Developmental expression of HERV-R (ERV3) and HERV-K in human tissue

The human endogenous retroviruses (HERVs), ERV3 (HERV-R) and HERV-K, are both known to be transcriptionally active in human placenta. In the case of ERV3 there is also indirect evidence for its participation in cellular differentiation. In this study we examined the expression of ERV3 (HERV-R) and HERV-K in human normal fetal tissues by in situ hybridization. The highest level of ERV3 env expression was detected in primitive adrenal cortex. Elevated levels of expression were also found in the following developing tissues: kidneys (tubules), tongue, heart, liver, and central nervous system. Tissue-specific expression was found for HERV-K rec (former cORF) but not for pol/int transcripts. The highest rec expression was found in placenta and levels slightly higher than sense control were found in the rest of the tissues examined. Pol/Int was not possible to quantitate. It appears that ERV3 is expressed in an organ-specific way during embryogenesis and might suggest a possible role in the development and differentiation of human tissues.