Bjornstad syndrome is caused by changes (mutations) in the BCS1L gene. This gene codes for a chaperone protein that is a member of the AAA family of ATPases. An ATPase is an enzyme that uses ATP (adenosine triphosphate), the main energy source in cells, to drive chemical reactions. This particular ATPase is involved in the assembly of complex III in the mitochondrial electron transport chain. The electron transport chain is responsible for generating the energy that cells require. Complex III also produces reactive oxygen species. When present in high numbers, these reactive compounds will cause damage to tissue. Even though this abnormal protein leads to a decrease in the activity of complex III, the complex will produce more reactive oxygen species.
The hair follicles and cells of the inner ear are particularly sensitive to these reactive oxygen species. This is thought to be the reason why hair changes and hearing loss are part of this syndrome.
Bjornstad syndrome is inherited in an autosomal recessive pattern. Recessive genetic disorders occur when an individual inherits a non-working gene from each parent. If an individual receives one working gene and one non-working gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. The risk for two carrier parents to both pass the non-working gene and, therefore, have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier, like the parents, is 50% for each pregnancy. The chance for a child to receive working genes from both parents is 25%. The risk is the same for males and females.