Mechanism identified for cancer associated with loss of BRCA1 and BRCA2

A new mechanism has been reported by which BRCA gene loss may accelerate cancer-promoting chromosome rearrangements. The new findings explain how the loss of BRCA1 or BRCA2 function impairs homologous recombination (HR), a normally accurate repair process used to fix DNA breaks, and actually stimulates faulty, error-prone HR repair.

Inherited mutations in the BRCA1 or BRCA2 tumor suppressor genes are by far the most frequent contributors to hereditary cancer risk in the human population, often causing breast or ovarian cancer in young women of child-bearing age. Attempts to test the role that the BRCA genes play in regulating a repair process associated with genome duplication have proven frustratingly difficult in living mammalian cells.

Described in Nature (2014; doi:10.1038/nature13295), the discovery by investigators at Beth Israel Deaconess Medical Center (BIDMC) in Boston, Massachusetts, could ultimately provide clinicians with valuable new information to help them ascertain risk and guide patient treatment when faced with BRCA mutations of uncertain significance. The findings are also potentially valuable developing cancer therapeutics.

"Mutations in the BRCA genes cause breast and ovarian cancers that affect thousands of women throughout the [United States] and around the world, often striking them in the prime of life," said senior author Ralph Scully, MB BS, PhD, a leader in the Breast Cancer Oncology program in BIDMC's Cancer Center and Associate Professor of Medicine at Harvard Medical School. "For almost two decades, scientists have been striving to better understand the tumor suppressor functions of BRCA1 and BRCA2."

Potentially harmful breaks in DNA strands commonly occur during DNA replication, a prerequisite to cell division. These breaks occur when the replication fork that duplicates the genome stalls at sites of DNA damage. If not properly repaired, the breaks can promote genomic instability, leading to cancer and other diseases.

"Some years ago, we and others suggested that BRCA1 and BRCA2 regulate homologous recombination at sites of stalled replication," explained Scully. "We believe that this function is critical to how these genes suppress breast and ovarian cancer. Until now, we haven't had the tools necessary to study in molecular detail the HR processes at sites of replication fork stalling in the chromosomes of a living mammalian cell."

To solve this problem, first author Nicholas Willis, PhD, a postdoctoral fellow in the Scully laboratory, created a new tool by harnessing a protein-DNA complex that evolved in bacteria.

"We found that the Escherichia coli Tus/Ter complex can be engineered to induce site-specific replication fork stalling and chromosomal HR in mouse cells," explained Willis. "In its essence, E coli bacteria—a standard model organism in science—has evolved a very simple system to arrest replication forks in a site-specific manner."

This system, he explained, is composed of short DNA elements called Ter sites, 21-23 base pairs in length and tightly bound by the protein Tus. "Tus binds these Ter elements with extremely high affinity and, upon replication fork approach, acts as a barrier to fork progression along the DNA. Tus/Ter effectively sets up a 'road block' and stalls the replication fork."

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