Alport syndrome is a genetic disorder of type IV collagen that impairs the structure and integrity of the glomerular basement membrane (GBM) and, in doing so, increases the risk of kidney failure. This syndrome has gone through a sea-change, both in our understanding of how genetics impact the disease and how various mutations affect the GBM. No longer can Alport syndrome be described as a disease of boys and young men accompanied by female carriers. Women are at increased risk not only of kidney failure, but extrarenal manifestations as well. Autosomal dominant Alport syndrome (ADAS), once thought to be a rare disorder, is now known to make up a significant portion of cases, including among patients with inherited or drug-resistant forms of focal segmental glomerulosclerosis (FSGS). Additionally, adults are being diagnosed with Alport syndrome at increasing rates as more attention is paid to the varied presentations of Alport syndrome and broader availability of genetic testing. This disorder is no longer in the realm of the pediatric nephrologist alone. All nephrologists must understand how Alport syndrome can present in their patients and be comfortable making the diagnosis and prescribing optimal treatments. This chapter represents a brief review of the pathogenesis, clinical presentation, and current treatments for Alport syndrome.

Alport syndrome is a disorder of type 4 collagen, a variety of collagen that forms a mesh-like network rather than fibrils.¹ It is a main component of the GBM within the glomerular capillary wall.¹ There are six α-chains of type 4 collagen (α1-α6), each encoded by a separate gene (COL4A1-COL4A6). These α-chains come together as trimers; however, there are only three combinations possible among these six chains. The combination of one chain each of α3, α4, and α5 chains (α345) comprises the trimer of type 4 collagen in the mature GBM. The presence of one chain of each protein is required for the others to combine. Thus, if one is not produced because of a mutation, the other two α-chains are degraded and not secreted into the GBM. Once the α-chains trimerize, eight trimers are then combined into a tetrahexamer before secretion by the podocyte¹ (Figure 21.1). These complex properties of type 4 collagen contribute to the GBM’s elasticity and resilience.² The α345 trimer of type 4 collagen is also present in other organs in the body, including the eye and the ear, and this distribution accounts for the extrarenal manifestations of Alport syndrome, including hearing loss and eye abnormalities.³

The genes responsible for producing the α3 and α4 chain of type 4 collagen lie on chromosome 2 and the α5 chain on the X chromosome. Mutations involving COL4A3 or COL4A4 result in autosomal forms of the disease. Mutations in
COL4A5 on the X chromosome result in the X-linked form. The severity of Alport syndrome in each patient depends on several factors: the type of mutation present, the affected gene, and the sex of the patient.⁴

Mutations that lead to lack of production of a protein (eg, large deletions, nonsense mutations) will have the most severe manifestations because no α3, α4, or α5 chains of type 4 collagen are produced.⁵ In this situation, the kidney reverts to producing a trimer of α1 and α2 chains (α112), which is normally present in the fetal kidney but downregulated shortly before birth to be replaced by the α345 network.⁶ The α112 network of type 4 collagen is thinner and less elastic than the α345 network.⁷ This contributes to the thin appearance of the Alport GBM as well as the porous nature of the GBM to red blood cells, leading to microscopic hematuria. Missense mutations, in which a single amino acid is substituted on one of the α-chains, will often lead to the complete α-chain being produced. Missense mutations may negatively impact the ability of the α-chains to trimerize or may alter the ability of the trimer to fold properly. In such cases, the α345 trimer may have difficulty being secreted by the podocyte and can lead to podocyte stress and possible apoptosis of the cell. Other missense mutations allow the trimer to form and the tetrahexamer to assemble and be secreted from the cell into the GBM. Missense mutations in which the α345 network of type 4 collagen is not found
in the GBM will have more severe manifestations than those where the α345 network is present, even if abnormal.⁵ In a large Japanese cohort of patients, those with the α5 chain of type 4 collagen present in the GBM had a median time to kidney failure of 55 years compared to 29 years in those where it was absent.⁵

Mutations in COL4A5 on the X chromosome that lead to X-linked Alport syndrome (XLAS) cause more severe manifestations in males.⁸ As the sex chromosomes in males consist of one X and one Y chromosome, there is unopposed production of the disordered α5 chain. Females with XLAS present in a heterogeneous manner because of X-inactivation, an occurrence at birth in which there is random inactivation of one X chromosome in order to equalize gene dosage. Thus, in women with a mutation in COL4A5, approximately half of cells at birth will still produce normal α5 chains of type 4 collagen. How these cells go on to populate the affected organs, including the kidneys, partially determines the severity of the disease.⁹,¹⁰

Mutations of COL4A3 or COL4A4 on chromosome 2 will lead to autosomal forms of AS. If both copies of the gene are affected, autosomal recessive AS (ARAS) develops. If only one copy of either gene contains a mutation, ADAS may develop. ADAS, for reasons not completely clear, has a heterogeneous presentation. Digenic AS, in which one gene for COL4A3 and one for COL4A4 contains a mutation, can have more severe manifestations than ADAS.¹¹,¹²

The presence of the abnormal type IV collagen proteins in the GBM leads to significant histopathologic changes seen on kidney biopsy, which progress in a fairly uniform manner in males with XLAS. The earliest finding is a uniformly thin GBM seen on electron microscopy because of replacement of the α345 trimers of type 4 collagen with the α112 trimers (Figure 21.2A).¹³ This is followed by areas of intermittent thickening of the GBM as the podocytes add disorganized layers of basement membrane in response to the altered forces transmitted across the abnormal GBM (Figure 21.2B).¹⁴ This leads to the typical finding of intermittent areas of thickening and thinning with lamellations or disorganized GBM layers with empty space between them.¹³ As this happens, podocyte effacement is seen, especially in the areas of GBM thickening.¹³ Mesangial cell filopodia can be seen infiltrating the GBM, possibly another attempt to stabilize the GBM.¹⁵ These
processes induce podocytes and mesangial cells to release cytokines that initiate an acute reparative process, which eventually leads to glomerulosclerosis through the upregulation of pro-fibrotic mediators such as transforming growth factor-β (TGF-β).¹⁶ In the tubulointerstitium, a hallmark feature is the presence of foam cells on light microscopy (LM) (Figure 21.3). These lipid-laden macrophage-derived cells often co-localize with recruited T-cells.¹⁷ Eventually, tubulointerstitial fibrosis is seen as kidney failure progresses.

In recent years, the term “thin basement membranes” has been encouraged to be used as a pathologic description and not as the name of a disease. Individuals with thin basement membranes identified on electron microscopy may have early Alport syndrome because of a COL4A5 variant or may have ADAS because of a heterozygous variant in COL4A3 or COL4A4. Identifying both of these examples as someone with “thin basement membrane disease,” which was previously thought to have a benign course with essentially no risk of progression, may lead to lack of appropriate follow-up and monitoring of at-risk patients.⁴ Thus, all patients with pathogenic variants in COL4A3, COL4A4, or COL4A5 should be described as having Alport syndrome with variable penetrance.

Routine immunofluorescence microscopy is usually nonspecific. Specific staining for type 4 collagen chains is widely available. Males with XLAS and any patient with ARAS in whom the type IV collagen α345 trimer is not secreted from the cell will demonstrate negative staining for any of these three chains. Women with XLAS may have a mosaic pattern of type 4 collagen α345 staining reflecting X-inactivation.³ Approximately 30% of males with XLAS who have mutations that lead to production of an abnormal α345 trimer that can still be secreted into the GBM have normal immunostaining; thus, presence of α345 chains of type 4 collagen by immunofluorescence microscopy does not exclude the diagnosis of Alport syndrome.⁷ Patients with ARAS will demonstrate staining for the α5 chain of type 4 collagen in Bowman capsule as a component of the α556 trimer within
this structure. Finally, patients with ADAS have normal immunostaining for α345 chains of type IV collagen.