X-linked Alport syndrome in females

Identification of four novel mutations in the gene identified in Chinese patients with X-linked Alport syndrome

Alport syndrome (AS) is an inherited progressive nephropathy caused by mutations in one or two of the type IV collagen novel chains (α3, α4 and α5), which are encoded by COL4A3, COL4A4 and COL4A5, respectively. To date, three genetic forms of AS have been reported, including X-linked AS, autosomal recessive AS, and autosomal dominant AS, and ~80% of patients have X-linked AS caused by mutations in COL4A5. In the present study, four novel and one previously reported COL4A5 mutations were identified using targeted next-generation sequencing in Chinese patients suspected of having AS. The results were confirmed by Sanger sequencing, which revealed two novel missense mutations resulting in the substitution of various glycine residues in a collagenous domain containing Gly-X-Y triplet sequence repeats [c.4198G>C, p.(Gly1400Arg) and c.3428G>T, p.(Gly1143Val)], a previously reported missense mutation [c.3071G>A, p.(Gly1024Glu)], a splice site mutation (c.2146+2T>A) and one frameshift mutation [c.1810delC (p.Thr605Ilefs*13)]. After analyzing the affected family members, it was shown that the identified mutations were associated with severe clinical phenotypes. These results broaden the known spectrum of mutations of the COL4A5 gene associated with AS and may have implications for genetic diagnosis, therapy and genetic counseling of affected families.

De novo mutations in COL4A5 identified by whole exome sequencing in 2 girls with Alport syndrome in Korea

Alport syndrome (ATS) is an inherited glomerular disease caused by mutations in one of the type IV collagen novel chains (α3, α4, and α5). ATS is characterized by persistent microscopic hematuria that starts during infancy, eventually leading to either progressive nephritis or end-stage renal disease. There are 3 known genetic forms of ATS, namely X-linked ATS, autosomal recessive ATS, and autosomal dominant ATS. About 80% of patients with ATS have X-linked ATS, which is caused by mutations in the type IV collagen α5 chain gene, COL4A5. Although an 80% mutation detection rate is observed in men with X-linked ATS, some difficulties do exist in the genetic diagnosis of ATS. Most mutations are point mutations without hotspots in the COL4A3, COL4A4, and COL4A5 genes. Further, there are insufficient data on the detection of COL4A3 and COL4A4 mutations for their comparison between patients with autosomal recessive or dominant ATS. Therefore, diagnosis of ATS in female patients with no apparent family history can be challenging. Therefore, in this study, we used whole-exome sequencing (WES) to identify mutations in type IV collagen in 2 girls with glomerular basement membrane structural changes suspected to be associated with ATS; these patients had no relevant family history. Our results revealed de novo c.4688G>A (p.Arg1563Gln) and c.2714G>A (p.Gly905Asp) mutations in COL4A5. Therefore, we suggest that WES is an effective approach to obtain genetic information in ATS, particularly in female patients without a relevant family history, to detect unexpected DNA variations.

The variable course of women with X-linked Alport Syndrome

X-linked Alport syndrome (XLAS) arises from mutations in the COL4A5 gene encoding the α5-chain of type IV collagen and is associated with hematuria, ocular abnormalities and high-tone sensorineural hearing loss. Nearly all affected males have decreased kidney function resulting in end-stage renal disease (ESRD) as early as the second decade of life. It was long thought that affected females had a benign outcome; however, in recent decades, it has become quite clear that they too are at risk for developing nephrotic syndrome, decreased kidney function and ESRD. We report two young females presenting with microscopic hematuria and proteinuria diagnosed with XLAS on renal biopsy. Both developed nephrotic-range proteinuria and progressive renal insufficiency. Additionally, both developed extra-renal manifestations of XLAS. The ultrastructural and immunofluorescence features on kidney biopsy were instrumental in making the diagnosis of heterozygous XLAS as neither patient had a family history of AS.

Expert guidelines for the management of Alport syndrome and thin basement membrane nephropathy

Few prospective, randomized controlled clinical trials address the diagnosis and management of patients with Alport syndrome or thin basement membrane nephropathy. Adult and pediatric nephrologists and geneticists from four continents whose clinical practice focuses on these conditions have developed the following guidelines. The 18 recommendations are based on Level D (Expert opinion without explicit critical appraisal, or based on physiology, bench research, or first principles-National Health Service category) or Level III (Opinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees-U.S. Preventive Services Task Force) evidence. The recommendations include the use of genetic testing as the gold standard for the diagnosis of Alport syndrome and the demonstration of its mode of inheritance; the need to identify and follow all affected members of a family with X-linked Alport syndrome, including most mothers of affected males; the treatment of males with X-linked Alport syndrome and individuals with autosomal recessive disease with renin-angiotensin system blockade, possibly even before the onset of proteinuria; discouraging the affected mothers of males with X-linked Alport syndrome from renal donation because of their own risk of kidney failure; and consideration of genetic testing to exclude X-linked Alport syndrome in some individuals with thin basement membrane nephropathy. The authors recognize that as evidence emerges, including data from patient registries, these guidelines will evolve further.

Basement membranes in development and disease

Basement membranes (BMs) are specializations of the extracellular matrix that act as key mediators of development and disease. Their sheet like protein matrices typically serve to separate epithelial or endothelial cell layers from underlying mesenchymal tissues, providing both a biophysical support to overlying tissue as well as a hub to promote and regulate cell-cell and cell-protein interactions. In the latter context, the BM is increasingly being recognized as a mediator of growth factor interactions during development. In this review, we discuss recent findings regarding the structure of the BM and its roles in mediating the normal development of the embryo, and we examine congenital diseases affecting the BM which impact embryonic development and health in later life.

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.

Familial hematuria

Hematuria is a common presenting complaint in pediatric nephrology clinics and often has a familial basis. This teaching article provides an overview of causes, diagnosis, and management of the major forms of familial hematuria, Alport syndrome, and thin basement membrane nephropathy.

Renal transplantation in patients with Alport syndrome

For pediatric kidney transplant physicians, two aspects of Alport syndrome set the disease apart from other causes of terminal renal failure. First, an understanding of the genetics of Alport syndrome is needed to make appropriate decisions regarding potential related kidney donors to Alport patients requiring renal transplantation. Second, renal transplantation for Alport syndrome may be complicated by post-transplant anti-GBM nephritis, a problem that is nearly unique to this disease. This review discusses these aspects of Alport syndrome and attempts to provide rational recommendations for clinicians.

Hereditary nephritis mimicking immune complex-mediated glomerulonephritis

The defining ultrastructural features of hereditary nephritis are "basket weave" lamellation or thinning of glomerular basement membranes. Electron-dense deposits are not seen and immunofluorescence (IF) is generally negative. In this study, we report 5 cases of hereditary nephritis in which substantial amounts of glomerular electron-dense deposits were identified on electron microscopy, with corresponding positive IF staining in 4 cases, suggesting immune complex-mediated glomerulonephritis. However, no case had histological evidence of glomerular endocapillary or extracapillary proliferation or leukocyte infiltration typical of active glomerulonephritis. Four cases were diagnosed at outside institutions simply as forms of glomerulonephritis without considering the possibility of hereditary nephritis and were sent for consultation in contemplation of possible immunosuppressive therapy. All patients had negative serologies and no known underlying infectious or autoimmune disease; 4 patients had family history of hematuria or renal disease. The glomerular electron-dense deposits were predominantly mesangial (4 cases) and intramembranous (4 cases), as well as subepithelial (2 cases) or subendothelial (1 case). Corresponding IF positivity for immune reactants was identified in 4 cases, and IgG was the predominant immunoglobulin deposited. A characteristic feature was the tendency for deposits to form between the complex layers of glomerular basement membrane material, favoring a process of nonspecific entrapment of immune reactants within the thickened, lamellated basement membrane. In all cases, a diagnosis of hereditary nephritis was confirmed by demonstration of the characteristic loss of immunoreactivity for the alpha5 subunit of collagen IV (4 cases) or Goodpasture's antigen (1 case) in renal or epidermal basement membranes. These cases expand the spectrum of unusual pathological findings in hereditary nephritis and emphasize the potential for hereditary nephritis to mimic immune complex glomerulonephritis.

Thin glomerular basement membrane nephropathy: incidence in 3471 consecutive renal biopsies examined by electron microscopy

CONTEXT

Thin glomerular basement membrane (GBM) nephropathy is often equated with benign familial hematuria, although it may occur sporadically and may not always be benign. Thin GBM nephropathy is reported to occur in at least 1% of the population, although its reported incidence varies considerably in different studies because there are presently no uniform criteria for its diagnosis by electron microscopy (EM).

OBJECTIVE

To determine the incidence of thin GBM nephropathy in a large sample of renal biopsies using a basic methodology that can easily be applied in any diagnostic EM laboratory.

DESIGN

Direct measurements of GBM thickness were made from electron micrographs at 3 specified points along each of 10 randomly selected glomerular capillaries to determine an average GBM thickness for each of 50 males and 50 females, ages 9 to 80 years, with minimal-change nephropathy or acute interstitial nephritis, without hematuria. The means of the average GBM thickness values were 330 +/- 50 (SD) nm in the males and 305 +/- 45 nm in the females; normal ranges for each sex were defined as being within 2 SD of these means. The average GBM thickness was then similarly determined for renal biopsies with suspected thin GBMs examined from January 2000 to December 2004; a total of 3471 renal biopsies were examined by EM during this period.

SETTING

Academic medical center renal pathology/EM laboratory.

RESULTS

Excluding biopsies with immunoglobulin A nephropathy, which is known to be frequently associated with thin GBMs, and biopsies with Alport syndrome, 67 biopsies (1.9% of total) had an average GBM thickness below the sex-specific normal range. Of these, 37 biopsies were performed specifically because of hematuria and had an average GBM thickness of 121 to 215 nm (mean, 185 +/- 20 nm). The remaining 30 (0.9%) biopsies, with average GBM thicknesses of 143 to 227 nm (mean, 190 +/- 20 nm), represent cases of incidentally discovered thin GBM nephropathy.

CONCLUSIONS

Based on the frequency of incidentally discovered cases and taking into account excluded cases and biopsies (eg, with diabetic nephropathy) in which diagnosis of incidental thin GBM nephropathy by EM is not possible, the incidence of thin GBM nephropathy in our population is estimated to be between 1% and 2%. Diseases most often associated with incidental thin GBM nephropathy were focal segmental glomerulosclerosis (10 cases) and minimal-change nephropathy (5 cases).

The Alport nephropathy: clinicopathological correlations

The alleged dominance of diffuse attenuation of the glomerular basement membrane (GBM) in young children and females with Alport's Syndrome (AS) suggests that it might be the initial ultrastructural manifestation of type IV collagen defects. We carried out a 'blind' review of 130 renal biopsies obtained from 100 patients with AS, emphasizing the electron microscopy changes, and related the findings to the clinical presentation and outcome. The intracapillary distribution of (1) thickened, (2) attenuated and (3) normal GBM was assessed individually as: none (grade 0), <25% (grade 1), 25-50% (grade 2) and >50% (grade 3). Deafness was defined as persistent loss of > or =30 dBs. Proteinuria was measured as protein/creatinine ratios in early morning urine. Heavy proteinuria (> or =200 mg/mmol) correlated significantly with the presence of segmental and global glomerulosclerosis and foam cells. Comparing grades 0+1 vs. 3 GBM changes, using a 2x2 chi(2) test, there were significant correlations between grade 3 GBM thickening and male sex (P =0.005), heavy proteinuria (P =0.02) and deafness (P <0.001). GBM thickening did not correlate with age at the initial biopsy, but repeat biopsies demonstrated increasing thickening with age. The grades of GBM attenuation did not correlate with either age at biopsy or sex. In 11 biopsies with atypical lamina densa changes in thickened GBM segments, there were no differences in clinicopathological correlations compared with classical biopsies. Our data indicate that diffuse GBM attenuation can be an ultrastructural variant of the Alport nephropathy, but do not support the contention that it is the initial lesion.

Histopathology, ultrastructure, and clinical phenotypes in thin glomerular basement membrane disease variants

Recent genetic studies indicate that Alport syndrome and thin glomerular basement membrane disease (TMD) may both be due to COL4A3, COL4A4, and COL4A5 mutations, but there is continuing uncertainty concerning the diagnosis and management of patients without classic family history and symptoms. We examined kidney pathology and collagen alpha 3 to alpha 5(IV) expression in a series of 16 patients who presented with overlapping signs between TMD and Alport nephritis. All patients presented with hematuria, and 11 also had proteinuria, of whom 5 had nephrotic range proteinuria. Only 9 had family history of hematuria. In 9 of 16 (60%) we found premature glomerulosclerosis in the renal biopsies. Three of 16 had predominantly wide, lamellated glomerullar basement membranes (GBM), and in these, alpha 3 to alpha 5(IV) was absent in glomeruli or skin, diagnostic of Alport nephritis. One patient (12) had a very wide GBM with intramembranous lucencies but no lamellation. Skin biopsy was collagen alpha 5(IV) positive. Nine of 16 patients had predominantly thin GBM by electron microscopy, and 3 had thin and slightly lamellated GBM. Collagen alpha 3 to alpha 5(IV) expression in the kidney or skin biopsy was present in all of the latter 12 patients. Three patients had end-stage renal disease, 7 patients had hypertension, and 1 patient had chronic renal failure. We found that of the 16 patients with presumed TMD, 3 had X-linked Alport nephritis, 2 appeared to have autosomal recessive Alport nephritis, and the remaining patients had either an Alport or a TMD variant. The latter had histologic and/or clinical evidence of progressive renal disease, including premature glomerulosclerosis, hypertension, sustained proteinuria, and either thin or slight GBM lamellation focally, and preserved alpha 3 to alpha 5(IV) expression. These patients have a TMD variant, but an Alport variant with a potentially transmissible severe defect different from benign hematuria cannot be excluded.

A comparison of the clinical, histopathologic, and ultrastructural phenotypes in carriers of X-linked and autosomal recessive Alport's syndrome

Previous series that described phenotypes in carriers of Alport's syndrome did not distinguish genetically between carriers of X-linked and autosomal recessive disease. In this study, modes of inheritance in unselected families with Alport's syndrome associated with two city and two provincial hospitals were determined using microsatellite markers, and carriers of disease haplotypes were identified within these families. All 47 carriers (100%) from 18 families with X-linked Alport's syndrome had dysmorphic hematuria on phase-contrast microscopy, but few developed renal failure (3 of 40 carriers; 8%), clinical hearing loss (2 of 45 carriers; 4%), retinopathy (1 of 30 carriers; 3%), or lenticonus (0 of 30 carriers; 0%). Eleven of the 14 carriers (79%) from 2 families with autosomal recessive disease had dysmorphic hematuria, but none had renal failure, clinical hearing loss, retinopathy, or lenticonus. Urinary red blood cell counts in carriers of X-linked Alport's syndrome were greater than those in carriers of autosomal recessive disease (P < 0.0001), but the frequency of proteinuria and hypertension and levels of proteinuria were not different. There was more tubulointerstitial damage in carriers of X-linked disease (P = 0.012); however, carriers of autosomal recessive disease had more widespread and more uniform thinning of the glomerular basement membrane (P < 0.0001) and less lamellation (P < 0.04).

Approach to the diagnosis of thin basement membrane nephropathy in females with the use of antibodies to type IV collagen

CONTEXT

Thin basement membrane nephropathy is recognized by a diffusely thin glomerular basement membrane (GBM) ultrastructurally. In contrast to Alport syndrome (AS), there is no GBM thickening, lamellation, or granular inclusions. Morphologically, there is overlap between thin basement membrane nephropathy and AS in female patients in whom there might be only thin GBM and no pathognomonic findings of AS.

OBJECTIVE

To determine if the use of antibodies to collagen IV is helpful in making the distinction between thin basement membrane nephropathy and AS in female patients with primarily thin GBMs.

DESIGN

We examined renal biopsies from 9 adult female patients with thin GBMs for the presence of alpha1, alpha3, alpha4, and alpha5 chains of type IV collagen by immunofluorescence.

RESULTS

In 2 patients with segmental GBM staining, no suggestion for AS was found on physical examination or in their family history. In the remaining 7 patients with normal GBM staining, 4 had family members with end-stage renal disease of unknown etiology, raising the suspicion of X-linked or autosomal-recessive AS. Three patients were presumed to have thin basement membrane nephropathy.

CONCLUSION

Segmental GBM staining for alpha3, alpha4, and alpha5 chains of type IV collagen raises the suspicion of AS in the presence of adequate controls and other supporting evidence. Normal GBM staining for alpha3, alpha4, and alpha5 chains of type IV collagen, however, does not exclude AS.

Alport syndrome. An inherited disorder of renal, ocular, and cochlear basement membranes

Alport syndrome (AS) is a genetically heterogeneous disease arising from mutations in genes coding for basement membrane type IV collagen. About 80% of AS is X-linked, due to mutations in COL4A5, the gene encoding the alpha 5 chain of type IV collagen (alpha 5[IV]). A subtype of X-linked Alport syndrome (XLAS) in which diffuse leiomyomatosis is an associated feature reflects deletion mutations involving the adjacent COL4A5 and COL4A6 genes. Most other patients have autosomal recessive Alport syndrome (ARAS) due to mutations in COL4A3 or COL4A4, which encode the alpha 3(IV) and alpha 4(IV) chains, respectively. Autosomal dominant AS has been mapped to chromosome 2 in the region of COL4A3 and COL4A4. The features of AS reflect derangements of basement membrane structure and function resulting from changes in type IV collagen expression. The primary pathologic event appears to be the loss from basement membranes of a type IV collagen network composed of alpha 3, alpha 4, and alpha 5(IV) chains. While this network is not critical for normal glomerulogenesis, its absence appears to provoke the overexpression of other extracellular matrix proteins, such as the alpha 1 and alpha 2(IV) chains, in glomerular basement membranes, leading to glomerulosclerosis. The diagnosis of AS still relies heavily on histologic studies, although routine application of molecular genetic diagnosis will probably be available in the future. Absence of epidermal basement membrane expression of alpha 5(IV) is diagnostic of XLAS, so in some cases kidney biopsy may not be necessary for diagnosis. Analysis of renal expression of alpha 3(IV)-alpha 5(IV) chains may be a useful adjunct to routine renal biopsy studies, especially when ultrastructural changes in the GBM are ambiguous. There are no specific therapies for AS. Spontaneous and engineered animal models are being used to study genetic and pharmacologic therapies. Renal transplantation for AS is usually very successful. Occasional patients develop anti-GBM nephritis of the allograft, almost always resulting in graft loss.