Biology

Dr. Arvind Panday
AACR Postdoctoral Fellow Department of Medicine Beth Israel Deaconess Medical Center Harvard Medical School, Boston

Astract:

Genomic instability is an important cause of tumorigenesis. A significant source of genomic instability arises during DNA replication when replication forks stall after encountering damage DNA template. If correctly processed, the stalled fork may be restarted or repaired in an error-free manner. If fork processing is dysfunctional, this can lead to misrepair of the stalled fork, aberrant replication restart and genomic instability. To quantify repair initiated upon fork stalling, Scully lab adapted the Escherichia coli Tus/Ter replication fork arrest complex to induce site-specific replication fork stalling on a mammalian chromosome. Primary cells lacking BRCA1 respond to the Tus/Ter replication fork barrier by forming small (<10 kb) tandem duplications (TDs). TDs are head-to-toe local duplications of a chromosome segment and are the frequently observed structural variation in the cancer genome. TDs in primary cells lacking BRCA1 are induced specifically by a Tus/Ter block but not by a conventional double strand break (DSB), indicating specificity for the stalled fork response. Intriguingly, breast and ovarian cancers lacking BRCA1 similarly acquire large numbers of small (~10 kb) TDs, which we have termed “Group 1” TD. Group 1 TDs in BRCA1-linked cancer are enriched at key cancer gene loci, inactivating tumor suppressor genes or boosting oncogene expression via enhancer duplication. In both the Tus/Ter system and in human cancer, Group 1 TDs are associated with BRCA1 loss but not loss of BRCA2. Thus, the Tus/Ter system recapitulates the BRCA1-specific regulation of Group 1 TD formation noted in human breast and ovarian cancer.

I find that stalled fork motor protein—FANCM (product of the Fanconi anemia [FA] group M gene) performs three functions in Tus/Ter-induced repair: it supports error-free “short tract” gene conversion (STGC); it suppresses error-prone “long tract” gene conversion (LTGC); and it acts synergistically with BRCA1 to suppresses TD formation. I identified the domains of Fancm required for “STGC support”, “LTGC suppression” and “TD suppression”, and found that these functions of FANCM are genetically separable. Further, I have discovered a novel synthetic lethal interaction between BRCA1 and FANCM loss in primary mouse ES cells. My goal is to analyze the mechanisms of synthetic lethality in Brca1/Fancm mutant. The advent of PARP inhibitors for therapy of BRCA-linked cancers has shown the incredible power of synthetic lethal interactions in cancer therapy. I believe that FANCM inhibition holds similar promise for therapy of BRCA1 mutant cancers.