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

• Main Authors: Natalia Rusetska, Paulina Kober, Sylwia Katarzyna Król, Joanna Boresowicz, Maria Maksymowicz, Jacek Kunicki, Wiesław Bonicki, Mateusz Bujko
• Format: Article
• Language: English
• Published: MDPI AG 2021-02-01
• Series: Journal of Clinical Medicine
• Subjects: DNA methylation; LINE-1; L1-ORF1p; pituitary tumor; gonadotropinoma; invasive growth
• Online Access: https://www.mdpi.com/2077-0383/10/4/560
• ISSN: 2077-0383

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 (<i>L1-ORF1</i> and <i>L1-ORF2</i>) was analyzed with qRT-<i>PCR</i> 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 <i>L1-ORF1</i> and <i>L1-ORF2</i> expression. Treatment of αT3-1 gonadotroph cells with 5-Azacytidine clearly increased the level of <i>L1-ORF1</i> and <i>L1-ORF2</i> mRNA; however, its effect on LβT2 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.