Multipoint Imprinting Analysis Indicates a Common Precursor Cell for Gonadal and Nongonadal Pediatric Germ Cell Tumors

Pediatric germ cell tumors (GCTs) commonly arise at extragonadal sites. It has been proposed that nongonadal GCTs arise from ectopic primordial germ cells that have aberrantly migrated during embryogenesis. During a time between their migration and development to mature gametes, primordial germ cells are characterized by their lack of imprinting, which can be assessed by the evaluation of allelic gene expression and DNA methylation in differentially methylated control regions. To elucidate the cellular origin of nongonadal GCTs, we evaluated the imprinting status of 21 gonadal and 21 nongonadal pediatric GCTs. Allele-specific H19 and IGF-2 expression was assessed with reverse transcription-PCR followed by digestion at polymorphic restriction sites. DNA methylation was evaluated after bisulfite modification, PCR amplification, and restriction digestion at a consistently methylated CpG dinucleotide within the 5 flanking region of the SNRPN gene. These results were compared with genetic gains and losses determined by comparative genomic hybridization. Seven of 15 informative tumors showed biallelic H19 expression, and 8 of 17 informative tumors showed biallelic IGF-2 expression. The frequency of biallelic gene expression was comparable in gonadal and nongonadal GCTs. Sixteen of 19 gonadal GCTs and 17 of 21 nongonadal GCTs showed absence of methylation of SNRPN consistent with loss of imprinting. One testicular GCT and three nongonadal GCTs showed a somatic methylation pattern. Two ovarian teratomas and one mediastinal teratoma showed only methylated SNRPN, consistent with entry into meiosis. Twenty-one of 22 non-GCT control samples showed a somatic methylation pattern. Gonadal and nongonadal germ cell tumors are derived from primordial germ cells that have consistently lost the imprinting of SNRPN and partly lost imprinting of H19 and IGF-2. Because the imprinting pattern of the latter genes differs from that found in testicular GCTs of adult patients, our data suggest that pediatric GCTs arise from a different stage of germ cell development.