One of the most remarkable characteristics of germ cell tumors is that they are developmental cancers, meaning that they closely resemble phenomena that occur during embryonic and germ cell development. It is only natural, then, that comprehensive knowledge about developmental biology drives the process of uncovering relevant disease biomarkers with a high likelihood of actual clinical use. This was the case for the classical serum markers AFP and HCG (secreted during embryogenesis), pluripotency factors (such as OCT3/4 and SOX2/17) and embryonic microRNAs (miR-371a-3p), which proved to be true biomarkers of germ cell neoplasms.
Pursuing this strategy, the evolutionary well-known event of X-chromosome inactivation in mammalian cells also resulted in another putative biomarker: XIST. This long non-coding RNA inactivates extra X-chromosome material in female cells, a process that is retained in testicular germ cell tumors as they have a super numerical X-chromosome constitution (because of the initial polyploidization step). Hence, in this work we explored and validated a demethylated XIST fragment (i.e., related to expression of the gene) as a biomarker of these tumors. Two different quantitative methodologies were applied, both with high sensitivity, including high-resolution melting analyses. Importantly, this biomarker was particularly useful in Seminomas, for which informative serum markers commonly used in the clinic are often detected in the normal range. Therefore, the demethylated XIST fragment in serum/plasma could be a promising biomarker for the clinical management of these patients.
Besides applications in the germ cell tumor field, we also demonstrated a valuable use of the demethylated XIST fragment for assessing spermatogenesis extent in testicular parenchyma samples. XIST has been shown to be only and specifically expressed in males during spermatogenesis when the germ cells enter meiosis. This is in line with the higher amount of demethylated XIST promoter identified by us in this study, being of relevance because infertility is a frequent side effect from cancer treatments, with a severe impact on cancer survivors’ quality of life. This novel finding may help to overcome the limitations of the time consuming and often inaccurate Johnsen’s score as evaluated by Pathologists to estimate spermatogenesis efficiency in clinical practice. Accordingly, we are pursuing an evaluation of our findings in seminal plasma samples.
Written by: João Lobo, MD, Resident in Pathology, PhD student, Cancer Biology & Epigenetics Group, Research Center, Portuguese Oncology Institute of Porto (IPO-Porto) & PhD student in Molecular Pathology and Genetics, Lecturer in Pathology, Master Degree in Medicine, Biomedical Sciences Institute Abel Salazar, University of Porto (ICBAS-UP) & PhD student, Looijenga Group, Princess Máxima Center (PMC) for Pediatric Oncology, Utrecht.
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