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Type IV collagen is also an important component of the organ of Corti, an inner ear structure that transforms sound waves into nerve impulses for the brain. Alterations in type IV collagen may result in abnormal inner ear function, which can lead to hearing loss. In addition, type IV collagen plays a role in the eye, where it helps maintain the shape of the lens and the normal color of the retina. Mutations found in Alport syndrome may affect the shape of the lenses and the color of the retina.
In approximately 15 percent of cases, Alport syndrome results from mutations in both copies of the COL4A3 or COL4A4 gene and is inherited in an autosomal recessive pattern. The parents of an individual with the autosomal recessive form of this condition each have one copy of the mutated gene and are called carriers. Some carriers are unaffected and others develop a less severe condition called thin basement membrane nephropathy, which is characterized by hematuria.
Alport syndrome has an autosomal dominant pattern of inheritance in about 5 percent of cases. People with this form of Alport syndrome have one mutation in either the COL4A3 or COL4A4 gene in each cell. It remains unclear why some individuals with one mutation in the COL4A3 or COL4A4 gene have autosomal dominant Alport syndrome and others have thin basement membrane nephropathy.
Most individuals diagnosed with an autosomal dominant Alport syndrome have an affected parent. In some cases, the disorder may result from a de novo gene mutation. The proportion of cases of autosomal dominant Alport syndrome that result from de novo gene mutations in unknown. Each child of an individual with autosomal dominant Alport syndrome has a 50% chance of inheriting the mutation.
Additional information about the different ways that Alport syndrome may be inherited (and how other family members might be impacted) can be accessed through GeneReviews. A genetics professional can help to better understand the risks for individuals and family members to develop Alport syndrome.