7-Dehydrocholesterol is a biosynthetic precursor to previtamin D 3 in
human skin and to cholesterol in tissues. Although the level of
7-dehydrocholesterol is normally low in human tissues and fluids, it is
significantly elevated in a number of human disorders, including Smith
Lemli-Opitz syndrome (SLOS) and cerebrotendinous xanthomatosis.
The Smith-Lemli-Opitz syndrome (SLOS) is a common birth defect syndrome
characterized biochemically by low plasma cholesterol levels and high
concentrations of the cholesterol precursor 7dehydrocholesterol. It
is a metabolic and developmental disorder caused by mutations
in the gene encoding the enzyme 7-dehydrocholesterol reductase
(Dhcr7).
In SLOS, the tissue cholesterol and total sterol levels are markedly
reduced while the concentrations of 7-dehydrocholesterol are greatly
elevated. This elevation in 7-dehydrocholesterol inhibits the activity
of Hmgcr, thus further exacerbating the cholesterol deficit.
Cholesterol in mammals is synthesized from acetate with the final two
steps being the conversion of lathosterol to 7-dehydrocholesterol and
7-dehydrocholesterol to cholesterol.
7-Dehydrocholesterol
