Dipanjan Chowdhury


Division of Genomic Stability and DNA Repair
Department of Radiation Oncology
Dana Farber Cancer Institute
Jimmy Fund Building, Room 517
44 Binney Street
Boston, MA 02115
Tel: (617) 582-8639
Fax: (617) 582-8213
Email: Dipanjan_Chowdhury@dfci.harvard.edu

Down regulating DNA repair- phosphatases and micro RNAs

Activation of DNA repair pathways have been well studied for decades with very limited research on down regulation. Most importantly, the physiological relevance of down modulating DNA repair is still unclear. We have focused on two aspects of down regulation, one is dephosphorylation of repair proteins via phosphatases, and second is decreased expression of repair factors  via micro RNAs (miRNAs).

 

Double stranded DNA breaks (DSBs) are critical for cell health as a single unrepaired DSB is sufficient for inducing apoptosis.  The cellular response to DSBs is initiated by the phosphatidylinositol-3 kinase-like family of kinases. A vast network of ~700 proteins including DNA repair and replication proteins are phosphorylated by these kinases in response to DNA damage.  How does this signaling cascade get switched off? Recently we have identified a role for the PP2A-like phosphatases (PP2AC and PP4C) in the DSB response and observed that these phosphatases dephosphorylate a key DNA repair protein, the histone variant, H2AX.  Importantly, dephosphorylation of H2AX and ‘turning off’ DNA damage induced signaling is necessary for efficient DNA repair and cell viability.  We reported that PP4 is significantly over expressed in malignant tissues from lungs and breast and PP4-deficiency specifically impacts repair of DNA-replication mediated damage. However, there are few confirmed substrates of PP4C and very limited understanding of its cellular role. We have recently observed that PP4 controls the activity of a well established DNA repair/replication factor, replication protein A2 (RPA2). Our goal is to continue these studies and use  a combination of sophisticated mass spectrometry and a novel biochemical strategy to identify new substrates of PP4C.

 

MiRNAs are small non-coding RNAs that typically dampen gene expression and are mis-expressed in many cancer cells. Although miRNAs are being widely studied in different cellular process, to date there is limited information regarding their role in DNA repair. Ectopic overexpression of miRNAs downregulating DNA repair proteins could sensitize cancer cells to DNA-damaging agents. Alternatively, tumors that delete these miRNAs may develop resistance to conventional cancer therapy. This idea gets some credence from our recent report which clearly demonstrates that miR-24, downregulates the expression of a key DSB repair protein, H2AX and impedes DSB repair. This is the first example of a miRNA regulating the efficiency of DNA repair. We have done multiple screens to identify miRNAs impacting DNA repair, and have identified miRNAs that are rapidly downregulated in response to DNA damage. We adapted a biochemical strategy of co-immunoprecipitating miRNA/mRNA interactions to identify physiologically relevant targets of miRNAs.  Conversely, we are devising new strategies to identify miRNAs targeting specific DNA repair factors.  We find that some of these downregulate DSB repair factors/tumor suppressors. Importantly, as predicted the expression of specific miRNAs correlate with therapeutic response of cancer patients, underlining their clinical relevance. A role for miRNAs in DNA repair is pertinent for understanding DNA replication and recombination. 

 

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BBS webpage updated 12/02/2009