Characterization of novel reverse transcriptase encoding human endogenous retroviral sequences similar to type A and type B retroviruses: differential transcription in normal human tissues.

The polymerase chain reaction was used to amplify genomic DNA and reverse-transcribed RNA from human lymphocytes, using primers derived from conserved regions within the retroviral reverse transcriptase. Sequencing of 33 cloned amplification products revealed that a variety of sequences with similarity to mouse mammary tumor virus, mouse intracisternal A particle, and human endogenous retrovirus K10 were detected with this primer pair. The sequences were divided into six subgroups, with a nucleotide sequence dissimilarity of about 25% between the subgroups. Members within five of the subgroups were most closely related to human endogenous retrovirus K10 and mouse mammary tumor virus, whereas sequences of the sixth subgroup also showed similarity to mouse intracisternal A particle. Ten of the sequences had open reading frames with preference for silent mutations at conserved sites. Southern blot analysis showed that some HML (human endogenous MMTV-like) subgroups (HML-4 and HML-5) were present in a few copies (about 5), whereas others (HML-1 to HML-3 and HML-6) were present in at least 10 to 20 copies per genome. Northern (RNA) blot analysis revealed that several of the subgroups are differentially expressed in human normal tissues. A complex pattern of transcripts from about 12 to 1.4 kb was found in several of the tissues tested. However, the most abundant expression was detected in lung (all subgroups), skeletal muscle (HML-4 and HML-5), placenta (HML-2 and HML-5), and kidney (HML-2, HML-3 and HML-5). Expression of reverse transcriptase sequences in human tissues may have biological consequences. The described sequences are similar to elements which cause carcinoma and are immunoregulatory in mice. It remains to be seen whether human sequences also have such functions.