize the diagnostic measures in the early stages of Fabry nephropathy and to assess their value as outcome measures for therapeutic decisions.

What Do We Know?

The glomerular barrier is a complex biological membrane with high filtration rates of water (despite the absence of water channels), non-restricted passage of small and middle-sized molecules, and almost total restriction of serum albumin and larger proteins (15). Alpha galactosidase A (α-Gal A) deficiency results in lysosomal accumulation of globotriaosylceramide (GL-3) in all kidney cell types and over time can cause irreversible functional damage to the glomerular barrier (13). Kidney cell GL-3 inclusions have been detected in fetuses affected with Fabry’s, especially in podocytes (16, 17), and Gubler et al showed abundant GL-3 inclusions in all glomerular cells and in vessels in 3 children 8 to 12 years old without proteinuria (13), albeit with a more heterogeneous distribution in females. Arteriopathy was present in some children with Fabry’s. More recently, renal lesions in biopsies from 9 Fabry children with normal GFR aged 7 to 18 years have been described (8). There were large numbers of GL-3 inclusions in podocytes and distal tubules in all patients. Importantly, there was segmental foot process effacement, consistent with podocyte injury in all cases including those without proteinuria or microalbuminuria. Four of the 9 children had arteriopathy, similar to the lesions described by Gubler et al (13). It is noteworthy that all Fabry pediatric patients in this study were symptomatic with acroparaesthesiae and more than 80% (7-8/9) of patients also had gastrointestinal symptoms, opthalmological findings and autonomic dysfunction with hypohydrosis prior to the renal biopsies. These 9 patients would therefore fall into a group of children with Fabry presenting with early onset of significant and severe symptoms. In this study, 3/9 patients in their adolescent years did not have microalbuminuria at baseline (Female aged 14 years; males aged 11 years and 18 years respectively), presumably indicating a more slowly progressive disease despite their observed renal histopathological changes. Two thirds of the patients (6/9), however, did have microalbuminuria at baseline.

With limited evidence based literature currently available with respect to the progression of renal disease, microalbuminuria seems to be a sensitive and noninvasive marker to evaluate renal disease in young children, which can be easily performed by every treating Fabry clinician worldwide. Wilcox et al showed that urinary protein levels in the microalbuminuria or proteinuria ranges are present in almost all adult Fabry registry patients where this was measured (4). Thus, in adults microalbuminuria cannot be a precise predictor of serious Fabry nephropathy, since not all patients, especially females would be expected to progress to advanced kidney disease (4). On the other hand, earlier onset of microalbuminuria in children may well prove to be of stronger predictive value.

In adult patients with Fabry disease, the natural progression of renal disease has been delineated rather extensively (1-3). Recently, Schiffmann et al have shown that the yearly decline in estimated GFR (eGFR) is −3.0 ml/min/1.73 m² in males with baseline eGFR >60 ml/min/1.73 m² and −6.8 ml/min/1.73 m² in males with advanced CKD at baseline (3). Progression rates for females were 0.9 and −2.1 ml/min/1.73 m², respectively. As previously demonstrated by Branton et al (1), more rapid progression was found in patients who had higher baseline proteinuria and/or chronic renal insufficiency at baseline.

No such large data sets on progression rates are available for Fabry children. The contribution of children, both boys and girls, to the total number of Fabry patients included in the registries (18, 19) is only around 20%, and asymptomatic young subjects may not be included at some centers. Therefore, the present registry data might be biased towards more severely affected children. Among children and adolescents with Fabry disease included in four published analyses (18-21), kidney dysfunction, primarily proteinuria, appeared to be more common in girls (18% of 97) than in boys (8% of 127). In the renal biopsy study of 9 symptomatic pediatric patients (7 males, 2 females; 7-8 years) who all had electron microscopically-detected lesions, mean albumin-creatinine ratio was increased at 38 mg/mg (range 5.3-104.3 mg/mg; and mean of 53mg/mg in 6/9 patients who had microalbuminuria) whereas measured glomerular filtration rate (Iohexol-GFR) was normal in all patients (8).

It is generally recommended that the eGFR slope should be monitored as an important outcome measure in Fabry nephropathy. However, recent reports demonstrate that eGFR, usually by the Modification of Diet in Renal Disease (MDRD) formula in adults (22) and the Schwartz formula (23-25) in children (26, 19, 10), may result in a substantial overestimation of the true GFR in adult male patients with Fabry disease (14, 27). Most of these methodologies rely on serum creatinine, which is measured in different ways including kinetic and Endpoint Jaffe method and enzymatic assays. Variations in serum creatinine measured by different assay methods can cause significant differences in estimated GFR values (28) and these discrepancies are even greater in children (29, 30). The new Schwartz 09 formula (31) using creatinine measured by the enzymatic method reported that nearly 80% of eGFR values were within 30% of the measured GFR and similar results are also seen with the Counahan-Barrat method first described in 1976 (32). Measured GFR (mGFR) is only rarely performed (33, 27) presumably due to the nature of these investigations being time consuming procedures requiring IV’s to be placed, with some using radioactive probes. On the other hand, Schwartz has concluded that, for children, “at this time, there is no dependable substitute for an accurately determined GFR, and Iohexol plasma disappearance offers the best combination of safety, accuracy, and reproducible precision” (34). Based on this, it may be reasonable to recommend a baseline GFR measurement prior to ERT initiation in children with correlation with the serum creatinine determined eGFR and repeated GFR measurement as clinically indicated. Likewise, a better standardization of albumin measurements is to be considered since urine albumin has been measured by nephelometry (8) or by routine radioimmunoassay (21) which can give negative results.

ERT has been shown to rapidly reduce GL-3 in endothelial and mesangial cells (35) in adult patients and, if started early, it may prevent or slow renal dysfunction (12, 36, 37). However, once proteinuria is established, responsiveness to ERT is incomplete and proteinuria generally does not normalize (38) although it has been demonstrated that enzyme substitution therapy will reach podocytes and renal endothelial cells (39). Studies of ERT in children (23-48 weeks) has shown that treatment was safe and well tolerated (9-11). ERT generally resulted in stable or reduced microalbuminuria and no worsening in eGFR in those with abnormal baseline values.

What Do We Not Know?

The renal biopsy studies in pediatric patients strongly suggest that Fabry nephropathy, similar to many other slowly progressive chronic kidney diseases, such as diabetic nephropathy or chronic hypertension, may progress in clinical silence. However, the number of patients in the Fabry renal biopsy studies is very small (less than 10 patients), and represent a cohort presenting early with a severe phenotype. Two thirds of these patients in the Tondel study presented with microalbuminuria as discussed earlier. We do not know how and at what pace the disease progresses in individual Fabry patients. For example, in Fabry males with kidney disease susceptibility, serious renal disease may manifest as early as the teenage years or as late as the 7th or 8^th decade of life (4). Proteinuria is not a sensitive marker of early injury, since advanced lesions have been detected in biopsies from normoalbuminuric Fabry patients (13, 8). A fraction of Fabry patients develop GFR loss before proteinuria (4). In addition, hyperfiltration may complicate detection of GFR loss since, if GFR is normal, this could represent a relative decline (40). Furthermore, we do not know when to initiate treatment and how much ERT is enough for each patient and if reduction in small amounts of albumin in the urine is a surrogate for treatment efficacy. There are as yet no studies that allow us to conclude that, given current dosing guidelines, ERT initiated in childhood will prevent or markedly delay the development of serious kidney disease in adulthood.

What Can We Do With What We Know

Renal biopsy is part of routine standard of care for many childhood and adult renal diseases to determine diagnosis, prognosis or to provide guidelines for management. Biopsies are regularly performed for their value as predictors of disease progression and to adjust treatment options in chronic kidney conditions in children, such as lupus nephritis, nephrotic syndrome, kidney transplant rejection, etc. (41, 42). Renal biopsy is considered in some centres to be clinically indicated if the outcome is reassurance of patients with long standing hematuria or low grade proteinuria and normal GFR (43). While Fabry nephropathy carries a far greater risk of progression to renal failure than these other disorders, renal biopsy has not yet become the mainstream of routine clinical care.

Careful measurement of urinary albumin excretion, estimation of GFR, and monitoring blood pressure must be included in the standard care of Fabry patients and, for reasons stated above, baseline formal measurement of GFR should be strongly considered. Some centers in the United States and in Norway include renal biopsy as part of their standard care of children with Fabry Disease. These biopsies are used to determine the extent of kidney involvement by the disease at baseline and, assuming that early treatment in children may represent the best way to correct early kidney damage and prevent progressive disease, whether ERT should be initiated. Despite currently available ERT protocols, heterogeneity of disease manifestations and progression in Fabry nephropathy does not support the strategy of uniform ERT dosage or other treatment for all patients (one size fits all). This was reflected in the study by Schiffmann et al where more frequent ERT resulted in slowing the GFR decline (38). An early intervention and dosing study over a five year period in young treatment-naïve male patients with Fabry Disease is currently being undertaken to address this (44). Although the benefit of serial renal biopsies in children with respect to therapeutic benefits of Fabry Disease has not been established, it is an important question that requires further evaluation before it is recommended as best practice in the management of Fabry Disease.

It is recommended to systematically evaluate all children in registries and to regularly and precisely measure albumin excretion rates. Total urinary protein (mg/24 hours) or microalbuminuria (micrograms/minute) obtained in at least 2 out of 3 consecutively timed urine collections are regularly measured in Fabry reference centers.

As mentioned, measured GFR methodologies require intravenous cannulation and sampling, and radioisotopic methods should be avoided in children. Estimated GFR calculated by the Counahan-Barratt and Schwartz 09 methodology are acceptable methods to calculate eGFR in children and are useful for both monitoring disease and to also assess treatment effects. However, it is important to consider measured GFR at baseline and monitoring when treatment modalities such as ERT are initiated in children.

Renal biopsy could potentially give us insight about disease progression and responsiveness to treatment. Although when to do a follow-up biopsy is not yet clear, studies show that marked treatment effects were present in many patients within 5 to 54 months of ERT in adult patients (45, 36). Currently, in the absence of adequate data on the timing of ERT and the benefits of renal biopsies, it is essential to perform meticulous and regular follow-up assessments particularly non-invasive renal function tests such as three consecutive early morning urine samples for microalbuminuira, arguably the most sensitive and readily available marker of early renal disease.

Key Research Questions

The identification of biomarkers of disease vulnerability, progression and treatment adequacy is a necessity. Discovery of non-invasive biomarkers will ease repetitive renal biopsy monitoring of patients without adding a risk. Structural-functional relationship studies based on information from renal biopsies and careful measurement of albuminuria and GFR are efficient tools to identify such predictors in chronic renal disease (46). Early predictors of disease progression can be chosen as primary endpoints of clinical trials and as guides to monitor ERT efficiency. Nevertheless, long-term studies are required to answer how ERT may affect the natural history of Fabry nephropathy in children.