COL4A3/COL4A4 mutations producing focal segmental glomerulosclerosis and renal failure in thin basement membrane nephropathy

Carriers of Autosomal Recessive Alport Syndrome with Thin Basement Membrane Nephropathy Presenting as Focal Segmental Glomerulosclerosis in Later Life

Collagen IV nephropathies (COL4Ns) comprise benign familial microscopic hematuria, thin basement membrane nephropathy (TBMN), X-linked Alport syndrome (AS) and also autosomal recessive and dominant AS. Apart from the X-linked form of AS, which is caused by hemizygous mutations in the COL4A5 gene, the other entities are caused by mutations in the COL4A3 or COL4A4 genes. The diagnosis of these conditions used to be based on clinical and/or histological findings of renal biopsies, but it is the new molecular genetics approach that revolutionised their investigation and proved particularly instrumental, especially, in many not so clear-cut cases. More recently, the spectrum of COL4N has expanded to include late onset focal segmental glomerulosclerosis (FSGS) that develops on top of TBMN in later life. Also, other reports showed that some patients with a primary diagnosis of familial FSGS proved to have variants in COL4 genes. In the presence of a renal biopsy picture of FSGS and in the absence of either electron microscopy studies or molecular genetic studies that point to TBMN and COL4N, the patient and his family may be mistakenly diagnosed with hereditary FSGS leading to unnecessary further investigations, erroneous family counselling and improper corticosteroid treatment. TBMN is a frequent finding in the general population, and according to several recent reports, it may be the underlying cause and the explanation for many familial and sporadic cases of late-onset FSGS with non-nephrotic proteinuria. This is an important new finding that needs widespread recognition. It is anticipated that the molecular genetic analysis with next generation sequencing will certainly offer timely correct diagnosis.

Co-Inheritance of Functional Podocin Variants with Heterozygous Collagen IV Mutations Predisposes to Renal Failure

BACKGROUND/AIMS

A subset of patients who present with proteinuria and are diagnosed with focal segmental glomerulosclerosis (FSGS) have inherited heterozygous COL4A3/A4 mutations and are also diagnosed with thin basement membrane nephropathy (TBMN-OMIM: 141200). Two studies showed that co-inheritance of NPHS2-p.Arg229Gln, a podocin variant, may increase the risk for proteinuria and renal function decline.

METHODS

We hypothesized that additional podocin variants may exert a similar effect. We studied genetically a well-characterized Cypriot TBMN patient cohort by re-sequencing the NPHS2 coding region. We also performed functional studies in cell culture experiments, investigating the interaction of podocin variants with itself and with nephrin.

RESULTS

Potentially disease-modifying podocin variants were searched for by analyzing NPHS2 in 35 'severe' TBMN patients. One non-synonymous variant, p.Glu237Gln, was detected. Both variants, p.Arg229Gln and p.Glu237Gln, were tested in a larger cohort of 122 TBMN patients, who were categorized as 'mild' or 'severe' based on the presence of microscopic hematuria alone or combined with chronic renal failure and/or proteinuria. Seven 'severe' patients carried either of the 2 variants; none was present in the 'mild' patients (p = 0.05, Pearson χ(2)). The 7 carriers belong in 2 families segregating mutation COL4A3-p.Gly1334Glu. Inheritance of the wild-type (WT) and mutant alleles correlated with the phenotype (combined concordance probability 0.003). Immunofluorescence (IF) experiments after dual co-transfection of WT and mutant podocin suggested altered co-localization of mutant homodimers. IF experiments after co-transfection of WT podocin and nephrin showed normal membrane localization, while both podocin variants interfered with normal trafficking, demonstrating perinuclear staining. Immunoprecipitation experiments showed stronger binding of mutant podocin to WT podocin or nephrin.

CONCLUSION

The results support the hypothesis that certain hypomorphic podocin variants may act as adverse genetic modifiers when co-inherited with COL4A3/A4 mutations, thus predisposing to FSGS and severe kidney function decline.

Co-existence of thin basement membrane nephropathy with other glomerular pathologies; a single center experience

Background: The co-existence of thin basement membrane nephropathy (TBMN) and another glomerular pathology portends a worse prognosis than TBMN alone.

Objectives: The purpose of our study was to investigate the prevalence of TBMN and associated glomerular pathologies at our institution.

Patients and Methods: We reviewed all renal biopsies performed at Saint Louis University hospital over a 7-year period. We excluded all post transplant biopsies, and biopsies showing diabetic glomerulopathy, membranoproliferative glomerulopathy, membranous glomerulopathy, and biopsies where no electron microscopy or immunofluorescent studies were done. All other biopsies were included.

Results: A total of 634 biopsies were included in the study. The prevalence of TBMN was 47 (7.4%), of whom 17 (36.2%) had TBMN alone. In the remaining 30 (63.8%) patients TBMN was associated with other glomerular pathologies: IgAN 9 (19.1%) and FSGS 9 (19.1%). We found significantly higher prevalence of IgAN in patients with TBMN versus all biopsies (19.1% vs. 7.7%, respectively, P = 0.002). We found significant similarities in biopsy indications for TBMN and IgAN group.

Conclusions: Around two thirds of the cases of TBMN were associated with other glomerular pathologies. The prevalence of IgAN, but not focal segmental glomerulosclerosis, was significantly higher in patients with TBMN as compared to the general renal biopsy specimens.

Combinatorial Conflicting Homozygosity (CCH) analysis enables the rapid identification of shared genomic regions in the presence of multiple phenocopies

Background

The ability to identify regions of the genome inherited with a dominant trait in one or more families has become increasingly valuable with the wide availability of high throughput sequencing technology. While a number of methods exist for mapping of homozygous variants segregating with recessive traits in consanguineous families, dominant conditions are conventionally analysed by linkage analysis, which requires computationally demanding haplotype reconstruction from marker genotypes and, even using advanced parallel approximation implementations, can take substantial time, particularly for large pedigrees. In addition, linkage analysis lacks sensitivity in the presence of phenocopies (individuals sharing the trait but not the genetic variant responsible). Combinatorial Conflicting Homozygosity (CCH) analysis uses high density biallelic single nucleotide polymorphism (SNP) marker genotypes to identify genetic loci within which consecutive markers are not homozygous for different alleles. This allows inference of identical by descent (IBD) inheritance of a haplotype among a set or subsets of related or unrelated individuals.

Results

A single genome-wide conflicting homozygosity analysis takes <3 seconds and parallelisation permits multiple combinations of subsets of individuals to be analysed quickly. Analysis of unrelated individuals demonstrated that in the absence of IBD inheritance, runs of no CH exceeding 4 cM are not observed. At this threshold, CCH is >97% sensitive and specific for IBD regions within a pedigree exceeding this length and was able to identify the locus responsible for a dominantly inherited kidney disease in a Turkish Cypriot family in which six out 17 affected individuals were phenocopies. It also revealed shared ancestry at the disease-linked locus among affected individuals from two different Cypriot populations.

Conclusions

CCH does not require computationally demanding haplotype reconstruction and can detect regions of shared inheritance of a haplotype among subsets of related or unrelated individuals directly from SNP genotype data. In contrast to parametric linkage allowing for phenocopies, CCH directly provides the exact number and identity of individuals sharing each locus. CCH can also identify regions of shared ancestry among ostensibly unrelated individuals who share a trait. CCH is implemented in Python and is freely available (as source code) from http://sourceforge.net/projects/cchsnp/.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1360-4) contains supplementary material, which is available to authorized users.

Frequency of COL4A3/COL4A4 mutations amongst families segregating glomerular microscopic hematuria and evidence for activation of the unfolded protein response. Focal and segmental glomerulosclerosis is a frequent development during ageing

Familial glomerular hematuria(s) comprise a genetically heterogeneous group of conditions which include Alport Syndrome (AS) and thin basement membrane nephropathy (TBMN). Here we investigated 57 Greek-Cypriot families presenting glomerular microscopic hematuria (GMH), with or without proteinuria or chronic kidney function decline, but excluded classical AS. We specifically searched the COL4A3/A4 genes and identified 8 heterozygous mutations in 16 families (28,1%). Eight non-related families featured the founder mutation COL4A3-p.(G1334E). Renal biopsies from 8 patients showed TBMN and focal segmental glomerulosclerosis (FSGS). Ten patients (11.5%) reached end-stage kidney disease (ESKD) at ages ranging from 37-69-yo (mean 50,1-yo). Next generation sequencing of the patients who progressed to ESKD failed to reveal a second mutation in any of the COL4A3/A4/A5 genes, supporting that true heterozygosity for COL4A3/A4 mutations predisposes to CRF/ESKD. Although this could be viewed as a milder and late-onset form of autosomal dominant AS, we had no evidence of ultrastructural features or extrarenal manifestations that would justify this diagnosis. Functional studies in cultured podocytes transfected with wild type or mutant COL4A3 chains showed retention of mutant collagens and differential activation of the unfolded protein response (UPR) cascade. This signifies the potential role of the UPR cascade in modulating the final phenotype in patients with collagen IV nephropathies.

Natural history of genetically proven autosomal recessive Alport syndrome

BACKGROUND

Autosomal recessive Alport syndrome (ARAS) is a rare hereditary disease caused by homozygous or compound heterozygous mutations in either the COL4A3 or COL4A4 genes. Failure to diagnose ARAS cases is common, even if detailed clinical and pathological examinations are carried out. As the mutation detection rate for ARAS is unsatisfactory, we sought to develop more reliable diagnostic methods and provide a better description of the clinicopathological characteristics of this disorder.

METHODS

A retrospective analysis of 30 genetically diagnosed patients with ARAS in 24 pedigrees was conducted. The mutation detection strategy comprised three steps: (1) genomic DNA analysis using polymerase chain reaction (PCR) and direct sequencing; (2) mRNA analysis using reverse transcription (RT)-PCR to detect RNA processing abnormalities; (3) semi-quantitative PCR using capillary electrophoresis to detect large heterozygous deletions.

RESULTS

Using the three-step analysis, we identified homozygous or compound heterozygous mutations in all patients. Interestingly, 20% of our ARAS patients showed normal expression of α5 in kidney tissue. The median age of developing end-stage renal disease was 21 years.

CONCLUSIONS

The strategy described in this study improves the diagnosis for ARAS families. Although immunohistochemical analysis of α5 can provide diagnostic information, normal distribution does not exclude the diagnosis of ARAS.

Multiple kidney cysts in thin basement membrane disease with proteinuria and kidney function impairment

Background

Some patients with thin basement membrane disease (TBMD) develop proteinuria, hypertension and different degrees of CKD, besides the persistent microhaematuria characteristic of the disease. Little is known about factors associated with this unfavourable outcome.

Methods

We reviewed clinical, pathological and radiological features of 32 patients with biopsy-proven TBMD. Patients were divided in two groups: those with persistent normal kidney function and negative or minimal proteinuria (n = 16) and those with persistent proteinuria >0.5 g/day (n = 16).

Results

Patients with proteinuria had a worse kidney function at baseline than those with negative proteinuria. Global or segmental glomerulosclerosis, together with interstitial fibrosis, was found in 37% of patients with proteinuria. All proteinuric patients were treated with renin–angiotensin system blockers. At the end of follow-up (198 months in proteinuric patients and 210 months in patients with negative proteinuria) the prevalence of hypertension was 68% in proteinuric patients (12% at baseline), compared with 12 and 6%, respectively, in non-proteinuric patients. A slow decline of renal function was observed in proteinuric patients, although no patient developed end-stage kidney disease. Ultrasound studies showed bilateral kidney cysts in nine patients (56%) with proteinuria. Cysts were bilateral and countless in six patients, and bilateral but with a limited number of cysts in the three remaining patients. No cysts were found in patients with negative proteinuria.

Conclusions

Some patients with TBMD develop hypertension, proteinuria and CKD. Multiple bilateral kidney cysts were found in a majority (56%) of these patients. Further studies are needed to investigate the pathogenesis and the influence on long-term outcome of this TBMD-associated multiple kidney cysts.

COL4A4-related nephropathy caused by a novel mutation in a large consanguineous Saudi family

INTRODUCTION

Collagen type IV related nephropathies are due to the defects in collagen IV genes COL4A3, COL4A4, or COL4A5 and comprise a spectrum of phenotypes ranging from Alport Syndrome (AS) to its mild variants, termed as familial haematuria or thin basement membrane nephropathy. Classical AS is a progressive renal disease presenting with a triad of progressive hematuric nephritis and typical extra-renal complications, such as sensorineural hearing loss (SNHL) and variable ocular anomalies. The mode of inheritance in AS is X-linked in 85%, autosomal recessive in 15%, and autosomal dominant in rare cases.

OBJECTIVES

This study aims to identify underlying mutation in multiple individuals from a large consanguineous Saudi family with inherited nephropathy, including our index patient who manifested all the features of classical AS.

PATIENTS AND METHODS

Patients were diagnosed by nephrologists and clinical geneticists. All the individuals underwent clinical, audiological and ophthalmological evaluation. Blood samples were collected after written informed consent. DNA extraction, homozygosity mapping and PCR amplification followed standard methodologies.

RESULTS

The disease locus was mapped to 2q36.3, where both COL4A3 and COL4A4 reside. Sanger sequencing of COL4A3 and COL4A4 revealed an underlying novel homozygous disease-causing COL4A4 mutation (c.2420delG; p.G807fsX60) in the affected proband. Considerable phenotypic variability segregating with this COL4A4 mutation in our study family is documented. The homozygous mutants were manifesting end-stage renal disease (ESRD) in their adolescence, while the heterozygous carrier members were presenting with considerable phenotypic heterogeneity ranging from intermittent hematuria to late onset ESRD. In addition, there is a relatively severe involvement of the ear (SNHL) and eye in the homozygotes than the heterozygotes. Fertility problems were also noted in both of the homozygous females.

CONCLUSION

Identification of the causative mutation is an efficient strategy for conclusive molecular diagnosis in the patients and to establish genotype/phenotype correlation. It is important to study and evaluate asymptomatic carriers, to predict prognosis of the disease and to obviate the need for another renal biopsy in at-risk related family members. While an accurate genetic diagnosis of AS provides valuable information for genetic counseling in the extended family members, it can also facilitate future prenatal diagnosis and planning for pre-implantation genetic diagnosis.

Genetic testing can resolve diagnostic confusion in Alport syndrome

Alport syndrome (AS) is a familial glomerular disorder resulting from mutations in the genes encoding several members of the type IV collagen protein family. Despite advances in molecular genetics, renal biopsy remains an important initial diagnostic tool. Histological diagnosis is challenging as features may be non-specific, particularly early in the disease course and in females with X-linked disease. We present three families for whom there was difficulty in correctly diagnosing AS or thin basement membrane nephropathy as a result of misinterpretation of non-specific and incomplete histology. We highlight the importance of electron microscopy and immunofluorescence in improving diagnostic yield and also the hazard of interpreting a descriptive histological term as a diagnostic label. Molecular genetic testing allows a definitive diagnosis to be made in index patients and at-risk family members.

Evidence for activation of the unfolded protein response in collagen IV nephropathies

Thin-basement-membrane nephropathy (TBMN) and Alport syndrome (AS) are progressive collagen IV nephropathies caused by mutations in COL4A3/A4/A5 genes. These nephropathies invariably present with microscopic hematuria and frequently progress to proteinuria and CKD or ESRD during long-term follow-up. Nonetheless, the exact molecular mechanisms by which these mutations exert their deleterious effects on the glomerulus remain elusive. We hypothesized that defective trafficking of the COL4A3 chain causes a strong intracellular effect on the cell responsible for COL4A3 expression, the podocyte. To this end, we overexpressed normal and mutant COL4A3 chains (G1334E mutation) in human undifferentiated podocytes and tested their effects in various intracellular pathways using a microarray approach. COL4A3 overexpression in the podocyte caused chain retention in the endoplasmic reticulum (ER) that was associated with activation of unfolded protein response (UPR)-related markers of ER stress. Notably, the overexpression of normal or mutant COL4A3 chains differentially activated the UPR pathway. Similar results were observed in a novel knockin mouse carrying the Col4a3-G1332E mutation, which produced a phenotype consistent with AS, and in biopsy specimens from patients with TBMN carrying a heterozygous COL4A3-G1334E mutation. These results suggest that ER stress arising from defective localization of collagen IV chains in human podocytes contributes to the pathogenesis of TBMN and AS through activation of the UPR, a finding that may pave the way for novel therapeutic interventions for a variety of collagenopathies.

Nephrotic-range proteinuria in an infant with thin basement membrane nephropathy

Thin basement membrane nephropathy (TBMN) with heterozygous COL4A3/COL4A4 mutations is considered to be a cause of benign familial hematuria. The disease has been believed to have excellent prognosis and TBMN in early childhood is rarely associated with nephrotic-range proteinuria. Furthermore, the presence of proteinuria in patients with TBMN is associated with autosomal-dominant Alport syndrome, which has poorer prognosis in later life. We present an infant case of nephrotic-range proteinuria associated with TBMN caused by heterozygous COL4A4 mutation. A previously healthy 3-year-old boy developed microhematuria and nephrotic-range proteinuria. Renal pathology simply revealed thinning of the glomerular basement membrane (GBM) and mutational analysis revealed a novel heterozygous mutation in COL4A4. He was treated with lisinopril for 1.5 years, which resolved his proteinuria and hematuria. At the most recent follow-up at 6.5 years of age, urinalysis and kidney function were completely normal, without requiring medication. However, transient but repeated moderate to nephrotic-range proteinuria and microscopic hematuria occurred in association with other illnesses. This case highlights the spectrum of phenotypes that may be apparent in an infant with TBMN. Thinning of the GBM can cause transient nephrotic-range proteinuria, particularly in the early stages of TBMN.

Molecular genetics of familial hematuric diseases

The familial hematuric diseases are a genetically heterogeneous group of monogenic conditions, caused by mutations in one of several genes. The major genes involved are the following: (i) the collagen IV genes COL4A3/A4/A5 that are expressed in the glomerular basement membranes (GBM) and are responsible for the most frequent forms of microscopic hematuria, namely Alport syndrome (X-linked or autosomal recessive) and thin basement membrane nephropathy (TBMN). (ii) The FN1 gene, expressed in the glomerulus and responsible for a rare form of glomerulopathy with fibronectin deposits (GFND). (iii) CFHR5 gene, a recently recognized regulator of the complement alternative pathway and mutated in a recently revisited form of inherited C3 glomerulonephritis (C3GN), characterized by isolated C3 deposits in the absence of immune complexes. A hallmark feature of all conditions is the age-dependent penetrance and a broad phenotypic heterogeneity in the sense that subsets of patients progress to added proteinuria or proteinuria and chronic renal failure that may or may not lead to end-stage kidney disease (ESKD) anywhere between the second and seventh decade of life. In addition to other excellent laboratory tools that assist the clinician in reaching the correct diagnosis, the molecular analysis emerges as the gold standard in establishing the diagnosis in many cases of doubt due to equivocal findings that complicate the differential diagnosis. Recent work led to the description of candidate genetic modifiers which confer a variable risk for progressing to chronic renal failure when co-inherited on the background of a primary glomerulopathy. Finally, more families are still waiting to be studied and more genes to be mapped and cloned that are responsible for other forms of heritable hematuric diseases. The study of such genes and their protein products will likely shed more light on the structure and function of the glomerular filtration barrier and other important glomerular components.

How benign is hematuria? Using genetics to predict prognosis

Hematuria is a common presenting feature of glomerular disease and is sometimes associated with kidney failure later in life. Where isolated microscopic hematuria occurs in children and young adults, an underlying monogenic disorder, such as Alport syndrome or thin basement membrane nephropathy, is frequently responsible. In this review, these and other diseases, which often present with isolated microscopic hematuria, including hereditary angiopathy, nephropathy, aneurysms, and muscle cramps (HANAC) syndrome, IgA nephropathy, and CFHR5 nephropathy, are discussed together with the associated molecular pathology, clinical features, and prognosis. Genetic testing for these conditions used in clinical practice can provide important diagnostic and prognostic information that is relevant to the patient and their family, particularly when kidney transplantation is considered.

New developments in steroid-resistant nephrotic syndrome

Nephrotic syndrome is a disorder of the glomerular filtration barrier, a highly specialised tri-layer structure with unique functional properties. Recent advances emanating from the field of molecular genetics have revealed the podocyte as probably the central player in the control of glomerular filtration. More specifically, the cell-cell junction between adjacent podocyte foot processes, namely, the slit diaphragm, has been revealed to be made up of a sophisticated multi-protein complex which dynamically controls foot process architecture via signalling to the actin cytoskeleton. Key genes that have been identified from the study of inherited nephrotic syndromes include those encoding nephrin, podocin, TRPC6 (transient receptor potential canonical channel-6) and α-actinin-4, and more remain to be found. It is now possible to identify genetic causes underlying a proportion of nephrotic syndromes presenting at any age. The next big challenge for clinicians and researchers is to translate the molecular information learnt into the understanding of acquired, non-inherited forms of the disease and to guide therapeutic options. In this regard several exciting advances have been made, both in understanding the molecular mechanisms of current therapies and in revealing circulating plasma factors and the molecular pathways they trigger in the podocyte, that could be targeted by novel therapies.

Epistatic role of the MYH9/APOL1 region on familial hematuria genes

Familial hematuria (FH) is explained by at least four different genes (see below). About 50% of patients develop late proteinuria and chronic kidney disease (CKD). We hypothesized that MYH9/APOL1, two closely linked genes associated with CKD, may be associated with adverse progression in FH. Our study included 102 thin basement membrane nephropathy (TBMN) patients with three known COL4A3/COL4A4 mutations (cohort A), 83 CFHR5/C3 glomerulopathy patients (cohort B) with a single CFHR5 mutation and 15 Alport syndrome patients (cohort C) with two known COL4A5 mild mutations, who were categorized as “Mild” (controls) or “Severe” (cases), based on renal manifestations. E1 and S1 MYH9 haplotypes and variant rs11089788 were analyzed for association with disease phenotype. Evidence for association with “Severe” progression in CFHR5 nephropathy was found with MYH9 variant rs11089788 and was confirmed in an independent FH cohort, D (cumulative p value = 0.001, odds ratio = 3.06, recessive model). No association was found with APOL1 gene. Quantitative Real time PCR did not reveal any functional significance for the rs11089788 risk allele. Our results derive additional evidence supporting previous reports according to which MYH9 is an important gene per se, predisposing to CKD, suggesting its usefulness as a prognostic marker for young hematuric patients.

Incidence of end-stage renal disease in the Turkish-Cypriot population of Northern Cyprus: a population based study

BACKGROUND

This is the first report of the incidence and causes of end-stage renal disease (ESRD) of the Turkish-Cypriot population in Northern Cyprus.

METHODS

Data were collected over eight consecutive years (2004-2011) from all those starting renal replacement therapy (RRT) in this population. Crude and age-standardised incidence at 90 days was calculated and comparisons made with other national registries. We collected DNA from the entire prevalent population. As an initial experiment we looked for two genetic causes of ESRD that have been reported in Greek Cypriots.

RESULTS

Crude and age-standardised incidence at 90 days was 234 and 327 per million population (pmp) per year, respectively. The mean age was 63, and 62% were male. The age-adjusted prevalence of RRT in Turkish-Cypriots was 1543 pmp on 01/01/2011. The incidence of RRT is higher than other countries reporting to the European Renal Association - European Dialysis and Transplant Association, with the exception of Turkey. Diabetes is a major cause of ESRD in those under 65, accounting for 36% of incident cases followed by 30% with uncertain aetiology. 18% of the incident population had a family history of ESRD. We identified two families with thin basement membrane nephropathy caused by a mutation in COL4A3, but no new cases of CFHR5 nephropathy.

CONCLUSIONS

This study provides the first estimate of RRT incidence in the Turkish-Cypriot population, describes the contribution of different underlying diagnoses to ESRD, and provides a basis for healthcare policy planning.

A novel COL4A3 mutation causes autosomal-recessive Alport syndrome in a large Turkish family

BACKGROUND

Alport syndrome (AS) is a genetically heterogeneous disorder that is characterized by hematuria, progressive renal failure typically resulting in end-stage renal disease, sensorineural hearing loss, and variable ocular abnormalities. Only 15% of cases with AS are autosomal recessive and are caused by mutations in the COL4A3 or COL4A4 genes, encoding type IV collagen.

METHODS

Clinical data in a large consanguineous family with four affected members were reviewed, and genomic DNA was extracted. For mapping, 15 microsatellite markers flanking COL4A3, COL4A4, and COL4A5 in 16 family members were typed. For mutation screening, all coding exons of COL4A3 were polymerase chain reaction- amplified and Sanger-sequenced from genomic DNA.

RESULTS

The disease locus was mapped to chromosome 2q36.3, where COL4A3 and COL4A4 reside. Sanger sequencing revealed a novel mis-sense mutation (c.2T>C; p.M1T) in exon 1 of COL4A3. The identified nucleotide change was not found in 100 healthy ethnicity-matched controls via Sanger sequencing.

CONCLUSIONS

We present a large consanguineous Turkish family with AS that was found to have a COL4A3 mutation as the cause of the disease. Although the relationship between the various genotypes and phenotypes in AS has not been fully elucidated, detailed clinical and molecular analyses are helpful for providing data to be used in genetic counseling. It is important to identify new mutations to clarify their clinical importance, to assess the prognosis of the disease, and to avoid renal biopsy for final diagnosis.

Challenge in pathologic diagnosis of Alport syndrome: evidence from correction of previous misdiagnosis

Background

Pathologic studies play an important role in evaluating patients with Alport syndrome besides genotyping. Difficulties still exist in diagnosing Alport syndrome (AS), and misdiagnosis is a not-so-rare event, even in adult patient evaluated with renal biopsy.

Methods

We used nested case–control study to investigate 52 patients previously misdiagnosed and 52 patients initially diagnosed in the China Alport Syndrome Treatments and Outcomes Registry e-system.

Results

We found mesangial proliferative glomerulonephritis (MsPGN, 26.9%) and focal and segmental glomerulosclerosis (FSGS, 19.2%) were the most common misdiagnosis. FSGS was the most frequent misdiagnosis in female X-linked AS (fXLAS) patients (34.8%), and MsPGN in male X-linked AS (mXLAS) patients (41.2%). Previous misdiagnosed mXLAS patients (13/17, 76.5%) and autosomal recessive AS (ARAS) patients (8/12, 66.7%) were corrected after a second renal biopsy. While misdiagnosed fXLAS patients (18/23, 78.3%) were corrected after a family member diagnosed (34.8%) or after rechecking electronic microscopy and/or collagen-IV alpha-chains immunofluresence study (COL-IF) (43.5%) during follow-up. With COL-IF as an additional criterion for AS diagnosis, we found that patients with less than 3 criteria reached have increased risk of misdiagnosis (3.29-fold for all misdiagnosed AS patients and 3.90-fold for fXLAS patients).

Conclusion

We emphasize timely and careful study of electronic microscopy and COL-IF in pathologic evaluation of AS patients. With renal and/or skin COL-IF as additional criterion, 3 diagnosis criteria reached are the cutoff for diagnosing AS pathologically.

Evidence that NPHS2-R229Q predisposes to proteinuria and renal failure in familial hematuria

BACKGROUND

Familial hematuria (FH) is associated with at least two pathological entities: thin basement membrane nephropathy (TBMN), caused by heterozygous COL4A3/COL4A4 mutations, and C3 nephropathy caused by CFHR5 mutations. It is now known that TBMN patients develop proteinuria and changes of focal segmental glomerulosclerosis when biopsied. End-stage kidney disease (ESKD) is observed in 20% of carriers, at ages 50-70. A similar progression is observed in CFHR5 nephropathy. Recent evidence suggests that NPHS2-R229Q, a podocin polymorphism, may contribute to proteinuria in TBMN and to micro-albuminuria in the general population.

CASE-DIAGNOSIS/TREATMENT

NPHS2-R229Q was screened in a Cypriot FH cohort. 102 TBMN patients with three known COL4 mutations and 45 CFHR5 male patients with a single mutation were categorized as "Mild" or "Severe", based on the presence of microhematuria only, or proteinuria and chronic kidney disease. Nine R229Q carriers were found in the "Severe" category and none in the "Mild" (p=0.010 for genotypic association; p=0.043 for allelic association, adjusted for patients' relatedness), thus supporting the possible contribution of 229Q allele in disease progress.

CONCLUSIONS

Our results offer more evidence that in patients with FH, NPHS2-R229Q predisposes to proteinuria and ESKD. R229Q may be a good prognostic marker for young hematuric patients.

A miR-1207-5p binding site polymorphism abolishes regulation of HBEGF and is associated with disease severity in CFHR5 nephropathy

Heparin binding epidermal growth factor (HBEGF) is expressed in podocytes and was shown to play a role in glomerular physiology. MicroRNA binding sites on the 3′UTR of HBEGF were predicted using miRWalk algorithm and followed by DNA sequencing in 103 patients diagnosed with mild or severe glomerulopathy. A single nucleotide polymorphism, miRSNP C1936T (rs13385), was identified at the 3′UTR of HBEGF that corresponds to the second base of the hsa-miR-1207-5p seed region. When AB8/13 undifferentiated podocytes were transfected with miRNA mimics of hsa-miR-1207-5p, the HBEGF protein levels were reduced by about 50%. A DNA fragment containing the miRSNP allele-1936C was cloned into the pMIR-Report Luciferase vector and co-transfected with miRNA mimics of hsa-miR-1207-5p into AB8/13 podocytes. In agreement with western blot data, this resulted in reduced luciferase expression demonstrating the ability of hsa-miR-1207-5p to directly regulate HBEGF expression. On the contrary, in the presence of the miRSNP 1936T allele, this regulation was abolished. Collectively, these results demonstrate that variant 1936T of this miRSNP prevents hsa-miR-1207-5p from down-regulating HBEGF in podocytes. We hypothesized that this variant has a functional role as a genetic modifier. To this end, we showed that in a cohort of 78 patients diagnosed with CFHR5 nephropathy (also known as C3-glomerulopathy), inheritance of miRSNP 1936T allele was significantly increased in the group demonstrating progression to chronic renal failure on long follow-up. No similar association was detected in a cohort of patients with thin basement membrane nephropathy. This is the first report associating a miRSNP as genetic modifier to a monogenic renal disorder.

The role of molecular genetics in diagnosing familial hematuria(s)

Familial microscopic hematuria (MH) of glomerular origin represents a heterogeneous group of monogenic conditions involving several genes, some of which remain unknown. Recent advances have increased our understanding and our ability to use molecular genetics for diagnosing such patients, enabling us to study their clinical characteristics over time. Three collagen IV genes, COL4A3, COL4A4, and COL4A5 explain the autosomal and X-linked forms of Alport syndrome (AS), and a subset of thin basement membrane nephropathy (TBMN). A number of X-linked AS patients follow a milder course reminiscent of that of patients with heterozygous COL4A3/COL4A4 mutations and TBMN, while at the same time a significant subset of patients with TBMN and familial MH progress to chronic kidney disease (CKD) or end-stage kidney disease (ESKD). A mutation in CFHR5, a member of the complement factor H family of genes that regulate complement activation, was recently shown to cause isolated C3 glomerulopathy, presenting with MH in childhood and demonstrating a significant risk for CKD/ESKD after 40 years old. Through these results molecular genetics emerges as a powerful tool for a definite diagnosis when all the above conditions enter the differential diagnosis, while in many at-risk related family members, a molecular diagnosis may obviate the need for another renal biopsy.

Familial C3 glomerulopathy associated with CFHR5 mutations: clinical characteristics of 91 patients in 16 pedigrees

BACKGROUND AND OBJECTIVES

Complement factor H and related proteins (CFHR) are key regulators of the alternative complement pathway, where loss of function mutations lead to a glomerulopathy with isolated mesangial C3 deposits without immunoglobulins. Gale et al. (12) reported on 26 patients with the first familial, hematuric glomerulopathy caused by a founder mutation in the CFHR5 gene in patients of Cypriot descent living in the United Kingdom. CFHR5 nephropathy is clinically characterized by continuous microscopic hematuria whereas some patients present with additional episodes of synpharyngitic macrohematuria, associated with infection and pyrexia. A subgroup of patients, particularly men, develop additional proteinuria, hypertension, and chronic renal disease or ESRD.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We herewith expand significantly on the study by Gale et al., reporting on histologic, molecular, and clinical findings in 91 patients from 16 families with the same founder mutation.

RESULTS

Eighty-two patients (90%) exhibited microscopic hematuria; 51 (62%), exhibited only microscopic hematuria, whereas the remaining 31 additionally had proteinuria (38%); 28 proteinuric patients developed chronic renal failure (CRF). Among carriers of CFHR5 mutation aged >50 years, 80% of the men and 21% of the women developed CRF; 18 developed ESRD (14 men [78%], 4 women [22%]).

CONCLUSIONS

The diagnosis of CFHR5-related, isolated C3 glomerulopathy was established in 2009 using newly described mutation analysis after decades of follow-up with unclear diagnoses, occasionally confused with IgA nephropathy. This larger patient cohort establishes the clinical course, significant variable expressivity, and marked gender difference regarding the development of CRF and ESRD.

X-linked Alport syndrome in Hellenic families: phenotypic heterogeneity and mutations near interruptions of the collagen domain in COL4A5

The X-linked Alport syndrome (ATS) is caused by mutations in COL4A5 and exhibits a widely variable expression. Usually ATS is heralded with continuous microhematuria which rapidly progresses to proteinuria, hypertension and chronic or end-stage renal disease (ESRD) by adolescence, frequently accompanied by sensorineural deafness and ocular complications. Milder forms of ATS also exist. We studied 42 patients (19M, 23F) of nine Hellenic families suspected clinically of X-linked ATS who presented with marked phenotypic heterogeneity. We identified mutations in COL4A5 in six families. Two males with nonsense mutation E228X reached ESRD by ages 14 and 18. Frameshift mutation 2946delT followed the same course with early onset renal involvement and deafness. However, two males with the milder missense mutation G624D, reached ESRD after 39 years and one patient showed thin basement membrane nephropathy (TBMN). Another 5/8 affected males with missense mutation P628L also developed ESRD between 30 and 57 years, while three exhibit only mild chronic renal failure (CRF). The data support previous findings that certain mutations are associated with milder phenotypes and confirm that mutation G624D may be expressed as TBMN with familial hematuria. Similar conclusions apply for missense mutation P628L. Interestingly, mutations G624D and P628L are near the 12th natural interruption of COL4A5 triple helical domain, which may explain the milder phenotype.

Identification of a mutation in complement factor H-related protein 5 in patients of Cypriot origin with glomerulonephritis

BACKGROUND

Complement is a key component of the innate immune system, and variation in genes that regulate its activation is associated with renal and other disease. We aimed to establish the genetic basis for a familial disorder of complement regulation associated with persistent microscopic haematuria, recurrent macroscopic haematuria, glomerulonephritis, and progressive renal failure.

METHODS

We sought patients from the West London Renal and Transplant Centre (London, UK) with unusual renal disease and affected family members as a method of identification of new genetic causes of kidney disease. Two families of Cypriot origin were identified in which renal disease was consistent with autosomal dominant transmission and renal biopsy of at least one individual showed C3 glomerulonephritis. A mutation was identified via a genome-wide linkage study and candidate gene analysis. A PCR-based diagnostic test was then developed and used to screen for the mutation in population-based samples and in individuals and families with renal disease.

FINDINGS

Occurrence of familial renal disease cosegregated with the same mutation in the complement factor H-related protein 5 gene (CFHR5). In a cohort of 84 Cypriots with unexplained renal disease, four had mutation in CFHR5. Overall, we identified 26 individuals with the mutation and evidence of renal disease from 11 ostensibly unrelated kindreds, including the original two families. A mutant CFHR5 protein present in patient serum had reduced affinity for surface-bound complement. We term this renal disease CFHR5 nephropathy.

INTERPRETATION

CFHR5 nephropathy accounts for a substantial burden of renal disease in patients of Cypriot origin and can be diagnosed with a specific molecular test. The high risk of progressive renal disease in carriers of the CFHR5 mutation implies that isolated microscopic haematuria or recurrent macroscopic haematuria should not be regarded as a benign finding in individuals of Cypriot descent.

FUNDING

UK Medical Research Council and Wellcome Trust.

Novel heterozygous COL4A3 mutation in a family with late-onset ESRD

Thin basement membrane nephropathy (TBMN) and Alport syndrome (ATS) are genetically heterogeneous conditions characterized by structural abnormalities in the glomerular basement membrane (GBM). TBMN presents with hematuria, minimal proteinuria, and normal renal function. Although TBMN is an autosomal dominant disease (COL4A3 and COL4A4), ATS can be inherited X-linked (COL4A5), autosomal recessive, or autosomal dominant (both COL4A3 and COL4A4). The clinical course of TBMN is usually benign, whereas ATS typically results in end-stage renal disease (ESRD). Nevertheless, there is a broad spectrum of clinical phenotypes caused by mutations in COL4A3 or COL4A4. We report an Italian family who presented with hematuria and mild proteinuria. Mutational analysis showed a novel heterozygous mutation p.G291E in exon 15 of the COL4A3 gene. Many different mutations in COL4A3 and COL4A4 that cause TBMN have already been identified, but most genetic variability in these genes has been found to cause autosomal ATS. A valid genotype-phenotype correlation for TBMN or ATS is not yet known. Therefore, it is important to identify new mutations by direct sequencing to clarify their clinical importance, to assess the prognosis of the disease, and to avoid renal biopsy.

Basement membranes and human disease

In 1990, the role of basement membranes in human disease was established by the identification of COL4A5 mutations in Alport's syndrome. Since then, the number of diseases caused by mutations in basement membrane components has steadily increased as has our understanding of the roles of basement membranes in organ development and function. However, many questions remain as to the molecular and cellular consequences of these mutations and the way in which they lead to the observed disease phenotypes. Despite this, exciting progress has recently been made with potential treatment options for some of these so far incurable diseases.

Nucleotide variations in the NPHS2 gene in Greek children with steroid-resistant nephrotic syndrome

Mutations in the NPHS2 gene, encoding podocin, are a major cause of autosomal-recessive steroid-resistant nephrotic syndrome (SRNS) in childhood, accounting for up to 30% of sporadic and 20-40% of familial cases. Among 22 Greek children with a clinical diagnosis of SRNS, mutation analysis was performed in all eight NPHS2 gene exons, using denaturing gradient gel electrophoresis and DNA sequencing. The frequency of all nucleotide variations found in patients was also evaluated in 100 unrelated samples (18-30 years) with no known history of nephrotic disease. Three pathogenic genotypes (R138Q/R138Q, R229Q/A295T, and R168H/R168H) accounted for 3/14 (21%) of sporadic patients; the A295T mutation in exon 8 (c.883G>A) is novel and predicted in silico to be pathogenic. Among the familial cases, a single patient was heterozygous for R229Q. Several known polymorphisms were found, including the in cis variants IVS3-46C>T plus IVS3-21C>T, IVS7+7A>G A and exonic variants S96S (c.288C>T), A318A (c.954T>C), and L346L (c.1038A>G), with allele frequencies comparable to those in other populations. A novel substitution (IVS3-17C>T) was found in two related patients, but in no controls. In conclusion, podocin mutations do not appear to be a major cause of SRNS in Greek children, although the study cohort was small. However, NPHS2 gene analysis could still be considered in Greek SRNS patients to support appropriate management. The present study also contributes potentially useful observations for the clinical management of SRNS patients.

Screening for mutations in kidney-related genes using SURVEYOR nuclease for cleavage at heteroduplex mismatches

SURVEYOR is a new mismatch-specific plant DNA endonuclease that is very efficient for mutation scanning in heteroduplex DNA. It is much faster, cheaper, more sensitive, and easier to perform than other "traditional" mutation detection methods such as single-strand conformation polymorphism analysis, denaturing high-performance liquid chromatography, heteroduplex analysis, and phage resolvases. This is the first comprehensive report on the use of SURVEYOR for screening genes implicated in a spectrum of inherited renal diseases. Of the 48.2 kb screened, 44 variations were identified, accounting for one variation per 1.1 kb. The re-sequencing of multiple samples did not reveal any variation that had not been identified by SURVEYOR, attesting to its high fidelity. Additionally, we tested this enzyme against 15 known variants, 14 of which it identified, thus showing a sensitivity of 93%. We showed that the genetic heterogeneity of renal diseases can be easily overcome using this enzyme with a high degree of confidence and no bias for any specific variations. We also showed for the first time that SURVEYOR does not demonstrate any preference regarding mismatch cleavage at specific positions. Disadvantages of using SURVEYOR include enhanced exonucleolytic activity for some polymerase chain reaction products and less than 100% sensitivity. We report that SURVEYOR can be used as a mutation detection method with a high degree of confidence, offering an excellent alternative for low-budget laboratories and for the rapid manipulation of multiple genes.

An update on the treatment options for focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is not a disease but a lesion initially affecting the podocyte. Various factors may induce 'secondary' FSGS, including defects in molecules that contribute to the podocyte slit diaphragm permselectivity to albumin. They do not represent indications for immunosuppression and require symptomatic treatment only, comprising angiotensin 2 and endothelin antagonists. Primary (idiopathic) FSGS is possibly but not certainly of immunologic origin, owing to an elusive glomerular permeability factor (GPF), explaining relapse on a renal transplant and justifying an immunosuppressive treatment. The best prognostic feature of primary nephrotic FSGS is its response to corticosteroids. Alkylating agents are mostly ineffective in steroid-resistant forms. An association of corticosteroids and cyclosporine A (CsA) remains the mainstay of treatment, with a good tolerability when CsA dosage is low. A definite advantage of tacrolimus on CsA has not yet been established. Sirolimus appears ineffective and potentially harmful. Azathioprine is not indicated. A number of mostly uncontrolled trials indicate that mycophenolate mofetil might find an adjunctive place in the treatment. Plasmapheresis is of no avail outside the special case of relapse in a transplanted kidney. Immunoabsorption of the GPF has not led to practical treatment options. Anecdotal reports on rituximab are as yet too few to determine whether this monoclonal anti-CD20 antibody will find a place in the treatment of primary FSGS.

Clinico-pathological correlations in 127 patients in 11 large pedigrees, segregating one of three heterozygous mutations in the COL4A3/ COL4A4 genes associated with familial haematuria and significant late progression to proteinuria and chronic kidney disease from focal segmental glomerulosclerosis

BACKGROUND

Heterozygous mutations in the COL4A3/ COL4A4 genes are currently thought to be responsible for familial benign microscopic haematuria and maintenance of normal long-term kidney function.

METHODS

We report on 11 large Cypriot pedigrees with three such mutations. A total of 236 at-risk family members were genetically studied, and 127 (53.8%) carried a heterozygous mutation. Clinico-pathological correlations were available in all of these patients. Renal biopsies in 21 of these patients all showed various stages of focal, segmental glomerulosclerosis (FSGS). Thirteen of these biopsies were also studied with EM and showed thinning of the glomerular basement membrane.

RESULTS

Mutation G1334E (COL4A3) was found in six pedigrees, mutation G871C (COL4A3) in four and mutation 3854delG (COL4A4) in one pedigree. Clinical and laboratory correlations in all 127 mutation carriers (MC) showed that microscopic haematuria was the only urinary finding in patients under age 30. The prevalence of 'haematuria alone' fell to 66% between 31 and 50 years, to 30% between 51 and 70 and to 23% over age 71. Proteinuria with CRF developed on top of haematuria in 8% of all MC between 31 and 50 years, to 25% between 51 and 70 years and to 50% over 71 years. Altogether 18 of these 127 MC (14%) developed ESRD at a mean age of 60 years. Two members with different mutations married, and two of their children inherited both mutations and developed adolescent, autosomal recessive Alport syndrome (ATS), confirming that these mutations are pathogenic.

CONCLUSIONS

Our data confirm for the first time a definite association of heterozygous COL4A3/COL4A4 mutations with familial microscopic haematuria, thin basement membrane nephropathy and the late development of familial proteinuria, CRF, and ESRD, due to FSGS, indicating that the term 'benign familial haematuria' is a misnomer, at least in this cohort. A strong hypothesis for a causal relationship between these mutations and FSGS is also made. Benign familial haematuria may not be so benign as commonly thought.

Incidence of thin basement membrane nephropathy in 990 consecutive renal biopsies examined with electron microscopy

Thin basement membrane nephropathy is one of the main causes of hematuria in both children and adults. It is often associated with a family history and its true incidence is unknown. Accurate diagnosis of thin basement membrane nephropathy relies on the presence of attenuated glomerular basement membranes, a finding that can be appreciated only by examination in the electron microscope. In Cyprus the department of electron microscopy has received 990 consecutive renal biopsies for diagnosis. The aim of this study is to define the incidence of thin basement membrane nephropathy in this population sample based on the results of electron microscopy.

Alport syndrome and thin glomerular basement membrane nephropathy: a practical approach to diagnosis

CONTEXT

Alport syndrome and thin glomerular basement membrane nephropathy (TBMN) are genetically heterogeneous conditions characterized by structural abnormalities in the glomerular basement membrane and an initial presentation that usually involves hematuria. Approximately 40% of patients with TBMN are heterozygous carriers for autosomal recessive Alport syndrome, with mutations at the genetic locus encoding type IV collagen alpha(3) [alpha(3)(IV)] and alpha(4) chains. However, although the clinical course of TBMN is usually benign, Alport syndrome, particularly the X-linked form with mutations in the locus encoding the alpha(5) chain of type IV collagen [alpha(5)(IV)], typically results in end-stage renal disease. Electron microscopy is essential to diagnosis of TBMN and Alport syndrome on renal biopsy, although electron microscopy alone is of limited value in distinguishing between TBMN, the heterozygous carrier state of X-linked Alport syndrome, autosomal recessive Alport syndrome, and even early stages of X-linked Alport syndrome.

OBJECTIVES

To review diagnostic pathologic features of each of the above conditions, emphasizing the need for immunohistology for alpha(3)(IV) and alpha(5)(IV) in addition to electron microscopy to resolve this differential diagnosis on a renal biopsy. The diagnostic value of immunofluorescence studies for alpha(5)(IV) on a skin biopsy in family members of patients with Alport syndrome also is reviewed.

DATA SOURCES

Original and comprehensive review articles on the diagnosis of Alport syndrome and TBMN from the past 35 years, primarily the past 2 decades, and experience in our own renal pathology laboratory.

CONCLUSIONS

Although Alport syndrome variants and TBMN do not show characteristic light microscopic findings and can be difficult to differentiate from each other even by electron microscopy, using a combination of electron microscopy and immunohistology for alpha(3)(IV) and alpha(5)(IV) enables pathologists to definitively diagnose these disorders on renal biopsy in most cases.

Autosomal dominant Alport syndrome: molecular analysis of the COL4A4 gene and clinical outcome

BACKGROUND

Alport syndrome is a clinically and genetically heterogeneous nephropathy characterized by glomerular basement membrane lesions often associated with hearing loss and ocular anomalies. While the X-linked and the autosomal recessive forms are well known, the autosomal dominant form is not well acknowledged.

METHODS

We have clinically investigated 38 patients with a diagnosis of autosomal dominant Alport syndrome belonging to eight different families. The analysis of the COL4A4 gene was performed by denaturing high performance liquid chromatography and automated DNA sequencing.

RESULTS

In our cohort of patients, only 24.3% (9/37) reached end-stage renal disease, at the mean age of 51.2 years. Four patients had hearing loss (13.3%) and none ocular changes. Molecular analysis revealed eight novel private COL4A4 gene mutations: three frameshift, three missense and two splice-site mutations.

CONCLUSIONS

These data indicate autosomal dominant Alport syndrome as a disease with a low risk of ocular and hearing anomalies but with a significant risk to develop renal failure although at an older age than the X-linked form. We were unable to demonstrate a genotype-phenotype correlation. Altogether, these data make difficult the differential diagnosis with the benign familial haematuria due to heterozygous mutations of COL4A4 and COL4A3, especially in young patients, and with the X-linked form of Alport syndrome in families where only females are affected. A correct diagnosis and prognosis is based on a comprehensive clinical investigation in as many family members as possible associated with a broadly formal genetic analysis of the pedigree.

COL4A3 founder mutations in Greek-Cypriot families with thin basement membrane nephropathy and focal segmental glomerulosclerosis dating from around 18th century

Mutations in the COL4A3/COL4A4 genes of type IV collagen account for about 40% of cases of thin basement membrane nephropathy, a condition that is estimated to affect 1% or more of the general population. We recently described 10 Cypriot families with familial hematuria and thin basement membrane nephropathy in the presence of focal segmental glomerulosclerosis, with founder mutations on COL4A3 gene. Seven of the families carried mutation G1334E on haplotype K, and another three carried mutation G871C on haplotype Ky. In this report we performed extension of the haplotypes with additional polymorphic markers, 12 for haplotype K and 22 for haplotype Ky, to estimate the linkage disequilibrium value between the mutation and flanking noncommon markers. Haplotype Ky extended to 13.71 Mb, but we did not attempt further analysis owing to the small number of chromosomes. Haplotype K extended to 3.83 Mb, thereby suggesting that it was a much older event compared to mutation G871C. Mutation G1334E was calculated to be about 5-10 generations old with a possible origin between 1693 and 1818 AD, during the Ottoman ruling of the island. Both mutations are clustered in specific geographic regions with apparently formerly isolated populations, although mutation G1334E has been detected elsewhere on the island. The identification of founder mutations in large families with microscopic hematuria greatly facilitates presymptomatic diagnosis and provides useful information on the history of the population, while it may also assist in association studies in search for disease modifier genes.

Alzheimer disease pathology as a host response

Identification of amyloid-beta and tau as the major protein components of senile plaques and neurofibrillary tangles, respectively, led to an exponential increase in investigations of these proteins and their corresponding metabolic pathways in Alzheimer disease (AD). The presumptions inherent in most studies and in the dogma of the amyloid cascade concept are that these hallmark lesions in AD brains contain molecules that drive the disease process, and that the proteinaceous accumulations are themselves toxic. On the other hand, the lesions of AD are, by definition, end-stage, and their relationship to the clinical disease is inconsistent; this has long been known but, generally, has not been acknowledged until relatively recently. Some recent attempts to address the etiology and pathogenesis of AD discard the pathology and focus on the interplay between invisible toxic intermediates, that is, amyloid-beta oligomers and the synapse. The concept that the hallmark lesions may be nontoxic (something we have long suggested) is slowly gaining acceptance. We favor the interpretation that senile plaques and neurofibrillary tangles represent a host response to an upstream pathophysiologic process, and that the therapeutic targeting of lesions, including toxic intermediates, will succeed only in the event that the host response is directly deleterious. Therefore, renewed efforts aimed at elucidating fundamental age-related processes such as oxidative stress and/or inflammatory mediators are warranted.

Novel COL4A3 mutations in African American siblings with autosomal recessive Alport syndrome

We describe a novel mutational study in 2 African American siblings with autosomal recessive Alport syndrome. Both siblings were compound heterozygotes for 2 abnormal DNA sequences in exon 49 of the COL4A3 gene, p.Arg1496X (CGA-->TGA) and p.Arg1516X (CGA-->TGA). These are nonsense mutations in the noncollagenous domain resulting in premature termination codons and have not been previously reported. In an African American population in which autosomal recessive Alport syndrome is rarely seen, complete sequencing of the COL4A3 and COL4A4 genes may be necessary to identify the underlying mutation and confirm the diagnosis.

COL4A3/COL4A4 mutations link familial hematuria and focal segmental glomerulosclerosis. glomerular epithelium destruction via basement membrane thinning?

The recent description of multiple gene defects in hereditary podocytopathies and in hereditary glomerular basement membrane diseases has dramatically improved the current state of our knowledge on the renal glomerular filtration barrier. Recently described mutations in collagen IV and laminin in patients with hematuria and severe nephrotic syndrome add to other experimental data supporting the hypothesis that the glomerular basement membrane (GBM) may also have a significant role in protein filtration, a function previously attributed exclusively to the podocytes. Collagen IV heterozygous mutations were thought to cause only a mild form of renal disease (thin basement membrane nephropathy--TBMN). However, data from our laboratory show that many patients who carry such mutations may later on in life develop focal and segmental glomerulosclerosis, on top of the TBMN and the microscopic hematuria, a situation that frequently progresses to chronic renal failure or even end-stage renal disease. The role of unknown modifier genes may explain the heterogeneity of symptoms in TBMN and other glomerular diseases and in particular the selected development of chronic renal failure. The molecular communication between GBM and podocytes may also be a key factor in the search for these major genetic modifiers while their understanding may improve novel drug design for glomerular diseases.