Yaacov Frishberg

Mutations in the Mitochondrial Seryl-tRNA Synthetase Cause Hyperuricemia, Pulmonary Hypertension, Renal Failure in Infancy and Alkalosis, HUPRA Syndrome

An uncharacterized multisystemic mitochondrial cytopathy was diagnosed in two infants from consanguineous Palestinian kindred living in a single village. The most significant clinical findings were tubulopathy (hyperuricemia, metabolic alkalosis), pulmonary hypertension, and progressive renal failure in infancy (HUPRA syndrome). Analysis of the consanguineous pedigree suggested that the causative mutation is in the nuclear DNA. By using genome-wide SNP homozygosity analysis, we identified a homozygous identity-by-descent region on chromosome 19 and detected the pathogenic mutation c.1169A>G (p.Asp390Gly) in SARS2, encoding the mitochondrial seryl-tRNA synthetase. The same homozygous mutation was later identified in a third infant with HUPRA syndrome. The carrier rate of this mutation among inhabitants of this Palestinian isolate was found to be 1:15. The mature enzyme catalyzes the ligation of serine to two mitochondrial tRNA isoacceptors: tRNA(Ser)(AGY) and tRNA(Ser)(UCN). Analysis of amino acylation of the two target tRNAs, extracted from immortalized peripheral lymphocytes derived from two patients, revealed that the p.Asp390Gly mutation significantly impacts on the acylation of tRNA(Ser)(AGY) but probably not that of tRNA(Ser)(UCN). Marked decrease in the expression of the nonacylated transcript and the complete absence of the acylated tRNA(Ser)(AGY) suggest that this mutation leads to significant loss of function and that the uncharged transcripts undergo degradation.

Hyperostosis-hyperphosphatemia syndrome: a congenital disorder of O-glycosylation associated with augmented processing of fibroblast growth factor 23.

Two hyperphosphatemic patients with mutations in GALNT3 showed low intact FGF23 levels with marked increase of processed C‐terminal fragments. FGF23 protein has three O‐linked glycans and FGF23 with incomplete glycosylation is susceptible to processing. Silencing GALNT3 resulted in enhanced processing of FGF23. Decreased function of FGF23 by enhanced processing is the cause of hyperphosphatemia in patients with GALNT3 mutation.

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.

Mutations of NPHP2 and NPHP3 in infantile nephronophthisis

Nephronophthisis is an autosomal recessive chronic tubulointerstitial disease that progresses to end-stage renal disease (ESRD) in about 10% of cases during infancy. Mutations in the INVS (NPHP2) gene were found in a few patients with infantile nephronophthisis. Mutations of NPHP3, known to be associated with adolescent nephronophthisis, were found in two patients with early-onset ESRD. Here we screened 43 families with infantile nephronophthisis (ESRD less than 5 years of age) for NPHP2 and NPHP3 mutations and determined genotype-phenotype correlations. In this cohort there were 16 families with NPHP2 mutations and NPHP3 mutations in seven. Three patients carried only one heterozygous mutation in NPHP3. ESRD arose during the first 2 years of life in 16 of 18 patients with mutations in NPHP2, but in only two patients with mutations in NPHP3. Renal morphology, characterized by hyper-echogenic kidneys on ultrasound and tubular lesions with interstitial fibrosis on histology, was similar in the two patient groups. The kidney sizes were highly diverse and ultrasound-visualized cysts were present in a minority of cases. Extra-renal anomalies were found in 80% of the entire cohort including hepatic involvement (50%), cardiac valve or septal defects (20%) and recurrent bronchial infections (18%). We show that NPHP3 mutations in both infantile and adolescent nephronophthisis point to a common pathophysiological mechanism despite their different clinical presentations.

Primary hyperoxaluria type III gene HOGA1 (Formerly DHDPSL) as a possible risk factor for idiopathic calcium oxalate urolithiasis

BACKGROUND AND OBJECTIVES Primary hyperoxaluria types I and II (PHI and PHII) are rare monogenic causes of hyperoxaluria and calcium oxalate urolithiasis. Recently, we described type III, due to mutations in HOGA1 (formerly DHDPSL), hypothesized to cause a gain of mitochondrial 4-hydroxy-2-oxoglutarate aldolase activity, resulting in excess oxalate. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS To further explore the pathophysiology of HOGA1, we screened additional non-PHI-PHII patients and performed reverse transcription PCR analysis. Postulating that HOGA1 may influence urine oxalate, we also screened 100 idiopathic calcium oxalate stone formers. RESULTS Of 28 unrelated hyperoxaluric patients with marked hyperoxaluria not due to PHI, PHII, or any identifiable secondary cause, we identified 10 (36%) with two HOGA1 mutations (four novel, including a nonsense variant). Reverse transcription PCR of the stop codon and two common mutations showed stable expression. From the new and our previously described PHIII cohort, 25 patients were identified for study. Urine oxalate was lower and urine calcium and uric acid were higher when compared with PHI and PHII. After 7.2 years median follow-up, mean eGFR was 116 ml/min per 1.73 m(2). HOGA1 heterozygosity was found in two patients with mild hyperoxaluria and in three of 100 idiopathic calcium oxalate stone formers. No HOGA1 variants were detected in 166 controls. CONCLUSIONS These findings, in the context of autosomal recessive inheritance for PHIII, support a loss-of-function mechanism for HOGA1, with potential for a dominant-negative effect. Detection of HOGA1 variants in idiopathic calcium oxalate urolithiasis also suggests HOGA1 may be a predisposing factor for this condition.

Dent’s disease manifesting as focal glomerulosclerosis: Is it the tip of the iceberg?

Dent’s disease is an X-linked proximal tubulopathy. It often manifests in childhood with symptoms of Fanconi syndrome and low-molecular-weight proteinuria. We describe four boys from three unrelated families whose only presenting symptoms of Dent’s disease were nephrotic-range proteinuria and histological findings of focal segmental and/or global glomerulosclerosis. In all families, a causal mutation in the CLCN5 gene, encoding a voltage-gated chloride transporter and chloride-proton exchanger, was identified. All three mutations are pathogenic: two are novel (p.Asp727fs and p.Trp122X), and one is a recurrent mutation, p.R648X. Given the atypical phenotype of these patients with Dent’s disease, it is possible that this clinical entity is markedly underdiagnosed and that our report represents only the tip of the iceberg. The diagnosis of Dent’s disease should be considered in all patients with nephrotic-range proteinuria without hypoalbuminemia or edema. Establishing the diagnosis of Dent’s disease will prevent the administration of unnecessary immunosuppressive medications with their undesirable side effects.

Risk Factors for Cardiovascular Disease in Children and Young Adults after Renal Transplantation

Despite good outcomes in pediatric renal transplantation, life expectancy is reduced, mostly as a result of accelerated atherosclerosis. A comprehensive evaluation of cardiac status and risk factors for cardiovascular disease was performed in 60 patients after renal transplantation (age 3 to 29 yr; mean 15.8). Posttransplantation diabetes was diagnosed in 7%. Half of the patients did not engage in any physical activity, and this was associated with increased body mass index. Uncontrolled hypertension was found in 13% of patient, and 53% were on antihypertensive medications. BP index was associated with left ventricular mass index (LVMI). Dyslipidemia was relatively uncommon, with hypercholesterolemia found in 15% and elevated LDL cholesterol found in 10% of patients. Hyperhomocysteinemia was frequent (58%); in most patients, it was not due to folate or B(12) deficiency. Lipid and homocysteine abnormalities were associated with cyclosporine therapy. Echocardiography demonstrated normal LVMI in 93% of patients, although LVMI was higher than in healthy control subjects. Cardiac troponin I was normal in all patients, but N-terminal pro-brain natriuretic peptide was elevated in 35% and was associated with LVMI and renal function. Although present cardiac status is relatively normal in pediatric renal transplantation patients, cardiac risk factors are common, and strategies to prevent cardiovascular disease need to be developed.

A comprehensive study of cardiovascular risk factors, cardiac function and vascular disease in children with chronic renal failure

BACKGROUND Cardiovascular disease causes major morbidity and is an important determinant of premature death in the paediatric chronic kidney disease (CKD) population. It is composed of three separate, although interrelated, disease processes: atherosclerosis, arteriosclerosis (i.e. medial vascular calcifications) and myocardial disease. Myocardial consequences of atherosclerosis barely exist in children, thus providing a good opportunity to investigate the role that kidney disease plays in the development of cardiovascular disease. METHODS We assessed 70 patients, aged 4 months to 18 years, with chronic kidney disease stages 3-5, for known risk factors of cardiovascular disease and for additional laboratory and clinical variables which may have an impact on this disease process. Carotid artery ultrasound was used to evaluate vascular structure and function, whereas myocardial disease was assessed by echocardiography. RESULTS Traditional risk factors, although present in this cohort, did not accumulate with progression of chronic kidney disease. Non-traditional risk factors increased in number and severity in correlation with the stage of CKD. The main myocardial abnormalities were left ventricular hypertrophy and diastolic dysfunction. Vascular function tests correlated with calcium-phosphate metabolism variables, homocysteine and time-averaged serum uric acid. CONCLUSIONS This study shows that children with CKD are exposed to risk factors and demonstrate signs of cardiovascular disease already at a young age. The possible role of uric acid and homocysteine in the evolution of cardiovascular disease is discussed. Further studies looking at possible interventions to prevent cardiovascular morbidity and mortality in this high risk population are needed.

Maternal and infantile hypercalcemia caused by vitamin-D-hydroxylase mutations and vitamin D intake.

BackgroundHypercalcemia is caused by many different conditions and may lead to severe complications. Loss-of-function mutations of CYP24A1, encoding vitamin D-24-hydroxylase, have recently been identified in idiopathic infantile hypercalcemia and in adult kidney stone disease. The aim of this study was to investigate the genetics and clinical features of both infantile and maternal hypercalcemia.MethodsWe studied members of four unrelated Israeli families with hypercalcemia, namely, one woman during pregnancy and after delivery and three infants. Clinical and biochemical data were obtained from probands’ medical charts. Genomic DNA was isolated from peripheral blood and CYP24A1 was sequenced.ResultsTypical symptoms of hypercalcemia associated with the intake of recommended doses of vitamin D developed in the infants and pregnant woman. Four different loss-of-function CYP24A1 mutations were identified, two of which are reported here for the first time (p.Trp134Gly and p.Glu315*). The infants from families 1 and 2, respectively, were found to be compound heterozygotes, and the infant from family 3 and the pregnant woman were found to be homozygous.ConclusionsThis is the first report of maternal hypercalcemia caused by a CYP24A1 mutation, showing that not only infants are at risk for this complication. Our findings emphasize the importance of recognition, genetic diagnosis and proper treatment of this recently identified hypercalcemic disorder in this era of widespread vitamin D supplements.

Recurrent Nephrotic Syndrome in Homozygous Truncating NPHS2 Mutation Is Not Due to Anti‐Podocin Antibodies

Mutations in NPHS2 are a common cause of focal segmental glomerulosclerosis (FSGS). It was initially assumed that FSGS caused by a genetically defective protein in the native kidney would not recur after transplantation; however, description of three patients with NPHS2 missense mutations challenged the validity of this assumption. A possible mechanism of recurrence in cases with stop‐codon mutations is the formation of auto‐antibodies against the truncated protein. In this case report, we describe a 9‐year‐old girl with the R138X NPHS2 mutation who presented with recurrent nephrotic syndrome 4 years after renal transplantation from a deceased donor, and was treated with plasmapheresis with a partial response. Renal histology did not demonstrate glomerular immunoglobulin deposition and an extensive search for anti‐podocin antibodies based on indirect Western blot with recombinant podocin, was negative, as was the test for glomerular permeability factor (Palb). Taken together these findings confirm the possibility of post transplantation nephrotic syndrome in patients with NPHS2 mutations. Lack of immunoglobulin deposition, absence of circulating anti‐podocin antibodies, and normal Palb suggest that other, unknown pathogenetic mechanisms are implicated.