Many novel anticancer targets are being identified as a result of the human genome project and through a greater understanding of the molecular basis of disease. A Conference symposium was held on September 11 to discuss such targets and the advances in chemistry that were required to produce novel, highly specific and non-toxic drug species for patient benefit.
Interfering with telomerase and telomere maintenance mechanisms may be a sufficient
“red flag” to tumour cells and shortening of telomeres per se may
not be necessary for cell crisis and death, suggested Professor Laurence Hurley
(Arizona cancer centre, Tucson, US).
Telomerase (see Panel) had been proposed as a cancer-specific target and there
were several ways in which this enzyme, and the telomeres on which it operated,
could be modulated, he suggested. One particular method of inhibiting telomerase
and telomerase-related processes was to target the telomeric DNA. Made up of
guanine-rich nucleotide repeats (TTAGGG), telomeric DNA formed a unique type
of secondary structure, known as a G-quadruplex.
| Telomeres, telomerase and G-quadruplexes | |
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Commenting on the role of G-quadruplexes, Professor Hurley suggested that although
their presence in vivo had not been unequivocally demonstrated, there were many
findings to support their physiological formation. It seemed possible that these
structures might function as biological switches in processes such as transcriptional
regulation, replication arrest, telomere end protection and telomerase inhibition.
Over the years, a number of agents had been developed to bind to G-quadruplexes,
said Professor Hurley. These compounds inhibited telomerase activity through
blocking telomere extension for which single-stranded DNA was required, he said.
In a programme of rational design, a series of cationic porphyrin compounds
were synthesised including 5,10,15,20-tetra(N-methyl-4- pyridyl)porphine chloride
(TMPyP4). A sterically constrained analogue (TMPyP2), inactive for quadruplex
binding, was available as a control compound.
TMPyP4 was not only found to be a potent inhibitor of telomerase activity but,
unexpectedly, also caused the cellular level of the telomerase protein to fall.
“This downregulation of h-TERT [the catalytic subunit of telomerase] was
puzzling,” said Professor Hurley. Furthermore, a known upstream regulator
of telomerase, c-myc, was also found to be downregulated. Effects of TMPyP4
on the cell cycle were ruled out and it became apparent that the most likely
explanation rested with the promoter region of the c-myc gene. The promoter,
a “nuclease hypersensitive region”, was formed from predominantly
purine bases on one strand and pyrimidines on the other. As such, TMPyP4 could
be interacting with G-quadruplexes within the promoter region that possibly
formed during transcriptional events, he suggested. DNA microarray studies confirmed
that there was a three-fold reduction in c-myc gene expression in the presence
of TMPyP4. Other genes with comparable promoter regions, c-fos and c-myb, were
similarly affected.
Thus, c-myc, overexpressed in many cancers, might actually be the more important
target in the antitumour activity of these compounds, suggested Professor Hurley.
Indeed, TMPyP4 had been shown to significantly reduce the weight increase of
a c-myc dependent tumour compared with TMPyP2.