Arno Fuchshuber

NPHS2, encoding the glomerular protein podocin, is mutated in autosomal recessive steroid-resistant nephrotic syndrome.

Familial idiopathic nephrotic syndromes represent a heterogeneous group of kidney disorders, and include autosomal recessive steroid-resistant nephrotic syndrome, which is characterized by early childhood onset of proteinuria, rapid progression to end-stage renal disease and focal segmental glomerulosclerosis. A causative gene for this disease, NPHS2, was mapped to 1q25–31 and we report here its identification by positional cloning. NPHS2 is almost exclusively expressed in the podocytes of fetal and mature kidney glomeruli, and encodes a new integral membrane protein, podocin, belonging to the stomatin protein family. We found ten different NPHS2 mutations, comprising nonsense, frameshift and missense mutations, to segregate with the disease, demonstrating a crucial role for podocin in the function of the glomerular filtration barrier.

Patients with Mutations in NPHS2 (Podocin) Do Not Respond to Standard Steroid Treatment of Nephrotic Syndrome

Nephrotic syndrome (NS) represents the association of proteinuria, hypoalbuminemia, edema, and hyperlipidemia. Steroid-resistant NS (SRNS) is defined by primary resistance to standard steroid therapy. It remains one of the most intractable causes of ESRD in the first two decades of life. Mutations in the NPHS2 gene represent a frequent cause of SRNS, occurring in approximately 20 to 30% of sporadic cases of SRNS. On the basis of a very small number of patients, it was suspected that children with homozygous or compound heterozygous mutations in NPHS2 might exhibit primary steroid resistance and a decreased risk of FSGS recurrence after kidney transplantation. To test this hypothesis, NPHS2 mutational analysis was performed with direct sequencing for 190 patients with SRNS from 165 different families and, as a control sample, 124 patients with steroid-sensitive NS from 120 families. Homozygous or compound heterozygous mutations in NPHS2 were detected for 43 of 165 SRNS families (26%). Conversely, no homozygous or compound heterozygous mutations in NPHS2 were observed for the 120 steroid-sensitive NS families. Recurrence of FSGS in a renal transplant was noted for seven of 20 patients with SRNS (35%) without NPHS2 mutations, whereas it occurred for only two of 24 patients with SRNS (8%) with homozygous or compound heterozygous mutations in NPHS2. None of 29 patients with homozygous or compound heterozygous mutations in NPHS2 who were treated with cyclosporine A or cyclophosphamide demonstrated complete remission of NS. It was concluded that patients with SRNS with homozygous or compound heterozygous mutations in NPHS2 do not respond to standard steroid treatment and have a reduced risk for recurrence of FSGS in a renal transplant. Because these findings might affect the treatment plan for childhood SRNS, it might be advisable to perform mutational analysis of NPHS2, if the patient consents, in parallel with the start of the first course of standard steroid therapy.

No evidence for genotype/phenotype correlation in NPHS1 and NPHS2 mutations

Primary steroid-resistant nephrotic syndrome (SRNS) is characterized by childhood onset of proteinuria and progression to end-stage renal disease. In 26% of cases it is caused by recessive mutations in NPHS2 (podocin). Congenital nephrotic syndrome (CNS) is caused by mutations in NPHS1 (nephrin) or NPHS2. In three families mutations in NPHS1 and NPHS2 had been reported to occur together, and these tri-allelic mutations were implicated in genotype/phenotype correlations. To further test the hypothesis of tri-allelism, we examined a group of 62 unrelated patients for NPHS1 mutations, who were previously shown to have NPHS2 mutations; 15 of 62 patients had CNS. In addition, 12 CNS patients without NPHS2 mutation were examined for NPHS1 mutations. Mutational analysis yielded three different groups. (1) In 48 patients with two recessive NPHS2 mutations (11 with CNS), no NPHS1 mutation was detected, except for 1 patient, who had one NPHS1 mutation only. This patient was indistinguishable clinically and did not have CNS. (2) In 14 patients with one NPHS2 mutation only (4 with CNS), we detected two additional recessive NPHS1 mutations in the 4 patients with CNS. They all carried the R229Q variant of NPHS2. The CNS phenotype may be sufficiently explained by the presence of two NPHS1 mutations. (3) In 12 patients without NPHS2 mutation (all with CNS), we detected two recessive NPHS1 mutations in 11 patients, explaining their CNS phenotype. We report ten novel mutations in the nephrin gene. Our data do not suggest any genotype/phenotype correlation in the 5 patients with mutations in both the NPHS1 and the NPHS2 genes.

Congenital nephrotic syndrome of the Finnish type: linkage to the locus in a non-Finnish population.

Congenital nephrotic syndrome of the Finnish type (CNF) is inherited as an autosomal recessive trait. The biochemical basis of the disease is unknown, although a lesion in the glomerular basement membrane is strongly suggested. Recently, the CNF locus was assigned to chromosome 19q12–q13.1 on the basis of linkage analysis in Finnish families. The high incidence of the disease in Finland, as well as the demonstration of linkage disequilibrium in the Finnish study, strongly suggests a founder effect based on a common ancient mutation in this population. We confirm linkage of the CNF locus to the same chromosomal region in seven non-Finnish CNF families without evidence of linkage disequilibrium. Our results show that the same gene seems to be affected in both Finnish and non-Finnish CNF populations. However, in the latter the mutation-carrying chromosomes descend from different ancestors without evidence of a founder effect.

Identification of the First Gene Locus (SSNS1) for Steroid-Sensitive Nephrotic Syndrome on Chromosome 2p

Disease mechanisms of steroid-sensitive nephrotic syndrome (SSNS) remain unknown. Whereas gene identification has furthered the understanding of pathomechanisms in steroid-resistant nephrotic syndrome (SRNS), not even a gene locus is known for SSNS. Total genome linkage analysis was performed in a consanguineous SSNS kindred to identify a gene locus for SSNS. Homozygosity mapping identified a locus for SSNS on chromosome 2p12-p13.2 between markers D2S292 and D2S289 (multipoint LOD score Z(max) = 3.01 at D2S145). The first gene locus for SSNS, as a first step to detect the responsible gene, was thus identified. There was clear evidence for genetic locus heterogeneity upon examination of ten additional families with SSNS.

Lack of large, homozygous deletions of the nephronophthisis 1 region in Joubert syndrome type B

Abstract.Joubert syndrome type B (JSB) is a developmental disorder of the nephronophthisis (NPH) complex with multiple organ involvement, including NPH, coloboma of the eye, aplasia of the cerebellar vermis, and the facultative symptoms of psychomotor retardation, polydactyly, and neonatal tachypnea. In isolated autosomal recessive NPH type 1 (NPH1), homozygous deletions have been described as causative in more than 80% of patients. Since different combinations of the extrarenal symptoms with NPH occur in JSB, a contiguous gene deletion syndrome in the NPH1 genetic region would seem a highly likely cause for JSB. We therefore examined 11 families with JSB for the presence of extended deletions at the NPH1 locus. Genomic DNA was examined using four consecutive polymerase chain reaction (PCR) markers that are deleted in NPH1 and three PCR makers flanking the NPH1 deletion. In all seven markers examined, there was no homozygous deletion detected in any of the 11 JSB families studied. Since these markers saturate the NPH1 deletion region at high density, this finding excludes the presence of large homozygous deletions of the NPH1 region in these JSB families, making it unlikely that deletions of the NPH1 region are a primary cause for JSB.

mut0 methylmalonic acidemia: eleven novel mutations of the methylmalonyl CoA mutase including a deletion-insertion mutation.

Methylmalonic aciduria (MMA) is an autosomal‐recessive disorder caused by inadequate function of methylmalonyl‐CoA mutase (MCM), a nuclear‐encoded, mitochondrial enzyme that uses adenosylcobalamin as a cofactor. Biochemical cell studies have delineated phenotypic variants: mut0 phenotypes in which there is no detectable enzymatic activity and mut‐ phenotypes in which there is residual cobalamin‐dependent activity. Mutation screening in MMA has led to the detection of 30 disease‐specific mutations. In 14 patients with the mut0 phenotype we found 11 novel mutations (K54X, A137V, F174S, 620insA, G203R, Q218H, A535P, H627R, 2085delG and 2270del4/ins5), 6 of them homozygous, consisting of 1 nonsense, 6 missense, 1 splice site, and 3 frame shift mutations. The position in relation to different functional domains in MCM allow for an interpretation of the identified mutations. Hum Mutat 16:179, 2000.

A gene locus for steroid-resistant nephrotic syndrome with deafness maps to chromosome 14q24.2

Steroid-resistant nephrotic syndrome (SRNS) leads to end-stage renal disease (ESRD) in childhood or young adulthood. Positional cloning for genes causing SRNS has opened the first insights into the understanding of its pathogenesis. This study reports a genome-wide search for linkage in a consanguineous Palestinian kindred with SRNS and deafness and detection of a region of homozygosity on chromosome 14q24.2. Multipoint analysis of 12 markers used for further fine mapping resulted in a LOD score Z(max) of 4.12 (theta = 0) for marker D14S1025 and a two-point LOD score of Z(max) = 3.46 (theta = 0) for marker D14S77. Lack of homozygosity defined D14S1065 and D14S273 as flanking markers to a 10.7 cM interval. The identification of the responsible gene will provide new insights into the molecular basis of nephrotic syndrome and sensorineural deafness.

Immune Response to the 23-Valent Pneumococcal Polysaccharide Vaccine in Lymphoma Patients and Patients with Chronic Renal Diseases

Lymphoma patients and patients suffering from chronic renal diseases exhibit an increased risk for pneumococcal infections. Therefore, vaccination with the polyvalent pneumococcal vaccine of such patients is recommended (1). This pneumococcal vaccine consists of purified capsular polysaccharides prepared from 23 types of Streptococcus pneumoniae. A “protective antibody level” is still not clearly defined. For routine estimations, we assume an at least fourfold rise in antibodies to the 23-valent vaccine as a positive immune response when measured in an ELISA using the derivatized whole vaccine as antigen four weeks post immunization as compared with the preimmunization level. Antibodies developed to individual capsular polysaccharides and to the cell wall polysaccharide (discussed as a contaminant of the vaccine) were estimated to compare their relative immunogenicity.