Abstract
Monocarboxylate transporter 8 (MCT8) deficiency or the Allan-Herndon-Dudley Syndrome (AHDS) is an X-linked psychomotor disability syndrome with around 320 clinical cases described worldwide. SLC16A2 gene mutations, encoding the thyroid hormone (TH) transporter MCT8, result in intellectual disability due to impaired TH uptake in the developing brain. MCT8 deficiency is a multi-organ affecting disease with a predominant neuronal cell-based pathology, with the glial component inadequately investigated. However, deficiency in myelin, a key component of white matter (WM) enabling fast nerve conduction, is a TH-dependent hallmark of the disease. Nevertheless, analysis of the myelin status in AHDS patients has led to conflicting interpretations. The majority of individual case studies reported delayed myelination, that was restored later in life. In contrast, post-mortem studies and high-resolution MRIs detected WM (micro-) abnormalities throughout adolescence, suggesting permanent hypomyelination. Thus, interpretations vary depending on methodology to investigate WM microstructure. Further, it is unknown whether the mutation within the MCT8 is linked to the severity of the myelin deficiency. Consequently, terminology is inconsistent among reports, and AHDS is occasionally misdiagnosed as another WM disorder. The evolutionary conserved TH signaling pathway that promotes the generation of myelinating oligodendrocytes enabled deciphering how the lack of MCT8 might affect myelinogenesis. Linking patient findings on myelination to those obtained from models of MCT8 deficiency revealed underlying pathophysiological mechanisms, but knowledge gaps remain, notably how myelination progresses both spatially and temporally in MCT8 deficiency. This limits predicting how myelin integrity might benefit therapeutically, and when to initiate. A recurrent observation in clinical trials is the absence of neurological improvement. Testing MCT8-independent thyromimetics in models, and evaluating treatments used in other demyelinating diseases, despite different etiologies, is crucial to propose new therapeutic strategies combatting this devastating disease.
Highlights
The Allan-Herndon-Dudley Syndrome (AHDS, OMIM #300523) is a very rare, X-linked psychomotor disability syndrome first described in 1944, almost exclusively affecting boys [1]
We employed a similar search strategy to [34] using the following search terms: “monocarboxylate transporter 8 (MCT8)” and “mutation,” “solute carrier 16A2 (SLC16A2)” and “mutation,” “Allan–Herndon–Dudley syndrome,” “AHDS” and “case report,” “AHDS” and “magnetic resonance imaging (MRI)” and “myelination.” We found an additional 18 peer-reviewed articles reporting on 61 patients for whom MRI was used to assess myelin status in the brain (Table 1)
This review underlines the degree of inconsistency existing in the literature on how to classify AHDS amongst the diverse group of white matter (WM) disorders
Summary
The Allan-Herndon-Dudley Syndrome (AHDS, OMIM #300523) is a very rare, X-linked psychomotor disability syndrome first described in 1944, almost exclusively affecting boys [1]. The two patients examined in Anik et al clearly illustrate the discrepancy among patients as a function of age [50]: the two boys of the second investigated family are 6 months and 6 years old, but only the older boy displays hypomyelination In this last case, the myelin status differs extensively despite the brothers sharing the same 2.8 kb deletion in the SLC16A2 gene that results in a complete loss of MCT8 uptake activity due to the lack of exons 3 and 4 [50]. The myelin status differs extensively despite the brothers sharing the same 2.8 kb deletion in the SLC16A2 gene that results in a complete loss of MCT8 uptake activity due to the lack of exons 3 and 4 [50] This case prompts the question whether or not a genotype-phenotype relationship exists between the type of gene mutation, its impact on MCT8 protein structure and function, and the severity of the myelin deficiency.
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