Background: Thyroid nodules are common in the general population with the vast majority being benign. The challenge with management is how to distinguish the malignant from the benign tumours. The most accurate diagnostic method is fine-needle aspiration biopsy. However, indeterminate results are seen in approximately 30% of cases. Recent studies have shown a difference in expression of certain microRNAs (miRNAs) between normal thyroid tissue and tumour tissue and have highlighted their potential as diagnostic markers. The aim of this study was to examine the differential expression of 7 miRNAs known to be associated with papillary thyroid cancer (PTC) in our Thyroid tissue bank samples.

Methods: Total RNA was extracted from histologically confirmed PTC and non-cancer thyroid tissue from patients (n=23) who had a thyroidectomy at Royal Prince Alfred Hospital. miRNA was isolated from frozen tissue (13 matched PTC and normal thyroid) as well as formalin-fixed, paraffin embedded (FFPE) tissue (10 matched PTC and normal thyroid) using Qiagen and Roche FFPE miRNA extraction kits respectively. cDNA synthesis was performed with spike in controls and primers obtained from Exiqon, miR-625 was used as a loading control. Results were expressed as fold change from corresponding normal tissue.

Results: Compared with corresponding normal tissue, miR-222 was up-regulated (20 fold, P<0.0005) and miR-7 and -144 were down-regulated (>2 fold, P<0.001) in all PTC samples. miR-34b was also down-regulated (>2 fold) but only in frozen PTC samples. The expression of miR-126 and let-7g was not different.

Conclusions: This study showed a similar profile of change in miRNA expression in the RPA hospital cohort to reported studies. The utility of FFPE tissue for examination of miRNA profile was also confirmed. The magnitude of change in miR-222 expression suggests that it may be a useful marker for PTC. Further work using miRNA array analysis may reveal other future targets.