Photo courtesy of NIGMS
Investigators say they have identified mutations that cause Fanconi anemia and, in the process, gained new insight into interstrand crosslink (ICL) repair.
The researchers studied two patients who had Fanconi anemia with no known genetic cause.
Genomic sequencing revealed that one patient had a mutation in RAD51, and the other had mutations in UBE2T.
These genes—and others linked to Fanconi anemia in previous studies—contribute to ICL repair, which fixes a misplaced attachment between two strands of DNA.
Caused by chemical agents, ICLs block the replication of DNA, making it impossible for cells to accurately copy their genomes as they divide. The ICL repair process uses multiple enzymes that cut away the connection between the DNA strands, freeing them up and allowing the cells to grow.
The genome is at constant risk of forming ICLs, and defects in the ICL repair pathway can produce a constellation of symptoms associated with Fanconi anemia—a predisposition to cancer, bone marrow failure, infertility, and developmental defects.
Via two different studies, Agata Smogorzewska, MD, PhD, of The Rockefeller University in New York, New York, and her colleagues unearthed new discoveries relating to the disease and the pathway.
“Our work began, as it often does, with samples and histories from patients,” Dr Smogorzewska said. “In these cases, we had two patients who each represented a sort of mystery. They had symptoms of Fanconi anemia but no genetic cause yet identified.”
With the RAD51 research, which was published in Molecular Cell, the investigators set out to determine the cause of the Fanconi anemia-like symptoms in a girl in the university’s International Fanconi Anemia Registry.
When they sequenced the protein-coding genes in her genome, the researchers found a mutation in one of two copies of the RAD51 gene.
The RAD1 protein was already known to be important for another DNA repair process—homologous recombination, in which a missing section of DNA is replaced using its sister strand as a template. Homologous recombination is thought to be used during the last step of ICL repair, after the crosslink has been cut.
Because only one copy of the RAD51 gene was partially defective, the patient’s cells could still perform homologous recombination but not ICL repair.
To show that the defective copy of the RAD51 gene was indeed responsible for the patient’s symptoms, the investigators genetically engineered the girl’s own cells to remove the defect, which restored their ability to fix ICLs.
Further experiments on the patient’s cells led the researchers to suspect that RAD51 plays a role outside of homologous recombination, by tamping down the activity of two enzymes that degrade the DNA at the ICL. When RAD51 is defective, these enzymes—DNA2 and WRN—become overly destructive.
With the UBE2T study, published in Cell Reports, the investigators analyzed genomic data from another patient in the International Fanconi Anemia Registry.
They found compound heterozygous mutations in UBE2T—a large genomic deletion in the paternally derived allele and a large duplication in the maternally derived allele.
While it was already known that UBE2T is involved in activating ICL repair, the researchers said the discovery that these mutations could produce Fanconi anemia revealed that UBE2T is an irreplaceable player in the pathway.
“Although we have discovered new causes for this devastating but very rare genetic disease, the implications of this work go much further,” Dr Smogorzewska said.
“By identifying new disruptions to this repair pathway, we can better understand the mechanisms of an event that is crucial to every cell division—a process that occurs constantly within the human body throughout a lifetime.”
