Structure, function and evolution of the Helix-hairpin-Helix DNA glycosylase superfamily: Piecing together the evolutionary puzzle of DNA base damage repair mechanisms.

The Base Excision Repair (BER) pathway is a highly conserved DNA repair system targeting chemical base modifications that arise from oxidation, deamination and alkylation reactions. BER features lesion-specific DNA glycosylases (DGs) which recognize and excise modified or inappropriate DNA bases to produce apurinic/apyrimidinic (AP) sites and coordinate AP-site hand-off to subsequent BER pathway enzymes. The DG superfamilies identified have evolved independently to cope with a wide variety of nucleobase chemical modifications. Most DG superfamilies recognize a distinct set of structurally related lesions. In contrast, the Helix-hairpin-Helix (HhH) DG superfamily has the remarkable ability to act upon structurally diverse sets of base modifications. The versatility in substrate recognition of the HhH-DG superfamily has been shaped by motif and domain acquisitions during evolution. In this paper, we review the structural features and catalytic mechanisms of the HhH-DG superfamily and draw a hypothetical reconstruction of the evolutionary path where these DGs developed diverse and unique enzymatic features.

Unique Hydrogen Bonding of Adenine with the Oxidatively Damaged Base 8-Oxoguanine Enables Specific Recognition and Repair by DNA Glycosylase MutY.

DNA repair protein MutY employs specific interactions to differentiate OG:A basepairs from canonical G:C and T:A basepairs. Prior work from our lab has focused on understanding the structural requirements of OG on lesion recognition and catalysis, and we have shown that MutY relies on the exocyclic 2-amino group of OG to identify and distinguish OG:A from other basepairs. Additionally, we’ve shown that OG binding induces conformational changes that influence A excision.

This new work uses structure-activity relationships (SARs) to identify the structural features of A that influence OG:A recognition, verification, base excision, and overall cellular repair. We correlate observed in vitro MutY activity on A analogue substrates with their experimental and calculated acidities to provide mechanistic insight into the factors influencing MutY base excision efficiency. Our results herein can be used to guide future design of MutY/MUTYH specific probes to monitor the activity, or lack thereof, of MutY/MUTYH variants. These results can also applied toward the development of MUTY/MUTYH specific inhibitors that may find utility in cancer therapeutics.

Designer Fluorescent Adenines Enable Real-Time Monitoring of MUTYH Activity

The human DNA base excision repair enzyme MUTYH (MutY homolog DNA glycosylase) excises undamaged adenine that has been misincorporated opposite the oxidatively damaged 8-oxoG, preventing transversion mutations and serving as an important defense against the deleterious effects of this damage. Mutations in the MUTYH gene predispose patients to MUTYH-associated polyposis and colorectal cancer, and MUTYH expression has been documented as a biomarker for pancreatic cancer. Measuring MUTYH activity is therefore critical for evaluating and diagnosing disease states as well as for testing this enzyme as a potential therapeutic target. However, current methods for measuring MUTYH activity rely on indirect electrophoresis and radioactivity assays, which are difficult to implement in biological and clinical settings. Herein, we synthesize and identify novel fluorescent adenine derivatives that can act as direct substrates for excision by MUTYH as well as bacterial MutY. When incorporated into synthetic DNAs, the resulting fluorescently modified adenine-release turn-on (FMART) probes report on enzymatic base excision activity in real time, both in vitro and in mammalian cells and human blood. We also employ the probes to identify several promising small-molecule modulators of MUTYH by employing FMART probes for in vitro screening.

The DNA repair enzyme MUTYH potentiates cytotoxicity of the alkylating agent MNNG by interacting with abasic sites

Inherited defects in the DNA repair gene MUTYH lead to cancer, proof that MUTYH has a critical role in preventing cancer in normal cells. In a new study from the David Lab, MUTYH is shown to have a new role that implicates it in the response to a common class of chemotherapy drugs, alkylating agents.

Cancer cells evolve resistance to chemotherapy drugs by a number of mechanisms, including upregulating DNA repair enzymes such as BRCA1, which helps cancer cells survive DNA damaging chemotherapy agents. Surprisingly, MUTYH does not help repair alkylating agent DNA damage, but instead enhance alkylating agent toxicity. This study uncovers the underlying molecular mechanism of this activity, which involves MUTYH stimulating cells to create more toxic DNA repair intermediates. By uncovering the molecular mechanism, this research suggests that MUTYH has both a role in preventing DNA mutations that cause cancer, and a separate role in helping kill cancer cells that are treated with chemotherapy drugs, thus the loss of MUTYH is a “double-whammy”. Tests to determine if cancer patients have normal versus functionally-deficient MUTYH may alter chemotherapy treatment choices if these results can be generalized to clinical practice.

Detection of OG:A Lesion Mispairs by MutY Relies on a Single His Residue and the 2-Amino Group of 8-Oxoguanine

MutY glycosylase excises adenines misincorporated opposite the oxidatively damaged lesion, 8-oxo-7,8-dihydroguanine (OG), to initiate base excision repair and prevent G to T transversion mutations. Successful repair requires MutY recognition of the OG:A mispair amidst highly abundant and structurally similar undamaged DNA base pairs. Herein we use a combination of in vitro and bacterial cell repair assays with single-molecule fluorescence microscopy to demonstrate that both a C-terminal domain histidine residue and the 2-amino group of OG base are critical for MutY detection of OG:A sites. These studies are the first to directly link deficiencies in MutY lesion detection with incomplete cellular repair. These results suggest that defects in lesion detection of human MutY (MUTYH) variants may prove predictive of early-onset colorectal cancer known an MUTYH-associated polyposis. Furthermore, unveiling these specific molecular determinants for repair makes it possible to envision new MUTYH-specific cancer therapies.