Endocrine Abstracts

Introduction: Paired box gene 8 (PAX8) is a key transcription factor required both for normal fetal thyroid development and maintenance of the differentiated thyroid phenotype, mediating transcriptional activation of SLC5A5, TG, and TPO, and synergizing with NKX2-1 at the TG promoter. Heterozygous PAX8 mutations are a rare but well-recognized cause of congenital hypothyroidism (CH) due to thyroid dysgenesis (TD), and are classically associated with thyroid hypoplasia. However, a spectrum of thyroidal morphologies and biochemical severities have been reported, with infrequent association of urogenital tract malformations. We report diverse clinical phenotypes in 4 kindreds harbouring different heterozygous PAX8 mutations, with supporting laboratory functional characterization of the mutant PAX8.

Case Reports: P1 harbours a novel PAX8 missense mutation (p.I34F) associated with severe CH, thyroid hypoplasia and hypospasias. P2 is a female child harbouring a truncating mutation (p.R207*) previously reported in the context of Mayer-Rokitansky-Kuster-Hauser (MRKH) syndrome. Here, p.R207* is newly associated with mild CH, and absent Tc-99m uptake in an ultrasonographically normal sized thyroid gland-in-situ (GIS) without associated urogenital tract defects. P3 exhibits severe CH with a dysplastic thyroid associated with PAX8 c.162C>A, p.S54R; the same amino acid mutation due to a different nucleotide change (c.162C>G) has previously been reported in individuals with TD. P4a and P4b are sisters harbouring PAX8 p.S59R associated either with permanent GIS CH or mild CH with hemiagenesis. This contrasts with previously reported cases in whom p.S59R is associated with severe GIS CH and partial iodide organification defect, or goitrous CH with cryptorchidism and hydrocele.

Laboratory Data: p.S54R has previously been characterized in detail (1). Here, functional evaluation of PAX8 p.S59R, p.R207* and p.I34F demonstrated that the mutant proteins were expressed, but exhibited impaired transactivation of both TPO and TG promoters in luciferase reporter assays, compared to wild-type PAX8. Co-transfection studies with NKX2-1 achieved at least partial rescue of TG promoter transactivation with both missense mutations but not p.R207*, supporting a role for the PAX8 carboxyterminus in this interaction. Homology modelling suggests steric hindrance due to the p.S59R and p.I34F mutations may indirectly affect PAX8-DNA interactions.

Conclusion: Our studies characterize a novel PAX8 mutation (p.I34F) associated with both thyroid and urogenital tract pathology, and yield further insights into PAX8-NKX2-1 synergism. Additionally, we expand the clinical phenotypes associated with reported PAX8 mutations, demonstrating isolated CH with possible impaired SLC5A5 function in MRKH-associated p.R207*, and thyroid hemiagenesis with p. S59R. 1. Hermanns et al (2013) Thyroid 23, 791