Freson Kathleen, Thys Chantal, Wittevrongel Christine, Proesmans Willem, Hoylaerts Marc F, Vermylen Jos, Van Geet Chris
Center for Molecular and Vascular Biology, Leuven, Belgium.
Hum Mol Genet. 2002 Oct 15;11(22):2741-50. doi: 10.1093/hmg/11.22.2741.
Pseudohypoparathyroidism Ib (PHPIb), characterized by parathyroid hormone-resistant hypocalcemia and hyperphosphatemia, is caused by a deregulation in the imprinting status of the GNAS1 cluster, comprising exons XL, NESP55 and 1A and the coding exons of Gsalpha. Differences in methylation of exon 1A and sporadically also of exons XL and NESP55 were found and thought to result in long-range effects on Gsalpha expression, limited to the proximal renal tubules. The exact imprinting defect is not precisely localized, and the expected differences in Gsalpha protein level and function are mainly hypothetical. We describe a PHPIb patient with lack of methylation of the exon XL and 1A promoters, and biallelic methylation of the NESP55 promoter. Platelets of this patient show a functional Gs defect, decreased cAMP formation upon Gs-receptor stimulation, normal Gsalpha sequence but reduced Gsalpha protein levels. Transcriptional deregulation between the now biallelically active promoters of both exon 1A and exon 1 of Gsalpha could explain the decreased Gsalpha expression in platelets and presumably in the proximal renal tubules. We found decreased NESP55 and increased XLalphas protein levels in platelets, in agreement with the methylation status of their corresponding first exons. In a megakaryocytic cell line MEG-01, exon 1A is methylated on both alleles, in contrast to the normally maternally methylated exon 1A in leukocytes. Experimental demethylation of exon 1A in MEG-01 cells led to reduced Gsalpha expression, in agreement with the observations in the patient. Platelet studies may therefore allow easy evaluation of disturbances of the GNAS1 cluster in PHPIb patients.
假性甲状旁腺功能减退Ib型（PHPIb）的特征为甲状旁腺激素抵抗性低钙血症和高磷血症，由GNAS1基因簇印记状态失调引起，该基因簇包括外显子XL、NESP55和1A以及Gsα的编码外显子。已发现外显子1A以及偶尔外显子XL和NESP55的甲基化存在差异，并认为这会对Gsα表达产生远距离影响，且局限于近端肾小管。确切的印记缺陷位置并不精确，Gsα蛋白水平和功能的预期差异主要是假设性的。我们描述了一名PHPIb患者，其外显子XL和1A启动子缺乏甲基化，而NESP55启动子呈双等位基因甲基化。该患者的血小板显示出功能性Gs缺陷，Gs受体刺激后cAMP生成减少，Gsα序列正常但蛋白水平降低。Gsα外显子1A和外显子1的双等位基因活性启动子之间的转录失调可解释血小板中Gsα表达降低的原因，推测在近端肾小管中也是如此。我们发现血小板中NESP55蛋白水平降低而XLαs蛋白水平升高，这与其相应首个外显子的甲基化状态一致。在巨核细胞系MEG - 01中，外显子1A在两个等位基因上均被甲基化，这与白细胞中通常母系甲基化的外显子1A不同。MEG - 01细胞中外显子1A的实验性去甲基化导致Gsα表达降低，这与在患者中的观察结果一致。因此，血小板研究可能有助于轻松评估PHPIb患者中GNAS1基因簇的紊乱情况。
