Ataxia with isolated vitamin E deficiency: a Japanese family carrying a novel mutation in the alpha-tocopherol transfer protein gene

α-Tocopherol transfer protein (α-TTP)

α-Tocopherol transfer protein (α-TTP) is so far the only known protein that specifically recognizes α-tocopherol (α-Toc), the most abundant and most biologically active form of vitamin E, in higher animals. α-TTP is highly expressed in the liver where α-TTP selects α-Toc among vitamin E forms taken up via plasma lipoproteins and promotes its secretion to circulating lipoproteins. Thus, α-TTP is a major determinant of plasma α-Toc concentrations. Familial vitamin E deficiency, also called Ataxia with vitamin E deficiency, is caused by mutations in the α-TTP gene. More than 20 different mutations have been found in the α-TTP gene worldwide, among which some missense mutations provided valuable clues to elucidate the molecular mechanisms underlying intracellular α-Toc transport. In hepatocytes, α-TTP catalyzes the vectorial transport of α-Toc from the endocytotic compartment to the plasma membrane (PM) by targeting phosphatidylinositol phosphates (PIPs) such as PI(4,5)P₂. By binding PIPs at the PM, α-TTP opens the lid covering the hydrophobic pocket, thus facilitating the release of bound α-Toc to the PM.

A Case of Ataxia with Isolated Vitamin E Deficiency Initially Diagnosed as Friedreich's Ataxia

Ataxia with isolated vitamin E deficiency (AVED) is a rare autosomal recessive condition that is caused by a mutation in the alpha tocopherol transfer protein gene. It is almost indistinguishable clinically from Friedreich's ataxia but with appropriate treatment its devastating neurological features can be prevented. Patients can present with a progressive cerebellar ataxia, pyramidal spasticity, and evidence of a neuropathy with absent deep tendon reflexes. It is important to screen for this condition on initial evaluation of a young patient presenting with progressive ataxia and it should be considered in patients with a long standing ataxia without any diagnosis in view of the potential therapeutics and genetic counselling. In this case report we present a patient who was initially diagnosed with Friedreich's ataxia but was later found to have AVED.

Intracellular transport of fat-soluble vitamins A and E

Vitamins are compounds that are essential for the normal growth, reproduction and functioning of the human body. Of the 13 known vitamins, vitamins A, D, E and K are lipophilic compounds and are therefore called fat-soluble vitamins. Because of their lipophilicity, fat-soluble vitamins are solubilized and transported by intracellular carrier proteins to exert their actions and to be metabolized properly. Vitamin A and its derivatives, collectively called retinoids, are solubilized by intracellular retinoid-binding proteins such as cellular retinol-binding protein (CRBP), cellular retinoic acid-binding protein (CRABP) and cellular retinal-binding protein (CRALBP). These proteins act as chaperones that regulate the metabolism, signaling and transport of retinoids. CRALBP-mediated intracellular retinoid transport is essential for vision in human. α-Tocopherol, the main form of vitamin E found in the body, is transported by α-tocopherol transfer protein (α-TTP) in hepatic cells. Defects of α-TTP cause vitamin E deficiency and neurological disorders in humans. Recently, it has been shown that the interaction of α-TTP with phosphoinositides plays a critical role in the intracellular transport of α-tocopherol and is associated with familial vitamin E deficiency. In this review, we summarize the mechanisms and biological significance of the intracellular transport of vitamins A and E.

Molecular, clinical and peripheral neuropathy study of Tunisian patients with ataxia with vitamin E deficiency

Ataxia with vitamin E deficiency is an autosomal recessive cerebellar ataxia caused by mutations in the α-tocopherol transfer protein coding gene localized on chromosome 8q, leading to lower levels of serum vitamin E. More than 91 patients diagnosed with ataxia with vitamin E deficiency have been reported worldwide. The majority of cases originated in the Mediterranean region, and the 744delA was the most common mutation among the 22 mutants previously described. We examined the clinical and molecular features of a large cohort of 132 Tunisian patients affected with ataxia with vitamin E deficiency. Of these patients, nerve conduction studies were performed on 45, and nerve biopsy was performed on 13. Serum vitamin E was dramatically reduced for 105 of the patients analysed. Molecular analysis revealed that 91.7% of the patients (n = 121) were homozygous for the 744delA mutation. Three other mutations were detected among the remaining patients (8.3%, n = 11) in the homozygous state. Two were previously reported (400C>T and 205-1G>T), and one was novel (553+1T>A). Age of onset was 13.2 ± 5.9 years, with extremes of 2 and 37 years. All described patients exhibited persistent progressive cerebellar ataxia with generally absent tendon reflexes. Deep sensory disturbances, pyramidal syndrome and skeletal deformities were frequent. Head tremor was present in 40% of the patients. Absence of neuropathy or mild peripheral neuropathy was noted in more than half of the cohort. This is the largest study of the genetic, clinical and peripheral neuropathic characteristics in patients with ataxia and vitamin E deficiency. The 744delA mutation represents the most common pathological mutation in Tunisia and worldwide, likely because of a Mediterranean founder effect. Our study led us to suggest that any patient displaying an autosomal recessive cerebellar ataxia phenotype with absent tendon reflexes and minor nerve abnormalities should first be screened for the 744delA mutation, even in the absence of a serum vitamin E measurement.

Ataxia with vitamin E deficiency: update of molecular diagnosis

Ataxia with vitamin E deficiency (AVED) is a rare autosomal recessive neurodegenerative disease, due to mutations in TTPA gene (Arita et al. in Biochem J 306(Pt. 2):437-443, 1995; Hentati et al. in Ann Neurol 39:295-300, 1996), which encodes for alpha-TTP, a cytosolic liver protein that is presumed to function in the intracellular transport of alpha-tocopherol. This disease is characterized clinically by symptoms with often striking resemblance to those of Friedreich ataxia. The neurological symptoms include ataxia, dysarthria, hyporeflexia, and decreased vibration sense, sometimes associated with cardiomyopathy and retinitis pigmentosa (Mariotti et al. in Neurol Sci 25:130-137, 2004). Vitamin E supplementation improves symptoms and prevents disease progress (Doria-Lamba et al. in Eur J Pediatr 165(7):494-495, 2006). Over 20 mutations have been identified in patients with AVED. In the present paper we summarize the recent findings on molecular genetic of this disease including the list of the known mutations.

Effect of bilayer phospholipid composition and curvature on ligand transfer by the alpha-tocopherol transfer protein

We report here our preliminary investigations on the mechanism of alpha-TTP-mediated ligand transfer as assessed using fluorescence resonance energy transfer (FRET) assays. These assays monitor the movement of the model alpha-tocopherol fluorescent derivative ((R)-2,5,7,8-tetramethyl-chroman-2-[9-(7-nitro-benzo[1,2,5]oxadiazol-4-yl amino)-nonyl]-chroman-6-ol; NBD-Toc) from protein to acceptor vesicles containing the fluorescence quencher TRITC-PE. We have found that alpha-TTP utilizes a collisional mechanism of ligand transfer requiring direct protein-membrane contact, that rates of ligand transfer are greater to more highly curved lipid vesicles, and that such rates are insensitive to the presence of anionic phospholipids in the acceptor membrane. These results point to hydrophobic features of alpha-TTP dominating the binding energy between protein and membrane.

Isolated vitamin E deficiency mimicking distal hereditary motor neuropathy in a 13-year-old boy

We report an atypical neurophysiologic pattern of isolated vitamin E deficiency in a 13-year-old boy. Electroneurography- electromyography, somatosensory evoked potentials, serum vitamin E concentration and genetic analysis of the alpha-tocopherol transfer protein gene were performed. Nerve conduction study failed to show peripheral neuropathy whereas needle electromyography of distal muscles demonstrated chronic neurogenic motor unit potentials. Both clinical and neurophysiologic data fulfilled the criteria of distal hereditary motor neuropathy. Later on, somatosensory-evoked potential displayed absence of spinal and central response. The serum vitamin E level was low, and the patient was found to be homozygous for a 513insTT mutation in exon 3 of the alpha-tocopherol transfer protein gene. To our knowledge this is the first case of isolated deficiency of vitamin E that presents the classic neurophysiologic and clinical features of distal hereditary motor neuropathy.

A novel delins mutation in the alpha-TTP gene in a family segregating ataxia with isolated vitamin E deficiency

Ataxia with isolated vitamin E deficiency is a rare autosomal recessive neurodegenerative disease due to mutations in the alpha-tocopherol transfer protein gene. In ataxia with isolated vitamin E deficiency, the biochemical hallmark is the low plasmatic levels of vitamin E and, in most of the patients, vitamin E supplementation allows a stabilization of the neurologic conditions. We have investigated the genetic cause of ataxia and reduced levels of vitamin E, and apolipoproteins A1 and B in a 16-y-old patient. Results revealed that our propositus is a compound heterozygote for the c.227_229delinsATT/c.744delA mutations in the alpha-tocopherol transfer protein gene, each inherited from one of the two parents. His sister is also a compound heterozygote for both mutations, and she presents a biochemical pattern similar to that of his brother. After receiving the vitamin E supplementation, plasmatic levels of vitamin E and apolipoprotein A1 have been normalized in the propositus. The detected mutations would justify the undetectable levels of vitamin E, but would not explain the also decreased levels of the apolipoproteins, as neither that after treatment with vitamin E, the levels of apolipoprotein B do not become normal. These findings suggest that other genes may play a role in producing this atypical biochemical profile.

Alpha-Tocopherol Transfer Protein (alpha-TTP): Insights from Alpha-Tocopherol Transfer Protein Knockout Mice

Alpha-tocopherol transfer protein (alpha-TTP) is a liver cytosolic transport protein that faciliates alpha-tocopherol (alpha-T) transfer into liver secreted plasma lipoproteins. Genetic defects in alpha-TTP, like dietary vitamin E deficiency, are associated with infertility, muscular weakness and neurological disorders. Both human and alpha-TTP deficient (alpha-TTP(-/-)) mice exhibit severe plasma and tissue vitamin E deficiency that can be attenuated by sufficient dietary alpha-T supplementations. In this review, we summarize the literature concerning studies utilizing the alpha-TTP(-/-) mice. Levels of vitamin E in the alpha-TTP(-/-) mice do not appear to be directly related to the amounts of dietary alpha-T or to the levels of alpha-TTP protein in tissues. The alpha-TTP(-/-) mice appear to present a good model for investigating the specific role of alpha-T in tissue vitamin E metabolism. Furthermore, alpha-TTP(-/-) mice appear to be useful to elucidate functions of alpha-TTP beyond its well recognized functions of transferring alpha-T from liver to plasma lipoprotein fractions.

Structure and Function of α‐Tocopherol Transfer Protein: Implications for Vitamin E Metabolism and AVED

Human alpha-tocopherol transfer protein (alpha-TTP) plays a central role in vitamin E homeostasis: mutations in the protein are a cause of a progressive neurodegenerative disorder known as ataxia with vitamin E deficiency (AVED). Despite normal dietary intake of vitamin E, affected individuals suffer from a relative deficiency of this essential lipophilic antioxidant. Disease-associated mutations in alpha-TTP impair its ability to prevent the degradation and excretion of alpha-T. Recently, we and others solved the crystal structures of alpha-TTP bound to a molecule of (2R, 4'R, 8'R)-alpha-T, which has led to a better understanding of the molecular basis of its biochemical activity. Surprisingly, the ligand was found buried in the hydrophobic core of the protein, completely sequestered from the aqueous milieu. In this chapter, the implications of the structure of alpha-TTP bound to its ligand regarding the mechanism of alpha-T retention are discussed. A comparison to a crystal structure of the apo form of alpha-TTP indicates a possible specific conformational change that allows the entry and exit of the ligand. The effect of known disease-associated point mutations is examined in light of the crystal structure as well as recent biochemical studies. Despite the knowledge gained from these studies, the exact molecular mechanism by which alpha-TTP retains alpha-T remains enigmatic and will likely prove a fruitful area for future research.

The α‐Tocopherol Transfer Protein

Almost a century ago, plant extracts were documented to be critical for the fertility of rodents. This activity was later ascribed to vitamin E, a term comprising a number of structurally related plant lipids that function as fat soluble antioxidants. The alpha-tocopherol transfer protein (TTP) is a critical regulator of vitamin E status that stimulates the movement of vitamin E between membrane vesicles in vitro and facilitates the secretion of tocopherol from hepatocytes. Heritable mutations in the ttpA gene cause ataxia with vitamin E deficiency (AVED), an autosomal recessive disorder characterized by low plasma vitamin E levels and progressive neurodegeneration. This chapter summarizes recent advances in our understanding of the molecular and physiological aspects of TTP activity.

Vitamin E deficiency induced neurological disease in common variable immunodeficiency: two cases and a review of the literature of vitamin E deficiency

Vitamin E deficiency causes a neurological disorder characterised by sensory loss, ataxia and retinitis pigmentosa due to free radical mediated neuronal damage. Symptomatic vitamin E deficiency has been reported in genetic defects of the vitamin E transport protein and in malabsorption complicating cholestasis, abetalipoproteinaemia, celiac disease, cystic fibrosis and small bowel resection. There are no reports to date of vitamin E deficiency in patients with primary immunodeficiencies. We describe two CVID patients with the associated enteropathy who developed neurological disease because of vitamin E deficiency, suggesting a possible predisposition to developing this complication. We recommend that all CVID patients with evidence of an enteropathy be screened for vitamin E deficiency, as early detection and consequent treatment may prevent, halt or reverse the neurological sequelae.

Crystal structure of human alpha-tocopherol transfer protein bound to its ligand: implications for ataxia with vitamin E deficiency

Human alpha-tocopherol (alpha-T) transfer protein (ATTP) plays a central role in vitamin E homeostasis, preventing degradation of alpha-T by routing this lipophilic molecule for secretion by hepatocytes. Mutations in the gene encoding ATTP have been shown to cause a severe deficiency in alpha-T, which results in a progressive neurodegenerative spinocerebellar ataxia, known as ataxia with vitamin E deficiency (AVED). We have determined the high-resolution crystal structure of human ATTP with (2R,4'R,8'R)-alpha-T in the binding pocket. Surprisingly, the ligand is sequestered deep in the hydrophobic core of the protein, implicating a large structural rearrangement for the entry and release of alpha-T. A comparison to the structure of a related protein, Sec14p, crystallized without a bona fide ligand, shows a possibly relevant open conformation for this family of proteins. Furthermore, of the known mutations that cause AVED, one mutation, L183P, is located directly in the binding pocket. Finally, three mutations associated with AVED involve arginine residues that are grouped together on the surface of ATTP. We propose that this positively charged surface may serve to orient an interacting protein, which might function to regulate the release of alpha-T through an induced change in conformation of ATTP.

Myoclonic dystonia as unique presentation of isolated vitamin E deficiency in a young patient

We describe a young patient affected by vitamin E deficiency with mutation in the tocopherol transfer protein alleles and the unique presentation as myoclonic dystonia, which was practically the only symptom for 6 years before ataxia became evident. Vitamin E supplementation markedly improved both symptoms. This unusual clinical phenotype must be considered, because isolated vitamin E deficiency is eminently treatable.

Delayed-onset ataxia in mice lacking alpha -tocopherol transfer protein: model for neuronal degeneration caused by chronic oxidative stress

alpha-Tocopherol transfer protein (alpha-TTP) maintains the concentration of serum alpha-tocopherol (vitamin E), one of the most potent fat-soluble antioxidants, by facilitating alpha-tocopherol export from the liver. Mutations of the alpha-TTP gene are linked to ataxia with isolated vitamin E deficiency (AVED). We produced a model mouse of AVED by deleting the alpha-TTP gene, which showed ataxia and retinal degeneration after 1 year of age. Because the brain alpha-TTP functions in maintaining alpha-tocopherol levels in the brain, alpha-tocopherol was completely depleted in the alpha-TTP(-/-) mouse brain, and the neurological phenotype of alpha-TTP(-/-) mice is much more severe than that of wild-type mice when maintained on an alpha-tocopherol-deficient diet. Lipid peroxidation in alpha-TTP(-/-) mice brains showed a significant increase, especially in degenerating neurons. alpha-Tocopherol supplementation suppressed lipid peroxidation and almost completely prevented the development of neurological symptoms. This therapy almost completely corrects the abnormalities in a mouse model of human neurodegenerative disease. Moreover, alpha-TTP(-/-) mice may prove to be excellent animal models of delayed onset, slowly progressive neuronal degeneration caused by chronic oxidative stress.

Vitamin E: non-antioxidant roles

Vitamin E was originally considered a dietary factor of animal nutrition especially important for normal reproduction. The significance of vitamin E has been subsequently proven as a radical chain breaking antioxidant that can protect the integrity of tissues and play an important role in life processes. More recently alpha-tocopherol has been found to possess functions that are independent of its antioxidant/radical scavenging ability. Absorption in the body is alpha-tocopherol selective and other tocopherols are not absorbed or are absorbed to a lesser extent. Furthermore, pro-oxidant effects have been attributed to tocopherols as well as an anti-nitrating action. Non-antioxidant and non-pro-oxidant molecular mechanisms of tocopherols have been also described that are produced by alpha-tocopherol and not by beta-tocopherol. alpha-Tocopherol specific inhibitory effects have been seen on protein kinase C, on the growth of certain cells and on the transcription of some genes (CD36, and collagenase). Activation events have been seen on the protein phosphatase PP2A and on the expression of other genes (alpha-tropomyosin and Connective Tissue Growth Factor). Non-antioxidant molecular mechanisms have been also described for gamma-tocopherol, delta-tocopherol and tocotrienols.