Genome integrity is widely recognized as playing a crucial role in cell health and efficacy. What is less understood is how somatic genome variance between cells in a population impacts the function of the whole system. This question is particularly relevant in the case of neurons, where many individual cells each assume an essential role in a neural network. Indeed, there are several lines of evidence indicating that maintaining genome integrity is critically important to neurons. An average wild-type neuron may be required to maintain genome integrity for decades or even, in rare cases, for over a century. This longevity raises the question of whether or not a failure to maintain genome integrity over time may in some way contribute to age-associated loss of cognitive function. Thus, whole genome sequencing (WGS) data of single young and aged neurons could implicate mechanisms of mutagenesis which underlie age-associated phenotypes.
One of the main challenges in addressing neuronal mutations is the senescent state of mature neurons. Conducting WGS experiments on post-mitotic cell types typically requires error-prone single cell whole genome amplification (SC-WGA) technologies. To circumvent this, our lab utilizes somatic cell nuclear transfer (SCNT) in the mouse, which allows us to amplify the genome evenly in a way that is minimally mutagenic. Although there are clever strategies for mitigating the drawbacks of SC-WGA, to the best of our knowledge SCNT remains the only method to assay all classes of mutation across the entire genome. SCNT thus provides a high-resolution map of individual genomes which complement the high-throughput studies performed in human neurons by SC-WGA. Our lab previously published on this approach applied to mitral and tufted neurons, finding over 100 unique neuronal mutations per single mature neuron (Hazen and Faust et. al. Neuron 2016). More recently, we have extend our SCNT approach to assay genomes of rod photoreceptors of varying ages (p6-2.5 years), which are exposed to UV and are responsible for a highly polygenic set of retinal disorders. This work provides evidence that (1) neurons in mice accumulate mutations at an annual rate comparable to humans, (2) there are both recurring and unique mechanisms of mutation across different neuronal subtypes, and (3) the most deleterious classes of mutation occur in highly expressed genes (Duran et. al. Manuscript in prep)