Positive selection for new disease mutations in the human germline: evidence from the heritable cancer syndrome multiple endocrine neoplasia type 2B

Abstract

Multiple endocrine neoplasia type 2B (MEN2B) is a highly aggressive thyroid cancer syndrome. Since almost all sporadic cases are caused by the same nucleotide substitution in the RET proto-oncogene, the calculated disease incidence is 100-200 times greater than would be expected based on the genome average mutation frequency. In order to determine whether this increased incidence is due to an elevated mutation rate at this position (true mutation hot spot) or a selective advantage conferred on mutated spermatogonial stem cells, we studied the spatial distribution of the mutation in 14 human testes. In donors aged 36-68, mutations were clustered with small regions of each testis having mutation frequencies several orders of magnitude greater than the rest of the testis. In donors aged 19-23 mutations were almost non-existent, demonstrating that clusters in middle-aged donors grew during adulthood. Computational analysis showed that germline selection is the only plausible explanation. Testes of men aged 75-80 were heterogeneous with some like middle-aged and others like younger testes. Incorporating data on age-dependent death of spermatogonial stem cells explains the results from all age groups. Germline selection also explains MEN2B's male mutation bias and paternal age effect. Our discovery focuses attention on MEN2B as a model for understanding the genetic and biochemical basis of germline selection. Since RET function in mouse spermatogonial stem cells has been extensively studied, we are able to suggest that the MEN2B mutation provides a selective advantage by altering the PI3K/AKT and SFK signaling pathways. Mutations that are preferred in the germline but reduce the fitness of offspring increase the population's mutational load. Our approach is useful for studying other disease mutations with similar characteristics and could uncover additional germline selection pathways or identify true mutation hot spots.

Figure 1. Distribution of the MEN2B mutations in 14 human testes.

Each testis is cut into 6 slices, and each slice is further dissected into 32 approximately equal pieces. The mutation frequency per million genomes (pmg) in each piece is represented by the color code in the lower left-hand corner of the figure. Above each testis is the identification number and the age of the donor. The testes are organized by age: the left-hand column is the youngest age group (19 to 23 years), the middle column is the middle-aged group (36 to 68 years), and the right-hand column is the oldest age group (75 to 80 years).

Figure 2. Distribution of the Apert syndrome c.755C>G mutation for the oldest age group (75–80 years).

These are the same testes as the oldest age group shown in Figure 1. For the Apert mutation, unlike the MEN2B mutation in Figure 1, all of the older testes have substantial mutation clusters. The color code is the same as in Figure 1.