Telomerase can extend the proliferative capacity of human myoblasts, but does not lead to their immortalization

Normal cells in culture display a limited capacity to divide and reach a non-proliferative state called cellular senescence. Spontaneous escape from senescence resulting in an indefinite life span is an exceptionally rare event for normal human cells and viral oncoproteins have been shown to extend the replicative life span but not to immortalize them. Telomere shortening has been proposed as a mitotic clock that regulates cellular senescence. Telomerase is capable of synthesizing telomere repeats onto chromosome ends to block telomere shortening and to maintain human fibroblasts in proliferation beyond their usual life span. However, the consequence of telomerase expression on the life span of human myoblasts and on their differentiation is unknown. In this study, the telomerase gene and the puromycin resistance gene were introduced into human satellite cells, which are the natural muscle precursors (myoblasts) in the adult and therefore, a target for cell-mediated gene therapy. Satellite cells expressing telomerase were selected, and the effects of the expression of the telomerase gene on proliferation, telomere length, and differentiation were investigated. Our results show that the telomerase-expressing cells are able to differentiate and to form multinucleated myotubes expressing mature muscle markers and do not form tumors in vivo. We also demonstrated that the expression of hTERT can extend the replicative life of muscle cells although these failed to undergo immortalization.