Genetic Structure Of Murine Retroviruses

所有哺乳动物物种都含有大量的内源性基因组逆转录病毒元素，这些元素是在进化过程中通过种系感染获得的。在人类中，逆转录病毒元件约占基因组的8%，其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，但有些含有复制能力强的外源性逆转录病毒特征的功能病毒基因的完整补体。尽管大多数内源性逆转录病毒在宿主体内的表达在很大程度上受到抑制，但逆转录病毒或其基因产物在发育和某些生理或病理状态下的产生是有充分文献记载的。关于逆转录病毒表达的控制以及逆转录病毒表达对宿主生理病理的影响，我们所知甚少。一组广泛研究的内源性逆转录病毒是那些在小鼠中引起多性mlv的病毒。多嗜性mulv是由亲生态mulv与近交系小鼠基因组中的内源性包膜序列重组而成。该病毒表现出改变的感染宿主范围，并利用不同于生态型mulv使用的受体的细胞表面受体。在一些情况下，多性mulv直接涉及发病机制，包括诱导增殖性、免疫和神经系统疾病。感染后产生的变异导致混合逆转录病毒感染。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性性和生态性mulv后，我们观察到对多性病毒的传染性传播有深远的影响，并伴有非常迅速的神经系统疾病的诱导，而单独接种任何一种病毒都没有观察到。混合感染导致神经系统疾病的一个常见影响是多性病毒在中枢神经系统（CNS）外周组织中的传播大大增强。这种现象是由嗜生态病毒颗粒内多性病毒基因组的假分型介导的。在将生态嗜性mulv与许多不同的多性mulv混合接种的小鼠中，神经系统疾病是明显的，这表明神经致病性可能是多性mulv的一般特性。我们研究的另一个方面涉及到与重组多嗜性mlv中发现的序列同源的内源性序列家族的表征，以及参与重组的内源性逆转录病毒的精确鉴定。不同的亲生态病毒与不同的内源性原病毒特异性地重组产生重组体。确定参与重组的精确序列对于了解促进这一过程的内源性病毒的特征至关重要。由于内源性病毒非常相似，以及病毒在复制过程中的快速进化，这种鉴定一直是难以捉摸的。我们已经在NFS/N小鼠中分离出了大多数可能产生重组病毒的内源性原病毒。大多数前病毒含有编码包膜蛋白受体结合区域的完整序列，并且可以通过该区域内的序列异质性来区分。此外，确定了三大类内源性前病毒，其中包括一组新的内源性病毒，它们具有迄今为止所描述的多嗜性前病毒的祖病毒的特性。通过序列异质性区分前病毒的能力有助于明确识别接种Moloney MLV （M-MLV）后参与多嗜性MLV生成的前病毒，Moloney MLV是一种诱导淋巴细胞白血病的生态嗜性MLV。我们的分析表明，大多数m - mlv衍生的重组病毒仅由三种内源性前病毒产生，其中一种对应于新描述的祖前病毒组的成员。此外，多嗜性mlv通常是由涉及多个原病毒的环境基因的多次重组事件产生的。后一种观察结果表明，有缺陷的原病毒可以相互补充，产生具有生态亲和性mlv的功能性重组病毒，并且重组mlv可能在m - mlv感染的宿主中进化成更精通复制和/或更具毒性的形式。

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.