[Molecular and pathological analyses of newly established transgenic rats carrying human endogenous retrovirus gene,...

Endogeneous retroviruses are known to be integrated in eukaryotic genome as proviral DNA similarly to infectious retroviruses. They are present in many kinds of living things, but their functions, especially those in humans, are unclear. To investigate the function of human endogeneous retroviruses, we chose endogeneous retrovirus type 3 (ERV3) which is a single copy type of human endogeneous retrovirus with mRNA expression in organ tissues in vivo. The full provirus genome of ERV3 was subcloned as a transgene and two lines of transgenic rats carrying ERV3 gene (ERV3 rats) were established. One line of ERV3 rats, ETR5, showed tandem insertion of multiple copies of the transgene and expressed ERV3 mRNA in various organs. High levels of the mRNA expression were detected in both lacrimal and salivary glands. In placentas, where ERV3 is expected to express at high levels in humans, the mRNA expression was evident from 12 days gestation in ETR5 rats. Another line, ETR16, showed a single copy insertion and expressed ERV3 mRNAs only in the lacrimal and salivary glands. By Northern analysis, the expected size (3.5 kb) of ERV3 env transcription as was already shown in human tissues was confirmed in ETR16, but high expression of an additional large transcript (4.0 kb) was detected in ETR5. The result of RT-PCR analysis of the transcript in ETR5 indicated that the tandem insertion in ETR5 genome probably caused mis-promoting and mis-terminal poly(A) splicing in the 3'LTR, resulting in the extension of ERV3 env transcript to 4.0 kb mRNA with an addition of nontranslated LTR sequence. By Western blot using an antiserum against oligopeptides synthesized from ERV3 env sequences, protein product of the transgene was shown to be an 85 kDa band in the lacrimal gland of ETR5. Although no pathological significance was evident in these transgenic rats under conditions without any treatment, ERV3 rats may be a suitable model for analysis of the ERV3 function in vivo.