Chromosomal instability (CIN) is a common feature of many malignancies including papillary and follicular thyroid cancer. During mitosis, there are multiple stages which, if dysregulated, may result in CIN. One critical stage is the regulation of sister-chromatid separation at the metaphase-anaphase transition. Three key protein complexes regulate this process: separase, cohesins and securin (also known as pituitary tumor transforming gene). In vivo, most tumours studied demonstrate elevated securin levels and in vitro experiments have demonstrated that overexpression of human securin results in incomplete sister-chromatid separation and delayed mitoses. Interestingly, securin null cell lines also demonstrate rapid chromosomal loss and instability. We sought to determine the relative levels of securin and separase mRNA and protein in normal and tumourous thyroid tissue in conjunction with pituitary, oesophageal and colorectal samples (in total normal = 48 samples, tumour = 98). Quantitative real-time RT PCR was performed to measure mRNA levels, and protein was quantitated by immunoblotting and densitometry. Securin expression was significantly elevated in almost all the thyroid tumour samples, and other tumours, relative to the normals, both at the mRNA (average 4 fold increase, p<0.05) and protein level (average 3.8 fold increase, p<0.01). However, separase mRNA levels stayed consistently low with no significant increase above that observed in the normal tissue. Separase protein levels were also very low or undetectable in most samples. Transfection experiments where the levels of securin and separase were modulated against each other demonstrated no change in the degree of double-stranded DNA breaks as measured by the comet assay, although this does not preclude the formation of other types of DNA lesions. Since overexpression of securin in vitro results in chromosomal aberrations consistent with CIN, the imbalance in securin:separase ratios observed in vivo in this study may provide a possible mechanism for CIN in thyroid and other tumour types.
22 - 24 Mar 2004
British Endocrine Societies