MCT8 Deficiency in Females

Abstract

Context: Monocarboxylate transporter (MCT) 8 facilitates thyroid hormone (TH) transport across the blood-brain barrier. Pathogenic variants in SLC16A2 cause MCT8 deficiency (Allan-Herndon-Dudley syndrome), characterized by intellectual and motor disability and abnormal thyroid function tests. MCT8 deficiency typically affects males due to its X-linked inheritance.

Objective: Here, we report 8 female patients with heterozygous pathogenic variants in SLC16A2 who presented with variable neurocognitive impairment, behavioral problems, and TH function abnormalities.

Methods: We performed X-chromosome inactivation studies in female patients in whom heterozygous pathogenic variants in SLC16A2 were identified. The effect of SLC16A2 variants on TH transport was assessed in transfected cells and patient-derived fibroblasts.

Results: In all patients (mean age 8.6 years; range, 2.3-25 years) routine care genetic analyses identified heterozygous variants in SLC16A2 (p.(R445C), p.(N193I), p.(G276R), t(X;20), resulting in a breakpoint in intron 1, t(X;19), resulting in a breakpoint in SLC16A2, p.(I562Sfs566*), p.(G221R)). All missense variants showed substantially reduced MCT8-mediated TH uptake in transiently transfected cells. X-chromosome inactivation studies in patient cells showed skewed X-inactivation in all 7 evaluated individuals. In 5 out of 7 evaluated cases, MCT8-mediated 3,5,3'-triiodothyronine (T3) uptake in patient-derived fibroblasts was impaired to a similar degree as in fibroblasts derived from male patients with MCT8 deficiency.

Conclusion: Female patients with heterozygous pathogenic variants in SLC16A2 and skewed X-chromosome inactivation may present with variable neuro(psycho)logical, behavioral, and thyroid function test abnormalities. Female patients presenting with neurocognitive impairment and abnormal TH function tests (low free thyroxine and/or high total T3 concentrations) should be tested for genetic variants in SLC16A2.

Keywords: MCT8; monocarboxylate transporter 8; neurocognitive impairment; skewed X-chromosome inactivation; thyroid hormone; thyroid hormone transport.

Figure 1.

Magnetic resonance imaging (MRI) studies in patient P1 (female), her affected brother (patient P1B) and patient P4 (female). A, FLAIR-, T1-, and T2-weighted MRI of the brain of patient P1 performed at age 21 years, showing mild cortical atrophy without signs of leukodystrophy. B, FLAIR-, T1-, and T2-weighted MRI of the brain of the brother of patient P1 (patient IB) performed at age 25 years, showing signs of mild cortical atrophy without obvious signs of leukodystrophy. C, T1- and T2-weighted MRI of the brain of patient P4 at age 1 year show a small anterior cranial fossa and 6 to 7 month delayed myelination with nonmyelination in the superior and middle frontal gyrus. The applied MRI sequences are indicated in each image.

Figure 2.

In vitro functional studies confirmed pathogenicity of identified mutations. A, T3 and T4 uptake studies in COS-1 cells transiently transfected with wild-type (WT) MCT8 or indicated mutants and the intracellular thyroid hormone (TH)-binding protein CRYM in 30 minutes at 37 °C. Data are expressed relative to WT MCT8 and are corrected for background TH uptake in empty vector transfected control cells. Results are presented as means ± SEM (N = 3-4). One-way analysis of variance with Dunnett post hoc tests were used to assess for statistically significant differences compared to healthy controls (*P < .05; **P < .005; ***P < .0005; ****P < .0001). B, Representative immunoblot analyses of MCT8 protein in total lysates (left) and the surface biotinylated fraction (right) of COS-1 cells transfected with indicated constructs. GAPDH (glyceraldehyde-3-phosphate dehydrogenase) was used as a loading control. Immunoblot analyses of G221R have been performed separately. C, Immunocytochemistry in transfected JEG-3 cells to visualize WT and mutant MCT8 (green) subcellular localization. Zona occludens (ZO-1; red) was used as a membrane marker and DAPI (4′,6-diamidino-2-phenylindole; blue) as a nuclear marker.

Figure 3.

Ex vivo analyses of MCT8 function in patient-derived fibroblasts. A, Triiodothyronine (T3) uptake in patient and control fibroblasts in the absence (black) and presence (white) of 10-µm silychristin (SC), a specific inhibitor of MCT8. Data are expressed relative to the amount of T3 accumulation in control cells in the absence of SC. B, Silychristin-induced reduction in T3 uptake in control and patient fibroblasts. Data are derived from A. One-way analysis of variance with Dunnett post hoc tests were used to assess for statistically significant differences compared to healthy controls (*P < .05; **P < .005; ***P < .0005; ****P < .0001). C, Correlation plot of patient’s IQ (divided into no, mild, and moderate intellectual disability (ID)) vs MCT8 function, defined as percentage silychristin-induced reduction in T3 uptake shown in B. In case of multiple available measurements, the mean ± SEM are displayed.

Figure 4.

Serum thyroid function tests in female patients in comparison to asymptomatic carriers and noncarriers. A, Serum free thyroxine (T4); B, total triiodothyronine (T3); C, total 3,3,5′-triiodothyronine (rT3) concentrations; D, total T3/total rT3; and E, total T3/free T4 ratios; and F, thyrotropin (TSH) concentrations in female patients (P), asymptomatic female carriers (C) and noncarriers (NC). One-way analysis of variance with Tukey post hoc tests were used to test for statistically significant differences between groups (ns, not significant; *P < .05; **P < .005; ***P < .0005; ****P < .0001).