Genetic Structure Of Murine Retroviruses

所有哺乳动物物种在进化过程中都含有大量通过生殖系感染获得的内源性基因组逆转录病毒元件。在人类中，逆转录病毒元件约占基因组的8%，它们的数量接近编码基因的总数。许多内源性逆转录病毒元件是有缺陷的，但有些包含具有复制能力的外源逆转录病毒所特有的完整的功能病毒基因。虽然大多数内源性逆转录病毒在宿主中的表达在很大程度上受到抑制，但在发育过程中以及在某些生理或病理状态下逆转录病毒或其基因产物的产生是有充分证据的。关于逆转录病毒表达的控制或这种表达对宿主生理或病理的影响，人们知之甚少。一组被广泛研究的内源性逆转录病毒是那些在小鼠中引起多嗜性MLV的病毒。多向性MuLV是由近交系小鼠基因组中存在的生态型MuLV与内源性包膜序列重组而成的。这些病毒表现出感染宿主范围的改变，并利用与生态型MuLV所利用的受体不同的细胞表面受体。在一些情况下，多向MULV直接与发病机制有关，包括诱导增殖性、免疫性和神经系统疾病。感染后产生的变异会导致逆转录病毒的混合感染。该项目的一个方面涉及研究逆转录病毒在混合感染中的相互作用。在小鼠中同时接种多嗜性和生态性MuLV后，我们观察到了对多嗜性病毒感染性传播的深刻影响，同时伴随着在单独接种其中任何一种病毒后不能观察到的非常迅速的神经疾病的诱导。混合感染导致神经系统疾病的一个常见影响是多嗜性病毒在中枢神经系统(CNS)周围组织中的传播大大增强。这一现象是由生态型病毒颗粒内多嗜性病毒基因组的伪分型所介导的。作为生态型MuLV和许多不同的多向性MuLV的混合物接种的小鼠中，神经疾病是明显的，这表明神经致病性可能是多变性MuLV的一般特性。我们研究的另一个方面涉及鉴定与重组多嗜性MLV中发现的序列同源的内源序列家族，以及精确识别参与重组的内源性逆转录病毒。不同的亲生病毒与不同的内源性前病毒特异性重组产生重组子。确定参与重组的准确序列对于了解促进这一过程的内源性病毒的特征是至关重要的。由于内源性病毒非常相似，以及病毒在复制过程中进化的速度很快，这种鉴定一直难以捉摸。我们已经分离出了可能导致重组病毒的大部分内源性前病毒。大多数前病毒含有编码包膜蛋白受体结合区的完整序列，并可通过该区域内的序列异质性来区分。此外，鉴定了三个主要的内源性前病毒组，其中包括一组新的内源性病毒，这些病毒具有迄今已描述的多嗜性前病毒的祖细胞特性。通过序列异质性区分前病毒的能力有助于明确识别在接种Moloney MLV(M-MLV)后参与产生多嗜性MLV的前病毒。Moloney MLV是一种诱导淋巴细胞白血病的生态MLV。我们的分析表明，大多数M-MLV来源的重组病毒只来自三个内源性前病毒，其中一个对应于新描述的前体前病毒组中的一个成员。此外，多向MLV通常是由涉及多个前病毒env基因的多个重组事件产生的。后一种观察结果表明，缺陷型前病毒可能相互补充，产生具有生态亲和性MLV的功能重组体，并且重组MLV可能在M-MLV感染的宿主中进化为更擅长复制和/或更毒力的形式。

All mammalian species contain a very large number of endogenous genomic retrovirus elements acquired by germline infection over the course of evolution. In humans retroviral elements represent approximately 8% of the genome and their number approaches the total number of encoded genes. Many endogenous retrovirus elements are defective, however some contain an entire complement of functional viral genes characteristic of replication-competent exogenous retroviruses. Although the expression of most endogenous retroviruses is largely suppressed in the host, the production of retroviruses or their gene products during development and during certain physiological or pathological states is well documented. Little is known about the control of retrovirus expression or the influence of such expression on the physiology or pathology of the host. An extensively investigated group of endogenous retroviruses are those giving rise to polytropic MLVs in mice. Polytropic MuLVs are formed by recombination of ecotropic MuLVs with endogenous envelope sequences present in the genomes of inbred mouse strains. The viruses exhibit an altered infectious host range and utilize a cell surface receptor distinct from the receptor utilized by ecotropic MuLVs. In several instances polytropic MuLVs have been directly implicated in pathogenesis, including the induction of proliferative, immunological, and neurological disorders. The generation of variants after infection results in a mixed retrovirus infection. One aspect of the project involves studies of the interactions of retroviruses in mixed infections. Upon co-inoculation of polytropic and ecotropic MuLVs in mice we have observed profound effects on the infectious spread of the polytropic virus, concomitant with a very rapid induction of neurological disease not observed after inoculation with either virus alone. A common effect of mixed infections resulting in neurological disease is a greatly enhanced spread of the polytropic virus in tissues peripheral to the central nervous system (CNS). This phenomenon is mediated by pseudotyping of polytropic viral genomes within ecotropic virus particles. Neurological disease is evident in mice inoculated as a mixture of ecotropic MuLVs with a number of different polytropic MuLVs, suggesting that neuropathogenicity may be a general property of polytropic MuLVs. Another aspect of our studies involves the characterization of the family of endogenous sequences that are homologous to the sequences found in recombinant polytropic MLVs and the precise identification of those endogenous retroviruses that participate in recombination. Different ecotropic viruses specifically recombine with different endogenous proviruses to give rise to the recombinants. Determination of the precise sequences that participate in recombination is essential to understanding characteristics of the endogenous viruses that facilitate this process. Such identification has been elusive because of the very close similarity of the endogenous viruses as well as the rapid rate of evolution of the viruses during replication. We have isolated most of the endogenous proviruses in NFS/N mice that could potentially give rise to the recombinant viruses. Most of the proviruses contain intact sequences encoding the receptor-binding region of the envelope protein and could be distinguished by sequence heterogeneity within that region. Furthermore, three major groups of endogenous proviruses were identified that included a new group of endogenous viruses that have properties of progenitors to the polytropic proviruses that have thus far been described. The ability to distinguish the proviruses by sequence heterogeneity has facilitated the unambiguous identification of proviruses that participate in the generation of polytropic MLVs after inoculation of Moloney MLV (M-MLV), an ecotropic MLV that induces lymphocytic leukemia. Our analyses indicate that the majority of M-MLV-derived recombinant viruses arise from only three endogenous proviruses, one of which corresponds to a member of the newly described group of progenitor proviruses. Furthermore, polytropic MLVs are frequently generated by multiple recombination events involving the env genes of more than one provirus. The latter observation suggests that defective proviruses may complement one another to generate functional recombinants with ecotropic MLVs and that the recombinant MLVs may evolve in the M-MLV-infected host to more replication-proficient and/or more virulent forms.