DIFFERENTIAL EXPRESSION OF HUMAN ENDOGENOUS RETROVIRAL SEQUENCES SIMILAR TO MOUSE MAMMARY-TUMOR VIRUS IN NORMAL PERIPHERAL-BLOOD MONONUCLEAR-CELLS

Mouse mammary tumor virus (MMTV) is a retrovirus that causes breast ca ncer in certain strains of mice. In a previous study we identified, by sequencing clones from human lymphocytes, six groups with similaritie s to MMTV. Using a primer pair derived from pol sequences conserved wi thin types A, B, and D retroviruses and probes from the six human MMTV -like (HML-1 to HML-6) groups in an internally controlled hybridizatio n assay we investigated the normal variation of expression in PBMCs. V ariations occurred within all groups but was most significant within g roup HML-1, where hybridization signals differed by more than 500-fold between individuals. Groups HML-2 and HML-3 showed consistently stron ger hybridization signals than groups HML-1 and HML-5, while group HML -6 resulted in weak signals for all individuals. Stringent hybridizati on of the amplified cDNA to 20 individual HML clones also demonstrated a marked heterogeneity of expression. Hybridization signals from some groups and sequences were found to be correlated, either in a positiv e or negative fashion. RNA isolated from PBMCs collected from two dono rs at four different time points (in the morning and in the afternoon on the same day, repeated 1 week later) was also analyzed using the si x hml probes. A small variation in hybridization signals was seen in s amples collected on the same day, but a larger difference was observed in samples taken 1 week later. The correlations and the differences i n the expression of HMLs between individuals implicate a complex trans criptional regulation system of these sequences.