Collagens are a family of proteins that are collectively the most abundant protein in the body, and can be found throughout the various connective tissues.
Each member of the family is named with a Roman numeral, and if mutated or absent, can lead to problems in the tissues where that particular collagen is found.
Alport syndrome occurs as a result of mutations in Type IV collagen, which is particularly important in the glomerulus of the kidney, the eye, and the cochlea, and that’s why the symptoms of Alport syndrome are specific to those tissues.
Type IV collagen is a sheet-like structure found in all basement membranes and serves to support cells and form barrier.
The three basement membrane layers are the lamina lucida, lamina densa (where type IV collagen is), and lamina reticularis.
Now within the kidneys, there are glomeruli, which filter the blood and that together with a tubule forms a nephron.
These glomeruli happen to have a basement membranes, called the glomerular basement membrane, or GBM, and that GBM, along with the fenestrated, meaning has pores, capillary endothelium and the podocyte slit diaphragm, forms a selective filter, meaning that water and certain other plasma components can escape the capillary, forming the filtrate that will become urine, but red blood cells and most proteins stay in the glomerular capillary.
In Alport syndrome, kidney function is normal through early childhood, but over time, the missing or nonfunctional type IV collagen causes the GBM to become thin and overly porous.
This allows red blood cells to pass right through from the capillary to the urinary filtrate leading to microscopic hematuria, which is where red blood cells are seen in the urine under a microscope, and this might eventually lead to gross hematuria, where the red blood cells can be seen with the naked eye.
Over time, excessive amounts of protein start to get through the filter, resulting in proteinuria, or protein in the urine.