Maple syrup urine disease
Jan. 08, 2023
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Adult Refsum disease (ARD) is a rare, autosomal recessive disorder that most frequently manifests in young adults as a variable combination of early-onset retinitis pigmentosa, anosmia, peripheral polyneuropathy, cerebellar ataxia, sensorineural hearing loss, and ichthyosis. Adult Refsum disease is caused by a defect in the catabolism of phytanic acid, a dietary branched chain fatty acid (BCFA), which leads to its toxic overaccumulation in the body. Although its neurologic phenotypes are often irreversible by the time of diagnosis, appropriate dietary interventions can result in clinically relevant neurologic improvements in some people with adult Refsum disease.
• Adult Refsum disease (ARD) is a rare, autosomal recessive disorder caused by an impaired ability to breakdown the branched chain fatty acid phytanic acid that can accumulate to toxic levels in tissues.
• Adult Refsum disease typically presents in early adulthood as a variable combination of retinitis pigmentosa, anosmia, peripheral polyneuropathy, cerebellar ataxia, sensorineural hearing loss, and ichthyosis.
• Adult Refsum disease patient typically have loss-of-function variants in the PHYH gene encoding phytanoyl-CoA alpha-hydroxylase, an enzyme involved in phytanic acid catabolism.
• In humans, phytanic acid is solely acquired from dietary sources, primarily ruminant meats and fats, dairy products, and certain fish.
• Adult Refsum disease is managed by life-long dietary avoidance of phytanic acid and dietary management is supplemented by lipid apheresis when acute lowering of phytanic acid levels is indicated.
Adult Refsum disease (ARD), originally referred to as “heredoataxia hemeralopica polyneuritiformis” (99) and then "heredopathia atactica polyneuritiformis” (100), is a neurologic syndrome first described by Norwegian neurologist Sigvald Refsum (21).The 4 Norwegian cases originally reported by Refsum had what is now considered to be a diagnostic tetrad of clinical findings that include retinitis pigmentosa, peripheral polyneuropathy, cerebellar ataxia, and a high cerebrospinal fluid protein concentration without pleocytosis (100). Although Jan Cammermeyer suggested that its pathology might results from a disturbance in lipid metabolism (100; 15; 16), the nature of the pathogenesis and pathophysiology was unknown until 1963 (118). At that time, postmortem studies of liver and kidney tissue from a patient diagnosed with this disorder revealed fatty infiltrates composed mainly of neutral lipids, providing the first evidence that it was a lipidosis (59). More than half of the total fatty acids isolated from liver lipids were a single, unusual species not previously reported in human tissues and subsequently identified as phytanic acid (3,7,11,15-tetramethylhexadecanoic acid). The discovery that a phytanoyl CoA hydroxylase enzyme is involved in phytanic acid alpha-oxidation was followed by the discovery that adult Refsum disease is caused by a phytanoyl-coenzyme A hydroxylase deficiency (55). The phytanoyl-coenzyme A hydroxylase gene (now known as PHYH) was cloned in 1997 (53; 84).
Adult Refsum disease is frequently referred to as “Refsum disease” and should not be confused with the peroxisome biogenesis disorder "infantile Refsum disease." Scotto and colleagues found elevated levels of phytanic acid in 3 infants and suggested that this "infantile phytanic acid storage disease" was a variant of adult Refsum disease (113). Other biochemical defects not found in typical adult Refsum disease patients were present in these cases, including elevated plasma levels of very long-chain fatty acids and pipecolic acid, decreased plasmalogen synthesis, and abnormal subcellular catalase distribution (72). As a result of these additional findings, it is now recognized that phytanic acid accumulation in these children is not the primary defect but is secondary to a peroxisome biogenesis disorder. Zellweger spectrum disorder is the modern term encompassing Zellweger syndrome (severe), neonatal adrenoleukodystrophy (moderate), and infantile Refsum disease (milder), which are all caused by inherited biallelic defects in PEX genes responsible peroxisome assembly, structure, and downstream functions (09).
Left untreated, adult Refsum disease is a degenerative condition that most typically presents in early adulthood with a subset (but rarely all) of the following clinical findings: retinitis pigmentosa, anosmia/microsmia, sensory motor neuropathy, hearing loss, cerebellar ataxia, and ichthyosis (Table 1). Cardiac arrhythmias and cardiomyopathy are often observed in adults, whereas short metacarpals and metatarsals are often present at birth (Table 1). Elevated plasma concentrations of phytanic acid are required for diagnosis. Carriers of adult Refsum disease deleterious variants do not manifest clinical signs or symptoms of disease and generally have normal plasma phytanic acid levels.
• Retinitis pigmentosa+
• Cerebellar ataxia+
• Peripheral polyneuropathy (motor and sensory)+
• Cardiac involvement (nonspecific electrocardiogram abnormalities)++
• Symptoms of cranial nerve involvement:
- Neurogenic hearing loss++
- Abnormal pupillary reflex+++
• Skeletal malformations (short metacarpals and metatarsals)+++
• Skin changes (dry skin, ichthyosis)+++
• Increased CSF protein without pleocytosis+
• Elevated plasma phytanic acid concentration+
Clinical course. Night blindness (nyctalopia) is typically the first symptom noticed by adult Refsum disease patients. Other early manifestations include ataxia and other cerebellar signs, frequently overlooked as simply "clumsiness" in the initial stages of the disease. Subsequently, patients develop a peripheral neuropathy that eventually leads to wasting and distal paralysis. The first appearance of symptoms can range from early childhood to the sixth decade, but they usually begin during the second or third decade of life (116). The disease is progressive with gradual deterioration if untreated. Even without treatment, acute exacerbation of symptoms followed by nearly complete remission is not uncommon. These exacerbations are frequently associated with stress, such as pregnancy or an infection. Respiratory failure is the cause of death in some patients, and sudden death has been observed in several cases (101). With proper treatment, the life expectancy of adult Refsum disease patients is normal.
Primary clinical features. Patients with adult Refsum disease are always found to have retinitis pigmentosa of the "salt-and-pepper" type (20). The degree of retinal involvement and the extent of the visual field defect may depend on the stage of the disease. Retinal changes, once developed, are generally unresponsive to dietary treatment (43). Nevertheless, there was a report of a 51-year-old patient with adult Refsum disease who underwent electroretinography before and after beginning a phytanic acid-restricted diet (07). Although their post-intervention 30 Hz flicker electroretinogram demonstrated significantly improved waveform amplitudes and implicit times in both eyes suggested improved retinal function, there was a lack of measured improvement in Snellen visual acuity.
The peripheral neuropathy of adult Refsum disease is of the mixed motor and sensory type and is chronic and progressive if untreated (133). Symptoms are generally first noted in the distal lower extremity and then in the small muscles of the hand; involvement is usually symmetrical. Deep sensation is disturbed, and deep tendon reflexes are diminished. Electrophysiologic studies reveal slowed motor nerve conduction velocities (69; 76). Although the possibility has been raised that the ataxia present in all cases is secondary to this polyneuropathy, it is generally thought that the severity of the symptoms cannot be explained on this basis alone and that patients have true cerebellar ataxia. Although cerebellar ataxia is a component of Refsum’s original diagnostic tetrad, 2 siblings with clinical and biochemical features of adult Refsum disease, but without cerebellar involvement, have been reported (34).
Other clinical findings. Most patients have evidence of cardiac involvement (136). Common findings include tachycardia, gallop rhythm, systolic murmur, and enlargement of the heart (103). Evidence of conduction disturbances, sinus tachycardia, nonspecific ST-segment and T-wave (ST-T) changes, or myocardial damage is often present on electrocardiograms. For this reason, it has been proposed that fatal cardiac arrhythmias may be responsible for some cases of sudden death reported in untreated adult Refsum disease (136). Symptoms, such as neurogenic hearing loss, anosmia, disturbed pupillary reflex, and miosis, indicate cranial nerve involvement. Hearing loss, like retinal degeneration, is usually unresponsive to treatment. More than half of patients exhibit skeletal malformations, including nonspecific (pes cavus or hammer toes) or specific (symmetrical hyperplasia and hypoplasia of fingers, toes, metacarpals, and metatarsals) deformities (145). Many patients also have skin changes, which range in severity from dry skin to ichthyosis. Hyperkeratosis and accumulation of fat droplets within basal keratinocytes are noted histologically (98; 83).
Late-onset adult Refsum disease presenting as a leukoencephalopathy. A single case of a patient with no neurologic symptoms prior to 69 years of age has been reported (08). She presented with balance difficulties, memory problems, and gait apraxia, but no clear cerebellar ataxia. Brain MRI showed a leukoencephalopathy involving the periventricular white matter, subcortical area, and the brainstem with relative sparing of juxtacortical U fibers. Electromyography showed no polyneuropathy, and her electroretinogram was normal. Cerebrospinal fluid (CSF) showed 2 white cells per cubic mm and slightly increased protein content of 0.46 g/L. Plasma lipid analysis showed significantly elevated phytanic acid levels but normal pristanic and very long-chain fatty acid levels. Genetic analysis revealed that she was homozygous for a frameshift variant that resulted in a loss-of-function PHYH allele.
Although this can happen without apparent cause, the exacerbation and remission of adult Refsum disease symptoms have been associated with stress, concurrent illness, or pregnancy. In addition, a paradoxical rise in the plasma phytanic acid concentration and clinical relapse has been observed when a patient on dietary therapy experiences a sudden weight loss. In these situations, plasmapheresis to lower circulating phytanic acid may be useful. If neurologic deficits have not progressed to the point of irreversibility, aggressive therapy (eg, weekly plasma exchange) may yield some improvement.
GK, a fictitious 55-year-old male, developed nyctalopia (night blindness) in childhood that worsened in his late teens. A diagnosis of retinitis pigmentosa was made at 33 years of age. At the same age, the patient first showed signs of mild hearing loss. The patient has had high foot arches since childhood and dry skin for many years. At age 40, he underwent a comprehensive series of medical examinations. The patient showed an impaired walking ability with feet slapping and an inability to stand on his heels. There was bilateral calf atrophy, and his leg muscles were weak. His knee and tricep reflexes were trace, and reflexes were absent at his ankles. Physical examination showed bilateral pes cavus. The patient had a decreased response to pinprick below the elbows and knees bilaterally, his vibratory sense was absent below the knees, and there was decreased position sense in the toes. He had palpable ulnar and superficial peroneal nerves. A left sural nerve biopsy showed chronic neuropathy with loss of myelinated fibers. Nerve conduction velocities were slow, and there was decreased amplitude of sensory potentials for right median and ulnar nerve. He was not ataxic, and the results of a Romberg test were negative.
At the time of examination, his visual, auditory, and olfactory senses were evaluated. He displayed pupillary abnormalities and iris atrophy in both eyes. Fundoscopic examination revealed bilateral diffuse retinal pigment epithelial degeneration, mid‐peripheral pigment clumps, and retinal arteriolar narrowing. Electroretinograms were nearly extinguished in both eyes and there was a severe bilateral constriction of his visual fields. A slit lamp examination showed bilateral subcapsular lens opacity in both eyes. Pure tone audiogram revealed bilateral hearing loss at middle and high frequencies. Auditory brainstem evoked responses were recorded and suggested subtle bilateral auditory nerve involvement. The patient showed evidence of moderate microsmia in a smell identification test.
To make a molecular diagnosis, a series of biochemical and genetic tests were conducted. Serum phytanic acid level was determined to 416 µg/ml (normal is less than 3 µg/ml). CSF examination showed an elevation of protein to 127. Clinical whole exome sequencing indicated the presence of 2 deleterious loss-of-function alleles in the PHYH coding sequence, one missense variant affecting a conserved residue near an Fe(II) binding site and one nonsense variant in the second exon. A molecular diagnosis of adult Refsum disease is made.
After diagnosis, patient was started immediately on a low phytanic acid diet. He also underwent a long course of plasmapheresis, with improvement in his phytanic acid levels and in his neuropathy. When seen at 52 years of age, his vision loss had worsened, but his hearing had remained stable.
• Adult Refsum disease is caused by elevated tissue concentrations of phytanic acid, most frequently secondary to a defect in phytanoyl-CoA hydroxylase (PHYH), a peroxisomal enzyme in the phytanic acid alpha-oxidation pathway.
• Genetic defects affecting PEX7, a protein required for the peroxisomal import of PHYH protein, are also a recognized cause of adult Refsum disease, but are far less common than cases caused by PHYH deficiency (126).
Accumulation of phytanic acid in plasma and tissues. Although the concentration of phytanic acid in plasma of normal individuals is nearly undetectable, it can account for up to 30% of the total plasma fatty acids in some patients with adult Refsum disease. The hepatic and renal fatty infiltrates found in patients are neutral lipids containing significant quantities of phytanic acid. Plasma levels presumably reflect both the dietary intake and the tissue stores.
Origin of phytanic acid. The observation that phytanic acid structurally resembles phytol, the side chain of chlorophyll, gave rise to the hypothesis that the diet is the main source of this fatty acid (117). Studies in rats and humans have shown that both dietary phytol and dietary phytanic acid are efficiently absorbed and that phytol is efficiently converted to phytanic acid in vivo (117). Nevertheless, chlorophyll-bound phytol is not well absorbed in either rats or humans, suggesting that green vegetables are not a primary dietary source of phytanic acid (22). In contrast, ruminant fats and dairy products are considered a major source of phytanic acid in human diets (80; 81). Rumen bacteria efficiently release the phytol side chain of chlorophyll; phytol is then either absorbed or converted to phytanic acid. It was observed that erythrocyte phytanic acid levels in great apes and other nonhuman primates were higher than in humans when all were on a phytanic acid-poor diet (139; 88). Colon volume relative to small bowel volume is significantly higher in great apes than in humans. Although colonic absorption of nutrients is relatively minor in humans under normal circumstances, it is of major importance to the health of some non-human primates. Thus, it was hypothesized that hindgut fermentation of plant materials in the colons of great apes and other nonhuman primates studied is most likely responsible for their higher erythrocyte phytanic acid levels relative to humans when all are on phytanic acid-poor diets (139; 88).
Phytanic acid degradation pathway. Cultured primary skin fibroblasts obtained from donors with adult Refsum disease have an impaired ability to degrade phytanic acid (114; 140). As a result of the methyl branch on the 3-carbon of phytanic acid, catabolism by the classic beta-oxidation pathway is not possible. Instead, a single carbon is initially removed in an alpha-oxidation process, shifting the position of the methyl groups and allowing further degradation by beta-oxidation. Successive rounds of beta-oxidation yield alternating 3- and 2-carbon fragments; however, both the enzymatic steps and the subcellular localization of the alpha-oxidation pathway have been controversial. A series of studies conducted in the 1960s by Steinberg and colleagues revealed that in both rats and humans phytanic acid is first converted to alpha-hydroxyphytanic acid and is then decarboxylated, forming pristanic acid (2,6,10,14-tetramethylpentadecanoic acid) and carbon dioxide (116). Pristanic acid is then degraded by beta-oxidation, first via the peroxisomal pathway and, ultimately, in mitochondria (129). Evidence suggests subsequent mitochondrial oxidation requires long-chain acyl-CoA dehydrogenase (132). Both the 3R, 7R, 11R- and 3S, 7R, 11R- stereoisomers of phytanic acid are found in nature, and both can be converted to the corresponding 2R, 6R, 10R- and 2S, 6R, 10R- stereoisomers of pristanic acid by the alpha-oxidation pathway (32). However, beta-oxidation of pristanate requires the S-configuration about carbon 2. The enzyme alpha-methylacyl-CoA racemase (AMACR), which is found in peroxisomes and mitochondria and catalyzes the interconversion of 2R and 2S, is, thus, required for the complete disposition of dietary phytanic acid.
Because people with inherited defects in peroxisome biogenesis have an impaired capacity to degrade phytanic acid, it was hypothesized that the alpha-oxidation pathway is localized in peroxisomes. Studies published since 1994 have clarified both the biochemical steps and the subcellular location of this pathway. The first step is activation of phytanic acid to its coenzyme A thioester (phytanoyl-CoA) in peroxisomes, a reaction catalyzed by both long- and very long-chain acyl-CoA synthetases (138; 137; 120). Phytanoyl-CoA is converted to alpha-hydroxyphytanoyl-CoA by phytanoyl-CoA hydroxylase (PHYH), a 2-oxoglutarate- and Fe++-requiring dioxygenase (85). PHYH is a peroxisomal enzyme that contains a functional peroxisome targeting signal 2 and requires the PEX7 protein, the peroxisome targeting signal 2 (PTS2) receptor, for proper targeting to the organelle surface and subsequent import in the peroxisomal matrix (84). Intraperoxisomal sterol carrier protein-2 (SCP-2) may increase the specificity of PHYH for phytanic acid relative to other fatty acid substrates (90). Subsequent decarboxylation of alpha-hydroxyphytanoyl-CoA yields an aldehyde, pristanal, and formyl-CoA (23; 130). This reaction proceeds by a unique mechanism similar to that catalyzed by 3-hydroxy-3-methylgluatryl-CoA lyase (HMG-CoA lyase) and requires thiamine pyrophosphate (TPP) as a cofactor. Because this cofactor is not known to be required for any other peroxisomal enzyme, a study was conducted that was found that although peroxisomes do contain thiamine pyrophosphate, they cannot phosphorylate thiamine and, therefore, must take up the cofactor in phosphorylated form (38). Formyl-CoA was found to be unstable at pH greater than 5.5, spontaneously breaking down to formate and CoA under physiologic conditions (23). Formate is ultimately translocated to mitochondria and converted to carbon dioxide. Pristanal must be oxidized to pristanic acid and reactivated to its CoA thioester for subsequent degradation by peroxisomal beta-oxidation. Pristanic acid reactivation may require long-chain acyl-CoA synthetase (120). Under experimental conditions, pristanal formation from alpha-hydroxyphytanoyl-CoA was detectable only when nicotinamide adenine dinucleotide (NAD) was excluded from reaction mixtures, suggesting the involvement of a dehydrogenase (130). Most investigators now believe that the complete degradation of phytanic acid (conversion of phytanic acid to formate and pristanic acid and the subsequent beta-oxidation of pristanic acid) occurs in peroxisomes (128). However, some evidence exists, advancing the theory that pristanal dehydrogenase may be microsomal. Fatty aldehyde dehydrogenase (FALDH), the microsomal enzyme defective in the Sjögren-Larsson syndrome, was found to oxidize pristanal; furthermore, phytanic acid oxidation was reported to be defective in cultured skin fibroblasts from patients (129). In contrast, Jansen and colleagues reported normal phytanic acid oxidation and residual pristanal dehydrogenase activity in cultured skin fibroblasts from people with Sjögren-Larsson syndrome who completely lacked FALDH (54). This issue was subsequently resolved when it was reported that alternative splicing of the murine Aldh3A2 gene yields four variant forms of Faldh, one of which is targeted exclusively to peroxisomes (03). The organization of mouse and human genes are similar, and these authors found that the peroxisomal variant, FALDH-V, plays an essential role in the efficient degradation of branched chain fatty acids and that it protects cells from the damage induced by lipid peroxidation. The more common variant, FALDH-N, was localized to the endoplasmic reticulum (microsomes) and did not catalyze the oxidation of pristanal. Furthermore, these authors detected pristanal dehydrogenase activity in peroxisomal fractions isolated from rat liver. Interestingly, FALDH has been found to be required for the conversion of dietary phytol to phytanic acid (127).
An important role for liver-type fatty acid-binding protein (FABP1 or L-FABP) in phytanic acid metabolism has been established (04; 05). Overexpression of Fabp1 (L-FABP) in mice enhanced cellular phytanic acid uptake and stimulated phytanic acid alpha-oxidation but had little effect on esterification of the branched-chain fatty acid. Conversely, murine cells in which the Fabp1 gene was disrupted exhibited decreased phytanic acid uptake and decreased alpha-oxidation rates. These findings suggest that other tissue specific FABP isoforms may be important for the metabolism of phytanic acid in tissues pathologically affected in adult Refsum disease (eg, the nervous system and heart).
Omega-oxidation provides an alternative means to degrade phytanic acid (64; 65; 66; 134). In this fatty acid oxidation pathway, the terminal (omega) methyl group is oxidized to a carboxylic acid, yielding a dicarboxylic acid that can be partially degraded from the omega end by beta-oxidation. Evidence from rats and humans suggests that the overall contribution of omega-oxidation to phytanic acid degradation is minor (64; 65). Humans on Western diets typically consume 50 to 100 mg of phytanic acid daily (41; 143). Wierzbicki and colleagues have estimated the omega-oxidation capacity in adult Refsum disease patients by measuring urinary excretion of the metabolite 3-methyladipic acid and found it to be around 7 mg/day (143). In vitro studies using liver microsomes confirmed that the omega-oxidation pathway is functional in humans (66; 147); the reactions are NADPH-dependent and involve specific P450 enzymes (64; 65; 66). In addition, phytanic acid is a good substrate for human UDP-glucuronosyltransferases, suggesting that some phytanic acid might be eliminated by this mechanism (75).
The regulation of peroxisomal phytanic acid and pristanic acid metabolism may in part be controlled by the amount of free fatty acid substrate versus substrate thioesterified to CoA. Peroxisomes contain a novel isozyme of acyl-CoA thioesterase (ACOT6), which is specific for these branched-chain fatty acids (141). As noted above, phytanoyl-CoA and pristanoyl-CoA and not the free fatty acids are the substrates for peroxisomal alpha- and beta-oxidation, respectively.
Enzyme defect in adult Refsum disease. For nearly 2 decades, it was suspected that the metabolic defect in adult Refsum disease was at the level of PHYH enzyme activity. Evidence for this hypothesis was obtained in 1997, when a liver biopsy from an adult Refsum disease patient was found to have no detectable PHYH enzyme activity as compared to controls (55). The cDNA encoding this enzyme was independently cloned by 2 laboratories (53; 84), and loss-of-function (LOF) variants in the cDNA obtained from at least 22 different adult Refsum disease patients have been identified (53; 51; 52; 84; 17). About 67% were missense variants, and the rest included insertions, deletions, and splice-site variants that resulted in a large deletion through the skipping of exon 3.
Mukherji and colleagues conducted structure-function analyses of clinically observed PHYH deleterious variants and found that although most of these deleterious variants caused impaired phytanoyl-CoA hydroxylation, one such variant did not (89). PHYH containing a missense variant (P29S) was fully active, but its location near the amino terminus suggests that impaired targeting to or proteolytic processing within peroxisomes (ie, removal of N-terminal PTS2 signal by the peroxisomal TYSND1 protease) produced the clinical disease (86). Subsequently, the crystal structure of PHYH was solved to 2.5 Å resolution, and many disease-causing variants were found to cluster around the binding pockets for either Fe++ or 2-oxoglutarate (82).
Wierzbicki and colleagues used linkage analysis to study 8 genetically informative families that included 17 patients with a clinical diagnosis of adult Refsum disease. Four families (with 8 affected members) exhibited linkage to chromosome region 10p13, the adult Refsum disease locus; however, in 3 other families (including 9 affected individuals), linkage was excluded (144). These investigators found linkage to the chromosome 6q22-24 region in 2 of the latter families and subsequently identified their defective gene as PEX7 (126). Horn and colleagues published a detailed report of an adult Refsum disease patient with PEX7 deficiency and found their clinical phenotypes to be indistinguishable from those caused by PHYH deficiency. They proposed that adult Refsum disease be subdivided into type 1 (PHYH deficiency) and type 2 (PEX7 deficiency) (46).
PEX7 deficiency is the recognized cause of rhizomelic chondrodysplasia punctata type 1 (RCDP1), a disorder that presents at birth with a considerably more severe clinical phenotype (28; 01). PEX7p, the protein product of the PEX7 gene, is a cytoplasmic receptor for proteins with peroxisome targeting signal 2 (PTS2), an amino acid sequence residing near the amino terminus. In the absence of PEX7 protein, PTS2-containing enzymes are not targeted to peroxisomes, resulting in specific biochemical abnormalities. Known PTS2 proteins include phytanoyl-CoA hydroxylase (PHYH), alkyl-dihydroxyacetone phosphate synthase (AGPS or ADHAPS), and peroxisomal 3-oxoacyl-CoA thiolase 1 (ACAA1) (70). Deficiencies in ADHAPS and peroxisomal thiolase were also found in the 2 families with PEX7 variants (126). A case report involving affected twins further suggests that a broad phenotypic spectrum for PEX7 deficiency exists and that other tests of peroxisome biochemistry may be indicated in the workup of newly identified adult Refsum disease patients (79). It is possible that the remaining patients whose defects have not yet been identified have deleterious variants in the HACL1 gene encoding alpha-hydroxyphytanoyl-CoA lyase (HACL1), the enzyme following PHYH in the alpha-oxidation pathway. This enzyme was purified from rat liver and cDNA encoding its human ortholog was cloned (36). At present, patients with germline mutations in the HACL1 gene have not been reported in the literature.
Pathogenesis. All evidence from biochemical studies suggests that elevated plasma and tissue levels of phytanic acid are directly or indirectly responsible for the clinical manifestations of adult Refsum disease. Furthermore, reducing the body burden of phytanic acid with dietary therapy leads to clinical improvement (106). Several potential pathogenic mechanisms have been suggested. Young and colleagues investigated the effects of phytanic acid on cultured retinal cells and found that it readily incorporated into cellular phospholipids, resulting in increased membrane fluidity but no increase in susceptibility to lipid peroxidation (148). They reported that phytanic acid did not competitively inhibit alpha-tocopherol uptake, weakening the hypothesis that phytanic acid in membranes interferes with vitamin E function. Phytanoyl-CoA, not free phytanic acid, is the substrate for incorporation into phospholipids, triacylglycerol, and other lipids. Phytanoyl-CoA (and pristanoyl-CoA) was found to exhibit high affinity (Kd near 11 nM) for the nuclear receptor, peroxisome proliferator-activated receptor alpha (PPAR-alpha), whereas the respective free fatty acids showed only weak binding (48). PPAR-alpha activation upregulates the expression of genes encoding enzymes involved in fatty acid beta-oxidation and other lipid metabolic pathways; however, the possibility that PPAR-alpha activation contributes to the pathogenesis of adult Refsum disease remains to be explored. Tang and colleagues incubated prostate PC-3 cells with phytanic acid and found decreased rates of cell proliferation that were associated with increased levels of the phytanate ester of retinol, suggesting that this compound could contribute to adult Refsum disease pathogenesis (124).
Mice deficient in sterol carrier protein-x, which encodes both the putative phytanic acid-binding protein, Scp2, and the 3-ketoacyl-CoA thiolase (Scpx) required for peroxisomal beta-oxidation of pristanic acid, accumulated phytanic acid in phospholipids of myocardial membranes when maintained on a phytol-enriched diet (87). This was associated with bradycardia, impaired AV nodal and intraventricular impulse conduction, and a high incidence of sudden death, strengthening the hypothesis that arrhythmias may explain sudden death in untreated adult Refsum disease patients. Foulon and colleagues investigated the substrate specificity of PHYH and found that a variety of 3-methyl-branched fatty acyl-CoAs were hydroxylated by this enzyme (and presumably were further metabolized by alpha-oxidation) (37). Thus, accumulation of 3-methyl fatty acids other than phytanic acid could contribute to adult Refsum disease pathogenesis.
Elevated phytanic acid levels may alter the behavior of the plasma membrane ion transporters. Wiersbicki and colleagues initially reported that the maximal velocity of the erythrocyte sodium-lithium countertransporter was affected by increasing plasma phytanic acid concentrations (142). Studies done using purified synaptic vesicles from young rat brain cortex revealed that membrane synaptic Na/K ATPase activity was decreased when either phytanic acid (100 to 200 µm) or its metabolite, pristanic acid (50 to 200 µm), were present in assays (13; 14). These observations led Busanello and colleagues to conclude that phytanic and pristanic acids may impair synaptic neurotransmission.
Increasing evidence suggests that phytanic acid affects mitochondrial function. Schonfeld and Wojtczak reported that exposure of isolated brain mitochondria to phytanic acid resulted in release of cytochrome c, suggesting that phytanic acid activates the mitochondrial route of apoptosis (102; 111). Schonfeld and coworkers also found that very low concentrations of phytanic acid de-energized brain mitochondria reduced state 3 respiration partly due to inhibition of the ADP/ATP carrier and sensitized the mitochondria to rapid permeability transition (110). Subsequent studies by this group indicated that phytanic acid activates intracellular calcium stores, producing mitochondrial depolarization and generating reactive oxygen species (ROS) (57). They concluded that these factors contribute to the short-term toxicity of phytanic acid and may induce the onset of neurodegeneration. Further studies demonstrated that the phytanic acid-induced increase in reactive oxygen species was associated with partial inhibition of the mitochondrial electron transport chain, most likely by changing membrane fluidity (112). In agreement with this finding, Komen and colleagues reported that phytanic acid decreased ATP synthesis and mitochondrial membrane potential in human skin fibroblasts (63). Nagai reported that in addition to its effect on mitochondrial dysfunction in Neuro2a cells, phytanic acid activated histone deacetylase activity, reducing histone acetylation and promoting cell death (91). The cytotoxic effect of phytanic acid was abolished by sodium butyrate, by a caspase-9 inhibitor, and by apicidin, a Hdac2- and 3-specific inhibitor. Nagai concluded that the toxicity of phytanic acid depends on activation of the Hdac2 and 3 subtypes.
Busanello and colleagues used homogenates of young rat brain cortex to investigate the in vitro effects of phytanic acid and pristanic acid on mitochondrial function (13; 14). Increasing concentrations of phytanic acid decreased respiratory chain activity at complexes I, II, 1-III, II-III, and IV, while increasing concentrations of pristanic acid decreased activity at complexes I, II, and II-III. Pristanic acid treatment also decreased mitochondrial carbon dioxide production from acetate, whereas phytanic acid was without effect. Subsequent work by these investigators showed that both phytanic and pristanic acids behave as uncouplers of oxidative phosphorylation (11; 12). Reiser and coworkers incubated mixed cultures of hippocampal neurons, astrocytes, and oligodendrocytes with phytanic or pristanic acid to assess effects on reactive oxygen species production and cellular calcium signaling (105; 68). They found that although reactive oxygen species production was increased by both branched-chain fatty acids, pristanic acid caused a more robust increase. In addition to effects on reactive oxygen species production, Reiser and colleagues showed that both phytanic acid and pristanic acid caused dramatic increases in cellular Ca++ concentrations. The increases were found to be mediated by the inositol triphosphate signaling cascade. Subsequently, studies revealed the involvement of a G-protein coupled receptor, GPR40, in this process. HEK293 cells overexpressing GPR40 had increased Ca++ levels when incubated with phytanic or pristanic acid, but not when incubated with the unbranched fatty acid palmitate. Leipnitz and colleagues also investigated oxidative damage caused by incubation of homogenates of rat brain cerebellum and cortex with phytanic acid (74). Phytanic acid increased thiobarbituric acid-reacting substances in both cortical and cerebellar homogenates, an effect that was prevented by the antioxidants alpha-tocopherol and melatonin. Increased carbonyl content and sulfhydryl oxidation was also observed. Glutathione levels were decreased by phytanic acid, and this effect was reversed by antioxidants. These investigators subsequently showed similar phenomena occurred in heart mitochondria, and they proposed that phytanic acid-induced disturbances of cellular energy and redox homeostasis may contribute to the cardiomyopathy seen in many adult Refsum disease patients (42).
Incubation of vascular smooth muscle cells with phytanic acid-induced TNF-alpha activation and secretion markedly increased inducible nitric-oxide synthase mRNA and protein and promoted apoptosis (50). Dhaunsi and coworkers reported that treatment of smooth muscle cells with phytanic acid increased NADPH oxidase activity (25). Phytanic acid also increased total and phosphorylated EGFR, and treatment of cells with EGFR inhibitor blocked the phytanate-induced enhancement of NADPH oxidase activity. Phytanic acid has been found to be a physiological ligand for peroxisome proliferator-activated receptor alpha (PPAR-alpha), which leads to transcriptional upregulation of many proteins including liver fatty acid binding protein (29; 146; 151). Subsequently, it was reported that PHYH is upregulated in cells incubated with phytanic acid (151; 150) or dehydroepiandrosterone (24). This physiologic response, which may be necessary to maintain a low body burden of phytanic acid, would not be operative in adult Refsum disease patients with PHYH deleterious variants. Further studies suggesting a phytanic acid/PPAR interaction were subsequently reported. Heim and colleagues showed that physiologic concentrations of phytanic acid enhanced 2-deoxy-D-glucose uptake in rat hepatocytes by increasing mRNA expression of glucose transporter-1 and -2, and glucokinase (45). Lampen and colleagues found that phytanic acid in combination with retinoic acid receptor ligands induced intestinal retinoic acid hydroxylase and retinoic acid metabolism (71). Schluter and colleagues reported that phytanic acid induced differentiation of both white and brown adipose and that it also induced expression of the uncoupling protein-1 (UCP1) mRNA in brown adipose tissue (108; 107; 109). UCP1 expression was enhanced by co-transfection of brown adipocytes with a retinoid X receptor expression vector, a finding supported by the observation of Radominska-Pandya and Chen, who found that phytanic acid is a natural ligand for retinoid X receptor beta (96).
PHYH was found to bind to the immunophilin FKBP52, suggesting that it might have a role in cellular signaling pathways (18). Lee and colleagues identified a previously undescribed brain-specific protein, PHYH-AP1, that also binds to PHYH in a yeast 2-hybrid system (73). Subsequently, these investigators reported that PHYH-AP1 (now called BAP4) also interacts with brain-specific angiogenesis inhibitor 1 (BAI1) (61). They postulate that PHYH interacts with BAI1 through BAP4 and that this interaction may be involved in the development of the central neurologic symptoms of adult Refsum disease such as retinitis pigmentosa, nerve deafness, and cerebellar ataxia. Selective overexpression of PHYH-AP1 in the heart (atrium) of a transgenic mouse model produced tachycardia and increased susceptibility to arrhythmia; this may be related to arrhythmias associated with ARD (60). These investigators found evidence that visual stimulation is essential for expression of this protein and that PHYH-AP1 may be involved in developmental regulation of photoreceptor function (02). This same group also identified a novel, long-chain fatty acyl-CoA synthetase (mLACS) expressed primarily in brain and testis as being a second protein that interacts with murine Phyh (58). Inhibition of this acyl-CoA synthetase blocked proliferation of cultured neuronal cells, suggesting possible relevance to the development of neurologic symptoms in adult Refsum disease. A yeast 2-hybrid screen conducted by Chen and colleagues identified PHYH as a protein interacting with human coagulation factor VIII, and further reported that PHYH overexpression decreased factor VIII production significantly in factor VIII-producing cells (19).
A report focused on using transcriptomic, metabolomic, and proteomic analyses of cultured fibroblasts from 6 healthy control donors and 5 donors with adult Refsum disease treated with phytol (140). Applying a newly developed fibroblast‐specific genome‐scale model to these data, 53 metabolites were predicted to discriminate between healthy controls and adult Refsum disease patients. In addition to highlighted defects in phytanic acid metabolism, several of these metabolites were linked to amino acid metabolism.
Mouse model of adult Refsum disease. A mouse model of adult Refsum disease with a biallelic deletion of exons 4 to 7 of the Phyh gene was reported in 2008 (33). On a mixed (Swiss/129SVJ/FVB) genetic background, these homozygous Phyh-null (knockout) mice displayed no phenotypic abnormalities when reared under normal laboratory conditions and fed normal rodent chow, which contains very low levels of phytanic acid and its precursor, phytol. When knockout mice were fed a diet supplemented with 0.25% phytol, the animals lost weight due to lipoatrophy with loss of white adipose tissue. Plasma, liver, kidney, testis, and cerebellum showed significantly elevated levels of phytanic acid. Plasma levels of other lipids, including cholesterol, triacylglycerol, free fatty acids, and total fatty acids, were decreased in knockout mice on the phytol diet. Histologic analysis of livers from knockout mice revealed steatosis, hepatocyte degeneration, and inflammatory infiltrates; the steatosis was microvesicular on a 0.1% phytol diet and macrovesicular on a 0.25% phytol diet. Histologic analysis of testes revealed that knockout mice on either 0.1% or 0.25% phytol diet lacked the full complement of spermatogenic cells. Brains of Phyh-null mice fed the 0.25% phytol diet showed prominent reactive astrocytosis and a striking loss of cerebellar Purkinje cells.
Homozygous Phyh-null mice fed phytol exhibited neuromuscular function abnormalities, with an increased number of paw slips while moving on a grid and absent trunk curl (33). These mice had an unsteady gait with reduced paw print areas (both forepaws and hind paws) and reduced base of support for the hind paws. Peripheral motor nerve conduction velocity was decreased in phytol-fed knockout mice, due to increased latency in action potentials. However, no gross evidence for demyelination was observed in sciatic nerve by histology and staining for myelin basic protein. Further studies of this mouse model could yield additional insights into the pathophysiology of adult Refsum disease.
Although Ferdinandusse and colleagues found essentially no effects of either 0.1% or 0.25% phytol diet on wild-type mice (33), other investigators reported that normal mice fed a diet supplemented with 0.5 or 1.0% phytol exhibited midzonal hepatocellular necrosis and periportal hepatocellular fatty vacuolation (78). In that study, high phytanic and pristanic acid levels correlated with increased PPAR-alpha-mediated responses, including reduced body weight, hepatomegaly, and peroxisome proliferation. A study in healthy sheep subject to dietary phytol supplementation found that plasma cholesterol and phospholipid concentrations decreased during the phytol treatment period, whereas triglyceride concentration increased (26). Furthermore, the plasma concentrations of amino acids changed during the treatment period (serine and glycine levels increased, whereas glutamate level decreased).
Genetics. Richterich and colleagues conducted formal genetic studies on a total of 37 patients and concluded that this disorder was inherited in an autosomal recessive manner based on the high parental consanguinity rate, absence of disease in parents, and observed number of affected sibs (103). A similar pattern of inheritance has been observed in patients subsequently diagnosed with adult Refsum disease. Cultured fibroblasts from obligate heterozygotes show an intermediate reduction in their ability to oxidize phytanic acid, consistent with an autosomal recessive mode of inheritance. The PHYH gene (NCBI Refseq NM_006214.4), defective in adult Refsum disease, is located at chromosome band 10p13 (chr10:13277799-13300064 in human genome build GRCh38/ng38). The PEX7 gene (NCBI Refseq NM_000288.4) is located at chromosome band 6q23.3 (chr6:136822592-136913934 in human genome build GRCh38/ng38).
Adult Refsum disease is a rare genetic disorder; Refsum reviewed the existing literature in 1975 and found a total of 73 reported cases (101). In 1995, Steinberg estimated the total number of confirmed adult Refsum disease cases to be about 150 (116). In 2015, it was reported that the incidence is around one in a million in the United Kingdom (136).
Prenatal diagnosis of adult Refsum disease can be established by measuring phytanic acid alpha-oxidation in cultured amniocytes or chorionic villus cells. Nevertheless, taking the rarity of the disorder and the rather late age of onset of symptoms into consideration, it is difficult to identity at-risk pregnancies unless both biological parents are both known to be asymptomatic carriers of deleterious variants in PHYH and/or have adult Refsum disease themselves.
Adult Refsum disease must be distinguished primarily from (i) other neurologic syndromes that resemble it but that do not have elevated phytanic acid and (ii) other peroxisomal disorders with increased phytanic acid levels. Neurologic disorders that might be considered in the differential diagnosis include: Friedreich ataxia, retinitis pigmentosa, multiple sclerosis, Dejerine-Sottas syndrome, Charcot-Marie-Tooth syndrome, nonspecific heredoataxia syndromes, abetalipoproteinemia, Tangier disease, amyotrophic lateral sclerosis, Sjogren-Larsson syndrome, Spielmeyer-Vogt disease, and Tay-Sachs disease. PHARC (polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataract), first described in 2008, is a novel adult Refsum disease-like disorder caused by deleterious variants in the ABHD12 gene; this disorder should also be considered in the differential diagnosis (35). A finding of elevated plasma phytanic acid would rule out all these disorders and, particularly in an adolescent or an adult, would indicate a diagnosis of adult Refsum disease. Two sisters who presented with an acute demyelinating polyneuropathy suggestive of familial Guillain-Barré syndrome were subsequently found to have adult Refsum disease based on elevated phytanic acid levels (131). The differential diagnosis of hereditary neuropathies in adult patients has been reviewed (94).
Phytanic acid accumulation is not unique to adult Refsum disease. Impaired ability to degrade phytanic acid leading to elevated plasma levels is also observed in peroxisome biogenesis disorders, which include Zellweger spectrum disorder (ZSD) and rhizomelic chondrodysplasia punctata (RCDP) (72). Depending on disease severity, patients with Zellweger spectrum disorder can also have elevated plasma concentrations of very long-chain fatty acids (VLCFAs) and pipecolic acid and lowered erythrocyte plasmalogen levels. Cultured primary skin fibroblasts from patients with Zellweger spectrum disorder can also exhibit decreased plasmalogen synthesis. Importantly, signs and symptoms of peroxisome biogenesis disorders are typically present at birth, especially for the intermediate and severe cases. Alpha-methylacyl-CoA racemase (AMACR) deficiency can also present as an adult-onset sensory motor neuropathy; however, pristanic acid and phytanic acid levels are elevated in this disorder (30). Mildly elevated plasma phytanic acid was also found in 1 reported case of sterol carrier protein x (SCPx) deficiency (31); this patient presented with leukoencephalopathy, dystonia, and motor neuropathy and had markedly elevated plasma pristanic acid levels. Thus, a diagnosis of adult Refsum disease can be made if patients exhibit the tetrad of clinical features originally described by Refsum, have elevated plasma phytanic acid or inability to degrade phytanic acid, and have no other biochemical abnormalities suggesting more generalized peroxisome dysfunction.
Because the PHYH enzyme deficient in most adult Refsum disease patients requires thiamine pyrophosphate as a cofactor, it has been suggested that patients with untreated thiamine deficiency may have elevated phytanic acid levels (115). Classical thiamine (vitamin B1) deficiency (beriberi) is associated with polyneuropathy. Wernicke encephalopathy, an acute neuropsychiatric disorder that occurs as a result of thiamine deficiency, most commonly in people with chronic heavy alcohol use, is more commonly seen in developed countries. Thiamine deficiency is also found in patients with inborn errors of metabolism, such as thiamine-responsive megaloblastic anemia. Nevertheless, phytanic acid levels have not been extensively studied in patients with beriberi, Wernicke encephalopathy, or thiamine-responsive megaloblastic anemia.
Patients presenting with symptoms of night blindness, gait disturbance, or peripheral neuropathy should be evaluated for adult Refsum disease. A complete neurologic examination is indicated to evaluate signs of peripheral motor or sensory neuropathy, ataxia, and cranial nerve dysfunction. Ophthalmologic examination should be performed to ascertain whether the patient has the salt-and-pepper type of retinitis pigmentosa typical of adult Refsum disease, any visual field defects, miosis, or abnormal pupillary reflexes. Olfactory functional assessment tools such as the quantitative University of Pennsylvania Smell Identification Test may be useful; in a study of 16 adult Refsum disease patients, all were found to have complete anosmia or grossly impaired smell function despite a median 15 years of dietary treatment (40). Skin and skeletal systems should be evaluated for the changes sometimes seen in adult Refsum disease; Jayaram and Downes noted that the presence of hand or foot abnormalities in a patient with autosomal recessive or simplex retinitis pigmentosa is suggestive of adult Refsum disease (56). All symptoms of this disease do not develop simultaneously and may be several years between the onsets of different symptoms. Thus, diagnosis is often difficult, and early misdiagnosis is not uncommon.
The primary laboratory test is measurement of the plasma phytanic acid concentration by gas chromatography or gas chromatography coupled to mass spectrometry (GC-MS). A novel method using proton magnetic resonance spectroscopy (MRS) to identify and quantitate plasma phytanic acid levels was reported (93). Analysis of plasma from a donor with adult Refsum disease yielded results similar to those obtained by gas chromatography, suggesting the possibility that noninvasive MRS diagnostic methods could be developed. If other peroxisomal diseases must be ruled out, plasma very long-chain fatty acids (VLCFAs) and pristanic acid levels as well as plasmalogen synthesis activity in cultured fibroblast should be evaluated. Determining the rates of phytanic acid catabolism in cultured primary fibroblasts is useful to confirm diagnosis. PHYH gene sequencing should not be required but could be useful in atypical cases (06). An electrocardiogram should be obtained and evaluated for the nonspecific changes sometimes seen in ARD. Evaluation of cerebrospinal (CSF) protein and cellular content may be helpful but is probably not necessary if the plasma phytanic acid level is elevated.
Current standard of care. In humans, phytanic acid is entirely of exogenous origin; therefore, control of dietary intake of this fatty acid decreases its accumulation in people with adult Refsum disease. Ingestion of both phytanic acid and free phytol, which can be converted to phytanic acid in people, should be minimized. Patients have a small residual capacity to degrade phytanic acid, and, as a result, these dietary measures will allow the depletion of body stores. Daily consumption of phytanic acid in people on typical Western diets is 50 to 100 mg and estimated residual degradation capacity in patients is 7 to 30 mg/day (41; 143). Thus, dietary intake must be significantly less than 30 mg daily to facilitate mobilization and elimination of stored phytanic acid.
The main dietary sources of phytanic acid are dairy products, ruminant meats and fats, and fatty fish. For effective management, these must be eliminated from the diet. Green vegetables, due to their high chlorophyll content, were not permitted in the early dietary trials; however, due to the poor bioavailability of chlorophyll-bound phytol, this is probably not necessary. Although phytanic acid is present in many foods, good information on food phytanic acid content is limited. More detailed information on dietary management of adult Refsum disease and the amount of phytanic acid in various foodstuffs can be found elsewhere (77; 80; 81; 49; 10; 104). Although non-leafy vegetables typically have a very low phytanic acid content, phytyl fatty acid esters (trans-phytol esterified with a fatty acid) that could contribute to phytanic acid body burden have been reported in some vegetables (67).
It is often difficult to evaluate response to treatment because reduction in plasma phytanic acid levels and improvement in clinical symptoms are generally not rapid. The overall plasma concentration is determined by both the dietary intake and the release of fatty acid from endogenous stores. The amount of phytanic acid stored as triglyceride in adipose can be high, and, as a result, the process of mobilization and clearance can be slow. Sudden weight loss in patients on a low phytanic acid diet can lead to a paradoxical rise in plasma concentration and clinical relapse, indicating that the contribution of adipose mobilization can be significant.
Plasmapheresis has been used in some cases, generally with success, as an adjunct to dietary therapy (77; 49; 44; 39). Plasmapheresis can be particularly useful in the early management of the disease or in cases of clinical relapses, but it is not a practical substitute for lifelong dietary treatment. Therapeutic apheresis using membrane differential filtration has been shown to be safe and effective in long-term management of adult Refsum disease (122). Zolotov and colleagues reported that long-term lipid apheresis was beneficial in 4 patients with severe adult Refsum disease whose symptoms progressed despite their compliance with a low phytanic acid diet (149). Kohlschutter and colleagues used lipid apheresis as an adjunct to dietary management of a 14-year-old female with night blindness (62). After 30 months of combined therapy, the patient was maintained only on diet. Their work suggests that aggressive measures to keep phytanic acid levels low may prevent more serious sequelae of adult Refsum disease.
Although dietary changes should begin immediately on diagnosis of adult Refsum disease to ensure the most favorable outcome, they are generally are not initiated until some irreversible vision or hearing deficit is already present. Although many symptoms of the disease respond favorably to treatment, the progression of retinal changes and hearing loss generally slow down or stabilize, but there is no compelling evidence that they improve in significant numbers of patients (119). Nevertheless, there was a report of a 51‐year‐old patient with adult Refsum disorder who underwent electroretinography before and after beginning a phytanic acid‐restricted diet and showed significantly improved waveform amplitudes and implicit times, despite no change evident in visual acuity (07).
Multiple reports indicate that some patients may benefit from cochlear implantation (97; 92; 121; 47). Aggressive therapy – dietary changes and weekly plasma exchange for 9 months – in a newly-diagnosed 42-year-old female patient with classical symptoms led to near normalization of both cervical vestibular evoked myogenic potential and ocular vestibular evoked myogenic potential latencies (125). In addition to improvement in her vestibular neuropathy, peripheral nerve function and mobility improved as well.
Future therapeutic prospects. Perera and colleagues treated 2 brothers (ages 48 and 50 years) with adult Refsum disease with the intestinal lipase inhibitor orlistat (95). Despite appropriate dietary management and plasmapheresis, their plasma phytanic acid levels remained 10-fold greater than the upper limit of normal. In addition, both siblings had progressive neurologic and dermatologic symptoms. The patients then began orlistat treatment (120 mg prior to meals) along with dietary treatment and plasmapheresis. After following this regimen for several years, plasma phytanic acid levels were reduced by more than 50%, and both patients reported stabilization or improvement of some neurologic symptoms. Further investigation of this approach may be warranted.
Several investigators have proposed therapeutic hypothesis aimed at augmenting phytanic acid metabolic activities in patients with adult Refsum disease through small molecule treatments. Kemp and coworkers investigated enhancing the fatty acid omega-oxidation pathway as an alternative means of catabolizing phytanic acid in patients (135). Although no drugs are currently available, they speculated that compounds causing upregulation of specific P450 isozymes could in theory increase the rate of omega-oxidation of phytanic acid. Overall, further investigation of fenofibrate and specific P450 activators in the treatment of adult Refsum disease seem warranted.
High levels of phytanic acid have been associated with life-threatening cardiac arrhythmia and peripheral neuropathy, requiring emergency plasmapheresis (77). Without therapy, half of untreated patients died before 30 years of age. Although Steinberg noted that since the institution of dietary therapy 30 years ago there have been no deaths directly attributable to adult Refsum disease (116), some cases go undiagnosed until later adulthood when large tissue stores of phytanic acid has accumulated in multiple organs and can increase the risk of an early death. Dietary therapy (with or without supplemental plasmapheresis) is effective in decreasing the severity of peripheral nerve dysfunction and ataxia, decreasing the plasma phytanic acid and CSF protein concentrations, and improving the electrocardiogram. Although therapy is thought to produce little or no improvement in existing retinal or hearing deficits, evidence is available that long-term dietary treatment may prevent progression the of nervous system damage.
Stress, including pregnancy, has been reported to exacerbate symptoms of adult Refsum disease (116), but no specific details regarding the effect of adult Refsum disease on pregnancy or outcome of pregnancy was reported until 2017 (123). The mother’s presentation was unusual in that she was symptomatic by age 3. She had salt and pepper retinitis pigmentosa, sensorineural hearing loss, ichthyosis, mild developmental delay, and bony abnormalities of the hands and feet. A diagnosis of adult Refsum disease was confirmed at age 10 by biochemical and genetic analyses. Her plasma phytanic acid level were not well controlled by diet, and plasmapheresis was occasionally necessary. She became pregnant at age 27. Her prepregnancy weight was 75.6 kg. During the first trimester, she experienced epigastric pain and lower abdominal pain. Fetal ultrasound indicated normal fetal development. Following advice from a metabolic physician and a metabolic dietitian, she was able to keep her phytanic acid levels at a higher than desirable, but a consistent, level. At 5 months, she had gained 7 kg. In the third trimester she presented with sinus tachycardia (mean, 120/min) and shortness of breath. Through the remainder of her pregnancy she had breathlessness, fluctuating hypertension, dry skin, and pruritus, all of which improved postpartum. Labor was induced at term, and during labor she was given Polycal® and Calogen® supplements to reduce the risk of acute metabolic decompensation. She gave birth to a healthy daughter, whose phytanic acid level was normal at 6 weeks of age. However, she suffered from postpartum depression and had poor appetite. This was of concern as release of phytanic acid from adipose tissue during starvation typically elevates plasma levels. Other postpartum issues included episodes of fatigue and general weakness, dyspareunia that required a refashioning of episiotomy, elevated blood pressure, headaches, dizziness, persistent episodes of nausea, vomiting, and epigastric abdominal pain. The latter symptoms were eventually attributed to irritable bowel syndrome. The child was reportedly healthy and developing normally.
In 2019, a case report was published of a woman with adult Refsum disease who was homozygous for a PHYH loss-of-function variant and pregnant several times with fetuses homozygous for the same variant (27). The woman was born to a family with several affected members and was herself diagnosed with adult Refsum disease at age 21. Although the first clinical examination revealed she has characteristic bilateral shortening of metatarsals, no neurologic signs were observed. The diagnosis was established by biochemical (plasma phytanic acid 91 μmol/L; controls < 6.5 μmol/L; no elevation of pristanic acid) and genetic testing, where she was homozygous for a deleterious PHYH splice junction variant. Afterwards, she was treated by phytanic acid dietary restriction. She married her first cousin carrying the same deleterious PHYH variant and was pregnant 7 times. A prenatal diagnosis was carried out in her first pregnancy, which revealed the fetus homozygous for the deleterious PHYH variant. This was followed by a miscarriage a few days after a trophoblast biopsy. In the second pregnancy, genetic testing revealed the fetus was a carrier of the deleterious PHYH variant, and the pregnancy proceeded without any special events. In the third pregnancy, genetic testing indicated the fetus homozygous for the deleterious PHYH variant, and there was a medical termination of the pregnancy. In the fourth pregnancy, genetic testing indicated the fetus was homozygous for the deleterious PHYH variant, and a baby girl was born. In the subsequent 3 pregnancies, no prenatal testing was performed, and another girl homozygous for the deleterious PHYH variant was born. During all her pregnancies, dietary therapy was essentially unchanged, and body weight and plasma PA levels were monitored. Despite some weight gain (10 kg added during the first 4 ti 5 months of the fourth pregnancy), the mother presented no adverse event during delivery and postpartum periods. Ultrasound examinations did not reveal abnormalities of the fetuses with homozygous PHYH deleterious variants. All fetal viability, biometric indices, morphological parameters, and annexes evaluated were normal. The 2 females affected by adult Refsum disease were born without special events (in particular, without shortened metatarsals), but their plasma phytanic acid levels were slightly elevated. The 2 newborns were fed with phytanic acid-deficient milk and immediately placed on a low‐phytanic acid diet. Their plasma phytanic acids were normal at 3 weeks and 4 months at age.
Exacerbation of symptoms has been associated with stress and prolonged fasting, and, thus, caution should be exercised if an adult Refsum disease patient must undergo anesthesia. In patients with neuropathy, careful perioperative positioning is necessary to avoid nerve compression. Due to the risk of electrocardiogram (ECG) change, cardiac perioperative monitoring is necessary.
All contributors' financial relationships have been reviewed and mitigated to ensure that this and every other article is free from commercial bias.
Erika Augustine MD MS
Dr. Augustine of Kennedy Krieger Institute, Johns Hopkins University, and University of Rochester Medical Center received consulting fees from Amicus Therapeutics, Exicure, and Taysha Gene Therapies, a clinical trial agreement as Central Rater from Neurogene Inc, and an honorarium as a member of the Data Safety and Monitory Board for PTC Therapeutics.See Profile
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