A new method for estimating high mutation rates in cultured cells

Fluctuation analysis allows for the determination of mutation rates in cell cultures in vitro. As originally described by Luria and Delbruck and extended by Lea and Coulson and by Capizzi and Jameson, this analysis has been useful in estimating mutation rates in cultured cells where the frequency of mutational events is low. However. in cultures where high mutation rates and multiple independent mutation events occur, leading to the accumulation of many mutant cells, these standard methods may not apply. Here, we present a new method for the estimation of mutation rates based on the assumption that multiple events may contribute to the accumulation of mutant cells. We compared mutation rates determined by Lea and Coulson's and by Capizzi and Jameson's methods with those determined by our method using experimental and stimulated data from our studies of immunoglobulin gene mutation and isotype switching in B lymphocyte cultures. The three methods resulted in very different calculated rates when many mutants were present in the culture, such as when mutation rates were high, while only small differences in calculated rates were found when mutants were rare. Unlike previous fluctuation analysis calculations, our method is applicable for the estimation of both low and high rates.