Daniella Magen

Mitochondrial Hsp60 Chaperonopathy Causes an Autosomal-Recessive Neurodegenerative Disorder Linked to Brain Hypomyelination and Leukodystrophy

Hypomyelinating leukodystrophies (HMLs) are disorders involving aberrant myelin formation. The prototype of primary HMLs is the X-linked Pelizaeus-Merzbacher disease (PMD) caused by mutations in PLP1. Recently, homozygous mutations in GJA12 encoding connexin 47 were found in patients with autosomal-recessive Pelizaeus-Merzbacher-like disease (PMLD). However, many patients of both genders with PMLD carry neither PLP1 nor GJA12 mutations. We report a consanguineous Israeli Bedouin kindred with clinical and radiological findings compatible with PMLD, in which linkage to PLP1 and GJA12 was excluded. Using homozygosity mapping and mutation analysis, we have identified a homozygous missense mutation (D29G) not previously described in HSPD1, encoding the mitochondrial heat-shock protein 60 (Hsp60) in all affected individuals. The D29G mutation completely segregates with the disease-associated phenotype. The pathogenic effect of D29G on Hsp60-chaperonin activity was verified by an in vivo E. coli complementation assay, which demonstrated compromised ability of the D29G-Hsp60 mutant protein to support E. coli survival, especially at high temperatures. The disorder, which we have termed MitCHAP-60 disease, can be distinguished from spastic paraplegia 13 (SPG13), another Hsp60-associated autosomal-dominant neurodegenerative disorder, by its autosomal-recessive inheritance pattern, as well as by its early-onset, profound cerebral involvement and lethality. Our findings suggest that Hsp60 defects can cause neurodegenerative pathologies of varying severity, not previously suspected on the basis of the SPG13 phenotype. These findings should help to clarify the important role of Hsp60 in myelinogenesis and neurodegeneration.

A Loss-of-Function Mutation in NaPi-IIa and Renal Fanconi's Syndrome

We describe two siblings from a consanguineous family with autosomal recessive Fanconis syndrome and hypophosphatemic rickets. Genetic analysis revealed a homozygous in-frame duplication of 21 bp in SLC34A1, which encodes the renal sodium-inorganic phosphate cotransporter NaPi-IIa, as the causative mutation. Functional studies in Xenopus laevis oocytes and in opossum kidney cells indicated complete loss of function of the mutant NaPi-IIa, resulting from failure of the transporter to reach the plasma membrane. These findings show that disruption of the human NaPi-IIa profoundly impairs overall renal phosphate reabsorption and proximal-tubule function and provide evidence of the critical role of NaPi-IIa in human renal phosphate handling.

Low Infection Rates and Prolonged Survival Times of Hemodialysis Catheters in Infants and Children

BACKGROUND AND OBJECTIVES Hemodialysis (HD) catheter-related complications are regarded as the main cause of HD failure in infants and children with ESRD. In this study, we determined HD catheter infection rates and survival times in children. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS We analyzed demographic, clinical, laboratory, and microbiologic data on all infants and children with ESRD who received HD therapy through a tunneled central venous catheter (CVC) in our Pediatric Dialysis Unit between January 2001 and December 2009. Our strict care of HD-CVCs makes no use of any kind of prophylactic antibiotic therapy. RESULTS Twenty-nine children with ESRD (median age, 10 years) received HD through a CVC, for a total of 22,892 days during the study period. Eleven (38%) children were infants (<1 year of age) who received HD for a cumulative 3779 days (16% of total). Fifty-nine CVCs were inserted, of which 13 (22%) were in infants. There were 12 episodes of CVC infection-a rate of 0.52/1000 CVC days. Four (33%) episodes occurred in infants-a rate of 1.06/1000 CVC days. Only three (5%) of the CVCs were removed because of infection. Median catheter survival time for all children was 310 days and for infants was 211 days. CONCLUSIONS Very low CVC infection rates (one infection per 5 CVC years) and prolonged CVC survival times (around 1 year) are achievable in infants and children with ESRD receiving HD therapy by adhering to a strict catheter management protocol and without using prophylactic antibiotic therapy.

Fanconi-Bickel Syndrome and Autosomal Recessive Proximal Tubulopathy with Hypercalciuria (ARPTH) Are Allelic Variants Caused by GLUT2 Mutations

CONTEXT Many inherited disorders of calcium and phosphate homeostasis are unexplained at the molecular level. OBJECTIVE The objective of the study was to identify the molecular basis of phosphate and calcium abnormalities in two unrelated, consanguineous families. PATIENTS The affected members in family 1 presented with rickets due to profound urinary phosphate-wasting and hypophosphatemic rickets. In the previously reported family 2, patients presented with proximal renal tubulopathy and hypercalciuria yet normal or only mildly increased urinary phosphate excretion. METHODS Genome-wide linkage scans and direct nucleotide sequence analyses of candidate genes were performed. Transport of glucose and phosphate by glucose transporter 2 (GLUT2) was assessed using Xenopus oocytes. Renal sodium-phosphate cotransporter 2a and 2c (Npt2a and Npt2c) expressions were evaluated in transgenically rescued Glut2-null mice (tgGlut2-/-). RESULTS In both families, genetic mapping and sequence analysis of candidate genes led to the identification of two novel homozygous mutations (IVS4-2A>G and R124S, respectively) in GLUT2, the gene mutated in Fanconi-Bickel syndrome, a rare disease usually characterized by renal tubulopathy, impaired glucose homeostasis, and hepatomegaly. Xenopus oocytes expressing the [R124S]GLUT2 mutant showed a significant reduction in glucose transport, but neither wild-type nor mutant GLUT2 facilitated phosphate import or export; tgGlut2-/- mice demonstrated a profound reduction of Npt2c expression in the proximal renal tubules. CONCLUSIONS Homozygous mutations in the facilitative glucose transporter GLUT2, which cause Fanconi-Bickel syndrome, can lead to very different clinical and biochemical findings that are not limited to mild proximal renal tubulopathy but can include significant hypercalciuria and highly variable degrees of urinary phosphate-wasting and hypophosphatemia, possibly because of the impaired proximal tubular expression of Npt2c.

Plasmapheresis in a very young infant with atypical hemolytic uremic syndrome

Abstract Atypical hemolytic uremic syndrome (HUS) is a heterogeneous group of disorders, the pathogenesis of which is unclear. Plasma transfusions and plasmapheresis are widely used modes of therapy for adults with this life-threatening syndrome. There is very limited experience in using plasmapheresis therapy in children and infants with atypical HUS. Plasmapheresis, which is considered a relatively safe procedure in adults and older children, may be hazardous in neonates and very young infants and can result in severe complications. We report a 2-month-old infant with idiopathic atypical HUS, who was successfully treated with a 1-month course of plasmapheresis during the acute phase of the disease. Appropriate preparations as well as several adjustments were made in order to meet the special needs of this very young infant who, to the best of our knowledge, is the youngest reported patient with atypical HUS to undergo plasmapheresis. Plasmapheresis therapy of the infant was not associated with any complications of the procedure and resulted in marked clinical improvement. We conclude that plasmapheresis in neonates and in very small infants is technically feasible, can be performed without major complications, and may be of benefit in individual cases.

Erratum to: Autosomal recessive lissencephaly with cerebellar hypoplasia is associated with a loss-of-function mutation in CDK5

Lissencephaly comprises a heterogeneous group of developmental brain disorders of varying severity, involving abnormal cortical gyration. We studied a highly consanguineous Israeli Moslem family with a lethal form of autosomal recessive lissencephaly with cerebellar hypoplasia (LCH). Using microarray-based homozygosity mapping in the reported family, combined with whole exome sequencing in one affected infant, we identified a homozygous splice site mutation g.IVS8+1G>A in cyclin-dependent kinase 5 (CDK5), causing complete skipping of exon 8, and leading to a frame shift and premature stop codon (p.V162SfsX19). The mutation co-segregated with the disease phenotype in all 29 study participants (4 patients and 25 healthy relatives), and was not identified in 200 ethnically matched control chromosomes. The p.V162SfsX19 mutation causes lack of endogenous CDK5 expression in affected dermal fibroblasts and brain tissue at the mRNA and protein levels, consistent with nonsense-mediated mRNA decay. Functional analysis of the p.V162SfsX19 mutation, using a yeast complementation assay, showed loss-of-function of the mutant CDK5 gene product, thereby implicating its role in the pathogenesis of autosomal recessive LCH in the studied family.

Loss of CD55 in Eculizumab-Responsive Protein-Losing Enteropathy

CD55 (complement decay-accelerating factor) inhibits the alternative and classical arms of the complement pathway. Three patients with protein-losing enteropathy and a genetic variant predicted to result in loss of function of CD55 had a response to eculizumab.

A Dominant Mutation in Nuclear Receptor Interacting Protein 1 Causes Urinary Tract Malformations via Dysregulation of Retinoic Acid Signaling

Congenital anomalies of the kidney and urinary tract (CAKUT) are the most common cause of CKD in the first three decades of life. However, for most patients with CAKUT, the causative mutation remains unknown. We identified a kindred with an autosomal dominant form of CAKUT. By whole-exome sequencing, we identified a heterozygous truncating mutation (c.279delG, p.Trp93fs*) of the nuclear receptor interacting protein 1 gene (NRIP1) in all seven affected members. NRIP1 encodes a nuclear receptor transcriptional cofactor that directly interacts with the retinoic acid receptors (RARs) to modulate retinoic acid transcriptional activity. Unlike wild-type NRIP1, the altered NRIP1 protein did not translocate to the nucleus, did not interact with RARα, and failed to inhibit retinoic acid-dependent transcriptional activity upon expression in HEK293 cells. Notably, we also showed that treatment with retinoic acid enhanced NRIP1 binding to RARα RNA in situ hybridization confirmed Nrip1 expression in the developing urogenital system of the mouse. In explant cultures of embryonic kidney rudiments, retinoic acid stimulated Nrip1 expression, whereas a pan-RAR antagonist strongly reduced it. Furthermore, mice heterozygous for a null allele of Nrip1 showed a CAKUT-spectrum phenotype. Finally, expression and knockdown experiments in Xenopus laevis confirmed an evolutionarily conserved role for NRIP1 in renal development. These data indicate that dominant NRIP1 mutations can cause CAKUT by interference with retinoic acid transcriptional signaling, shedding light on the well documented association between abnormal vitamin A levels and renal malformations in humans, and suggest a possible gene-environment pathomechanism in this disease.

Genetic disorders of renal phosphate transport.

To the Editor: In their comprehensive review of genetic disorders of renal phosphate transport, Prié and Friedlander (June 24 issue)1 claim that the role of NPT2a in renal phosphate handling has been shown by the association between heterozygous mutations in NPT2a and renal phosphate leak.2 However, Virkki et al.3 found no dominant negative effect of these NPT2a variants, which indicates that they are polymorphisms. Lapointe et al.4 also identified NPT2a variants in patients with renal phosphate leak, but ruled out their functional significance by the finding of normal renal phosphate balance in family members carrying the same variants. The major contribution of NPT2a to renal phosphate balance was definitively established in a recent article in the Journal that reported autosomal recessive hypophosphatemic rickets with renal Fanconi’s syndrome secondary to a loss-of-function mutation in SLC34A1 (also known as NPT2a), the gene that encodes NPT2a (also known as NaPi-IIa).5

MPGN type I induced by granulocyte colony stimulating factor.

Abstract We report a girl with severe congenital neutropenia who has received long-term granulocyte-colony stimulating factor (G-CSF) therapy and has developed macroscopic hematuria, proteinuria, and decreased renal function associated with biopsy-proven membranoproliferative glomerulonephritis (MPGN) type I. Temporary discontinuation of G-CSF therapy as well as the use of glycosylated G-CSF has resulted in improvement in renal manifestations. We postulate that the MPGN was G-CSF- induced. Long-term G-CSF therapy should be used with great caution and close surveillance of kidney function.