没有人类就不会存在 病毒 因为病毒蛋白在人类胚胎的发育过程中起着关键作用。 然而，有时它们会以疾病的形式构成生存威胁，就像当前的 COVID-19 大流行一样。 具有讽刺意味的是，病毒占我们基因组的约 8%，这是在进化过程中获得的，使我们“实际上是一个嵌合体”。
毫无疑问，2020 年最臭名昭著、最可怕的词是“病毒'。 小说 冠状病毒 是造成当前前所未有的 COVID-19 疾病和世界经济几近崩溃的原因。 所有这一切都是由一个微小的粒子引起的，它甚至不被认为是“完全”活着的，因为它在宿主外部处于非功能状态，而只有在感染宿主后才能在内部永久存在。 更令人惊讶和震惊的是， 人类 自古以来就一直携带病毒“基因”，目前病毒基因约占人类基因组的 8% (1)。 从这个角度来看，只有约 1% 的人类基因组在功能上活跃，负责制造决定我们是谁的蛋白质。
人与人之间关系的故事 病毒 started 20-100 million years ago when our ancestors got infected by 病毒. Each endogenous retrovirus family is derived from a single infection of the germline cells by an exogenous retrovirus that after integrating into our ancestor, expanded and evolved (2). The propagation followed by the horizontal transfer from parents to offspring and today we have these viral genomes embedded in our DNA as human endogenous retroviruses (HERVs). This is a continuous process and may even be happening at the moment. Over the course of evolution, these HERVs acquired mutations, became stabilised in the human genome and lost their ability to cause the disease. The endogenous 逆转录病毒 are not only present in humans but are omnipresent in all living organisms. All these endogenous retroviruses grouped into three classes (Class I, II and III) occurring across different animal species exhibit a phylogenetic relationship based on their sequence similarity (3) as depicted in Figure below. HERVs belong to the Class I group.
Of the various embedded retroviruses present in the human genome, a classic example worth mentioning here, is that of a retroviral protein that is highly fusogenic envelope protein called syncytin, (5) whose original function in the 病毒 was to fuse with host cells to cause infection. This protein has now been adapted in humans to form placenta (fusion of cells to make multinucleated cells) that not only provides food to foetus from the mother during pregnancy but also protects the foetus from the mother’s immune system due to the immunosuppressive nature of the syncytin protein. This particular HERV has proven to be beneficial to the human race by defining its very existence.
HERVs have also been implicated in providing innate immunity to the host by preventing further infection from related viruses or reducing the severity of the disease upon re-infection by similar type of 病毒. A 2016 review by Katzourakis and Aswad (6) describes that endogenous viruses can act as regulatory elements for genes that control immune function, thereby leading to immunity development. In the same year, Chuong et al (7) demonstrated that certain HERVs act as regulatory enhancers by modulating the expression of IFN (interferon) inducible genes thereby providing innate immunity. HERV expression products can also act as pathogen-associated molecular patterns (PAMPs), triggering the cellular receptors responsible for host first line of defences (8-10).
HERV 的另一个有趣方面是它们中的一些表现出插入多态性，即由于插入事件，基因组中存在不同数量的拷贝。 对属于不同种族的 20 名受试者的研究显示，所有受试者的插入多态性模式在 0-87% 之间 (11)。 这可能会通过激活某些沉默的基因而导致疾病。
某些 HERV 也已被证明与自身免疫性疾病如多发性硬化症的发展有关 (12)。 在正常生理条件下，HERV表达受到严格调控，而在病理条件下，由于外部/内部环境的变化、激素变化和/或微生物相互作用会导致HERV表达失调，从而导致疾病。
HERV 的上述特征表明它们不仅在人类基因组中的存在是不可避免的，而且它们具有通过激活或抑制免疫系统来调节免疫系统稳态的能力，从而在以下方面产生不同的影响（从有益到引起疾病）主机。
The COVID-19 pandemic is also caused by a retrovirus SARS-nCoV-2, that belongs to the influenza family, and it may be plausible that, during the course of evolution, genomes related to this family of 病毒 got integrated into the human genome and are now present as HERVs. It is surmised that these HERVs might exhibit different polymorphisms, as mentioned above, among people of different ethnicity. These polymorphisms may be in the form of differential copy number of these HERVs and/or presence or absence of mutations (changes in the genome sequence) accumulated over a period of time. This variability in the integrated HERVs may offer an explanation for the differential mortality rates and the severity of COVID-19 disease in different countries effected by the pandemic.
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