THE TELOMERE LENGTHENING MECHANISM IN TELOMERASE-NEGATIVE IMMORTAL HUMAN-CELLS DOES NOT INVOLVE THE TELOMERASE RNA SUBUNIT

According to the telomere hypothesis of senescence, the progressive sh ortening of telomeres that occurs upon division of normal somatic cell s eventually leads to cellular senescence. The immortalisation of huma n cells is associated with the acquisition of a telomere maintenance m echanism which is usually dependent upon expression of the enzyme telo merase. About one third of in vitro immortalised human cell lines, how ever, have no detectable telomerase but contain telomeres that are abn ormally long, The nature of the alternative telomere maintenance mecha nism (referred to as ALT, for Alternative Lengthening of Telomeres) th at must exist in these telomerase-negative cells has not been elucidat ed, It has previously been shown that abnormal lengthening of yeast te lomeres may occur due to mutations in the yeast telomerase RNA gene. T hat this is not the mechanism of the abnormally long telomeres in ALT cell lines was demonstrated by the finding that seven of seven ALT lin es have wild-type human telomerase RNA (hTR) sequence, including a nov el polymorphism that is present in 30% of normal individuals, We found that two ALT cell lines have no detectable expression of the hTR gene , This shows that the ALT mechanism in these cell lines is not depende nt on hTR, Expression of exogenous hTR via infection of these cells wi th a recombinant hTR-adenovirus vector did not result in telomerase ac tivity, indicating that their lack of telomerase activity is not due t o absence of hTR expression, We conclude that the ALT mechanism is not dependent on the expression of hTR, and does not involve mutations in the hTR sequence.