Tao Qian, Huang He, Geiman Theresa M, Lim Chai Yen, Fu Li, Qiu Guo-Hua, Robertson Keith D
Tumor Virology Laboratory, Johns Hopkins Singapore, Level 5 Clinical Research Center (MD11), NUS, 10 Medical Drive, Singapore 117597.
Hum Mol Genet. 2002 Sep 1;11(18):2091-102. doi: 10.1093/hmg/11.18.2091.
ICF syndrome (immunodeficiency, centromere instability and facial anomalies) is a recessive human genetic disorder resulting from mutations in the DNA methyltransferase 3B (DNMT3B) gene. Patients with this disease exhibit numerous chromosomal abnormalities, including anomalous decondensation, pairing, separation and breakage, primarily involving the pericentromeric regions of chromosomes 1 and 16. Global levels of DNA methylation in ICF cells are only slightly reduced; however, certain repetitive sequences and genes on the inactive X chromosome of female ICF patients are significantly hypomethylated. In the present report, we analyze the molecular defect of de novo methylation in ICF cells in greater detail by making use of a model Epstein-Barr virus (EBV)-based system and three members of the unique cellular cancer-testis (C-T) gene family. Results with the EBV-based system indicate that de novo methylation of newly introduced viral sequences is defective in ICF syndrome. Limited de novo methylation capacity is retained in ICF cells, indicating that the mutations in DNMT3B are not complete loss-of-function mutations or that other DNMTs cooperate with DNMT3B. Analysis of three C-T genes (two on the X chromosome and one autosomal) revealed that loss of methylation from cellular gene sequences is heterogeneous, with both autosomal and X chromosome-based genes demonstrating sensitivity to mutations in DNMT3B. Aberrant hypomethylation at a number of loci examined correlated with altered gene expression levels. Lastly, no consistent changes in the protein levels of the DNA methyltransferases were noted when normal and ICF cell lines were compared.
ICF综合征（免疫缺陷、着丝粒不稳定和面部异常）是一种隐性人类遗传疾病，由DNA甲基转移酶3B（DNMT3B）基因突变引起。患有这种疾病的患者表现出许多染色体异常，包括异常解聚、配对、分离和断裂，主要涉及1号和16号染色体的着丝粒周围区域。ICF细胞中的DNA甲基化总体水平仅略有降低；然而，女性ICF患者失活X染色体上的某些重复序列和基因显著低甲基化。在本报告中，我们利用基于爱泼斯坦-巴尔病毒（EBV）的系统和独特的细胞癌-睾丸（C-T）基因家族的三个成员，更详细地分析了ICF细胞中从头甲基化的分子缺陷。基于EBV系统的结果表明，新引入的病毒序列的从头甲基化在ICF综合征中存在缺陷。ICF细胞保留了有限的从头甲基化能力，这表明DNMT3B中的突变不是完全功能丧失突变，或者其他DNMT与DNMT3B协同作用。对三个C-T基因（两个位于X染色体上，一个位于常染色体上）的分析表明，细胞基因序列甲基化的缺失是异质性的，常染色体和基于X染色体的基因都显示出对DNMT3B突变的敏感性。在所检测的多个位点上异常的低甲基化与基因表达水平的改变相关。最后，当比较正常细胞系和ICF细胞系时，未发现DNA甲基转移酶蛋白水平有一致的变化。
