Smith–Lemli–Opitz syndrome: Phenotype, natural history, and epidemiology^†

INTRODUCTION

Smith–Lemli–Opitz syndrome (OMIM 270400) was the first human multiple malformation syndrome attributed to an inborn error of sterol synthesis [Irons et al., 1993; Tint et al., 1994]. The syndrome was first described in 1964 by Smith, Lemli, and Opitz who reported three male patients with distinctive facial features, intellectual disability, microcephaly, developmental delay, and hypospadias [Smith et al., 1964]. The severity of physical defects correlates with the severity of the cholesterol deficiency [Tint et al., 1995; Ryan et al., 1998], whereas behavioral abnormalities occur even in the least physically affected individuals, and can include self-injury, aggressiveness, autistic behaviors, severe sleep cycle disturbances, and hyperactivity [Nowaczyk et al., 1998; Ryan et al., 1998; Tierney et al., 2000].

Deficient cholesterol synthesis in SLOS is caused by abnormally low activity of 3β-hydroxysteroid-Δ7-reductase (7-dehydrocholesterol reductase, DHCR7), the enzyme responsible for conversion of 7DHC to cholesterol and, in a parallel step, of 7-dehydrodesmosterol to desmosterol (Fig. 1). This enzymatic deficiency results in the accumulation of the precursor 7-dehydrocholesterol (7DHC) and its isomer, 8-dehydrocholesterol (8DHC).

Deficient cholesterol synthesis in SLOS is caused by abnormally low activity of 3β-hydroxysteroid-Δ7-reductase (7-dehydrocholesterol reductase, DHCR7), the enzyme responsible for conversion of 7DHC to cholesterol and, in a parallel step, of 7-dehydrodesmosterol to desmosterol (Fig. 1). This enzymatic deficiency results in the accumulation of the precursor 7-dehydrocholesterol (7DHC) and its isomer, 8-dehydrocholesterol (8DHC).

In 1998, three teams characterized the human DHCR7 gene and identified mutations in SLOS individuals [Fitzky et al., 1998; Moebius et al., 1998; Wassif et al., 1998; Waterham et al., 1998]. To date, more than 130 DHCR7 mutations have been reported in individuals with SLOS representing a continuum of clinical severity [http://www.hgmd.cf.ac.uk/ac/gene.php?gene=DHCR7] For a detailed review of the current molecular knowledge, the readers are referred to the article by Hennekam and Waterham in this issue of Seminars in Medical Genetics.

In this review, we update the clinical features of SLOS and its natural history based on the most recent publications in the field of clinical dysmorphology and fetopathology as well as review the current understanding of the pathophysiology of SLOS. For a review of therapeutic approaches to SLOS, the reader is referred to the review by Svoboda et al. in this issue of Seminars in Medical Genetics.

PHENOTYPE

SLOS has protean clinical manifestations of variable severity. The availability of a reference standard diagnostic test has enabled the broadening of the SLOS phenotype so as to include more severely affected individuals with severe malformations, novel malformation patterns (e.g., cyclopia, renal agenesis), individuals with minimal intellectual disability or behavioral abnormalities, and those with subtle clinical findings who had been previously diagnosed with idiopathic intellectual disability, attention deficit-hyperactivity disorder, or autism. More recently, reports have appeared of individuals with normal intelligence with documented DHCR7 mutations and elevated plasma 7DHC. [Eroglu et al., 2011; Nowaczyk, personal observations]. While the majority of familial cases present similar phenotypic severity, significant intrafamilial variability has been observed.

More recently, reports have appeared of individuals with normal intelligence with documented DHCR7 mutations and elevated plasma 7DHC. While the majority of familial cases present similar phenotypic severity, significant intrafamilial variability has been observed.

Before the discovery of the biochemical defect, SLOS was divided into two groups based on clinical severity: a classic form (SLOS type I), and a severe form (SLOS type II) [Bialer et al., 1987; Curry et al., 1987; Donnai et al., 1986]. Phenotypic severity, based on the degree and number of physical malformations, can be quantified using a physical severity score that was initially developed by Bialer et al. [1987] and subsequently modified by Kelley and Hennekam [2000]. The physical severity score is obtained as follows: categories of cerebral, ocular, oral, skeletal, and genital defects are identified (Table I); the scores for the 10 categories are summed, and the scale normalized by dividing by the maximum score of 20 and multiplying by 100. Based on the physical severity score, the SLOS phenotype can be divided into three categories: mild (<20), classical (20–50), and severe (>50). A strong correlation of the severity score was found with biochemical parameters [Cunniff et al., 1997; Ryan et al., 1998; Kratz and Kelley, 1999; Nowaczyk et al., 2012]. There exists an inverse correlation between serum concentration of cholesterol and clinical severity [Tint et al., 1995; Cunniff et al., 1997; Yu et al., 2000b; Nowaczyk et al., 2012], and mortality is particularly high in patients with the lowest cholesterol concentrations (<7 mg/dl) [Tint et al., 1995]. An unusual case of a patient with low 7DHC/cholesterol levels and severe phenotype was recently reported [Koo et al., 2010], which emphasizes the important role of endogenous cholesterol synthesis in embryonic period.

Although the SLOS severity score is useful for measuring the severity of the pattern of malformations in affected individuals, it does not consider the severity of intellectual disability, behavioral disturbances, or feeding difficulties, which are important in the day-to-day management of individuals with SLOS [Krakowiak et al., 2003; Witsch-Baumgartner et al., 2000; Yu et al., 2000a; Langius et al., 2003].

Physical Features

SLOS is most commonly associated with prenatal and postnatal growth retardation, microcephaly, moderate to severe intellectual disability, and multiple major and minor malformations including characteristic facial features, cleft palate, abnormal gingivae, cardiac defects, postaxial polydactyly, 2–3 toe syndactyly, hypospadias, and undervirilization of the genitalia in males [Cunniff et al., 1997; Ryan et al., 1998; Krajewska-Walasek et al., 1999; Kelley and Hennekam, 2000]. Individuals with milder forms may have only minimal facial characteristics, hypotonia, 2–3 toe syndactyly, and mild to minimal developmental delay [Nowaczyk et al., 1998; Eroglu et al., 2011].

The characteristic facial features of severe SLOS are microcephaly, bitemporal narrowing, ptosis, short nasal ridge, short nose with anteverted nares, micrognathia, epicanthal folds, and capillary hemangiomas over the nasal root that extend onto the glabella (Fig. 2). The ears are low-set and are posteriorly rotated but are otherwise normal. Cleft palate is present in 40–50% of affected individuals [Cunniff et al., 1997] and may contribute to feeding and growth problems. The neck is often short with redundant skin at the nape. The characteristic facial appearance may be subtle in some individuals (Fig. 2) but, when assessed objectively, is present in even the least severely affected individuals [Nowaczyk et al., 2012]. The severity of the dysmorphic features correlates with the severity of both the biochemical and physical abnormalities [Nowaczyk et al., 2012]. In individuals with cleft lip/palate or holoprosencephaly the most characteristic facial features of SLOS may be less apparent [Kelley et al., 1996; Worthington and Goldblatt, 1997; Weaver et al., 2010]. Characteristic oral findings include a high and narrow hard palate, broad and ridged alveolar ridges, and redundancy of sublingual tissues [Donnai et al., 1986; Curry et al., 1987; Kelley et al., 1996; Pizzo et al., 2008]. Although submucosal and U-shaped cleft palates are common, cleft lip is uncommon. A mild manifestation of cleft palate that is frequently observed in SLOS is a bifid uvula [Porter and Herman, 2011].

Y-shaped cutaneous syndactyly of the second and third toes is the most common physical finding in SLOS and the most characteristic clinical finding in affected individuals (Fig. 3) [Cunniff et al., 1997; Curry et al., 1987; Ryan et al., 1998; Krajewska-Walasek et al., 1999]. However, syndactyly may be a subtle finding and only noticeable from the plantar aspect [Nowaczyk et al., 1998]. Although considered a normal variant, given the wide phenotypic spectrum for SLOS, syndactyly of the second and third toes in combination with other malformations, behavioral disturbances, or cognitive problems should prompt consideration of SLOS. Occasionally, patients with SLOS also have cutaneous syndactyly of the fingers. Bilateral or unilateral postaxial polydactyly can be present in the hands or feet, or both (Fig. 4), and occurs in 25–50% of affected individuals [Gorlin et al., 1990; Cunniff et al., 1997; Lin et al., 1997]. The thumb may be short and proximally placed, the first metacarpals short and the thenar eminences hypoplastic; the index finger often has a subtle “zigzag” alignment of the phalanges (Fig. 5). Less common findings include clinodactyly, hammer toes, and dorsiflexed halluces [Pinsky and DiGeorge, 1965; Opitz, 1969].

Central nervous system anomalies include agenesis or hypoplasia of the corpus callosum, cerebellar hypoplasia, increased ventricular size, decreased frontal lobe size, and cerebellar malformations [Kelley et al., 1996; Ryan et al., 1998; Caruso et al., 2004]. Microcephaly occurs in 80–84% of individuals with SLOS. Approximately 5% of individuals have various forms of the holoprosencephaly [Chasalow et al., 1985; Kelley et al., 1996; Weaver et al., 2010] including the mildest form such as single central incisor (Fig. 6).

Congenital heart defects occur in approximately 50% of individuals with SLOS [Cunniff et al., 1997; Lin et al., 1997]. Endocardial cushion defects and hypoplastic left heart are most common. Anomalous pulmonary venous return, when compared with an unselected series of individuals with SLOS, also occurs more commonly than in the general population [Lin et al., 1997]. Abnormal pulmonary lobation and pulmonary hypoplasia are common in more severely affected individuals [Donnai et al., 1986; Curry et al., 1987; Bialer et al., 1987; Quélin et al., 2012].

Gastrointestinal malformations associated with SLOS include Hirschsprung disease, pyloric stenosis, and malrotation, especially in more severely affected individuals [Dallaire and Fraser, 1966; Patterson et al., 1983; Lipson and Hayes, 1984; Donnai et al., 1986; Bialer et al., 1987; Curry et al., 1987; Quélin et al., 2012]. Gastroesophageal reflux, formula intolerance, and constipation are frequent day-to-day gastrointestinal problems. Liver disease is variable and can range from severe cholestasis (generally in those who are more severely affected) [Natowicz and Evans, 1994; Ryan et al., 1998; Koo et al., 2010] to mild/moderate stable elevation of liver transaminases [Rossi et al., 2005].

Approximately one quarter of affected individuals have renal anomalies, most commonly renal hypoplasia or agenesis, renal cortical cysts, hydronephrosis, and structural abnormalities of the collecting system [Curry et al., 1987; Ryan et al., 1998; Kratz and Kelley, 1999; Nowaczyk et al., 2001].

The external genitalia in the 46,XY male range from normal to severely undervirilized, with a characteristically female appearance (Fig. 7). Up to 25% of individuals with SLOS have a 46,XY karyotype with a female genital phenotype [Lin et al., 1997]. Hypospadias or bilateral cryptorchidism occurs in 50% of males with SLOS [Gorlin et al., 1990; Lin et al., 1997].

The external genitalia in the 46,XY male range from normal to severely undervirilized, with a characteristically female appearance (Fig. 7). Up to 25% of individuals with SLOS have a 46,XY karyotype with a female genital phenotype. Hypospadias or bilateral cryptorchidism occurs in 50% of males with SLOS.

Internal genitalia of 46,XY individuals with sex-reversal may include a blind vaginal pouch and rudimentary uterus; testes are usually palpable in the inguinal region [Dallaire and Fraser, 1966; Bialer et al., 1987; Donnai et al., 1986; Curry et al., 1987]. Female individuals with SLOS usually have normal genitalia, but there may be hypoplasia of the labia minora and majora. Bicornuate uterus and septate vagina may occur in 46,XX females [Lowry et al., 1968]. Because genital abnormalities are easier to recognize in males than females, males are more likely to be assessed for suspected SLOS. Other genitourinary findings include a persistent urogenital sinus and posterior labial fusion without clitoromegaly in a female with a 46,XX karyotype [Chemaitilly et al., 2003] and precocious puberty in girls with SLOS [Starck et al., 1999; Irons, 2007]. No gonadal dysgenesis was observed in fetopathological examination of fetuses with SLOS of either sex [Quélin et al., 2012].

Congenital cataracts are present in approximately 20% of affected individuals [Finley et al., 1969; Cunniff et al., 1997; Lin et al., 1997] and can develop acutely [Goodwin et al., 2008]. Other ophthalmologic manifestations include ptosis, strabismus, optic atrophy, and optic nerve hypoplasia [Atchaneeyasakul et al., 1998].

Skin photosensitivity, which is common in SLOS, appears to be UVA mediated [Anstey, 2001; Martin et al., 2001]. Photosensitivity can be severe and can result from even brief exposure to sunlight.

NATURAL HISTORY

Prenatal

Prenatal findings of SLOS may include intrauterine growth retardation, major malformations of the brain, heart, kidneys, or limbs, and female-appearing genitalia or severe hypospadias in an XY fetus. Additional non-specific findings may include increased nuchal translucency, cystic hygroma, non-immune hydrops, and cleft palate. Prenatal diagnosis of SLOS may be made following detection of abnormal ultrasound findings, often in combination with very low maternal unconjugated estriol levels [Chasalow et al., 1985; Rossiter et al., 1995]. However, no pattern of malformation is pathognomonic. Furthermore, ultrasound examination may be normal, especially in fetuses affected with mild SLOS. Early detection of multiple malformations was noted in only 10% of one case series [Goldenberg et al., 2004]. Intrauterine growth restriction (IUGR), including isolated IUGR, was the most frequent ultrasound finding, detected in 67–100% of affected fetuses [Goldenberg et al., 2004; Quélin et al., 2012].

The combination of low concentrations of unconjugated estriol, β-HCG, and alpha-fetoprotein on routine maternal serum testing at 15–20 weeks' gestation suggests the possible diagnosis of SLOS [Rossiter et al., 1995]. Low maternal serum concentrations of unconjugated estriol may warrant further investigation, especially when associated with abnormal ultrasonographic findings suggestive of SLOS. However, the estriol level correlates with severity and can be as high as 0.67 MoM in mild forms of the disorder [Kratz and Kelley, 1999].

The pattern of malformations observed in pregnancy terminations differs from that observed in individuals diagnosed with SLOS postnatally: there is a higher incidence of pulmonary segmentation defects, renal anomalies, intestinal malrotation, and CNS anomalies [Quélin et al., 2012]. No anomalies of gonadal differentiation or genital anomalies were observed in this case series. Dysmorphic features characteristic of SLOS can be observed as early as 15 weeks of gestation (Fig. 8) [Quélin et al., 2012].

Infancy and Childhood

Infants and children with SLOS are frequently described as irritable, inconsolable and hypersensitive to stimulation [Nwokoro and Mulvihill, 1997; Ryan et al., 1998; Opitz, 1999]; their sleep pattern is severely disrupted [Ryan et al., 1998; Zarowski et al., 2011].

Abnormalities of intestinal motility (manifest by vomiting, gastroesophageal reflux, feeding intolerance, formula intolerance), food allergies, and pyloric stenosis are frequent and contribute significantly to morbidity [Kelley and Hennekam, 2000; Haas et al., 2005]. Slow growth and poor weight gain are typical. Gastrostomy tube placement is required in many cases. In general, infants with the more severe phenotype have more feeding problems. Constipation is a common problem. At any age, individuals with SLOS are generally below average in height and weight. A hypermetabolic state requiring caloric intake significantly in excess of the calculated need based on weight and level of activity is seen in many individuals [Kelley and Hennekam, 2000]. SLOS-specific growth curves have been established and are available for clinical use [Lee et al., 2012].

Adrenal insufficiency, although rare, can result in severe electrolyte abnormalities, often in the setting of physiological stress [Anderson et al., 1998; Chemaitilly et al., 2003]. However, individuals with mild to moderate SLOS have an adequate glucocorticoid response even in the setting of compensated glucocorticoid insufficiency [Bianconi et al., 2011]. Low serum concentrations of testosterone can occur in severely affected males [Chasalow et al., 1985].

Cardiorespiratory problems can occur secondary to malformations of the heart or respiratory tract, including the trachea or larynx [Leal-Pavey, 2004]. An increased frequency of upper- and/or lower-respiratory infections is seen particularly in infancy and early childhood. Laryngomalacia and tracheomalacia are common and may result in sleep apnea [Nwokoro and Mulvihill, 1997]. Persistent pulmonary hypertension has been reported in one individual with SLOS [Katheria et al., 2010]. Systemic hypertension also can be a clinical problem [Kelley and Hennekam, 2000; Nowaczyk et al., 2001].

Airway access for anesthesia may be difficult because of micrognathia, but no other significant anesthetic problems have been reported [Choi and Nowaczyk, 2000; Kelley and Hennekam, 2000; Matveevskii et al., 2006]. Recurrent infections such as otitis media, upper respiratory tract infections and pneumonia occur commonly, especially in the more severely affected individuals. Hypotonia, central or secondary to muscle hypoplasia, is universal in infancy, but it improves with age often leading to hypertonia, contractures, and orthopedic complications in non-ambulatory children. Many children cannot tolerate any exposure to sunlight; others can tolerate varying periods of exposure if properly clothed and protected with a UVA- and UVB-protection sunscreen.

Depression and other psychiatric complications have been observed in older individuals [Irons, 2007]. Older individuals with SLOS often appear older than their chronological age (Fig. 9).

EPIDEMIOLOGY

SLOS is most prevalent among populations of northern and central European origin. It appears to be extremely rare among Asian [Tsukahara et al., 1998] and African populations. Estimates of the incidence of SLOS vary considerably depending on the diagnostic criteria and the reference population. The incidence of severe, clinically diagnosed SLOS in a completely ascertained population of newborns in the Czech Republic was estimated to be >1 in 10,000 [Opitz, 1999], and the incidence of severe, biochemically confirmed SLOS in Slovakia was estimated to be 1 in 15,000 to 1 in 20,000 [Bzduch et al., 2000]. For more heterogenous populations the incidence is lower and is estimated to be 1 in 20,000 in British Columbia [Lowry and Yong, 1980], 1 in 26,500 in Ontario, Canada [Nowaczyk et al., 2001], 1 in 30,000 in New England [Opitz et al., 1994], and 1:70,000 in the United Kingdom [Ryan et al., 1998]. A prospective population surveillance study in Canada found a minimum incidence of 1 in 70,358 live births, with a minimum prevalence of approximately 1 in 950,000 [Nowaczyk et al., 2004]. Based on biochemical testing, Kelley [1997] estimated an incidence of SLOS of 1:50,000 in the United States. However, these estimates may be artificially low because of ascertainment bias that favors the diagnosis of cases with the classical SLOS phenotype and underestimates the contribution of severe cases that result in fetal demise and mild cases with subtle phenotypes [Langius et al., 2003; Nowaczyk et al., 2006].

Population surveys have found that the carrier frequency of the most common SLOS mutation c.964-1G>C (IVS8-1G>C) is approximately 1% [Battaile et al., 2001; Yu et al., 2000a; Nowaczyk and Waye, 2001]. Given that this mutation represents <30% of the alleles in surveys of affected individuals, the carrier rate for SLOS mutations may be as high as 1 in 30 [Battaile et al., 2001]. In Poland, the combined carrier frequency of p.W151X and p.V326L was found to be 2.4% [Ciara et al., 2006; Jezela-Stanek et al., 2010]. In other European populations, the combined carrier frequency of c.964-1G>C and p.W151X is 1.0–2.3%, with a decreasing westward gradient across Europe [Witsch-Baumgartner et al., 2008].

In a meta-analysis of studies assessing DHCR7 mutation frequencies, Kelley and Herman [2001] found overrepresentation of four common missense alleles and underrepresentation of two null alleles, c.964-1G>C and p.W151X. They concluded that incomplete ascertainment of null alleles due to prenatal or neonatal death likely leads to skewing of allele frequency in the surviving genotyped individuals. Nowaczyk et al. [2006] and Opitz et al. [2002] also concluded that a high frequency of fetal loss occurs. Population-based prenatal screening for SLOS estimated a second trimester prevalence of 1 in 101,000 [Craig et al., 2006], a finding much lower than that expected for the estimates for carrier rates and suggestive of early pregnancy loss. Similarly, although the c.964-1G>C carrier frequency is 0.7% in African Americans [Wright et al., 2003], few African American cases have been reported.

Such high carrier frequencies for DHCR7 mutations suggest the possibility of a heterozygote advantage. In a recent review, Porter and Herman [2011] presented several plausible mechanisms for heterozygote advantage including increased vitamin D levels providing protection against vitamin D-deficient rickets and autoimmune diseases, or increased resistance to certain infections resulting from increased 7DHC concentration in cell membranes. In the skin, 7DHC is the substrate for formation of previtamin D₃. It is plausible that in Northern European populations heterozygosity for a null DHCR7 mutation may have provided protection against the development of vitamin D-deficient rickets [Kelley and Hennekam, 2000]. Decreased vitamin D levels have been implicated as a contributing factor for a number of autoimmune disorders [Cantorna and Mahon, 2004; Fernandes de Abreu et al., 2009] and multiple sclerosis [Munger et al., 2004]; thus, elevated 7DHC with resultant higher vitamin D levels may protect against these diseases. The alteration of cellular membrane composition brought on by the increase of 7DHC content may reduce the infectivity of certain microbes such as the hepatitis B virus [Rodgers et al., 2009] or Mycoplasma bacteria [Pieters, 2001].

DIAGNOSIS

The clinical diagnosis of SLOS is confirmed by demonstrating elevated 7DHC levels in the serum. 7DHC can also be measured in other tissues, including cultured fibroblasts, amniocytes or chorionic villi as well as in amniotic fluid. Although many individuals with SLOS have hypocholesterolemia, normal cholesterol levels do not exclude the diagnosis of SLOS. The standard measurement method used in many hospital laboratories measures total cholesterol (cholesterol and its precursors, 7DHC and 8DHC) and may result in spurious elevation of “cholesterol,” because of the presence of significant amounts of 7DHC and 8DHC. In cases with borderline 7DHC levels, sterol measurement in skin fibroblasts or amniocytes cultured in cholesterol-depleted medium may be helpful. 7DHC may be minimally elevated in individuals without SLOS on psychotropic medications such as haloperidol leading to a “false positive” result [Kelley and Hennekam, 2000].

In equivocal cases with low levels of plasma 7DHC and normal levels of cholesterol, the diagnosis can be performed by demonstration of two DHCR7 mutations. Sequencing of exons 6–9 identifies approximately 85% of DHCR7 mutations [Porter, 2008]. If both mutations are not identified, exons 3–5 can then be sequenced. Exons 1 and 2 are noncoding. In a small number of biochemically positive individuals, only a single heterozygous coding mutation has been identified [Yu and Patel, 2005]. Molecular testing identifies mutations in as many as 96% of patients [Witsch-Baumgartner et al., 2001]. A diagnostic algorithm for SLOS has been published [Porter, 2008].

Prenatal diagnosis of SLOS can be performed by demonstration of elevated 7DHC levels as measured by either gas chromatography–mass spectroscopy or liquid chromatography–tandem mass spectroscopy analysis of amniotic fluid or chorionic villus samples [Abuelo et al., 1995; Mills et al., 1996; Kelley, 1995; Tint et al., 1998; Griffiths et al., 2008]. During pregnancy dehydrosteroids (abnormal 7DHC-derived steroids) are produced by the placenta [Shackleton et al., 1999]. These steroids can be detected in maternal serum, and are excreted in the urine of women pregnant with a fetus affected with SLOS. Non-invasive prenatal diagnosis measuring these metabolites is possible [Shackleton et al., 2007; Griffiths et al., 2008]. Molecular prenatal diagnosis of SLOS is available in families with known mutations [Loeffler et al., 2002; Waye et al., 2007].

GENETICS AND GENETIC COUNSELING

SLOS, like most conditions associated with enzyme deficiency, is inherited in an autosomal recessive pattern. Carrier couples have a 25% risk of having an affected child of either sex. Because of a high carrier frequency in various populations, we recommend that for the purposes of risk evaluation a 2% carrier rate be used [Witsch-Baumgartner et al., 2004; Porter, 2008]. Because there is considerable overlap between the ranges of serum concentration of cholesterol and 7DHC in carriers and non-carriers, carrier status cannot be determined by measuring the serum concentration of either compound [Kelley, 1995]. Biochemical testing of fibroblasts has been successful in carrier detection [Shefer et al., 1997]. Carrier testing is possible by molecular genetic analysis when the disease-causing mutations are known in the family.

PATHOGENESIS

It is not known how the underlying biochemical defect of abnormal cholesterol synthesis results in the protean and multiple abnormalities in body structure and function observed in SLOS. It is likely that the clinical manifestations result from deficient cholesterol or deficient total sterols, the toxic effects of either 7DHC or compounds derived from 7DHC, or a combination of these factors. Cholesterol has multiple biological functions: it is a major lipid component of cellular membranes including myelin; it is an important structural component of lipid rafts which play a major role in signal transduction; it is a biosynthetic precursor of bile acids, steroid hormones and neuroactive steroids and oxysterols; and addition of the cholesterol moiety is required for sonic hedgehog (SHH) signaling

Cholesterol has multiple biological functions: it is a major lipid component of cellular membranes including myelin; it is an important structural component of lipid rafts which play a major role in signal transduction; it is a biosynthetic precursor of bile acids, steroid hormones and neuroactive steroids and oxysterols; and addition of the cholesterol moiety is required for sonic hedgehog (SHH) signaling

(Fig. 10). The developing fetal brain is dependent on the endogenous synthesis of cholesterol: 90% of sterols in the newborn mouse are of fetal origin [Tint et al., 2006]. Given these multiple biological functions of cholesterol, it is unlikely that a single pathological mechanism underlies the myriad malformations and clinical problems found in affected individuals.

Cholesterol is a major lipid component of plasma membranes and a structural component of lipid rafts. Substitution of 7DHC for cholesterol may alter the physiochemical properties and function of cellular membranes in several ways. The fluidity and caveolae formation of cell membranes are affected by altered sterol composition [Tulenko et al., 2006; Gondre-Lewis et al., 2006; Staneva et al., 2010; Ren et al., 2011]. Lipid rafts are liquid-ordered subdomains composed of cholesterol, sphingolipids, and proteins that play a major role in signal transduction and membrane trafficking. In comparison with cholesterol, 7DHC stabilizes lipid rafts in model membranes [Xu et al., 2001; Megha et al., 2006]. In addition to altered membrane sterol composition, increased levels of dolichol and ubiquinone synthesis could further alter membrane fluidity, permeability, and function [Pappu et al., 2006]. Signaling by the human serotonin(1A) receptor has been shown to be impaired by altered membrane cholesterol concentration [Singh et al., 2007; Paila et al., 2008]; other neurotransmitters may also be affected.

7DHC and 7-DHC-derived sterol species may have toxic effects. The known toxic effects of 7DHC include impairment of intracellular cholesterol transport that may decrease intracellular cholesterol bioavailabily [Wassif et al., 2002, 2003] and increased degradation rate of hydroxymethyl glutaryl coenzyme A reductase (HMG-CoA reductase) [Fitzky et al., 2001] with a resultant further decrease in sterol synthesis in [Steiner et al., 2000]. Other 7DHC-derived steroids and oxysterols may have unique biological functions that contribute to the SLOS cognitive or behavioral phenotype. Abnormally decreased levels of 24(S)-hydroxycholesterol and increased levels of 27-hydroxycholesterol as well as novel oxysterols, such as 27-hydroxy-7DHC and 27-hydroxy-8DHC have been found in serum from individuals with SLOS [Björkhem et al., 2009; Wassif et al., 2003]. These oxysterols' biological activity may have a functional role in the development of the SLOS phenotype. 7DHC is highly reactive and its metabolites may be cytoxic to neurons [Korade and Kenworthy, 2008; Xu et al., 2012]. Abnormal dehydrocholesterol metabolites are transported into the mitochondria and participate in numerous subsequent enzymatic reactions leading to the formation of abnormal dehydrosteroid and dehydroketosteroid species [Shackleton et al., 2002; Shinkyo et al., 2011]. Dehydrocholesterol analogs of pregnenolone, pregnanetriol, DHEA and androstenediol have been identified in SLOS individuals [Shackleton et al., 2002] as have been dehydrosteroid derivatives of neuroactive steroids as well as novel oxysterols [Wassif et al., 2003; Marcos et al., 2004]. Neuroactive steroids are modulatory ligands for neurotransmitter and nuclear steroid hormone receptors and have functional roles in neurogenesis, neuroprotection, and myelination. In contrast to cholesterol, neuroactive steroids can cross the blood–brain barrier, thus providing a hypothetical mechanism that could explain anecdotal reports of rapid behavioral effects reported in association with dietary cholesterol supplementation.

Some of the malformations associated with SLOS are consistent with impaired SHH functioning [Porter et al., 1996]. SHH plays an important role in embryonic pattern formation of the central nervous system, facial structures and limbs. Mutations in SHH have shown to cause holoprosencephaly, a malformation observed in SLOS. SHH is cholesterol modified, secreted from a signaling cell, and binds to a receptor called Patched (PTCH) in the responding cell. PTCH then regulates transmembrane signaling in the responding cell by relieving inhibition of Smoothened (SMO). A number of mechanisms by which SHH signaling might be impaired in SLOS have been postulated [Koide et al., 2006]. In addition, PTCH-mediated transport of vitamin D3 (a metabolic product of 7DHC) modulates SMO function, and oxysterols stimulate SHH signaling [Bijlsma et al., 2006; Corcoran and Scott, 2006; Dwyer et al., 2007].

SUMMARY

SLOS is both common and unique; common because both its incidence and the carrier rate for DHCR7 mutations are high in many populations and unique because of the role cholesterol and its metabolites play in embryogenesis and in normal postnatal functions. It was the first metabolic defect of sterol synthesis identified and the first enzymatic defect responsible for a malformation syndrome with an inverse relationship between the cholesterol level and the severity of physical manifestations. SLOS is relatively common with a carrier frequency that may be as high as 2% in certain Caucasian populations, and prenatal diagnosis is available and feasible. Care of patients with SLOS is not confined to the management of families living with the chronic and profound cognitive, behavioral and physical handicaps of their children, but also includes diagnosis and management of pregnancy losses and newborn deaths, and of emotional and psychiatric issues in the carriers. Knowledge of the various aspects of SLOS will improve the care of affected patients and their families. More investigations are needed to understand the pathophysiology of this common defect of endogenous cholesterol synthesis.