Molecular Genetic Pathogenesis
Basement membranes, the sheet-like structures that support epithelial and endothelial cells, display heterogeneity in protein composition, ultrastructural features, and function. Basement membranes are composed of several major and minor glycoprotein constituents. Type IV collagen is present ubiquitously in basement membranes, where it is the major collagenous component. Type IV collagen molecules secreted by endothelial and epithelial cells self-associate into polygonal networks, which interact with laminin networks, as well as with nidogen, proteoglycans, and other glycoproteins, to form basement membranes.
It is now clear that the tissue pathology and clinical features of Alport syndrome result from abnormalities of basement membrane expression of the collagen α3, α4, α5, and possibly α6(IV) chains. These chains are usually absent from or under-expressed in the basement membranes of individuals with Alport syndrome, such that the networks that they form are absent or, if present, defective in structure and function.
In the normal developing kidney, collagen α1(IV) and collagen α2(IV) chains predominate in the primordial glomerular basement membrane (GBM) of immature glomeruli. The formation of capillary loops within the maturing glomeruli is associated with the appearance of collagen α3, α4, and α5(IV) chains in the GBM. As glomerular maturation progresses, the α3, α4, and α5(IV) chains become the predominant type IV collagen chains in GBM. This process has been referred to as "isotype switching." Although an isotype switch does not occur in most males with Alport syndrome, glomerular development otherwise proceeds normally and the GBM of young animals and children with Alport syndrome exhibits a normal trilaminar appearance by electron microscopy. These glomeruli exhibit normal capacities for filtration and for selective permeability, as demonstrated by the normal glomerular filtration rates and absence of overt proteinuria that are characteristic of early Alport syndrome in both humans and animals. Therefore, it appears that proteinuria and renal insufficiency, as well as sensorineural deafness, come about as the result of processes initiated by the absence of the collagen α3-α4-α5(IV) network, rather than arising directly from the absence of this network.
The most straightforward demonstration of the consequences arising directly from the absence of the collagen α3-α4-α5(IV) network from basement membranes may be anterior lenticonus, in which the anterior lens capsule lacks the strength to maintain the normal conformation of the lens. Microhematuria, the first and invariable renal manifestation of Alport syndrome, probably reflects GBM thinning and a tendency to develop focal ruptures caused by the absent or defective expression of the collagen α3-α4- α5(IV) network. Episodic gross hematuria precipitated by infections, which is not uncommon during the first two decades of life, may reflect increased susceptibility of the Alport GBM to proteolysis.
The processes that bring about GBM thickening, proteinuria, and renal insufficiency in males with XLAS and in both males and females with ARAS remain undefined, although there are some clues to what may be occurring. Unlike other glomerulopathies, Alport syndrome is characterized by the accumulation of the collagen α1(IV) and α2(IV) chains, along with types V and VI collagen, in the GBM. These proteins appear to spread from their normal subendothelial location, occupying the full width of the GBM. As Alport glomeruli undergo sclerosis, the collagen α1(IV) and α2(IV) chains disappear from the GBM, but type V and type VI collagen persist and, in fact, continue to accumulate. It is possible that the altered expression of the collagen α1(IV) and α2(IV) chains, type V collagen and type VI collagen represents a compensatory response to the loss of the collagen α3(IV), α4(IV), and α5(IV) chains from GBM, or it may simply reflect altered gene expression resulting from changes in signaling from the extracellular matrix to the nucleus. In transgenic mice with ARAS caused by partial deletion of COL4A3, renal mRNA levels for the collagen α1(IV) and α2(IV) chains progressively increase, suggesting activation of these genes. Whatever the underlying mechanism, the unrestrained deposition of certain collagens in GBM may contribute to glomerulosclerosis in Alport syndrome.
COL4A3
Gene structure. COL4A3 contains 52 exons (NM_000091.3). For a detailed summary of gene and protein information, see Table A, Gene.
Benign variants. Several benign variants in COL4A3 have been described.
Pathogenic variants. Relatively few COL4A3 pathogenic variants have been reported. However, pathogenic variants in this gene appear to exhibit the same variety as seen in COL4A5, with a predilection for glycine substitutions in the collagenous domains of the collagen α3(IV) chain.
Normal gene product. The gene products of COL4A3, COL4A4, and COL4A5 are, respectively, the α3, α4, and α5 chains of type IV collagen [α3(IV), α4(IV), and α5(IV)]. The six type IV collagen α chains share basic structural features and show extensive sequence homology. The major structural features of α(IV) chains are: a collagenous domain of approximately 1400 residues containing the repetitive triplet sequence glycine (Gly)-X-Y, in which X and Y represent a variety of other amino acids; a carboxy-terminal non-collagenous (NC1) domain of approximately 230 residues; and a non-collagenous aminoterminal sequence of 15-20 residues. Approximately 20 interruptions of the collagenous triplet sequence are present in the collagenous domain. The NC1 domains each contain 12 completely conserved cysteine residues, which participate in intrachain and interchain disulfide bonds.
Type IV collagen chains form heterotrimers through associations between their COO- NC1 domains, associated with folding of the collagenous domains into triple helices.
Type IV collagen heterotrimers form networks through several types of intermolecular interaction. These include end-to-end linkages between the COO- NC1 domains of two heterotrimers, covalent interactions between four heterotrimers at their NH- ends, and lateral associations between heterotrimers via binding of the COO- domains to sites along the collagenous region of another heterotrimer. Disulfide bonds involving conserved cysteine residues are critical to the interactions between NC1 domains. These various linkages between type IV collagen molecules produce a nonfibrillar polygonal assembly that serves as scaffolding for the deposition of other matrix glycoproteins and for cell attachment.
Abnormal gene product. The abnormalities of type IV collagen expression observed in individuals with XLAS and ARAS indicate that a pathogenic variant affecting one of the chains involved in the putative collagen 3- 4- 5(IV) network can prevent basement membrane expression not only of that chain but of the other two chains as well. Similarly, a pathogenic variant involving the collagen 5(IV) chain can interfere with basement membrane expression of collagen 6(IV). The mechanisms that produce these effects remain under investigation. It is likely that at least some pathogenic variants interfere in various ways with the formation of trimeric type IV collagen molecules, leading to degradation of normal chains that have been prevented from forming trimers or that have formed abnormal trimers. This kind of process accounts for abnormal type I collagen deposition in bone in osteogenesis imperfecta. Most of the available data suggest that COL4A5 pathogenic variants do not suppress transcription of COL4A3, COL4A4, and COL4A6.
COL4A4
Gene structure COL4A4 includes 48 exons (NM_000092.4). For a detailed summary of gene and protein information, see Table A, Gene.
Benign variants. Several benign variants in COL4A4 have been described.
Pathogenic variants. Relatively few COL4A4 pathogenic variants have been reported. However, pathogenic variants in this gene appear to exhibit the same variety as COL4A5, with a predilection for glycine substitutions in the collagenous domains of the collagen α4(IV) chain.
Normal gene product. See COL4A3.
Abnormal gene product. See COL4A3.
COL4A5
Gene structure. COL4A5 consists of 51 exons. For a detailed summary of gene and protein information, see Table A, Gene.
Benign variants. A variety of benign variants in COL4A5 have been described.
Pathogenic variants. Of the several hundred reported COL4A5 pathgoenic variants, an estimated 20% are large rearrangements, predominantly deletions [Jais et al 2000]. Missense variants account for approximately 35%-40%, approximately 15% are splice-site variants, and 25%-30% are nonsense variants or small frameshifting deletions or insertions that result in premature stop codons.
The great majority of missense COL4A5 variants 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 the collagen α5(IV) chain by another amino acid. Such pathogenic variants are thought to interfere with the normal folding of the mutant collagen α5(IV) chain into triple helices with other type IV collagen α chains. Glycine lacks a side chain, making it p.Cys1564Ser (c.4692G>A) 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. 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 collagen α1(I) chain account for the majority of pathogenic variants causing osteogenesis imperfecta and are common in other genetic disorders of collagen. Abnormally folded collagen triple helices exhibit increased susceptibility to proteolytic degradation. The position of the substituted glycine, or the substituting amino acid itself, may influence the effect of the pathogenic variant on triple helical folding and ultimately the impact of the pathogenic variant on the severity of the clinical phenotype.
Table 3.
Selected COL4A5 Pathogenic Variants
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DNA Nucleotide Change |
Predicted Protein Change |
Reference Sequences |
c.4692G>C |
p.Cys1564Ser |
NM_000495.3 NP_000486.1 |
c.4946T>G |
p.Leu1649Arg |
c.5030G>A |
p.Arg1677Gln |
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Note on variant classification: Variants listed in the table have been provided by the author. GeneReviews staff have not independently verified the classification of variants.
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Note on nomenclature: GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen.hgvs.org). See Quick Reference for an explanation of nomenclature.
Normal gene product. See COL4A3.
Abnormal gene product. Rare missense variants in COL4A5 involve critical residues in the carboxy-terminal NC1 domain of the collagen α5(IV) chain — for example, one of the twelve conserved cysteine moieties. The loss of one of these cysteines would eliminate a disulfide bond, which could interfere with the formation of triple helices, or with the construction of networks involving collagen α5(IV) chains.