Towards a mechanistic understanding of the role of TERRA in the alternative lengthening mechanism of telomeres
Date issued
Authors
Editors
Journal Title
Journal ISSN
Volume Title
Publisher
License
Abstract
Telomeres are the nucleoprotein structures that shelter the chromosome ends from illegitimate
repair and degradation. Due to the inability of replication to fully duplicate DNA molecules,
telomeres shorten with every cell cycle, causing replicative senescence. To counteract telomere
erosion, different mechanisms of telomere maintenance are adopted. The most common one is
via telomerase, a reverse transcriptase that synthesizes de novo telomeric DNA. In the absence
of telomerase, an alternative lengthening mechanism of telomeres, namely ALT, promotes
telomere maintenance via homology-directed repair (HDR).
Both telomerase and ALT are used by human cancer cells to lengthen their telomeres with the
former being adopted in 85-90% of the cases and the latter in the remaining portion. Because of
this, the two mechanisms represent potential targets for anti-cancer therapies.
TERRA is the long non-coding RNA transcribed at telomeres by RNA pol II in a variety of organisms.
The exact function of the transcript is still unknown but gathering evidence proposes its
involvement in several processes, including telomere maintenance. Emerging data validate the
importance of TERRA, and the RNA-DNA hybrids it forms at telomeres, especially in ALT. Indeed,
the transcript is upregulated in this type of cancer cells and telomeric RNA-DNA hybrids appear
to be key triggers of telomeric HDR. Little is known about the source of the lncRNA
increased abundance in ALT and how TERRA RNA-DNA hybrids promote telomere lengthening
via recombination. Uncovering these hidden aspects might be relevant to further understand the
alternative lengthening mechanism of telomeres and develop more efficient therapeutics against
cancer.
In this study, Saccharomyces cerevisiae post-senescent ALT-survivors, which lengthen their
telomeres in a similar manner to human ALT, were employed to address the questions: is TERRA
required for ALT and why is it required? In this regard, TERRA levels were measured in survivors
to see if they upregulate the transcript like their human counterpart. Following, the regulation of
the lncRNA expression was characterized.
Interestingly, TERRA abundance resulted to be increased in survivors in a manner that resembled
human ALT. The main source of this upregulation seemed to be impaired degradation. Moreover,
TERRA expression was regulated in a cell cycle- and telomere length-dependent manner, with
the transcript peaking in early S-phase and at short telomeres, respectively. The increase of the
lncRNA at short telomeres is believed to trigger HDR and telomere elongation.
In pre-survivor cells, telomere erosion triggers senescence, whose rate is negatively influenced
by the abundance of TERRA RNA-DNA hybrids. Since survivors likewise shorten their telomeres,
they were monitored for the presence of a senescence-like phenotype and how telomeric hybrids
might affect it. Surprisingly, ALT-survivors senesced in response to telomere shortening and the
pace was negatively regulated by the amount of TERRA RNA-DNA hybrids. This phenomenon is
named hereafter “post-crisis senescence”.
Overall, this study shows that TERRA might be relevant in ALT to promote lengthening via HDR
and avert senescence. The data here presented are expected to broaden the current knowledge
of telomere transcription in ALT and emphasize its relevance as target for therapeutic
applications.