Prion protein and susceptibility to kainate-induced seizures: genetic pitfalls in the use of PrP knockout mice.

Prion protein (PrP) is a cell surface glycoprotein which is required for susceptibility to prion infection and disease. However, PrP is expressed in many different cell types located in numerous organs. Therefore, in addition to its role in prion diseases, PrP may have a large variety of other biological functions involving the nervous system and other systems. We recently showed that susceptibility to kainate-induced seizures differed in Prnp(-/-) and Prnp(+/+) mice on the C57BL/10SnJ background. However, in a genetic complementation experiment a PrP expressing transgene was not able to rescue the Prnp(+/+) phenotype. Thus the apparent effect of PrP on seizures was actually due to genes flanking the Prnp(-/-) gene rather that the Prnp deletion itself. We discuss here several pitfalls in the use of Prnp(-/-) genotypes expressed in various mouse genetic backgrounds to determine the functions of PrP. In particular, the use of Prnp(-/-) mice with heterogeneous mixed genetic backgrounds may have weakened the conclusions of many previous experiments. Use of either co-isogenic mice or congenic mice with more homogeneous genetic backgrounds is now feasible. For congenic mice, the potential problem of flanking genes can be mitigated by the use of appropriate transgene rescue experiments to confirm the conclusions.