PAX8 is one of the best characterized thyroid-specific transcription factors that play a role in development and physiology of the thyroid gland. PAX8 causes, when mutated, congenital hypothyroidism in both mice and humans, although mode of inheritance is different among the two species (mice, recessive; humans, dominant). Genetically engineered PAX8-deficient mice show abnormalities in early proliferation of thyroid precursor cells, resulting in markedly severe thyroid hypoplasia. In humans, clinical phenotypes of mutation carriers are highly variable, ranging from severe congenital hypothyroidism with thyroid hypoplasia to normal thyroid function with a morphologically normal gland. Several genetic epidemiology studies have revealed that PAX8 mutation is a relatively rare cause of human congenital hypothyroidism, observed in about 2 % of cases. Nonetheless, PAX8 mutation is the leading genetic cause of congenital hypothyroidism showing dominant inheritance. Thus, clinicians should consider PAX8 mutations when he/she encounters a patient with positive family history of congenital hypothyroidism. This review focuses on the molecular genetics of human congenital hypothyroidism due to PAX8 mutations.
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