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BACKGROUND: Allan-Herndon-Dudley syndrome (AHDS) is a severe psychomotor disability disorder that also manifests characteristic abnormal thyroid hormone (TH) levels. AHDS is caused by inactivating mutations in monocarboxylate transporter 8 (MCT8), a specific TH plasma membrane transporter widely expressed in the central nervous system (CNS). MCT8 mutations cause impaired transport of TH across brain barriers, leading to insufficient neural TH supply. There is currently no successful therapy for the neurological symptoms. Earlier work has shown that intravenous (IV), but not intracerebroventricular adeno-associated virus serotype 9 (AAV9) -based gene therapy given to newborn Mct8 knockout (Mct8(−/y)) male mice increased triiodothyronine (T(3)) brain content and partially rescued TH-dependent gene expression, suggesting a promising approach to treat this neurological disorder. METHODS: The potential of IV delivery of AAV9 carrying human MCT8 was tested in the well-established Mct8(−/y)/Organic anion-transporting polypeptide 1c1 (Oatp1c1)(−/ −) double knockout (dKO) mouse model of AHDS, which, unlike Mct8(−/y) mice, displays both neurological and TH phenotype. Further, as the condition is usually diagnosed during childhood, treatment was given intravenously to P30 mice and psychomotor tests were carried out blindly at P120–P140 after which tissues were collected and analyzed. RESULTS: Systemic IV delivery of AAV9-MCT8 at a juvenile stage led to improved locomotor and cognitive functions at P120–P140, which was accompanied by a near normalization of T(3) content and an increased response of positively regulated TH-dependent gene expression in different brain regions examined (thalamus, hippocampus, and parietal cortex). The effects on serum TH concentrations and peripheral tissues were less pronounced, showing only improvement in the serum T(3)/reverse T(3) (rT(3)) ratio and in liver deiodinase 1 expression. CONCLUSION: IV administration of AAV9, carrying the human MCT8, to juvenile dKO mice manifesting AHDS has long-term beneficial effects, predominantly on the CNS. This preclinical study indicates that this gene therapy has the potential to ameliorate the devastating neurological symptoms in patients with AHDS.

Allan-Herndon-Dudley syndrome (AHDS) is a severe X-linked intellectual and psychomotor disability disorder accompanied by abnormal thyroid hormone (TH) levels. AHDS is caused by inactivating mutations in the monocarboxylate transporter 8 (MCT8), a specific TH transporter widely expressed in the central nervous system. MCT8 gene mutations cause impaired transport of TH across brain barriers, leading to insufficient neural TH supply. There is currently no successful therapy for the neurological symptoms. AAV9-based gene therapy is a promising approach to treat monogenic neurological disorders. Here, the potential of this approach was tested in the well-established double knockout (dKO) Mct8-/y;Oatp1c1-/- mouse model of AHDS, which displays disease-relevant neurological and TH phenotypes. Systemic intravenous delivery of AAV9-MCT8 at a juvenile stage led to improved locomotor and cognitive function, as well as rescue of T3-brain content and T3-related gene expression. This preclinical study indicates that this gene therapy may improve the neurological symptoms of AHDS patients.