Sjögren-Larsson Syndrome (SLS)

Sjögren-Larsson syndrome (SLS) is an autosomal recessive neurocutaneous disorder characterized by intellectual disability, spastic diplegia or tetraplegia, and ichthyosis.

Most individuals with SLS are born preterm and have an erythematous hyperkeratotic appearance at birth. The skin gradually becomes dry, thickened and scaly with the appearance of a brownish yellow hyperkeratosis. The ichthyosis is widespread but tends to spare the central face. Pruritus is a prominent feature that is not found in most other types of ichthyosis. Rarely, the ichthyosis may first appear in some patients after the first year of life.

The first neurologic signs are delay in achieving motor milestones and the appearance of spasticity. Cognitive and developmental delay is typically obvious by the time the child is 1-2 years of age. Spastic diplegia is much more common that tetraplegia. Walking is almost invariably impaired and affected individuals typically require crutches or walkers to ambulate. These symptoms gradually progress to a plateau phase later in childhood. Seizures may develop during infancy or later in childhood. Additional signs include characteristic ophthalmologic abnormalities with crystalline inclusions (so called glistening white dots) on the macular region of the retina, which are pathognomonic for SLS, and many patients complain of photophobia. Delayed speech with pseudobulbar dysarthria and skeletal abnormalities (short stature, kyphoscoliosis) are commonly seen.

SLS results from deficient activity of fatty aldehyde dehydrogenase (FALDH), an enzyme that oxidizes medium- and long-chain fatty aldehydes to fatty acids. Fatty aldehydes are generated during metabolism of several lipids including sphingolipids, ether glycerolipids, leukotriene B4, isoprenols (farnesol and geranylgeraniol) and aldehydes generated from oxidative stress. FALDH is also necessary for the complete oxidation of fatty alcohol to fatty acid. The gene encoding FALDH is ALDH3A2 located on chromosome 17p11.2.

The pathophysiologic mechanisms causing the manifestations of SLS are thought to be due to accumulation of long-chain fatty alcohols and fatty aldehydes in cellular membranes. Long-chain aldehydes can modify macromolecules by forming covalent Schiff base adducts with certain lipids (phosphatidylethanolamine) and proteins. Fatty aldehyde and alcohol accumulation may lead to alteration of the epidermal water barrier and increased transepidermal water loss, subsequently resulting in ichthyosis. Brain MRI reveals white matter disease and MR spectroscopy detects lipid accumulation in myelin, which may be responsible for the neurologic symptoms of spasticity and intellectual disability.

SLS has an estimated incidence of <1 case per 250,000 births worldwide, though the disease is more common in certain geographical locations (e.g. northern Sweden).

There is no curative treatment for SLS. The symptomatic treatment of ichthyosis may include the use of retinoids, topical moisturizing lotions and keratolytic agents to help remove scales. Spasticity may benefit from Botox injections and surgical procedures to lengthen tendons. Seizures typically respond to anticonvulsants. Fat modified diets have had inconsistent success in helping the ichthyosis and shown no effect on the neurologic symptoms.

Smith-Lemli-Opitz Syndrome (SLOS)

Smith-Lemli-Opitz Syndrome (SLOS) is an autosomal recessive intellectual disability/malformation syndrome caused by an abnormality in cholesterol metabolism resulting from deficiency of the enzyme 7-dehydrocholesterol (7-DHC) reductase.

SLOS is characterized by prenatal and postnatal growth retardation, microcephaly, variable intellectual disability, and malformations. The clinical spectrum is wide and affected individuals have been described with normal development and only minor malformations.

The most commonly observed features include:

• Characteristic facial features
• Microcephaly
• Cardiac defects
• Postaxial polydactyly
• 2-3 syndactyly of the toes
• Growth retardation
• Cleft palate
• Genital anomalies including in some cases ambiguous genitalia or gender reversal

SLOS is estimated to affect between 1/15,000 and 1/60,000 individuals; although research shows that the carrier frequency is as high as 1 in 30. The diagnosis of SLOS is based on the presence of clinical features and detection of elevated blood concentration of 7-DHC. Blood concentrations of cholesterol are usually low, though can be in the normal range in approximately 10% of affected individuals. DHCR7 is the gene associated with SLOS. Mutations in this gene lead to deficiency of the enzyme 7-dehydrocholesterol delta-7-reductase, which causes decreased synthesis of cholesterol and accumulation of the cholesterol precursors 7-DHC and 8-dehydrocholesterol (8-DHC). DNA mutation analysis is available and detects biallelic mutations in approximately 96% of known cases.

Management of SLOS typically includes cholesterol supplementation, early intervention and physical/occupational/speech therapies for identified disabilities; consultation with a nutritionist and feeding specialist are is often useful, and gastrostomy placement not uncommon. Consultation with other medical specialists depends on organ systems involved in particular individuals. There is no proven effective specific therapy.

Sitosterolemia

Sitosterolemia is a rare inherited plant sterol storage disease, in which the metabolic defect in the affected patient causes hyperabsorption of sitosterol and other plant sterols from the GI tract, decreased hepatic secretion of plant sterols (eg. sitosterol and campesterol) and cholesterol with subsequent decreased elimination, and altered cholesterol synthesis. The disease is characterized by tendon and tuberous xanthomas and a tendency for premature coronary atherosclerosis, along with hemolytic anemia and platelet abnormalities (e.g. macrothrombocytopenia). Increases of the plant sterol levels are found in the blood and multiple tissues. Patients may also present with moderate to high cholesterol levels since plant sterols and cholesterol are absorbed by the same mechanism. Xanthomas and arteries of affected patients have increased levels of these sterols, especially sitosterol, campesterol, sigmasterol, and their five alpha derivatives (i.e. 5α-stanols –example sitostanol). If misdiagnosed and left untreated, Sitosterolemia results in a significant increase in morbidity and mortality.

Sitosterolemia is caused by homozygous or compound heterozygous mutations in one of two adenosine triphosphate binding cassette (ABC) genes, ABCG8 and ABCG5 located on human chromosome 2p21, which results in hyper-absorption and decreased hepatic secretion of plant sterols. The active pumping back into the intestine of passively absorbed plant sterols is disrupted, and hepatic secretion of the resultant accumulation of these sterols is decreased. More recent studies suggest that inadequate cholesterol production in Sitosterolemia is due to abnormal down-regulation of early, intermediate and late enzymes in the cholesterol biosynthetic pathway.

Common features of Sitosterolemia include tendon and tuberous xanthomas, early coronary vascular disease, chronic hemolytic anemia and thrombocytopenia. Only 80-100 cases have been reported worldwide. This is an extremely rare disease, but likely to be under diagnosed. Patients may be misdiagnosed with familial hypercholesterolemia. Both genders and all races are equally affected by this disease.

The usual clinical test for cholesterol is not useful to diagnose Sitosterolemia, because it does not distinguish among the different sterols. Only specialized chromatographic analysis can separate and quantitate the different sterols in the plasma using gas-chromatography or gas chromatography/mass spectrometry. In affected patients, plant sterol concentrations can be as high as 10 to 20 mg/dL. Normal values of sitosterol are less than 1 mg/dL.

With the recent identification of the disease genes (ABCG8, ABGC5), molecular diagnosis is now theoretically possible, but is not routinely available.

A CBC is also indicated, and may reveal hemolytic anemia and platelet abnormalities.

Treatment of Sitosterolemia may include dietary changes, such as restricting sources of both shellfish and plant-based sterols including vegetable fats, nuts, seeds, chocolate, etc. The diet is quite restrictive, but references for acceptable commercial products, possible menus and recipes are available for guidance. Dietary treatment may no longer be necessary, since Ezetimibe has been shown to effectively lower the concentrations of plant sterols in the plasma. Outpatient care includes monitoring plant sterol concentrations in blood to monitor the effectiveness of treatment.

Mevalonate Kinase Deficiency (MKD)
Mevalonic aciduria (MVA)
Hyperimmunoglobulinemia D syndrome (HIDS)

Mevalonate Kinase Deficiency (MKD) is an autosomal recessive inherited disorder caused by deficiency of the enzyme mevalonate kinase (ATP: mevalonate 5-phosphotransferase) causing a defect in cholesterol biosynthesis. This deficiency can lead to one of two ends of a clinical spectrum: mevalonic aciduria (MVA) and hyperimmunoglobulinemia D with periodic fever syndrome (HIDS).

Mutations in the MVK gene which lead to reduced activity of MVK are the underlying cause of both MVA and HIDS.

MVA is caused by homozygous or compound heterozygous disease-causing mutations in the MVK gene, localized to chromosome 12q24. MVA is characterized biochemically by accumulation of mevalonic acid and its metabolites such as mevalonolactone. Most patients with HIDS are compound heterozygotes for missense mutations in the MVK gene. In HIDS patients, MVK may have a residual activity of 5-15%. In contrast, no residual activity is present in MVA patients.

Patients severely affected with MVA present from birth with congenital malformations (microcephaly, dolicocephaly and wide irregular fontanels, low set posteriorly rotated ears, downslanting palpebral fissures, blue sclerae, and central cataracts). Cholestatic liver disease may be present.

Cardinal manifestations from late infancy include mild to severe intellectual disability, recurrent crises of fever, vomiting and diarrhea, failure to thrive, hypotonia and myopathy. Some hematological abnormalities may include anemia, leukocytosis, and thrombocytopenia.

In childhood, short stature and ataxia due to progressive cerebellar atrophy and ocular involvement with uveitis, cataracts and tapetoretinal degeneration become predominant findings.

Frequent crisis characterized by fever, vomiting and diarrhea are often accompanied by arthralgia, subcutaneous edema and a morbilliform rash. The severity and frequency of attacks may decline over the years.

At the other end of the clinical spectrum, HIDS is characterized by recurrent febrile attacks that usually begin before the end of the first year of life. The fever may be provoked by vaccination, minor trauma, surgery or stress and is often associated with abdominal pain, vomiting, diarrhea and cervical lymphadenopathy. Other common symptoms include hepatosplenomegaly, headache, arthralgia and rashes. Most patients display no malformations or neurological abnormalities. However, a subgroup of HIDS patients may also develop neurologic abnormalities including intellectual disability, ataxia, ocular symptoms and epilepsy, suggesting that mevalonate kinase deficiency represents a spectrum of disorders rather than two completely distinct clinical syndromes.

The diagnosis of MVA is established by the detection of elevated excretion of mevalonic acid in urine typically detected by urine organic acid analysis in the setting of clinical features of the condition. The diagnosis of MVA can be confirmed by demonstration of deficient MVK enzyme activity and/or by mutation analysis of the MVK gene. Mevalonic acid and metabolite levels in HIDS are often normal especially between episodes, and individuals may have increased immunoglobulins IgD and IgA. The diagnosis of HIDS can be confirmed mutation analysis of the MVK gene as demonstration of deficient MVK enzyme activity is not widely available and may be challenging in HIDS.

Approximately 30 patients have been reported with MVA and approximately 180 with HIDS.

There is no established therapeutic regime for patients with MVA and treatment has been largely supportive. Recently, successful disease modifying liver and hematopoietic stem cell transplantation has been reported in one child with MVA. With regards to HIDS, pharmacologic mediators of inflammation such as Etanercept, Anakinra, and Adalimumab have been increasingly used, apparently with some success, in attempts to treat HIDS.

Cerebrotendinous Xanthomatosis (CTX)

Cerebrotendinous Xanthomatosis (CTX) is an autosomal recessive inherited disorder of bile acid synthesis.

CTX is characterized typically by infantile-onset diarrhea, childhood-onset cataracts, adolescent-to young adult-onset tendon xanthomas, and progressive neurologic dysfunction. The neurologic dysfunction can include dementia, psychiatric disturbances, pyramidal and/or cerebellar signs, and seizures.

Chronic diarrhea may be the first clinical manifestation. Xanthomas appear in the second or third decade of life and can occur in the Achilles tendon, the extensor tendons of the elbow and hand, the patellar tendon, and the neck tendons. Some individuals may demonstrate intellectual impairment from early infancy while others have normal intellectual function until puberty. Neuropsychiatric symptoms may include behavioral changes, hallucinations, agitation, aggression, depression, and suicide attempts.

CTX is diagnosed by a combination of clinical features and biochemical testing, often with confirmation by DNA mutation analysis. Biochemical abnormalities in CTX include elevated blood and tissue cholestanol concentration, decreased chenodeoxycholic acid; and increased concentration of certain bile alcohols and their glycoconjugates.

CTX is inherited in an autosomal recessive manner. It is estimated to affect 1/50,000 individuals. CYP27A1 is the only gene known to be associated with CTX. Molecular genetic testing of CYP27A1 is available.

Long term treatment of individuals with CTX with chenodeoxycholic acid (CDCA) can normalize plasma and CSF concentration of cholestanol, and may lead to improvement in neurophysiologic findings. Early treatment can prevent most, if not all, clinical complications. Replacement of CDCA inhibits the conversion of cholesterol to cholestanol and reduces the production of bile alcohols. Cholic acid is an alternative treatment, but there is far less experience with this regimen and regulatory approval for this treatment is pending. CDCA is currently used off label to treat CTX.

Methylsterol Oxidase Deficiency (SC4MOL)

C4-Methylsterol Oxidase Deficiency (MOD) is a rare genetic disorder of sterol synthesis characterized by microcephaly, developmental delay, congenital cataracts, growth retardation, arthralgias and severe psoriasiform skin disease. The disease was recently identified and only 4 patients from 3 families are known. Two patients developed dramatic psoriasiform ichthyosis beginning at 5-6 years of age that proved resistant to standard treatment. Two other siblings with MOD deficiency showed severe skin disease since infancy. Patients also had evidence of inflammatory joint disease.

MOD is caused by mutations in SC4MOL, a gene in the PSORS9 region on 4q32-34, that encodes sterol C4 methyl oxidase that catalyzes demethylation of C4-methylsterols in the cholesterol synthesis pathway. Analysis of plasma and skin sterols shows marked elevation of 4-methyl- and 4,4’-dimethyl-sterols, indicating a deficiency in the first step of sterol C4 demethylation in cholesterol biosynthesis. Serum cholesterol levels are variably reduced or normal. C4-methylsterols are meiosis activating sterols and thus potentially affect cell proliferation in both skin and blood. Inhibition of sterol C4 methyl oxidase also significantly alters the immune regulation in immunocytes. Elevations in IgE and IgA may be seen. Additional studies also demonstrated diminished epidermal growth factor receptor (EGFR) signaling and disrupted vesicular trafficking in cells from the affected patients. These findings suggest that methylsterols play an important role by their influence on cell proliferation, intracellular signaling, vesicular trafficking and immune response.

Therapy for MOD is not established. The metabolic block in C4-methyl oxidase suggests that accumulation of C4-methylsterols may be the underlying cause of symptoms. Attempts have been made to reduce cholesterol precursors and restore cholesterol content in tissues. One patient with severe skin disease responded dramatically to treatment with a combination of oral and topical cholesterol and simvastatin with near resolution of skin symptoms, improved growth and reduction in the blood concentration of methylsterols. This suggests that compensation of the primary enzyme deficiency by reducing methylsterol accumulation and increasing cholesterol availability might prove to be a treatment applicable to all patients with MOD.

The incidence of MOD is not known. As seen in many inborn errors of metabolism, it is likely that the disease is under recognized.

MOD is inherited as an autosomal recessive disorder. Affected patients inherit two copies of the mutant SC4MOL gene, one from each parent, who are heterozygous carriers. Carriers do not have symptoms, but may have mildly increased methylsterols in blood.

Dolichol Metabolism Disorders (DMDs)

Dolichol Metabolism Disorders (DMDs) are a group of rare genetic diseases of dolichol synthesis and metabolism. Dolichols are aliphatic lipid molecules that are needed for attaching sugars to proteins and other lipids, forming glycoproteins and glycolipids. The abnormal synthesis or utilization of dolichols constitutes a newly described group of inborn errors of metabolism that bridge sterol metabolism and congenital disorders of glycosylation (CDGs). In the last decade, seven inherited disorders in the metabolism of the dolichols have been described, with more disorders likely to be identified in this pathway. All seven known dolichol disorders lead to hypo-glycosylated target proteins, which allows for the identification of these disorders in the diagnostic laboratory.

The cholesterol pathway generates a number of nonsterol isoprene compounds, the most prominent represented by ubiquinone and the dolichols (Figure 1). Isoprene (2-methyl-1,3-butadiene), one of the most abundant molecular building blocks in nature, is represented in the proximal pathway of cholesterol biosynthesis in the form of isopentenyl phosphate (IPP). Condensation of IPP with an additional activated isoprene, dimethylallyl diphosphate, yields geranyl diphosphate, which is further metabolized to farnesyl diphosphate and eventually ubiquinone and the dolichols. The latter are structurally similar, yet diverse, long-chain unsaturated intermediates that terminate in a free alcohol moiety. This dolichol may undergo biological activation to produce both mono- and diphosphate dolichols, the latter conjugating with various carbohydrates (glucose, galactose, mannose). Activated dolichol sugars serve as the carbohydrate donor to growing oligosaccharide chains of post-translationally modified proteins (glycoproteins) and lipids (glycolipids). These post-translational modifications encompass N-linked glycosylation (N=the nitrogen side-group of the amino acids asparagine or arginine), O-linked glycosylation (O=the oxygen in the alcohol side groups of serine, threonine or tyrosine) and C-linked glycosylation (C=a carbon in the side chain of tryptophan). Once the sugar is donated, the dolichol carrier is recycled for further intracellular reactions.

Glycoproteins and dolichol-dependent glycolipids are important for the function of many tissues and organs. Individuals with DMDs have deficiency of an enzyme that is necessary for making dolichols, which results in failure to make glycoproteins and certain glycolipids. The lack of these glycosylated molecules in DMD individuals usually causes symptoms involving multiple organ systems. Symptoms include:

• Dysmorphic facies
• Brain cerebellar or cortical atrophy
• Developmental delay/intellectual disability
• Microcephaly
• Seizures
• Stroke-like episodes
• Hypotonia
• Muscular dystrophy
• Ataxia/spasticity
• Ophthalmologic abnormalities (cortical visual impairment retinitiis pigmentosa, nystagmus, optic atrophy, coloboma, cataracts, glaucoma, micro-ophthalmia)
• Cardiomyopathy or congenital heart defects
• Cardiac dysfunction
• Failure to thrive and feeding problems
• Liver dysfunction
• Coagulopathy
• Microcytic anemia
• Frequent infections
• Ichthyosis
• Skeletal abnormalities

DMDs are inherited in an autosomal recessive fashion. Parents are heterozygous carriers and have a 25% (1 in 4) chance of passing along two DMD genes to their child, who is affected by DMD.

Owing to the overlapping spectrum of symptoms and the lack of diagnostically available enzyme tests, DMDs are initially suspected by demonstrating defective glycosylation of serum proteins, usually glycosylated transferrin. The specific diagnosis of a DMD requires genetic sequencing of the known genes causing these diseases.

There is no specific therapy for the DMDs. Symptoms are treated as they arise using conventional therapy.

Peroxisome Biogenesis Disorders (Zellweger Spectrum Disorders)

Peroxisome Biogenesis Disorders (PBDs; also known as Zellweger Spectrum Disorders) are a group of rare genetic diseases that have impaired biogenesis of peroxisomes. PBDs are characterized by defective peroxisomal protein import resulting in mislocalization of peroxisomal enzymes to the cytosol where they are not functional and rapidly degraded. This results in impaired peroxisomal metabolism of a number of molecules including very long-chain fatty acids, phytanic acid (a dietary branched-chain fatty acid), ether glycerolipids (plasmalogens), pipecolic acid (lysine metabolite), bile acids and other metabolites.

The suspicion of a PBD is usually confirmed by measuring the peroxisomal metabolites in blood and urine, and identifying multiple biochemical abnormalities. Defective peroxisomal b-oxidation results in elevated very long-chain fatty acids, pristanic acid and phytanic acid, which can be measured in blood. Defective synthesis of plasmalogen lipids, which are important in myelin membranes, can be detected as reduced plasmalogens in erythrocyte membranes. Impaired synthesis of mature bile acids from cholesterol results in accumulation of abnormal bile acid intermediates. Defective peroxisomal lysine degradation results in pipecolic acid accumulation. Impaired peroxisomal a-oxidation causes phytanic acid accumulation.

The many biochemical abnormalities in PBD individuals usually cause severe neurologic symptoms with multiple organ involvement. Symptoms may include a combination of:

• Dysmorphic facies with dolichocephaly, frontal prominence and large anterior fontanelle
• Hypotonia
• Seizures
• Dysplastic brain abnormalities, cortical atrophy and/or white matter disease
• Spasticity
• Visual impairment, cataracts or blindness
• Deafness or hearing impairment
• Developmental delay/intellectual disability
• Failure to thrive, feeding problems and growth delay
• Adrenal insufficiency
• Liver dysfunction, cirrhosis
• Bone abnormalities and osteoporosis

Patients with PBDs have historically been diagnosed as Zellweger syndrome, neonatal adrenoleukodystrophy and infantile Refsum disease, which corresponds to the most severe, intermediate and mildest phenotypes, respectively. These diseases were named before their shared peroxisomal etiology was appreciated. It is now generally agreed that these diagnostic categories represent a continual spectrum of severity. Some patients cannot be readily classified into one of the 3 diagnostic categories, hence their referral as “Zellweger spectrum disorders”. The PBDs should be distinguished from single protein (enzyme) defects in peroxisomal metabolism, which affect one pathway only and are usually not associated with multiple biochemical defects.

PBDs are caused by mutations in one of at least 12 identified PEX genes that code for peroxisomal proteins necessary for peroxisome biogenesis. PEX gene products include cytosolic peroxisomal protein receptors, peroxisomal membrane docking proteins and membrane transporters that function in peroxisomal import of proteins and metabolites. Proteins destined for the peroxisome carry a peroxisomal targeting signal sequence consisting of one of at least two specific amino acid sequences. Cytosolic receptors recognize the signaling sequence, bind the proteins and transport them to the peroxisome membrane where they interact with docking proteins and transporters that import them into the peroxisomal lumen. Failure to localize these proteins to the peroxisome results in impaired metabolic function.

There are generally poor genotype-phenotype correlations within the PBDs. Severe and mild phenotypes can be caused by many of the different PEX genes and mutations within each PEX gene can give rise to the wide phenotypic spectrum. This complicates the ability to select any specific gene for targeted sequencing based on the clinical phenotype. PEX1 mutations account for the greatest percentage of PBD patients but other PEX genes are also causative.

PBDs are inherited in an autosomal recessive fashion. Parents are heterozygous carriers and have a 25% (1 in 4) chance of passing along two PBD genes to their child, who is consequently affected by PBD. Heterozygous carriers do not exhibit sufficient peroxisomal biochemical abnormalities and must be diagnosed using molecular means.

Owing to the spectrum of symptoms, PBD patients are typically diagnosed using biochemical tests of peroxisomal function. The finding of abnormalities in two or more different peroxisomal functions is necessary to diagnose PBDs. The most readily available tests include measurement of plasma very long-chain fatty acids (elevated); phytanic and/or pristanic acid (elevated if consuming dairy products); and erythrocyte plasmalogen levels (reduced). Identifying elevated pipecolic acid in plasma or urine, or abnormal bile acid intermediates is also diagnostically useful. The specific genetic diagnosis of PBDs requires gene sequencing of the known genes causing these diseases.

The advent of newborn screening for X-linked adrenoleukodystrophy by measuring elevated very long-chain fatty acids in dried blood spots promises to simultaneously identify PBD newborn infants as a screening by-product.

There is no specific therapy for the underlying genetic defect in PBD. Symptoms are treated as they arise using conventional therapies. Treatment is available for some symptoms, for example, adrenal insufficiency (adrenal hormone replacement), hearing impairment (hearing aids, cochlear implant), seizures (anticonvulsants), and osteoporosis (bisphosphonates). Attempts to treat individual peroxisomal functions, such as accumulation very long-chain fatty acids or phytanic acid using dietary approaches, have so far had minimal clinical efficacy. It is likely that a combination of therapeutic approaches may be more promising for treatment of the post-natal symptoms.