Fanconi anemia

Fanconi pancytopenia

:\tNineteen genes currently involved (for which bi-allelic inactivating mutations are retrieved in affected individual): FANCA, FANCB, FANCC, BRCA2 (FANCD1), FANCD2, FANCE, FANCF, FANCG (XRCC9), FANCI, BRIP1 (BACH1\/FANCJ) FANCL, FANCM, PALB2 (FANCN), RAD51C (FANCO), SLX4 (FANCP), ERCC4 (FANCQ\/XPF) RAD51 (FANCR), BRCA1 (FANCS), UBE2T (FANCT)

Autosomal recessive and X-linked (for FANCB); the estimated prevalence is 1 to 5 cases for million people; with a heterozygous carrier frequency of around 1\/300 people.

227650 , 227645

D005199

84 Fanconi anemia

C0015625

Diagnosis is based on standard, ie on Giemsa stained metaphases, cytogenetic analysis of the level of chromosome fragility following exposure to a DNA crosslinking agent exposure. Due to the risk of mosaicism linked to the spontaneous reversion of the mutation in the highly replicative hematopoietic precursors, cytogenetic analysis must be performed on fibroblasts and not simply on lymphocytes culture stimulated in vitro. An eventual confirmation of the diagnosis could be realized at molecular level thanks to the identification of the complementation group and, eventually, of the mutation(s) in the involved gene.

Androgens and steroids to improve hematopoietic function. Bone marrow transplantation to palliate the BMF and in the case of AML.

The BMF, which is the central characteristic of the syndrome, is progressive and culminate in pancytopenia and, in a fraction of the cases, in AML.

The clinical presentation of the FA is extremely heterogenous. It is generally admitted that BMF appears at around 7 years. Most patients die from marrow aplasia (haemorrhage, sepsis), and others from malignancies; MDS and AML in FA bear a very poor prognosis (median survival of about 6 mths); survival is also poor in the case of a squamous cell carcinoma.
Significant phenotypic differences were found between the various complementation groups. In FA group A, patients homozygous for null mutations had an earlier onset of anemia and a higher incidence of leukemia than those with mutations producing an altered protein. FA group G patients had more severe cytopenia and a higher incidence of leukemia. FA group C patients had less somatic abnormalities, which, in reverse, were more frequent in the rare groups FA-D, FA-E, and FA-F. FA group G patients patients and patients homozygous for null mutations in FANCA are high-risk groups with a poor hematologic outcome and should be considered as candidates both for frequent monitoring and early therapeutic intervention.
There may also be a certain degree of clinical heterogeneity.according to the degree of mosaicism. Therefore, clinical manifestations may be variable within a given family, according to the stage of embryonic development at which the somatic reverse mutation occurred.

Spontaneous elevated levels of chromatid and chromosome gaps and breaks, presence of abnormal figures, in particular triradials and quadriradials.
Hypersensitivity to the clastogenic effects of DNA crosslinking agents, like mitomycin C, diepoxybutane or cis-Platin.

Clonal abnormalities were reported in MDS and AML: in particular: -5\/del(5q) and -7\/del(7q) .

In response to DNA damage and together with several other partners involved in DNA damage signaling and cell cycle checkpoint activation, the FANC proteins work long a linear pathway to cope with the replication stress induced by the presence of DNA lesions and help in the replication rescue by homologous recombination based mechanism. Briefly, FANCA, FANCB, FANCC, FANCE, FANCF, FANCG and FANCL (with other companion proteins) assemble on FANCM and meet UBE2T to monoubiquitinate FANCD2 and FANCI. Following their monoubiquitination, the FANCD2\/FANCI heterodimer assembles into subnuclear foci where in a yet undetermined manner participates to and\/or coordinates the elimination of the lesions and the restart of the stalled replication fork thanks to the action of the other component of the FANC pathway, which include structure specific endonucleases (XPF, SLX4) and homologous recombination proteins (RAD51, BRCA1, BRCA2, ...)
The pathway, or some of its components, participate also to transcription regulation, epigentics, production\/response to inflammatory and stress induced cytokines and interferons.

The gene spans 80kB and contains 43 exons. FANCA is the most frequently mutated among the 19 known FANC genes: it accounts for more than 60% of the FA patients worldwide. Alternative splice results in the production of several transcripts variants encoding different protein isoforms. The most representative protein is a polypeptide of 1455-amino acids weighting approximatively 163 kDa. Present in both cytoplasms and nucleus, the protein possesses a nuclear localization signal but lacks of other known regulatory motifs and any biochemical function was ascribed to it. FANCA participates to the nuclear FANCcore complex that hosts the E3 ligase (FANCL) activity that, in collaboration with the E2 UBE2T, monoubiquitinates FANCD2 and FANCI in response to DNA damage. FANCA interacts directly with FANCG and FAAP20.

FAAP95

FANCB is constituted by 10 exons spanning 77kB. Alternative splicing results in two transcript variants encoding a same protein of 859-amino acids with a MW of 98 kDa. Any biochemical function was reported for the protein. FANCB aggregates with FANCL and FAAP100 in a sub-complex that participates to the FANCcore complex to mediate FANCD2 and FANCI monoubiquitination in response to DNA damage. FANCB stabilizes FANCL and needs FANCA to translocate into thAe nucleus. Mutations in FANCB are associated to both Fanconi anemia and X linked VACTERL with hydrocephalus syndromes.

FANCC has been the first FANC gene to be cloned. It contains 14 exons and codes an ORF of 1677 bp which translation results in a protein of 558aa, weighting about 63kDa. The protein, present in both cytoplasm and nucleus, interacts with FANCE and FANCF, a subgroup participating to the FANCcore complex. Any direct biochemical function was reported for FANCC.

FANCD1, XRCC11

The gene contains 27 exons, coding a mRNA which translation results in a protein of 3418aa, weighting about 385kDa. The protein is involved in the homologous recombination process. FANCD1\/BRCA2 contains several repetitions of a 70 aa motif called the BRC motif that mediate RAD51 interaction. Indeed, FANCD1\/BRCA2 is the cargo that target RAD51 to ssDNA stretches covered by RPA at DBS. It interacts with several proteins involved in DNA metabolism, including FANCD2, FANCN\/PALB2, POLH and some components of the TREX-2 complex. FANCD1\/BRCA2 inherited mutations are associated to the recessive syndrome Fanconi anemia while carriers of one inactivated allele are at risk for breast and ovarian cancer predisposition following the somatic loss-of-function of the wild-type allele.

The gene contains 44 exons. FANCD2 encodes a 1,451-amino acid nuclear protein. As several other FANC proteins, FANCD2 had no known functional domains. With its major partner, FANCI, FANCD2 is the target of the Ubiquitin-ligase activity of the the FANCcore complex. In presence of DNA damage or replication stress, FANCD2 is monoubiquitinated on K561 and targeted to subnuclear foci where it colocalize with several DNA repair proteins. It is phosphorylated by both ATM and ATR. The protein participate to both replication safeguard and chromosome fragile sites integrity maintenance. Interacts directly or indirectly with several proteins, including, FANCI, FANCE. USP1, MEN1, BRCA1, BRCA2, phosphorylated FANCG, FAN1 and DCLRE1B\/Apollo.

The gene contains 10 exons. FANCE protein is constituted by 536 amino-acids weighting approximatively 59kDa. It contains two Nuclear Localization Signal (NLS). FANCE forms with FANCC and FANCF a FANCcore complex sub-complex. It is required for FANCC nuclear accumulation and connects the FANCcore complex to FANCD2 allowing the FANCL\/UBE2T-mediated FANCD2 monoubiquitination. It is phosphorylated by CHK1 in response to DNA damage. As several other FANC proteins, FANCE had no known biochemical functions.

FANCF is an intron-less gene. The protein, long of 374aa, weights 42kDa. FANCF is predominantly nuclear, where it interacts with FANCE and FANCC, a subgroup participating to the FANCcore complex. As a FANCcore complex participant, FANCF is involved in FANCD2 and FANCI monoubiquitination. FANCF had no known biochemical functions.

XRCC9

The gene codes at least two mRNA of 2.2 and 2.5 kb, which translation results in a major proteins of 622 aa, weighting 68kDa. It participates to the FANCcore complex and its phosphorylation on serine 7 is mandatory for its function inside the complex. Nevertheless, as for several other FANC proteins any biochemical function has been attributed to FANCG. As for the other components of the FANCcore complex, its presence inside the complex is mandatory for FANCD2 and FANCI monoubiquitination and targeting to subnuclear foci.

The gene contains 38 exons. The FANCI protein is long of 1328aa, weights 50kDa and contains 3 NLS. FANCI is phosphorylated by ATM\/ATR and is monoubiquitinated by the FANCcore complex on the lys523. It is considered as a functional homolog of FANCD2. The two proteins forms a heterodimer that, following their DNA damage- or replication stress -induced monoubiquitination, relocalizes to subnuclear foci to optimally restore DNA and rescue replication in a yet undetermined manner.

The gene encodes a protein of 1249aa with a molecular mass de 141kDa. Memeber of the RecQ DEAH helicase family, FANCJ interact with BRCA1 participating to the DNA double-strand breaks repair by homologous recombination. Germline mutations in FANCJ are associated to breast and ovarian cancer susceptibility. Biallelic inheritance results in a Fanconi anemia-like phenotype.

FANCJ

The gene encodes a protein of 1249aa with a molecular mass de 141kDa. Member of the RecQ DEAH helicase family, FANCJ interact with BRCA1 participating to the DNA double-strand breaks repair by homologous recombination. Germline mutations in FANCJ are associated to breast and ovarian cancer susceptibility. Biallelic inheritance results in a Fanconi anemia-like phenotype

PHF9, FAAP43

It codes a proteins of 373 aa, weighting 43 kDa, containing 3 putative WD40 motifs and a PHD zync finger motif. The protein could be retrieved in both cytoplasm and nucleus. FANCL is the catalytic subunit of the FANCore complex. It has the E3 ubiquitin ligase activity necessary for FANCD2 and FANCI monoubiquitination. It mediate ubiquitin release from UBE2T and UBE2W.

FAAP250

It code for a protein of 2048aa. Contains an N-terminal helicase domain ans possess the ability to translocate on duplex DNA. It belongs to the DEAD box helicase family. It is hyperphosphorylated by ATR in response of DNA damage FANCM is thought be the transporter of the FANCcore complex along the DNA and, so, it participates de facto to both FANCD2 and FANCI optimal monoubiquitination.

FANCN

FANCN contains 13 exons and encodes for a protein of 1186 aa having a molecular mass of about 130kDa. The protein participates to homologous recombination in collaboration with its major partner BRCA2. It interacts also with BRCA1, RAD51, RAD51C and POLH. Monoallelic PALB2 mutations confer predisposition to breast and pancreatic cancers. hereditary bi-allelic mutations in FANCN result in Fanconi anemia.

FANCO

Member of the RAD51 gene family, involved in homologous recombination repair of damaged DNA and in meiotic recombination. RAD51C encodes a major 1.3 kB mRNA translated in a protein of 376 aa, weighting approximatively 45kDa. It interact with several DNA repair proteins, including RAD51 and PALB2. It participates to several complexes with RAD51B, RAD51D and XRCC2 or with XRCC3. The monoallelic inheritance of RAD51C is associated to breast and ovarian cancers predispostion. The biallelic, recessive, inheritance of RAD51C mutations result in a Fanconi anemia-like syndrome.

FANCP

The protein is constituted by 1834 aa which weights about 200kDa. Component of the SLX1-SLX4 structure-specific endonuclease, it is the docking platform of a complex assembling two other structure specific enducleases: XPF-ERCC1 and MUS81-EME1. SLX4 is also associated to MSH2\/MSH3, the telomere binding complex TRF2-RAP1 and the kinase PLK1. FANCP is required DNA repair, chromosome fragile sites maintenance and for replication fork failure rescue.

FANCQ, XPF

The gene contains 11 exons spanning more than 28 kb. The gene encodes 3 mRNA of 2.4, 3.8 and 7 kb, which translation results in a protein of 905 aa, having a mass of about 110 kDa. The protein interacts primarily with with ERCC1 making up the ERCC1-XPF-5 structure specific endonuclease. The protein also interacst with FANCP\/SLX4. Biallelic inactivating mutation in this gene have been associated to Fanconi anemia, xeroderma pigmentosum, cockayne syndrome and XFE progeroid syndrome.

FANCR, RAD51A

Belonging to the RAD51 family, this gene is encodes several transcript variant, the major being a 1.8kb mRNA which translation results in a protein of 339aa weighting 37kDa which plays a central role in homologous recobination repair and in meiotic recombination. It interacts with BRCA1, BRCA2, RPA, and several other DNA repair proteins. The only Fanconi anemia patient associated to RAD51 mutation bears a de novo mutation which created a dominant-negative variant. Mutations in RAD51 have been also associated to breast cancer suceptibility and to the congenital Mirror Movements 2 syndrome.

FANCS

FANCT

The gene contains 7 exons. Two transcript variants encodes different protein isoforms, the major is a protein of 197 amino acids weighting approximatively 22kDa. UBE2T is an E2-conjugating enzyme that collaborates with FANCL, the E3 ubiquitin ligase hosted by the FANC core complex, for the monoubiquitination of FANCD2 and FANCI. It interact with FANCL and BRCA1.

http:\/\/clinfo.rockefeller.edu\/fanconi\/ptrecrt.html International Fanconi Anemia Registry (IFAR)

http:\/\/www.fanconi.org\/ Fanconis Anemia International Registryhttp:\/\/asso.orpha.net\/AFMF\/AFMF.html Association Francaise de la Maladie de Fanconi