Recurrent deep intronic mutations in the SLC12A3 gene responsible for Gitelman's syndrome

BACKGROUND AND OBJECTIVES

Gitelman's syndrome (GS) is an autosomal recessive renal tubular disorder caused by mutations in the SLC12A3 gene encoding the thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC). Despite meticulous sequencing of genomic DNA, approximately one-third of GS patients are negative or heterozygotes for the known mutations.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Because blood leukocytes express NCC mRNA, we evaluate whether deep intronic mutations contribute to GS patients with uniallelic or undetectable SLC12A3 mutations. Twenty-nine patients with GS (men/women = 16/13), including eight negative and 21 uniallelic SLC12A3 mutations from 19 unrelated families, and normal controls were enrolled in an academic medical center. Analysis of cDNA from blood leukocytes, sequencing of the corresponding introns of genomic DNA for abnormal transcript, and analysis of NCC protein expression from renal biopsy were performed.

RESULTS

We identified nine Taiwan aboriginal patients carrying c.1670-191C→T mutations in intron 13 and 10 nonaboriginal patients carrying c.2548+253C→T mutations in intron 21 from 14 families (14/19). These two mutations undetected in 100 healthy subjects created pseudoexons containing new premature termination codons. Haplotype analysis with markers flanking SLC12A3 revealed that both mutations did not have founder effects. Apical NCC expression in the DCT of renal tissue was markedly diminished in two patients carrying deep intronic mutations.

CONCLUSIONS

Deep intronic mutations in SLC12A3 causing defective NCC expression can be identified with the RNA-based approach in patients with GS. c.1670-191C→T and c.2548+253C→T are hot spot mutations that can be screened in GS patients with uniallelic or negative SLC12A3 mutations.

Severe Alport syndrome in a young woman caused by a t(X;1)(q22.3;p36.32) balanced translocation

The course of renal involvement and hearing loss is much milder in most female X-linked Alport syndromes than in male patients. We examined the molecular mechanism of development of the disease in a female patient with severe Alport syndrome. The patient showed heavy proteinuria, hematuria, neurosensory hearing loss and primary amenorrhea. Renal biopsy findings of electron microscopy and immunostaining of the alpha5 chain of type IV collagen indicated a female X-linked Alport syndrome. G-banding chromosomal analysis showed a t(X;1)(q22.3;p36.32) balanced translocation. Analysis of the collagen type IV (COL4A5) gene by genomic DNA sequencing, complementary DNA (cDNA) sequencing and multiplex ligation-dependent probe amplification assay showed no mutations or deletions/duplications of the gene. However, fluorescence in situ hybridization using the probes for exon 1 and exon 51 of the COL4A5 gene showed disruption of one copy of the gene. Replication R-banding chromosomal analysis indicated preferential inactivation of the normal X chromosome. This is the first report of severe Alport syndrome in a female patient carrying a balanced translocation between the chromosome X and 1 producing the disruption of one copy of COL4A5 gene and silencing of the other copy because of preferential inactivation of the normal X chromosome. Chromosomal abnormalities should be considered in female patients with severe forms of Alport syndrome.

Branchio-oto-renal syndrome caused by partial EYA1 deletion due to LINE-1 insertion

A 7-year-old Japanese girl with conductive deafness and preauricular fistulae developed proteinuria. She had renal insufficiency, and ultrasound revealed bilateral small kidneys. These findings indicated that she had branchio-oto-renal (BOR) syndrome. In the present patient, we identified, by using multiplex ligation-dependent probe amplification (MLPA) analysis, a heterozygous EYA1 gene deletion comprising at least exons 5 to 7. In her parents, we did not detect any deletion in EYA1 by MLPA, so the deletion was a de novo mutation. PCR analysis and sequencing of patient DNA revealed a heterozygous approximately 17 kb EYA1 deletion starting from the eight last bases of exon 4 and proceeding to base 1,217 of intron 7. Furthermore, in place of this deleted region was inserted a 3756-bp-long interspersed nuclear elements-1 (LINE-1, L1). Accordingly, RT-PCR showed that exons 4-7 were not present in EYA1 mRNA expressed from the mutated allele. Although there are reports of L1 element insertion occurring in various human diseases, this is the first report of a large EYA1 deletion in combination with L1 element insertion.

HNF1B alterations associated with congenital anomalies of the kidney and urinary tract

Hepatocyte nuclear factor 1beta (HNF1beta) abnormalities have been recognized to cause congenital anomalies of the kidney and urinary tract (CAKUT), predominantly affecting bilateral renal malformations. To further understand the spectrum of HNF1beta related phenotypes, we performed HNF1B gene mutation and deletion analyses in Japanese patients with renal hypodysplasia (n = 31), unilateral multicystic dysplastic kidney (MCDK; n = 14) and others (n = 5). We identified HNF1B alterations in 5 out of 50 patients (10%). De novo heterozygous complete deletions of HNF1B were found in 3 patients with unilateral MCDK. Two of the patients showed contralateral hypodysplasia, whereas the other patient showed a radiologically normal contralateral kidney with normal renal function. Copy number variation analyses showed 1.4 Mb microdeletions involving the whole HNF1B gene with breakpoints in flanking segmental duplications. We also identified 1 novel truncated mutation (1007insC) and another missense mutation (226G>T) in patients with bilateral hypodysplasia. HNF1B alterations leading to haploinsufficiency affect a diverse spectrum of CAKUT. The existence of a patient with unilateral MCDK with normal renal function might provide genetic insight into the etiology of these substantial populations of only unilateral MCDK. The recurrent microdeletions encompassing HNF1B could have a significant impact on the mechanism of HNF1B deletions.

Gastric hyposecretion in esophageal squamous-cell carcinomas

Recently, gastric fundic atrophy is reported to be an independent risk factor for esophageal squamous-cell carcinoma (ESCC). The aim of this study is to investigate the acid secretory level in ESCC in a case-control study. From April 2004 to March 2008, 100 consecutive subjects with early ESCC and 100 age- and sex-matched asymptomatic controls were prospectively enrolled. Gastrin-stimulated acid output was assessed by endoscopic gastrin test. Conditional regression analyses were used to adjust for other potential confounders. Multivariate analyses revealed a strong association between profound hypochlorhydria and ESCC with odds ratio (95% confidence interval): 6.0 (1.9-18.4). The association remained significant after adjusting for the effect of gastric atrophy as a covariate. The association became stronger as the ESCC developed more distal site of the esophagus. This study indicates that profound hypochlorhydria is a strong independent risk factor for ESCC even after adjusting for the influence of gastric atrophy.

Reactive nitrogen oxide species induce dilatation of the intercellular space of rat esophagus

OBJECTIVE

Dilatation of the intercellular space (DIS) of the esophageal epithelium is recognized as one of the earliest histological changes in gastroesophageal reflux disease patients. At the human gastroesophageal junction, reactive nitrogen oxide species (RNOS) are generated luminally through the entero-salivary re-circulation of dietary nitrate. In cases with gastroesophageal reflux, the site of luminal RNOS generation may shift to the distal esophagus. The aim of this study was to investigate whether luminal RNOS exposure could be involved in the pathogenesis of DIS.

MATERIAL AND METHODS

Rat esophageal mucosa was studied with an Ussing chamber model. On the luminal side of the chamber, RNOS were generated by the acidification of physiologic concentrations of sodium nitrite (1.0 or 5.0 mM). Esophageal barrier function was assessed by means of electrophysiological transmembrane resistance and membrane permeability by means of (3)H-mannitol flux. The dimensions of the intercellular spaces were assessed by using transmission electron microscopy.

RESULTS

Administration of acid plus sodium nitrite induced DIS of the esophageal epithelium, and this ultrastructural morphological change was accompanied by a concomitant decrease in the transmembrane resistance and an increase in the epithelial permeability. The DIS induced by luminal RNOS was also confirmed in an in vivo exposure model.

CONCLUSIONS

The present animal study indicates that the RNOS generated by the acidification of salivary nitrite in the presence of refluxed gastric acid in the esophagus could be a luminal factor that is responsible for the induction of DIS. Further studies are warranted to investigate the clinical relevance of the present findings to the human situation.

A family with X-linked benign familial hematuria

Gene mutations in COL4A5 located on Xq22 are believed to cause X-linked Alport syndrome, whereas mutations in COL4A3 and COL4A4 located on chromosome 2 are associated with autosomal inherited Alport syndrome or benign familial hematuria. A family with benign familial hematuria caused by COL4A5 mutation, implying X-linked transmission, is reported here for the first time. This result suggests that COL4A5 should be added to the list of causative genes for benign familial hematuria, although the mechanism(s) by which the same mutation leads to the distinct phenotypes, i.e. X-linked Alport syndrome or benign familial hematuria, remains unknown.

Rituximab in refractory nephrotic syndrome

The aim of this study was to establish the efficacy and safety of rituximab in refractory nephrotic syndrome (NS). Members of the International Paediatric Nephrology Association were asked to retrospectively fill in a questionnaire with details on the use of rituximab in their centres. We divided the data into three groups: group 1, patients with steroid-dependent and frequently relapsing NS; group 2, with steroid-resistant NS; group 3, with post-transplant recurrence of NS. Seventy questionnaires from 25 centres described the outcome of 28, 27 and 15 patients in groups 1, 2 and 3, respectively. Of these, 82% of patients in group 1, 44% of patients in group 2 and 60% of patients in group 3 had a good initial response. Side effects were observed in 27% of the patients, and these were mostly acute reactions. We present a large multicentre series of children with refractory NS. Children in group 1 showed the best response. The good initial response in group 3 can be biased by the accompanying treatments that were administered at the same time as rituximab. Controlled prospective trials are required to establish the value of rituximab in idiopathic NS.

Autosomal dominant pseudohypoaldosteronism type 1 with a novel splice site mutation in MR gene

Background

Autosomal dominant pseudohypoaldosteronism type 1 (PHA1) is a rare inherited condition that is characterized by renal resistance to aldosterone as well as salt wasting, hyperkalemia, and metabolic acidosis. Renal PHA1 is caused by mutations of the human mineralcorticoid receptor gene (MR), but it is a matter of debate whether MR mutations cause mineralcorticoid resistance via haploinsufficiency or dominant negative mechanism. It was previously reported that in a case with nonsense mutation the mutant mRNA was absent in lymphocytes because of nonsense mediated mRNA decay (NMD) and therefore postulated that haploinsufficiency alone can give rise to the PHA1 phenotype in patients with truncated mutations.

Methods and Results

We conducted genomic DNA analysis and mRNA analysis for familial PHA1 patients extracted from lymphocytes and urinary sediments and could detect one novel splice site mutation which leads to exon skipping and frame shift result in premature termination at the transcript level. The mRNA analysis showed evidence of wild type and exon-skipped RT-PCR products.

Conclusion

mRNA analysis have been rarely conducted for PHA1 because kidney tissues are unavailable for this disease. However, we conducted RT-PCR analysis using mRNA extracted from urinary sediments. We could demonstrate that NMD does not fully function in kidney cells and that haploinsufficiency due to NMD with premature termination is not sufficient to give rise to the PHA1 phenotype at least in this mutation of our patient. Additional studies including mRNA analysis will be needed to identify the exact mechanism of the phenotype of PHA.

Cyclosporine and steroid therapy in children with steroid-resistant nephrotic syndrome

We conducted a prospective, multicenter trial to evaluate the efficacy and safety of a 12-month course of cyclosporine in children with steroid-resistant nephrotic syndrome (SRNS). Thirty-five patients were enrolled, of whom 28 had minimal change or diffuse mesangial proliferation (MC/DMP), and seven had focal segmental glomerulosclerosis (FSGS). All patients received cyclosporine and prednisolone; patients with FSGS additionally received methylprednisolone pulse therapy (MPT). The dose of cyclosporine was adjusted to maintain a trough level of 120-150 ng/ml during the initial 3 months of treatment, followed by 80-100 ng/ml during months 4-12. The primary end point was the remission rate at month 12. Remission was achieved in 23 of 28 (82.1%) patients in the MC/DMP group and in six of the seven (85.7%) patients in the FSGS group. Follow-up renal biopsies were performed in 26 patients (nine at month 12, 17 at month 24), and cyclosporine-related nephrotoxicity was detected in one (3.8%). Major adverse events comprised severe bacterial infections (two patients) and posterior reversible encephalopathy syndrome (one patient). In conclusion, a high remission rate was achieved in our patient cohort using a combined cyclosporine/ prednisolone treatment regimen in children with SRNS who had MC/DMP and a combined cyclosporine/prednisolone plus MPT regimen in children who had FSGS.

A deep intronic mutation in the SLC12A3 gene leads to Gitelman syndrome

Many mutations have been detected in the SLC12A3 gene of Gitelman syndrome (GS, OMIM 263800) patients. In previous studies, only one mutant allele was detected in approximately 20 to 41% of patients with GS; however, the exact reason for the nonidentification has not been established. In this study, we used RT-PCR using mRNA to investigate for the first time transcript abnormalities caused by deep intronic mutation. Direct sequencing analysis of leukocyte DNA identified one base insertion in exon 6 (c.818_819insG), but no mutation was detected in another allele. We analyzed RNA extracted from leukocytes and urine sediments and detected unknown sequence containing 238bp between exons 13 and 14. The genomic DNA analysis of intron 13 revealed a single-base substitution (c.1670-191C>T) that creates a new donor splice site within the intron resulting in the inclusion of a novel cryptic exon in mRNA. This is the first report of creation of a splice site by a deep intronic single-nucleotide change in GS and the first report to detect the onset mechanism in a patient with GS and missing mutation in one allele. This molecular onset mechanism may partly explain the poor success rate of mutation detection in both alleles of patients with GS.

Single dose of rituximab for refractory steroid-dependent nephrotic syndrome in children

We conducted a multicenter prospective trial to evaluate the efficacy, safety and pharmacokinetics of a single dose of rituximab (375 mg/m(2) body surface area) for the treatment of children with refractory steroid-dependent nephrotic syndrome (SDNS). All patients (n = 12) were able to discontinue steroids at a median of 74 days after treatment. The frequency of relapses per 6 months was significantly reduced and the steroid-free period per 6 months was significantly increased after treatment compared with those before treatment. The condition in nine of the patients (75%) relapsed at a median of 129 days after treatment, and seven patients were given additional rituximab due to steroid dependency. Most of the relapses developed simultaneously with recovery of B-cells. However, three patients (25%) did not have a relapse with B-cell recovery and the disease was kept in remission for more than 1 year. None of the patients developed life-threatening adverse events. This is the first report of a prospective study of a single dose of rituximab for refractory SDNS. Treatment with a single dose of rituximab may be effective for refractory SDNS, but its efficacy to prevent relapses was transient in most of the patients.

In vivo and in vitro splicing assay of SLC12A1 in an antenatal salt-losing tubulopathy patient with an intronic mutation

Type I Bartter syndrome (BS), an inherited salt-losing tubulopathy, is caused by mutations of the SLC12A1 gene. While several intronic nucleotide changes in this gene have been detected, transcriptional analysis had not been conducted because mRNA analysis is possible only when renal biopsy specimens can be obtained or occasionally when mRNA is expressed in the leukocytes. This report concerns a type I BS patient due to compound heterozygosity for the SLC12A1 gene. Genomic DNA sequencing disclosed the presence of two novel heterozygous mutations of c.724 + 4A > G in intron 5 and c.2095delG in intron 16, but it remains to be determined whether the former would be likely to influence the transcription. In this report, we conducted both in vivo assay of RT-PCR analysis using RNA extracted from the proband's urinary sediments and in vitro functional splicing study by minigene construction, and obtained evidence that this intronic mutation leads to complete exon 5 skipping. To the best of our knowledge, this is the first study to use non-invasive methods for both an in vivo assay and an in vitro functional splicing assay of inherited kidney disease. These analytical assays could be adapted for all inherited kidney diseases.

Minimal change nephrotic syndrome associated with immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome

Several studies have suggested that T cell-producing permeability factors might lead to proteinuria in minimal change nephrotic syndrome (MCNS). However, it is still unclear whether T-cell abnormalities cause MCNS. Immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome is a rare disorder of the immune regulation system, which leads to severe autoimmune phenomena including autoimmune enteropathy, atopic dermatitis with high levels of serum immunoglobulin E (IgE), type 1 diabetes mellitus (T1DM), and severe infection such as sepsis, which frequently result in death within the first 2 years of life. This disease is caused by mutations in the FOXP3 gene that result in the defective development of regulatory T (Treg) cells. This report describes a 5-year-old boy with IPEX syndrome with a 3 bp deletion in the FOXP3 gene (c.748-750delAAG, p.250K.del) and a paucity of CD4(+) CD25(+) FOXP3(+) T cells. The boy's condition was complicated by MCNS in addition to many IPEX-related manifestations, such as atopic dermatitis, T1DM, enteropathy, sepsis and hemolytic anemia. This is the first report of IPEX syndrome complicated by MCNS, and our findings imply that Treg cell dysfunction may be crucial for the development of MCNS.

Predisposition to relapsing nephrotic syndrome by a nephrin mutation that interferes with assembly of functioning microdomains

Minimal-change disease (MCD) is the most common cause of nephrotic syndrome (NS) and is characterized only by minor morphological alterations in podocytes. A subtype of MCD arises from mutations in nephrin, a major component of the slit diaphragm (SD). Idiopathic MCD is a complex trait where interactions of genetic and immunological factors are implicated. However, the pathogenic mechanisms remain unclear. Here we studied the molecular basis for familial NS characterized by frequent relapses and minimal-change histology. Our previous mutational analysis revealed that the two affected children were compound heterozygotes for nephrin variants C265R and V822M (Kidney Int., 2008). When heterologously expressed, these variants exhibited normal metabolic half-life and raft binding. C265R exhibited substantial ER retention, reflecting an intracellular trafficking defect. In contrast, V822M was able to reach the plasma membrane, but was restricted in lateral diffusion as well as trafficking at the cell surface. Clustering of V822M failed to evoke a maximum tyrosine-phosphorylation and actin reorganization, suggesting the inability to assemble into functioning membrane microdomains. Our results suggest that C265R and V822M compose a dysfunctional SD complex due to their mixed defects comprising reduced cell surface targeting and ineffective assembly of signaling microdomains. The defective SD likely confers a susceptibility to immunogenic stimuli and predisposes to a relapsing phenotype.

Membranous nephropathy associated with thyroid-peroxidase antigen

A 6-year-old previously healthy Japanese girl was found to have dipstick 2+ proteinuria and a goiter based on the results of a routine school medical examination. Her serum free-thyroxine level was 4.98 ng/dL (normal range 0.95-1.74 ng/dL), thyroid-stimulating hormone (TSH) was less than 0.003 microU/mL (0.34-3.88 microU/mL), anti-microsomal (anti-thyroid-peroxidase) antibody was 1600 T (up to 100), anti-thyroglobulin antibody was 400 T (up to 100), and TSH-receptor antibody was 84% (up to +/-10%). These results are consistent with a diagnosis of Graves' disease. Electron microscopy examination of a renal biopsy specimen revealed electron-dense deposits located in the subepithelial spaces, and immunofluorescence microscopy examination demonstrated bright granular stainings of immunoglobulin G along the glomerular capillary walls. These findings are characteristic of membranous nephropathy. To investigate the relationship between the membranous nephropathy and Graves' disease, we carried out a second immunofluorescence study, which revealed that the immunoglobulin G granular deposits corresponded to glomerular granular staining of thyroid-peroxidase, whereas staining for thyroglobulin was absent. It was therefore assumed that the deposition of immune complexes mediated by thyroid-peroxidase had caused the membranous nephropathy in this patient. This is the first report of membranous nephropathy associated with Graves' disease in which deposits of thyroid-peroxidase, rather than thyroglobulin, have been confirmed in the kidney.

Analysis of body responses to an accelerating platform by the largest-Lyapunov-exponent method

Various disciplines have benefited from the advent of high-performance computing in achieving practical solutions to their problems, and the area of health care is no exception to this. Non-linear signal-processing tools have been developed to understand the hidden complexity of the time series, and these will help clinicians in diagnosis and treatment. Postural study helps the elderly and people with a balancing problem due to various pathological conditions. In elderly subjects, falls are common and may result in injury. Correct postural balance is basic to well-being and it influences our daily life significantly. These postural signals are non-stationary; they may appear to be random in the time scale and it is difficult to observe the subtle changes for the human observer. Hence, more hidden information can be obtained from the signal using non-linear parameters. In this paper, ten young normal subjects are subjected to the balancing platform whose acceleration is gradually increased from 1 m/s2 to 5 m/s2 to study the postural response. The ankle front-back acceleration and ankle pitch angular velocity sensor data were studied using the largest Lyapunov exponent (LLE). The results show that for higher acceleration of the platform the ankle movement follows a particular rhythm, resulting in a lower Lyapunov exponent. During lower acceleration of the balancing platform, this value is higher because of the random movement of the ankle. In this work, the pattern of the body response was studied using LLE values for different accelerations using ankle data as the base signal for the normal subjects.