Smith-Lemli-Opitz syndrome (OMIM 270400) (SLOS) is caused by inherited enzy
matic deficiency of 3 beta -hydroxysterol-Delta (7)-reductase (7-dehydrocho
lesterol-Delta (7)-reductase, DHCR7). SLOS is diagnosed clinically by the d
emonstration of elevated levels of 7-dehydrocholesterol (7DHC) in body flui
ds or tissues. SLOS is associated with mental retardation of variable degre
e and severe behavior abnormalities. The physical abnormalities range from
minor facial anomalies to lethal malformations of the central nervous syste
m, heart, kidneys, and other organs. The exact incidence of SLOS is not kno
wn. Although there exist estimates of the incidence of SLOS ranging from 1
in 20,000 to 1 in 60,000, no prospective studies of the incidence of SLOS,
based on the clinical data and biochemical diagnosis of SLOS, have been per
formed. Five unrelated cases of SLOS were diagnosed in Ontario during a 12-
month period. The diagnoses were made based on the demonstration of elevate
d 7DHC in plasma or amniotic fluid. The birth rate for Ontario for that per
iod was 132,000 births, The incidence of SLOS in Ontario was at least 1 in
26,500 pregnancies in 1999-2000. Given that 86% of the population of Ontari
o is of European origin, the incidence of SLOS in the Ontario population of
European origin was at least 1 in 22,700. As infants with mild forms of SL
OS born during this period may remain undiagnosed, these numbers likely are
underestimates. This observation has implications for prenatal and newborn
screening for this potentially treatable inherited disorder. (C) 2001 Wile
y-Liss, Inc.