**3.4 Lathosterolosis**

Lathosterolosis (OMIM 607330) results from impaired 3-hydroxysteroid-5-desaturase (SC5D) activity [90]. In the Kandutsch-Russel synthetic pathway, SC5D catalyses the conversion of lathosterol to 7-dehydrocholesterol (7DHC)

in the enzymatic step immediately preceding the defect in SLOS, whereas in the Bloch pathway of cholesterol synthesis, SC5D catalyses the conversion of cholesta-7,24-dienol to 7-dehydrodesmosterol [90].

To date, deleterious missense mutations of *SC5D* have been reported in six patients from three families [91–95]. The clinical features include microcephaly, facial dysmorphism, bitemporal narrowing, ptosis, cataracts, anteverted nares, micrognathia, postaxial polydactyly, syndactyly, ambiguous genitalia, nonneuronal mucolipidosis, global developmental delay, intellectual impairment, hepatic cirrhosis, and early lethality [91–95]). One surviving patient who developed end-stage hepatic failure and received a liver transplantation had improvement of lathosterolosis symptoms [96]. Another patient had a milder clinical phenotype of microcephaly and learning defects with cataracts [91] highlighting the possible under-diagnosis of the syndrome without plasma sterol analysis.

Plasma cholesterol levels are normal with accumulation of lathosterol in plasma and in cultured fibroblasts, and lamellar inclusions within cellular lysosomes [95]. *Sc5d*−/− pups are stillborn and demonstrate craniofacial malformations including cleft palate and limb defects such as postaxial polydactyly [94].

### **3.5 Smith-Lemli-Opitz syndrome**

Smith-Lemli-Opitz syndrome (SLOS) is the prototypical inborn error of cholesterol biosynthesis first described in 1964 [97]. It is by far the most common disorder in this group, with an incidence of approximately 1/40,000 although this can range from 1/70,000 to 1/10,000 depending on the population in question [98]. The carrier frequency can range from approximately 1:100 in North American Caucasians to 1:50–1:30 in various Central European populations [99]. While these carrier rates would imply a far greater incidence than is clinically observed, there is thought to be a level of misdiagnosis or non-diagnosis in mildly-affected patients, and *in utero* prenatal demise is estimated to affect 42–88% of conceptuses [100], mostly in the first trimester [98, 101].

SLOS has a broad range of phenotypic variabilities: mild cases can comprise minor physical abnormalities and behavioural or learning difficulties through a wide spectrum to a severe phenotype comprising major and life-limiting congenital abnormalities. Cognition can range from near-normal [102] to profound intellectual impairment, and on MRI, up to 96% of SLOS patients have a structural brain abnormality [103]. There is a correlation of atypical sterol profiles with both intellectual impairment and brain malformations, particularly abnormalities of the septum pellucidum and corpus callosum [103]. CNS myelination is normal despite its high proportion of cholesterol content and the mostly *in situ* synthesis of cholesterol in the CNS [104]. As well as intellectual impairment, patients are often diagnosed with language delays or impairment, autistic spectrum disorder and sleep disturbances, and can engage in self-harm. Global developmental delay, hypotonia and failure to thrive are common [105–107]. The most common physical manifestation reported with SLOS is that of 2,3 toe syndactyly, and a combination of this with other structural or cognitive symptoms should suggest a possible SLOS diagnosis for investigation [108]. Limb anomalies are common, including polydactyly, short proximal thumbs and a single palmar crease [105–107]. Other structural malformations that can occur include microcephaly, cleft palate, bifid uvula and characteristic facies with micrognathia, ptosis and broad nasal tip with anteverted nares [105–107]. This facial dysmorphia can be less recognisable in older patients [107]. Congenital abnormalities can also affect the heart and lungs, gastrointestinal tract and genitalia [105–107]. Patients with SLOS often have severe ultraviolet photosensitivity [109].

**141**

*Human Cholesterol Biosynthesis Defects DOI: http://dx.doi.org/10.5772/intechopen.87150*

mediation [120].

Europe [98, 99].

reproductive options.

and investigation of human SLOS [138, 139].

The final steps of the post-squalene cholesterol biosynthesis pathway are conversion of 7-dehydrodesmosterol to desmosterol and 7-dehydrocholesterol (7-DHC)

Over 100 mutations in *DHCR7* have been identified in SLOS [114] with no clear genotype-phenotype correlations [115, 116], although some mutations are associated with more mild phenotypes due to some residual enzyme activity [117]. There is a significant correlation between SLOS patient phenotype and maternal genotype for *ApoE* and *ABCA1* [118, 119]. These correlations are positive for amelioration of SLOS symptomatology and pathogenesis and with the potential for therapeutic

As well as being the precursor to cholesterol, 7-DHC is also the precursor to vitamin D with exposure of cutaneous 7-DHC to ultraviolet B and subsequent synthesis to vitamin D by the liver and kidney. Increased levels of circulating vitamin D are seen in patients with SLOS [121], despite their increased photosensitivity and ensuing limited sun exposure. One of the primary theories for a possible heterozygous advantage of *DHCR7* mutations is that of protection against vitamin D deficiency [105], particularly given the greater carrier rate seen in populations of northern

A prenatal diagnosis can be obtained via molecular or biochemical analysis (e.g. of amniotic fluid sterols [122]); however, non-invasive techniques can also identify pregnancies requiring SLOS investigation. Measurement of a low maternal serum unconjugated estriol (uE3), particularly when combined with abnormal sonography results, can be utilised for prenatal screening although this can yield false positive results and uE3 levels can also be predictive for other disorders [101]. Baseline screening for a SLOS-affected pregnancy is also possible noninvasively via serial measurement of steroids such as pregnanetriol in maternal urine [123, 124]. Abnormal plasma sterol ratios in unaffected heterozygotes [125] mean that carrier status may be determined prior to pregnancy for increased

Current treatment protocols for SLOS usually involve endogenous cholesterol supplementation with or without adjunct therapies such as simvastatin [126]. There is broad anecdotal evidence throughout the literature as to the positive benefit of cholesterol supplementation for patient growth, overall health (including improved photosensitivity and response to infection) and behaviour, as well as measurable changes towards typical plasma sterols [127–129]. These improvements have been reported following initiation of cholesterol treatment in both children and adults [130], although with greater rate of improvement with earlier intervention [131]. Limitations to the efficacy of cholesterol treatment certainly exist, such as cholesterol's inability to cross the blood-brain barrier in any practical quantity (which makes the apparent behavioural improvements reported interesting). The real value of cholesterol supplementation is yet to be definitively determined as trials of increased dietary cholesterol both with and without placebo controls have yielded very mixed results [132–134]. Antioxidant [135, 136] and virus vector [137] therapies have also been explored as an avenue for improved patient outcomes for SLOS and other disorders of cholesterol synthesis. Both mouse and rat models of null and hypomorphic alleles in *DHCR7* have been useful homologues for characterisation

7-dehydrocholesterol reductase, DHCR7) enzyme, encoded by the *DHCR7* gene at 11q13.4. Increased levels of 7-DHC and decreased levels of cholesterol led to SLOS being identified as a disorder of sterol biosynthesis in 1993 [110, 111]. This altered plasma profile is a useful diagnostic tool for SLOS, and there is evidence of a


to cholesterol. The latter is catalysed by the 3β-hydroxysteroid-Δ<sup>7</sup>

relationship between serum sterols and disease severity [112, 113].

#### *Human Cholesterol Biosynthesis Defects DOI: http://dx.doi.org/10.5772/intechopen.87150*

*Apolipoproteins, Triglycerides and Cholesterol*

7,24-dienol to 7-dehydrodesmosterol [90].

**3.5 Smith-Lemli-Opitz syndrome**

in the enzymatic step immediately preceding the defect in SLOS, whereas in the Bloch pathway of cholesterol synthesis, SC5D catalyses the conversion of cholesta-

To date, deleterious missense mutations of *SC5D* have been reported in six patients from three families [91–95]. The clinical features include microcephaly, facial dysmorphism, bitemporal narrowing, ptosis, cataracts, anteverted nares, micrognathia, postaxial polydactyly, syndactyly, ambiguous genitalia, nonneuronal mucolipidosis, global developmental delay, intellectual impairment, hepatic cirrhosis, and early lethality [91–95]). One surviving patient who developed end-stage hepatic failure and received a liver transplantation had improvement of lathosterolosis symptoms [96]. Another patient had a milder clinical phenotype of microcephaly and learning defects with cataracts [91] highlighting the possible

Plasma cholesterol levels are normal with accumulation of lathosterol in plasma and in cultured fibroblasts, and lamellar inclusions within cellular lysosomes [95]. *Sc5d*−/− pups are stillborn and demonstrate craniofacial malformations including

Smith-Lemli-Opitz syndrome (SLOS) is the prototypical inborn error of cholesterol biosynthesis first described in 1964 [97]. It is by far the most common disorder in this group, with an incidence of approximately 1/40,000 although this can range from 1/70,000 to 1/10,000 depending on the population in question [98]. The carrier frequency can range from approximately 1:100 in North American Caucasians to 1:50–1:30 in various Central European populations [99]. While these carrier rates would imply a far greater incidence than is clinically observed, there is thought to be a level of misdiagnosis or non-diagnosis in mildly-affected patients, and *in utero* prenatal demise is estimated to affect 42–88% of conceptuses [100], mostly in the first trimester [98, 101]. SLOS has a broad range of phenotypic variabilities: mild cases can comprise minor physical abnormalities and behavioural or learning difficulties through a wide spectrum to a severe phenotype comprising major and life-limiting congenital abnormalities. Cognition can range from near-normal [102] to profound intellectual impairment, and on MRI, up to 96% of SLOS patients have a structural brain abnormality [103]. There is a correlation of atypical sterol profiles with both intellectual impairment and brain malformations, particularly abnormalities of the septum pellucidum and corpus callosum [103]. CNS myelination is normal despite its high proportion of cholesterol content and the mostly *in situ* synthesis of cholesterol in the CNS [104]. As well as intellectual impairment, patients are often diagnosed with language delays or impairment, autistic spectrum disorder and sleep disturbances, and can engage in self-harm. Global developmental delay, hypotonia and failure to thrive are common [105–107]. The most common physical manifestation reported with SLOS is that of 2,3 toe syndactyly, and a combination of this with other structural or cognitive symptoms should suggest a possible SLOS diagnosis for investigation [108]. Limb anomalies are common, including polydactyly, short proximal thumbs and a single palmar crease [105–107]. Other structural malformations that can occur include microcephaly, cleft palate, bifid uvula and characteristic facies with micrognathia, ptosis and broad nasal tip with anteverted nares [105–107]. This facial dysmorphia can be less recognisable in older patients [107]. Congenital abnormalities can also affect the heart and lungs, gastrointestinal tract and genitalia [105–107]. Patients with SLOS often have severe ultraviolet

under-diagnosis of the syndrome without plasma sterol analysis.

cleft palate and limb defects such as postaxial polydactyly [94].

**140**

photosensitivity [109].

The final steps of the post-squalene cholesterol biosynthesis pathway are conversion of 7-dehydrodesmosterol to desmosterol and 7-dehydrocholesterol (7-DHC) to cholesterol. The latter is catalysed by the 3β-hydroxysteroid-Δ<sup>7</sup> -reductase (or 7-dehydrocholesterol reductase, DHCR7) enzyme, encoded by the *DHCR7* gene at 11q13.4. Increased levels of 7-DHC and decreased levels of cholesterol led to SLOS being identified as a disorder of sterol biosynthesis in 1993 [110, 111]. This altered plasma profile is a useful diagnostic tool for SLOS, and there is evidence of a relationship between serum sterols and disease severity [112, 113].

Over 100 mutations in *DHCR7* have been identified in SLOS [114] with no clear genotype-phenotype correlations [115, 116], although some mutations are associated with more mild phenotypes due to some residual enzyme activity [117]. There is a significant correlation between SLOS patient phenotype and maternal genotype for *ApoE* and *ABCA1* [118, 119]. These correlations are positive for amelioration of SLOS symptomatology and pathogenesis and with the potential for therapeutic mediation [120].

As well as being the precursor to cholesterol, 7-DHC is also the precursor to vitamin D with exposure of cutaneous 7-DHC to ultraviolet B and subsequent synthesis to vitamin D by the liver and kidney. Increased levels of circulating vitamin D are seen in patients with SLOS [121], despite their increased photosensitivity and ensuing limited sun exposure. One of the primary theories for a possible heterozygous advantage of *DHCR7* mutations is that of protection against vitamin D deficiency [105], particularly given the greater carrier rate seen in populations of northern Europe [98, 99].

A prenatal diagnosis can be obtained via molecular or biochemical analysis (e.g. of amniotic fluid sterols [122]); however, non-invasive techniques can also identify pregnancies requiring SLOS investigation. Measurement of a low maternal serum unconjugated estriol (uE3), particularly when combined with abnormal sonography results, can be utilised for prenatal screening although this can yield false positive results and uE3 levels can also be predictive for other disorders [101]. Baseline screening for a SLOS-affected pregnancy is also possible noninvasively via serial measurement of steroids such as pregnanetriol in maternal urine [123, 124]. Abnormal plasma sterol ratios in unaffected heterozygotes [125] mean that carrier status may be determined prior to pregnancy for increased reproductive options.

Current treatment protocols for SLOS usually involve endogenous cholesterol supplementation with or without adjunct therapies such as simvastatin [126]. There is broad anecdotal evidence throughout the literature as to the positive benefit of cholesterol supplementation for patient growth, overall health (including improved photosensitivity and response to infection) and behaviour, as well as measurable changes towards typical plasma sterols [127–129]. These improvements have been reported following initiation of cholesterol treatment in both children and adults [130], although with greater rate of improvement with earlier intervention [131]. Limitations to the efficacy of cholesterol treatment certainly exist, such as cholesterol's inability to cross the blood-brain barrier in any practical quantity (which makes the apparent behavioural improvements reported interesting). The real value of cholesterol supplementation is yet to be definitively determined as trials of increased dietary cholesterol both with and without placebo controls have yielded very mixed results [132–134]. Antioxidant [135, 136] and virus vector [137] therapies have also been explored as an avenue for improved patient outcomes for SLOS and other disorders of cholesterol synthesis. Both mouse and rat models of null and hypomorphic alleles in *DHCR7* have been useful homologues for characterisation and investigation of human SLOS [138, 139].

#### **3.6 Desmosterolosis**

Desmosterolosis (OMIM 602398) is currently the final inborn error of cholesterol biosynthesis and is caused by defective enzymatic function of 3-hydroxysterol-delta 24-reductase (DHCR24). This reaction causes the reduction of the C-24 bond in the aliphatic side chain of cholesterol [140]. Reduction of the C-24 bond catalysed by DHCR24 can occur at different times in the cholesterol synthetic pathway: this step occurs early in the Kandutsch-Russel cholesterol synthetic pathway [6] but is the penultimate step in the Bloch pathway of cholesterol synthesis [5].

While first described in 1998, the molecular mechanisms of desmosterolosis were not characterised until 2001 [140, 141]. To date, only nine cases have been reported and clinical features include SLOS-like dysmorphism, thick alveolar ridges, gingival nodules, cleft palate, short limbs, severe congenital heart defect, atherosclerosis, arthrogryposis, ambiguous genitalia, microcephaly, agenesis of the corpus callosum, global developmental delay and intellectual impairment [141–147]. The diagnosis of desmosterolosis is made by demonstrating elevated levels of desmosterol by GC-MS analysis, with serum cholesterol levels usually normal [141, 142]. Reported *DHCR24* pathogenic mutations thus far have all been missense mutations.

A targeted mouse model for desmosterolosis has been generated, and *Dhcr24*<sup>−</sup>/<sup>−</sup> mice are viable with some postnatal growth retardation and infertility [148]. Pharmacological inhibitors of DHCR24 have been developed for studies in rat models [135, 149, 150]. Treatment of pregnant rats with these inhibitors of sterol-D24-reductase is teratogenic and produces cataracts, CNS abnormalities, genitourinary and skeletal anomalies [149–151].
