Biological Responses to DNA Damage
DNA Polymerase Kappa (Polκ)
The past five years have witnessed an explosion of information regarding the number and types of DNA polymerases in eukaryotic cells. A new class of error-prone DNA polymerases (the Y-family) has been discovered and is now known to synthesize DNA past sites of template base damage (translesion DNA synthesis) The Friedberg lab is characterizing in detail one such polymerase called DNA polymerase kappa (Polκ). Polκ can bypass several forms of DNA damage in vitro. Thymine glycol bypass is particularly interesting because it is an abundant form of oxidative DNA damage in DNA. Polκ is highly expressed in several tissues that are have high oxidative loads.
Polκ is highly expressed in the testis in a cell-specific manner. Multiple transcripts are expressed in the testis suggesting the existence of multiple protein isoforms. We are investigating a possible role(s) of Polκin normal spermatogenesis, using both the mouse and C. elegans as model systems.
Nucleotide Excision Repair in Yeast
The Friedberg lab is continuing studies on nucleotide excision repair (NER) in the yeast S. cerevisiae More than 30 proteins are required for NER, which can form large multiprotein complexes. We are presently focusing on the Rad1/Rad10 protein sub-complex, a structure-specific DNA endonuclease that functions both in NER and in genetic recombination.
Mice Defective in NER
The Friedberg lab has generated mouse strains mutant for various genes required for NER. These include mice defective in the Xpa, Xpc, and Csa genes. Recent studies on Xpc mice suggest a role of XPC protein in a novel DNA repair mechanism.
A Mouse Model for Bloom Syndrome
Several mouse models for Bloom syndrome have been reported, each believed to disrupt Blm gene function (i.e., null alleles). However, animals bearing these unique alleles exhibit distinct phenotypes. In one model, homozygous mutant animals exhibit embryonic lethality while in the other, homozygosity yields viable, fertile animals. Evaluation of spleen cells isolated from viable animals reveals an inverse correlation between the quantity of BLM protein and the level of chromosome instability. Moreover, these mice exhibit a similar genotypic relationship for tumor predisposition. The Blm mouse is a valuable tool for introducing chromosome instability into other strains of mice.
Roberts syndrome (RS) is a rare developmental disorder characterized by tetraphocomelia (symmetrical limb reduction). Cells and cell lines derived from RS patients show a phenomenon evident in mitotic chromosomes which has been previously referred to as heterochromatic repulsion (HR), heterochromatic splaying (HS) or premature centromere separation (PCS). The Friedberg lab has used chromosome transfer to map the gene causative for the RS phenotype first by complementation with a new technique of transient microcell-mediated chromosome transfer and confirmation of the results by traditional stable microcell-mediated chromosome transfer. Sub-chromosomal localization has utilized translocations and haplotype analysis. Candidate genes are being evaluated.
The clinical features of TTD include brittle hair and nails, and dry scaly skin. TTD is associated with defects in nucleotide excision repair (NER) due to mutations in XPD, XPB or an unidentified gene TTDA. XPD and XPB are subunits of TFIIH, the multiprotein complex involved in NER and transcription. The Friedberg lab is using microcell-mediated chromosome transfer to map the gene causative for TTDA by complementation of the UV-sensitivity of cell lines.