ATPase H+ transporting V0 subunit a4
The ATP6V0A4 gene provides instructions for making a part (subunit) of a large protein complex known as vacuolar H+-ATPase (V-ATPase). V-ATPases are a group of similar complexes that act as pumps to move positively charged hydrogen atoms (protons) across membranes. Because acids are substances that can "donate" protons to other molecules, this movement of protons helps regulate the relative acidity (pH) of cells and their surrounding environment. Tight control of pH is necessary for most biological reactions to proceed properly.
The V-ATPase that includes the subunit produced from the ATP6V0A4 gene is found in the inner ear and in nephrons, which are the functional structures within the kidneys. The kidneys filter waste products from the blood and remove them in urine. They also reabsorb needed nutrients and release them back into the blood. Each nephron consists of two parts: a renal corpuscle (also known as a glomerulus) that filters the blood, and a renal tubule that reabsorbs substances that are needed and eliminates unneeded substances in urine. The V-ATPase is involved in regulating the amount of acid that is removed from the blood into the urine, and also in maintaining the proper pH of the fluid in the inner ear (endolymph).
Several ATP6V0A4 gene mutations have been identified in people with renal tubular acidosis with deafness, a disorder involving excess acid in the blood (metabolic acidosis), bone weakness, and hearing loss caused by changes in the inner ear (sensorineural hearing loss).
Mutations in the ATP6V0A4 gene impair the function of the V-ATPase proton pump. As a result, the kidneys are less able to control the acidity of the blood, which leads to bone weakness caused by loss of bone minerals (demineralization) and other consequences of metabolic acidosis. The body's capability to control the pH of the fluid in the inner ear is also impaired, resulting in sensorineural hearing loss.
ATP6V0A4 gene mutations have also been identified in people with renal tubular acidosis who have normal hearing when diagnosed, usually in infancy or early childhood. However, hearing loss sometimes does not develop until adulthood in renal tubular acidosis with deafness (described above), so some of these individuals are later found to have that disorder.