The observed human sperm mutation frequency cannot explain the achondroplasia paternal age effect

Abstract

The lifelong spermatogonial stem cell divisions unique to male germ cell production are thought to contribute to a higher mutation frequency in males. The fact that certain de novo human genetic conditions (e.g., achondroplasia) increase in incidence with the age of the father is consistent with this idea. Although it is assumed that the paternal age effect is the result of an increasing frequency of mutant sperm as a man grows older, no direct molecular measurement of the germ-line mutation frequency has been made to confirm this hypothesis. Using sperm DNA from donors of different ages, we determined the frequency of the nucleotide substitution in the fibroblast growth factor receptor 3 (FGFR3) gene that causes achondroplasia. Surprisingly, the magnitude of the increase in mutation frequency with age appears insufficient to explain why older fathers have a greater chance of having a child with this condition. A number of alternatives may explain this discrepancy, including selection for sperm that carry the mutation or an age-dependent increase in premutagenic lesions that remain unrepaired in sperm and are inefficiently detected by the PCR assay.

Fig 1.

The accuracy of the assay is shown by the relationship between the median value of the measured counts (• with 95% C.I. indicated; in some cases the C.I. is too small to be seen) and the actual number of G1138A genomes added to blood DNA. Points falling along the diagonal would indicate a perfect assay. (Inset) An expansion of the data for the lower count values.

Fig 2.

Relationship between donor age and the frequency of the G1138A mutation in sperm DNA. Each small dash represents one donor. The count value is taken as the median of the mean values obtained in 3–6 independent experiments on each donor. The open red squares show the average G1138A frequency (along with the 95% C.I.) for all individuals within the following age intervals: <25, 5-year age intervals (beginning with 25–29 and ending with 60–64) and >64. The plot was created by drawing a line (gray) between adjacent average values for the different age intervals.

Fig 3.

Age-dependent fold increase in the frequency of the G1138A mutations in sperm DNA (•) and sporadic achondroplasia cases (9, 11) (▴). Birth values are based on the observed-to-expected ratio of sporadic achondroplasia cases for fathers in different age groups (7). In each plot, the value for the youngest age interval is taken as 1 and divided into the values for the remaining age intervals to calculate the fold change. The plot was constructed by drawing a line between adjacent points.