Cytochrome P450 Family 7 Subfamily B Member 1
The CYP7B1 gene is a member of the cytochrome P450 gene family. Enzymes produced from cytochrome P450 genes are involved in the formation and breakdown of various molecules and chemicals within cells. The CYP7B1 gene provides instructions for making an enzyme called oxysterol 7-alpha-hydroxylase. This enzyme is produced primarily in the liver and the brain. In the liver, oxysterol 7-alpha-hydroxylase is involved in the pathway that breaks down a waxy, fat-like substance called cholesterol to form a bile acid called chenodeoxycholic acid. Bile acids are a component of a digestive fluid called bile that digests fats.
In the brain, oxysterol 7-alpha-hydroxylase is also involved in a pathway that converts cholesterol to hormones called neurosteroids. Neurosteroids increase nerve cell activity (excitability) and promote cell survival and communication between nerve cells. The enzyme primarily converts the neurosteroid dehydroepiandrosterone (DHEA) into 7-hydroxy-DHEA. Oxysterol 7-alpha-hydroxylase helps maintain normal cholesterol levels in the brain and, by producing neurosteroids through altering existing hormones within the pathway, regulates the effects of neurosteroids on the brain.
At least 37 mutations in the CYP7B1 gene have been found to cause spastic paraplegia type 5A. This condition is characterized by muscle stiffness (spasticity) and severe weakness of the lower limbs (paraplegia), typically beginning in adolescence. Most CYP7B1 gene mutations change single protein building blocks (amino acids) in the oxysterol 7-alpha-hydroxylase enzyme. Such changes reduce the enzyme's activity. Other mutations result in a complete loss of functional enzyme.
Reduced oxysterol 7-alpha-hydroxylase enzyme activity does not seem to affect cholesterol breakdown or bile acid production in the liver. Another pathway in the liver can perform these functions, which may explain why reduction of oxysterol 7-alpha-hydroxylase activity does not impact liver function.
In the brain, a decrease in enzyme activity results in an accumulation of cholesterol and alters neurosteroid production triggered by oxysterol 7-alpha-hydroxylase. Abnormal levels of neurosteroids impairs cell survival, likely leading to nerve cell death. The abnormal buildup of cholesterol in the brain probably also contributes to the death of nerve cells. The loss of these cells results in the deterioration of nervous system functions (neurodegeneration) and causes the movement problems, weakness, and other signs and symptoms characteristic of spastic paraplegia type 5A.
cytochrome P450 7B1
cytochrome P450, subfamily VIIB (oxysterol 7 alpha-hydroxylase), polypeptide 1