Alport syndrome is a genetic disorder characterized by glomerulonephritis, endstage kidney disease, and hearing loss. Alport syndrome can also affect the eyes. The presence of blood in the urine (hematuria) is almost always found in this condition.
It was first identified in a British family by Dr. Cecil A. Alport in 1927.
Alport syndrome is caused by mutations in COL4A3, COL4A4, and COL4A5, collagen biosynthesis genes. Mutations in any of these genes prevent the proper production or assembly of the type IV collagen network, which is an important structural component of basement membranes in the kidney, inner ear, and eye. Basement membranes are thin, sheet-like structures that separate and support cells in many tissues. When mutations prevent the formation of type IV collagen fibers, the basement membranes of the kidneys are not able to filter waste products from the blood and create urine normally, allowing blood and protein into the urine. The abnormalities of type IV collagen in kidney basement membranes cause gradual scarring of the kidneys, eventually leading to kidney failure in many people with the disease.
Alport syndrome can have different inheritance patterns that are dependent on the genetic mutation.
In most people with Alport syndrome, the condition is inherited in an X-linked pattern, due to mutations in the COL4A5 gene. A condition is considered X-linked if the gene involved in the disorder is located on the X chromosome. In males, who have only one X chromosome, one altered copy of the COL4A5 gene is sufficient to cause severe Alport syndrome, explaining why most affected males eventually develop kidney failure. In females, who have two X chromosomes, a mutation in one copy of the COL4A5 gene usually results in blood in the urine, but most affected females do not develop kidney failure. A striking characteristic of X-linked inheritance is that fathers cannot pass X-linked diseases to their sons.
Alport syndrome can be inherited in an autosomal recessive pattern if both copies of the COL4A3 or COL4A4 gene, located on chromosome 2, have been mutated. Most often, the parents of a child with an autosomal recessive disorder are not affected but are carriers of one copy of the altered gene.
Criteria for the clinical diagnosis
Gregory et al, 1996, give the following 10 criteria for the diagnosis of Alport syndrome, 4 of the 10 criteria must be met:
Family history of nephritis of unexplained haematuria in a first degree relative of the index case or in a male relative linked through any numbers of females.