Insertional polymorphisms of full-length endogenous retroviruses in humans

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Human endogenous retrovirus K (HERV-K) is distinctive among the retroviruses in the human genome in that many HERV-K proviruses were inserted into the human germline after the human and chimpanzee lineages evolutionarily diverged [1, 2]. However, all full-length endogenous retroviruses described to date in humans are sufficiently old that all humans examined were homozygous for their presence [1]. Moreover, none are intact; all have lethal mutations [1, 3, 4]. Here, we describe the first endogenous retroviruses in humans for which both the full-length provirus and the preintegration site alleles are shown to be present in the human population today. One provirus, called HERV-K113, was present in about 30% of tested individuals, while a second, called HERV-K115, was found in about 15%. HERV-K113 has full-length open reading frames (ORFs) for all viral proteins and lacks any nonsynomymous substitutions in amino acid motifs that are well conserved among retroviruses. This is the first such endogenous retrovirus identified in humans. These findings indicate that HERV-K remained capable of reinfecting humans through very recent evolutionary times and that HERV-K113 is an excellent candidate for an endogenous retrovirus that is capable of reinfecting humans today.