Modelling thyroid embryogenesis using embryonic stem cells

Congenital hypothyroidism (CH) is the most frequent of the rare endocrine diseases (e.g. Addison's disease, Cushing's syndrome, Congenital adrenal hyperplasia.), which affects 1:2000 – 4000 newborns. If not immediately diagnosed after birth, thyroid hormones deficiency causes severe defects in brain and skeletal development leading to a complex clinical scenario called cretinism. CH can be due to a defective synthesis of thyroid hormones (dyshormonogenesis) or an abnormal embryonic development of the gland. Data obtained using knockout mouse models have shown the pivotal role of four specific transcription factors (NKX2.1, PAX8, FOXE1 and HHEX) for the correct organogenesis or function of the gland. Although mutations in those genes have been identified in few cases of CH patients, the pathogenetic mechanisms remain still elusive in the vast majority of CH cases (95%).<p>For the identification of new genes and molecular events controlling thyroid organogenesis it would be useful to develop an in vitro cellular model to recapitulate thyroid embryogenesis in a dish. Embryonic Stem Cells (ESCs) have recently emerged as system model to recapitulate the embryogenesis of several tissues in vitro.<p>Induced overexpression of defined transcription factors has been shown to have a directing effect on the differentiation of pluripotent stem cells into specific cell types. In this thesis I show that a transient overexpression of the transcription factors NKX2.1 and PAX8 is sufficient to direct the differentiation of murine ESCs into thyroid follicular cells (TFC) and promotes in vitro self- assembly of TFC into three-dimensional follicular structures, when associated to a subsequent thyrotropin (TSH) treatment. Cells differentiated by this protocol showed significant iodide organification activity, a hallmark of thyroid tissue function. Importantly, athyroid mice grafted with mESC-derived thyroid follicles show normalization of plasma T4 levels with concomitant decrease of plasma TSH. In addition, a full normalization of body temperature at 4 weeks after transplantation was observed. Together, these data clearly demonstrate that grafting of our mESC-derived thyroid cells rescues the hypothyroid state and triggers symptomatic recovery along with the normalization of plasma hormone concentrations. The high efficiency of TFC differentiation and follicle morphogenesis in our system will provide an unprecedented opportunity for future studies to decipher regulatory mechanisms involved in embryonic thyroid development, a major research need towards an improved understanding of the molecular mechanisms underlying congenital hypothyroidism, the most common congenital endocrine disorder in humans.Doctorat en Sciences biomédicales et pharmaceutiques