In the human genome, HERV-K(HML2) is the most recently endogenized retrovirus 42 Through its interaction with CRM1 nuclear export factor [4, 6], Rec is involved in exporting un-43 spliced HERV-K RNA from the nucleus [3, 7]. In addition, Rec can directly modulate cell signalling 44 via binding to the promyelocytic leukemia zinc finger (PLZF) protein, a transcriptional repressor of 45 the c-MYC proto-oncogene [8]. Rec also binds the TZFP (testicular zinc-finger protein) and the hSGT 46 2 of 23 (human small glutamine-rich tetratricopeptide repeat protein), which are transcriptional repressors 47 of the androgen receptor [9, 10]. While it is evident that Rec must have first evolved to facilitate 48 HERV-K replication, there is speculation that the protein may have been coopted for physiological 49 function, such as spermatogenesis [6, 11] and defence against viral infections in the early human 50 embryo [12].
51It has been reported that at least 18 HERV-K genomic loci can be transcribed in healthy human 52 tissues and express potentially-coding Rec or Np9 mRNAs (alone or in combination with retroviral 53 genes), though the landscape of transcribed HERV-K loci differs considerably between cell/tissue 54 types [13]. Rec RNA/protein expression has been observed in human embryonic tissues, placenta and 55 retina [14, 15]. Interestingly, HERV-K is expressed in both germinal-and pluripotent stem cells, 56 providing a special link between these cell types [16]. While HERV-K transcription has been observed 57 at low levels in various normal tissues, it is strongly induced upon environmental stress, such as 58 ultraviolet irradiation (e.g. UVB and UVC), starvation or viral infection that may manifest the global 59 DNA hypomethylation [17][18][19][20][21]. Furthermore, many studies have reported robust mRNA 60 upregulation of several genomic loci of HERV-K in various disease states, including certain 61 autoimmune diseases and a number of cancers, especially lung, breast cancers, germ cell tumours 62 and melanoma [18, 22, 23] and reviewed in [24, 25]. Moreover, transcriptional activation and 63 translation of various HERV-K products [22, 26] and even virus-like particles have been observed in 64 certain cancer cells such as teratoma, embryonic carcinoma and melanomas [3,[27][28][29].
65Whether and how the overexpression of HERV-K products may contribute to the etiology or 66 progression of these diseases has been the subject of many studies, debates, and speculations. Several 67 mechanisms have been proposed by which HERV-K might contribute to disease. The activated LTRs 68 could act as alternative promoters and de-regulate tumor suppressor genes or proto-oncogenes 69 (reviewed in [35, 36]). Furthermore, the Env protein of HERV-K, via its fusogenic property, is capable 70 to induce cell-cell fusion and could therefore contribute to tumour invasiveness [18, 37, 38]. Env could 71 also have oncogenic properties through direct interference with cellular signalling pathways (e.g.
72RSS/MEK/ERK) [39, 40]. Indeed, the overexpressi...