Philip Hanawalt

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Hanawalt has been a productive researcher in the field of DNA repair since his pioneering discovery of repair replication in E. coli in 1963. He also first demonstrated repair replication in mycoplasmata and in a eukaryote and he has developed a number of important experimental approaches for studying repair, beginning with the BrdUrd density labeling method for resolving semiconservatively replicated DNA from parental DNA containing repair patches. Hanawalt’s approach was used to validate the widely-used phenomenon of unscheduled DNA synthesis as a measure of DNA repair, and also to document the first example of a human hereditary disease due to a deficiency in DNA repair. Other significant research contributions from Hanawalt’s laboratory have included: the demonstration of preferential mutagenesis by N-methyl nitrosoguanidine at DNA replication forks and its application to mapping genes; early evidence for membrane association of DNA replication complexes in E. coli and in mammalian cells; discovery of long-patch excision-repair in E. coli and the demonstration that it is an inducible component of the RecA-LexA regulatory circuit; discovery of a gene controlling nucleoside uptake in E. coli; development of permeabilized bacterial and mammalian cell systems to study excision-repair pathways; demonstration of enhanced survival of UV irradiated Simian Virus 40 upon treating the host cells with low doses of UV or chemical carcinogens; discovery that the repair enzyme, T4 endonuclease V, operates processively on damaged DNA; discovery that certain types of damage in transfecting plasmid DNA markedly enhances the efficiency of stable transformation in human cells; and the discovery that UV irradiation of short sequences of nucleotides can result in their ligation through pyrimidine dimerization, providing a plausible mechanism for prebiotic assembly of high molecular weight duplex DNA.

In 1982 Hanawalt and his colleagues reported the first example of intragenomic DNA repair heterogeneity: chemical adducts in alpha DNA in African green monkey kidney cells were not as efficiently repaired as in the genome overall. Hanawalt and his colleagues then discovered that repair of some types of damage is selective; active genes are preferentially repaired, and in fact a special repair pathway, termed transcription-coupled repair (TCR), operates on the transcribed strands of expressed genes. TCR was documented in mammalian cells, in E. coli, and in yeast chromosomal and plasmid borne genes. The discovery of TCR in Hanawalt’s laboratory has had profound implications for the fields of mutagenesis, environmental carcinogenesis, aging, and risk assessment.

The prototype recQ gene was discovered in E. coli in Hanawalt’s laboratory, and we now know of five homologues in humans including the genes mutated in the cancer prone hereditary diseases: Bloom’s syndrome, Werner’s syndrome, and Rothman Thompson syndrome.

More recent studies have focused upon the regulation of TCR and the global genomic nucleotide excision repair (GGR) pathway. Features of the TCR pathway (defective in Cockayne syndrome) include the possibility of "gratuitous TCR" at transcription pause sites in undamaged DNA. The GGR pathway was shown to be controlled through the SOS stress response in E. coli and through the activated product of the p53 tumor suppressor gene in human cells. These regulatory systems particularly affect the efficiency of repair of the predominant UV-induced photoproduct, the cyclobutane pyrimidine dimer, as well as that of chemical carcinogen DNA adducts, such as benzo(a)pyrene diol-epoxide and benzo(g)chrysene. Rodent cells (typically lacking the p53-controlled GGR pathway) are unable to carry out efficient GGR of some lesions. Therefore, caution should be exercised in the interpretation of results from such systems for risk assessment in genetic toxicology.