Enigmatic Mammalian DNA Methyltransferase Homologue

描述(申请人提供)：哺乳动物基因组含有约3×10甲基化的胞嘧啶残基。基因组甲基化模式对基因组的稳定性、印记基因的等位基因特异性表达、雌性X染色体的失活以及占哺乳动物基因组45%以上的转座子及其残留物的转录抑制是必不可少的。全球基因组去甲基化对分化的细胞是致命的，局部从头甲基化或去甲基化参与了人类的各种发育异常和癌症的发生。DNA(胞嘧啶-5)-甲基转移酶建立和维持甲基化模式。这些酶高度保守，通常显示10个保守基序，其催化功能从结晶学研究中已知。哺乳动物和许多其他类群(包括一些基因组尚未甲基化的动物)表达一种蛋白质(DNA甲基转移酶-2或DNMT2)，从序列和结构的每一个细节来看，它都是DNA甲基转移酶，但在体外不显示+转甲基酶活性，连接DNMT2的胚胎干细胞也没有表现出可检测到的基因组甲基化模式异常。与已知的DNA甲基转移酶不同，DNMT2以不可逆的共价复合体的形式裂解并结合到DNA上。这可能是通过与1A型拓扑异构酶活性部位相关的基序来调节的，这在其他甲基转移酶同系物中是没有的。最近发现，DNMT2基因是Moloney相关的小鼠白血病病毒在小鼠白血病和淋巴瘤中整合的复发部位，而在人类白血病和淋巴瘤中，含有人DNMT2基因的染色体区域(10P12-10p13)经常发生重排。据报道，DNMT2在某些癌症中表达下调。我们将进行生化和遗传学实验，以确定DNMT2在正常细胞中的生物学功能，并将确定DNMT2基因是否在人类肿瘤疾病中发生改变。

DESCRIPTION (provided by applicant): The mammalian genome contains about 3 x 10 methylated cytosine residues. Genomic methylation patterns are essential for genome stability, for allele-specific expression of imprinted genes, for X chromosome inactivation in females, and for transcriptional repression of the transposons and their remnants that make up more than 45% of the mammalian genome. Global genome demethylation is lethal to differentiated cells, and focal de novo methylation or demethylation is involved in a variety of developmental abnormalities and carcinogenesis in humans. Methylation patterns are established and maintained by DNA (cytosine-5)-methyltransferases. These enzymes are highly conserved and typically display 10 conserved motifs whose catalytic functions are known from crystallographic studies. Mammals and many other taxa (including some whose genomes are not known to be methylated) express a protein (DNA methyltransferase-2 or Dnmt2) that must be a DNA methyltransferase by every detail of sequence and structure but which does not display +transmethylase activity in vitro, nor do embryonic stem cells that tack Dnmt2 display detectable abnormalities of genomic methylation patterns. Unlike known DNA methyltransferases, Dnmt2 cleaves and binds to DNA in an irreversible covalent complex. This may be mediated by a motif related to the active site of type 1A topoisomerases which is not found in other methyltransferase homologues. The Dnmt2 gene has very recently been shown to be a recurrent site of integration of a Moloney-related murine leukemia virus in mouse leukemia and lymphoma, and the chromosomal region that contains the human DNMT2 gene (10p12-10p13) is frequently rearranged in human leukemia and lymphoma. DNMT2 has also been reported to be downregulated in certain carcinomas. We will perform biochemical and genetic experiments that will determine the biological functions of Dnmt2 in normal cells, and will determine whether the Dnmt2 gene is altered in neoplastic diseases of humans.