Positive correlation between DNA polymerase α-primase pausing and mutagenesis within polypyrimidine/polypurine microsatellite sequences

Microsatellite DNA sequences are ubiquitous in the human genome, and mutation rates of these repetitive sequences vary with respect to DNA sequence as well as length. We have analyzed polymerase-DNA interactions as a function of microsatellite sequence, using polypyrimidine/polypurine di- and tetranucleotide alleles representative of those found in the human genome. Using an in vitro primer extension assay and the mammalian DNA polymerase α-primase complex, we have observed a polymerase termination profile for each microsatellite that is unique to that allele. Interestingly, a periodic termination profile with an interval size (9-11 nucleotides) unrelated to microsatellite unit length was observed for the [TC]₂₀ and [TTCC]₉ templates. In contrast, a unit-punctuated polymerase termination profile was found for the longer polypurine templates. We detected strong polymerase pauses within the [TC]₂₀ allele at low reaction pH which were eliminated by the addition of deaza-dGTP, consistent with these specific pauses being a consequence of triplex DNA formation during DNA synthesis. Quantitatively, a strand bias was observed in the primer extension assay, in that polymerase synthesis termination is more intense when the polypurine sequence serves as the template, relative to its complementary polypyrimidine sequence. The HSV-tk forward mutation assay was utilized to determine the corresponding polymerase α-primase error frequencies and specificities at the microsatellite alleles. A higher microsatellite polymerase error frequency (50×10^-4 to 60×10^-4) was measured when polypurine sequences serve as templates for DNA synthesis, relative to the polypyrimidine template (18×10^-4). Thus, a positive correlation exists between polymerase α-primase pausing and mutagenesis within microsatellite DNA alleles.