A Genomic Analysis of Subclinical Hypothyroidism in Hippocampus and Neocortex of the Developing Rat Brain

Hypothyroidism during pregnancy and the early postnatal period has severe neurological consequences for the developing offspring. The impact of milder degrees of perturbation of the thyroid axis as encompassed in conditions of subclinical hypothyroidism and hypothyroxinemia, however, has not been established. The present investigation examined the effects of graded levels of hypothyroidism, from subclinical to severe, on global gene expression in the developing rodent brain. Thyroid hormone insufficiency was induced by administration of propylthiouracil (PTU) to pregnant rats via drinking water from gestational day 6 until sacrifice of pups prior to weaning. In the first study a specialised microarray, the Affymetrix Rat Neurobiology array RN_U34, was used to contrast gene expression in the hippocampus of animals exposed to 0 or 10 ppm (10 mg/l) PTU, a treatment producing severe hypothyroidism. In the second study, a more complete genome array (Affymetrix Rat 230A) was used to compare gene expression in the neocortex and hippocampus of postnatal day (PN) 14 animals experiencing graded degrees of thyroid hormone insufficiency induced by delivery of 0, 1, 2 or 3 ppm PTU to the dam. Dose‐dependent up‐ and down‐regulation were observed for gene transcripts known to play critical roles in brain development and brain function. Expression levels of a subset of approximately 25 genes in each brain region were altered at a dose of PTU (1 ppm) that induced mild hypothyroxinemia in dams and pups. These data indicate that genes driving important developmental processes are sensitive to relatively modest perturbations of the thyroid axis, and that the level of gene expression is related to the degree of hormone reduction. Altered patterns of gene expression during critical windows of brain development indicate that thyroid disease must be viewed as a continuum and that conditions typically considered ‘subclinical’ may induce structural and functional abnormalities in the developing central nervous system.

妊娠期和产后早期的甲状腺功能减退对发育中的后代具有严重的神经学影响。然而，亚临床甲状腺功能减退和低甲状腺素血症等情况下甲状腺轴较轻微紊乱的影响尚未确定。本研究检测了从亚临床到严重不同程度的甲状腺功能减退对发育中的啮齿动物大脑整体基因表达的影响。从妊娠第6天起通过饮用水给怀孕大鼠施用丙硫氧嘧啶（PTU），直至幼崽断奶前处死，以此诱导甲状腺激素不足。在第一项研究中，使用一种专门的微阵列，即Affymetrix大鼠神经生物学阵列RN_U34，对比暴露于0或10ppm（10mg/L）PTU（一种导致严重甲状腺功能减退的处理）的动物海马中的基因表达。在第二项研究中，使用一种更完整的基因组阵列（Affymetrix大鼠230A），比较产后第14天（PN14）的动物在其母鼠接受0、1、2或3ppm PTU处理而出现不同程度甲状腺激素不足时其新皮质和海马中的基因表达。观察到已知在大脑发育和大脑功能中起关键作用的基因转录本存在剂量依赖性的上调和下调。在母鼠和幼崽中诱导轻度低甲状腺素血症的PTU剂量（1ppm）下，每个大脑区域中约25个基因的一个子集的表达水平发生了改变。这些数据表明，驱动重要发育过程的基因对甲状腺轴相对轻微的紊乱敏感，并且基因表达水平与激素减少的程度有关。在大脑发育的关键窗口期基因表达模式的改变表明，甲状腺疾病必须被视为一个连续体，并且通常被认为是“亚临床”的情况可能会导致发育中的中枢神经系统出现结构和功能异常。