Back in 1927 a Swiss pediatrician, Guido Fanconi was visited by distraught
parents of three siblings. all three siblings appeared to be suffering from
pernicious anemia and had physical abnormalities. Unfortunately there wasn't
much that dr. Fanconi could do for the siblings back in 1927 and they
passed away not long after their visit to his practice. determined to understand
as much as he could about what ailed the three siblings dr. Fanconi
personally conducted an autopsy on the siblings and wrote a 23 page paper
describing the symptoms of the patients. dr. Fanconi's commitment to
understanding the disorder that had plagued the siblings continued for 40
years culminating in the disorder - Fanconi anemia - being named after him. dr.
Fanconi was not alone in his work multiple talented researchers joined him
and by the 1960s the disorder was recognized or characterized as having
high levels of chromosome breakage that was spontaneous. the exaggerated
chromosomal breakage could also be induced by exposure to agents that cause
cross-linking of DNA bases, agents such as Mitomycin C.
This feature is subsequently used in diagnoses of FA patients. Lynn and David
Frohnmayer received devastating news in 1983. they learned that their two
daughters Katie and Kristen have FA. at this point in time it is known that FA
is a rare genetic disorder that causes bone marrow to fail in producing blood
cells and also leads to early onset cancers. frightened and feeling lonely
they decide that rather than become victims of the disease they would fight
their fight took the form of trying to discover as much about the disease as
possible by supporting research into FA their dedicated commitment to trying to
understand the molecular basis of Fanconi anemia starts to pay dividends
with the first gene found to be associated with the disorder FANCC
being identified in 1992 so what is Fanconi anemia? to get into that we
should know that DNA can and does get damaged on a daily basis DNA can be
damaged in various ways the phosphate groups all the ribose sugar all the
nitrogenous base - so the adenines, cytosines thymine and guanines can encounter damage
the whole nucleotide can also encounter damage the source of damage can be
external for example ultraviolet light from Sun can affect nitrogenous bases in
DNA by compressing adjacent cytosines and adjacent thymines to form what are known
as cyclobutane dimers. the source of damage could also be internal for
example did you know that hydrolytic reactions in the cell can result in
cytosines being converted to Uracil overtime? uracil pairs with adenine
while Cytosine would have been paired with guanine so the cell has a mechanism for
removing conversions of cytosines to Uracil.
DNA damage is initially sensed by proteins that phosphorylate other
proteins to modify them for particular activity so they're called kinases.
these initial DNA damage sensing proteins are ataxia telangiectasia
mutated or ATM for short and ataxia telangiectasia and rad three related protein
or ATR. ataxia ataxia telangiectasia mutated and ATM and rad3 related
kinases in turn phosphorylate hundreds of other proteins that are needed for
appropriate DNA damage sensing and repair the Fanconi anemia pathway comprises 23
proteins that contribute to repairing DNA when it is damaged it also assists
during DNA replication and in other cellular processes during DNA
replication the DNA in a Cell is copied - what we call replicated in what is known
as a semi conservative manner this simply means that one strand of the
double helix serves as a template for synthesis of the new strand. well in
order to achieve this a replication fork is formed while the existing DNA is
being copied to build the new or nascent strand nucleotides are
incorporated as the replication fork moves along but addition of nucleotides
can be impeded by obstacles such as one, if there are damaged bases or if there
are DNA : protein complexes or if there are DNA: RNA hybrids which are called R-
loops and certain structures can also halt replication fork movement. DNA
structures such as G quadruplexes and fragile sites can impede the movement of
the replication fork. but the main barrier that the FA pathway deals with
is the removal of cross-linking that sometimes occur within the strands of
DNA called interstrand crosslinks interstrand crosslinks can occur
through internal, that is endogenous sources or external, that is exogenous
sources. the endogenous sources of interstrand crosslinks include
aldehydes and nitrous acid that are produced in the body as a result of
metabolism. the exogenous sources of interstrand crosslinks could be
things like Psolarens and cisplatin that are used in cancer chemotherapy. if an
interstrand crosslinks occurs in DNA then the FA pathway is activated
during S phase in the cell cycle. so proteins called UHRF1 and FANCM-MHF1-
MHF2 complex senses interstrand crosslinks during the S phase in the
cell cycle. UHRF1 protein recruits FANCI a protein called FANCI and FANCD2
FANCD2 the FANCM-MHF1-MHF2 complex recruit what is known as the Fanconi
anemia core complex. the FA core complex that is recruited by the FANCM-MHF1-
MHF2 complex is an E3 ligase and as you know ligases add... joins two things
together so the E3 ligase ubiquitinates FANCI and FAND2
Hot off the press in 2020 just this year, Tan et al shows that ataxia telangiectasia and rad three
related protein ATR phosphorylate three residues on FANCI and this is
actually what enables it to be monoubiquitinated. so the core complex is
composed of eight Fanconi anemia proteins and three Fanconi anemia
associated protein so they're given the abbreviation 'FAAP' standing for Fanconi
anemia associated proteins the core complex proteins are FANC A, B, C E, F G, L, M
and then the three Fanconi associated proteins FAAP 20, FAAP24 and FAAP 100 the
core complex proteins actually consist of 3 sub complexes so you have BL 100 Fanconi
anemia proteins when we say the names we say FANC as in FANC so you just
hear people say FANC A, FANC B, FANCC so they are referring to the
the Fanconi Anemia proteins. so it would be BL 100 which are FANCB, FANCL and
the FAAP 100 so that's a sub complex and this sub complex guides the assembly of
the whole complex. then FANC CEF forms the sub complex that forms a bridge for
the interaction with the target FANCI- FANCD2 and then the target FANCI-
FANCD2 which gets mono ubiquitinated and then there is... A
AG20, which are FANCA, FANCG and FAAP20 that and this complex guides
localization of the proteins to the nucleus.I should mention at this point
that the this understanding of the core complex assembly is based on... from van
Twest et al, 2017 molecular cell paper however knowledge and insights into the
structure is rapidly evolving take for example a look at this core complex
assembly that's been deposited in the protein data bank by Shakeel et al. it's it
was published in Nature just in just last year 2019 and just highlights the
importance of keeping an eye on the literature for more up-to-date and
in-depth look. This is just a broad overview of the pathway so let's
continue. the E3 ligase is assisted by a ubiquitin conjugating enzyme UBE2T also
known as FANCT. so once they core complex ubiquitinates FANCI-FANCD2,
after the mono ubiquitination of FANCD2 and FANCI a host of
proteins interact in this pathway beginning with the recruitment of
a scaffolding protein. the scaffolding protein is FANCP it's also known as
SLX4, you may come across it as SLX4 that is needed in subsequent steps
of the biochemical pathway the core complex recruits
endonucleases these are proteins that cut DNA to cut the interstrand crosslinks
the endonucleases are FANCQ, FANCQ is also known as XPF and it's also
known as ERCC4, so watch out for those names
so these these three endonucleases cleave the interstrand crosslinks and
leads to a DNA adduct and a double strand break in the DNA where the
the interstrand crosslinkswas the DNA adduct is bypassed with the help of DNA
exonucleases rev 1 rev 3 and rev7 rev7 is also known as FANC V so
the three exonucleus do this by forming what is known as a translesion
synthesis complex. the resulting double-stranded break from the cut by
mus81, FANCU and SLX1 is repaired by homologous recombination. various
proteins play a role in this part so pay attention three other exonucleases are :
CTIP, MRN and XO1 and they cut the double strand break that resulted to
create a single-stranded DNA with three prime overhang the single-stranded DNA
is coated initially by a protein called replication protein A or RPA for short
so then another protein another Franconia anemia protein FANCR - a
recombinase - recognizes the replication protein A covered single-stranded DNA
this leads to the removal of RPA and stimulation of a recombination filament
the recombination filament formation is assisted by rad51C it's also called
FANCO, FANCJ, partner and localizer of BRCA2 also known as FANCN
BRCA2 - also known as FANCD1 and XRCC1 also known as FANCU. the
recombination filament searches for homologous bases to finally repair the
interstrand cross-link. so with that I thank you for watching if you found this
helpful please give with a thumbs up or if you have any
comments or questions you can leave it in the comments below all the best with
your FA related research