Ionizing radiation and genetic risks: XII. The concept of “potential recoverability correction factor” (PRCF) and its use for predicting the risk of radiation-inducible genetic disease in human live births

Abstract

Genetic risks of radiation exposure of humans are generally expressed as expected increases in the frequencies of genetic diseases over those that occur naturally in the population as a result of spontaneous mutations. Since human data on radiation-induced germ cell mutations and genetic diseases remain scanty, the rates derived from the induced frequencies of mutations in mouse genes are used for this purpose. Such an extrapolation from mouse data to the risk of genetic diseases will be valid only if the average rates of inducible mutations in human genes of interest and the average rates of induced mutations in mice are similar. Advances in knowledge of human genetic diseases and in molecular studies of radiation-induced mutations in experimental systems now question the validity of the above extrapolation. In fact, they (i) support the view that only in a limited number of genes in the human genome, induced mutations may be compatible with viability and hence recoverable in live births and (ii) suggest that the average rate of induced mutations in human genes of interest from the disease point of view will be lower than that assumed from mouse results. Since, at present, there is no alternative to the use of mouse data on induced mutation rates, there is a need to bridge the gap between these and the risk of potentially inducible genetic diseases in human live births. In this paper, we advance the concept of what we refer to here as “the potential recoverability correction factor” (PRCF) to bridge the above gap in risk estimation and present a method to estimate PRCF. In developing the concept of PRCF, we first used the available information on radiation-induced mutations recovered in experimental studies to define some criteria for assessing potential recoverability of induced mutations and then applied these to human genes on a gene-by-gene basis. The analysis permitted us to estimate unweighted PRCFs (i.e. the fraction of genes among the total studied that might contribute to recoverable induced mutations) and weighted PRCFs (i.e. PRCFs weighted by the incidences of the respective diseases). The estimates are: 0.15 (weighted) to 0.30 (unweighted) for autosomal dominant and X-linked diseases and 0.02 (weighted) to 0.09 (unweighted) for chronic multifactorial diseases. The PRCF calculations are unnecessary for autosomal recessive diseases since the risks projected for the first few generations even without using PRCFs are already very small. For congenital abnormalities, PRCFs cannot be reliably estimated. With the incorporation of PRCF into the equation used for predicting risk, the risk per unit dose becomes the product of four quantities (risk per unit dose= P×(1/DD)×MC×PRCF) where P is the baseline frequency of the genetic disease, 1/DD is the relative mutation risk per unit dose, MC is the mutation component and PRCF is the disease-class-specific potential recoverability correction factor instead of the first three (as has been the case thus far). Since PRCF is a fraction, it is obvious that the estimate of risk obtained with the revised risk equation will be smaller than previously calculated values.