Type IV collagen
Collagen IV (ColIV or Col4) is a type of collagen found primarily in the basal lamina. The collagen IV C4 domain at the C-terminus is not removed in post-translational processing, and the fibers link head-to-head, rather than in parallel. Also, collagen IV lacks the regular glycine in every third residue necessary for the tight, collagen helix. This makes the overall arrangement more sloppy with kinks. These two features cause the collagen to form in a sheet, the form of the basal lamina. Collagen IV is the more common usage, as opposed to the older terminology of "type-IV collagen". Collagen IV exists in all metazoan phyla.
There are six human genes associated with it:
The alpha-3 subunit (COL4A3) of collagen IV is thought to be the antigen implicated in Goodpasture syndrome, wherein the immune system attacks the basement membranes of the glomeruli and the alveoli upon the antigenic site on the alpha-3 subunit becomes unsequestered due to environmental exposures.
Goodpasture syndrome presents with nephritic syndrome and hemoptysis. Microscopic evaluation of biopsied renal tissue will reveal linear deposits of Immunoglobulin G by immunofluorescence. This is classically in young adult males.
Mutations to the genes coding for collagen IV lead to Alport syndrome. This will cause thinning and splitting of the glomerular basement membrane. It will present as isolated hematuria, sensorineural hearing loss, and ocular disturbances and is passed on genetically, usually in an X-linked manner, although there are rarer autosomal forms.
Liver fibrosis and cirrhosis are associated with the deposition of collagen IV in the liver. Serum Collagen IV concentrations correlate with hepatic tissue levels of collagen IV in subjects with alcoholic liver disease and hepatitis C and fall following successful therapy.
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