Alport syndrome is a genetic condition which causes kidney disease, hearing loss, and eye abnormalities. The disease was first characterized by A. Cecil Alport, a British doctor, in 1927 and affects approximately 1 in 50,000 newborns.
Causes of Alport syndrome
Alport syndrome can be caused by a mutation in the genes COL4A3, COL4A4, and COL4A5, which each provide instructions to make a portion of a protein called type 4 collagen, a connective tissue protein.
There are three types of Alport syndrome: X-linked Alport syndrome (XLAS), autosomal recessive Alport syndrome (ARAS), and autosomal dominant Alport syndrome (ADAS). Each type has a different pattern of inheritance.
The most common form of Alport syndrome is X-linked or sex-linked Alport, meaning that the mutated gene is located on the X-chromosome. The symptoms of this type of Alport syndrome are more severe in men than in women, since men only have one X chromosome.
Symptoms of Alport syndrome
Type 4 collagen is involved in the formation of small structures in the kidney called the glomeruli, which are responsible for filtering the blood and producing urine. Mutations in genes encoding for type 4 collagen allow blood and protein into the urine and cause gradual scarring of the kidney which can lead to kidney disease and kidney failure. Patients with Alport syndrome always have kidney symptoms, usually proteinuria (protein in the urine) or hematuria (blood in the urine). These symptoms usually show up when patients are children.
Type 4 collagen is also important in the organ of Corti, a structure in the inner ear, which transforms sound into nerve impulses. Mutations in the genes encoding for type 4 collagen cause this structure to be formed abnormally, leading to hearing loss.
However, this symptom is never present at birth in patients with Alport syndrome and only develops as the children age, usually appearing in late childhood or early adolescence. Most patients with this symptom are deaf by age 40.
In the eyes, type 4 collagen is responsible for maintaining the normal color of the retina and the shape of the lens. Abnormal lens shape occurs in about 15 to 20 percent of Alport syndrome patients. About 5 percent of patients may also develop a hole in the macular region of the retina, which causes central vision to become hazy or distorted.
Diagnosis of Alport syndrome
There are several methods of diagnosing Alport syndrome. A complete medical history and physical exam can indicate the early symptoms of the disease. Blood and urine analysis can indicate abnormalities in how the body filters waste products.
Skin or kidney biopsies can be used to test for abnormalities in type 4 collagen by staining the tissue for the protein and examining the tissue under a microscope.
Hearing tests can determine whether patients are experiencing hearing loss, though this test on its own cannot definitively point to Alport syndrome.
The lens and retina abnormalities caused by Alport syndrome are visible in a dilated eye exam.
The surest method of diagnosing Alport syndrome is genetic testing for mutations in COL4A3, COL4A4, and COL4A5 genes. This genetic test can be done on a tissue biopsy, a kidney biopsy, or a blood sample.
Treatment of Alport syndrome
Treatments for Alport syndrome are targeted toward maintaining kidney function and limiting the formation of scar tissue. Angiotensin-converting enzyme (ACE) inhibitors and statins can be useful treatments, but many patients will develop kidney disease and need either dialysis or a kidney transplant.
Prognosis of Alport syndrome
Most male Alport syndrome patients reach end-stage kidney disease and kidney failure by their 40s or 50s, which can reduce lifespan if not managed with dialysis or a kidney transplant. Most women with Alport syndrome have a normal lifespan.
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