剂量补偿和MSL复合物研究进展

剂量补偿效应(Dosage compensation effect)广泛存在于两性真核生物,是基于性别决定、平衡不同性别间基因转录水平的遗传效应。MSL复合物(Male-specific lethal complex)是果蝇剂量补偿机制的核心,它乙酰化雄性果蝇X染色体上一些特定的位点,双倍激活X连锁活跃基因的转录,从而弥补雄性果蝇只具有单一条X染色体的不足。目前,已对果蝇MSL复合物各主要成分进行了结构分析,大体了解了各组分间的相互作用位点,并对该复合物的识别机制进行了大量的研究。与果蝇不同,哺乳动物是通过雌性个体一条X染色体的失活来实现剂量补偿。虽然哺乳动物MSL复合物的组成已被鉴定,但对其功能的研究还处于初步阶段。迄今为止,对硬骨鱼类剂量补偿及MSL复合物的研究极少。文章概括了线虫、果蝇和哺乳动物各物种剂量补偿机制的异同,综述了果蝇MSL复合物及其剂量补偿机制作用机理的研究进展,并提出有待解决的问题,同时利用同线性分析发现了不同鱼类msl3基因的多样性,为今后继续研究各物种的剂量补偿机制提供基础资料和研究方向。     

剂量补偿和MSL复合物研究进展

剂量补偿效应(Dosage compensation effect)广泛存在于两性真核生物, 是基于性别决定、平衡不同性别间基因转录水平的遗传效应。MSL复合物(Male-specific lethal complex)是果蝇剂量补偿机制的核心, 它乙酰化雄性果蝇X染色体上一些特定的位点, 双倍激活X连锁活跃基因的转录, 从而弥补雄性果蝇只具有单一条X染色体的不足。目前, 已对果蝇MSL复合物各主要成分进行了结构分析, 大体了解了各组分间的相互作用位点, 并对该复合物的识别机制进行了大量的研究。与果蝇不同, 哺乳动物是通过雌性个体一条X染色体的失活来实现剂量补偿。虽然哺乳动物MSL复合物的组成已被鉴定, 但对其功能的研究还处于初步阶段。迄今为止, 对硬骨鱼类剂量补偿及MSL复合物的研究极少。文章概括了线虫、果蝇和哺乳动物各物种剂量补偿机制的异同, 综述了果蝇MSL复合物及其剂量补偿机制作用机理的研究进展, 并提出有待解决的问题, 同时利用同线性分析发现了不同鱼类msl3基因的多样性, 为今后继续研究各物种的剂量补偿机制提供基础资料和研究方向。     

Dosage Compensation Regulatory Proteins and the Evolution of Sex Chromosomes in Drosophila

In the fruitfly Drosophila melanogaster, the four male-specific lethal (msl) genes are required to achieve dosage compensation of the male X chromosome. The MSL proteins are thought to interact with cis-acting sites that confer dosage compensation to nearby genes, as they are detected at hundreds of discrete sites along the length of the polytene X chromosome in males but not in females. The histone H4 acetylated isoform, H4Ac16, colocalizes with the MSL proteins at a majority of sites on the D. melanogaster X chromosome. Using polytene chromosome immunostaining of other species from the genus Drosophila, we found that X chromosome association of MSL proteins and H4Ac16 is conserved despite differences in the sex chromosome karyotype between species. Our results support a model in which cis-acting regulatory sites for dosage compensation evolve on a neo-X chromosome arm in response to the degeneration of its former homologue.     

Modulation of Heterochromatin by Male Specific Lethal Proteins and roX RNA in Drosophila melanogaster Males

The ribonucleoprotein Male Specific Lethal (MSL) complex is required for X chromosome dosage compensation in Drosophila melanogaster males. Beginning at 3 h of development the MSL complex binds transcribed X-linked genes and modifies chromatin. A subset of MSL complex proteins, including MSL1 and MSL3, is also necessary for full expression of autosomal heterochromatic genes in males, but not females. Loss of the non-coding roX RNAs, essential components of the MSL complex, lowers the expression of heterochromatic genes and suppresses position effect variegation (PEV) only in males, revealing a sex-limited disruption of heterochromatin. To explore the molecular basis of this observation we examined additional proteins that participate in compensation and found that MLE, but not Jil-1 kinase, contributes to heterochromatic gene expression. To determine if identical regions of roX RNA are required for dosage compensation and heterochromatic silencing, we tested a panel of roX1 transgenes and deletions and find that the X chromosome and heterochromatin functions are separable by some mutations. Chromatin immunoprecipitation of staged embryos revealed widespread autosomal binding of MSL3 before and after localization of the MSL complex to the X chromosome at 3 h AEL. Autosomal MSL3 binding was dependent on MSL1, supporting the idea that a subset of MSL proteins associates with chromatin throughout the genome during early development. The broad localization of these proteins early in embryogenesis supports the idea of direct action at autosomal sites. We postulate that this may contribute to the sex-specific differences in heterochromatin that we, and others, have noted.     

Study of dosage compensation in Drosophila

Using a sensitive RT-QPCR assay, we analyzed the regulatory effects of sex and different dosage compensation mutations in Drosophila. To validate the assay, we showed that regulation for several genes indeed varied with the number of functional copies of that gene. We then confirmed that dosage compensation occurred for most genes we examined in male and female flies. Finally, we examined the effects on regulation of several genes in the MSL pathway, presumed to be involved in sex-dependent determination of regulation. Rather than seeing global alterations of either X chromosomal or autosomal genes, regulation of genes on either the X chromosome or the autosomes could be elevated, depressed, or unaltered between sexes in unpredictable ways for the various MSL mutations. Relative dosage for a given gene between the sexes could vary at different developmental times. Autosomal genes often showed deranged regulatory levels, indicating they were in pathways perturbed by X chromosomal changes. As exemplified by the BR-C locus and its dependent Sgs genes, multiple genes in a given pathway could exhibit coordinate regulatory modulation. The variegated pattern shown for expression of both X chromosomal and autosomal loci underscores the complexity of gene expression so that the phenotype of MSL mutations does not reflect only simple perturbations of genes on the X chromosome.     

Implications of the gene balance hypothesis for dosage compensation

Dosage compensation refers to the equal expression between the sexes despite the fact that the dosage of the X chromosome is different in males and females. In Drosophila there is a twofold upregulation of the single male X. In triple X metafemales, there is also dosage compensation, which occurs by a two-thirds downregulation. There is a concomitant reduction in expression of many autosomal genes in metafemales. The male specific lethal (MSL) complex is present on the male X chromosome. Evidence is discussed showing that the MSL complex sequesters a histone acetyltransferase to the X chromosome to mute an otherwise increased expression by diminishing the histone acetylation on the autosomes. Several lines of evidence indicate that a constraining activity occurs from the MSL complex to prevent overcompensation on the X that might otherwise occur from the high level of acetylation present. Together, the evidence suggests that dosage compensation is a modification of a regulatory inverse dosage effect that is a reflection of intrinsic gene regulatory mechanisms and that the MSL complex has evolved in reaction in order to equalize the expression on both the X and autosomes of males and females.