Alport syndrome, a serious genetic condition, is characterized by kidney disease, hearing loss, and eye abnormalities. There are several types of Alport syndrome, depending on where the disease-causing mutation — in type 4 collagen genes — is located in the genome.
Chromosomes and inheritance
The human genome, which contains the information necessary to make all the proteins in the body (and much more), is packaged into 23 discrete chromosome pairs (46 chromosomes) in every cell. So each gene is present as two copies: one in each of the chromosome pairs. A mutation in one gene copy in one chromosome does not always cause a disease if the other gene copy (in the paired chromosome) is healthy.
The sex chromosomes, X and Y, determine a person’s biological gender – two copies of the X chromosome mean a person will be born female, while one copy of the X chromosome and one of the Y chromosome means a person will be born male.
Other chromosomes are called autosomal chromosomes, and do not determine gender.
Inherited mutations can be present on a sex chromosome or on one or more of the autosomal chromosomes.
X-linked Alport syndrome (XLAS)
XLAS is the most common type of Alport syndrome, representing about 80 percent of diagnosed cases. In XLAS, the mutated disease-causing gene is situated on the X-chromosome (COL4A5). Men, having only one X-chromosome, are affected more severely, because they have only one copy of this gene.
Because women have two X-chromosomes, they usually need to inherit a disease-causing gene from both parents to develop Alport, which is less likely to occur. Most often, women with XLAS only inherit one copy of the disease-causing mutation, which results in much milder symptoms. But they will be disease carriers, and have a 50 percent chance of passing Alport to their children.
Men with XLAS will develop kidney failure at some point in their lives. For women, kidney failure is less likely but can occur.
Autosomal recessive Alport syndrome (ARAS)
ARAS is the second most common type of Alport syndrome, affecting the genes COL4A3 and COL4A4 on chromosome 2. As it is an autosomal recessive disease here, people only develop this type of Alport if they inherit two copies of the disease-causing mutation, one from each parent, on the autosomal chromosomes. A patient’s parents may or may not have disease symptoms, depending on whether they themselves have one or two copies of the disease-causing mutation.
Autosomal recessive disease affects men and women equally.
Autosomal dominant Alport syndrome (ADAS)
ADAS is the rarest form of Alport syndrome, found in about 5 percent of all cases. A single inherited copy of the mutated gene — also COL4A3 and COL4A4 — can cause the disease. Generally, at least one parent will have shown symptoms of Alport syndrome. ADAS is comparable to XLAS in symptoms, although men and women experience similar disease progression and severity, and kidney failure typically occurs later in life.
Other information
The type of Alport syndrome does not change the diagnosis or treatment of the disease, but affects the risk of patients passing the disease onto their children.
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