Congenital thyrotropin deficiency--from discovery to molecular biology, postgenome and preventive medicine.

IT is well known that the human glycoprotein hormone family is comprised of four members: thyro-tropin (TSH) and gonadotropin (Gn), which includes follicle stimulating hormone (FSH), luteinizing hormone (LH) and chorionic gonadotropin (CG) [1, 2]. As shown in Fig. 1, synthesis and secretion of TSH, FSH and LH from the pituitary gland are regulated by the hypothalamic releasing hormones, thyrotropin releasing hormone (TRH) and Gn releasing hormone (GnRH) (luteinizing hormone releasing hormone (LHRH)) as well as by the negative feedback mechanism involving the peripheral hormones, thyroid hormones and sex steroid hormones. The CG synthesized in the placenta is functionally most like LH and essential for the maintenance of pregnancy. The glycoprotein hormones are composed of two non-covalently linked α-and β-subunits (chains) as a heterodimer. The amino acid sequence of α (common α) subunits are identical in all glycoprotein hormones, while those of β subunits are homologous to each other yet each is unique and these differences confer the specific biological activities. Complex carbohydrate side chains are attached to the α-and β-subunits. Isolation, cloning and sequence analysis of the DNAs for glyco-protein hormones and their transcription factors have been performed extensively. Congenital glycoprotein hormone deficiency is classified into three types: isolated deficiency with lack of a single hormone, combined (multiple) deficiency (partial hypopituitarism) associated with lack of other pituitary hormones, and panhypopituitarism. On the other hand, hypothyroidism and hypogonadism are classified into four types according to the site of impairment in the hypothalamic-pituitary-thyroidal and gonadal axis (Fig. 1). In the case of hypothyroidism, three types are known: central including hypothalamic (tertiary) and pituitary (secondary), primary (impairment of the thyroid gland itself) and peripheral (resistance to thyroid hormone). In 1971, the first case of congenital glycoprotein hormone deficiency was reported by Miyai et al. [3] as a familial isolated TSH deficiency with congenital hypothyroidism (cretinism). The case was well documented by sophisticated clinical laboratory tests such as immunoassay and provocation tests. About two decades later, introduction of molecular biology techniques paved the way for the gene analyses of the patient. We performed molecular cloning of hTSH β subunit gene [4, 5] and demonstrated that a single base substitution in the CAGYC region of the TSH β-subunit was responsible for this disorder [6, 7]. Based on our subsequent studies on wild and mutant types of glycoprotein hormones with site directed mutagenesis of the CAGYC region in the β-subunits and CMGCC region in the …