46
with that situation. DDR is formed by several mechanisms which are activated to
detect DNA damage, signal its presence, and mediate its repair (Jackson and Bartek
2009). Several DNA repair mechanisms, damage tolerance processes, and pathways
to check the quality of the DNA (called cell-cycle checkpoints) are members of this
sophisticated DDR. Animal models (including genetically engineered mice and
human skin xenografts) and cell lines have been widely used to investigate the key
role of the DDR mechanisms in UV-induced skin cancer.
4.2.2.2.1 Repair of Photoproducts in Human Cells
The main mechanism responsible for the elimination of thymine dimers is called
nucleotide excision repair (NER). This pathway involves three steps: (1) the recognition of the lesion (actually, the recognition of the distortion generated in the DNA
structure by the lesion); (2) the removal of a small area of the damaged DNA strand;
and (3) the resynthesis of DNA over the deleted region using the undamaged strand
as a template (Spivak 2015). NER reduces the number of mutations, by removing
most of the UV-lesions before DNA duplication begins. In fact, failures in this
repair pathway generate xeroderma pigmentosum which is associated with an
increased (>1000 fold) cancer incidence for all types of skin cancers (Griffiths et al.
1998). NER has been studied for many years, and its functions and components are
known in detail.
4.2.2.2.2 DNA Damage Tolerance Pathway
Although NER is a very efficient mechanism, excessive DNA damage can cause a
saturation of this pathway initiating DNA duplication with unrepaired DNA lesions,
jeopardizing the DNA sequence’s accuracy and cell survival. When the DNA duplication machinery abuts a photoproduct, the continuity of DNA synthesis is challenged and, if not resolved, can induce cell death. To avoid this situation, cells
trigger DNA damage tolerance pathways. Translesion DNA synthesis (TLS) is an
alternative DNA replication pathway (Bertolin et al. 2015). TLS can use damaged
DNA as template to allow the progression of DNA duplication over the lesion. TLS
is not a repair mechanism, as the lesion is kept in the DNA molecule to be eliminated later (Fig. 4.4). Polymerase eta, one of the principal proteins of TLS, can
recognize a thymine dimer and add the correct bases (two adenines), unblocking
DNA replication. When polymerase eta is absent, a set of alternative TLS polymerases can recognize and bypass UV-lesions, but they do not respect the bases complementarity (i.e., these backup polymerases incorporate random nucleotides, lacking
base complementarity). As a result, the TLS process avoids cell death but can induce
mutations on the DNA; thus cell viability is preserved at the expense of increased
genomic instability (Bertolin et al. 2015). The relevance of polymerase eta after UV
radiation is evince in xeroderma pigmentosum variant (XPV) patients which do not
express this protein. XPV is an inherited genetic disorder characterized by extreme
M. B. Federico
with that situation. DDR is formed by several mechanisms which are activated to
detect DNA damage, signal its presence, and mediate its repair (Jackson and Bartek
2009). Several DNA repair mechanisms, damage tolerance processes, and pathways
to check the quality of the DNA (called cell-cycle checkpoints) are members of this
sophisticated DDR. Animal models (including genetically engineered mice and
human skin xenografts) and cell lines have been widely used to investigate the key
role of the DDR mechanisms in UV-induced skin cancer.
4.2.2.2.1 Repair of Photoproducts in Human Cells
The main mechanism responsible for the elimination of thymine dimers is called
nucleotide excision repair (NER). This pathway involves three steps: (1) the recognition of the lesion (actually, the recognition of the distortion generated in the DNA
structure by the lesion); (2) the removal of a small area of the damaged DNA strand;
and (3) the resynthesis of DNA over the deleted region using the undamaged strand
as a template (Spivak 2015). NER reduces the number of mutations, by removing
most of the UV-lesions before DNA duplication begins. In fact, failures in this
repair pathway generate xeroderma pigmentosum which is associated with an
increased (>1000 fold) cancer incidence for all types of skin cancers (Griffiths et al.
1998). NER has been studied for many years, and its functions and components are
known in detail.
4.2.2.2.2 DNA Damage Tolerance Pathway
Although NER is a very efficient mechanism, excessive DNA damage can cause a
saturation of this pathway initiating DNA duplication with unrepaired DNA lesions,
jeopardizing the DNA sequence’s accuracy and cell survival. When the DNA duplication machinery abuts a photoproduct, the continuity of DNA synthesis is challenged and, if not resolved, can induce cell death. To avoid this situation, cells
trigger DNA damage tolerance pathways. Translesion DNA synthesis (TLS) is an
alternative DNA replication pathway (Bertolin et al. 2015). TLS can use damaged
DNA as template to allow the progression of DNA duplication over the lesion. TLS
is not a repair mechanism, as the lesion is kept in the DNA molecule to be eliminated later (Fig. 4.4). Polymerase eta, one of the principal proteins of TLS, can
recognize a thymine dimer and add the correct bases (two adenines), unblocking
DNA replication. When polymerase eta is absent, a set of alternative TLS polymerases can recognize and bypass UV-lesions, but they do not respect the bases complementarity (i.e., these backup polymerases incorporate random nucleotides, lacking
base complementarity). As a result, the TLS process avoids cell death but can induce
mutations on the DNA; thus cell viability is preserved at the expense of increased
genomic instability (Bertolin et al. 2015). The relevance of polymerase eta after UV
radiation is evince in xeroderma pigmentosum variant (XPV) patients which do not
express this protein. XPV is an inherited genetic disorder characterized by extreme
M. B. Federico