Anemia Proteins Function Cooperatively with PTEN Tumor Suppressor

Fanconi anemia (FA) is a genetic disease characterized by bone marrow failure and increased cancer risk. There are 17 genes responsible for FA, one of them being the breast-cancer susceptibility gene BRCA2. Proteins encoded by these genes are involved in the recognition and repair of damaged DNA; genetic defects leave them unable to carry this out.

The FA proteins function primarily in DNA inter-strand crosslink (ICL) repair. Investigators at the University of Rhode Island (Kingston, USA) have examined the role of PTEN (Phosphatase and tensin homolog) phosphatase in this process. PTEN, which is missing in 60 to 70% of metastatic cancers in humans, is the name of a phospholipid phosphatase protein, and gene that encodes it. The PTEN gene acts as a tumor suppressor gene thanks to the role of its protein product in regulation of the cycle of cell division, preventing cells from growing and dividing too rapidly.

The investigators reported in the November 7, 2016, online edition of the journal Scientific Reports that PTEN played an important role in ICL repair, as PTEN-deficient cells, like FA patient cells, exhibited increased sensitivity to ICL-mediated cytotoxicity and displayed increased levels of chromosome structural aberrations following ICL exposure. PTEN function in ICL repair was independent of its lipid phosphatase activity yet dependent on its protein phosphatase activity. PTEN deficiency led to increased mutagenic ICL repair.

"The PTEN gene codes for a phosphatase - an enzyme that removes phosphate groups from proteins," said senior author Dr. Niall Howlett, associate professor of cell and molecular biology at the University of Rhode Island. "Many Fanconi anemia proteins have phosphate groups attached to them when they become activated. However, how these phosphate groups are removed is poorly understood. So we performed an experiment to determine if Fanconi anemia and PTEN were biochemically linked. By testing if cells with mutations in the PTEN gene were also sensitive to DNA crosslinking agents, we discovered that Fanconi anemia patient cells and PTEN-deficient cells were practically indistinguishable in terms of sensitivity to these drugs. This strongly suggested that the Fanconi anemia proteins and PTEN might work together to repair the DNA damage caused by DNA crosslinking agents."

"Before this work, Fanconi anemia and PTEN were not even on the same radar," said Dr. Howlett. "This is really important to understanding how this disease arises and what its molecular underpinnings are. The more we can find out about its molecular basis, the more likely we are to come up with strategies to treat the disease. We can now predict that if a patient has cancer associated with mutations in PTEN, then it is likely that the cancer will be sensitive to DNA crosslinking agents. This could lead to improved outcomes for patients with certain types of PTEN mutations."

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