Greally J M, Gray T A, Gabriel J M, Song L, Zemel S, Nicholls R D
Department of Genetics, Yale University School of Medicine, 333 Cedar Street, New Haven, CT 06520, USA.
Proc Natl Acad Sci U S A. 1999 Dec 7;96(25):14430-5. doi: 10.1073/pnas.96.25.14430.
Nuclear matrix binding assays (NMBAs) define certain DNA sequences as matrix attachment regions (MARs), which often have cis-acting epigenetic regulatory functions. We used NMBAs to analyze the functionally important 15q11-q13 imprinting center (IC). We find that the IC is composed of an unusually high density of MARs, located in close proximity to the germ line elements that are proposed to direct imprint switching in this region. Moreover, we find that the organization of MARs is the same at the homologous mouse locus, despite extensive divergence of DNA sequence. MARs of this size are not usually associated with genes but rather with heterochromatin-forming areas of the genome. In contrast, the 15q11-q13 region contains multiple transcribed genes and is unusual for being subject to genomic imprinting, causing the maternal chromosome to be more transcriptionally silent, methylated, and late replicating than the paternal chromosome. We suggest that the extensive MAR sequences at the IC are organized as heterochromatin during oogenesis, an organization disrupted during spermatogenesis. Consistent with this model, multicolor fluorescence in situ hybridization to halo nuclei demonstrates a strong matrix association of the maternal IC, whereas the paternal IC is more decondensed, extending into the nuclear halo. This model also provides a mechanism for spreading of the imprinting signal, because heterochromatin at the IC on the maternal chromosome may exert a suppressive position effect in cis. We propose that the germ line elements at the 15q11-q13 IC mediate their effects through the candidate heterochromatin-forming DNA identified in this study.
核基质结合分析（NMBA）将某些DNA序列定义为基质附着区域（MAR），这些区域通常具有顺式作用的表观遗传调控功能。我们使用NMBA分析了功能上重要的15q11 - q13印记中心（IC）。我们发现该IC由异常高密度的MAR组成，这些MAR紧邻被认为在此区域指导印记转换的种系元件。此外，我们发现尽管DNA序列存在广泛差异，但在同源小鼠基因座处MAR的组织方式是相同的。这种大小的MAR通常不与基因相关，而是与基因组中形成异染色质的区域相关。相比之下，15q11 - q13区域包含多个转录基因，并且因受到基因组印记影响而不同寻常，这导致母本染色体比父本染色体在转录上更沉默、甲基化程度更高且复制更晚。我们认为IC处广泛的MAR序列在卵子发生过程中组织成异染色质，这种组织方式在精子发生过程中被破坏。与该模型一致，对晕核进行的多色荧光原位杂交表明母本IC与基质有强烈关联，而父本IC则更松散，延伸到核晕中。该模型还为印记信号的传播提供了一种机制，因为母本染色体上IC处的异染色质可能在顺式作用中发挥抑制性位置效应。我们提出15q11 - q13 IC处的种系元件通过本研究中鉴定的候选异染色质形成DNA来介导它们的作用。
