Invasive and Noninvasive Nonfunctioning Gonadotroph Pituitary Tumors Differ in DNA Methylation Level of LINE-1 Repetitive Elements

Purpose: Epigenetic dysregulation plays a role in pituitary tumor pathogenesis. Some differences in DNA methylation were observed between invasive and noninvasive nonfunctioning gonadotroph tumors. This study sought to determine the role of DNA methylation changes in repetitive LINE-1 elements in nonfunctioning gonadotroph pituitary tumors. Methods: We investigated LINE-1 methylation levels in 80 tumors and normal pituitary glands with bisulfite-pyrosequencing. Expression of two LINE-1 open reading frames (L1-ORF1 and L1-ORF2) was analyzed with qRT-PCR in tumor samples and mouse gonadotroph pituitary cells treated with DNA methyltransferase inhibitor. Immunohistochemical staining against L1-ORF1p was also performed in normal pituitary glands and tumors. Results: Hypomethylation of LINE-1 was observed in pituitary tumors. Tumors characterized by invasive growth revealed lower LINE-1 methylation level than noninvasive ones. LINE-1 methylation correlated with overall DNA methylation assessed with HM450K arrays and negatively correlated with L1-ORF1 and L1-ORF2 expression. Treatment of alpha T3-1 gonadotroph cells with 5-Azacytidine clearly increased the level of L1-ORF1 and L1-ORF2 mRNA; however, its effect on L beta T2 cells was less pronounced. Immunoreactivity against L1-ORF1p was higher in tumors than normal tissue. No difference in L1-ORF1p expression was observed in invasive and noninvasive tumors. Conclusion: Hypomethylation of LINE-1 is related to invasive growth and influences transcriptional activity of transposable elements.

Automated summary

The study found that hypomethylation of LINE-1 in pituitary tumors is associated with invasive growth and influences the transcriptional activity of transposable elements. Tumors with invasive growth had significantly lower levels of methylation compared to noninvasive ones, and methylation of LINE-1 correlated with overall DNA methylation. Treating gonadotroph cells with a DNA methyltransferase inhibitor led to increased expression of LINE-1 open reading frames, with more pronounced effects in certain cell types. Immunoreactivity against LINE-1 in tumors was higher compared to normal tissue, but no difference was observed between invasive and noninvasive tumors.