Administrative Office St. Joseph's Hospital Site, L301-10 50 Charlton Avenue East HAMILTON
Administrative Office
St. Joseph's Hospital Site, L301-10
50 Charlton Avenue East
HAMILTON, Ontario, CANADAL8N 4A6
PHONE: (905) 521-6141
FAX: (905) 521-6142
Smith-Lemli-Opitz Syndrome: Molecular Diagnostic Service
The typical patient with SLOS has a characteristic facial phenotype (microcephaly, short nose, epicanthal folds, and small chin), various degrees of cleft palate, syndactyly of toes 2 and 3, feeding difficulties, failure to thrive, behavioral problems and mental retardation, ranging from profound to borderline. In males, anomalous external genitalia are common and the severity of malformations may extend from the mild hypospadias, undescended testes, and micropenis to complete sex reversal in males (i.e., male chromosomes with female external genitalia). Behavioral abnormalities include hyperactivity, self-injury, abnormal sleep patterns, tactile hypersensitivity of hands and feet, hypersensitivity to certain types of sound and visual stimuli, and food aversion. Autistic-spectrum behaviors feature prominently in the SLOS phenotype. Medical complications in patients with SLOS include cholestatic liver disease, adrenal insufficiency, recurrent infections, gastrointestinal allergies, gastroesophageal reflux, pyloric stenosis, and skin photosensitivity. Growth failure is common. The clinical diagnosis of SLOS requires confirmation by demonstrating elevated serum levels of 7DHC. Retrospective or postmortem biochemical diagnosis can be performed using frozen or formalin preserved tissue samples, neonatal blood spots, stored amniotic fluid (AF), or plasma. For additional information about the clinical aspects of SLOS please see the Quarterly Newsletter Issue No. 44, Summer 1998.
The DHCR7 gene was identified and characterized in 1998. DHCR7 maps to chromosome 11q13, spans approximately 14 kb, and encodes a protein of 475 amino acid residues. DHCR7 contains nine exons, the first two of which are non-coding. The DHCR7 mRNA transcript is present in all body tissues although it is most abundant in the adrenal gland, liver, testes, and brain. The predicted molecular mass of normal DHCR7 is 55kD and it is structurally related to many other plant and sterol reductases. This protein is localized in the membrane of the endoplasmic reticulum and has nine putative transmembrane domains. The large fourth cytoplasmic loop likely contains the active site of the enzyme and the binding site for NADPH. The C-terminus is predicted to be within the lumen of the endoplasmic reticulum (ER). To date, 84 different DHCR7 mutations have been identified in more than 200 SLOS patients. More than
85% of the known SLOS alleles are missense mutations that alter conserved amino acid residues located within or near the putative transmembrane domains, or within the highly conserved 4th cytoplasmic loop or C-terminal ER domains. The remaining SLOS alleles include a small number of frameshift mutations, nonsense mutations, and splice site mutations.
For rapid and reliable detection of the most common SLOS mutations, we have developed a panel of allele-specific assays based on the amplification refractory mutation system (ARMS). These tests are designed to detect the five most common SLOS alleles, specifically IVS8-1G® C (g.9447G>C), T93M (c.278C>T), W151X (c.453G>A), V326L (c.976C>T), and R404C (c.1210C>T). Collectively, these account for approximately 70% of the mutant alleles in SLOS patients. IVS8-1G® C is the most common among Caucasians of northwestern European descent, and approximately 1 in 100 individuals in the North American Caucasian population are carriers of this mutation. In central European populations, the W151X mutation reaches similarly high frequencies. R404C is common among French Canadians, T93M is common among those of Mediterranean descent, and V326L is present in several populations. Automated fluorescent DNA sequencing of the exons and exon/intron boundaries are used to identify rare or private DHCR7 mutations. This sequencing strategy detects >95% of all known SLOS alleles. Since first offering the diagnostic service in 2000, we have identified the DHCR7 mutations in more than fifty SLOS patients and their families. Many of the Canadian SLOS patients have been identified with the help of the Canadian Paediatric Surveillance Program. Twenty different DHCR7 mutations have been encountered, including two mutations that had not been reported in the literature. We have also determined the carrier frequency for the most common SLOS mutation in the Canadian population: approximately 1% of all Caucasian Canadians carry the IVS8-1G® C mutation, suggesting that approximately 1 in 60 Caucasian Canadians are carriers for SLOS.
We have established a comprehensive clinical service for patients with SLOS and their families. Biochemical testing for 7DHC is used to confirm suspected cases of SLOS, followed by DNA testing to identify the specific DHCR7 mutations. Subsequently DNA testing can be used to identify carriers of SLOS mutations, and thereby identify couples at risk for having affected children. Clinical genetic consultation and follow up of patients with SLOS, the initiation and monitoring of cholesterol therapy, and comprehensive medical care for patients with SLOS are available through the McMaster University Clinical Genetic Clinic. Genetic counseling for the nuclear and extended families is provided by a genetic counselor with an expertise in SLOS. Prenatal diagnosis of SLOS can be made through biochemical measurement of 7DHC levels in amniotic fluid after 15 weeks of gestation. Direct mutation analysis can be performed using cells from amniotic fluid, cultured amniocytes, or cultured cells obtained by chorionic villus sampling. Prenatal counseling and diagnosis are available through the Prenatal Diagnosis Clinic with the back up of the clinical geneticist specializing in SLOS and the biochemical and molecular laboratories. To date, prenatal diagnosis has been provided for several families at risk for having children with SLOS.
For additional information regarding the SLOS service at McMaster University please contact Ms. Laura Hunnisett, M.Sc., Genetic Counselor, McMaster University Medical Center, Rm 3N20, 1200 Main St. W., Hamilton, Ontario, L8S 4J9; e-mail: ; Phone: 905.521.5085; Fax: 905.521.2651.
Malgorzata J.M. Nowaczyk, MD, FRCPC, FCCMG
John S. Waye, PhD
Discipline of Genetics
Hamilton Regional Laboratory Medicine Program