Mutations in the human laminin beta2 (LAMB2) gene and the associated phenotypic spectrum

Whole-exome sequencing reveals genetic variants associated with chronic kidney disease characterized by tubulointerstitial damages in North Central Region, Sri Lanka

OBJECTIVES

The familial clustering observed in chronic kidney disease of uncertain etiology (CKDu) characterized by tubulointerstitial damages in the North Central Region of Sri Lanka strongly suggests the involvement of genetic factors in its pathogenesis. The objective of the present study is to use whole-exome sequencing to identify the genetic variants associated with CKDu.

METHODS

Whole-exome sequencing of eight CKDu cases and eight controls was performed, followed by direct sequencing of candidate loci in 301 CKDu cases and 276 controls.

RESULTS

Association study revealed rs34970857 (c.658G > A/p.V220M) located in the KCNA10 gene encoding a voltage-gated K channel as the most promising SNP with the highest odds ratio of 1.74. Four rare variants were identified in gene encoding Laminin beta2 (LAMB2) which is known to cause congenital nephrotic syndrome. Three out of four variants in LAMB2 were novel variants found exclusively in cases.

CONCLUSION

Genetic investigations provide strong evidence on the presence of genetic susceptibility for CKDu. Possibility of presence of several rare variants associated with CKDu in this population is also suggested.

Integrating Activities of Laminins that Drive Basement Membrane Assembly and Function

Studies on extracellular matrix proteins, cells, and genetically modified animals have converged to reveal mechanisms of basement membrane self-assembly as mediated by γ1 subunit-containing laminins, the focus of this chapter. The basic model is as follows: A member of the laminin family adheres to a competent cell surface and typically polymerizes followed by laminin binding to the extracellular adaptor proteins nidogen, perlecan, and agrin. Assembly is completed by the linking of nidogen and heparan sulfates to type IV collagen, allowing it to form a second stabilizing network polymer. The assembled matrix provides structural support, anchoring the extracellular matrix to the cytoskeleton, and acts as a signaling platform. Heterogeneity of function is created in part by the isoforms of laminin that vary in their ability to polymerize and to interact with integrins, dystroglycan, and other receptors. Mutations in laminin subunits, affecting expression or LN domain-specific functions, are a cause of human diseases that include those of muscle, nerve, brain, and kidney.

Genetic causes of proteinuria and nephrotic syndrome: impact on podocyte pathobiology

In the past 20 years, multiple genetic mutations have been identified in patients with congenital nephrotic syndrome (CNS) and both familial and sporadic focal segmental glomerulosclerosis (FSGS). Characterization of the genetic basis of CNS and FSGS has led to the recognition of the importance of podocyte injury to the development of glomerulosclerosis. Genetic mutations induce injury due to effects on the podocyte's structure, actin cytoskeleton, calcium signaling, and lysosomal and mitochondrial function. Transgenic animal studies have contributed to our understanding of podocyte pathobiology. Podocyte endoplasmic reticulum stress response, cell polarity, and autophagy play a role in maintenance of podocyte health. Further investigations related to the effects of genetic mutations on podocytes may identify new pathways for targeting therapeutics for nephrotic syndrome.

Retrospective mutational analysis of NPHS1, NPHS2, WT1 and LAMB2 in children with steroid-resistant focal segmental glomerulosclerosis - a single-centre experience

The aim of our study was to examine NPHS1, NPHS2, WT1 and LAMB2 mutations, previously reported in two thirds of patients with nephrotic syndrome with onset before the age of one year old. Genomic DNA samples from Polish children (n=33) with Steroid-Resistant Nephrotic Syndrome (SRNS) due to focal segmental glomerulosclerosis (FSGS), manifesting before the age of 13 years old, underwent retrospective analysis of NPHS1, NPHS2, WT1 (exons 8, 9 and adjacent exon/intron boundaries) and LAMB2. No pathogenic NPHS1 or LAMB2 mutations were found in our FSGS cohort. SRNS-causing mutations of NPHS2 and WT1 were detected in 7 of 33 patients (21%), including those with nephrotic syndrome manifesting before one year old: five of seven patients. Four patients had homozygous c.413G>A (p.Arg138Gln) NPHS2 mutations; one subject was homozygous for c.868G>A (p.Val290Met) NPHS2. A phenotypic female had C>T transition at position +4 of the WT1 intron 9 (c.1432+4C>T) splice-donor site, and another phenotypic female was heterozygous for G>A transition at position +5 (c.1432+5G>A). Genotyping revealed a female genotypic gender (46, XX) for the first subject and male (46, XY) for the latter. In addition, one patient was heterozygous for c.104dup (p.Arg36Profs*34) NPHS2; two patients carried a c.686G>A (p.Arg229Gln) NPHS2 non-neutral variant. Results indicate possible clustering of causative NPHS2 mutations in FSGS-proven SRNS with onset before age one year old, and provide additional evidence that patients with childhood steroid-resistant nephrotic syndrome due to focal segmental glomerulosclerosis should first undergo analysis of NPHS2 coding sequence and WT1 exons 8 and 9 and surrounding exon/intron boundary sequences, followed by gender genotyping.

Rare hereditary COL4A3/COL4A4 variants may be mistaken for familial focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is a histological lesion with many causes including inherited genetic defects with significant proteinuria being the predominant clinical finding at presentation. Mutations in COL4A3 and COL4A4 are known to cause Alport syndrome, thin basement membrane nephropathy, and to result in pathognomonic glomerular basement membrane findings. Secondary FSGS is known to develop in classic Alport Syndrome at later stages of the disease. Here, we present seven families with rare or novel variants in COL4A3 or COL4A4 (six with single and one with two heterozygous variants) from a cohort of 70 families with a diagnosis of hereditary FSGS. The predominant clinical findings at diagnosis were proteinuria associated with hematuria. In all seven families, there were individuals with nephrotic range proteinuria with histologic features of FSGS by light microscopy. In one family, electron microscopy showed thin glomerular basement membrane, but four other families had variable findings inconsistent with classical Alport nephritis. There was no recurrence of disease after kidney transplantation. Families with COL4A3 and COL4A4 variants that segregated with disease represent 10% of our cohort. Thus, COL4A3 and COL4A4 variants should be considered in the interpretation of next-generation sequencing data from such patients. Furthermore, this study illustrates the power of molecular genetic diagnostics in the clarification of renal phenotypes.

Nephrotic and nephritic syndrome in the newborn

Glomerular disorders in infancy can include nephrotic and nephritic syndromes. Congenital nephrotic syndrome (CNS) is most commonly caused by single gene mutations in kidney proteins, but may also be due to congenital infections or passive transfer of maternal antibodies that target kidney proteins. Prenatal findings of increased maternal serum α-fetoprotein and enlarged placenta suggest CNS. Neonatal nephritis is rare; its causes may overlap with those of CNS and include primary glomerulonephritis, systemic disease, infections, and transplacental transfer of maternal antibodies. These syndromes in the neonate can cause significant morbidity and mortality, making urgent diagnosis and treatment necessary.

Distinct functions of the laminin β LN domain and collagen IV during cardiac extracellular matrix formation and stabilization of alary muscle attachments revealed by EMS mutagenesis in Drosophila

BACKGROUND

The Drosophila heart (dorsal vessel) is a relatively simple tubular organ that serves as a model for several aspects of cardiogenesis. Cardiac morphogenesis, proper heart function and stability require structural components whose identity and ways of assembly are only partially understood. Structural components are also needed to connect the myocardial tube with neighboring cells such as pericardial cells and specialized muscle fibers, the so-called alary muscles.

RESULTS

Using an EMS mutagenesis screen for cardiac and muscular abnormalities in Drosophila embryos we obtained multiple mutants for two genetically interacting complementation groups that showed similar alary muscle and pericardial cell detachment phenotypes. The molecular lesions underlying these defects were identified as domain-specific point mutations in LamininB1 and Cg25C, encoding the extracellular matrix (ECM) components laminin β and collagen IV α1, respectively. Of particular interest within the LamininB1 group are certain hypomorphic mutants that feature prominent defects in cardiac morphogenesis and cardiac ECM layer formation, but in contrast to amorphic mutants, only mild defects in other tissues. All of these alleles carry clustered missense mutations in the laminin LN domain. The identified Cg25C mutants display weaker and largely temperature-sensitive phenotypes that result from glycine substitutions in different Gly-X-Y repeats of the triple helix-forming domain. While initial basement membrane assembly is not abolished in Cg25C mutants, incorporation of perlecan is impaired and intracellular accumulation of perlecan as well as the collagen IV α2 chain is detected during late embryogenesis.

CONCLUSIONS

Assembly of the cardiac ECM depends primarily on laminin, whereas collagen IV is needed for stabilization. Our data underscore the importance of a correctly assembled ECM particularly for the development of cardiac tissues and their lateral connections. The mutational analysis suggests that the β6/β3/β8 interface of the laminin β LN domain is highly critical for formation of contiguous cardiac ECM layers. Certain mutations in the collagen IV triple helix-forming domain may exert a semi-dominant effect leading to an overall weakening of ECM structures as well as intracellular accumulation of collagen and other molecules, thus paralleling observations made in other organisms and in connection with collagen-related diseases.

Next-generation sequencing to dissect hereditary nephrotic syndrome in mice identifies a hypomorphic mutation in Lamb2 and models Pierson's syndrome

The study of mutations causing the steroid-resistant nephrotic syndrome in children has greatly advanced our understanding of the kidney filtration barrier. In particular, these genetic variants have illuminated the roles of the podocyte, glomerular basement membrane and endothelial cell in glomerular filtration. However, in a significant number of familial and early onset cases, an underlying mutation cannot be identified, indicating that there are likely to be multiple unknown genes with roles in glomerular permeability. We now show how the combination of N-ethyl-N-nitrosourea mutagenesis and next-generation sequencing could be used to identify the range of mutations affecting these pathways. Using this approach, we isolated a novel mouse strain with a viable nephrotic phenotype and used whole-genome sequencing to isolate a causative hypomorphic mutation in Lamb2. This discovery generated a model for one part of the spectrum of human Pierson's syndrome and provides a powerful proof of principle for accelerating gene discovery and improving our understanding of inherited forms of renal disease.

Rapid detection of monogenic causes of childhood-onset steroid-resistant nephrotic syndrome

BACKGROUND AND OBJECTIVES

In steroid-resistant nephrotic syndrome (SRNS), >21 single-gene causes are known. However, mutation analysis of all known SRNS genes is time and cost intensive. This report describes a new high-throughput method of mutation analysis using a PCR-based microfluidic technology that allows rapid simultaneous mutation analysis of 21 single-gene causes of SRNS in a large number of individuals.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

This study screened individuals with SRNS; samples were submitted for mutation analysis from international sources between 1996 and 2012. For proof of principle, a pilot cohort of 48 individuals who harbored known mutations in known SRNS genes was evaluated. After improvements to the method, 48 individuals with an unknown cause of SRNS were then examined in a subsequent diagnostic study. The analysis included 16 recessive SRNS genes and 5 dominant SRNS genes. A 10-fold primer multiplexing was applied, allowing PCR-based amplification of 474 amplicons in 21 genes for 48 DNA samples simultaneously. Forty-eight individuals were indexed in a barcode PCR, and high-throughput sequencing was performed. All disease-causing variants were confirmed via Sanger sequencing.

RESULTS

The pilot study identified the genetic cause of disease in 42 of 48 (87.5%) of the affected individuals. The diagnostic study detected the genetic cause of disease in 16 of 48 (33%) of the affected individuals with a previously unknown cause of SRNS. Seven novel disease-causing mutations in PLCE1 (n=5), NPHS1 (n=1), and LAMB2 (n=1) were identified in <3 weeks. Use of this method could reduce costs to 1/29th of the cost of Sanger sequencing.

CONCLUSION

This highly parallel approach allows rapid (<3 weeks) mutation analysis of 21 genes known to cause SRNS at a greatly reduced cost (1/29th) compared with traditional mutation analysis techniques. It detects mutations in about 33% of childhood-onset SRNS cases.

Genetic testing for nephrotic syndrome and FSGS in the era of next-generation sequencing

The haploid human genome is composed of three billion base pairs, about one percent of which consists of exonic regions, the coding sequence for functional proteins, also now known as the “exome”. The development of next-generation sequencing makes it possible from a technical and economic standpoint to sequence an individual’s exome but at the cost of generating long lists of gene variants that are not straightforward to interpret. Various public consortiums such as the 1000 Genomes Project and the NHLBI Exome Sequencing Project have sequenced the exomes and a subset of entire genomes of over 2500 control individuals with ongoing efforts to further catalogue genetic variation in humans.¹ The use of these public databases facilitates the interpretation of these variant lists produced by exome sequencing and, as a result, novel genetic variants linked to disease are being discovered and reported at a record rate. However, the interpretation of these results and their bearing on diagnosis, prognosis, and treatment is becoming ever more complicated. Here, we discuss the application of genetic testing to individuals with focal and segmental glomerulosclerosis (FSGS), taking a historical perspective on gene identification and its clinical implications along with the growing potential of next-generation sequencing.

Lens extrusion from Laminin alpha 1 mutant zebrafish

We report analysis of the ocular lens phenotype of the recessive, larval lethal zebrafish mutant, lama1^a69/a69. Previous work revealed that this mutant has a shortened body axis and eye defects including a defective hyaloid vasculature, focal corneal dysplasia, and loss of the crystalline lens. While these studies highlight the importance of laminin α1 in lens development, a detailed analysis of the lens defects seen in these mutants was not reported. In the present study, we analyze the lenticular anomalies seen in the lama1^a69/a69 mutants and show that the lens defects result from the anterior extrusion of lens material from the eye secondary to structural defects in the lens capsule and developing corneal epithelium associated with basement membrane loss. Our analysis provides further insights into the role of the lens capsule and corneal basement membrane in the structural integrity of the developing eye.

The importance of podocyte adhesion for a healthy glomerulus

Podocytes are specialized epithelial cells that cover the outer surfaces of glomerular capillaries. Unique cell junctions, known as slit diaphragms, which feature nephrin and Neph family proteins in addition to components of adherens, tight, and gap junctions, connect adjacent podocyte foot processes. Single gene disorders affecting the slit diaphragm result in nephrotic syndrome in humans, characterized by massive loss of protein across the capillary wall. In addition to specialized cell junctions, interconnecting podocytes also adhere to the glomerular basement membrane (GBM) of the capillary wall. The GBM is a dense network of secreted, extracellular matrix (ECM) components and contains tissue-restricted isoforms of collagen IV and laminin in addition to other structural proteins and ECM regulators such as proteases and growth factors. The specialized niche of the GBM provides a scaffold for endothelial cells and podocytes to support their unique functions and human genetic mutations in GBM components lead to renal failure, thus highlighting the importance of cell-matrix interactions in the glomerulus. Cells adhere to ECM via adhesion receptors, including integrins, syndecans, and dystroglycan and in particular the integrin heterodimer α3β1 is required to maintain barrier integrity. Therefore, the sophisticated function of glomerular filtration relies on podocyte adhesion both at cell junctions and at the interface with the ECM. In health, the podocyte coordinates signals from cell junctions and cell-matrix interactions, in response to environmental cues in order to regulate filtration and as our understanding of mechanisms that control cell adhesion in the glomerulus develops, then insight into the effects of disease will improve. The ultimate goal will be to develop targeted therapies to prevent or repair defects in the filtration barrier and to restore glomerular function.

Genes and podocytes - new insights into mechanisms of podocytopathy

After decades of primarily morphological study, positional cloning of the NPHS1 gene was the landmark event that established aberrant podocyte genetics as a pivotal cause of malfunction of the glomerular filter. This ended any uncertainty whether genetic mutation plays a significant role in hereditary nephrotic syndromes (NS) and confirmed podocytes as critical players in regulating glomerular protein filtration. Although subsequent sequencing of candidate genes chosen on the basis of podocyte biology had less success, unbiased analysis of genetically informative kindreds and syndromic disease has led to further gene discovery. However, the 45 genes currently associated with human NS explain not more than 20-30% of hereditary and only 10-20% of sporadic cases. It is becoming increasingly clear both from genetic analysis and phenotypic data - including occasional response to immunosuppressive agents and post-transplant disease recurrence in Mendelian disease - that monogenic inheritance of abnormalities in podocyte-specific genes disrupting filter function is only part of the story. Recent advances in genetic screening technology combined with increasingly robust bioinformatics are set to allow identification and characterization of novel disease causing variants and more importantly, disease modifying genes. Emerging data also support a significant but incompletely characterized immunoregulatory component.

Congenital nephrotic syndrome with prolonged renal survival without renal replacement therapy

BACKGROUND

Infants with congenital nephrotic syndrome (CNS) develop severe nephrotic syndrome that is resistant to medical therapy, and bilateral nephrectomy is recommended toward the end of the first year of life followed by renal replacement therapy. CNS infants in New Zealand have been observed to exhibit a different course to those with the typical Finnish mutation.

METHODS

A database of CNS children at our center was retrospectively examined. All cases diagnosed between 1975 and 2011 were reviewed. Demographic data, clinical features, genetic mutations, treatment, and outcome were extracted from clinical records.

RESULTS

Thirty-five patients with CNS, 23 children of Maori descent, and 12 Caucasians . Fourteen had died of either bacterial sepsis or intracranial thrombosis. Maori children had displayed a highly variable and protracted timeline to end-stage renal disease (ESRD) with median renal survival of 30 years versus 0.7 years in Caucasian patients. Mutation analysis of NPHS1 showed a founder mutation in the Maori population.

CONCLUSIONS

Congenital nephrotic syndrome in New Zealand Maori children exhibit a different clinical course to Caucasian children and have a mutation that was first described in this ethnic group.

Allelic heterogeneity contributes to variability in ocular dysgenesis, myopathy and brain malformations caused by Col4a1 and Col4a2 mutations

Collagen type IV alpha 1 and 2 (COL4A1 and COL4A2) are present in nearly all basement membranes. COL4A1 and COL4A2 mutations are pleiotropic, affecting multiple organ systems to differing degrees, and both genetic-context and environmental factors influence this variable expressivity. Here, we report important phenotypic and molecular differences in an allelic series of Col4a1 and Col4a2 mutant mice that are on a uniform genetic background. We evaluated three organs commonly affected by COL4A1 and COL4A2 mutations and discovered allelic heterogeneity in the penetrance and severity of ocular dysgenesis, myopathy and brain malformations. Similarly, we show allelic heterogeneity in COL4A1 and COL4A2 biosynthesis. While most mutations that we examined caused increased intracellular and decreased extracellular COL4A1 and COL4A2, we identified three mutations with distinct biosynthetic signatures. Reduced temperature or presence of 4-phenylbutyrate ameliorated biosynthetic defects in primary cell lines derived from mutant mice. Together, our data demonstrate the effects and clinical implications of allelic heterogeneity in Col4a1- and Col4a2-related diseases. Understanding allelic differences will be valuable for increasing prognostic accuracy and for the development of therapeutic interventions that consider the nature of the molecular cause in patients with COL4A1 and COL4A2 mutations.

Arg kinase signaling in dendrite and synapse stabilization pathways: memory, cocaine sensitivity, and stress

The Abl2/Arg nonreceptor tyrosine kinase is enriched in dendritic spines where it is essential for maintaining dendrite and synapse stability in the postnatal mouse brain. Arg is activated downstream of integrin α3β1 receptors and it regulates the neuronal actin cytoskeleton by directly binding F-actin and via phosphorylation of substrates including p190RhoGAP and cortactin. Neurons in mice lacking Arg or integrin α3β1 develop normally through postnatal day 21 (P21), however by P42 mice exhibit major reductions in dendrite arbor size and complexity, and lose dendritic spines and synapses. As a result, mice with loss of Arg and Arg-dependent signaling pathways have impairments in memory tasks, heightened sensitivity to cocaine, and vulnerability to corticosteroid-induced neuronal remodeling. Therefore, understanding the molecular mechanisms of Arg regulation may lead to therapeutic approaches to treat human psychiatric and neurodegenerative diseases in which neuronal structure is destabilized.

The renal biopsy in the genomic era

Renal biopsy was introduced in the 1950s. By 1980 the pathologic diagnostic criteria for the majority of medical kidney diseases known today, including pediatric diseases, were established using light, electron microscopy and immunohistochemistry. However, it has become clear that there are limitations in the morphologic evaluation, mainly because a given pattern of injury can be caused by different aetiologies and, conversely, a single aetiology may present with more than one histological pattern. An explosion in kidney disease research in the last 20-30 years has brought new knowledge from bench to bedside rapidly and resulted in new molecular and genetic tools that enhance the diagnostic and prognostic power of the renal biopsy. Genomic technologies such as polymerase chain reaction (PCR), in situ hybridization and oligonucleotide microarrays, collectively known as genomics, detect single or multiple genes underscoring the pathologic changes and revealing specific causes of injury that may require different treatment. The aims of this review are to (1) summarize current recommendations for diagnostic renal biopsies encompassing light microscopy, immunofluorescence or immunohistochemistry and electron microscopy; (2) address the limitations of morphology; (3) show current contributions of genomic technologies adjunct to the renal biopsy, and provide examples of how these may transform pathologic interpretation into molecular disease phenotypes.

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.

The glomerular basement membrane as a barrier to albumin

The glomerular basement membrane (GBM) is the central, non-cellular layer of the glomerular filtration barrier that is situated between the two cellular components--fenestrated endothelial cells and interdigitated podocyte foot processes. The GBM is composed primarily of four types of extracellular matrix macromolecule--laminin-521, type IV collagen α3α4α5, the heparan sulphate proteoglycan agrin, and nidogen--which produce an interwoven meshwork thought to impart both size-selective and charge-selective properties. Although the composition and biochemical nature of the GBM have been known for a long time, the functional importance of the GBM versus that of podocytes and endothelial cells for establishing the glomerular filtration barrier to albumin is still debated. Together with findings from genetic studies in mice, the discoveries of four human mutations affecting GBM components in two inherited kidney disorders, Alport syndrome and Pierson syndrome, support essential roles for the GBM in glomerular permselectivity. Here, we explain in detail the proposed mechanisms whereby the GBM can serve as the major albumin barrier and discuss possible approaches to circumvent GBM defects associated with loss of permselectivity.

Laminin β2 gene missense mutation produces endoplasmic reticulum stress in podocytes

Mutations in the laminin β2 gene (LAMB2) cause Pierson syndrome, a severe congenital nephrotic syndrome with ocular and neurologic defects. LAMB2 is a component of the laminin-521 (α5β2γ1) trimer, an important constituent of the glomerular basement membrane (GBM). The C321R-LAMB2 missense mutation leads to congenital nephrotic syndrome but only mild extrarenal symptoms; the mechanisms underlying the development of proteinuria with this mutation are unclear. We generated three transgenic mouse lines, in which rat C321R-LAMB2 replaced mouse LAMB2 in the GBM. During the first postnatal month, expression of C321R-LAMB2 attenuated the severe proteinuria exhibited by Lamb2(-/-) mice in a dose-dependent fashion; proteinuria eventually increased, however, leading to renal failure. The C321R mutation caused defective secretion of laminin-521 from podocytes to the GBM accompanied by podocyte endoplasmic reticulum (ER) stress, likely resulting from protein misfolding. Moreover, ER stress preceded the onset of significant proteinuria and was manifested by induction of the ER-initiated apoptotic signal C/EBP homologous protein (CHOP), ER distention, and podocyte injury. Treatment of cells expressing C321R-LAMB2 with the chemical chaperone taurodeoxycholic acid (TUDCA), which can facilitate protein folding and trafficking, greatly increased the secretion of the mutant LAMB2. Taken together, these results suggest that the mild variant of Pierson syndrome caused by the C321R-LAMB2 mutation may be a prototypical ER storage disease, which may benefit from treatment approaches that target the handling of misfolded proteins.

NPHS2 p.V290M mutation in late-onset steroid-resistant nephrotic syndrome

BACKGROUND

The most frequently mutated gene of steroid-resistant nephrotic syndrome (SRNS) is NPHS2. Current guidelines propose the sequencing of all NPHS2 exons only in childhood-onset SRNS.

METHODS

A cohort of 38 Hungarian patients with childhood-onset nephrotic-range proteinuria was screened for NPHS2 mutations. The frequency of the p.V290M mutation in late-onset SRNS was examined in the French and PodoNet cohorts.

RESULTS

Of the 38 Hungarian patients screened, seven carried NPHS2 mutations on both alleles, of whom two-diagnosed with proteinuria through school screening programs at the age of 9.7 and 14 years, respectively-did not develop nephrotic syndrome in childhood. The first, an 18-year-old boy, homozygous for p.V290M, has never developed edema. The second, a 31-year-old woman-compound heterozygous for p.V290M and p.R138Q-was first detected with hypoalbuminemia (<30 g/l) and edema at the age of 24.3 and 27.5 years, respectively. Both patients currently have a normal glomerular filtration rate. The mutation p.V290M was carried by three of the 38 patients in the Hungarian cohort, by two of the 95 patients with late-onset SRNS in the PodoNet cohort and by none of the 83 patients in the French cohort.

CONCLUSIONS

We propose that not only the p.R229Q variant, but also the p.V290M mutation should be screened in Central and Eastern European patients with late-onset SRNS.

A molecular genetic analysis of childhood nephrotic syndrome in a cohort of Saudi Arabian families

Nephrotic syndrome (NS) is a renal disease characterized by heavy proteinuria, hypoalbuminemia, edema and hyperlipidemia. Its presentation within the first 3 months of life or in multiple family members suggests an underlying inherited cause. To determine the frequency of inherited NS, 62 cases (representing 49 families with NS) from Saudi Arabia were screened for mutations in NPHS1, NPHS2, LAMB2, PLCE1, CD2AP, MYO1E, WT1, PTPRO and Nei endonuclease VIII-like 1 (NEIL1). We detected likely causative mutations in 25 out of 49 families studied (51%). We found that the most common genetic cause of NS in our cohort was a homozygous mutation in the NPHS2 gene, found in 11 of the 49 families (22%). Mutations in the NPHS1 and PLCE1 genes allowed a molecular genetic diagnosis in 12% and 8% of families, respectively. We detected novel MYO1E mutations in three families (6%). No mutations were found in WT1, PTPRO or NEIL1. The pathogenicity of novel variants was analyzed by in silico tests and by genetic screening of ethnically matched control populations. This is the first report describing the molecular genetics of NS in the Arabian Peninsula.

Glomerular Filtration Barrier Assembly: An insight

A glomerulus is the network of capillaries that resides in the Bowman's capsule that functions as a filtration unit of kidney. The glomerular function ensures that essential plasma proteins are retained in blood and the filtrate is passed on as urine. The glomerular filtration assembly is composed of three main cellular barriers that are critical for the ultrafiltration process, the fenestrated endothelium, glomerular basement membrane and highly specialized podocytes. The podocytes along with their specialized junctions "slit diaphragm" form the basic backbone of this filtration assembly. The presence of high amounts of protein in urine a condition commonly referred as proteinuria indicates a defective glomerular filtration barrier. Various glomerular disorders including Nephrotic syndrome are characterized by significant alteration in the structure of podocytes that is associated with prolonged increase in the glomerular permeability leading to heavy proteinuria. Recent identification of proteins that are specifically localized at the slit diaphragm whose mutations and knockouts are known to result in loss of renal function has significantly advanced our understanding of the molecular makeup of this filtration assembly. The present review is an effort to summarize the recent developments in this field and highlight our understanding of the glomerular filtration barrier assembly.

First Japanese case of Pierson syndrome with mutations in LAMB2

Pierson syndrome (OMIM 609049) is typically characterized by congenital nephritic syndrome and peculiar ocular anomalies with microcoria. It is caused by mutations in LAMB2, which encodes laminin β2. Approximately 50 mutations of LAMB2 from approximately 40 unrelated families have been identified; however, most of them were from Western countries. Although three patients in Asia with mutations of LAMB2 have been reported, they were not typical cases. We report the first Japanese case of Pierson syndrome with proven causative LAMB2 mutations. She presented with congenital nephrotic syndrome and bilateral microcoria at birth, and developed end-stage renal disease at 2 months of age. This is the first report of a typical case from Asia. LAMB2 analysis by direct sequencing revealed the compound heterozygous mutations c.3974_3975insA (p.N1325KfsX1331, maternal, novel) in exon 25 and c.4519C>T (p.Q1507X, paternal) in exon 27. The phenotype due to LAMB2 mutations appears to be similar between different ethnic groups.

ARHGDIA: a novel gene implicated in nephrotic syndrome

Background

Congenital nephrotic syndrome arises from a defect in the glomerular filtration barrier that permits the unrestricted passage of protein across the barrier, resulting in proteinuria, hypoalbuminaemia, and severe oedema. While most cases are due to mutations in one of five genes, in up to 15% of cases, a genetic cause is not identified. We investigated two sisters with a presumed recessive form of congenital nephrotic syndrome.

Methods and results

Whole exome sequencing identified five genes with diallelic mutations that were shared by the sisters, and Sanger sequencing revealed that ARHGDIA that encodes Rho GDP (guanosine diphosphate) dissociation inhibitor α (RhoGDIα, OMIM 601925) was the most likely candidate. Mice with targeted inactivation of ARHGDIA are known to develop severe proteinuria and nephrotic syndrome, therefore this gene was pursued in functional studies. The sisters harbour a homozygous in-frame deletion that is predicted to remove a highly conserved aspartic acid residue within the interface where the protein, RhoGDIα, interacts with the Rho family of small GTPases (c.553_555del(p.Asp185del)). Rho-GTPases are critical regulators of the actin cytoskeleton and when bound to RhoGDIα, they are sequestered in an inactive, cytosolic pool. In the mouse kidney, RhoGDIα was highly expressed in podocytes, a critical cell within the glomerular filtration barrier. When transfected in HEK293T cells, the mutant RhoGDIα was unable to bind to the Rho-GTPases, RhoA, Rac1, and Cdc42, unlike the wild-type construct. When RhoGDIα was knocked down in podocytes, RhoA, Rac1, and Cdc42 were hyperactivated and podocyte motility was impaired. The proband's fibroblasts demonstrated mislocalisation of RhoGDIα to the nucleus, hyperactivation of the three Rho-GTPases, and impaired cell motility, suggesting that the in-frame deletion leads to a loss of function.

Conclusions

Mutations in ARHGDIA need to be considered in the aetiology of heritable forms of nephrotic syndrome.

Synaptic basal lamina-associated congenital myasthenic syndromes

Proteins associated with the basal lamina (BL) participate in complex signal transduction processes that are essential for the development and maintenance of the neuromuscular junction (NMJ). Most important junctional BL proteins are collagens, such as collagen IV (α3-6), collagen XIII, and ColQ; laminins; nidogens; and heparan sulfate proteoglycans, such as perlecan and agrin. Mice lacking Colq (Colq(-/-)), laminin β2 (Lamb2(-/-)), or collagen XIII (Col13a1(-/-)) show immature nerve terminals enwrapped by Schwann cell projections that invaginate into the synaptic cleft and decrease contact surface for neurotransmission. Human mutations in COLQ, LAMB2, and AGRN cause congenital myasthenic syndromes (CMSs) owing to deficiency of ColQ, laminin-β2, and agrin, respectively. In these syndromes the NMJ ultrastructure shows striking resemblance to that of mice lacking the corresponding protein; furthermore, the extracellular localization of mutant proteins may provide favorable conditions for replacement strategies based on gene therapy and stem cells.

Cell-matrix adhesion of podocytes in physiology and disease

Cell-matrix adhesion is crucial for maintaining the mechanical integrity of epithelial tissues. Podocytes--a key component of the glomerular filtration barrier--are exposed to permanent transcapillary filtration pressure and must therefore adhere tightly to the underlying glomerular basement membrane (GBM). The major cell-matrix adhesion receptor in podocytes is the integrin α3β1, which connects laminin 521 in the GBM through various adaptor proteins to the intracellular actin cytoskeleton. Other cell-matrix adhesion receptors expressed by podocytes include the integrins α2β1 and αvβ3, α-dystroglycan, syndecan-4 and type XVII collagen. Mutations in genes encoding any of the components critical for podocyte adhesion cause glomerular disease. This Review highlights recent advances in our understanding of the cell biology and genetics of podocyte adhesion with special emphasis on glomerular disease.

Laminin network formation studied by reconstitution of ternary nodes in solution

The polymerization of laminins into a cell-associated network is a key process in basement membrane assembly. Network formation is mediated by the homologous short arm tips of the laminin heterotrimer, each consisting of a globular laminin N-terminal (LN) domain followed by a tandem of laminin-type epidermal growth factor-like (LEa) domains. How the short arms interact in the laminin network is unclear. Here, we have addressed this question by reconstituting laminin network nodes in solution and analyzing them by size exclusion chromatography and light scattering. Recombinant LN-LEa1-4 fragments of the laminin α1, α2, α5, β1, and γ1 chains were monomeric in solution. The β1 and γ1 fragments formed the only detectable binary complex and ternary complexes of 1:1:1 stoichiometry with all α chain fragments. Ternary complex formation required calcium and did not occur at 4 °C, like the polymerization of full-length laminins. Experiments with chimeric short arm fragments demonstrated that the LEa2-4 regions of the β1 and γ1 fragments are dispensable for ternary complex formation, and an engineered glycan in the β1 LEa1 domain was also tolerated. In contrast, mutation of Ser-68 in the β1 LN domain (corresponding to a Pierson syndrome mutation in the closely related β2 chain) abolished ternary complex formation. We conclude that authentic ternary nodes of the laminin network can be reconstituted for structure-function studies.

Laminin α5 guides tissue patterning and organogenesis

Laminins (LM) are extracellular matrix molecules that contribute to and are required for the formation of basement membranes. They participate in the modulation of epithelial/mesenchymal interactions and are implicated in organogenesis and maintenance of organ homeostasis. Among the LM molecules, the LM α5 chain (LMα5) is one of the most widely distributed LM in the developing and mature organism. Its presence in some basement membranes during embryogenesis is absolutely required for maintenance of basement membrane integrity and thus for proper organogenesis. LMα5 also regulates the expression of genes important for major biological processes, in part by repressing or activating signaling pathways, depending upon the physiological context.

Glomerular basement membrane and related glomerular disease

The glomerular basement membrane (GBM) is lined by fenestrated endothelium from the capillary-lumen side and by interdigitating foot processes of the podocytes from the urinary- space side. These three layers of the glomerular capillary wall constitute the functional unit of the glomerular filtration barrier. The GBM is assembled through an interweaving of type IV collagen with laminins, nidogen, and sulfated proteoglycans. Mutations in genes encoding LAMB2, COL4A3, COL4A4, and COL4A5 cause glomerular disease in humans as well as in mice. In addition, laminin α5 mutation in podocytes leads to proteinuria and renal failure in mice. Moreover, more neoepitopes in Goodpasture's disease and for the first time alloepitopes in Alport post-transplantation nephritis have been located in the collagen α5(IV) NC1 domain. These discoveries underscore the importance of the GBM in establishing and maintaining the integrity of the glomerular filtration barrier.

Crystal structures of the network-forming short-arm tips of the laminin β1 and γ1 chains

The heterotrimeric laminins are a defining component of basement membranes and essential for tissue formation and function in all animals. The three short arms of the cross-shaped laminin molecule are composed of one chain each and their tips mediate the formation of a polymeric network. The structural basis for laminin polymerisation is unknown. We have determined crystal structures of the short-arm tips of the mouse laminin β1 and γ1 chains, which are grossly similar to the previously determined structure of the corresponding α5 chain region. The short-arm tips consist of a laminin N-terminal (LN) domain that is attached like the head of a flower to a rod-like stem formed by tandem laminin-type epidermal growth factor-like (LE) domains. The LN domain is a β-sandwich with elaborate loop regions that differ between chains. The γ1 LN domain uniquely contains a calcium binding site. The LE domains have little regular structure and are stabilised by cysteines that are disulphide-linked 1–3, 2–4, 5–6 and 7–8 in all chains. The LN surface is not conserved across the α, β and γ chains, but within each chain subfamily there is a striking concentration of conserved residues on one face of the β-sandwich, while the opposite face invariably is shielded by glycans. We propose that the extensive conserved patches on the β and γ LN domains mediate the binding of these two chains to each other, and that the α chain LN domain subsequently binds to the composite β-γ surface. Mutations in the laminin β2 LN domain causing Pierson syndrome are likely to impair the folding of the β2 chain or its ability to form network interactions.

Progress in pathogenesis of proteinuria

Aims. Proteinuria not only is a sign of kidney damage, but also is involved in the progression of renal diseases as an independent pathologic factor. Clinically, glomerular proteinuria is most commonly observed, which relates to structural and functional anomalies in the glomerular filtration barrier. The aim of this paper was to describe the pathogenesis of glomerular proteinuria. Data Sources. Articles on glomerular proteinuria retrieved from Pubmed and MEDLINE in the recent 5 years were reviewed. Results. The new understanding of the roles of glomerular endothelial cells and the glomerular basement membrane (GBM) in the pathogenesis of glomerular proteinuria was gained. The close relationships of slit diaphragm (SD) molecules such as nephrin, podocin, CD2-associated protein (CD2AP), a-actinin-4, transient receptor potential cation channel 6 (TRPC6), Densin and membrane-associated guanylate kinase inverted 1 (MAGI-1), α3β1 integrin, WT1, phospholipase C epsilon-1 (PLCE1), Lmx1b, and MYH9, and mitochondrial disorders and circulating factors in the pathogenesis of glomerular proteinuria were also gradually discovered. Conclusion. Renal proteinuria is a manifestation of glomerular filtration barrier dysfunction. Not only glomerular endothelial cells and GBM, but also the glomerular podocytes and their SDs play an important role in the pathogenesis of glomerular proteinuria.

Pierson syndrome in an adolescent girl with nephrotic range proteinuria but a normal GFR

BACKGROUND

Pierson syndrome, caused by mutations in the LAMB2 gene, was originally described as a combination of microcoria and congenital nephrotic syndrome, rapidly progressing to end-stage renal failure.

CASE-DIAGNOSIS/TREATMENT

We report a minor variant of Pierson syndrome in a teenage girl with severe myopia since early infancy and proteinuria first detected at age 6. At the age of 11 she was found to carry a unique homozygous non-truncating LAMB2 mutation in exon 2: c.T240G (p.S80R). Renal biopsy revealed mild diffuse mesangial sclerosis and residual expression of laminin β2. Today at age 14, on treatment with angiotensin-converting enzyme inhibitors and angiotensin receptor blockers, she continues to have nephrotic range proteinuria, but a normal glomerular filtration rate.

CONCLUSIONS

LAMB2 mutations should be considered in all patients with glomerular proteinuria and abnormal ocular phenotype, irrespective of age and disease severity.

RNA-binding protein IGF2BP2/IMP2 is required for laminin-β2 mRNA translation and is modulated by glucose concentration

Laminin-β2 (LAMB2) is a critical component of the glomerular basement membrane as content of LAMB2 in part determines glomerular barrier permeability. Previously, we reported that high concentrations of glucose reduce expression of this laminin subunit at the translational level. The present studies were undertaken to further define systems that control Lamb2 translation and the effect of high glucose on those systems. Complementary studies were performed using in vitro differentiation of cultured podocytes and mesangial cells exposed to normal and elevated concentrations of glucose, and tissues from control and diabetic rats. Together, these studies provide evidence for regulation of Lamb2 translation by IMP2, an RNA binding protein that targets Lamb2 mRNA to the actin cytoskeleton. Expression of Imp2 itself is regulated by the transcription factor HMGA2, which in turn is regulated by the microRNA let-7b. Elevated concentrations of glucose increase let-7b, which reduces HMGA2 expression, in turn reducing IMP2 and LAMB2. Correlative changes in kidney tissues from control and streptozotocin-induced diabetic rats suggest these control mechanisms are operative in vivo and may contribute to proteinuria in diabetic nephropathy. To our knowledge, this is the first time that translation of Lamb2 mRNA has been linked to the actin cytoskeleton, as well as to specific RNA-binding proteins. These translational control points may provide new targets for therapy in proteinuric disorders such as diabetic nephropathy where LAMB2 levels are reduced.

The role of cell-extracellular matrix interactions in glomerular injury

Glomerulosclerosis is characterized by excessive deposition of extracellular matrix within the glomeruli of the kidney, glomerular cell death, and subsequent loss of functional glomeruli. While in physiological situations the levels of extracellular matrix components are kept constant by a tight balance between formation and degradation, in the case of injury that results in fibrosis there is increased matrix deposition relative to its breakdown. Multiple factors control matrix synthesis and degradation, thus contributing to the development of glomerulosclerosis. This review focuses primarily on the role of cell-matrix interactions, which play a critical role in governing glomerular cell cues in both healthy and diseased kidneys. Cell-extracellular matrix interactions are made possible by various cellular receptors including integrins, discoidin domain receptors, and dystroglycan. Upon binding to a selective extracellular matrix protein, these receptors activate intracellular signaling pathways that can either downregulate or upregulate matrix synthesis and deposition. This, together with the observation that changes in the expression levels of matrix receptors have been documented in glomerular disease, clearly emphasizes the contribution of cell-matrix interactions in glomerular injury. Understanding the molecular mechanisms whereby extracellular matrix receptors regulate matrix homeostasis in the course of glomerular injury is therefore critical for devising more effective therapies to treat and ideally prevent glomerulosclerosis.

The glomerular basement membrane

The kidney's glomerular filtration barrier consists of two cells-podocytes and endothelial cells-and the glomerular basement membrane (GBM), a specialized extracellular matrix that lies between them. Like all basement membranes, the GBM consists mainly of laminin, type IV collagen, nidogen, and heparan sulfate proteoglycan. However, the GBM is unusually thick and contains particular members of these general protein families, including laminin-521, collagen α3α4α5(IV), and agrin. Knockout studies in mice and genetic findings in humans show that the laminin and type IV collagen components are particularly important for GBM structure and function, as laminin or collagen IV gene mutations cause filtration defects and renal disease of varying severities, depending on the nature of the mutations. These studies suggest that the GBM plays a crucial role in establishing and maintaining the glomerular filtration barrier.

Genetics of anterior segment dysgenesis disorders

PURPOSE OF REVIEW

Anterior segment dysgenesis (ASD) disorders encompass a spectrum of developmental conditions affecting the cornea, iris, and lens and are generally associated with an approximate 50% risk for glaucoma. These conditions are characterized by both autosomal dominant and recessive patterns of inheritance often with incomplete penetrance/variable expressivity. This article summarizes what is known about the genetics of ASD disorders and reviews recent developments.

RECENT FINDINGS

Mutations in Collagen type IV alpha-1 (COL4A1) and Beta-1,3-galactosyltransferase-like (B3GALTL) have been reported in ASD patients. Novel findings in other well known ocular genes are also presented, among which regulatory region deletions in PAX6 and PITX2 are most notable.

SUMMARY

Although a number of genetic causes have been identified, many ASD conditions are still awaiting genetic elucidation. The majority of characterized ASD genes encode transcription factors; several other genes represent extracellular matrix-related proteins. All of the involved genes play active roles in ocular development and demonstrate conserved functions across species. The use of novel technologies, such as whole genome sequencing/comparative genomic hybridization, is likely to broaden the mutation spectrums in known genes and assist in the identification of novel causative genes as well as modifiers explaining the phenotypic variability of ASD conditions.

Glomerular basement membrane composition and the filtration barrier

The glomerular basement membrane (GBM) is an especially thick basement membrane that contributes importantly to the kidney's filtration barrier. The GBM derives from the fusion of separate podocyte and endothelial cell basement membranes during glomerulogenesis and consists primarily of laminin-521 (α5β2γ1), collagen α3α4α5(IV), nidogens-1 and -2, and agrin. Of these nine proteins, mutations in the genes encoding four of them (LAMB2, COL4A3, COL4A4, and COL4A5) cause glomerular disease in humans as well as in mice. Furthermore, mutation of a fifth (Lama5) gene in podocytes in mice causes proteinuria, nephrotic syndrome, and progression to renal failure. These results highlight the importance of the GBM for establishing and maintaining a properly functioning glomerular filtration barrier.

Forced expression of laminin beta1 in podocytes prevents nephrotic syndrome in mice lacking laminin beta2, a model for Pierson syndrome

Pierson syndrome is a congenital nephrotic syndrome with ocular and neurological defects caused by mutations in LAMB2, the gene encoding the basement membrane protein laminin β2 (Lamβ2). It is the kidney glomerular basement membrane (GBM) that is defective in Pierson syndrome, as Lamβ2 is a component of laminin-521 (LM-521; α5β2γ1), the major laminin in the mature GBM. In both Pierson syndrome and the Lamb2(-/-) mouse model for this disease, laminin β1 (Lamβ1), a structurally similar homolog of Lamβ2, is marginally increased in the GBM, but it fails to fully compensate for the loss of Lamβ2, leading to the filtration barrier defects and nephrotic syndrome. Here we generated several lines of Lamβ1 transgenic mice and used them to show that podocyte-specific Lamβ1 expression in Lamb2(-/-) mice abrogates the development of nephrotic syndrome, correlating with a greatly extended lifespan. In addition, the more Lamβ1 was expressed, the less urinary albumin was excreted. Transgenic Lamβ1 expression increased the level of Lamα5 in the GBM of rescued mice, consistent with the desired increased deposition of laminin-511 (α5β1γ1) trimers. Ultrastructural analysis revealed occasional knob-like subepithelial GBM thickening but intact podocyte foot processes in aged rescued mice. These results suggest the possibility that up-regulation of LAMB1 in podocytes, should it become achievable, would likely lessen the severity of nephrotic syndrome in patients carrying LAMB2 mutations.

A missense LAMB2 mutation causes congenital nephrotic syndrome by impairing laminin secretion

Laminin β2 is a component of laminin-521, which is an important constituent of the glomerular basement membrane (GBM). Null mutations in laminin β2 (LAMB2) cause Pierson syndrome, a severe congenital nephrotic syndrome with ocular and neurologic defects. In contrast, patients with LAMB2 missense mutations, such as R246Q, can have less severe extrarenal defects but still exhibit congenital nephrotic syndrome. To investigate how such missense mutations in LAMB2 cause proteinuria, we generated three transgenic lines of mice in which R246Q-mutant rat laminin β2 replaced the wild-type mouse laminin β2 in the GBM. These transgenic mice developed much less severe proteinuria than their nontransgenic Lamb2-deficient littermates; the level of proteinuria correlated inversely with R246Q-LAMB2 expression. At the onset of proteinuria, expression and localization of proteins associated with the slit diaphragm and foot processes were normal, and there were no obvious ultrastructural abnormalities. Low transgene expressors developed heavy proteinuria, foot process effacement, GBM thickening, and renal failure by 3 months, but high expressors developed only mild proteinuria by 9 months. In vitro studies demonstrated that the R246Q mutation results in impaired secretion of laminin. Taken together, these results suggest that the R246Q mutation causes nephrotic syndrome by impairing secretion of laminin-521 from podocytes into the GBM; however, increased expression of the mutant protein is able to overcome this secretion defect and improve glomerular permselectivity.

Organogenesis of the kidney glomerulus: focus on the glomerular basement membrane

The glomerular basement membrane (GBM) is a crucial component of the kidney's filtration barrier that separates the vasculature from the urinary space. During glomerulogenesis, the GBM is formed from fusion of two distinct basement membranes, one synthesized by the glomerular epithelial cell (podocyte) and the other by the glomerular endothelial cell. The main components of the GBM are laminin-521 (α5β2γ1), collagen α3α4α5(IV), nidogen and the heparan sulfate proteoglycan, agrin. By studying mice lacking specific GBM components, we have shown that during glomerulogenesis, laminin is the only one that is required for GBM integrity and in turn, the GBM is required for completion of glomerulogenesis and glomerular vascularization. In addition, our results from laminin β2-null mice suggest that laminin-521, and thus the GBM, contribute to the establishment and maintenance of the glomerular filtration barrier to plasma albumin. In contrast, mutations that affect GBM collagen IV or agrin do not impair glomerular development or cause immediate leakage of plasma proteins. However, collagen IV mutation, which causes Alport syndrome and ESRD in humans, leads to gradual damage to the GBM that eventually leads to albuminuria and renal failure. These results highlight the importance of the GBM for establishing and maintaining a perfectly functioning, highly selective glomerular filter.

Basement membranes: cell scaffoldings and signaling platforms

Basement membranes are widely distributed extracellular matrices that coat the basal aspect of epithelial and endothelial cells and surround muscle, fat, and Schwann cells. These extracellular matrices, first expressed in early embryogenesis, are self-assembled on competent cell surfaces through binding interactions among laminins, type IV collagens, nidogens, and proteoglycans. They form stabilizing extensions of the plasma membrane that provide cell adhesion and that act as solid-phase agonists. Basement membranes play a role in tissue and organ morphogenesis and help maintain function in the adult. Mutations adversely affecting expression of the different structural components are associated with developmental arrest at different stages as well as postnatal diseases of muscle, nerve, brain, eye, skin, vasculature, and kidney.

Novel mutations in steroid-resistant nephrotic syndrome diagnosed in Tunisian children

Steroid-resistant nephrotic syndrome (NS) remains one of the most intractable causes of end-stage renal disease in the first two decades of life. Several genes have been involved including NPHS1, NPHS2, WT1, PLCE1, and LAMB2. Our aim was to identify causative mutations in these genes, in 24 children belonging to 13 families with NS manifesting with various ages of onset. We performed haplotype analysis and direct exon sequencing of NPHS1, NPHS2, PLCE1, LAMB2, and the relevant exons 8 and 9 of WT1. Ten different pathogenic mutations were detected in seven families concerning four genes (NPHS1 (3/7), LAMB2 (2/7), NPHS2 (1/7), and WT1 (1/7)). Five of the detected mutations were novel; IVS9+2 T>C and p.D616G in NPHS1; p.E371fsX16 in NPHS2, and p.E705X and p.D1151fsX23 in LAMB2. Nine of 24 patients failed to be categorized by mutational analysis. Our study extends the spectrum of abnormalities underlying NS, by reporting novel mutations in the NPHS1 and NPHS2 genes and the first cases of LAMB2 mutations in Tunisia. Congenital and infantile NS can be explained by mutations in NPHS1, NPHS2, WT1, or LAMB2 genes. The identification of additional genes mutated in NS can be anticipated.

A novel mutation in NPHS2 causing nephrotic syndrome in a Saudi Arabian family

We report a consanguineous family from Saudi Arabia with three affected children presenting with infantile nephrotic syndrome. In order to provide a molecular diagnosis, a genome-wide SNP analysis of the affected patients was performed. We identified a region of homozygosity on chromosome 1, containing the NPHS2 gene. Direct sequencing, by exon PCR, of NPHS2 identified a homozygous nucleotide change 385C > T within exon 3 in the three affected children, leading to a premature stop codon (Q129X). This homozygous truncating mutation in NPHS2 is novel and was associated with a severe clinical phenotype. Additional mutations in related genes NPHS1, PLCE1 and NEPH1 were not identified, excluding tri-allelism within these genes in this family.