Islam Mohammad Saiful, Namba Noriyuki, Ohata Yasuhisa, Fujiwara Makoto, Nakano Chiho, Takeyari Shinji, Miyata Kei, Nakano Yukako, Yamamoto Kenichi, Nakayama Hirofumi, Kitaoka Taichi, Kubota Takuo, Ozono Keiichi
Department of Pediatrics, Graduate School of Medicine, Osaka University, Suita, Japan.
Department of Pediatrics, Osaka Hospital, Japan Community Healthcare Organization, Osaka, Japan.
Endocr J. 2019 Jan 28;66(1):19-29. doi: 10.1507/endocrj.EJ18-0251. Epub 2018 Oct 25.
Monocarboxylate transporter 8 (MCT8) facilitates T3 uptake into cells. Mutations in MCT8 lead to Allan-Herndon-Dudley syndrome (AHDS), which is characterized by severe psychomotor retardation and abnormal thyroid hormone profile. Nine uncharacterized MCT8 mutations in Japanese patients with severe neurocognitive impairment and elevated serum T3 levels were studied regarding the transport of T3. Human MCT8 (hMCT8) function was studied in wild-type (WT) or mutant hMCT8-transfected human placental choriocarcinoma cells (JEG3) by visualizing the locations of the proteins in the cells, detecting specific proteins, and measuring T3 uptake. We identified 6 missense (p.Arg445Ser, p.Asp498Asn, p.Gly276Arg, p.Gly196Glu, p.Gly401Arg, and p.Gly312Arg), 2 frameshift (p.Arg355Profs64 and p.Tyr550Serfs17), and 1 deletion (p.Pro561del) mutation(s) in the hMCT8 gene. All patients exhibited clinical characteristics of AHDS with high free T3, low-normal free T4, and normal-elevated TSH levels. All tested mutants were expressed at the protein level, except p.Arg355Profs64 and p.Tyr550Serfs17, which were truncated, and were inactive in T3 uptake, excluding p.Arg445Ser and p.Pro561del mutants, compared with WT-hMCT8. Immunocytochemistry revealed plasma membrane localization of p.Arg445Ser and p.Pro561del mutants similar with WT-hMCT8. The other mutants failed to localize in significant amount(s) in the plasma membrane and instead localized in the cytoplasm. These data indicate that p.Arg445Ser and p.Pro561del mutants preserve residual function, whereas p.Asp498Asn, p.Gly276Arg, p.Gly196Glu, p.Gly401Arg, p.Gly312Arg, p.Arg355Profs64, and p.Tyr550Serfs17 mutants lack function. These findings suggest that the mutations in MCT8 cause loss of function by reducing protein expression, impairing trafficking of protein to plasma membrane, and disrupting substrate channel.
单羧酸转运蛋白8（MCT8）促进T3进入细胞。MCT8突变会导致艾伦 - 赫恩登 - 达德利综合征（AHDS），其特征为严重的精神运动发育迟缓以及甲状腺激素谱异常。我们研究了9例患有严重神经认知障碍且血清T3水平升高的日本患者中未被鉴定的MCT8突变对T3转运的影响。通过观察蛋白质在细胞中的位置、检测特定蛋白质以及测量T3摄取，在野生型（WT）或转染了突变型hMCT8的人胎盘绒毛膜癌细胞（JEG3）中研究了人MCT8（hMCT8）的功能。我们在hMCT8基因中鉴定出6个错义突变（p.Arg445Ser、p.Asp498Asn、p.Gly276Arg、p.Gly196Glu、p.Gly401Arg和p.Gly312Arg）、2个移码突变（p.Arg355Profs64和p.Tyr550Serfs17）以及1个缺失突变（p.Pro561del）。所有患者均表现出AHDS的临床特征，即游离T3水平高、游离T4水平低至正常、促甲状腺激素（TSH）水平正常至升高。除了截短的p.Arg355Profs64和p.Tyr550Serfs17外，所有测试的突变体均在蛋白质水平表达，并且与WT - hMCT8相比，在T3摄取方面无活性，但不包括p.Arg445Ser和p.Pro561del突变体。免疫细胞化学显示，p.Arg445Ser和p.Pro561del突变体与WT - hMCT8类似，定位于质膜。其他突变体未能大量定位于质膜，而是定位于细胞质中。这些数据表明，p.Arg445Ser和p.Pro561del突变体保留了残余功能，而p.Asp498Asn、p.Gly276Arg、p.Gly196Glu、p.Gly401Arg、p.Gly312Arg、p.Arg355Profs64和p.Tyr550Serfs17突变体缺乏功能。这些发现表明，MCT8中的突变通过降低蛋白质表达、损害蛋白质向质膜的运输以及破坏底物通道导致功能丧失。
