MOLECULAR GENETICS OF NUCLEOLAR DOMINANCE

描述(申请人提供)：核仁优势是一种表观遗传现象，发生在植物和动物的遗传杂交中，描述了由于选择性沉默另一个祖先的rRNA基因而导致只从一个亲本遗传的核糖体RNA(RRNA)基因的转录。发生在核仁优势中的沉默发生在数百万个碱基对的范围内，仅次于雌性哺乳动物的X染色体失活。核仁优势的独特之处在于，rRNA基因沉默的选择不是随机的，也不是由母亲或父亲的印记决定的。负责选择性地使核仁显性的一组亲本rRNA基因失活的机制尚不清楚。同样，对于其他数百个单等位基因、发育受控沉默的基因或在多种癌症中沉默的肿瘤抑制基因，是如何做出沉默决定的，目前尚不清楚。因此，了解核仁显性的分子机制在遗传学、发育生物学和医学中具有广泛的意义。在拟南芥中，拟南芥和拟南芥的异源四倍体杂交，来自拟南芥的rRNA基因是沉默的。在目前的资助下，我们确定了rRNA基因沉默所需的多种活性，包括从头DNA甲基转移酶、甲基胞嘧啶结合结构域蛋白、组蛋白去乙酰基酶和组蛋白甲基转移酶。重要的是，我们最近发现rRNA基因沉默是RNA依赖的，涉及siRNA指导的DNA甲基化途径的组成部分。由于siRNA与互补序列的同源配对具有区分亲本rRNA基因集的潜力，这一发现有可能揭示核仁显性的选择机制(S)。此外，与rRNA基因沉默有关的siRNAs来自基因间非编码RNA(NcRNA)前体，这是在细胞核中产生的最普遍的一类转录本，但人们对其了解最少。目前，我们还不知道siRNAs及其前体转录物，还是基因间隔区转录行为引起的染色质修饰是介导rRNA基因沉默的因素，验证这些替代假说是优先考虑的。结合遗传学、细胞学、生物信息学和生物化学的方法，我们的短期目标是了解非编码基因间RNA和参与rRNA基因沉默的染色质修饰活动之间的相互作用。这些努力将有助于我们的长期目标，即了解导致核仁优势的机制，以及能够在兆基规模上沉默染色体位点的机制。 公共卫生相关性：基因沉默在控制发育过程中特定细胞类型中被抑制的基因集、控制哺乳动物中X连锁基因的剂量以及确定动植物印记基因的亲本特异性表达方面发挥着重要作用。基因沉默还通过抑制转座子和逆转录病毒的活性在基因组防御中发挥重要作用。然而，基因沉默也有阴暗面，包括许多形式的癌症中肿瘤抑制基因的沉默。作为一种癌症治疗方法，能够特异性地去抑制肿瘤抑制基因将是可取的。同样，将去抑制多基因家族特定成员的能力作为治疗某些遗传性疾病的方法也是可取的。例如，由成人特有的珠蛋白基因突变引起的地中海贫血可能会通过去抑制功能性珠蛋白基因来缓解，这些基因在胎儿发育期间表达，但后来在发育过程中被沉默。因此，了解导致基因沉默的机制对基础科学和医学都具有重要意义。核仁显性中rRNA基因沉默的独特方面将有助于这一理解。

DESCRIPTION (provided by applicant): Nucleolar dominance is an epigenetic phenomenon that occurs in plant and animal genetic hybrids and describes the transcription of ribosomal RNA (rRNA) genes inherited from only one parent due to the selective silencing of the other progenitor's rRNA genes. The silencing that occurs in nucleolar dominance happens on a scale of millions of basepairs, second in scope only to X chromosome inactivation in female mammals. Unique aspects of nucleolar dominance are that the choice of rRNA genes to silence is not random nor is it dictated by maternal or paternal imprints. The mechanisms responsible for selectively inactivating one parental set of rRNA genes in nucleolar dominance are not clear. Likewise, it is not clear how silencing decisions are made for hundreds of other genes that display monoallelic expression, for genes that are subjected to developmentally controlled silencing, or for tumor suppressor genes that become silenced in numerous forms of cancer. Therefore, understanding the molecular mechanisms responsible for nucleolar dominance has broad relevance in genetics, developmental biology and medicine. In Arabidopsis suecica, the allotetraploid hybrid of A. thaliana and A. arenosa, the A. thaliana- derived rRNA genes are silenced. Under the current grant, we identified multiple activities required for rRNA gene silencing, including a de novo DNA methyltransferase, methylcytosine binding domain proteins, histone deacetylases and histone methyltransferases. Importantly, we recently found that rRNA gene silencing is RNA and Dicer-dependent, involving components of the siRNA-directed DNA methylation pathway. Because homologous pairing of siRNAs with complementary sequences has the potential to discriminate between parental sets of rRNA genes, this discovery has the potential to reveal the choice mechanism(s) in nucleolar dominance. Moreover, siRNAs implicated in rRNA gene silencing derive from intergenic non-coding RNA (ncRNA) precursors, which are the most prevalent class of transcripts generated in the nucleus, yet the least understood. At present, we do not know whether siRNAs, their precursor transcripts, or chromatin modifications resulting from the act of intergenic spacer transcription are what mediate rRNA gene silencing, and testing these alternative hypotheses is a priority. Using a combination of genetic, cytological, bioinformatic and biochemical approaches, our short-term goals are to understand the interplay between the non-coding intergenic RNAs and the chromatin modifying activities that are involved in rRNA gene silencing. These efforts will contribute to our long-term goals of understanding the mechanisms responsible for nucleolar dominance and the mechanisms capable of silencing chromosomal loci on a megabase scale. PUBLIC HEALTH RELEVANCE: Gene silencing plays an important role in controlling the sets of genes that are repressed in specific cell types during development, in controlling the dosage of X-linked genes in mammals and in determining the parent-specific expression of imprinted genes in animals and plants. Gene silencing also plays an important role in genome defense by suppressing the activity of transposons and retroviruses. However, gene silencing has a dark side, including the silencing of tumor suppressor genes in many forms of cancer. An ability to specifically derepress tumor suppressor genes would be desirable as a cancer treatment. Likewise, the ability to derepress specific members of multigene families would be desirable as a treatment for certain genetic disorders. For instance, thalassemias that result from mutations in adult-specific globin genes might be alleviated by derepression of functional globin genes that were expressed during fetal development but were then developmentally silenced. Understanding the mechanisms responsible for gene silencing therefore has relevance to basic science as well as medicine. The unique aspects of rRNA gene silencing in nucleolar dominance will contribute to this understanding.