Localization of the gene for X-linked Alport's syndrome

Telmisartan/hydrochlorothiazide: a new fixed dose combination

Many patients with hypertension require two or more antihypertensive drugs with complementary mechanisms of action to lower their blood pressure and attain the therapeutic goals specified in internationally accepted guidelines. Yet, these latter guidelines offer the choice of fixed dose combinations as possible first-step therapies. The angiotensin II type 1-receptor antagonist telmisartan and the diuretic hydrochlorothiazide are two antihypertensive agents that have a well-recognized clinical efficacy. Their combination was shown in randomized, controlled trials to be more effective than each agent alone in lowering blood pressure, due to a dual and synergistic mechanism. Indeed, combining telmisartan with hydrochlorothiazide enhances the antihypertensive efficacy of telmisartan in almost two-thirds of hypertensive patients with mild-to-moderate hypertension. The pharmacokinetics and -dynamics of the combination are similar to monotherapy, and the addition of hydrochlorothiazide to telmisartan does not modify the excellent tolerability profile of the drug.

Very-low-dose combination: a first-line choice for the treatment of hypertension?

Essential hypertension is a very heterogeneous disease and different pressor mechanisms might interact to increase blood pressure. It is therefore not surprising that antihypertensive drugs given as monotherapies normalize blood pressure in only a proportion of hypertensive patients. This is, for instance, the case for diuretics, angiotensin-converting enzyme (ACE) inhibitors and angiotensin II type 1 (AT1) receptor antagonists administered as single agents. The rationale for combining antihypertensive agents relates in part to the concept that the blood pressure-decreasing effect may be enhanced when two classes are coadministered. Also, combination treatment serves to counteract the counter-regulatory mechanisms that are triggered whenever pharmacologic intervention is initiated and act to limit the efficacy of the antihypertensive medication. For example, the compensatory increase in renin secretion induced by sodium depletion may become the predominant factor sustaining high blood pressure. Simultaneous blockade of the renin-angiotensin system, with either an ACE inhibitor or an AT1 receptor blocker, makes this compensatory hyper-reninaemia ineffective and allows maximum benefit from sodium depletion. The increased effectiveness obtained by combining a blocker of the renin-angiotensin system with a low dose of a diuretic is not obtained at the expense of reduced tolerability compared with the individual components administered alone. Fixed very-low-dose combinations containing an ACE inhibitor or an AT1 receptor blocker and a diuretic are therefore likely to become increasingly used, not only as second-line therapy, but also as first-line treatment. This is the case, for instance, for the fixed very-low-dose combination of the ACE inhibitor perindopril (2 mg) and the diuretic indapamide (0.625 mg), as this preparation is very effective in decreasing blood pressure while maintaining a tolerability that is similar to that of placebo.

The clinical spectrum of type IV collagen mutations

Clinical manifestations of type IV collagen mutations can vary from the severe, clinically and genetically heterogeneous renal disorder, Alport syndrome, to autosomal dominant familial benign hematuria. The predominant form of Alport syndrome is X-linked; more than 160 different mutations have yet been identified in the type IV collagen alpha 5 chain (COL4A5) gene, located at Xq22-24 head to head to the COL4A6 gene. The autosomal recessive form of Alport syndrome is caused by mutations in the COL4A3 and COL4A4 genes, located at 2q35-37. Recently, the first mutation in the COL4A4 gene was identified in familial benign hematuria. This paper presents an overview of type IV collagen mutations, including eight novel COL4A5 mutations from our own group in patients with Alport syndrome. The spectrum of mutations is broad and provides insight into the clinical heterogeneity of Alport syndrome with respect to age at renal failure and accompanying features such as deafness, leiomyomatosis, and anti-GBM nephritis.

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.

Identification of COL4A5 defects in Alport's syndrome by immunohistochemistry of skin

BACKGROUND

The COL4A3-COL4A4-COL4A5 network in the glomerular basement membrane is affected in the inherited renal disorder Alport's syndrome (AS). Approximately 85% of the AS patients are expected to carry a mutation in the X-chromosomal COL4A5 gene and 15% in the autosomal COL4A3 and COL4A4 genes. The COL4A5 chain is also present in the epidermal basement membrane (EBM). It is predicted that approximately 70% of the COL4A5 mutations prevent incorporation of this chain in basement membranes.

METHODS

We investigated whether or not COL4A5 defects could be detected by immunohistochemical analysis of the EBM. Punch skin biopsies were obtained from 22 patients out of 17 families and two biopsy specimens from healthy males were used as controls.

RESULTS

In four cases with the COL4A5 frameshift or missense mutations, the COL4A5 chain was either lacking from the EBM (male) or showed a focally negative pattern (female). In three other patients with a COL4A5 missense mutation, a COL4A3 and a COL4A4 mutation, respectively, the COL4A5 staining was normal. A (focally) negative EBM-COL4A5 staining was found in three patients of six families with a diagnosis of AS and in one family of a group of four families with possible AS.

CONCLUSIONS

The (focal) absence of COL4A5 in the EBM of skin biopsy specimens can be used for fast identification of COL4A5 defects. Combined with polymorphic COL4A5 markers, both postnatal and prenatal DNA diagnosis are possible in the family of the patient.

Ocular manifestations of autosomal recessive Alport syndrome

Ocular abnormalities are common in X-linked Alport syndrome, but they have not been studied in patients with the rarer autosomal recessive disease. We have examined the eyes of a family with autosomal recessive Alport syndrome. Four of the eight offspring of a consanguineous marriage had renal failure and deafness by the age of 20 years. The diagnosis of Alport syndrome was confirmed on the ultrastructural demonstration of a lamellated glomerular basement membrane (GBM) in one affected family member. Autosomal recessive inheritance was suggested by the lack of linkage to the COL4A5/COL4A6 locus, and by linkage to the COL4A3/COL4A4 locus. All four affected family members had anterior lenticonus (or had had a lens replacement for this) and the three who were examined had a dot-and-fleck retinopathy. Neither of the two unaffected offspring who were examined nor the father had these abnormalities. The ocular manifestations of autosomal recessive Alport syndrome are probably identical to those for the X-linked form. Although the mutations in these diseases affect genes for different type IV collagen chains, these chains occur together in the basement membranes of the kidney, eye and ear, and abnormalities in any one may result in the same clinical phenotype.

Ultrastructural immunocytochemistry of collagenous and non-collagenous proteins in fast-frozen, freeze-substituted, and low-temperature-embedded renal tissue in Alport syndrome

This paper describes the ultrastructural immunolocalization of the alpha 2 chain of collagen IV, laminin, and the amino terminal propeptide of collagen I (N-Pro I) in glomeruli of rapidly frozen, freeze-substituted, and low-temperature-embedded renal biopsy specimens from two cases of Alport disease and from normal kidneys. The alpha 2 chain of collagen IV is present in the whole thickness of the basement membrane in glomeruli of Alport patients, while it is limited to the subendothelial portion of the basement membrane of normal glomeruli. Laminin has the same distribution in both normal and Alport glomeruli, but is apparently more concentrated along the basement membrane of normal glomeruli. N-Pro I is localized in mesangial areas and in the basement membrane in Alport cases, while it is not detected in normal glomeruli. These data suggest complex rearrangements of major constituents of the glomerular basement membrane network and demonstrate early deposition of fibrillary collagen proteins in the matrix before the appearance of banded collagen fibres. This finding could be an indicator of early evolution towards glomerulosclerosis.

Alport's syndrome

Alport's syndrome (AS) is a progressive glomerulonephritis which is associated with high tone sensorineural deafness and characteristic eye signs. It accounts for 0.6% of all patients who start renal replacement therapy in Europe, and is most commonly inherited as an X linked disorder with a gene frequency of 1 in 5000. During the last six years several type IV collagen genes have been implicated in the aetiology of AS, and mutation detection studies are enabling genotype/phenotype correlations to be made, as well as facilitating carrier detection and prenatal diagnosis.

Alport's syndrome in 78 patients: epidemiological and clinical study

In a nationwide study in Finland, 78 patients, 38 male and 40 female in 25 families, were found to have Alport's syndrome, corresponding to 1 in 53,000 live births. This frequency of clinically manifest Alport's syndrome was much lower than expected from earlier reports. This first sign of the disease was most often haematuria, but was sometimes proteinuria or hearing loss. These signs were detected at a similar median age in both boys and girls, namely 6.2 and 6.0 years, respectively. The patients were followed up over a median period of 12.1 years (range 0.1 - 34.0 years). The clinical course of the disease was more severe in the male subjects than in the female subjects: 53% of the males and 13% of the females developed terminal renal failure at median ages of 24.9 and 31.1 years, respectively. At the last observation, 34% males and 78% females were free of renal insufficiency at median ages of 10.3 and 26.8 years. Hearing loss was detected in 74% of the males and 5% of the females. Regarding the rate of deterioration of renal function, no statistically significant difference was noticed between males and females. The routine use of dialysis and transplantations has dramatically changed the life expectancy of the patients.

Relationship between COL4A5 gene mutation and distribution of type IV collagen in male X-linked Alport syndrome. Japanese Alport Network

The renal immunohistochemical distribution of collagen IV chains was studied with a monoclonal antibody series recognizing the alpha 1(IV) to alpha 6(IV) chains in nine males with X-linked Alport syndrome whose COL4A5 mutation had been already identified. Two patients had a deletional mutation, six patients had a missense mutation and one patient had a splicing site mutation. The alpha 3(IV) to alpha 6(IV) chains were completely absent in the renal basement membrane of the two patients with a deletional mutation. On the contrary, in four of six patients with a missense mutation (substitution of a glycine within collagenous domain), antigenecity of the alpha 3(IV) to alpha 5(IV) chains was recognized in the glomerular basement membrane although it was weak. In addition, one of the remaining patients showed a normal histochemical pattern of all type IV collagen chains, while the rest one showed completely absent of the alpha 3(IV) to alpha 5(IV) chains at the same pattern of deletional mutation. One patient with a splice site mutation showed complete absence of the alpha 3(IV) to alpha 5(IV) chains from the glomerular basement membrane, but weak staining of the alpha 5(IV) and alpha 6(IV) chains from the Bowman's capsular basement membrane. Our observations indicated that there is variety in the staining of the alpha 3(IV) to alpha 6(IV) antibodies among male patients with COL4A5, mutations.

Clinical findings in obligate carriers of type I Usher syndrome

Seventeen obligate carriers from nine families with autosomal recessive Usher syndrome type I underwent otological, audiological, vestibular, and ophthalmological examination in order to identify possible manifestations of heterozygosity. Linkage studies were performed and six families showed linkage to chromosome region 11q13.5 while 3 families have so far failed to show linkage to the candidate regions. Eight obligate carriers had an abnormal pure-tone audiogram. Two different audiometric patterns could be distinguished when hearing loss was corrected for age and sex. Four carriers (24%) had significant sensorineural hearing loss (SNHL) which increased at higher frequencies. The other 13 carriers had SNHL of about 10 dB at 0.25 and 0.5 kHz, but less at higher frequencies. Vestibular findings were generally normal. Electro-oculography demonstrated a significant lower mean light peak/dark trough ratio in Usher type I carriers compared to normal control individuals. The methods used in this study were found not to be specific enough to clinically identify carriers of Usher type I syndrome. Nevertheless it is remarkable that a number of obligate carriers showed significant audiological and ophthalmological abnormalities.

A nonsense mutation in the COL4A5 collagen gene in a family with X-linked juvenile Alport syndrome

The X-linked form of Alport syndrome is associated with mutations in the COL4A5 gene encoding the alpha 5-chain of type IV collagen. By using PCR-amplification and direct sequencing we identified a novel mutation involving a deletion of the last two bases in the codon GGA for Glycine-1479 in exon 47 of the COL4A5 gene in a patient with a juvenile form of X-linked Alport syndrome with deafness. This two base deletion caused a shift in the reading frame and introduced a premature stop codon which resulted in an alpha 5(IV)-chain shortened by 202 residues and lacking almost the entire NC1 domain. The mutation was found to co-segregate with the disease in the family. The information of the sequence variation in this family was used to perform carrier detection and prenatal diagnosis by allele-specific oligonucleotide hybridization analysis and direct sequencing of PCR amplified exon 47. Prenatal diagnosis on chorionic villi tissue, obtained from one of the female carriers in the family, revealed a male fetus hemizygous for the mutated allele. A subsequent prenatal test in her next pregnancy revealed a normal male fetus. Prenatal diagnosis of Alport syndrome has not previously been reported.

Canine X chromosome-linked hereditary nephritis: a genetic model for human X-linked hereditary nephritis resulting from a single base mutation in the gene encoding the alpha 5 chain of collagen type IV

Many families with X-chromosome linked hereditary nephritis (HN) have mutations in the gene on the X chromosome that codes for the alpha 5 chain of collagen type IV. Canine X-linked HN is an animal model for human X-linked HN. To study the alpha 5(IV) gene in this model, we used the nucleotide sequence published for the human alpha 5(IV) cDNA to construct sets of primers covering approximately 95% of the complete cDNA. cDNA from both affected and normal dog kidneys was amplified by PCR in nine overlapping regions. The nucleotide sequence encoding the noncollagenous domain NC1 hybridized to the human X chromosome and was 93% identical at the DNA level and 97% identical at the protein level to the human alpha 5(IV) NC1 domain, confirming that the canine alpha 5(IV) cDNA had been amplified. Sequence analysis of the alpha 5(IV) cDNA detected a single nucleotide substitution, G-->T, in affected dogs, changing a codon for a conserved glycine residue (GGA) to a stop codon (TGA). When genomic DNA was amplified, the same abnormality was found in exon 35. Using the canine NC1 domain cDNA as a probe for Northern analysis, two transcripts of approximately 8.6 kb and approximately 6.7 kb were identified in both normal and affected male dog kidney RNA. However, the abundance of both transcripts was decreased by a factor of approximately 10 in the affected dog. These results establish at the molecular level that canine X-linked HN is a model for human X-linked HN. This model provides an opportunity to determine the efficacy of new therapies and to investigate the role of the alpha 5(IV) chain in type IV collagen assembly.

Inherited diseases of the glomerular basement membrane

The inherited diseases of the glomerular basement membrane include Alport's syndrome (AS), nail-patella syndrome, and thin basement membrane nephropathy. Classical AS is inherited in an X-linked manner and accounts for approximately 85% of the cases. Its manifestations include hematuria, sensorineural hearing loss, ocular defects, and a progression to renal failure. A defect(s) in the alpha 5 (IV) chain of type IV collagen is believed to be the etiology of classic AS, and alterations in its encoding gene localized to the X-chromosome have been elucidated. Although isolated cases of anti-glomerular basement membrane glomerulonephritis have been reported following renal transplantation in patients with AS, it is considered an effective form of renal replacement therapy. Less is known regarding the genetic basis of the autosomal-dominant form of AS, which apparently accounts for the remaining 15% of the cases. Nail-patella syndrome is characterized by nail dysplasia, patellar hypoplasia or aplasia, and nephropathy. It is inherited in an autosomal-dominant fashion with the gene locus assigned to the long arm of chromosome 9. Possible linkage between the COL5A1 gene and the gene for nail-patella syndrome has been suggested. Approximately 30% of the patients progress to end-stage renal failure. Renal transplantation has been successful in treating patients who progress to end-stage renal failure. Thin basement membrane nephropathy is an autosomal dominant trait that accounts for approximately 30% of the cases presenting as persistent, asymptomatic hematuria. The cause of thin basement membrane nephropathy is unknown at present. No decline in renal function is associated with thin basement membrane nephropathy.

COL4A5 gene deletion and production of post-transplant anti-alpha 3(IV) collagen alloantibodies in Alport syndrome

Mutations in the COL4A5 gene encoding the alpha 5(IV) chain of type IV collagen have been implicated as the primary defect in X-linked Alport syndrome. Several kinds of mutations have been reported so far, spanning point mutations to complete gene deletions. About 5% of Alport patients, who undergo renal transplantation, develop anti-glomerular basement membrane (GBM) nephritis, causing loss of allograft function. In one such patient, COL4A5 gene deletion was recently identified. In the present study, the GBM constituent, targeted by the anti-GBM alloantibodies from the patient who had complete COL4A5 gene deletion was identified. Its identity was determined on the basis of circulating antibody binding to various GBM constituents, domains of bovine type IV collagen and recombinant NC1 domain of human type IV collagen. These results establish, for the first time, the absence of the alpha 5(IV) chain in Alport GBM and, in the same patient, the production of an alloantibody that is targeted to a different chain of type IV collagen, the alpha 3(IV) chain. These findings provide further support for the hypothesis that: (1) anti-alpha 3(IV) collagen alloantibodies mediate the allograft glomerulonephritis; and (2) COL4A5 gene mutations cause defective assembly of the alpha 3(IV) collagen alloantibodies mediate the allograft glomerulonephritis; and (2) COL4A5 gene mutations cause defective assembly of the alpha 3(IV) chain in Alport GBM, as reflected by the production of anti-alpha 3(IV) alloantibodies.

Differential splicing of COL4A5 mRNA in kidney and white blood cells: a complex mutation in the COL4A5 gene of an Alport patient deletes the NC1 domain

PCR conditions were optimized to amplify the COL4A5 cDNA from lymphoblasts and kidney tissue. Sequencing of the COL4A5 mRNA isolated from the kidney of an Alport syndrome patient revealed two differences with the published sequence. One divergence, the insertion of an 18 bp sequence between exon 11 and 10 of the COL4A5 mRNA added two Gly-X-Y triplets to the COL4A5 sequence and was subsequently found in the mRNA of four normal kidney mRNA samples. This sequence was absent in all white blood cell RNA samples sequenced by us, indicating tissue specific splicing with the presence of an additional exon in kidney COL4A5 mRNA. This finding of differential splicing of COL4A5 mRNA in kidney and white blood cells might affect the use of white blood cell mRNA for the analysis of Alport mutations. Second, a complex mutation was detected in the mRNA from the AS patient introducing a premature stop codon in the message, deleting part of the triple helical domain and the complete NC domain. The mother of the patient was shown to be heterozygous for this mutation.

Alport syndrome: from bedside to genome to bedside

Alport syndrome is a genetic disorder of basement membranes manifested clinically by a progressive nephropathy and, in many families, sensorineural hearing loss and ocular lesions. During the 1980s evidence was amassed indicating type IV (basement membrane) collagen as the defective protein in Alport This hypothesis was confirmed in 1990 by the cloning of the X-chromosomal gene COL4A5, which encodes the alpha 5 chain of type IV collagen, and the discovery of mutations in this gene in many Alport kindreds. The results of results of recent studies suggest that the alpha 5(IV) chain forms a distinct collagenous network with the alpha 3 and alpha 4 chains of type IV collagen and that mutations in alpha 5(IV) may prevent the normal incorporation of alpha 3(IV) and alpha 4(IV) into basement membranes. Renal biopsy remains an important modality for making the diagnosis of Alport syndrome, but may eventually be replaced by molecular genetic techniques. Posttransplant anti-glomerular basement membrane nephritis occurs rarely in Alport patients and may be restricted to a subgroup with particular COL4A5 mutations. It is not clear why COL4A5 mutations result in glomerulosclerosis and renal failure, or whether this process may be slowed through dietary or pharmacologic intervention.

Alport-leiomyomatosis syndrome: an update

Alport-leiomyomatosis syndrome is a polygenic syndrome with a dominant X-linked inheritance pattern resulting from a large deletion in the 5' end of the COL4A5 gene coding for the type IV collagen alpha 5 chains. Hypothetically, the deletion extends beyond the 5' end and probably includes a second contiguous gene responsible for leiomyomatosis (the DL gene) and even a third one coding for congenital cataract (the CCT gene).

Deletion of the paired alpha 5(IV) and alpha 6(IV) collagen genes in inherited smooth muscle tumors

The gene encoding alpha 6(IV) collagen, COL4A6, was identified on the human X chromosome in a head-to-head arrangement and within 452 base pairs of the alpha 5(IV) collagen gene, COL4A5. In earlier studies, intragenic deletions of COL4A5 were detected in a subset of patients with Alport syndrome (AS), a hereditary defect of basement membranes. In some families, AS cosegregates with diffuse leiomyomatosis (DL), a benign smooth muscle tumor diathesis. Here it is shown that patients with AS-DL harbor deletions that disrupt both COL4A5 and COL4A6. Thus, type IV collagen may regulate smooth muscle differentiation and morphogenesis.

Mild phenotypic manifestation of a 7p15.3p21.2 deletion

A 28 month old girl with dysmorphic features was found to have an interstitial deletion of the short arm of chromosome 7p15.3-7p21.2. The patient had ptosis, dacryostenosis, pectus excavatum, short hands, and her development was normal or mildly delayed. Craniosynostosis and growth retardation, which were present in two other patients with similar deletions, were not present. Because of the mild manifestations, this case expands the clinical spectrum of the 7p15-7p21 deletion phenotype.

COL4A5 splice site mutation and alpha 5(IV) collagen mRNA in Alport syndrome

Mutations affecting the COL4A5 gene encoding the alpha 5 chain of type IV collagen, are involved in the pathogenesis of X-linked Alport syndrome. We used denaturing gradient gel electrophoresis (DGGE) to screen PCR amplified exons of COL4A5 for point mutations in a set of 18 Alport patients previously characterized by Southern blotting. One sequence variant was identified in the exon 38 region of a male Alport patient. Sequence analysis revealed a G to C transversion in the 5' intron splice donor site downstream from exon 38 (GT to CT). To determine the effect of the mutation on mRNA splicing, alpha 5(IV) cDNA was generated from total RNA of peripheral blood lymphocytes. Subsequent cDNA PCR yielded a product 81 base pairs shorter in the affected Alport patient, compared to normal controls. The absence of exon 38 from the alpha 5(IV) cDNA was confirmed by sequence analysis. The results demonstrated that the mutation leads to skipping of exon 38 in the processing of alpha 5(IV) pre-mRNA. The shortened transcript lacked 27 codons encoding a Gly-X-Y-repeat sequence with a preserved reading frame, enabling the translation of codons further downstream. Clinically, the patient presented with juvenile onset Alport syndrome, end-stage renal failure, and deafness. He had no ocular lesions. Typical ultrastructural changes of the glomerular basement membrane (GBM) were shown on electron microscopy. The patient developed anti-GBM antibodies after renal transplantation, however, renal function deteriorated only moderately.

Molecular genetics of Alport syndrome

Alport syndrome is a progressive hereditary kidney disease characterized by hematuria, sensorineural hearing loss and ocular lesions with structural defects in the glomerular basement membrane (GBM). The gene frequency has been estimated to be 1:5000. The disease is primarily X chromosome-linked, but autosomal forms have also been reported. The X-linked form has been shown to be caused by mutations in a recently identified alpha 5(IV) collagen chain gene (COL4A5). We have isolated cDNA clones for providing the entire primary structure of the human alpha 5(IV) chain. The gene has been located to the Xq22 region. Using antibodies against synthetic peptides, the alpha 5(IV) chain was shown to be located in the kidney only in the glomerular basement membrane. Thus far, the exon-intron structure has been determined for a large portion of the gene which probably has a size of over 200 kb. Numerous different mutations have been identified in the COL4A5 gene. The mutations include single base mutations, large deletions and other major rearrangements such as inversion and duplication. The consequences of the mutations observed can be considered sufficient to cause structural and functional defects in the type IV collagen molecule and, therefore, also the GBM network. This, in turn can explain the disruption of the GBM and hematuria occurring in these Alport patients. Alport syndrome is the first genetic basement membrane and kidney disease whose gene has been cloned. These recent results have enabled the development of antibodies and DNA probes for accurate diagnosis of Alport syndrome.

Alport syndrome: a genetic study of 31 families

Thirty one families with Alport syndrome including 3 families with associated syndromes were studied. The location of the COL4A5 gene, responsible for the Alport syndrome, was determined by linkage analysis with eight probes of the Xq arm and by a radiation hybrid panel. Concordant data indicated the localization of the Alport gene between DXS17 and DXS11. Four deletions and one single base mutation of the COL4A5 gene were detected. Homogeneity tests failed to show any evidence of genetic heterogeneity superimposed on clinical heterogeneity for ophthalmic signs and end-stage renal disease age.

Deletions of the COL4A5 gene in patients with Alport syndrome

Mutations in the COL4A5 gene encoding the alpha 5 chain of type IV collagen have been found in linkage with X-chromosomal Alport syndrome (AS). To identify COL4A5 mutations in patients from Germany with clinically defined AS, DNA from 20 unrelated patients was analyzed by conventional Southern blotting. By using full length alpha 5(IV) cDNA probes, large COL4A5 deletions could be detected in two patients. In one case, a 34 kb deletion affecting the 14 most 3' exons of the gene was observed. The second patient harbored a complete COL4A5 deletion. In both cases, functional alpha 5(IV) mRNA was unlikely to be present. Clinically, both patients developed end-stage renal failure before age 30. Furthermore, they had characteristic retinal flecks, and sensorineural hearing loss with typical changes on the audiogram. The patient with the complete deletion of COL4A5 lost the renal allograft due to an anti-GBM mediated glomerulonephritis.

Alport syndrome and diffuse leiomyomatosis: deletions in the 5' end of the COL4A5 collagen gene

Alport syndrome (AS) is an hereditary glomerulonephritis that is mainly inherited as a dominant X-linked trait. Structural abnormalities in the type IV collagen alpha 5 chain gene (COL4A5), which maps to Xq22, have recently been detected in several patients with AS. The association of AS with diffuse esophageal leiomyomatosis (DL) has been reported in 24 patients, most of them also suffering from congenital cataract. The mode of transmission and the location of the gene(s) involved in this association have not been elucidated. Southern blotting using cDNA probes spanning the whole COL4A5 and a 5' end COL4A5 genomic probe showed that three out of three patients with the DL-AS association had a deletion in the 5' part of the COL4A5 gene extending beyond its 5' end. This indicates that the same gene, COL4A5, is involved in classical AS and in DL-AS and that the transmission of DL-AS is X-linked dominant. These results also suggest that leiomyomatosis might be due to the alteration of a second gene involved in smooth muscle cell proliferation, which is located upstream of the COL4A5 gene, and that there might be a contiguous gene deletion syndrome, involving at least the genes coding for congenital cataract, DL and AS.

DNA rearrangements in the alpha 5(IV) collagen gene (COL4A5) of individuals with Alport syndrome: further refinement using pulsed-field gel electrophoresis

Alport syndrome (AS), an X-linked kidney disorder, has been shown to be caused by mutations in the gene for the alpha 5-chain of type IV collagen (COL4A5), which maps to Xq22. On the basis of the results of conventional Southern blot analysis of AS patient DNAs, we employed pulsed-field gel electrophoresis to characterize further three gene rearrangements at the 3'-end of alpha 5(IV). We were able to construct long-range restriction maps for all three of these patients and deduce the extent and nature of each rearrangement. One of these mutations is a 450-kb simple deletion that includes 12 kb of the alpha 5(IV) gene. A second mutation has been shown to be a direct duplication of 35 kb of alpha 5(IV) genomic DNA, and a third mutation involves a complex insertion/deletion event resulting in an overall loss of 25 kb.

X inactivation patterns in females with Alport's syndrome: a means of selecting against a deleterious gene?

The patterns of X chromosome inactivation in 43 females from families segregating classic Alport's syndrome (AS) (X linked hereditary nephritis with deafness) have been analysed. AS carrier females have a most variable clinical course. The aim of the study was to establish whether there was any correlation between the X inactivation pattern of a carrier female and the severity of her disease. No correlation was found in DNA derived from peripheral blood lymphocytes. However, it remains possible that differential patterns of X inactivation may occur in the tissues affected by AS, namely the basement membrane of the kidney, eye, and ear.

Different mutations in the COL4A5 collagen gene in two patients with different features of Alport syndrome

Alport syndrome is a hereditary renal disease in which progressive renal failure is often accompanied by sensorineural deafness and ocular abnormalities. Recently, mutations were detected in the type IV collagen alpha 5 chain gene in Alport syndrome patients. We searched for mutations in this gene in 18 unrelated patients, and in two patients abnormalities were detected. In the gene of patient BB we identified a complex deletion, which included the exons encoding the non-collagenous domain and part of the collagenous region. This patient showed early onset nephritis (end-stage renal disease at 17 years) with deafness. Within a year after receiving a kidney from an unrelated donor, he developed an antiglomerular basement membrane nephritis. In patient WJ a point-mutation was detected, changing a tryptophane into a serine in the non-collagenous domain. His clinical features are milder (renal failure at 33 years, no hearing loss), and a recent renal allograft did not provoke antiglomerular basement membrane disease. These initial data suggest that differences in the extent of disruption of the non-collagenous domain may correlate with the severity and/or heterogeneity of Alport syndrome and with the development of nephritis in renal allografts.

The pathogenesis of Alport syndrome involves type IV collagen molecules containing the alpha 3(IV) chain: evidence from anti-GBM nephritis after renal transplantation

Mutations in the COL4A5 collagen gene have been implicated as the primary defect in Alport syndrome, a heritable disorder characterized by sensorineural deafness and glomerulonephritis that progresses to end-stage renal failure. In the present study, the molecular nature of the defect in Alport glomerular basement membrane (GBM) was explored using anti-GBM alloantibodies (tissue-bound and circulating) produced in three Alport patients subsequent to renal transplantation. The alloantibodies bound to the alpha 3(IV)NC1 domain of type IV collagen and not to any other basement membrane component. In tissue sections, the alloantibodies bound specifically to peripheral GBM in normal kidney and the affected renal transplant but not to that of Alport kidney. These results establish that: the alpha 3 chain in type IV collagen molecules, the Goodpasture autoantigen, is the target alloantigen in post-transplant anti-GBM nephritis in patients with Alport syndrome, and that a molecular commonality exists in the pathogenesis of anti-GBM nephritis causing loss of renal allografts in patients with Alport syndrome and renal failure in patients with Goodpasture syndrome. These findings implicate: (1) defective assembly of type IV collagen molecules containing the alpha 3(IV) chain in Alport GBM; and (2) the existence of a mechanism linking the assembly of molecules containing the alpha 3(IV) chain with those containing the alpha 5(IV) chain.

Amplification of human polymorphic sites in the X-chromosomal region q21.33 to q24: DXS17, DXS87, DXS287, and α-glactosidase A

Methods for the PCR amplification of five polymorphic sites in the region Xq21.33 to Xq24 were developed and used to predict heterozygosity for Fabry disease in informative families. Clones containing polymorphic sites associated with DNA segments DXS17, DXS87, and DXS287, and the alpha-galactosidase A gene were isolated from genomic libraries. Surrounding nucleotide sequences and optimal conditions for amplification of each polymorphic site were determined. These amplifiable polymorphisms provided predictions of heterozygosity for Fabry disease and should be useful for diagnostic linkage analyses in Alport syndrome, X-linked cleft palate and ankyloglossia, Pelizaeus-Merzbacher disease, and X-linked agammaglobulinemia as well as sequence-tagged sites for gene mapping.

Type IV collagen of Engelbreth-Holm-Swarm tumor matrix: identification of constituent chains

The noncollagenous hexamer (NC1) domain of type IV collagen from Engelbreth-Holm-Swarm (EHS) sarcoma matrix was subjected to electrophoretic, amino-terminal amino acid sequence, and immunochemical analysis to determine which of the five known kinds of alpha(IV) chains are present. Electrophoretic analysis, whether by one-dimensional or two-dimensional electrophoresis, showed that nonlathyritic and lathyritic hexamer gave nearly identical patterns. Amino-terminal amino acid sequence analysis of hexamer subunits, transblotted from two-dimensional gels, revealed that the hexamer subunits were derived exclusively from the alpha 1 and alpha 2 chains. Western blots of hexamer subunits confirmed the sequence results, as the subunits. identified as alpha 1(IV) and alpha 2(IV) NC1 domains reacted with antibodies directed specifically against those subunits. Conversely, no reactivity of NC1 hexamer subunits was seen with Goodpasture serum, or with antibodies directed specifically against the alpha 3, alpha 4, and alpha 5 NC1 domains, confirming the lack of alpha 3, alpha 4, and alpha 5 chains. These results revealed that the type IV collagen component of the EHS sarcoma matrix is comprised exclusively of alpha 1 and alpha 2 chains. Its relative homogeneity simplifies, but restricts, interpretation of studies that employ it as a model type IV collagen because the studies would be based only on alpha 1 and alpha 2 chains.

Partial protein sequence of the globular domain of alpha 4(IV) collagen chain: sites of sequence variability and homology with alpha 2(IV)

The globular domain (NC) of alpha 4(IV) collagen chain was partially sequenced and compared with the NC domain of other collagen IV chains. The alpha 4(IV) NC domain was found to be most closely related to alpha 2(IV) NC domain but distinct from the NC domain of alpha 1(IV), alpha 2(IV), alpha 3(IV) and alpha 5(IV) collagen chains. Partial sequence, representing nearly one half of alpha 4(IV) NC domain, shows 56%, 69%, 51% and 54% identity with the corresponding NC domains of alpha 1(IV), alpha 2(IV), alpha 3(IV) and alpha 5(IV) collagen chains, respectively. A short, highly polar, region of variable sequence is found near the carboxy terminus of alpha 4(IV) NC domain. This sequence corresponds to a non-conserved region among NC domains, suggesting functional specialization at this site. It exhibits high surface probability with predicted structural differences among NC domains. These results confirm uniqueness of alpha 4(IV) NC domain and indicate its structural relatedness to other NC domains of collagen IV.

Long-range mapping of the gene for the human alpha 5(IV) collagen chain at Xq22-q23

The X-linked kidney disorder known as Alport syndrome (AS) has been shown to be due to mutations in the gene for an alpha 5 chain of type IV collagen that maps to Xq22-23. Using overlapping cDNA clones that represent approximately 90% of this gene and pulsed-field gel electrophoresis, we have constructed a 2.4-Mb long-range restriction map around the locus. All of the cDNA clones lie within a 360-kb segment of DNA bounded by CpG islands that contain sites for the rare-cutting enzymes BssHII, MluI, NotI, NruI, SalI, and SfiI. High-resolution PFGE mapping with XhoI shows that the gene is at least 110 kb in size and is one of the largest collagen genes characterized to date. This map will prove useful in the characterization of mutations in individuals affected with AS and will also provide information as to the location of other genes in the region.

Major rearrangements in the alpha 5(IV) collagen gene in three patients with Alport syndrome

The gene coding for the alpha 5 chian of type IV collagen (alpha 5(IV) collagen), which maps to Xq22, is a candidate gene for the X-linked dominant disease Alport syndrome (AS). Using three cDNA clones, covering the 3' end of the alpha 5(IV) collagen gene, 3 of 38 patients have been identified with mutations in this gene. Each of these patients shows a gross rearrangement of DNA: a deletion of at least 35 kb, an insertion/deletion event involving approximately 25 kb, and a duplication of at least 35 kb of DNA.

Hereditary abnormalities of renal basement membranes

The glomerular and tubular basement membranes are the principal barriers to filtration and re-absorption of water and molecules in the nephron. They are composed primarily of type IV collagen, laminin, fibronectin, sulphated proteoglycans and collagen type I. Three common inherited diseases are associated with abnormalities of basement membrane proteins: Alport's syndrome, thin basement membrane disease (TBMD) and adult polycystic kidney disease. In this review we describe the application of molecular biological techniques to the study of these conditions. Classic Alport's syndrome is an X-linked disorder with a lamellated glomerular basement membrane (GBM) which typically results in renal failure in males. Studies with sera from patients with Goodpasture's syndrome, or monoclonal antibodies specific for the Goodpasture antigen, show that the Goodpasture antigen is absent or masked in the kidneys of individuals with Alport's syndrome. There is some evidence to suggest that the Goodpasture antigen is best represented by the non-collagenous domain of the alpha 3 chain of type IV collagen, but that other non-collagenous regions may also contribute to the antigen. It is through these non-collagenous regions that the type IV collagen chains form the typical network, and the abnormality in Alport's syndrome interferes with this network formation. However, we have recently demonstrated that the gene for the non-collagenous domain of the alpha 3 collagen chain is present in individuals with Alport's syndrome. Furthermore, other groups have shown a defect in a novel type IV collagen chain, the alpha 5 chain, in 3 unrelated cases of Alport's syndrome.(ABSTRACT TRUNCATED AT 250 WORDS)

Nonsyndromal profound genetic deafness in childhood

About one-half of children with profound deafness have an autosomal recessive or autosomal dominant inherited type of deafness. The X-linked inherited types of deafness are rare. About one out of three profoundly deaf children has an autosomal recessive form of inherited deafness. At sometime during their life a syndromal diagnosis can be made in one out of four cases with an autosomal recessive form of deafness. Therefore in about 25% of all the children with profound deafness, a nonsyndromal autosomal recessive type of genetic deafness will be involved. It is still not clear how many different genes are responsible for this. The more severe the deafness in a child, the greater the chance that an autosomal recessive etiology is involved. The autosomal dominant inherited types of deafness are significantly more frequent in cases where the hearing loss in the best ear is less than 80-90 db. About one-half of the autosomal dominant inherited cases show a classical syndromal type of deafness based on clinical features. In the other half, some audiometrically recognizable types of deafness can be diagnosed after an autosomal dominant pattern of inheritance has been established. Additional genetic knowledge based on gene-linkage studies is needed to provide better tools for the more accurate diagnosis of genetic etiology in a profoundly deaf child. Adequate pedigrees are quite rare and such pedigrees are expected to become even more scarce as a result of a diminishing ratio of consanguineous marriages. It is necessary to start gene-linkage studies in these existing pedigrees to trace the genes responsible for this nonsyndromal type of profound genetic deafness in childhood.

The gene corresponding to the putative Goodpasture antigen is present in Alport's syndrome

Alport's syndrome is a heterogeneous group of inherited abnormalities of basement membranes that may result in progressive renal failure, defective hearing and lens abnormalities. The glomerular basement membrane (GBM) characteristically has areas of reduplication, lamellation and attenuation on electron microscopic examination. In the majority of affected males and some females, there is reduced or variable binding of serum from patients with anti-GBM disease (Goodpasture's syndrome) to these basement membranes. These sera contain antibodies directed against the Goodpasture antigen which has been thought to be located in the non-collagenous domain of the alpha3 chain of type IV collagen and is presumed to be important in cross-linking of the collagen molecules. The reduced staining for the Goodpasture antigen suggests that this structure is either absent or masked in Alport's syndrome. We have tested DNA from six unrelated individuals with Alport's syndrome. All had been transplanted for renal failure. The diagnosis of Alport's syndrome was made on the characteristic electron microscopic appearance of the renal basement membranes (n = 4), the presence of sensori-neural deafness (n = 4), a family history of Alport's syndrome (n = 5) and the presence of circulating inhibitable anti-GBM antibody activity post-transplant (n = 2). Oligonucleotides (20mers) corresponding to the 5' and 3' ends of the known 25 amino acid sequence for the putative Goodpasture antigen were used as primers for amplification of genomic DNA. The products were then blotted and probed with an intermediate 19-mer DNA. All Alport's patients contained a 75-bp fragment corresponding to the published peptide sequence for the non-collagenous domain of the alpha 3 chain of type IV collagen, suggesting that a large deletion of this region, the putative Goodpasture antigen, is unlikely to account for the defect in Alport's syndrome.

Characterization of the 3' half of the human type IV collagen alpha 5 gene that is affected in the Alport syndrome

We have determined the exon-intron structure of the 3' half of the gene for the human type IV collagen alpha 5 chain that is affected in X-chromosome-linked Alport syndrome. Six overlapping lambda phage genomic clones containing exons 1-14 (as counted from the 3' end) and two additional overlapping genomic clones containing exons 16-19 spanned a total of 60 kb, 9.5 kb of which were the 3' flanking region. The exon-intron structure was elucidated by restriction enzyme mapping, nucleotide sequencing, and heteroduplex analyses. The sequences of all of the 19 most 3' exons and their flanking sequences were determined from the genomic clones, with the exception of exon 15, which was sequenced after amplification from genomic DNA with the polymerase chain reaction. The results show that the genes for the alpha 5(IV) and alpha 1(IV) chains have an almost identical exon size pattern in the 3' half. In contrast, there is not a clear conservation of intron sizes between the two genes, although both genes may have a similar total size. The current results have allowed the identification of three mutations in the alpha 5(IV) gene in three kindreds with Alport syndrome, and the gene structure and sequencing data presented should facilitate the analysis of other as yet unidentified mutations in this heterogeneous genetic disease.

Single base mutation in alpha 5(IV) collagen chain gene converting a conserved cysteine to serine in Alport syndrome

We have identified a point mutation in the type IV collagen alpha 5 chain gene (COL4A5) in Alport syndrome. Variant PstI (Barker et al., 1990, Science 248, 1224-1227), and BglII restriction sites with complete linkage with the Alport phenotype have been found in the 3' end of the COL4A5 gene in the large Utah Kindred P. The approximate location of the variant sites was determined by restriction enzyme mapping, after which this region of the gene (1028 bp) was amplified with the polymerase chain reaction (PCR) from DNA of normal and affected individuals for sequencing analysis. The PCR products showed the absence or presence of the variant PstI and BglII sites in DNA from normal and affected individuals, respectively. DNA sequencing revealed a single base change in exon 3 (from the 3' end) in DNA from affected individuals, changing the TGT codon of cysteine to the TCT codon for serine. This single base mutation also generated new restriction sites for PstI and BglII. The mutation involves a cysteine residue that has remained conserved in the carboxyl-end noncollagenous domain (NC domain) of all known type IV collagen alpha chains from Drosophila to man. It is presumably crucial for maintaining the right conformation of the NC domain, which is important for both triple-helix formation and the formation of intermolecular cross-links of type IV collagen molecules.