The clinical features of thin basement membrane nephropathy

Thin basement membrane nephropathy (TBMN) is a common, lifelong condition affecting the kidneys that is characterized by microscopic glomerular hematuria, minimal or no proteinuria, and normal renal function. It often is discovered incidentally, and usually has an excellent prognosis. Many cases are familial and show autosomal-dominant inheritance. The defining characteristic is a glomerular basement membrane (GBM) that is thinned to about half its normal thickness on ultrastructural examination of the renal biopsy specimen. However, occasionally patients with TBMN develop marked proteinuria or renal impairment. It is unclear whether individuals with TBMN and impaired renal function represent part of the spectrum of TBMN associated with heterozygous COL4A3 or COL4A4 mutations, or if their disease is caused by mutations of other genes, or whether it is caused by a second coexistent renal lesion or is misdiagnosed Alport syndrome.     

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.     

Thin basement membrane nephropathy

Thin basement membrane nephropathy. Thin basement membrane nephropathy (TBMN) is the most common cause of persistent glomerular bleeding in children and adults, and occurs in at least 1% of the population. Most affected individuals have, in addition to the hematuria, minimal proteinuria, normal renal function, a uniformly thinned glomerular basement membrane (GBM) and a family history of hematuria. Their clinical course is usually benign. However, some adults with TBMN have proteinuria >500 mg/day or renal impairment. This is more likely in hospital-based series of biopsied patients than in the uninvestigated, but affected, family members. The cause of renal impairment in TBMN is usually not known, but may be due to secondary focal segmental glomerulosclerosis (FSGS) or immunoglobulin A (IgA) glomerulonephritis, to misdiagnosed IgA disease or X-linked Alport syndrome, or because of coincidental disease. About 40% families with TBMN have hematuria that segregates with the COL4A3/COL4A4 locus, and many COL4A3 and COL4A4 mutations have now been described. These genes are also affected in autosomal-recessive Alport syndrome, and at least some cases of TBMN represent the carrier state for this condition. Families with TBMN in whom hematuria does not segregate with the COL4A3/COL4A4 locus can be explained by de novo mutations, incomplete penetrance of hematuria, coincidental hematuria in family members without COL4A3 or COL4A4 mutations, and by a novel gene locus for TBMN. A renal biopsy is warranted in TBMN only if there are atypical features, or if IgA disease or X-linked Alport syndrome cannot be excluded clinically. In IgA disease, there is usually no family history of hematuria. X-linked Alport syndrome is much less common than TBMN and can often be identified in family members by its typical clinical features (including retinopathy), a lamellated GBM without the collagen alpha3(IV), alpha4(IV), and alpha5(IV) chains, and by gene linkage studies or the demonstration of a COL4A5 mutation. Technical difficulties in the demonstration and interpretation of COL4A3 and COL4A4 mutations mean that mutation detection is not used routinely in the diagnosis of TBMN.     

Incidence of renal failure and nephroprotection by RAAS inhibition in heterozygous carriers of X-chromosomal and autosomal recessive Alport mutations

We studied here the clinical course of heterozygous carriers of X-linked Alport syndrome and a subgroup of patients with thin basement membrane disease due to heterozygous autosomal recessive Alport mutations whose prognosis may be worse than formerly thought. We analyzed 234 Alport carriers, including 29 with autosomal recessive mutations. Using Kaplan-Meier estimates and log-rank tests, autosomal and X-linked carriers were found to have similar incidences of renal replacement therapy, proteinuria, and impaired creatinine clearance. Further, age at onset of renal replacement therapy did not differ between X-chromosomal and autosomal carriers. Both groups showed an impaired life expectancy when reaching renal replacement therapy. RAAS inhibition significantly delayed the onset of end-stage renal failure. Not only carriers of X-linked Alport mutations but also heterozygous carriers of autosomal recessive mutations were found to have an increased risk for worse renal function. The risk of end-stage renal disease in both groups affected life expectancy, and this should cause a greater alertness toward patients presenting with what has been wrongly termed 'familial benign hematuria.' Timely therapy can help to delay onset of end-stage renal failure. Thus, yearly follow-up by a nephrologist is advised for X-linked Alport carriers and patients with thin basement membrane nephropathy, microalbuminuria, proteinuria, or hypertension.     

The genetics of thin basement membrane nephropathy

The diagnosis of thin basement membrane nephropathy (TBMN) usually is made on the basis of the clinical features or the glomerular membrane ultrastructural appearance. Only now are we beginning to understand the genetics of TBMN and the role of diagnostic genetic testing. The similarity of clinical and glomerular membrane features first suggested TBMN might represent the carrier state for autosomal-recessive Alport syndrome. This was confirmed subsequently by the demonstration that 40% of families with TBMN have hematuria that segregates with the corresponding locus ( COL4A3/COL4A4 ), and identical mutations occur in both conditions. To date, about 20 COL4A3 and COL4A4 mutations have been shown in TBMN, and these mainly are single nucleotide substitutions that are different in each family. The families in whom hematuria does not appear to segregate with the COL4A3/COL4A4 locus cannot all be explained by de novo mutations, and nonpenetrant or coincidental hematuria. This suggests a further TBMN locus. In patients with persistent hematuria, testing for COL4A3 and COL4A4 mutations to diagnose TBMN is problematic because of the huge size of these genes, their frequent polymorphisms, and the likelihood of a further gene locus. It is far more practicable to perform genetic testing to exclude or confirm X-linked Alport syndrome because this condition is the major differential diagnosis of TBMN and has a very different prognosis.     

The epidemiology of thin basement membrane nephropathy

The prevalence of this basement membrane nephropathy (TBMN) may be approximated from the known frequencies of glomerular hematuria in the population, and from the prevalence of autosomal-recessive Alport syndrome and its known relationship to TBMN. These approaches confirm that TBMN affects more than 1% (but < 10%) of the population, making it the commonest inherited renal disease, and one of the commonest conditions affecting the kidney after infections, hypertension, and stones. TBMN is the most frequent cause of persistent glomerular hematuria. Although we do not advocate mass screening for hematuria to detect TBMN, we strongly support investigating hematuria that is discovered incidentally. Individuals with TBMN and isolated hematuria should be evaluated initially by a nephrologist and subsequently reviewed by their family doctor. Those with proteinuria, hypertension, or renal impairment are at risk for progressive renal impairment and should by examined carefully for features of Alport syndrome or an additional glomerular or tubulointerstitial lesion, undergo a renal biopsy examination, be treated symptomatically, and be monitored by a renal physician.     

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.     

Structure of the human type IV collagen gene COL4A3 and mutations in autosomal Alport syndrome

Mutations in either the COL4A3 or the COL4A4 genes, encoding the alpha3 and alpha4 chains of type IV collagen, are responsible for the autosomal-recessive form of Alport syndrome, a progressive hematuric nephropathy characterized by glomerular basement membrane abnormalities. Reported here are the complete COL4A3 exon-intron structure and a comprehensive screen for mutations of the 52 COL4A3 exons in 41 unrelated patients diagnosed as having autosomal Alport syndrome. This resulted in the identification of 21 mutations that are expected to be causative. Furthermore, it is shown that heterozygous COL4A3 missense mutations, when symptomatic, can be associated with a broad range of phenotypes, from familial benign hematuria to the complete features of Alport syndrome nephropathy.     

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.     

The R229Q mutation in <Emphasis Type="Italic">NPHS2</Emphasis> may predispose to proteinuria in thin-basement-membrane nephropathy

Thin-basement-membrane nephropathy (TBMN) is characterized by persistent dysmorphic hematuria, and the presence of proteinuria is a risk factor for renal impairment. TBMN is often due to mutations in the COL4A3 and COL4A4 genes, and this study determined whether additional mutations in genes encoding other structures in the glomerular filtration barrier contributed to the development of proteinuria. Fifty-six unrelated individuals with TBMN including 18 (32%) with proteinuria ≥ 300 mg/L and ten (18%) with proteinuria ≥ 500 mg/L were studied. Deoxyribonucleic acid (DNA) was screened for NPHS2 mutations and variants (R138Q and P375L) using single-stranded conformational analysis (SSCA) and for the R229Q mutation by sequencing. DNA was also screened for ACTN4 mutations. R229Q was more common in patients with TBMN and proteinuria ≥ 500 mg/L (p < 0.05), and a possible NPHS2 mutation (671G>A, R224H) was identified in one patient with proteinuria 700 mg/L. No other NPHS2 variants correlated with proteinuria, and no ACTN4 mutations were found. Individuals with TBMN and R229Q are carriers of the autosomal recessive forms of both Alport syndrome and familial focal segmental glomerulosclerosis (FSGS). The early demonstration of R229Q in individuals with TBMN may indicate those at increased risk of proteinuria and renal impairment.     

Segregation of hematuria in thin basement membrane disease with haplotypes at the loci for Alport syndrome

BACKGROUND: Inherited hematuria is common and is usually attributed to thin basement membrane disease (TBMD). The aim of this study was to determine how often hematuria in families with TBMD segregated with haplotypes at the chromosomal loci for autosomal recessive and X-linked Alport syndrome (COL4A3/COL4A4 and COL4A5, respectively). METHODS: The families of 22 individuals with TBMD on renal biopsy and with urinary glomerular red blood cell (RBC) counts of more than 50,000/mL were studied using phase-contrast microscopy of the urine and DNA microsatellite markers. Eighteen families had at least two members with hematuria. RESULTS: Hematuria segregated with or was consistent with segregation at the COL4A3/COL4A4 locus in eight (36%) families (P < 0.05 in 5 of these) and at the COL4A5 locus in four (18%) families (P < 0.05 in 2). The lack of segregation in the other 10 (45%) families may have occurred because of incomplete penetrance of the hematuria, de novo mutations, coincidental hematuria in other family members, or the presence of a novel gene locus. In four different families, three of which had hematuria that segregated with the COL4A3/COL4A4 locus, four family members with the hematuria haplotype had spouses with coincidental hematuria (4 of 29, 14%). However, none of their four offspring who had also inherited the hematuria haplotype had the clinical features of autosomal recessive Alport syndrome. CONCLUSIONS: Hematuria in families with TBMD commonly segregates with the COL4A3/COL4A4 locus and thus results from mutations in the same genes as autosomal recessive Alport syndrome. Sometimes TBMD may be confused with the carrier state for X-linked Alport syndrome. However, nearly half of the families in this study had hematuria that did not segregate with the loci for either autosomal recessive or X-linked Alport syndrome.     

COL4A4 mutation in thin basement membrane disease previously described in Alport syndrome

BACKGROUND: Carriers of autosomal-recessive and X-linked Alport syndrome often have a thinned glomerular basement membrane (GBM) and have mutations in the COL4A3/COL4A4 and COL4A5 genes respectively. Recently, we have shown that many individuals with thin basement membrane disease (TBMD) are also from families where hematuria segregates with the COL4A3/COL4A4 locus. This study describes the first COL4A4 mutation in an individual with biopsy-proven TBMD who did not have a family member with autosomal-recessive or X-linked Alport syndrome, inherited renal failure, or deafness. METHODS: The index case and all available family members were examined for dysmorphic hematuria> 50,000/mL using phase contrast microscopy and for segregation of hematuria with the COL4A3/COL4A4 and COL4A5 loci using DNA satellite markers. COL4A4 exons from the index case were then studied using the enzyme mismatch cleavage method, and exons that demonstrated abnormal cleavage products were sequenced. RESULTS: Hematuria in this family segregated with a haplotype at the COL4A3/COL4A4 locus (P = 0.031) but not with haplotypes at the COL4A5 locus. A mutation in COL4A4 that changed C to T resulting in an arginine residue being replaced by a stop codon (R1377X) was demonstrated in exon 44, which encodes part of the alpha 4(IV) collagen sequence close to the junction with the noncollagenous domain. This mutation was present in all five family members with hematuria, but not in the four unaffected family members, 33 unrelated individuals with TBMD, or 22 nonhematuric normals. CONCLUSIONS: R1377X has been described previously in a compound heterozygous form of autosomal-recessive Alport syndrome. Our observation is evidence that TBMD can represent a carrier state for autosomal-recessive Alport syndrome in at least some individuals.     

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

Mutations in the COL4A3/COL4A4 genes of type IV collagen have been found in approximately 40% of cases of thin basement membrane nephropathy, which is characterized by microscopic hematuria and is classically thought to cause proteinuria and chronic renal failure rarely. Here we report our observations of 116 subjects from 13 Cypriot families clinically affected with thin basement membrane nephropathy. These families first came to our attention because they segregated microscopic hematuria, mild proteinuria, and variable degrees of renal impairment, but a dual diagnosis of focal segmental glomerulosclerosis (FSGS) and thin basement membrane nephropathy was made in 20 biopsied cases. Molecular studies identified founder mutations in both COL4A3 and COL4A4 genes in 10 families. None of 82 heterozygous patients had any extrarenal manifestations, supporting the diagnosis of thin basement membrane nephropathy. During follow-up of up to three decades, 31 of these 82 patients (37.8%) developed chronic renal failure and 16 (19.5%) reached end-stage renal disease. Mutations G1334E and G871C were detected in seven and three families, respectively, and were probably introduced by founders. We conclude that these particular COL4A3/COL4A4 mutations either predispose some patients to FSGS and chronic renal failure, or that thin basement membrane nephropathy sometimes coexists with another genetic modifier that is responsible for FSGS and progressive renal failure. The findings presented here do not justify the labelling of thin basement membrane nephropathy as a benign condition with excellent prognosis.     

Nine novel <Emphasis Type="Italic">COL4A3</Emphasis> and <Emphasis Type="Italic">COL4A4</Emphasis> mutations and polymorphisms identified in inherited membrane diseases

Both thin basement membrane nephropathy (TBMN) and autosomal recessive Alport syndrome result from mutations in the COL4A3 and COL4A4 genes, and this study documents further mutations and polymorphisms in these genes. Thirteen unrelated children with TBMN and five individuals with autosomal recessive Alport syndrome were examined for mutations in the 52 exons of COL4A3 and the 47 coding exons of COL4A4 using single-stranded conformation polymorphism (SSCP) analysis. Amplicons producing different electrophoretic patterns were sequenced, and mutations were defined as variants that changed an amino acid but were not present in 50 non-hematuric normals. Three further novel mutations were identified. These were IVS 22-5 T>A in the COL4A3 gene in a consanguineous family with autosomal recessive Alport syndrome, and R1677C and R1682Q in the COL4A4 gene. In addition, six novel polymorphisms (G455G, I462I, G736G and IVS 38-8 G>A in COL4A3, and L658L and A1577A in COL4A4) were demonstrated. Many different COL4A3 and COL4A4 mutations cause TBMN and autosomal recessive Alport syndrome. The identification of polymorphisms in these genes is particularly important to enable diagnostic laboratories to distinguish mutations from uncommon normal variants.     

Thin glomerular basement membrane disease

The term thin glomerular basement membrane disease (TBMD) refers to a condition characterised by thinning of the GBM at electron microscopy examination and, clinically, by isolated hematuria, frequently occurring in other family members, with no extra-renal manifestations. Progression towards chronic renal failure (CRF), although rare, has been reported and blood pressure is high in 30-35% of cases during follow-up. TBMD is generally considered different from Alport syndrome since immunohistological investigation does not show abnormalities of type IV collagen alpha chains in the GBM, as frequently observed in Alport patients; moreover, in familial cases, the disease is transmitted as autosomal dominant trait, rarely observed in Alport syndrome. Genetic studies suggest that TBMD is a heterogeneous disease, but some cases may be related to mutations of COL4A3/COL4A4 genes, thus belonging to the spectrum of type IV collagen diseases. TBMD may arise with other glomerular diseases, most frequently IgA nephropathy, and it remains to be established whether these cases are a casual occurrence or whether a thinner than normal GBM predisposes to immune complex deposition.     

薄基底膜肾病并发局灶节段性肾小球硬化症一家系的COL4A3和COL4A4基因突变分析

目的 探讨薄基底膜肾病(TBMN)合并局灶节段性肾小球硬化症(FSGS)的遗传学机制.方法 对一病理学诊断为TBMN合并FSGS患者及其家系的COL4A3和COL4A4基因突变,应用与COL4A3和COL4A4基因连锁的微卫星标记连锁分析方法进行分析.PCR扩增COIAA3和COL4A4全部98个外显子后,直接测序筛查突变.同时测序排除已为公认的FSGS相关基因NPHS1、NPHS2、WT1、TRPC6、ACTN4、CD2AP突变导致FSGS的可能.结果 微卫星标记连锁分析显示此家系与COL4A3和COL4A4基因连锁.直接测序在此家系中发现疾病患者COL4A4基因1214位的鸟嘌呤突变为腺嘌呤,导致Ⅳ型胶原α4链第405位甘氨酸突变为谷氨酸,并且发现COL4A3基因一多态性IVS1-4C＞T.此多态性随疾病分布,可能与致病相关.未发现FSGS相关基因的突变.结论 此家系是在TBMN的基础上发生FSGS.Ⅳ型胶原α4链突变及随疾病分布的基因多态性是否导致TBMN合并FSGS或使其易感性增加尚待更多家系进一步研究.     

Thin basement membrane nephropathy and renal transplantation

The clinical implications of thin basement membrane nephropathy (TBMN) in renal transplantation must be considered from the perspectives of both the allograft recipient and the donor. Most individuals with TBMN have a benign course, but some develop end-stage renal failure (ESRF) and undergo transplantation. ESRF in patients with TBMN often results from the presence of additional glomerular or interstitial lesions and some of these, such as immunoglobulin (Ig)A disease, may recur in the renal allograft and affect outcome. In addition, individuals with TBMN always must be distinguished from those with glomerular membrane thinning due to Alport syndrome. This is not only to enable appropriate genetic counseling but also to anticipate the possible complication of posttransplant anti-glomerular basement membrane disease. From the perspective of the live renal donor, donation from individuals with TBMN (or carriers of X-linked Alport syndrome with thinned membranes) remains controversial because the risks remain unknown. Any effects of the thinned membranes themselves on allograft function are unclear. Further advances in our understanding of the clinical, pathologic, and molecular features of TBMN should result in improved assessment of potential live donors and help stratify those at risk for renal impairment.     

Persistent familial hematuria in children and the locus for thin basement membrane nephropathy

This study examined how often children with persistent familial hematuria were from families where hematuria segregated with the known genetic locus for the condition known as benign familial hematuria or thin basement membrane nephropathy (TBMN) at COL4A3/COL4A4. Twenty-one unrelated children with persistent familial hematuria as well as their families were studied for segregation of hematuria with haplotypes at the COL4A3/COL4A4 locus for benign familial hematuria and at the COL4A5 locus for X-linked Alport syndrome. Eight families (38%) had hematuria that segregated with COL4A3/COL4A4, and four (19%) had hematuria that segregated with COL4A5. At most, eight of the other nine families could be explained by disease at the COL4A3/COL4A4 locus if de novo mutations, non-penetrant hematuria or coincidental hematuria in unaffected family members was present individually or in combination. This study confirms that persistent familial hematuria is not always linked to COL4A3/COL4A4 (or COL4A5) and suggests the possibility of a further genetic locus for benign familial hematuria. This study also highlights the risk of excluding X-linked Alport syndrome on the basis of the absence of a family history or of kidney failure.     

Alport syndrome or progressive hereditary nephritis with hearing loss

Alport syndrome is an inherited disorder characterized by progressive hematuric nephritis with structural defects of the glomerular basement membrane, and sensorineural deafness. Ocular abnormalities are frequently associated. The incidence is approximatively 1/5000. The renal disease is severe in male patients and should be responsible for 2% of end-stage renal failure. Alport syndrome is heterogeneous at the clinical and genetic levels. It occurs as a consequence of structural abnormalities in type IV collagen, the major constituent of basement membranes. Six genetically distinct chains of type IV collagen have been identified. Mutations in the COL4A5 gene located at Xq22, and encoding the alpha 5(IV) chain are responsible for X-linked Alport syndrome whereas COL4A3 or COL4A4 located "head to head" on chromosome 2 are involved in the rarer autosomal forms of the disease.     

一例女性Fabry病并发薄基底膜肾病及其家系调查

目的 了解具有两种遗传性疾病,即Fabry病并发薄基底膜肾病（TBMN）的临床病理和基因突变特点以及家系患病情况。 方法 总结分析本院收治的1例41岁女性Fabry病并发TBMN患者的临床病理特征和基因突变情况,同时对家系成员进行调查及相关检测。 结果 先证者呈现典型的Fabry病的肾外临床表现,包括皮疹、神经痛、眩晕、耳鸣、肥厚型心肌病等,同时亦有蛋白尿、镜下血尿及高血压等肾脏受累表现;肾活检光镜下病理改变为局灶性节段性肾小球硬化（FSGS）,部分足细胞空泡变性;电镜下肾小球脏层上皮细胞胞质内多数髓磷脂小体形成,肾小球基底膜（GBM）弥漫性变薄,厚度为（216±31） nm。家系调查及基因突变检测显示先证者女儿除有典型Fabry病肾外表现外,亦有以血尿为主的肾脏症状。先证者的1个妹妹仅表现为镜下血尿。先证者及其女儿α-半乳糖苷酶 A（α-Gal A）活性分别为33和75活性单位（正常参考值为100~500活性单位）,且2人均携带新发现的GLA基因突变——1208ins21 bp及COL4A3基因多态性——c:3627 G>A（p:M1209I）。仅表现为镜下血尿的先证者的妹妹仅携带COL4A3基因的c:3627 G>A（p:M1209I）多态性,α-Gal A活性正常,无GLA基因突变。 结论 对于Fabry肾病患者呈现血尿,尤其是表现为家族性血尿时,应考虑并认真排除并发TBMN的可能。