As a key component of the
transforming growth factor beta (TGFB) pathway, which regulates the expression of thyroid-specific genes, tumor suppressor SMAD4 is crucial for thyroid development and function. Aberrant expression of SMAD4 in thyroid
tumor tissue was reported and mutations affecting the coding region have been detected, but a potential role of mutations in SMAD4 gene regulatory regions remains unexplored. The aim of this study was to analyze SMAD4 gene promoters in thyroid
tumors. A total of 76
thyroidectomy specimens were studied, including 42 malignant and 34 benign
tumors. The presence of mutations in four SMAD4 gene promoters was analyzed in thyroid
tumor tissue and peripheral blood by PCR and
DNA sequencing. The expression and intracellular localization of endogenous
SMAD4 protein in selected
tumor samples was studied by immunostaining and confocal microscopy. Of three novel variants detected, two were within promoter A (-204T/C and -5C/T) and one in promoter D (-180delA). Unlike somatic mutations previously detected in the nearby region, germline mutation -180delA in promoter D doesn't appear to affect SMAD4 expression in the thyroid
tumor tissue. However, all newly detected SMAD4 promoter variants affect predicted binding sites of
transcription factors involved in cell cycle regulation and should be further characterized functionally. Although not directly involved in
carcinogenesis, detected variants may alter SMAD4 transcriptional regulation to some extent. Considering that dosage dependence is of great importance for the role of
SMAD4 protein as a
tumor suppressor, potential clinical significance of SMAD4 gene promoter mutations is worth further investigation.