Biology of the congenitally hypothyroid hyt/hyt mouse

The hyt/hyt mouse has an autosomal recessive, fetal onset, characterized by severe hypothyroidism that persists throughout life and is a reliable model of human sporadic congenital hypothyroidism. The hypothyroidism in the hyt/hyt mouse reflects the hyporesponsiveness of the thyroid gland to thyrotropin (TSH). This is attributable to a point mutation of C to T at nucleotide position 1666, resulting in the replacement of a Pro with Leu at position 556 in transmembrane domain IV of the G protein-linked TSH receptor. This mutation leads to a reduction in all cAMP-regulated events, including thyroid hormone synthesis. The diminution in T3/T4 in serum and other organs, including the brain, also leads to alterations in the level and timing of expression of critical brain molecules, i.e. selected tubulin isoforms (M beta 5, M beta 2, and M alpha 1), microtubule associated proteins (MAPs), and myelin basic protein, as well as to changes in important neuronal cytoskeletal events, i.e. microtubule assembly and SCa and SCb axonal transport. In the hyt/hyt mouse, fetal hypothyroidism leads to reductions in M beta 5, M beta 2, and M alpha 1 mRNAs, important tubulin isoforms, and M beta 5 and M beta 2 proteins, which comprise the microtubules. These molecules are localized to layer V pyramidal neurons in the sensorimotor cortex, a site of differentiating neurons, as well as a site for localization of specific thyroid hormone receptors. These molecular abnormalities in specific cells and at specific times of development or maturation may contribute to the observed neuroanatomical abnormalities, i.e. altered neuronal process growth and maintenance, synaptogenesis, and myelination, in hypothyroid brain. Abnormal neuroanatomical development in selected brain regions may be the factor underlying the abnormalities in reflexive, locomotor, and adaptive behavior seen in the hyt/hyt mouse and other hypothyroid animals.