RESEARCH ARTICLE OFFICIAL JOURNAL www.hgvs.org Relevance of Different Cellular Models in Determining the Effects of Mutations on SLC16A2/MCT8 Thyroid Hormone Transporter Function and Genotype–Phenotype Correlation Yline Capri, 1,2,3 Edith C.H. Friesema, 4 Simone Kersseboom, 4 Renaud Touraine, 5 Aur ´ elie Monnier, 1,6 El´ eonore Eymard-Pierre, 1,6 Vincent Des Portes, 7 Giusseppe De Michele, 8 Angela F. Brady, 9 Odile Boespflug-Tanguy, 3,10,11 Theo J. Visser, 4 and Catherine Vaurs-Barriere 1,12 ∗ 1 INSERM, UMR 1103, CNRS 6293, GReD, Medical school, Clermont-Ferrand, France; 2 APHP, Genetic Department, Robert Debr ´ e University Hospital, Paris, France; 3 Universit ´ e Paris Diderot, Sorbonne Paris Cit ´ e, Robert Debr ´ e University Hospital, Paris, France; 4 Department of Internal Medicine, Erasmus University Medical Center, Rotterdam, The Netherlands; 5 Department of Clinical Chromosomal and Molecular Genetics, CHU St Etienne, France; 6 Medical Cytogenetic, Clermont-Ferrand University Hospital, Clermont-Ferrand, France; 7 Reference Center for Rare Intellectual Disabilities, Neuro-Paediatric Department, Debrousse Hospital, Lyon, France; 8 Dipartimento di Scienze Neurologiche, Universit ` a di Napoli Federico II, Napoli, Italy; 9 North West Thames Regional Genetics Service, Kennedy-Galton Centre, Northwick Park Hospital, Harrow, United-Kingdom; 10 APHP, Reference Center for Rare diseases “Leukodystrophies”, Pediatric Neurology and Metabolic Disorders Department, Robert Debr ´ e University Hospital, Paris, France; 11 INSERM U676, H ˆ opital Robert Debr ´ e, Paris, France; 12 Auvergne University, Medical School, Clermont-Ferrand, France Communicated by Madhuri Hegde Received 6 November 2012; accepted revised manuscript 25 March 2013. Published online 8 April 2013 in Wiley Online Library (www.wiley.com/humanmutation). DOI: 10.1002/humu.22331 ABSTRACT: SLC 16A2, the gene for the second member of the solute carrier family 16 (monocarboxylic acid transporter), located on chromosome Xq13.2, encodes a very efficient thyroid hormone transporter: monocarboxylate transporter 8, MCT8. Its loss of function is responsible in males for a continuum of psy- chomotor retardation ranging from severe (no motor acquisition, no speech) to mild (ability to walk with help and a few words of speech). Triiodothyronine uptake measurement in transfected cells and, more recently, patient fibroblasts, has been described to study the functional consequences of MCT8 mutations. Here, we describe three novel MCT8 mutations, including one mis- sense variation not clearly predicted to be damaging but found in a severely affected patient. Functional studies in fibroblasts and JEG3 cells demonstrate the usefulness of both cellular models in validating the deleterious effects of a new MCT8 mutation if there is still a doubt as to its pathogenicity. Moreover, the screen- ing of fibroblasts from a large number of patient fibroblasts and of transfected mutations has allowed us to demonstrate that JEG3 transfected cells are more relevant than fibroblasts in revealing a genotype–phenotype correlation. Hum Mutat 34:1018–1025, 2013. C  2013 Wiley Periodicals, Inc. KEY WORDS: SLC16A2; monocarboxylate transporter 8; MCT8; thyroid hormone Additional Supporting Information may be found in the online version of this article. ∗ Correspondence to: Catherine Vaurs-Barriere, GReD, INSERM U1103, 28 Place Henri Dunant, CLERMONT-FERRAND, 63000, France. E-mail: Catherine.BARRIERE@ u-clermont1.fr Contract grant sponsors: Foundation of Auvergne University; French Ministry of Health (DHOS). Introduction SLC16A2 (also referred to as MCT8; MIM #300095), the gene for the second member of the solute carrier family 16 (monocar- boxylic acid transporter), located on chromosome Xq13.2, encodes a very efficient thyroid hormone (TH) transporter: monocarboxy- late transporter 8 or MCT8 [Friesema et al., 2003; 2006]. This 12- transmembrane domain (TMD) protein allows triiodothyronine (T3) and thyroxine (T4) to enter and exit cells. MCT8 is expressed in cells of various tissues including neurons in the central nervous system, where it appears to play its most important role. Hem- izygous SLC16A2 loss-of-function mutations in males cause severe psychomotor retardation with massive axial hypotonia, poor head control, abnormal dystonic movements, profound intellectual im- pairment, and an absence of language acquisition [Dumitrescu et al., 2004; Friesema et al., 2004]. A milder phenotype has been known since 1944 as Allan–Herndon–Dudley syndrome (MIM #300523) [Stevenson et al., 1990], although it may vary in severity. In addition to the clinical and biological features of patients with SLC16A2 mutations, some authors have reported a nonspe- cific myelination delay [Holden et al., 2005; Namba et al., 2008] and spectroscopic abnormalities [Sijens et al., 2008]. More recently, the identification of SLC16A2 mutations in a series of patients initially considered as presenting with a severe Pelizaeus–Merzbacher-like disease (PMD) in their first years of life has confirmed this asso- ciation with delayed myelination [Gika et al., 2010; Vaurs-Barri` ere et al., 2009]. These patients are characterized by early-onset hy- potonia, nystagmus, progressive spasticity, dystonia, ataxia, and dif- fuse hypomyelination on brain magnetic resonance imaging (MRI), which tends to disappear with age (without a parallel clinical im- provement). A correlation between the SLC16A2 genotype and the severity of the patient’s phenotype has been suggested. In vitro, JEG3 hu- man placental choriocarcinoma cells transfected with mutant hu- man SLC16A2 cDNA show an almost complete loss of T3 transport C  2013 WILEY PERIODICALS, INC.,