Context: The pituitary-specific transcription factor, Pit-1, plays a key role in the development and differentiation of three pituitary cell types: somatotrophs, lactotrophs, and thyrotrophs. Several mutations of the human gene (called POU1F1) have been shown to be responsible for a phenotype of combined pituitary hormone deficiency (CPHD) involving GH, PRL and TSH.

Objective: We have identified a novel homozygous C to G mutation in exon 4 of POU1F1 gene (S179R) in a patient with this rare phenotype. We analyzed the functional consequences of this S179R mutation associated with a single amino acid change in the POU-specific domain.

Methods: Consequences of this mutation on transcriptional activities by transfection studies in T3 cells, DNA binding ability by electromobility shift assay, structural properties, and nuclear accumulation of POU1F1 were investigated.

Results: The transactivation capacity of this mutant was markedly decreased on the GH1, PRL, TSH, and POU1F1 genes. Interestingly, this mutation abolished the functional interaction of POU1F1 on the PRL promoter with the coactivator CBP, but not with the transcription factor LHX3. The S179R mutant displayed normal nuclear accumulation but a markedly decreased binding to a DNA response element in keeping with crystallographic data suggesting that the S179R mutation might interfere with DNA binding.

Conclusions: Together with previous data, our study indicates that both DNA binding and interaction with cofactors like CBP are critical for POU1F1 function, and that functional and structural properties of abnormal POU1F1 proteins are variously influenced by the type of mutations.