Picard Christophe, Silvy Monique, Gerard Corinne, Buffat Christophe, Lavaque Esteban, Figarella-Branger Dominique, Dufour Henri, Gabert Jean, Beckers Albert, Brue Thierry, Enjalbert Alain, Barlier Anne
Laboratory Interactions Cellulaires Neuroendocriniennes, UMR 6544 CNRS, Institut Fédératif Jean Roche, Faculté de Médecine Nord, Université de la Méditerranée, Marseille, France.
Int J Cancer. 2007 Sep 15;121(6):1245-52. doi: 10.1002/ijc.22816.
Gs alpha, the alpha-subunit of the heterotrimeric GTP-binding protein, is coded from the GNAS gene, which is imprinted in a tissue-specific manner. Gs alpha is paternally silenced in normal pituitary, but Gs alpha imprinting relaxation is found in some tumoral tissue. In addition, Gs alpha mRNA levels are high in some somatotroph adenomas not bearing the active Gs alpha mutant, the gsp oncogene. In this study, the impact of loss of imprinting on Gs alpha expression level and on tumoral phenotype has been investigated. We compared the expression and imprinting of 4 transcripts of GNAS locus (NESP55, XL alpha s, exon 1A, Gs alpha) of 60 somatotroph adenomas with those of 23 lactotroph adenomas. The paternal and maternal transcripts were quantified using allele-specific real-time PCR and FokI polymorphism. Moreover, the methylation of exon 1A DMR was analyzed. As is the case for the gsp oncogene, high Gs alpha expression in gsp- tumors was associated with smaller tumor size and better octreotide sensitivity. A strong imprinting relaxation (percentage of paternal Gs alpha expression >or=7.5%) was found only in gsp- tumors. The loss of Gs alpha imprinting was associated with a decrease in exon 1A mRNA expression. Unexpectedly, the methylation status of exon 1A DMR was not modified in relaxed tumors. Maternal Gs alpha mRNA level decreased with exon 1A level, and consequently the loss of Gs alpha imprinting did not induce the expected Gs alpha overexpression. Finally, XL alpha s mRNA level correlated with that of paternal Gs alpha and of NESP55 showing the complexity of gene regulation in the GNAS locus.
Gsα是异三聚体GTP结合蛋白的α亚基，由GNAS基因编码，该基因以组织特异性方式印记。Gsα在正常垂体中父源沉默，但在一些肿瘤组织中发现Gsα印记松弛。此外，在一些不携带活性Gsα突变体gsp癌基因的生长激素腺瘤中，Gsα mRNA水平较高。在本研究中，研究了印记缺失对Gsα表达水平和肿瘤表型的影响。我们比较了60例生长激素腺瘤和23例催乳素腺瘤的GNAS基因座4种转录本（NESP55、XLαs、外显子1A、Gsα）的表达和印记情况。使用等位基因特异性实时PCR和FokI多态性对父源和母源转录本进行定量。此外，分析了外显子1A差异甲基化区域（DMR）的甲基化情况。与gsp癌基因情况相同，gsp阴性肿瘤中高Gsα表达与肿瘤较小和奥曲肽敏感性较好相关。仅在gsp阴性肿瘤中发现强烈的印记松弛（父源Gsα表达百分比≥7.5%）。Gsα印记缺失与外显子1A mRNA表达降低相关。出乎意料的是，印记松弛的肿瘤中外显子1A DMR的甲基化状态未改变。母源Gsα mRNA水平随外显子1A水平降低，因此Gsα印记缺失并未诱导预期的Gsα过表达。最后，XLαs mRNA水平与父源Gsα和NESP55的水平相关，显示了GNAS基因座基因调控的复杂性。
