PRINCIPAL SCIENTIST
 Hang, B

 SCIENTISTS
 Guliaev, Anton B.

POSTDOCTORAL FELLOWS
 Wang, Ping

 


A broad variety of chemical carcinogens are implicated in initiation of cancer by reacting with DNA to modify bases. Cellular repair of such lesions is one of the crucial factors influencing human susceptibility to cancer, as is lack of repair. We are interested in how repair enzymes/pathways recognize and process specific exocyclic DNA adducts.



Exocyclic DNA adducts are a group of base modifications that can be produced by many environmental mutagens/carcinogens including vinyl compounds, benzene, glycidyl ethers, mucochloric acid and genotoxic therapeutic nitrosoureas such as BCNU. Exocyclic adducts such as etheno bases are also formed by endogenous metabolic processes. Some of these adducts have been identified in vivo and shown to be promutagenic in bacterial and mammalian cells. Our previous repair studies using defined oligonucleotides containing a site-directed exocyclic adduct have led to identifying a number of novel substrates for several DNA glycosylases and AP endonucleases. Combined with our data on the use of gene knockout mice, site-directed mutagenesis and structural approaches, these findings have enhanced our knowledge of substrate specificity of the enzymes studied.

The study on repair of exocylic adducts with various structure differences has been useful for understanding the structure-function relationship in enzyme recognition. We are currently investigating the effect on repair specificity and efficiency of such adduct features as ring size (5- vs. 6-member), number (1 vs. 2 extra rings), saturation (etheno vs. ethano), or addition of a functional group (e.g. –OH, -CH2OH). Since DNA glycosylases and AP endonucleases generally require double-stranded DNA as substrates, the effect on repair of DNA conformational changes due to adduct structure as well as neighbor or opposite bases is also analyzed in biochemical and structural studies. Besides the potential biological importance of this work for each adduct to be studied, we hope to gain more insight into the “big picture”: what structural features determine repair enzyme specificity and mechanism in chemically modified DNA?

The following are some specific projects that are ongoing or will be pursued:

• Identifying and characterizing repair activities that act on newly synthesized exocyclic adducts from such compounds as glycidaldehyde, BCNU, chlorohydroxyfuranones, and 2-OH-p-benzoquinone.

• Exploring the complete pathway for the repair of p-benzoquinone-derived bulky adducts, which is initiated by the endonucleolytic activity of the major human AP endonuclease.

• Examining the initial recognition by mismatch repair pathway of the synthesized exocyclic adducts as some of these lesions have similar structures to base-base mismatches. Dual recognition/repair by different pathways (e.g. base excision and mismatch repair) will be studied for their possible interaction as one pathway may backup/stimulate or shield repair from the other.

• Using computational methods, including molecular modeling of enzyme/substrate complexes, and site-directed mutagenesis, to aid in understanding how repair enzymes interact with adducted DNA.

Bo Hang
Staff Scientist /
Life Sciences Division

One Cyclotron Rd.
Mailstop: DONNER
Berkeley, CA 94720
tel: (510)495-2537
fax: (510)486-6488
email: Bo_Hang@lbl.gov

 

 

Selected Papers:

Guliaev, A.,B., Singer, B. and Hang, B. Chloroethylnitrosourea-derived ethano cytosine and adenine adducts are substrates for Escherichia coli glycosylases excising analogous etheno adducts. DNA Repair, In Press.

Guliaev, A.,B., Hang, B., and Singer, B. Structural insights by molecular dynamics simulations into specificity of the major human AP endonuclease toward the benzene-derived DNA adduct, pBQ-C. Nucleic Acids Res. 32, 2844-52 (2004).

Hang, B., Downing, G., Guliaev, A. and Singer, B. Novel activity of Escherichia coli mismatch uracil glycosylase (MUG) excising 8-(hydroxymethyl)-3,N4-
ethenocytosine, a potential product resulting from glycidaldehyde reaction. Biochemistry 41, 2158-65 (2002).

Guliaev, A.B., Hang, B. and Singer, B. Structural insights by molecular dynamics simulations into differential repair efficiency for ethano-A versus etheno-A adducts by the human alkylpurine-DNA N-glycosylase. Nucleic Acids Res. 30, 3778-87 (2002).

Rothwell, D.G., Hang, B., Gorman, M.A., Freemont, P.S., Singer, B. and Hickson, I.D. Substitution of Asp 210 in HAP1 (APE/Ref-1) eliminates endonuclease activity but stabilizes substrate binding. Nucleic Acids Res. 28, 2207-13 (2000).

Sági, J., Hang, B. and Singer, B. Sequence dependent repair of synthetic AP sites in 15-mer and 35-mer oligonucleotides: role of thermodynamic stability imposed by neighbor bases. Chem. Res. Toxicol. 12, 917-23 (1999).

Hang, B., Chenna, A., Sági, A. and Singer, B. Correlation between sequence-dependent glycosylase repair and the thermal stability of oligonucleotide duplexes containing 1,N6-ethenoadenine. J. Biol. Chem. 273, 33406-13 (1998).

Hang, B., Medina, M., Fraenkel-Conrat, H. and Singer, B. A 55-kDa protein isolated from human cells shows DNA glycosylase activity toward 3,N4-ethenocytosine and the G/T mismatch. Proc. Natl. Acad. Sci. USA 95, 13561-6 (1998).

Hang, B., Chenna, A., Sági, A. and Singer, B. (1998) Differential repair of the benzene-derived adduct, 1,N6-benzetheno-dA, by the major human AP endonuclease HAP1 and Escherichia coli exonuclease III and endonuclease IV. Carcinogenesis 19, 1339-43 (1998).

Hang, B., Rothwell, D.G., Sági, J., Hickson, I.D. and Singer, B. Evidence for a common active site for cleavage of an AP site and the benzene-derived exocyclic adduct, 3,N4-benzetheno-dC, in the major human AP endonuclease. Biochemistry 36, 15411-8 (1997).

Hang, B., Singer, B., Margison, G.P. and Elder, R.H. Targeted deletion of alkylpurine-DNA-N-glycosylase in mice eliminate repair of 1,N6-ethenoadenine and hypoxanthine but not of 3,N4-ethenocytosine or 8-oxoguanine. Proc. Natl. Acad. Sci. USA 94, 12869-74 (1997).

Hang, B., Chenna, A., Fraenkel-Conrat, H. and Singer, B. An unusual mechanism for the major human AP endonuclease involving 5' cleavage of DNA containing a benzene-derived exocyclic adduct in the absence of an AP site. Proc. Natl. Acad. Sci. USA 93, 13737-41 (1996).

Hang, B., Chenna, A., Rao, S. and Singer, B. 1,N6-ethenoadenine and 3,N4-ethenocytosine are excised by separate human DNA glycosylases. Carcinogenesis 17, 155-7 (1996).

 

Hang, B., Chenna, A., Guliaev, A.B. and Singer, B. Miscoding properties of 1,N6-ethanoadenine, a DNA adduct derived from reaction with the antitumor agent 1,3-bis(2-chloroethyl)-1-nitrosourea. Mutat. Res. 531(1-2), 191-203 (2003).

Singer, B., Medina, M., Zhang, Y., Wang, Z., Guliaev, A.B. and Hang, B. 8-(hydroxymethyl)3,N4-etheno-dC, a potential carcinogenic glycidaldehyde product, miscodes in vitro using mammalian polymerases. Biochemistry 41, 1778-85 (2002).

 

Hang, B. and Singer, B. Review: Protein-protein interactions involving DNA glycosylases. Chem. Res. Toxicol. 16, 1181-95 (2003).

Singer, B. and Hang, B. Commentary: Nucleic acid sequence and repair: role of adduct, neighbor bases and enzyme specificity. Carcinogenesis 21, 1071-8 (2000).

Singer, B. and Hang, B. Perspective: What structural features determine repair enzyme specificity and mechanism in chemically modified DNA? Chem. Res. Toxicol. 10, 713-32 (1997).