Chapter 20
Peroxisomal Disorders
Anita MacDonald and Eleanor Baldwin
Introduction
Peroxisomal disorders are a group of inherited metabolic heterogeneous disorders resulting from dysfunction of either peroxisomal biogenesis or peroxisomal functions [1, 2]. Peroxisomes are cellular organelles that have an important role in at least eight different metabolic pathways [1] including the β-oxidation of very long chain fatty acids (VLCFA); the production of plasmalogens (a class of phospholipids); and the synthesis of bile acids [3]. Peroxisomes normally metabolise as much as 20% of cellular oxygen [4].
Peroxisomal disorders are divided into two main categories:
- single peroxisomal (enzyme) protein deficiency, e.g. X-linked adrenoleukodystrophy and Refsum’s disease
- peroxisome biogenesis disorders (PBD) including Zellweger syndrome, neonatal adrenoleukodystrophy, infantile Refsum’s disease and rhizomelic chondrodysplasia punctata [3, 5] (Table 20.1). They vary in their age of onset, clinical symptoms, tissues affected and pathology.
Table 20.1 Peroxisomal disorders
| Group 1: Single peroxisomal enzyme (transporter) deficiencies X-linked adrenoleukodystrophy; (X-ALD/AMN) Refsum’s disease Rhizomelic chondrodysplasia punctata (type 2) (DHAPAT deficiency) Rhizomelic chondrodysplasia punctata (type 3) (alkyl-DHAP synthase deficiency) D-Bifunctional protein deficiency α-Methylacyl-CoA racemase deficiency Glutaryl-CoA oxidase deficiency |
| Group 2: Peroxisomal biogenesis disorders Zellweger syndrome Infantile Refsum’s disease Neonatal adrenoleukodystrophy Rhizomelic chondrodysplasia punctata (type 1) |
AMN, adrenomyeloneuropathy
DHAPAT, dihydroxyacetone phosphate acyltransferase; DHAP, dihydroxyacetone phosphate.
X-linked Adrenoleukodystrophy
Anita MacDonald
Genetics
X-linked adrenoleukodystrophy (X-ALD) is a rare, severe, neurodegenerative, X-chromosomal disorder with reduced peroxisomal VLCFA β-oxidation activity [6, 7]. It causes progressive brain and peripheral demyelination and adrenal insufficiency in males [8, 9]. It is characterised by accumulation of saturated and mono-unsaturated VLCFA, especially hexacosanoic acid (C26:0) and tetracosanoic acid (C24:0), primarily in the nervous system, adrenal cortex and testis [8]. It produces wide ranging and unpredictable clinical phenotypes with members of the same family presenting with different phenotypes [10].
It is estimated that 93% of X-ALD cases are caused by mutations in the ABCD1 gene [11, 12], encoding the peroxisomal protein ATP binding cassette transporter D1. It is suggested that this plays a crucial role in transporting VLCFA, or their CoA derivatives, into peroxisomes in humans [7]. More than 1000 distinct mutations in the defective gene are described [13]. The remaining 7% of cases are caused by a de novo mutation [14]. The disorder has been reported in all races and geographical locations. Estimates of the incidence of X-ALD are 1 in 15 000 [15] in males and 1 in 14 000 in females [16]. As females have two X chromosomes they don’t usually show the clinical features of X-ALD. There is no correlation between X-ALD phenotype, ABCD1 gene mutations [13] and concentrations of VLCFA in the plasma and fibroblasts [17], although a combination of genetic and environmental factors may modulate the outcome of the condition [18, 19].
Clinical features
There are at least six known clinical phenotypes which vary greatly with respect to expression, age of onset, rate of progression and therapy (Table 20.2). Pre-symptomatic boys may progress to one of several phenotypes between the age of 3 and 50 years. Presentation ranges from the rapidly progressive childhood cerebral form to the more slowly progressive adult form (adrenomyeloneuropathy, AMN) and variants without neurological involvement even within the same family.
Table 20.2 Clinical phenotypes of X-linked adrenoleukodystrophy
| Clinical phenotype | Age of presentation (years) | Relative frequency (%) | Features | Progression |
| Childhood cerebral ALD | 3–10 | 31–57 | Progressive behavioural, cognitive and neurological impairments resulting in total disability within 2 years. Addison’s disease. Behavioural changes, school failure, dementia, psychoses, paralysis, epilepsy, loss of vision, loss of speech | Rapid, rarely slowly |
| Adolescent ALD | 11–21 | Similar to childhood cerebral ALD. Resembles cerebral childhood ALD but with slower progression | Rapid, rarely slowly | |
| Adult cerebral ALD | >21 | 1–3 | Similar to childhood cerebral ALD. Symptoms resemble schizophrenia with dementia. Addison’s disease | Rapid, sometimes slowly |
| AMN | >18 | 25–40 | Symptoms include leg stiffness, clumsiness, progressive spastic paraparesis (stiffness, weakness and/or paralysis) of the lower extremities, ataxia, impotence. Primary adrenal failure | Slowly, sometimes rapidly |
| Addison’s disease only | >2 | 8–14 | Fatigue, hypotension, diffuse or focal bronzing of skin. May occur at any age | – |
| Pre- or symptomatic ALD | – | 4–10 | Genetic and biochemical disorder without any evident neurological involvement. Risk of developing neurological symptoms high, but some patients remain asymptomatic for many years | – |
AMN, adrenomyeloneuropathy.
Source: adapted from van Geel et al. [87].
Cerebral X-ALD (childhood or adulthood)
This causes a rapidly progressive inflammatory demyelination of the white matter within the brain [19, 20] and may involve autoimmune mechanisms [21], mostly causing severe disability. It usually occurs between 4 and 12 years [21] with a peak at 7–8 years, and less frequently in teenagers and adults [22]. Early symptoms in children are behavioural changes (withdrawn or hyperactive behaviour) and visual impairment. Brain magnetic resonance imaging (MRI) abnormalities precede symptoms [16], but unfortunately in many cases diagnosis is made only after significant deterioration in neurological function. In some cases the demyelinating process can stop spontaneously without further progression when it is not associated with disruption of the blood–brain barrier [23]. Death may occur within 5 years of the onset of neurological symptoms [24] but some patients survive for several years with severe neurological impairment and are bed ridden, blind and unable to eat or speak [25].
Symptoms in adults include psychosis, mania, depression, spastic paraparesis, epilepsy, optic atrophy and adrenal insufficiency [26]. It may be mistaken for a psychiatric disorder such as schizophrenia with dementia [27]. Addison’s disease may precede overt neurological involvement in about 80% of males [23]. The mechanisms leading to neuroinflammation are unknown but data suggest there may be a relationship between the expression of inflammation mediators, impairment of peroxisomal function and accumulation of VLCFA in the cerebral ALD brain [28]. Oxidative stress appears to play a role in the neurodegenerative features [29].
Adrenomyeloneuropathy
This is the slightly more common adult form with symptoms and signs of spinal cord involvement usually occurring between the ages of 30 and 40 years [30]. It causes a non inflammatory, slowly progressive distal axonopathy in the spinal cord tract and peripheral nerves, with patients surviving to their eighth decade [13]. About 80% of men with AMN have impaired adrenocortical function at the time neurological symptoms are first noted [23]. Approximately half the patients have normal MRI results although they are at risk for developing the cerebral form of X-ALD.
Addison only phenotype
These are male patients who have primary adrenocortical insufficiency without clinical or MRI evidence of neurological involvement [16].
Asymptomatic/normal MRI phenotype
These are males commonly diagnosed by family screening and they have the biochemical and gene abnormality of X-ALD. They are at high risk of developing one of the other phenotypes at a later stage.
Heterozygous females
Neurological symptoms may occur in up to 50% of women [31] and vary in severity from mild hyperflexia and vibratory sense impairment, with little or no functional disability, to severe paraparesis in middle age or later [13]. Cerebral involvement is rare and adrenal insufficiency occurs in less than 1% [23]. There is no correlation between clinical severity and VLCFA blood levels [9].
Diagnosis
The diagnosis of X-ALD is made with clinical findings, brain MRI, plasma concentrations of VLCFA and molecular genetic testing of ABCD1 gene [14]. 99.9% of hemizygous males and 85% of heterozygous female carriers will have increased levels of VLCFA in plasma [32], but a normal result does not exclude carrier status [33].
X-ALD is probably underdiagnosed [25] and may be misdiagnosed as attention deficit hyperactivity disorder in boys and as multiple sclerosis in men and women [31]. In female carriers, molecular analysis of the ABCD1 gene should provide a reliable diagnosis [33].
Genetic counselling of family members is essential [34] and molecular genetic testing has been used primarily to determine carrier status in at risk female relatives and for prenatal diagnosis when the nature of the familial mutation is known.
Treatment
Adrenal insufficiency is treated with appropriate steroid replacement therapy. The prognosis for patients with symptomatic childhood cerebral X-ALD is poor and specific treatment options are limited. For boys with asymptomatic childhood cerebral X-ALD treatment options include
- Lorenzo’s oil
- bone marrow transplant before the onset of neurological symptoms
The inability to predict the future clinical course in individual patients is a major problem when considering the choice of appropriate therapy.
Dietary treatment
Although VLCFA are obtained from dietary sources they are mainly derived from endogenous synthesis through elongation of medium and long chain fatty acids [6]. Consequently, diet therapy has been designed to limit the intake of C26:0 fatty acids and to decrease their synthesis [35] by competition for the microsomal elongation system. The diet is based on Lorenzo’s oil and a moderate fat restriction. The aim is to achieve normal plasma C26:0 concentrations [10]. The diet is summarised in Table 20.3.
Table 20.3 Summary of diet therapy used in adrenoleukodystrophy
| Lorenzo’s oil | Glycerol trioleate oil (GTO) | Moderately low fat diet | Vitamin and mineral supplementation | Essential fatty acids | Energy supplements |
| Description 4 parts GTO: 1 part GTE Dose 20% of energy intake Some boys need less than this to normalise C26:0 levels. Administration Give 2–3 times daily Give neat as a medicine, or mixed with skimmed milk and flavouring or fruit juice Can also be mixed with very low fat yoghurt Not ACBS prescriibable | Description Rich in oleic acid, free of C26:0 Dose No set dose Administration Use for frying potatoes, fish and meats, salad dressings Not ACBS prescribable | Description Give 15% of dietary energy as fat (try not to exceed 35% energy from fat) Note: No other dietary restrictions are necessary | Description Diet is low in fat soluble vitamins and commonly trace elements. Type of supplements Give comprehensive vitamin and mineral supplement, e.g. Fruitvits or Paediatric Seravit. Monitoring Vitamin and mineral status must be monitored annually | Description Diet is low in essential fatty acids Lorenzo’s oil leads to reduced levels of omega 6 and omega 3 fatty acids Type of supplements Give 1%–2% of total energy from essential fatty acid supplement It should provide a source of linoleic acid and α-linolenic acid in the ratio 4:1 to 10:1, e.g. walnut oil | Description Energy intake may be low Type of supplements Useful ACBS energy supplements include glucose polymers, glucose drinks (liquid Polycal) and fortified fat free fruit ‘juice’ drinks (Fortijuce, Paediasure Plus Juce) |
ACBS, Advisory Committee on Borderline Substances; GTE, glyceryl trierucate oil.
Lorenzo’s oil
Lorenzo’s oil (Table 20.4) is a blend of four parts glycerol trioleate (GTO) and one part glycerol trierucate (GTE) (the triacylglycerol forms of oleic acid C18:1n-9 and erucic acid C22:1n-9). It is therefore high in mono-unsaturated fatty acids. It was developed by Augusto and Micaela Odone to treat their son, Lorenzo, after he was diagnosed with ALD in the 1980s [36].
Table 20.4 Composition of Lorenzo’s oil
| Nutritional information | Composition (per 100 mL) |
| Energy kcal kJ | 807 3320 |
| Protein g | Nil added |
| Carbohydrate g | Nil added |
| Fat g | 89.7 |
| Typical fatty acid profile | g/100 g fatty acids |
| C16:0 | 0.8 |
| C17:0 | 0.16 |
| C17:1 | 0.16 |
| C18:0 | 2.4 |
| C18:1 | 73 |
| C18:2 | 3.25 |
| C18:3 | 0.16 |
| C20:1 | 0.48 |
| C22:0 | 0.02 |
| C22:1 | 19.1 |
| C24:1 | 0.36 |
| Other | 0.14 |
How does Lorenzo’s oil work?
Fatty acid synthesis and elongation are complex highly regulated processes. Endogenous VLCFA are synthesised in microsomes by a series of elongation steps [37] with mono-unsaturated and saturated VLCFA sharing the same microsomal enzyme system [38]. As Lorenzo’s oil contains a high proportion of long chain mono-unsaturated fatty acids it probably works by competitive inhibition of the elongation of the saturated docosanoic acid (C22:0) to hexacosanoic acid (C26:0) [39]. In untreated patients the rate of synthesis of C26:0 is increased and the ability to degrade saturated VLCFA is impaired [40] but, when Lorenzo’s oil is administered, sustained lowering of plasma C26:0 is achieved.
Dosage and use of Lorenzo’s oil
It is recommended that 20% of total energy intake is given from Lorenzo’s oil [41]. Examples of calculated daily dosage is given in Table 20.5. Lorenzo’s oil is 90% fat providing 8 kcal (34 kJ)/mL. Although this dose normalises plasma C26:0 levels, the figure of 20% is arbitrary. Because of an apparent dose–response effect of Lorenzo’s oil on lowering C26:0 concentrations, it has no benefit unless substantial and sustained lowering of C26:0 concentrations are achieved [10, 42]. The GTE component is a solid fat at room temperature. It is about 93% erucic acid which is purified from rapeseed oil. When GTO and GTE are combined a clear yellow liquid is produced, although the GTE can solidify and form white sediment at ambient temperatures.
- Lorenzo’s oil should be left at room temperature for 1 hour before use but otherwise should be stored in a refrigerator.
- The bottle should be shaken very well until the white sediment of GTE is evenly distributed throughout the oil so that a homogeneous dose of oil can be given.
- The daily amount of oil should be divided into two or three doses throughout the day. There is no evidence to suggest that its efficacy is affected if taken at different times to meals or in a single dose, although children may develop diarrhoea if the oil is taken in one dose.
- It is difficult to disguise the oily taste or consistency of Lorenzo’s oil. Many children take it as a medicine direct from a spoon or by syringe. Some prefer to take it mixed with skimmed milk and milk shake flavouring or fruit juice; unfortunately, it does not mix well with either.
- It may be easier to take the mixture in a covered cup or beaker to help mask the smell and the poorly dispersed fat. Others mix the oil with yoghurt or other low fat desserts.
- It is not recommended to cook with the oil.
Table 20.5 Suggested daily dosage of Lorenzo’s oil
| Age (years) | Estimated average requirement for energy kcal (kJ)/day [88] | Daily requirement Lorenzo’s oil (mL) (20% of energy intake) |
| 1–3 | 1230 (5150) | 30 |
| 4–6 | 1715 (7160) | 45 |
| 7–10 | 1970 (8240) | 50 |
| 11–14 | 2220 (9270) | 55 |
Lorenzo’s oil is not recommended before 18 months of age because it may lower levels of docosahexaenoic acid (DHA) which plays an important role in early retinal and brain development [42]. It is unclear at what age Lorenzo’s oil therapy could be discontinued in patients who remain neurologically normal and there are no recommendations about stopping diet. There is some evidence of benefit in adult patients with AMN [43] and heterozygous female carriers. It is not available on prescription (Advisory Committee on Borderline Substances, ACBS).
Glyceryl trioleate oil
Glyceryl trioleate oil (GTO) is often used in addition to Lorenzo’s oil, specifically as cooking oil. It is a pale yellow oil free of C26:0 and rich in oleic acid (Table 20.6). It is 90% fat providing 8 kcal (34 kJ)/mL. It is useful in the preparation of salad dressings, cakes and biscuits and for frying potatoes, crisps, fish and meats. It should be stored at 4°C under dry conditions. It is not currently available on ACBS prescription and is quite expensive to buy.
Table 20.6 Composition of glyceryl trioleate oil
| Nutritional information | Composition (per 100 mL) |
| Energy kcal kJ | 819 3367 |
| Protein g | Nil added |
| Carbohydrate g | Nil added |
| Fat g | 91 |
| Typical fatty acid profile | g/100 g fatty acids |
| C16:0 | 1 |
| C17:0 | 0.2 |
| C17:1 | 0.2 |
| C18:0 | 3 |
| C18:1 | 91 |
| C18:2 | 4 |
| C18:3 | 0.2 |
