Bodo B. Beck

Bone Mineral Content per Muscle Cross‐Sectional Area as an Index of the Functional Muscle‐Bone Unit

Bone densitometric data often are difficult to interpret in children and adolescents because of large inter‐ and intraindividual variations in bone size. Here, we propose a functional approach to bone densitometry that addresses two questions: Is bone strength normally adapted to the largest physiological loads, that is, muscle force? Is muscle force adequate for body size? To implement this approach, forearm muscle cross‐sectional area (CSA) and bone mineral content (BMC) of the radial diaphysis were measured in 349 healthy subjects from 6 to 19 years of age (183 girls), using peripheral quantitative computed tomography (pQCT). Reference data were established for height‐dependent muscle CSA and for the variation with age in the BMC/muscle CSA ratio. These reference data were used to evaluate results from three pediatric patient groups: children who had sustained multiple fractures without adequate trauma (n = 11), children with preterminal chronic renal failure (n = 11), and renal transplant recipients (n = 15). In all three groups mean height, muscle CSA, and BMC were low for age, but muscle CSA was normal for height. In the multiple fracture group and in renal transplant recipients the BMC/muscle CSA ratio was decreased (p < 0.05), suggesting that bone strength was not adapted adequately to muscle force. In contrast, chronic renal failure patients had a normal BMC/muscle CSA ratio, suggesting that their musculoskeletal system was adapted normally to their (decreased) body size. This functional approach to pediatric bone densitometric data should be adaptable to a variety of densitometric techniques.

The primary hyperoxalurias

The primary hyperoxalurias (PHs) are rare disorders of glyoxylate metabolism in which specific hepatic enzyme deficiencies result in overproduction of oxalate. Due to the resulting severe hyperoxaluria, recurrent urolithiasis or progressive nephrocalcinosis are principal manifestations. End stage renal failure frequently occurs and is followed by systemic oxalate deposition along with its devastating effects. Due to the lack of familiarity with PHs and their heterogeneous clinical expressions, the diagnosis is often delayed until there is advanced disease. In recent years, improvements in medical management have been associated with better patient outcomes. Although there are several therapeutic options that can help prevent early kidney failure, the only curative treatment to date is combined liver-kidney transplantation in patients with type I PH. Promising areas of investigation are being identified. Knowledge of the spectrum of disease expression, early diagnosis, and initiation of treatment before renal failure are essential to realize a benefit for patients.

Early angiotensin-converting enzyme inhibition in Alport syndrome delays renal failure and improves life expectancy.

Alport syndrome inevitably leads to end-stage renal disease and there are no therapies known to improve outcome. Here we determined whether angiotensin-converting enzyme inhibitors can delay time to dialysis and improve life expectancy in three generations of Alport families. Patients were categorized by renal function at the initiation of therapy and included 33 with hematuria or microalbuminuria, 115 with proteinuria, 26 with impaired renal function, and 109 untreated relatives. Patients were followed for a period whose mean duration exceeded two decades. Untreated relatives started dialysis at a median age of 22 years. Treatment of those with impaired renal function significantly delayed dialysis to a median age of 25, while treatment of those with proteinuria delayed dialysis to a median age of 40. Significantly, no patient with hematuria or microalbuminuria advanced to renal failure so far. Sibling pairs confirmed these results, showing that earlier therapy in younger patients significantly delayed dialysis by 13 years compared to later or no therapy in older siblings. Therapy significantly improved life expectancy beyond the median age of 55 years of the no-treatment cohort. Thus, Alport syndrome is treatable with angiotensin-converting enzyme inhibition to delay renal failure and therapy improves life expectancy in a time-dependent manner. This supports the need for early diagnosis and early nephroprotective therapy in oligosymptomatic patients.

ARHGDIA mutations cause nephrotic syndrome via defective RHO GTPase signaling

Nephrotic syndrome (NS) is divided into steroid-sensitive (SSNS) and -resistant (SRNS) variants. SRNS causes end-stage kidney disease, which cannot be cured. While the disease mechanisms of NS are not well understood, genetic mapping studies suggest a multitude of unknown single-gene causes. We combined homozygosity mapping with whole-exome resequencing and identified an ARHGDIA mutation that causes SRNS. We demonstrated that ARHGDIA is in a complex with RHO GTPases and is prominently expressed in podocytes of rat glomeruli. ARHGDIA mutations (R120X and G173V) from individuals with SRNS abrogated interaction with RHO GTPases and increased active GTP-bound RAC1 and CDC42, but not RHOA, indicating that RAC1 and CDC42 are more relevant to the pathogenesis of this SRNS variant than RHOA. Moreover, the mutations enhanced migration of cultured human podocytes; however, enhanced migration was reversed by treatment with RAC1 inhibitors. The nephrotic phenotype was recapitulated in arhgdia-deficient zebrafish. RAC1 inhibitors were partially effective in ameliorating arhgdia-associated defects. These findings identify a single-gene cause of NS and reveal that RHO GTPase signaling is a pathogenic mediator of SRNS.

Muscle Analysis by Measurement of Maximal Isometric Grip Force: New Reference Data and Clinical Applications in Pediatrics

Skeletal muscle development is one of the key features of childhood and adolescence. Determining maximal isometric grip force (MIGF) using a hand-held Jamar dynamometer is a simple method to quantify one aspect of muscle function. Presently available reference data present MIGF as a function of chronological age. However, muscle force is largely determined by body size, and many children undergoing muscle performance tests in the clinical setting suffer from growth retardation secondary to a chronic disorder. Reference data were established from simple regressions between age or log height and log MIGF in a population of 315 healthy children and adolescents aged 6 to 19 y (157 girls). These data were used to calculate age- or height-dependent SD scores (SDS) for MIGF in three pediatric patient groups. In renal graft recipients (n = 14), the age-dependent MIGF SDS was markedly decreased (−2.5 ± 1.9; mean ± SD). However, these patients had short stature (height SDS, −2.5 ± 1.2), and the height-dependent MIGF SDS was close to normal (−0.4 ± 1.5). Similarly, in cystic fibrosis patients (n = 13) age-dependent MIGF SDS was −1.6 ± 1.6, but height-dependent MIGF SDS was −0.5 ± 1.1. Children with epilepsy who were taking anticonvulsant therapy (n = 34) had normal stature, and consequently age- and height-dependent MIGF SDS were similar (0.4 ± 1.0 and 0.4 ± 0.8, respectively). In conclusion, MIGF determination provides information on an important aspect of physical development. Height should be taken into account to avoid misinterpretation.

C3 deposition glomerulopathy due to a functional Factor H defect

Factor H defect Sandra Habbig, Michael J. Mihatsch, Stefan Heinen, Bodo Beck, Mathias Emmel, Christine Skerka, Michael Kirschfink, Bernd Hoppe, Peter F. Zipfel and Christoph Licht Division of Pediatric Nephrology, Children’s Hospital of the University of Cologne, Cologne, Germany; Institute of Pathology, Basel, Switzerland; Leibniz Institute for Natural Products Research and Infection Biology, Jena, Germany; Department of Pediatric Cardiology, Children’s Hospital of the University of Cologne, Cologne, Germany; Institute of Immunology, University of Heidelberg, Heidelberg, Germany; Friedrich Schiller University, Jena, Germany; Division of Pediatric Nephrology, The Hospital for Sick Children; University of Toronto, Toronto, Ontario, Canada

Nephrocalcinosis and urolithiasis in children.

The incidence of adult urolithiasis has increased significantly in industrialized countries over the past decades. Sound incidence rates are not available for children, nor are they known for nephrocalcinosis, which can appear as a single entity or together with urolithiasis. In contrast to the adult kidney stone patient, where environmental factors are the main cause, genetic and/or metabolic disorders are the main reason for childhood nephrocalcinosis and urolithiasis. While hypercalciuria is considered to be the most frequent risk factor, several other metabolic disorders such as hypocitraturia or hyperoxaluria, as well as a variety of renal tubular diseases, e.g., Dents disease or renal tubular acidosis, have to be ruled out by urine and/or blood analysis. Associated symptoms such as growth retardation, intestinal absorption, or bone demineralization should be evaluated for diagnostic and therapeutic purposes. Preterm infants are a special risk population with a high incidence of nephrocalcinosis arising from immature kidney, medication, and hypocitraturia. In children, concise evaluation will reveal an underlying pathomechanism in >75% of patients. Early treatment reducing urinary saturation of the soluble by increasing fluid intake and by providing crystallization inhibitors, as well as disease-specific medication, are mandatory to prevent recurrent kidney stones and/or progressive nephrocalcinosis, and consequently deterioration of renal function.

Vitamin B6 in primary hyperoxaluria I: first prospective trial after 40 years of practice.

BACKGROUND AND OBJECTIVES Primary hyperoxaluria type I (PH I) is caused by deficiency of the liver-specific enzyme alanine-glyoxylate:aminotransferase (AGT). Many mutations are known to perturb AGT protein folding. Vitamin B6 (B6) is the only specific drug available for treatment. Although B6 has been used for >40 years, controlled data on B6 efficacy are lacking. Therefore, this study investigated the absolute and relative change of urinary oxalate (Uox) excretion under increasing dosages of B6, the first prospective trial to do so. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS B6 response was studied in 12 patients (7 male patients) with genetically confirmed PH I (3 Gly170Arg homozygous, 5 compound Gly170Arg and/or Phe152Ile heterozygous, and 4 negative for Gly170Arg and/or Phe152Ile mutations) and noncompromised renal function. Efficacy was defined as a 30% relative reduction in Uox excretion. B6 was administered orally starting at 5 mg/kg body weight per day and given in increments of 5 mg/kg every 6 weeks, up to a final dosage of 20 mg/kg per day at week 24. Uox and serum B6 levels were measured every 6 weeks. RESULTS Mean relative Uox reduction was 25.5%. Uox declined from 2.09±0.55 (mean±SD) at baseline to 1.52±0.60 mmol/1.73 m(2) per day (P=0.01) at week 24. Serum B6 levels increased from 22.5±8.7 to 1217±776 ng/ml (P<0.001). Six patients showed a ≥30% relative reduction of Uox at week 24. CONCLUSION This first prospective trial confirmed B6 efficacy in 50% of patients (three of three homozygous, one of five heterozygous, and two of four patients negative for the Gly170Arg and/or Phe152Ile mutations). Interestingly, no complete biochemical remission was observed, even in the homozygous Gly170Arg study participants. Future trials are necessary to learn more about genotype-related B6 response and B6 metabolism.

Polyhydramnios, Transient Antenatal Bartter’s Syndrome, and MAGED2 Mutations

BACKGROUND Three pregnancies with male offspring in one family were complicated by severe polyhydramnios and prematurity. One fetus died; the other two had transient massive salt-wasting and polyuria reminiscent of antenatal Bartters syndrome. METHODS To uncover the molecular cause of this possibly X-linked disease, we performed whole-exome sequencing of DNA from two members of the index family and targeted gene analysis of other members of this family and of six additional families with affected male fetuses. We also evaluated a series of women with idiopathic polyhydramnios who were pregnant with male fetuses. We performed immunohistochemical analysis, knockdown and overexpression experiments, and protein-protein interaction studies. RESULTS We identified a mutation in MAGED2 in each of the 13 infants in our analysis who had transient antenatal Bartters syndrome. MAGED2 encodes melanoma-associated antigen D2 (MAGE-D2) and maps to the X chromosome. We also identified two different MAGED2 mutations in two families with idiopathic polyhydramnios. Four patients died perinatally, and 11 survived. The initial presentation was more severe than in known types of antenatal Bartters syndrome, as reflected by an earlier onset of polyhydramnios and labor. All symptoms disappeared spontaneously during follow-up in the infants who survived. We showed that MAGE-D2 affects the expression and function of the sodium chloride cotransporters NKCC2 and NCC (key components of salt reabsorption in the distal renal tubule), possibly through adenylate cyclase and cyclic AMP signaling and a cytoplasmic heat-shock protein. CONCLUSIONS We found that MAGED2 mutations caused X-linked polyhydramnios with prematurity and a severe but transient form of antenatal Bartters syndrome. MAGE-D2 is essential for fetal renal salt reabsorption, amniotic fluid homeostasis, and the maintenance of pregnancy. (Funded by the University of Groningen and others.).

Novel findings in patients with primary hyperoxaluria type III and implications for advanced molecular testing strategies

Identification of mutations in the HOGA1 gene as the cause of autosomal recessive primary hyperoxaluria (PH) type III has revitalized research in the field of PH and related stone disease. In contrast to the well-characterized entities of PH type I and type II, the pathophysiology and prevalence of type III is largely unknown. In this study, we analyzed a large cohort of subjects previously tested negative for type I/II by complete HOGA1 sequencing. Seven distinct mutations, among them four novel, were found in 15 patients. In patients of non-consanguineous European descent the previously reported c.700+5G>T splice-site mutation was predominant and represents a potential founder mutation, while in consanguineous families private homozygous mutations were identified throughout the gene. Furthermore, we identified a family where a homozygous mutation in HOGA1 (p.P190L) segregated in two siblings with an additional AGXT mutation (p.D201E). The two girls exhibiting triallelic inheritance presented a more severe phenotype than their only mildly affected p.P190L homozygous father. In silico analysis of five mutations reveals that HOGA1 deficiency is causing type III, yet reduced HOGA1 expression or aberrant subcellular protein targeting is unlikely to be the responsible pathomechanism. Our results strongly suggest HOGA1 as a major cause of PH, indicate a greater genetic heterogeneity of hyperoxaluria, and point to a favorable outcome of type III in the context of PH despite incomplete or absent biochemical remission. Multiallelic inheritance could have implications for genetic testing strategies and might represent an unrecognized mechanism for phenotype variability in PH.