Retroviral Elements in Human Evolution and Neural Development

Abstract

Human embryogenesis and the development of its most unique product, the human brain, are believed to be precisely regulated by factors adopted during human evolution that differentiate us from other species. Nevertheless, increasing evidence shows an unthinkable “alien” factor may have contributed to the process. Pervasive horizontal gene transfer between species mediated by retroviruses is such a defining factor of evolution [1]. Retroviral infections occurred in germline cells and were able to transfer the genomic codes vertically from parent to offspring. These genes once integrated into the host chromosome, can get dispersed and exist in multiple mutated copies throughout the host genome. As a result, retroviral genes and other retro elements contribute to about 50% of the human genome. Of these, 20% belong to the group of LINEs and over 8% consists of HERVs which are relatively intact since they were acquired more recently [2]. From an evolutionary point of view, these retroviral elements have at least a few known functions that could benefit the human host. Generally, the vast amount of such “relic” genes in the genome can provide a specific buffer zone to preserve functional genes against further viral infections and other gene mutation causing events. The similarities of gene sequences and functions provide a more specific competition to limit further similar viral infections [3]. These functions are evidenced by the abnormal shares of mutations and translocations within the retroviral elements compared with other functional genes. Other functions of the HERV proteins lent to the host include the immune regulatory functions, such as an immunosuppressive function mediated by a domain located in the transmembrane subunit of the HERV-W [4,5]. In the present review, we focus on the effects of retroviral elements on human embryogenesis and neural development.