Endocrine Abstracts (2004) 7 OC18

Biological consequences of gain-of-function thyrotropin receptor (TSHR) mutant expression in adipose tissue

GRC Baker1, JW Gregory2, A Bakhsh1, PR Betts3, IA Hughes4 & M Ludgate1

1Medicine, UWCM, Cardiff, UK; 2Child Health, UWCM, Cardiff, UK; 3Paediatrics, University of Southampton, UK; 4Paediatric Endocrinology, University of Cambridge, UK.

Neonates harbouring germline gain-of-function TSHR mutations (M453T, L629F) have been reported with transient proptosis. This resolves once euthyroidism is achieved. In contrast children expressing TSHR mutations (V597L, I486T) fail to thrive, with weight gain and height between the 0.4th and 2nd centiles. We have previously demonstrated TSHR expression in preadipocytes undergoing adipogenesis. We now investigate the consequences of constitutively active TSHR mutation expression in adipose tissue.

We have used retroviral vectors (RV) to introduce wild type (WT) and gain-of-function TSHR mutants into human primary preadipocytes.

Initially we confirmed the presence of the relevant TSHR transcripts and protein in RV transduced preadipocyte cell lines including 3T3L1. We observed that the expressed TSHRs were sulphated, a requirement for functional signalling. We also demonstrated basal cAMP levels increased to 116% (WT), 126% (L629F) and 220% (M453T) of the non-transduced controls.

In human preadipocytes, the proliferation of the cells was significantly inhibited to 62-77% (WT), 40-66% (L629F) and 5-34% (M453T) of the non-transduced controls. No spontaneous adipogenesis was evident in human preadipocytes expressing WT or mutant TSHR. In differentiation induced with a PPAR gamma agonist, despite morphological changes characteristic of adipogenesis, mutant TSHR expressing cells did not accumulate lipid. This contrasted with the WT and non-transduced cells which displayed oil red O staining.

An 8-year-old girl harbouring TSHR V597L had weight and height below the 0.4th centile compared with 3 siblings and parents between the 9-25th centiles. Measurement of body composition by dual energy X-ray absorptiometry demonstrated that she had reduced body fat (19.1% of total body weight) compared to her unaffected 11-year-old twin sisters (22.1 & 25.4%) and 13-year-old sister (23.3%).

Although there are wide normal variations in growth and body composition, the combined clinical and in vitro finding suggest that the terminal stages of adipocyte differentiation are inhibited by gain-of-function TSHR mutation.

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