Biotin-Responsive Basal Ganglia Disease in Ethnic Europeans With Novel SLC19A3 Mutations FREE

Biotin-responsive basal ganglia disease (BBGD; OMIM 607483) was first described in 1998 in 10 patients, 8 of whom were of Saudi Arabian, 1 of whom was of Syrian, and 1 of whom was of Yemenite ethnic origin.¹ Patients with BBGD presented with subacute episodes of encephalopathy often triggered by febrile illness and characterized by confusion, epilepsy, external ophthalmoplegia, dysphagia, and generalized stiffness, which eventually led to coma and death. Administration of high doses of biotin during encephalopathies resulted in partial or complete improvement within days. If untreated, encephalopathies led to permanent dystonia. Brain magnetic resonance imaging (MRI) showed characteristic bilateral lesions of the caudate nuclei and putamen.

The disease was recessively inherited and the gene defect was mapped on 2q36.3. Two missense mutations were identified in the SLC19A3 gene that encodes hTHTR2, a second thiamine-transporter.² Because biotin is not a substrate for hTHTR2, the precise mechanism by which biotin rescues the clinical phenotype remains unknown.³ Since the first description of BBGD, only 1 additional case has been reported in Lebanon but without genetic confirmation.⁴

The family presented in this study has 2 affected children (a brother and a sister) and is of Portuguese ancestry. The parents were healthy and not related by blood.

Patient 1 is the eldest child of the family. At the age of 7 years, after a benign illness, he had presented, during a 1-month period, with confusion, inability to walk, loss of speech, swallowing dysfunction, and generalized epileptic seizures. The results of brain computed tomography and basic cerebrospinal fluid (CSF) analysis were normal. He had progressively recovered except for dystonia and epilepsy treated with carbamazepine. He was first seen in our department when he was 31 years old. Examination disclosed generalized dystonia of the upper limbs and face and pyramidal signs. Brain MRI showed signal abnormalities of the putamen and caudate nuclei (Figure 1A-C).

At the age of 33 years, after a simple viral infection, he presented for a few days with confusion, severe gait ataxia, mutism, swallowing dysfunction, gaze palsy, and bilateral ptosis. Brain MRI showed diffuse signal abnormalities in cortical and subcortical areas, thalami, and mesencephalon with no contrast enhancement (Figure 1D-F). Electroencephalography revealed diffuse slow waves. Seven days after admission, treatment with biotin was started at 100 mg 3 times daily. Three days later, he was transferred to the intensive care unit because of worsening of alertness and tachypnea. The same day, the biotin dosage was increased to 600 mg/d without adjunction of thiamine. Within the next 2 days, the patient improved and could be transferred to the neurology unit again. One week later, with biotin treatment, the patient had regained his baseline clinical status and his MRI findings had improved (Figure 1G-I).

Patient 2 is the 29-year-old sister of patient 1. She had shown routine development within a conventional course of schooling until the age of 12 years, when she presented with simple partial motor seizures of the right upper limb. At the age of 20 years, she exhibited, during a 1-month period, generalized seizures, loss of ambulation, swallowing dysfunction, and dysarthria. Since that time, generalized dystonia and epilepsy have been observed. Examination at the age of 27 years disclosed generalized dystonia that involved the face and upper limbs, dysarthria, mild cerebellar ataxia, and central gaze nystagmus. A brain MRI showed abnormalities of the caudate nuclei and putamen identical to those of her brother (Figure 2A-C). At the age of 29 years, shortly after BBGD was diagnosed in her brother, she was hospitalized because of worsening of epileptic crises and difficulties in swallowing. Biotin was introduced at 300 mg/d for 3 days and then increased to 600 mg/d for 3 subsequent days. Despite this regimen, her condition worsened, and a new MRI of the brain showed diffuse cortical and subcortical hyperintensities (Figure 2D-F). Rapid aggravation motivated her transfer to an intensive care unit. Thiamine (500 mg/d intravenously) was then added to the biotin, which led to a marked and rapid improvement within the following 24 hours. A new brain MRI showed disappearance of cortical and subcortical hyperintensities (Figure 2G-I).

The results of the following examinations performed between episodes of encephalopathy were normal in both patients: electromyography, optic funduscopy, measurement of plasma amino acid levels, measurement of urine organic acid levels, measurement of plasma homocysteine levels, measurement of lactate and pyruvate levels in blood and CSF, measurement of methyltetrahydrofolate levels in CSF, measurement of plasma copper levels, measurement of ceruloplasmin levels, and muscular biopsy with spectroscopic analysis of the respiratory chain. The results of analysis for the mitochondrial mutations MERRF, MELAS, NARP, ATP6, and ND6 were negative. During encephalopathies, serum lactate and pyruvate levels were normal in both patients. Amino acid chromatographies in blood and CSF and CSF lactate and pyruvate measurements were only performed in patient 1 and produced normal results.

Sequencing of SLC19A3 (eText) in patients 1 and 2 identified 3 novel variants present in the heterozygote state in both patients. The first was duplication of the thymine in position 74, which introduced a frameshift in exon 2 (c.74dupT/p.Ser26LeufsX19) and led to a premature termination codon (PTC). The second variant was a duplication of an adenine (c.980-38dupA) in intron 3 also found in 22 of 94 healthy white controls, which indicates that it is a polymorphism. The third variant, also located in intron 3, was an adenine-to-guanine substitution (c.980-14 A>G) 14 base pairs upstream of exon 4, not found in the population of control individuals (Figure 3). This variant was predicted to create a new acceptor splice site in intron 3. Its effect at the messenger RNA level was therefore investigated in fibroblasts from patients 1 and 2, previously treated or not with emetine, an anti–nonsense-mediated decay drug. This analysis revealed that exon 4 was skipped in both patients as a consequence of the c.980-14 A>G mutation (Figure 3). Skipping of exon 4 (r.980_1172del) also caused a PTC (p.Gly327AspfsX8). Treatment with emetin resulted in an increase of the aberrantly spliced transcript, which suggests that this transcript is degraded by the nonsense mediated decay (Figure 3). Direct sequencing of the DNA of the mother showed that she had the c.980-14 A>G intronic variant but not the c.74dupT mutation; these results confirm that each mutation was inherited from 1 parent.

The neurologic history of these patients is similar to that of the 19 patients from the Middle East described by Ozand et al.¹ Our patients exhibited recurrent episodes of subacute encephalopathy, sometimes triggered by febrile illness. The first episode appeared at the age of 7 years for patient 1 but not until the age of 20 years for patient 2. As in previous reports, these subacute encephalopathies were followed by permanent sequelae consisting of epilepsy and generalized dystonia. Brain MRI showed permanent lesions of the caudate and putamen similar to those previously described. However, our patients had some peculiarities: (1) both are of European (Portuguese) ancestry; (2) both presented with abnormal high signals of the cortex and subcortical areas reversible under treatment; (3) although patient 1 responded rapidly to high doses of biotin, patient 2 responded only to a combination of biotin and thiamine; and (4) the genetic analyses identified 2 novel mutations in SLC19A3, which created frameshifts in the protein sequence and PTC; these results thus confirm that the disease is caused by a loss of function of hTHTR2.

The efficacy of high doses of biotin in this disease remains enigmatic. Indeed, hTHTR2 belongs to the family of thiamine and folate transporters⁵ but has no homology with known biotin transporters.⁶ Furthermore, in vitro experiments confirmed that hTHTR2 can transport thiamine but not biotin, at least in several nonneuronal cell lines.³ It is known that biotin regulates gene expression⁷ through the biotinylation of histones,⁸ and previous studies have shown that expression of SLC19A3 depends on biotin levels.⁹ It is therefore likely that high doses of biotin increase the expression of SLC19A3, thus restoring some function of the mutated receptor by increasing its expression. Of note, patient 2 did not improve with high doses of biotin but only after the addition of thiamine, which reinforces the hypothesis that impaired thiamine transport plays a critical role.

Overall, BBGD is a panethnic condition that should be suspected in every patient with unexplained encephalopathy and signal abnormalities of the putamen and caudate nuclei. In such a condition, a therapeutic trial with thiamine and high doses of biotin can be livesaving.

Correspondence: Frédéric Sedel, MD, PhD, Federation of Nervous System Diseases, Hôpital de la Salpêtrière, 47 Blvd de l’Hôpital, 75651 Paris CEDEX 13, France (frederic.sedel@psl.aphp.fr).

Accepted for Publication: July 15, 2009.

Author Contributions: Dr Sedel had full access to all the data in the study and takes full responsibility for the integrity of the data and the accuracy of the data analysis. Drs Debs and Depienne participated equally in this work. Study concept and design: Debs, Depienne, Bellanger, Lyon-Caen, Brice, and Sedel. Acquisition of data: Debs, Rastetter, Degos, Galanaud, and Sedel. Analysis and interpretation of data: Debs, Depienne, Galanaud, and Sedel. Drafting of the manuscript: Debs, Depienne, and Sedel. Critical revision of the manuscript for important intellectual content: Debs, Rastetter, Bellanger, Degos, Galanaud, Lyon-Caen, Brice, and Sedel. Administrative, technical, and material support: Debs, Rastetter, Bellanger, and Sedel. Study supervision: Depienne, Lyon-Caen, Brice, and Sedel.

Financial Disclosure: None reported.

Funding/Support: This work was supported financially by Institut National de la Santé et de la Recherche Médicale (INSERM), Assistance-Publique–Hôpitaux de Paris, and Université Pierre et Marie Curie (Paris VI).

Additional Contributions: We thank the family for their participation and the Bank of IFR70 for DNA extraction. We also thank Monique Mercadiel for her technical help in fibroblast culture and emetine treatment. The experiments comply with current French law regarding humane treatment of patients.