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| Year : 2003 | Volume
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| Issue : 3 | Page : 276-289 |
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| Alport Syndrome and Thin Basement Membrane Nephropathy: Diseases Arising from Mutations in Type IV Collagen |
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Clifford E Kashtan
Department of Pediatrics, University of Minnesota Medical School, Minneapolis, Minnesota, USA
Click here for correspondence address and email
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How to cite this article:
Kashtan CE. Alport Syndrome and Thin Basement Membrane Nephropathy: Diseases Arising from Mutations in Type IV Collagen. Saudi J Kidney Dis Transpl 2003;14:276-89 |
How to cite this URL:
Kashtan CE. Alport Syndrome and Thin Basement Membrane Nephropathy: Diseases Arising from Mutations in Type IV Collagen. Saudi J Kidney Dis Transpl [serial online] 2003 [cited 2022 May 23];14:276-89. Available from: https://www.sjkdt.org/text.asp?2003/14/3/0/32998 |
 Introduction | |  |
Alport syndrome (AS) and thin basement membrane nephropathy (TBMN) are common causes of persistent hematuria. All cases of AS, and many instances of TBMN, arise from mutations in genes encoding isoforms of type IV collagen, the major collagenous component of basement membranes. In both disorders, hematuria is typically first observed in childhood. AS and TBMN may be difficult to distinguish on the basis of clinical features and conventional renal biopsy evaluation, especially early in life. This article will review current knowledge regarding the genetics and pathogenesis of AS and TBMN and describe the tools available to the clinician and pathologist for accurately diagnosing these conditions.
| Definitions | | |
AS is a progressive nephropathy that results from mutations in the type IV collagen genes COL4A3, COL4A4 or COL4A5. TBMN is a usually benign nephropathy that is frequently the result of heterozygous mutations in certain type IV collagen genes (COL4A3 or COL4A4), but may also arise from mutations in other, as yet unidentified, genes. Hematuria is common to both conditions, but the nephropathy of AS is progressive, leading to proteinuria, hypertension and ultimately, renal failure, and is frequently associated with extra-renal abnormalities, such as sensorineural deafness and ocular lesions. TBMN is clinically defined by the absence of extra-renal findings, and the development of proteinuria or hypertension is unusual. In both conditions, a family history of hematuria is common, although either condition can occur sporadically as the result of de novo mutations. The family history of a patient with AS frequently is positive for relatives with renal failure, while this is unusual in those with TBMN.
The term "benign familial hematuria", often used to describe families with TBMN, should probably be dropped. While most patients with TBMN have a non-progressive disorder, some patients with thin glomerular basement membranes develop proteinuria and renal insufficiency. For purposes of clarity, the definition of AS excludes patients with hereditary nephritis accompanied by macrothrombocytopenia (Epstein and Fechtner syndromes), who have been shown to have heterozygous mutations in non-muscle myosin heavy chain IIA (MYH9). [1]
| Type IV Collagen | | |
Type IV collagen is a family of six proteins, the α1(IV) - α6(IV) chains, each encoded by its own gene (COL4A1 - COL4A6). The six type IV collagen genes are distributed in pairs on three different chromosomes: COL4A1-COL4A2 on chromosome 13, COL4A3-COL4A4 on chromo-some 2, and COL4A5-COL4A6 on the X chromosome. The members of each pair are oriented in a 5'-5' manner, with the transcriptional initiation sites separated by DNA sequences containing regulatory elements.
Type IV collagen α chains possess a noncollagenous domain (NC1) at the carboxyterminal end, a central collagenous domain consisting of Gly-X-Y repeats, and a short amino-terminal non-collagenous (7S) domain. Type IV collagen α chains form trimers through associations between their carboxyterminal NC1 domains, which initiate folding of the collagenous domains into triple helices. Variable residues within the NC1 domains appear to dictate which chains can interact with each other. [2]
The Type IV collagen trimers associate with each other via carboxy- and amino-terminal interactions to create basement membrane networks. Evidence from a variety of sources indicates the existence of at least three type IV collagen hetero-trimers in mammalian basement membranes: α1(IV) 2 α2(IV), α3(IV)- α4(IV)- α5(IV), and α5(IV) 2 α6(IV). The αl(IV) 2 α2(IV) hetero-trimer is found in all basement membranes, although it is a relatively minor component of glomerular basement membranes (GBM), in which the α3(IV) α4(IV)- α5(IV) hetero-trimer predominates. The α3(IV)- α4(IV)- α5(IV) hetero-trimer also occurs in the Bowman's capsule and the basement membranes of distal and collecting tubules. The α5(IV) 2α6(IV) hetero-trimer is present in the Bowman's capsule and distal and collecting tubule basement membranes, but is not present in GBM. The epidermal basement membrane (EBM) contains α1(IV) 2 - α2(IV) and α5(IV) 2 α6(IV) hetero-trimers, but not the α3(IV)- α4(IV)- α5(IV) hetero-trimers.
There appear to be at least three rules governing the deposition of α3(IV), α4(IV) and α5(IV) chains in basement membranes. First, the α3(IV) and α4(IV) chains must form trimers with the α5(IV) chain in order to be incorporated into basement membranes. Second, the α3(IV) and α4(IV) chains must associate with each other to be deposited in the basement membranes. Third, the α5(IV) chain can partner with the α6(IV) chain in hetero-trimers. Some basement membranes, such as the Bowman's capsule and distal and collecting tubule basement membranes, have both α3(IV)- α4(IV)- α5(IV) and α5(IV) 2α6(IV) hetero-trimers. Others, such as epidermal basement membranes, have α5(IV)2 α6(IV) hetero-trimers, but not α3(IV)- α4(IV)- α5(IV) hetero-trimers.
These requirements have important consequences for patients with AS, which can arise from mutations in any of α3(IV), α4(IV) or α5(IV) chains. First, COL4A5 mutations that result in failure to synthesize an α5(IV) chain (deletions) or synthesis of a truncated or nonsensical chain (frameshift, premature stop codon, etc.), prevent the formation of trimers containing the α3(IV) and α4(IV) chains.
The result is that none of the three chains appears in basement membranes. Second, missense mutations in COL4A5, in particular those that result in replacement of a glycine residue in the collagenous domain of the α5(IV) chain by another amino acid, allow the formation of abnormally folded trimers that are highly susceptible to extracellular degradation before being incorporated into basement membranes. Again, the consequence of the mutation in the α5(IV) chain is the elimination of all three chains from basement membranes. In some instances, a missense mutation may allow formation of an abnormal network that is deposited in basement membranes.
Mutations in the α3(IV) and α4(IV) chains can have similar effects, particularly in basement membranes such as GBM in which the α6(IV) chain is not expressed, so that α5(IV) chains can form hetero-trimers with only the α3(IV) and α4(IV) chains. In the Bowman's capsule, distal and collecting tubule basement membranes, and epidermal basement membranes, the α5(IV) chain can be expressed even in the absence of the α3(IV) or α4(IV) chains, because these basement membranes express the α6(IV) chain, with which the α5(IV) chain can form trimers.
| Genetics | | |
There are three genetic forms of AS. The X-linked form (XLAS) results from mutations in COL4A5 and accounts for about 80% of patients with the disease. About 15% of patients have autosomal recessive AS (ARAS), which arises from mutations affecting both alleles of COL4A3 or COL4A4. The heterozygous parents of children with ARAS often have asymptomatic hematuria, although some have normal urinalyses. Finally, approximately 5% of patients have autosomal dominant AS (ADAS), due to heterozygous mutations in COL4A3 or COL4A4. Heterozygous mutations in COL4A3 or COL4A4 have also been found in families with TBMN. It is not yet clear why many, perhaps most, individuals with heterozygous mutations in these genes have asymptomatic hematuria, while others have progressive disease.
X-Linked Alport Syndrome
Several hundred COL4A5 mutations have been reported in XLAS families. These mutations are distributed throughout the gene, and with few exceptions each mutation is unique. About 20% of reported COL4A5 mutations are large rearrangements, predominantly deletions. [3],[4] Missense mutations account for about 35-40%, about 15% are splice-site mutations, and 25-30% are nonsense mutations or small frame-shifting deletions or insertions that result in premature stop codons. [3],[4] About 10 to 15% of COL4A5 mutations occur as spontaneous events in the proband, explaining why some patients with XLAS lack a family history of the disease.
The association of XLAS with leiomyomatosis of the esophagus and tracheobronchial tree has been reported in several dozen families. [5] All of these families exhibit large deletions that span the adjacent 5' ends of the COL4A5 and COL4A6 genes. [6],[7] These deletions involve varying lengths of COL4A5, but the COL4A6 breakpoint is always located in the second intron of the gene. [8],[9],[10] Leiomyomatosis does not occur in patients with deletions of COL4A5 and COL4A6 that extend beyond intron 2 of COL4A6. Mutations of COL4A6 alone do not appear to cause AS, consistent with the absence of the C6(IV) chain from normal GBM. [11],[12]
The great majority of missense COL4A5 mutations are guanine substitutions in the first or second position of glycine codons that result in the replacement of a glycine residue in the collagenous domain of α5(IV) by another amino acid. [3],[4] Such mutations are thought to interfere with the normal folding of the mutant C55(IV) chain into triple helices with other type IV collagen α chains. Glycine lacks a side chain, making it the least bulky of amino acids, and small enough to allow three glycine residues to fit into the interior of a tightly wound triple helix. [13] The presence of a bulkier amino acid in a glycine position presumably creates a kink or an unfolding in the triple helix. Glycine substitutions in the El1 chain of type I collagen account for the majority of mutations causing osteogenesis imperfecta, and are common in other genetic disorders of collagen. [14],[15] Abnormally folded collagen triple helices exhibit increased susceptibility to proteolytic degradation. [15] The position of the substituted glycine, or the substituting amino acid itself, may influence the effect of the mutation on triple helical folding, and ultimately the impact of the mutation on the severity of the clinical phenotype.
Autosomal Recessive Alport Syndrome
To date, mutations causing ARAS have been found in the COL4A3 gene in nearly 30 patients [16],[17],[18],[19],[20] and in the COL4A4 gene in 12 patients. [17],[21] Some of these patients are homozygous for their mutations and others are compound heterozygotes. As with COL4A5, there appear to be no mutation hot spots in COL4A3 or COL4A4. The mating of two individuals with asymptomatic hematuria due to heterozygous COL4A3 or COL4A4 mutations can result in a child who has mutations in both alleles of COL4A3 or COL4A4 and, as a result, ARAS.
The reported COL4A3 and COLA4 mutations in ARAS include nonsense, frameshift, splicing and missense mutations. As with XLAS and other heritable collagen disorders, a common type of mutation in ARAS is a glycine substitution in the collagenous domain of α3(IV) or α4(IV).
Autosomal Dominant Alport Syndrome
Heterozygous type IV collagen mutations have been described in several families with clearly defined ADAS. Heterozygous mutations have also been found in some patients with hematuria, proteinuria and, variably, renal insufficiency, who appear to have a progressive nephropathy, rather than typical TBMN. These mutations include a splicing mutation, a small deletion and three missense mutations in COL4A3 [20],[22],[23] and two missense mutations in COL4A4 [23],[24] It is not clear why some individuals with heterozygous COL4A3 or COL4A4 mutations are asymptomatic or exhibit only isolated microhematuria [20],[21],[23],[25],[26] while others have a progressive nephropathy. Several possibilities can be proposed: (a) the type and/or site of the mutation may be critical; (b) the presence of certain polymorphisms in these genes may influence the effect of a pathogenic mutation; [22] or (c) a polymorphism or mutation in another gene may modify the effect of the mutation. In some cases, a heterozygous missense mutation in COL4A3 or COL4A4 might be more detrimental than a deletion or nonsense mutation, because the mutant chain can then induce the degradation of normal chains with which it forms abnormal trimers.
Thin Basement Membrane Nephropathy
Heterozygous mutations in COL4A3 or COL4A4 have now been described in 13 families diagnosed with TBMN or benign familial hematuria. [25],[26],[27] In eight other families, the disease gene has been mapped to the region of chromosome 2 where the COL4A3 and COL4A4 genes reside. [28] The described mutations have included missense, frameshift and splicing alterations. Heterozygous mutations in COL4A3 or COL4A4 may result in GBM with approximately 50% of the normal content of the α3- α4- α5 network. This may result in a structurally weak GBM that is prone to rupture or tearing, producing hematuria.
It is clear that in many families TBMN results from heterozygous mutations in COL4A3 or COL4A4. However, in other families with TBMN or benign familial hematuria, linkage analysis has excluded COL4A3 and COL4A4 as the disease-causing gene, indicating that mutations at other, as yet undetermined loci, can cause TBMN. [28],[29]
| Clinical Renal Findings | | |
The cardinal finding of AS is hematuria. Affected males have persistent microscopic hematuria. Many also have episodic gross hematuria, precipitated by upper respiratory infections, during the first two decades of life. Hematuria has been discovered in the first year of life in affected boys, in whom it is probably present from birth. Boys who are free of hematuria during the first 10 years of life are unlikely to be affected.
Heterozygous females with XLAS typically have hematuria, but about 5% of obligate heterozygotes never manifest hematuria. Hematuria appears to be persistent in both males and females with ARAS. Approximately 50% or more of carriers of COL4A3 or COL4A4 mutations have hematuria. [20],[21]
Proteinuria is usually absent early in life but develops eventually in all males with XLAS and in both males and females with ARAS. Proteinuria increases progressively with age and may result in the nephrotic syndrome. Proteinuria occurs in many heterozygous females with XLAS; the prevalence increases with age.
Hypertension also increases in prevalence and severity with age. Like proteinuria, hypertension is much more likely to occur in affected males than in affected females with XLAS, but there are no gender differences in ARAS.
End-stage renal disease (ESRD) develops in all affected males with XLAS. The rate of progression to ESRD is determined primarily by the nature of the underlying COL4A5 mutation. [4] The rate of progression is fairly constant among affected males within a particular family, but there is significant interkindred variability. Significant intra-kindred variability in the rate of progression to ESRD has been reported in some families with missense COL4A5 mutations. [4]
While most affected females with XLAS survive into old age with mild renal disease, chronic renal insufficiency and ESRD can occur. Gross hematuria in childhood, nephrotic syndrome, diffuse GBM thickening observed by electron microscopy, sensorineural deafness and anterior lenticonus are features suggestive of progressive nephropathy in affected females [30] Many women with progressive nephropathy maintain adequate renal function until late in life. Both males and females with ARAS appear likely to progress to ESRD during the second or third decade of life.
| Renal Pathology | | |
There are no pathognomonic lesions by light microscopy in either AS or TBMN. Direct immunofluorescence is negative, serving to eliminate immune complex-mediated forms of glomerulonephritis from the differential diagnosis. Diagnostic information is provided by electron microscopy and by immunostaining for type collagen F1 chains.
Electron Microscopy (EM)
The earliest ultrastructural abnormality of the GBM seen in patients with AS is diffuse attenuation, with GBMs as thin as 100 nm or less. As diffuse GBM attenuation is also characteristic of TBMN, it may be difficult to differentiate AS and TBMN by EM in children. Since gradual splitting of the lamina densa and thickening of the GBM occurs in most AS patients, but not in patients with TBMN, AS should be suspected when the GBM exhibits mixed thinning and thickening, especially within a single capillary loop.
The cardinal ultrastructural feature of the kidney in AS is thickening of the GBM with splitting or fraying of the lamina densa. The thick segments can measure up to 1200 nm in width. The subepithelial aspect of the GBM is often markedly irregular in contour. The lamina densa is transformed into a heterogeneous network of membranous strands, which enclose clear electron-lucent areas that may contain round granules of variable density measuring 20-90 nm in diameter. These areas of "basketweaving" are often associated with podocytes exhibiting foot process fusion. The extent of GBM thickening and lamellation is genderand age-dependent. In males with XLAS or ARAS, and in females with ARAS, the percentage of capillary loops demonstrating these pathognomonic changes increases progressively with age. [31] Females with XLAS often exhibit a mixture of thin, thick and lamellated, and normal-appearing GBM, and the extent of thickening and lamellation does not show a strong correlation with advancing age. These changes also occur in patients with ADAS, but progress much more slowly, and may not be apparent until well into adulthood. [32]
Patients with TBMN typically exhibit diffuse thinning of the lamina densa and, perhaps as a result, of the GBM as a whole. The thickness of normal GBM is age- and gender-dependent. Both the lamina densa and the GBM increase rapidly in thickness between birth and two years of age, followed by gradual thickening throughout childhood and adolescence. [33] GBM thickness of adult men exceeds that of adult women. [34] Thus, it is important to consider the age and sex of the patient when evaluating GBM width.
Because a variety of techniques have been used to measure GBM width, there is no standard definition of "thin" GBM. The cut-off value in adults ranges from 250 nm to 330 nm, depending upon the technique used. [35],[36] For children, the cut-off is in the range of 200-250 nm (250 nm is within 2 SD of the mean at age 11). [37],[38],[39] The intra-glomerular variability in GBM width is small in TBMN. [35] Marked variability in GBM width within a glomerulus, in a patient with persistent microhematuria, should raise suspicion of AS.
Occasionally, patients with AS or TBMN will exhibit electron-dense mesangial or glomerular capillary wall deposits. This is consistent with reports of the co-occurrence of AS and TBMN with IgA nephropathy and membranoproliferative glomerulonephritis. [40],[41]
Some patients have been found to have both TBMN and glomerular obsolescence [42] Whether these associations represent cooccurrence of two conditions, or increased susceptibility of patients with abnormal GBM biochemistry and structure to immune complex trapping and capillary loop collapse, is unclear.
Immunohistology
In males with XLAS, the GBM, distal TBM and Bowman's capsules usually fail to express α1-α2,α3-α4-α5 trimers, but expression of the α1-α2 trimers is preserved and, in fact, increased. [43],[44],[45] Basement membranes of some males with XLAS exhibit normal, or reduced but positive, expression of α3- α4- α5 5 trimers. α5- α6 - trimers are typically not expressed in the Bowman's capsule or distal TBM of XLAS males. Women who are heterozygous for XLAS mutations frequently exhibit mosaic loss of GBM expression of α3- α4- α5 trimers, while expression of α1- α2 trimers is mosaically enhanced. [43],[44],[45],[46] Epidermal basement membranes (EBM) normally express the α1- α2 trimers and α5- α6 trimers, but not α3- α4- α5 trimers. Most males with XLAS show no EBM expression of α5- α6 trimers while female heterozygotes frequently display mosaic loss of expression of α5- α6 trimers.
In patients with ARAS, GBMs usually show no expression of α3- α4- α5 5 trimers, but α5- α6 trimers are expressed in the Bowman's capsule, distal TBM and EBM. [47] Therefore, XLAS and ARAS may be distinguishable by immunohistochemical analysis of renal biopsy specimens. The expression of type IV collagen chains in basement membranes of patients with ADAS has not been characterized. Expression of these chains in the renal basement membranes of individuals with TBMN has received little systematic study, but based on available literature (and personal experience), is indistinguishable from the normal findings. [48],[49]
Kits for immunostaining of biopsy specimens for collagen IV α chain expression are commercially available from two sources: Wieslab (http://www.wieslab.se/) and Shigei Medical Institute (http://www.shigei.or.jp/smri/smri1/1alport_e.htm).
Extrarenal Features of Alport Syndrome
Because the α3, α4 and α5 chains of collagen IV are expressed in specialized basement membranes throughout the body, many patients with AS have extrarenal abnormalities. The presence of these abnormalities in a patient with hematuria can strongly support, or even confirm, a diagnosis of AS.
Deafness
Eighty to 90% of XLAS males exhibit sensorineural hearing loss by age 40 years. [4]H Hearing loss in AS is never congenital. Audiologic evaluation frequently demonstrates hearing loss in late childhood, but in some families deafness is not detectable until relatively late in life. [50] In its early stages, the hearing deficit is detectable only by audiometry, with bilateral reduction in sensitivity to tones in the 2000 to 8000 Hz range. In XLAS males, and ARAS patients of either gender, the deficit is progressive and eventually extends to other frequencies, including those of conversational speech.
XLAS males with large rearrangements of COL4A5, nonsense mutations, frameshift mutations and splice site mutations have a risk of deafness of 50% at 10 years of age. [4] In those with missense mutations, the risk of deafness does not reach 50% until age 20. In females with XLAS, hearing loss is less frequent and tends to occur later in life. Virtually all patients with ARAS exhibit deafness, with no apparent gender differences. Deafness in AS is presumably a consequence of the loss of the α3(IV)- α4(IV)- α5(IV) network from basement membranes of the cochlea. [51]
Ocular Findings
Ocular lesions are common in AS, occurring in 30-40% of patients with XLAS. [4],[52] The spectrum of ocular lesions appears to be similar in XLAS and ARAS. [53],[54] These clinical abnormalities arise from proven or putative changes in the basement membranes of the eye.
Anterior lenticonus, in which the central portion of the lens protrudes into the anterior chamber, is virtually pathognomonic of AS. All reported patients with anterior lenticonus who have been adequately examined have exhibited evidence of chronic nephritis and sensorineural deafness. [55] The frequency of lenticonus in XLAS males was 13% in one large series. [4] Anterior lenticonus is almost entirely restricted to AS families with progression to ESRD before age 30 and deafness. [56] This is explained by the finding that lenticonus is significantly more common in patients with a COL4A5 deletion or mutation resulting in a premature stop codon than in patients with missense or splice site mutations. [4] The lesion is bilateral in about 75% of patients. It is far more common in affected males but can occur in females. Anterior lenticonus is absent at birth, usually appearing during the second to third decade of life. Attenuation and fracturing of the anterior lens capsule have been demonstrated by light and electron microscopy in patients with lenticonus. [57],[58],[59]
Another common ocular manifestation of AS is a maculopathy consisting of whitish or yellowish flecks or granulations in a perimacular distribution. [60] This lesion was found in about 14% of XLAS males in a large series. [4] In contrast to lenticonus, there does not appear to be any correlation of maculopathy with the type of COL4A5 mutation. [4] The maculopathy does not appear to be associated with any visual abnormalities.
Corneal endothelial vesicles (posterior polymorphous dystrophy) have been observed in AS patients by several investigators, [61],[62],[63] and may indicate defects in Descemet's membrane, the basement membrane underlying the corneal endothelium. Recurrent corneal erosion in AS patients has been attributed to alterations of the corneal epithelial basement membrane. [64],[65]
Leiomyomatosis
In patients with XLAS and leiomyomatosis, symptoms usually appear in late childhood and include dysphagia, post-prandial vomiting, retrosternal or epigastric pain, recurrent bronchitis, dyspnea, cough and stridor. [5] Affected females in these kindreds typically exhibit genital leiomyomas as well, causing clitoral hypertrophy with variable involvement of the labia majora and uterus. Bilateral posterior subcapsular cataracts also occur frequently in affected individuals in these kindreds.
Diagnostic Approach
Diagnosis of AS and TBMN relies on clinical examination, pedigree analysis, immunohistochemistry and electron microscopy. Routine application of molecular analysis to the differential diagnosis of hematuria awaits the advent of commercial testing services. With the available tools, a suspected diagnosis of AS can be confirmed in the great majority of cases, [66] and in most cases the genetic type of AS can also be determined. Similarly, most patients with TBMN can be accurately diagnosed when clinical examination is combined with careful documentation of family history, immunohistochemistry and electron microscopy. Patients diagnosed as having TBMN should continue to be monitored for the development of proteinuria and/or hypertension.
Besides potentially obviating the need for renal biopsy, molecular analysis would be most useful in several situations: (a) when a diagnosis of AS cannot be confirmed or excluded by conventional methods; (b) when the mode of inheritance of AS cannot be clearly established and (c) and for absolute confirmation or exclusion of the carrier state in an at-risk female.
About 75-80% of males with XLAS can be identified by the absence of expression of the E55(IV) chain in skin. [67] Therefore, renal biopsy may not be necessary for confirmation of a suspected diagnosis of XLAS. Examination of the skin by EM is not useful, since the epidermal basement membranes of AS patients do not exhibit ultrastructural changes.
What is the appropriate role for skin biopsy in the evaluation of a patient with hematuria who is suspected to have XLAS or TBMN? One approach to diagnosis would be to perform a skin biopsy with immuno-histochemical analysis of E5(IV) expression. Absence of E55(IV) expression in a male, or clearly mosaic E5(IV) expression in a female, is diagnostic of XLAS arising from a COL4A5 mutation. Normal expression of α5(IV) in the skin of a patient suspected of AS has several possible explanations: (a) the patient has a COL4A5 mutation that allows skin expression of α5(IV); [67], (b) the patient has ARAS or (c) the patient has TBMN or some other glomerular disorder. If skin biopsy is not diagnostic, the next step would be to perform a kidney biopsy. Analysis of type IV collagen expression in the kidney, combined with routine light, immunofluorescence and electron microscopy will, in most cases, allow differentiation of AS and TBMN from other renal diseases. A normal skin biopsy result in a patient suspected to have TBMN does not confirm the diagnosis, but does provide information in support of the diagnosis.
| Treatment of Alport Syndrome and Renal Transplantation | | |
Clinical therapeutic trials in AS have not been conducted. The availability of canine and murine models of AS should allow the testing of genetic or pharmacologic therapies, in order to select promising treatments for human trials. [68],[69],[70],[71],[72],[73],[74] Cyclosporine appeared to stabilize renal function in a small, uncontrolled study of Alport males; [75] confirmatory studies will need to be published before this therapeutic approach can be recommended.
At present, renal transplantation is the only available treatment for AS. Allograft survival rates in AS patients are equivalent to those in patients with other causes of ESRD. Anti-GBM glomerulonephritis involving the renal allograft is a rare but frequently catastrophic manifesttation of AS, occurring in 2-3% of transplanted males with XLAS. [76] AS patients with normal hearing or late progression to ESRD are at very low risk for allograft anti-GBM nephritis, as are females with XLAS. Allograft antiGBM nephritis has also been reported in patients with ARAS, but the magnitude of the risk is unknown. [77],[78]
The onset of allograft anti-GBM nephritis is usually within the first year following transplantation. Three-quarters of the allografts fail irreversibly, usually within a few weeks to months. Plasmapharesis and cyclophosphamide administration have been of limited benefit. Anti-GBM nephritis has recurred in most retransplanted patients, despite prolonged intervals between transplants, and absence of detectable circulating anti-GBM antibodies prior to retransplantation.
The pathogenesis of post-transplant anti-GBM nephritis in AS is presumably based upon exposure to antigens present in the donor GBM, for which the recipient has not established immune tolerance. [79] The target(s) of anti-GBM antibodies in some of these patients has been determined. Most of the patients with XLAS exhibit antibodies against the NCl domain of the C55(IV) chain, but antibodies against L3(IV) NCl have also been described. [80],[81],[82] In ARAS patients with anti-GBM nephritis, antibodies appear to target the L33(IV) NC1 domain. [78],[81] Allograft anti-GBM nephritis has been reported in two females with AS, both of whom proved to have ARAS due to COL4A3 mutations. [19],[78]
There is evidence to suggest that mutations in the COL4A5 gene that prevent expression of an immunogenic gene product, thereby preventing the establishment of tolerance for C55(IV), are associated with an increased risk for the development of post-transplant anti-GBM nephritis. [83] A recent review of the genetics of AS included seven patients with XLAS and allograft anti-GBM nephritis; six had large deletions of COL4A5, and the other had a splicing mutation. [3] Even if certain types of COL4A5 mutation confer a higher risk of developing allograft anti-GBM nephritis, such data is currently of limited value in planning transplantation. It is clear that AS patients with COL4A5 deletions can undergo renal transplantation without developing antiGBM nephritis, indicating that other factors, presently unknown, must influence the initiation and elaboration of the immune response to the allograft. [3],[84] At this time, the only way to determine whether a previously untransplanted AS patient will develop post-transplant anti-GBM nephritis is to perform the transplant, although as noted above, certain patients are at very low risk.
| Conclusion | | |
When evaluating a patient with persistent glomerular hematuria, the goal of diagnostic evaluation is to answer several critical questions. What is the patient's risk of progression to ESRD? Is the condition heritable? What is the mode of inheritance? The tools available to clinicians and pathologists including patient and family history, audiologic and ophthalmologic evaluation, examination of biopsy material using immuno-histochemistry and electron microscopy are usually sufficient to allow accurate diagnoses of AS and TBMN to be made, and to determine the mode of inheritance. Molecular genetic analysis will eventually be a very valuable adjunct to these tools.
| Acknowledgements | | |
This work was supported by the National Institutes of Health (RO1 DK57676 to C.E.K.).
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Correspondence Address:
Clifford E Kashtan
Department of Pediatrics, University of Minnesota Medical School, MMC 491, 420 Delaware Street SE, Minneapolis, Minnesota 55455
USA
 Source of Support: None, Conflict of Interest: None  | Check |
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