The chromosomes in our cells carry our genes which occur in pairs and are instructions to make proteins. One copy of each gene is inherited from the mother, and the other from the father. SLOS is caused by having a harmful change (mutation) in both copies of the DHCR7 gene. This causes lack of an enzyme (7-dehydrocholesterol reductase) which is needed to make cholesterol. Cholesterol is a waxy substance that is important for making cell structures. Cholesterol is necessary for development before and after birth. Mistakes in DHCR7 also leads to a buildup of 7-dehydrocholesterol (7-DHC) which is toxic to the body. Buildup of 7-DHC along with low cholesterol cause the features of SLOS.
SLOS is an autosomal recessive genetic condition. Recessive genetic conditions 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 on 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% with 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.
Parents who are close relatives (consanguineous) have a higher chance than unrelated parents to both carry the same harmful gene change, which increases the risk to have children with a recessive genetic condition.