|Year : 2019 | Volume
| Issue : 1 | Page : 56-60
Christy Cathreen Thomas
Department of Pediatrics, Division of Pediatric Nephrology, All India Institute of Medical Sciences, New Delhi, India
|Date of Web Publication||17-May-2019|
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Thomas CC. Journal Scan. Asian J Pediatr Nephrol 2019;2:56-60
| Can Readthrough Agents Offer Personalized Genomic Medicine to Patients With Cystinosis Due to Nonsense Mutations?|| |
Cystinosis, the most common cause of renal Fanconi syndrome in childhood, is caused by homozygous or compound heterozygous mutations in CTNS gene, which encodes the Lysosomal transport protein cystinosin. The consequent lysosomal accumulation of cystine has multisystemic manifestations, with an average life expectancy of 30 years. The current standard of care is oral and intraocular therapy with cysteamine, a sulfhydryl compound that chemically reduces cystine to form mixed disulfides that can exit the lysosome through an alternative route, PQLC2 channels. However, noncompliance to therapy is common because the drug is expensive and unpalatable, requires frequent dosing, and is associated with halitosis and other adverse effects. Further, despite significantly reduced intralysosomal cystine levels, end-stage renal disease (ESRD) is inevitable, suggesting that cystinosinb or CTNS may have functionally relevant nonchannel properties. Although the most common cause of cystinosis worldwide is a 57-kB deletion spanning exons 1–10 and an upstream region, 15% of cases have heterozygous or homozygous nonsense mutations, of which W138X is the most common.
A nonsense mutation causes insertion of a stop (or premature termination) codon, leading to generation of a truncated protein that is usually nonfunctional, and the mRNA is degraded rapidly by nonsense-mediated messenger RNA (mRNA) decay. Aminoglycoside antibiotics, such as geneticin (G418) and gentamicin, bind to ribosomes and promote insertion of near-cognate aminoacyl-transfer RNAs, thus inhibiting the termination of translation at stop codons. Hence, these agents, termed readthrough agents, ensure functional translation leading to increase in mRNA synthesis and reduced mRNA decay, which in turn, leading to increased synthesis of functioning protein. The efficacy of aminoglycosides as readthrough agents has previously been demonstrated in mouse models of Duchenne muscular dystrophy. In a recent paper, Brasell et al. demonstrated that G418 reads through CTNS mRNA, restores full-length CTNS mRNA and protein, and reduces pathologic cystine accumulation in patient fibroblasts harboring the nonsense W138X mutation.
The authors examined the levels of CTNS mRNA and intracellular half-cystine (cysteine), before and after therapy with geneticin, in fibroblasts harboring homozygous nonsense W138X mutation, homozygous 57-kB deletion, and compound heterozygous mutations of W138X and 57-kB deletion or an exon 10 frameshift mutation (1035insC). In cell lines with homozygous W138X mutation, therapy with geneticin was associated with significant increase in CTNS mRNA expression, from 14% to 38%-100% of values in normal fibroblasts, and decline in the levels of half cystine to 28%–44% of baseline values. CTNS mRNA expression and half-cystine levels were not altered in fibroblasts with homozygous 57-kB deletion. The reduction in half-cystine levels in fibroblasts with homozygous nonsense mutations was comparable to the reduction, to 36% of baseline value, achieved by adding cysteamine to fibroblast cell culture. The degree of increase in mRNA expression and reduction in half-cystine levels were less marked (to 36%–59% of baseline values) in fibroblasts harboring one heterozygous W138X mutation and a deletion or missense mutation on the other allele.
While these findings represent a breakthrough in terms of a novel genomic approach to therapy for cystinosis, practical considerations preclude the direct application of geneticin to human clinical studies. Geneticin and its analog gentamicin accumulate in proximal tubular cells, where their effects are desired, but have significant toxicity at doses that require to be administered. Further, other readthrough agents, such as ataluren that held promise of cure for nonsense mutations in cystic fibrosis and Duchenne muscular dystrophy, failed to show therapeutic efficacy in human studies. Future studies should examine less toxic aminoglycoside-derived readthrough agents in in vivo as well as in vitro models of disease before these can be considered for clinical studies in humans.
Brasell EJ, Chu L, El Kares R, Seo JH, Loesch R, Iglesias DM, et al. The aminoglycoside geneticin permits translational readthrough of the CTNS W138X nonsense mutation in fibroblasts from patients with nephropathic cystinosis. Pediatr Nephrol 2019;34:873-81.
| Role of Next-Generation Sequencing in Enabling The Diagnosis of End-Stage Renal Disease in Children and Young Adults|| |
The availability of next-generation, massive parallel sequencing has enabled improved understanding of the underlying disease in clinical and nonresearch settings. Unraveling the genetic basis of disease might create opportunities for prevention or targeted therapies in several scenarios. Whole-exome sequencing permits evaluation of all exons, around 1% of the human genome, that account for 85% of the relevant pathogenic genetic variations. ESRD in children and young adults is chiefly caused by various forms of congenital anomalies of kidneys and urinary tract (CAKUT), ciliopathies, and focal segmental glomerulosclerosis (FSGS). In the absence of a clear etiology, there are no recommendations to perform genetic testing to establish the diagnosis of chronic kidney disease (CKD). Biochemical, histological, and other evaluations have low yield in the presence of ESRD; for example, urine and serum biochemical parameters are not useful in the diagnosis of primary hyperoxaluria or hypomagnesemic hypercalciuria in a setting of low glomerular filtration rate. Further, with atypical clinical presentation, the diagnosis is often missed. Establishing accurate diagnosis of the etiology of renal disease might have significant implications for management, such as delaying progression to ESRD using specific therapy for cystinosis and coenzyme Q-related nephropathy, favoring combined liver–kidney rather than isolated kidney transplantation in patients with primary hyperoxaluria, facilitating peritransplant management with eculizumab in patients with atypical hemolytic-uremic syndrome mediated by inherited defects in complement regulation, avoiding peritransplant measures to prevent disease recurrence in patients with FSGS caused by inherited defects, and enabling choice of donors in patients with Alport syndrome. The major limiting factor in genetic diagnosis has been lack of access to testing outside research settings and the expenditure as well as limited expertise in bioinformatics and interpretation of results of testing.
A recent study from Boston Children's Hospital reports results of whole-exome sequencing in 104 patients who underwent renal transplantation between 2007 and 2017 and had onset of CKD before the age of 25 years. Based on the available clinical details, cases were assigned to one of the following six broad etiological categories: CAKUT, steroid-resistant nephrotic syndrome (SRNS), chronic glomerulonephritis (GN), renal cystic ciliopathy, nephrolithiasis or nephrocalcinosis, and ESRD of unknown etiology. Following whole-exome sequencing, variants were compared to human reference genome, and variations with low minor allele frequency (<1%) in any of the 396 genes, selected by the investigators considering the available data, were examined further using standard bioinformatic tools for pathogenicity prediction. The gene list included 41 genes known to be associated with isolated CAKUT, 50 genes associated with SRNS, 17 genes for chronic GN, 95 genes for renal cystic ciliopathies, and 37 genes for urinary stone disease; an additional 145 genes were included that have been reported to be associated with syndromic CAKUT and 11 genes to be associated with unspecified CKD. Clinician-scientists and geneticists with knowledge of the clinical phenotypes and pedigree, and experience with exome evaluation, performed variant calling. A likely genetic cause for the disease was obtained in 32.7% of individuals. The yield of genetic testing was highest for renal cystic ciliopathies (78%), ESRD of unknown etiology (44%), and SRNS (43%). The yield was low for patients with CAKUT (1%) and chronic GN (14%). The likelihood of finding a genetic cause was higher for patients with parental consanguinity, extrarenal manifestations, and family history of renal disease. Following reverse phenotyping, minor extrarenal manifestation was also detected in a few patients. The authors also highlight five cases in whom the management would have differed had the genetic diagnosis been available earlier.
In this single-center, retrospective study that included referred patients, the risk of selection bias was high which affects the generalizability of results. Whole-exome sequencing is known to miss large deletions, copy number variations, and mutations in promoter and intronic regions. The average coverage of the included genes was around 58×, which might be insufficient. Use of an approach targeting a panel of genes obviated the possibility of novel gene discovery. When extrapolated to the clinical setting, where clinical rather than whole-exome sequencing is likely to be ordered in a nonselected cohort, the yield of genetic testing is likely to be lower. Still, the manuscript has important messages for clinicians. It informs the utility of genetic testing in patients with CKD in whom etiology has not been established, particularly in the presence of family history or consanguinity, and emphasizes on the need for genetic testing in patients with CKD in whom an inherited defect is likely, such as those with SRNS and nephrolithiasis or nephrocalcinosis.
Mann N, Braun DA, Amann K, Tan W, Shril S, Connaughton DM, et al. Whole-exome sequencing enables a precision medicine approach for kidney transplant recipients. J Am Soc Nephrol 2019;30:201-15.
| An International Consensus On Imaging Of Kidney Cysts And Cystic Kidney Diseases In Children|| |
The diagnosis of cystic kidney disease is challenging, particularly in early childhood, because kidney cysts are more often caused by inherited conditions in than in adulthood where simple or acquired cysts predominate, and etiological classifications based on imaging are lacking. With increasing emphasis on antenatal ultrasonography, the frequency with which kidney cysts are diagnosed prenatally has increased. Despite detailed attention to the clinical, biochemical, and radiological features, it is often difficult to distinguish between cysts caused by dysplasia and those in inherited ciliopathies. An understanding of etiology has important implications for outcomes and management, including the intensity and frequency of renal and extrarenal imaging and clinical follow-up.
A recent consensus statement from a consortium of European experts in pediatric and adult nephrology and radiology and pediatric ultrasonology, termed “NEOCYST” (Network for Early Onset CYSTic Kidney Disease), is an important step toward establishing uniform standards for the choice, performance, and reporting of various imaging modalities in the diagnosis of kidney cysts detected in childhood. The statement provides diagnostic criteria for the most common cystic kidney diseases in childhood and adolescence, proposes operational approaches to diagnosis and follow-up in important clinical settings, and provides recommendations on the choice of imaging modalities for individual cystic diseases.
The statement emphasizes ultrasonography as the preferred study in initial and follow-up evaluation and monitoring of kidney cysts with or without associated underlying conditions. The indications of magnetic resonance imaging are limited to the following three scenarios: ruling out malignancy in a setting of complex cysts, evaluation for cysts and angiomyolipoma in tuberous sclerosis complex, and measurement of total kidney or cyst volume in clinical trials in autosomal polycystic kidney disease. Computed tomography is not recommended routinely due to the risk of radiation. Disease-specific recommendations are also provided, for simple cysts, multicystic dysplastic kidney, cystic dysplasia, hepatocyte nuclear factor-1β-associated disease, nephronophthisis, Bardet–Biedl syndrome, autosomal dominant and autosomal recessive polycystic kidney disease, tuberous sclerosis complex, complex cysts, cystic tumors, and acquired cystic kidney disease.
Gimpel C, Avni EF, Breysem L, Burgmaier K, Caroli A, Cetiner M, et al. Imaging of kidney cysts and cystic kidney diseases in children: An international working group consensus statement. Radiology 2019;290:769-82.
| Clinical Trial to Evaluate the Safety and Efficacy of Cinacalcet in Correcting Hyperparathyroidism in Children Receiving Dialysis|| |
Secondary hyperparathyroidism is a well-described component of mineral bone disease associated with advanced CKD and is linked to bony deformities, growth retardation, and vascular calcifications. Conventionally, the management of hyperparathyroidism is guided by serum levels of calcium, phosphorus, alkaline phosphatase, 25-hydroxyvitamin D, and parathormone (PTH), and involves dietary restriction of phosphorus intake; use of phosphate binders, 25-hydroxyvitamin D analogs, and/or active vitamin D; and parathyroidectomy. Cinacalcet is a calcimimetic that allosterically activates the calcium-sensing receptor on parathyroid gland to inhibit PTH production, and is conventionally used when other modalities fail and there is concomitant hypercalcemia. The Kidney Disease Improving Global Outcome guidelines suggest its use in secondary hyperparathyroidism in adults on dialysis (CKD Stage 5D). The Evaluation of Cinacalcet Hydrochloride Therapy to Lower Cardiovascular Events trial in adults with CKD 5D showed persistent hypocalcemia in the group of patients treated with cinacalcet. Unlike in adults in whom avoiding hypercalcemia, rather than hypocalcemia, is clinically more relevant due to its association with vascular calcification, hypocalcemia is quite undesirable in children because it lowers bone mineral density Z scores in growing cortical bones. Past studies on cinacalcet use in children are limited to small, single-center, retrospective or prospective observational studies and case reports.
In a multicentric, double-blind, placebo-controlled, randomized Phase 3 study, Warady et al. evaluated the efficacy and safety of cinacalcet in children, 6–18 years old, receiving hemodialysis or peritoneal dialysis stably for >2 months, who had secondary hyperparathyroidism (intact PTH [iPTH] >300 pg/ml), normocalcemia (corrected serum calcium ≥8.8 mg/dl), normal or elevated serum phosphorus (≥4.0 mg/dl if <12 years old or ≥3.5 mg/dL if ≥12 years old), and on stable doses, if any, of active vitamin D and/or recombinant growth hormone (ClinicalTrials.gov identifier: NCT01277510). Eligible patients were randomized to receive either cinacalcet or placebo during a 30-week, randomized, double-blind, placebo-controlled phase, followed by cinacalcet in a 30-week, open-label phase. After initiating therapy at ≤0.2 mg/kg of dry weight, the dose of intervention (cinacalcet or placebo) was titrated every 4 weeks, to a maximum of 180 mg or 4.2 mg/kg, to achieve the primary objective of ≥30% reduction from baseline in mean iPTH and secondary objective of iPTH <300 pg/ml. Other secondary end points were the percentage change in serum levels of calcium and phosphate every 4 weeks; nature, frequency, severity, and relationship to treatment of all adverse events; and change in growth velocity from baseline. Patients continued to receive calcium supplements, phosphate binders, or vitamin D analogs according to physicians' discretion.
The sample size was estimated at 50 patients/group; however, the occurrence of one patient's death after 43 patients had been enrolled, in 26 centers across 9 countries, led to premature termination of the study. Baseline characteristics were similar for 22 patients randomized to receive cinacalcet and 21 patients allocated to placebo. Before the study was held up, a significantly higher proportion of patients on cinacalcet versus placebo (55% vs. 19%) achieved the primary end point (P = 0.017). The proportions of patients who achieved iPTH ≤300 pg/ml (27% vs. 24%) were comparable. The mean maximum actual weight-adjusted daily cinacalcet dosage administered was 0.99 mg/kg/day. There were no significant differences (95% confidence intervals) between the groups in percentage change from baseline in levels of total serum calcium (−4 [−9, 1] %), phosphorus (−6 [−21, 8] %), and Ca × P product (−10 [−23, 3] %).
More than one therapy-emergent adverse events were observed in similar proportions of patients receiving cinacalcet or placebo (82% vs. 86%), most common of which were vomiting (32% vs. 24%), hypocalcemia (23% vs. 19%), nausea (18% vs. 14%), and hypertension (14% vs. 24%). Severe hypocalcemia was observed in 14% of patients receiving cinacalcet and none in the placebo group. The overall incidence of hypocalcemia was higher in patients receiving cinacalcet, particularly in the open-label extension phase, in which 40% of patients had hypocalcemia. The death observed during the study was in an adolescent girl with baseline QT prolongation who expired following acute diarrhea and had low corrected calcium (5.3 mg/dl).
This study was well designed, but its early discontinuation led to premature discontinuation of therapy in 31.8% of patients in the cinacalcet group versus 23.8% of patients receiving placebo. The power to detect the proposed difference in proportions achieving the primary end point, recalculated for the available sample size, was 82% at an alpha level of 0.05, suggesting adequate power to detect meaningful difference. However, the opportunity to study the effect on growth velocity and Tanner staging was lost due to the early termination.
In children with CKD on dialysis and uncontrolled secondary hyperparathyroidism, therapy with cinacalcet might be effective in controlling hyperparathyroidism, but requires close monitoring and timely intervention for hypocalcemia. Future studies should be planned with careful exclusion of children with baseline QT prolongation and close monitoring for hypocalcemia.
Warady BA, Iles JN, Ariceta G, Dehmel B, Hidalgo G, Jiang X, et al. A randomized, double-blind, placebo-controlled study to assess the efficacy and safety of cinacalcet in pediatric patients with chronic kidney disease and secondary hyperparathyroidism receiving dialysis. Pediatr Nephrol 2019;34:475-86.
| Evidence for Safety and Efficacy of Oral Enarodustat, A Hypoxia-Inducible Factor-Prolyl Hydroxylase Inhibitor, In the Therapy Of Anemia of CKD|| |
Anemia, a common complication of CKD, is caused by erythropoietin deficiency, functional iron deficiency, and decreased red blood cell survival, and is managed with oral or intravenous (IV) iron and subcutaneous or IV erythropoiesis-stimulating agents (ESAs). Improved understanding of renal regulation of erythropoiesis has led to the advent of agents that increase endogenous erythropoietin production, such as hypoxia-inducible factor-prolyl hydroxylase inhibitors (HIF-PHIs). HIF is a heterodimeric hypoxia-inducible protein that activates gene transcription of erythropoietin as well as hepcidin, which is involved in iron absorption and homeostasis. Hydroxylation at proline residues makes HIF susceptible to degradation by prolyl hydroxylase enzymes. Inhibitors of prolyl hydroxylase stabilized the levels of HIF. HIF-PHIs are expected to increase erythropoiesis as well as increase iron availability for proerythrocytes. Their potential for angiogenesis raises concerns of proliferative retinopathy and certain cancers. Available agents, such as roxadustat, daprodustat, molidustat, and vadadustat, have been demonstrated to increase hemoglobin levels in incident CKD patients in Phase 2 clinical trials.
In a multicentric, randomized, double-blind, placebo-controlled, parallel-arm clinical trial study conducted across 54 centers in Japan, adult patients with CKD not on dialysis were randomized to receive a novel HIF-PHI, enarodustat, at 2, 4, or 6 mg, or placebo, in a ratio of 1:1:1:1. The short-term efficacy and safety of therapy, given once daily, was assessed at 6 weeks. During a subsequent, open-label, uncontrolled phase of the study, the long-term safety and dose of the agent, required to maintain hemoglobin levels at 10–12 g/dl, was examined over 24 weeks. Patients were stratified into two groups: a correction group, comprising ESA-naïve patients (ESA not received for ≥12 weeks before screening visit) and a conversion group (ESA-treated patients on stable doses of ESA at screening visit). The primary end points were the rate of increase of hemoglobin per week in the correction group and the proportion of patients that maintained a hemoglobin level within ±1.0 g/dl of baseline in the conversion group.
Of 94 patients randomized in the correction group and 107 in the conversion group, 77 and 97 patients, respectively, completed the randomized clinical trial. In the correction group, the rate of rise of hemoglobin level per week was significantly higher than in placebo limb and increased in a dose-related manner across enarodustat groups. Values of the rate of increase in hemoglobin were −0.02 ± 0.03 g/dl/week in placebo arm and 0.14 ± 0.03 g/dl/week, 0.19 ± 0.03 g/dl/week, and 0.44 ± 0.04 g/dl/week in the 2-mg, 4-mg, and 6-mg arms, respectively. However, in the conversion group, hemoglobin levels could be maintained within ± 1.0 g/dl of baseline, by comparable proportions of patients receiving placebo or various doses of enarodustat; this included 54% of patients in the placebo arm and 81%, 70%, and 50% of patients receiving enarodustat at 2 mg, 4 mg, and 6 mg, respectively. The levels of hepcidin and ferritin were lower, and those of total iron-binding capacity higher, in patients randomized to receive enarodustat versus placebo, with significantly higher changes in these levels between baseline and 6 weeks, in patients receiving enarodustat at 4 mg and 6 mg compared to those receiving placebo. Adverse events, recorded in 5% of cases, were not related to drug therapy. There were no clinically significant changes in vital signs, 12-lead electrocardiogram, chest X-ray, fundoscopy, estimated glomerular filtration rate, or urine protein. Levels of vascular endothelial growth factor and erythropoietin were also similar between the groups. The open-label phase included 77 individuals in the correction group and 90 individuals in the conversion group, of whom 66 and 77 individuals completed the study, respectively. At the end of phase 2, 71% of individuals in the correction group and 79% in the conversion group maintained hemoglobin level within the target range, using mean prescribed doses of 3.6 mg/day and 3.7 mg/day, respectively.
The authors concluded that enarodustat corrects and maintains hemoglobin levels in anemic adult patients with CKD not on dialysis and is safe. The study indicates the potential for orally administered HIF inhibitors to effectively increase endogenous erythropoietin production and control iron metabolism while avoiding high ESA doses and iron overload. Future studies should examine prolonged therapy to address concerns of vascular proliferation and carcinogenic potential and reassess efficacy in patients converted from ESA to HIF inhibitors.
Akizawa T, Nangaku M, Yamaguchi T, Arai M, Koretomo R, Matsui A, et al. A placebo-controlled, randomized trial of enarodustat in patients with chronic kidney disease, followed by long-term trial. Am J Nephrol 2019;49:165-74.
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