One of the tasks of comparative oncology is the
identification of carcinogenic factors and mechanisms
of their influence on the cells of different,
phylogenetically distant multicellular organisms, as
well as of differences in sensitivity of these cells to
carcinogenic factors. Even though there are no reliable
data regarding the incidence of tumors in different
groups of multicellular organisms, some studies
indicate the possibility that tumor incidence might be
growing along with the growing complexity of
observed species and their place on the evolutionary
scale. [1,2,3,4]
The rationale behind such phenomenon could be
the stability of genome and the reliability of
mechanisms controlling the cell-cycle. For example,
simple multicellular organisms, such as yeasts, may
have simpler and more reliable machinery and more
effective repair mechanisms for damaged DNA
sequences. Unlike these, higher multicellular
organisms, like vertebrates, have a more complex but
less stable genome, as well as more complex
surveillance mechanisms, which could be taken into
account for a higher tumor incidence in vertebrates in
comparison with the lower multicellular organisms.
Yet, there are data that significant differences
regarding tumor incidence might be present across
different classes of vertebrates.
Thus, Effron et al. [5] presented the rate of
neoplasia at necropsy of captive wild animals of the
Zoological Society of San Diego collection. Neoplasia
was present at necropsy in 2.75% of 3,127 mammals,
1.89% of 5,957 birds, and 2.19% of 1,233 reptiles.
Interestingly, neoplasms were not detected during 198
necropsies of amphibians. The same authors argue that
the most common types of tumors differ greatly across
vertebrate classes as well [5]. Notably, lymphosarcoma
was the most common tumor registered in birds and
reptiles while various types of tumors, such as
adenomas, hepatoma and different lines of carcinomas
were registered in mammals.
Unfortunately, the research did not include tumor
incidence in cartilaginous fish and bony fish, though
there are data indicating a very low tumor incidence in
the former and particularly in the latter. Namely,
Hendricks et al. [6] failed to prove the presence of
tumors on 144 necropsies in brown bullheads
(Ictalurus nebulosus), but tumor incidence in the same
type of fish taken from polluted waters ranged 30% on
the sample of 532 [6].
Neoplasms do appear in all non-mammalian
vertebrates and this is important in and of itself since
these animals can serve as models to understand the
behaviour and trajectory of such tumors in mammals
and humans. Thus, an inter-disciplinary effort must
begin among collectors, zookeepers, veterinarians,
comparative pathologists in order to obtain
comprehensive documentation that would help us to
understand neoplasms more thoroughly not only in
non-mammals, but also in mammals and humans.
There are many reasons to believe that the
phenomenon of carcinogenesis follows the same or
similar postulates across all vertebrate classes. A large
number of studies also indicate the possibility that the
biological fundaments of carcinogenesis in vertebrates
are observable in invertebrates, as well. It is clearly,
therefore, that one of the primary steps leading
towards carcinogenesis in all vertebrates is the
destabilization of the genome and loss of cells' ability
to repair DNA damage. Genome destabilization ending
in carcinogenesis is, basically, the consequence of a
series of mutations or translocations, and comes
largely as a result of the influence of chemical agents,
viral infections, radiation and chronic inflammations
including auto-immune processes. What makes an
altered cell malignant is the activation of individual
genes or group of genes, otherwise normally
functional during embryogenesis. An incomplete and
inadequate ontogenic regression in terms of both time
and space results in the loss of sociability of the
altered cell, destruction of its environment and the
host. The described mechanism of cell alteration has
been verified in all classes of vertebrates, except for
cartilaginous fish where few cases of tumors have
been registered. Some of the reasons for considerable
variations regarding tumor incidence across vertebrate
classes, may be contained within genome stability, as
well as various possibilities for repairment of the DNA
damage [7,8].
Although some neoplasms are directly hereditary,
genetic predisposition is only one of factors affecting
the occurrence of all neoplasms. The tendency of
certain species to develop particular types of tumors is
a well-known aspect of comparative oncology [9].
Apart from a great similarity in cell organization
across vertebrates, there is an opinion that DNA and
mechanisms regulating the cell-cycle in lower
vertebrates are more stable and more resistant to the
influence of various carcinogens [10]. This explanation
is quite acceptable from the aspect of control
mechanisms of the cell-cycle and DNA stability as
being "the first line of defence" from malignant cell
alteration. The immune system could represent "the
second line" of anti-tumor defence. Due to the possible
significance of the immune system in anti-tumor
protection, differences in anti-tumor efficacy among
different groups of multicellular organisms could also
be of great influence for the incidence of manifesting
tumors [4]. However, it is very difficult to make a
viable conclusion as to how much the failure of the
"first", particularly "the second" line of defence really
contribute to even greater differences in tumor
incidence across various groups of multicellular
organisms. This dilemma may be solved by future
research in the field of comparative oncology,
particularly by developing disciplines such as
comparative tumor genetics and comparative tumor
immunology.
Answers to all your Biology Questions
