The Role of DNA Damage and Repair in Neurotoxicity Caused by Cancer Therapies - DNA Repair in Cancer Therapy - page 288

APE1siRNA and the adenovirus vector control, the
expression of APE1 was lower than APE1 levels in
cultures treated with scramble siRNA. When neurons
were infected with adenoviral vectors containing
a human wild-type APE1 construct or a C65 mutated
APE1 (which lacks redox activity), the levels of APE1
expression increased ( Figure 13.4 A). Exposing
neuronal cultures with normal levels of APE1 to 50
or 100

APE1 is important in attenuating the functional neuro-
toxicity of cisplatin.

SUMMARY

As discussed above, cognitive impairment (chemo-
brain) and/or peripheral neuropathy (CIPN) are major
side effects of cancer therapies ( Figure 13.1 ). These
neurotoxicities, which occur with a number of therapies
including ionizing radiation and chemotherapy, have
been well characterized in clinical studies and in animal
models. In general, the adverse effects on the central and
peripheral nervous systems occur in a significant subset
of patients, both children and adults, and in many cases
worsen with increased dosage and duration of therapy.
It seems likely that the use of multiple drugs or drugs
in combination with radiation can worsen neurotoxicity.
To date, the cellular mechanisms of neurotoxicity have
not been identified, and there are no proven effective
therapies to prevent or reverse neurotoxicity. Conse-
quently, understanding the mechanism for the toxicity
is an important priority for ongoing research. Indeed,
recent studies show that manipulating DNA repair
mechanisms is a promising and relatively novel
approach to addressing the neurotoxicity of agents that
produce DNA damage.
Two major clinical issues emerge when considering
that various cancer therapies produce these neurotox-
icities in a significant number of patients. First, there
is the issue of whether the toxicity is severe enough
for the therapy to be discontinued. In the case of
CIPN, depending on a number of variables including
the therapy, the patient might find the treatment to
be debilitating or quite painful, which could require
stopping or changing treatment. Second, as cancer
survival continues to improve, the impact of the
therapy on the continuing quality of life of the patient
becomes a more critical consideration. Strong evidence
supports the notion that neurotoxicity may or may not
resolve after therapy is discontinued, and the variables
influencing neurotoxicity resolution remain a mystery.
Therefore, it is exceedingly important to develop strat-
egies to minimize or prevent the occurrence of these
toxicities without compromising the efficacy of the
therapy.
Much work remains to develop these strategies. The
nature of the disease and the multiple approaches to
treat cancer suggest that there are multiple etiologies
for therapy-induced neurotoxicity. Clearly one goal of
future research is to attempt to define patient traits
that contribute to the development neurotoxicity. As
outlined above, augmenting DNA repair mechanisms
appears effective in animal models, but to date, the
ability to

M cisplatin for 24 hours significantly reduced
cell survival ( Figure 13.4 B). The cisplatin-induced
cytotoxicity was enhanced in cultures treated with
APE1siRNA ( Figure 13.4 B). Overexpressing either
wild-type APE1 or a mutant APE1 protein that has
no redox activity (C65-APE1) significantly attenuates
the effects of cisplatin on cell viability ( Figure 13.4 B)
in cultures pretreated with either scramble siRNA or
APE1siRNA. These results show that APE1 is neuro-
protective against the cytotoxic effects of cisplatin on
neurons, but that overexpression alone is not sufficient
to totally block the effects of cisplatin. Given the fact
that both the BER and the NER may be important
repair pathways for platinum, it will be important to
ascertain effects of both pathways on cisplatin-induced
neurotoxicity.
Given the data that cisplatin affects sensory neuronal
function (see above), we also examined the effects of
altering APE1 expression on the release of the neuro-
peptide CGRP from sensory neurons grown in culture
( Figure 13.5 ). This neuropeptide is synthesized and
released by a subset of small diameter sensory neurons.
In situ, the release of CGRP from peripheral endings of
sensory neurons results in vasodilation as a component
of neurogenic inflammation, whereas release from
central endings of sensory neurons contributes to the
perception of pain. Thus, examining the evoked release
of CGRP from sensory neurons represents a functional
endpoint for a subset of small diameter sensory
neurons. For these experiments, rat sensory neurons
in culture were exposed to cisplatin for 24 hours, then
evoked release measured using capsaicin as the evoking
stimulus. As can be seen in Figure 13.5 A, when sensory
neurons were treated with cisplatin for 24 hours, there
was a concentration-dependent reduction in the capsa-
icin-evoked release of CGRP. This decreased release was
observed in cells exposed to scramble siRNA, and
was more profound in cells where APE1 expression
was reduced ( Figure 13.5 B). Overexpressing either
wild-type APE1 or C65 APE1 in sensory neurons signif-
icantly attenuated the ability of cisplatin to reduce the
capsaicin-evoked release of CGRP ( Figure 13.5 C). In
contrast,

m

overexpressing a redox mutant APE1

(226 177 APE1) that did not have significant repair

activity did not affect the ability of cisplatin to reduce
peptide release ( Figure 13.5 C). Together, these data
provide strong evidence that the repair component of

þ

augment repair in humans remains