INSERM, Université Paris Descartes, Hôpital Saint Vincent de Paul, France.
J Med Genet. 2011 Jan;48(1):55-63. doi: 10.1136/jmg.2010.081356. Epub 2010 Oct 23.
Pseudohypoparathyroidism type Ib (PHP-Ib) is due to epigenetic changes at the imprinted GNAS locus, including loss of methylation at the A/B differentially methylated region (DMR) and sometimes at the XL and AS DMRs and gain of methylation at the NESP DMR.
To investigate if quantitative measurement of the methylation at the GNAS DMRs identifies subtypes of PHP-Ib.
In 19 patients with PHP-Ib and 7 controls, methylation was characterised at the four GNAS DMRs through combined bisulfite restriction analysis and quantified through cytosine specific real-time PCR in blood lymphocyte DNA.
A principal component analysis using the per cent of methylation at seven cytosines of the GNAS locus provided three clusters of subjects (controls n=7, autosomal dominant PHP-Ib with loss of methylation restricted to the A/B DMR n=3, and sporadic PHP-Ib with broad GNAS methylation changes n=16) that matched perfectly the combined bisulfite restriction analysis classification. Furthermore, three sub-clusters of patients with sporadic PHP-Ib, that displayed different patterns of methylation, were identified: incomplete changes at all DMRs compatible with somatic mosaicism (n=5), profound epigenetic changes at all DMRs (n=8), and unmodified methylation at XL in contrast with the other DMRs (n=3). Interestingly, parathyroid hormone concentration at the time of diagnosis correlated with the per cent of methylation at the A/B DMR.
Quantitative assessment of the methylation in blood lymphocyte DNA is of clinical relevance, allows the diagnosis of PHP-Ib, and identifies subtypes of PHP-Ib. These epigenetic findings suggest mosaicism at least in some patients.
假性甲状旁腺功能减退症 1b 型（PHP-Ib）是由于印记 GNAS 基因座的表观遗传改变引起的，包括 A/B 差异甲基化区域（DMR）去甲基化，有时还包括 XL 和 AS DMR 去甲基化以及 NESP DMR 甲基化增加。
研究 GNAS DMR 甲基化的定量测量是否可以确定 PHP-Ib 的亚型。
在 19 名 PHP-Ib 患者和 7 名对照者中，通过联合亚硫酸氢盐限制性分析（COBRA）对 GNAS 四个 DMR 的甲基化进行了特征描述，并通过血液淋巴细胞 DNA 中的胞嘧啶特异性实时 PCR 进行了定量。
使用 GNAS 基因座七个胞嘧啶的甲基化百分比进行的主成分分析提供了三个主题群（对照组 n=7、仅 A/B DMR 去甲基化的常染色体显性 PHP-Ib n=3、广泛 GNAS 甲基化改变的散发性 PHP-Ib n=16），与 COBRA 分类完全匹配。此外，还鉴定了散发性 PHP-Ib 患者的三个亚群，它们显示出不同的甲基化模式：所有 DMR 均存在不完全改变，符合体细胞镶嵌性（n=5）、所有 DMR 均存在深刻的表观遗传改变（n=8）以及与其他 DMR 相比，XL 未改变甲基化（n=3）。有趣的是，诊断时甲状旁腺激素浓度与 A/B DMR 的甲基化百分比相关。
对血液淋巴细胞 DNA 中甲基化的定量评估具有临床意义，可诊断 PHP-Ib，并确定 PHP-Ib 的亚型。这些表观遗传发现表明至少在一些患者中存在镶嵌现象。
