Incomplete methylation of a germ cell tumor (Seminoma) in a Prader-Willi male.

Background: Prader-Willi syndrome (PWS) is a multisystem genetic disorder characterized by lack of satiety leading to morbid obesity, variable degrees of mental retardation, behavior disorders, short stature, and hypogonadism. The underlying genetic cause for PWS is an imprinting defect resulting from a lack of expression of several paternally inherited genes embedded within the 15q11.2-q13 region. Although the clinical expression of hypogonadism in PWS is variable, there are no known cases of fertility in PWS men. In this paper, we described a pure, nearly diploid seminoma in an apparently 32 year-old infertile man with PWS due to maternal uniparental disomy (UPD) on chromosome 15. The development of a germ cell tumor in this subject was an unanticipated result. The aim of this study was to explore the origin of the germ cell tumor in this PWS male patient.

Methods: To explain the origin of the germ cell tumor (seminoma) in our PWS patient we have characterized the tumor for cell morphology and tumor type by pathological examination (H&E and immuno-stainings), evaluated its karyotype by chromosomal microarray analysis and confirmed its UPD origin by haplotype analysis. In addition, DNA methylation status of the PWS- and H19- imprinting centers in wild-type and affected fibroblasts, patient derived induced pluripotent stem cells (iPSCs), and PWS seminoma were determined by bisulfite DNA colony sequencing.

Results: To explain the apparent contradiction between the existence of a germ cell tumor and hypogonadism we first confirmed the germ cell origin of the tumor. Next, we determined the tumor chromosomal composition, and validated the presence of a maternal UPD in all examined cell types from this patient. Finally, we characterized the maternal imprints in the PWS and H19 imprinting centers in the tumor and compared them with patient's fibroblasts and iPSCs derived from them. Unpredictably, methylation was reduced to 50% in the tumor, while preserved in the other cell types.

Conclusion: We infer from this assay that the loss of methylation in the PWS-IC specifically in the tumor of our patient is most likely a locus-specific event resulting from imprint relaxation rather than from general resetting of the imprints throughout the genome during germ line specification.