Chapter 164. Disorders of Cholesterol Synthesis

The isoprenoid synthesis pathway (Fig. 164-1) leads to the synthesis of cholesterol (an important constituent of cell membranes and a precursor of steroid hormones and bile acids), haem A (a component of complex IV of the respiratory chain), ubiquinone (an electron carrier in the respiratory chain), dolichol (which is required for glycosylation of proteins), farnesyl-pyrophosphate, and geranylgeranyl-pyrophosphate (which are important for prenylation of proteins).1 Prenylation of proteins is important in many signaling cascades in the cell.

The cholesterol synthesis pathway is tightly regulated.1,2 When cholesterol levels in the cell are low, the transcription of the HMG-CoA reductase gene (HMGR), and probably the transcription of every other gene involved in the cholesterol synthesis pathway, is upregulated by sterol regulatory element binding proteins (particularly SREBP-2). High levels of sterols reduce the activity of the cholesterol-synthesizing enzymes by reducing SREBP-2 activity. High levels of sterols in the cell also lead to increased production of oxysterols, which act on the liver X receptor (LXR); this, in turn, drives the disposal of the excess cholesterol. LXR may also reduce cholesterol synthesis by downregulating SREBP-2.

Cholesterol is an important constituent of cell membranes and has important interactions with signaling proteins that are involved in embryogenesis and development (eg, the hedgehog proteins). Disorders of the pathway from lanosterol to cholesterol lead to dysmorphic features and to malformations of internal organs. In all but one of these disorders, psychomotor delay is prominent.

The first-line test for diagnosing an inborn error in the pathway from lanosterol to cholesterol is the analysis of sterols (from a plasma or tissue sample) by gas chromatography–mass spectrometry (GC-MS). Mevalonic acid can be detected by urine organic acid analysis (see below).

Clinical Presentation

Mevalonic aciduria (MA) is the severe form of mevalonate kinase deficiency; the hyper-IgD periodic fever syndrome (HIDS) is more benign. Both present in the first year of life with episodes of pyrexia for 3 to 5 days every 4 to 6 weeks; symptoms include diarrhea and vomiting, abdominal pain, cervical lymphadenopathy, hepatosplenomegaly, arthralgia, and a rash.3,4Children with MA have additional major problems, which may present before the fevers: Dysmorphic features include microcephaly, large fontanels, triangular face, low-set ears, down-slanting palpebral fissures, blue sclerae, and cataracts. Neurological manifestations include hypotonia, developmental delay, nystagmus, and progressive ataxia in the second year of life. Magnetic resonance imaging may show cerebral and/or cerebellar atrophy; agenesis of the cerebellar vermis may be apparent. A blood count may show normocytic anemia, leukocytosis, and thrombocytopenia. Transaminases and creatine kinase may be elevated. Ubiquinone levels are low. Babies with MA may die in infancy.

Metabolic Derangement and Pathophysiology

MA and HIDS are both caused by a deficiency of mevalonate kinase (which catalyzes step 4 in Fig. 164-1). In MA, the enzyme activity in white cells or cultured skin fibroblasts is virtually undetectable, whereas in HIDS it is 2% to 8% that of controls.5 The reduced activity of mevalonate kinase explains why the substrate for the enzyme, mevalonic acid, builds up. However, it is the shortage of one or more nonsterol products of the isoprenoid pathway rather than accumulation of mevalonic acid that enhances the secretion of proinflammatory cytokines. In the case of MA, the deficiency of sterol isoprenoids probably plays a role in problems such as the developmental delay and dysmorphism.

Genetics

MA (OMIM 610377) and hyper-IgD periodic fever syndrome (HIDS) (607622) are both autosomal recessive conditions caused by mutations in the MK gene, which encodes mevalonate kinase (251170) and is located on chromosome 12q24.5 The majority of patients with the HIDS phenotype are compound heterozygotes for a V377I mutation (which is found only in patients with HIDS), and a second mutation that can be one of the mutations found in patients with mevalonic aciduria. The V337I protein has mevalonate kinase activity, but its synthesis is temperature-dependent. Other fairly common mutations in the MK gene are H20P, I268T, and A334T; however, a wide range of mutations has been described. Prenatal diagnosis can be undertaken by measuring MK enzyme activity or mutation detection in chorionic villus, cultured villus cells, and amniotic fluid cells.

Diagnostic Tests

Measurement of urinary mevalonic acid by stable isotope dilution gas chromatography–mass spectrometry6 will readily detect patients with mevalonic aciduria, who excrete 1 to 56 mmol/mol creatinine but may not detect all patients with HIDS, who excrete only 0.005 to 0.040 mmol/mol creatinine even during an episode of fever (mevalonic acid is normally undetectable in urine). Most patients with HIDS have persistent elevation of plasma IgD (over 100 IU/ml) and/or IgA. The best tests, however, are measurement of MK activity in white blood cells/cultured skin fibroblasts and analysis of the MVK gene by sequencing the exons and flanking regions.

Treatment and Prognosis

Mevalonic aciduria can be fatal, but the prognosis for HIDS is good with symptoms tending to become less frequent and less severe with age.4 No specific treatment has been shown to be effective in all patients.7-9 Corticosteroids, colchicine, and cyclosporin seem to benefit a few. Simvastatin reduces duration of attacks in some patients but may make symptoms worse in others. A small number of patients have been successfully treated with etanercept, a soluble p75 TNFα receptor-Fc fusion protein. One patient has recently been successfully treated by allogenic bone marrow transplant.10

Clinical Presentation

The symptoms and signs of Smith-Lemli-Opitz syndrome (SLOS) vary enormously in severity, from intrauterine death in the first or second trimester to subtle abnormalities leading to diagnosis of the disorder in an adult whose educational attainment has been normal. Typical features of the disorder include intrauterine growth retardation with low maternal estriol levels; sometimes abnormalities are detected on ultrasound, such as increased nuchal translucency, polydactyly, and facial dysmorphism. At birth, a range of characteristic dysmorphic features may be obvious11,12: microcephaly; ptosis; anteverted nares; long philtrum; low-set, posteriorly rotated ears; micrognathia; cleft or high-arched palate; irregular alveolar ridges; 2/3 toe syndactyly; postaxial polydactyly; and short, proximally placed thumbs (eFigs. 164.1 and 164.2). Boys may have hypospadias and cryptorchidism (eFig. 164.3) or even ambiguous or female external genitalia. Hypotonia is common. Serious congenital malformations include airway problems, congenital heart disease (particularly atrioventricular septal defects and patent ductus arteriosus), pyloric stenosis, and Hirschsprung disease. Feeding problems and failure to thrive are common. However, the most common problem emerging in infancy and the preschool years is developmental delay often associated with communication difficulties (akin to autism) and behavior problems, including aggression and sleep disruption. Photosensitivity of the skin may also be a problem.

Metabolic Derangement and Pathophysiology

SLOS is caused by a deficiency of 7-dehydrocholesterol (7-DHC) reductase,13 which catalyzes the last (or penultimate) step in cholesterol synthesis (Fig. 164-1, right-hand pathway and left-hand pathway, respectively). Tissues of SLOS patients have an increased concentration of 7-DHC and 8-dehydrocholesterol, or 8-DHC (probably formed from 7-DHC by the Δ8-Δ7 isomerase working in reverse). There is a link between the biochemical severity as expressed by the ratio 7-DHC + 8-DHC/total sterol and the clinical severity score; however, it is still not clear whether accumulation of 7-DHC and 8-DHC or lack of cholesterol is the important factor in pathogenesis. There is evidence that the efficiency of transfer of maternal cholesterol to the fetus (determined by the maternal apo-E genotype) influences disease severity.

Genetics

SLOS (OMIM 270400) is an autosomal recessive disorder caused by mutations in the DHCR7 gene encoding 7-DHC reductase (3β-hydroxysterol Δ7-reductase), which is located on chromosome 11q13.14 In the Caucasian population, there is a common splice site mutation (IVS8-1G>C) that has a frequency of 30% of alleles. Prenatal diagnosis can be made by analyzing maternal urine for dehydro-estriol and dehydro-pregnanetriol15 by direct analysis of sterols in chorionic villus or amniotic fluid16,17 or by measurement of 7-DHC reductase enzyme activity in villus/cultured villus cells.18 However, for mild cases of SLOS, DNA analysis is the most reliable means of prenatal diagnosis.19

Diagnostic Tests

SLOS can be diagnosed by GC-MS analysis of plasma sterols, which shows (often markedly) elevated concentrations of 7-DHC and 8-DHC. The activity of 7-DHC reductase can be measured in white cells or fibroblasts. Fibroblasts from SLOS patients also accumulate 7-DHC when grown in lipoprotein-depleted medium. Mutations in the DHCR7 gene can be detected by sequencing of the exons and flanking regions.

Treatment and Prognosis

Postnatal treatment cannot be expected to reverse aberrant embryonic and fetal development. However, dietary cholesterol supplementation may help treat postnatal growth failure, learning difficulties, and behavior problems.20 There are anecdotal accounts of improvements in all of these parameters in individual patients but no consistent improvements in the limited clinical trials that have been undertaken.21,22 The HMG-CoA reductase inhibitor simvastatin reduces plasma concentrations of 7-DHC and 8-DHC,22 and, interestingly, plasma cholesterol concentrations rise; however, it is unclear whether trials will show any clinical improvements.

Clinical Presentation

CDPX2 can be detected during pregnancy when a scan shows severe chondrodysplasia (eFig. 164.4). The skeletal dysplasia is usually asymmetrical. CDPX2 can present in the neonatal period with ichthyosiform erythroderma (eFig. 164.5), developing into linear/whorled atrophic and pigmentary lesions in childhood (eFig. 164.6); hyperkeratosis; follicular atrophoderma (eFig. 164.7); coarse, lusterless hair; and cicatricial alopecia (eFig. 164.8) in adults.23 Affected girls have mild craniofacial dysmorphic features—a flat face with high forehead, a low nasal bridge, and malar hypoplasia (eFig. 164.9). The chondrodysplasia punctata may be detected by asymmetric limb shortening, scoliosis, or musculoskeletal pain; sometimes the presence of cataracts prompts investigation. Full assessment of a child with any of the above presentations requires careful dermatological and ophthalmological examination of the child and his or her mother, a skeletal survey, and analysis of plasma sterols by GC-MS. The bone disease and the cataracts are both typically asymmetric.

Metabolic Derangement and Pathophysiology

CDPX2 is caused by reduced activity of the sterol Δ8-Δ7 isomerase enzyme; the plasma concentration of its substrate, 8(9)-cholesten-3β-ol, is mildly raised (eFigs. 164.10 and 164.11).24 There is also a very modest buildup of 8-dehydrocholesterol (presumably formed from 8(9)-cholesten-3β-ol by the action of sterol Δ5 desaturase). Plasma cholesterol is normal or low normal. It is not clear if these biochemical changes are directly responsible for the clinical problems.

Genetics

CDPX2 (OMIM 302960) is an X-linked dominant disorder caused by a heterozygous mutation in the EBP gene encoding sterol Δ8-Δ7 isomerase and located on chromosome Xp11.23-11.22.25 Many cases are sporadic and due to a de novo (somatic) mutation.26 In some cases, there is evidence of gonadal mosaicism in the mother. When a girl inherits CDPX2 from her mother, her disease tends to be more severe than her mother’s. Affected male fetuses usually die in the first or second trimester. Confirmation of the diagnosis in a female fetus, terminated because of severe chondrodysplasia (eFig. 164.4), has been achieved by sterol analysis.27 Prenatal diagnosis is undertaken by DNA analysis.

Diagnostic Tests

GC-MS analysis of plasma sterols shows increased concentrations of 8(9)-cholesten-3β-ol and 8-dehydrocholesterol. This test is positive in the overwhelming majority of affected girls, but if there is a high index of suspicion (eg, from skeletal radiology plus clinical examination), it is wise to request sequence analysis of the coding exons and flanking intronic sequences of the EBP gene.

Treatment and Prognosis

The most severely affected girls die in utero. The majority, however, survive and have a normal life expectancy. Their problems (skeletal, skin, hair, lens) vary considerably in severity. Some require surgery for cataracts or scoliosis. Correction of scoliosis associated with hemidysplasia of vertebrae requires a special anterior strut graft and a posterior fusion.28 The majority of affected girls show normal psychomotor development. Any surviving male patients, however, show severe developmental delay.

Clinical Presentation

Hemidysplasia is the predominant feature of this disorder.29 One side of the body (usually the right) bears a large inflammatory skin naevus with waxlike scaling. This striking skin rash has a predilection for skin folds and stops abruptly at the midline. The hair and nails on that side of the body are also affected (causing unilateral alopecia and onychorrhexis). The same side of the body is affected by skeletal hypoplasia (eg scapular/clavicular/pelvic hypoplasia, limb reductions), and radiology shows calcific stippling of cartilages in the epiphyses in these areas. There is also ipsilateral hypoplasia of internal organs (brain, lungs, kidneys, thyroid, adrenals). When the left side of the body is affected, there is usually a major cardiac malformation (eg, single ventricle). The disorder is thought to be usually lethal in hemizygous males, as only two male patients with the features of CHILD syndrome have been described.

Metabolic Derangement and Pathophysiology

CHILD syndrome is caused by reduced activity of a 3β-hydroxysteroid dehydrogenase, which acts as a 4α-carboxysterol-C4-dehydrogenase as part of a multienzyme complex catalyzing the C4-demethylation of lanosterol and related C4-dimethyl sterols (step 16 in Fig. 164-1).30-32 The other components of the multienzyme complex are a C4-methyl oxidase and a C4-ketoreductase. In theory, deficiency of the 3β-hydroxysteroid dehydrogenase should lead to buildup of C4-methyl sterol precursors, but plasma concentrations of these cholesterol precursors appear normal or only minimally increased in girls with CHILD syndrome.

Genetics

CHILD syndrome (OMIM 308050) is inherited as an X-linked dominant trait; it is caused by a heterozygous mutation in the NSHDL gene, which encodes a 3β-hydroxysteroid dehydrogenase (see above) located on Xq28.30-32 At least 10 female patients have had their molecular defect defined. In one girl with features of CHILD syndrome, a heterozygous mutation was found in the EBP gene, mutations in which normally cause the CDPX2 phenotype (see above).

Diagnostic Tests

The only reliable diagnostic test is the sequencing of the exons and the flanking intronic sequences of the NSDHL gene. If no mutations are found, the next step is to check the EBP gene.

Treatment and Prognosis

Severe malformations can be lethal. Skin lesions have been treated with dermabrasion, but they recur. Orthopedic surgery has improved function in affected limbs. Squamous cell carcinoma arising in the skin lesion in a young woman has been reported.

Clinical Presentation

There are only two reported cases of desmosterolosis,33,34 and these are so different that the spectrum of clinical abnormalities that can be caused by this disorder cannot be specified accurately. The first reported case was a female infant who died shortly after birth and had multiple congenital malformations (eFigs. 164.12 and 164.13).33 These included macrocephaly, frontal bossing, a hypoplastic nasal bridge, small mouth, micrognathia, thickened alveolar ridges, gingival nodules, cleft palate, short limbs, ambiguous genitalia, total anomalous pulmonary venous drainage, splenomegaly, renal hypoplasia, and a short nonrotated small bowel. Postmortem radiology showed generalized osteosclerosis, rhizomelic shortening of all four limbs, and shortened malformed ribs (eFig. 164.14). Postmortem analysis of the brain showed an immature gyral pattern with poor development of the corpus callosum and gross dilatation of the ventricles. The second affected patient is a 3-year-old boy who had microcephaly, dysmorphic facial features, limb abnormalities, and profound developmental delay.34

Metabolic Derangement and Pathophysiology

Desmosterolosis is caused by deficiency of the enzyme 3β-hydroxysterol Δ24-reductase, which catalyzes the saturation of the C24-C25 double bond in desmosterol and other Δ24-sterols (step 13 in Fig. 164-1). Plasma and tissues of affected individuals contain abnormally high concentrations of desmosterol (eFigs. 164.15 and 164.16).

Genetics

Desmosterolosis is an autosomal recessive disorder caused by mutations in the DHCR24 gene encoding 3β-hydroxysterol Δ24-reductase, which is located on chromosome 1p33.1-p33. Sequencing of the DHCR24 gene in the two affected patients showed four different mutations.35

Diagnostic Tests

Diagnosis of desmosterolosis depends upon analysis of sterols in plasma or tissues, which reveals a higher than normal concentration of desmosterol. Mutations in the DHCR24 gene can be sought by sequencing the exons and neighboring parts of the introns.

Treatment and Prognosis

Insufficient data are available to comment. Desmosterolosis can cause stillbirth/neonatal death or survival with severe learning difficulties. No attempts at treatment have been reported.

Clinical Presentation

There are only two reported cases. The first is a girl with severe microcephaly with receding forehead, anteverted nares, micrognathia, prominent upper lip, high arched palate, postaxial hexadactyly of the left foot, syndactyly between the second and fourth toes and between the fifth toe and the extra digit.36 From early infancy, she suffered from cholestatic liver disease. During the first year of life, severe psychomotor delay became apparent. The second affected patient is a boy who presented at birth with SLOS-like features that included growth failure, microcephaly, ptosis, cataracts, short nose, micrognathia, prominent alveolar ridges, ambiguous genitalia, 2/3 toe syndactyly, and postaxial hexadactyly of the feet.36He showed failure to thrive, severe developmental delay, increasing hepatosplenomegaly, and increasing gingival hypertrophy, and he died early. Autopsy showed widespread storage of mucopolysaccharides and lipids within macrophages and (to a lesser extent) within parenchymal cells of all organ systems; there was also extensive demyelination of the cerebral white matter together with dystrophic calcification of the cerebrum, cerebellum, and brain stem.

Metabolic Derangement and Pathophysiology

Lathosterolosis is due to a deficiency in the enzyme sterol Δ5-destaurase, which converts lathosterol to 7-dehydrocholesterol (step 18 in Fig. 164-1). Increased levels of lathosterol can be detected in plasma and tissues, but understanding of how this leads to the clinical problems is very incomplete.

Genetics

Lathosterolosis (OMIM 607330) is an autosomal recessive disorder caused by mutations in the SC5DL gene encoding 3β-hydroxysterol Δ5-desaturase and located on chromosome 11q23.3.36,37 Sequence analysis of this gene in the two reported patients with lathosterolosis revealed three different disease-causing mutations.

Diagnostic Tests

Lathosterolosis is detected by analysis of sterols in plasma, tissues, or cultured cells. The diagnosis is confirmed by sequencing the coding exons and the flanking intronic sequences of the SC5D gene.

Treatment and Prognosis

Information on the natural history of this disorder is unavailable. Chronic cholestatic liver disease necessitates monitoring for failure to thrive due to long-chain-fat malabsorption and monitoring for fat-soluble vitamin malabsorption, which may require treatment.

Clinical Presentation

Greenberg skeletal dysplasia is also known as hydrops-ectopic calcification-moth-eaten (HEM) skeletal dysplasia. It is a rare disorder that causes early intrauterine death.38 Affected fetuses are severely hydropic, have short-limbed dwarfism, an unusual moth-eaten appearance to their long bones, a narrow thoracic cage, and ectopic calcification/ossification (eg, in laryngeal and costal cartilages; eFig. 164-17). Polydactyly and other skeletal malformations may be present.

Adults heterozygous for Greenberg skeletal dysplasia mutation can have an abnormality detectable on their blood film that is unimportant for them but could have significance for their offspring. This is the Pelger-Huet anomaly; the polymorphonuclear leukocytes have a reduced number of lobules on their nuclei.39

Metabolic Derangement and Pathophysiology

Fetuses with Greenberg skeletal dysplasia have a deficiency of the enzyme sterol Δ14-reductase, which catalyses the saturation of the C14-C15 double bond in an early step in the conversion of lanosterol to cholesterol (step 15 in Fig. 164-1). The enzyme deficiency leads to a buildup of a specific sterol, cholesta-8,14-dien-3β-ol (eFigs. 164.18 and 164.19), which has been used to diagnose Greenberg dysplasia in affected fetuses.20,21

Genetics

Greenberg skeletal dysplasia (OMIM 215140) is an autosomal recessive disorder caused by mutations in the LBR (lamin B receptor) gene located on chromosome 1q42. The lamin B receptor protein consists of an N-terminal domain that binds lamin B and DNA and a C-terminal domain with Δ14-sterol reductase activity. Mutations in both alleles of the LBR gene have been detected in fetuses with Greenberg skeletal dysplasia. Mutations of one allele have been detected in individuals with Pelger-Huet anomaly.

Diagnostic Tests

Fetal tissues from an infant with Greenberg dysplasia had increased amounts of cholesta-8,14-dien-3β-ol. Identification of this sterol requires careful differentiation (by mass spectrometry) from 8(9)-cholesten-3β-ol, which accumulates in fetuses with CDPX2. Definitive diagnosis is achieved by sequence analysis of the LBR gene.

Treatment and Prognosis

Greenberg dysplasia is fatal in utero. Heterozygotes with Pelger-Huet anomaly do not need treatment (eTable 164.1).