Slippage--misalignment: to what extent does it contribute to mammalian cell mutagenesis?

Mutations in mammalian genomes are the result of several mutagenic processes that are intrinsic to cell metabolism. Analysis of the mutation spectrum of a chromosomal gene is a valuable tool for assessing the contribution of these mechanisms to mutagenesis in the cell. We have studied the specificity of mutations induced by various mutagens in a cDNA hprt gene integrated in a chromosome of a mouse cell line. To understand the mechanisms underlying mammalian cell mutagenesis, we compiled a list of more than 250 sequenced hprt mutations that arose spontaneously or were induced by mutagens, and compared it with the published mutation data. There are at least two distinct processes of mutagenesis in eukaryotic cells: one is mispairing, while another is errors in translesion synthesis. The alkylating agent methylnitrosourea causes G:T mispairing; consequently, most mutations it induces are G to A transitions. The second process can occur spontaneously or be caused by exposure to X-rays, Trp-P2, a tryptophan pyrolysate, or acetylaminofluorene. A variety of premutagenic lesions are produced in DNA by these mutagens, but spectra of the mutations resemble each other, especially in the high frequency of deletions at the sites of short direct repeats. The slippage--misalignment mechanism accounted well for the greater part of the observed deletions. A similar spectrum of mutations was observed in the tumor suppressor gene APC from colorectal tumors; about 40% are deletions at the sites of short repeats. These findings led us to propose that slippage--misalignment is an ubiquitous mechanism of mutagenesis and is responsible for a significant proportion of spontaneous mutations in mammalian cells.