Published online December 31, 2007
PEDIATRICS Vol. 121 No. 1 January 2008, pp. e199-e202 (doi:10.1542/peds.2007-1247)
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EXPERIENCE & REASON

A Novel Monocarboxylate Transporter 8 Gene Mutation as a Cause of Severe Neonatal Hypotonia and Developmental Delay

Anastasios Papadimitriou, MDa, Alexandra Mihaela Dumitrescu, MD, PhDb, Antigone Papavasiliou, MDc, Andreas Fretzayas, MDa, Polyxeni Nicolaidou, MDa and Samuel Refetoff, MDd,e

a Third Department of Pediatrics, University of Athens School of Medicine, Attikon University Hospital, Athens, Greece
b Departments of Medicine
d Pediatrics
e Committees on Genetics and Molecular Medicine, and J.P. Kennedy Mental Retardation and Developmental Disabilities Center, University of Chicago, Chicago, Illinois
c Neurology Department, Penteli Children's Hospital, Athens, Greece

ABSTRACT

Monocarboxylate transporter 8 acts as a specific cell membrane transporter for thyroxine and especially triiodothyronine into target cells. It is expressed in brain neurons and in many other tissues. The monocarboxylate transporter 8 gene resides on chromosome Xq13.2. An 11-month-old male infant was referred because of severe hypotonia from early life and global developmental delay. Thyroid-function tests showed normal thyrotropin levels and the characteristic for the disorder, including high serum triiodothyronine and low thyroxine concentrations. Molecular analysis of the monocarboxylate transporter 8 gene showed that the patient was hemizygous for a novel missense mutation P537L. This case highlights the importance of determining thyroid hormone levels, especially triiodothyronine, in infants with severe neonatal hypotonia.

Key Words: monocarboxylate transporter 8 • neonatal hypotonia • developmental delay • thyroid hormones

Abbreviations: TH—thyroid hormone • CNS—central nervous system • T3—3,3',5-triiodothyronine • D1—type 1 5'-deiodinase • D2—type 2 5'-deiodinase • T4—thyroxine • rT3—3,3',5'-triiododothyronine • MCT8—monocarboxylate transporter 8 • TFT—thyroid-function test • L-T4—L-thyroxine

Accepted May 24, 2007.