Classical Alport's syndrome is an X-linked disease and is the most common form of Alport's syndrome (90% of patients), with an overall incidence of Alport's syndrome in the United States of 1 : 5000 to 1 : 10,000 (1-4). Patients show hematuria in childhood with progressive hearing loss in one third, and ocular defects and progression to renal failure in 30% to 40% by early adulthood. Anterior lenticonus is the most common eye defect.
Alport syndrome is due to mutations of collagen type IV (3-6). Collagen type IV is made up of heterotrimers of alpha chains. These six alpha chains are encoded by genes arranged in pairs on three different chromosomes: COL4A1 and COL4A2 are on chromosome 13; COL4A3 and COL4A4 are on chromosome 2; and COL4A5 and COL4A6 are on the X chromosome. The mutation in the classic form of Alport's occurs in the a5 (IV) collagen chain (COL4A5). The autosomal recessive form accounts for most of the remaining patients, and is due to mutations in both alleles of a3 or a4 type IV collagen genes (COL4A3 or COL4A4). Rare cases of autosomal dominant Alport's due to heterozygous mutations in COL4A3 or COL4A4 also occur, with a highly variable clinical course and reduced penetrance (7). Alport's syndrome and coexisting diffuse leiomyomatosis is linked to large gene deletions that span the adjacent 5' ends of the adjacent COL4A5 and COL4A6 genes (5).
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