Alport syndrome is an inherited disorder caused by a mutation in one of three type 4 collagen genes. These genes provide the instructions for making part of the type 4 collagen protein, which is essential to the structure and function of glomerular basement membranes, present in the kidneys, ears, and eyes.
Autosomal recessive Alport syndrome (ARAS) is one of three types of Alport syndrome, based on where the mutation is, and which gene is affected. The others are X-linked Alport syndrome (XLAS) and autosomal dominant Alport syndrome (ADAS).
ARAS is the second most common form of Alport syndrome, after XLAS. It is predicted to be the cause of about 15 percent of Alport syndrome cases.
The inheritance pattern of ARAS
ARAS is caused by mutations in either the COL4A3 or the COL4A4 genes. These genes are located on chromosome 2, of which both men and women have two copies. Therefore, everyone has two copies of each of these genes, one inherited from the mother and one from the father. A person develops ARAS if both copies of the same gene are faulty. If a person has only one mutated copy of COL4A3 or COL4A4, they are referred to as a carrier. A carrier will normally have no or very minor symptoms but a 50 percent risk of passing on this mutation to their children. If both parents are a carrier of the same mutation, their child has a 25 percent risk of developing ARAS, regardless of gender.
Clinical characteristics of ARAS
The clinical characteristics of ARAS are very similar to those of XLAS. However, unlike XLAS, the severity of the disease does not differ between men and women. The severity and speed of disease progression may differ due to the exact type of mutation in COL4A3 or COL4A4.
One of the first symptoms of ARAS is hematuria, or blood in the urine. Patients later develop kidney disease, which generally progresses faster than in ADAS. In most cases, this progresses to end-stage kidney disease or kidney failure by the time patients are in their mid-teens or early adulthood.
Progressive hearing loss can begin in late childhood or early adolescence. It often affects both ears, and hearing aids from a young age can often help patients.
Vision is not always affected, but patients may develop eye abnormalities. These can include:
- Anterior lenticonus, where an abnormally shaped lens can lead to cataract formation and vision problems;
- Corneal erosions, which affect the outer clear layer of the eye and which can lead to blurred vision, discomfort or pain, and light sensitivity
A carrier may also develop thin basement membrane nephropathy. This is characterized by hematuria, but otherwise, the patient experiences no problems with kidney function in general.
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