Fanconi-like crosslink repair in yeast
Genome Integrity
Fanconi-like crosslink repair in yeast
Danielle L Daee 0
Kyungjae Myung 0
0 Genome Instability Section, Genetics and Molecular Biology Branch, National Human Genome Research Institute, National Institutes of Health , 49 Convent Drive, Bethesda, MD 20892 , USA
Interstrand crosslinks covalently link complementary DNA strands, block replication and transcription, and can trigger cell death. In eukaryotic systems several pathways, including the Fanconi Anemia pathway, are involved in repairing interstrand crosslinks, but their precise mechanisms remain enigmatic. The lack of functional homologs in simpler model organisms has significantly hampered progress in this field. Two recent studies have finally identified a Fanconi-like interstrand crosslink repair pathway in yeast. Future studies in this simplistic model organism promise to greatly improve our basic understanding of complex interstrand crosslink repair pathways like the Fanconi pathway.
Fanconi anemia; Interstrand crosslink repair; Mph1; Chl1; Slx4; Msh2; Msh6; Mhf1; Mhf2
Background
DNA damaging agents such as nitrogen mustard [
1,2
],
formaldehyde [3], and cisplatin [
4
] generate many
lesions that inhibit proper DNA replication and
transcription. One such lesion, the interstrand crosslink
(ICL), covalently links two complementary DNA strands
and prevents their separation. Importantly, since both
strands are damaged, an undamaged template strand is
not available for repair. Due to these blocks and repair
challenges, ICLs are considered one of the most toxic
DNA lesions. It is estimated that the presence of just
one unrepaired ICL is sufficient to kill yeast or bacteria
[
5
] and approximately 40 unrepaired ICLs can kill
mammalian cells [
6
]. As a result of this high cytotoxicity,
crosslinking agents are common anticancer agents [
7
].
Outside of chemotherapies, ICLs can be induced by
exposures in the environment [
8
] and byproducts of
normal metabolic processes [
9,10
]. Thus, a clearer
understanding of the mechanisms of ICL repair will inform
our knowledge of both normal and cancer cells. This
article and another recent review [11] describe novel
findings in yeast that provide insight into the
mechanisms of eukaryotic ICL repair.
A yeast fanconi-like pathway emerges
Cells have the capacity to repair ICLs through highly
complex DNA repair mechanisms. ICL repair in the
prokaryotic system is relatively well defined. In
Escherichia coli, nucleotide excision repair (NER) creates
incisions on each side of the ICL. The resulting short
oligonucleotide is attached through the ICL, but is
displaced from the helix, revealing a gap. The gap is filled
in by homologous recombination (HR) or translesion
bypass synthesis (TLS), and the displaced
oligonucleotide/ICL adduct is removed by NER [
12
].
In lower eukaryotes, defects in most known DNA repair
pathways result in ICL sensitivity suggesting that
eukaryotic mechanisms are much more complex, involve
multiple repair pathways, and can occur in multiple
phases of the cell cycle. Several recent reviews address this
complexity in detail [
13-23
]. In the budding yeast
Saccharomyces cerevisiae, a G1-specific repair pathway involves
NER and TLS similar to the E. coli system [
24
].
Additionally, three independent epistasis groups (PSO2, RAD52,
and RAD18) are implicated in ICL repair [
25
], but each
pathway mechanism is not fully defined. Pso2 is an
exonuclease that may be important for cleaving ICL repair
intermediates [
26-30
]. HR proteins, including Rad52 and
Rad51, likely fill in gaps post-incision and/or repair double
strand breaks (DSBs) that arise during ICL repair. The
post replication repair (PRR) pathway may help fill in the
gaps after the incision and unhooking of ICLs.
In higher eukaryotes the Fanconi anemia (FA) DNA
repair pathway has emerged as a master-regulator of
downstream checkpoints and pathways of ICL repair [
13
].
This pathway was named for patients with the heritable,
recessive disorder caused by mutations in FA repair genes.
These mutations confer developmental defects, cancer
(See figure on previous page.)
Figure 1 Model for replication-associated interstrand crosslink repair in yeast. (A) Replication is stalled by an ICL, Rad5 polyubiquitylates
PCNA, and Mph1-mediated fork-reversal stabilizes the fork for repair (with Smc5/6 and Mhf1/2) and protects the repair intermediates from
collapsing into double strand breaks (DSBs). Downstream events of repair are mediated by Slx4 and Exo1. HR and TLS are important for
gapfilling steps. The figure key shows the putative human homologs in brackets. (B) The basic steps of ICL repair lead to various fragile intermediates
(ssDNA, single strand DNA) that can collapse into DSBs. Cell death is triggered if the DSB cannot be repaired.
predisposition, and marked sensitivity to ICL-forming
agents [
31
]. In the FA repair pathway, FANCM and
FAAP24 are thought to recognize blocked forks, activate
checkpoin (...truncated)