Date of Award
Doctor of Philosophy (PhD)
As the mean age of childrearing grows, the effect of parental age on genetic disease and child health becomes ever more important. A number of autosomal dominant disorders show a dramatic paternal age effect due to selfish mutations: substitutions that grant spermatogonial stem cells (SSCs) a selective advantage in the testes of the father but have a deleterious effect in offspring.
I present a mathematical model to analyse the normal function of the stem cell compartment, which provides a framework for SSC renewal and accommodates differences between animal systems. In order to model the SSC mutation accumulation, a Markov chain was used to model the probabilities of mutation and positive selection with cell divisions. This model provided average numbers of mutant sperm produced with increasing paternal age. The proportions of mutant to wildtype cells with increasing paternal age was used to generate a simulated population and observed/expected curves. These were then fitted against existing disease and sequencing data. The parameter for the probability of positive selection per division of a mutant cell was estimated. Incidence of the diseases was predicted closely for most disorders and was influenced by the site-specific mutation rate caused by hypermutable CpG sites and the number of mutable alleles. The incidence of disease was explained satisfactorily only when a combination of positive selection and the site-specific mutation rate were included in the analysis.
To provide experimental evidence for the hyposthesis that paternal age effect mutations present a selective advantage, I selected the mutation in the RET (REarranged during Transfection) gene that causes multiple endocrine neoplasia type 2B. SSCs were created by inducing differentiation to spermatogonia of induced pluripotent stem cells. Wildtype and mutant SSCs were generated by transfection with a plasmid containing the normal RET gene and the gene containing the disease mutation, respectively. Mutant SSCs showed increased proliferation in culture. This effect was counteracted when the mutant receptors were saturated with their ligand, GDNF (glial-derived neurotropic factor).
This research demonstrated theoretical and experimental evidence for positive selection in SSCs for multiple endocrine neoplasia type 2B and other paternal age effect syndromes.
Whelan, Eoin C., "Selfish Mutations: the Genetic Basis of the Paternal Age Effect" (2016). Biological Sciences Theses & Dissertations. 14.
Available for download on Thursday, October 18, 2018