Decrease in amplified telomeric sequences and induction of senescence markers by introduction of human chromosome 7...

Introduction of human chromosome 7 by microcell-mediated chromosome transfer suppresses indefinite division of SUSM-1, an in vitro established human fibroblast line. This cell line has unusually long telomeric sequences although it lacks detectable telomerase activity. Thus, we examined whether such telomeric sequences change upon introduction of chromosome 7 or its segments. In the microcell hybrids that stopped dividing by introduction of chromosome 7, the telomeric sequences were found to be lost or markedly diminished. Introduction of various fragments (2-40 Mb) of chromosome 7 contained in radiation hybrids gave similar results. On the other hand, the telomeric sequences were not altered significantly in the unsuppressed hybrids, a revertant of one suppressed clone, or subclones of SUSM-1 used as controls. In the suppressed microcell hybrids, the distribution of a mortality marker, mortalin, was changed to the cytosolic type of mortal cells from the immortal type of perinuclear fibres. Also, senescence-associated beta-galactosidase was induced to a level similar to that of normally senesced diploid fibroblasts. These results suggest that human chromosome 7 induces senescence in SUSM-1 by suppressing its telomere maintenance mechanism, which does not depend on telomerase.