It has long been known that cells acquire mutations over time. In fact, ongoing random mutation rates are such that no two cells in a human body are identical. Unlike most cell types, which die and are frequently replaced, human neurons can live for decades, carrying on their functions for the entire lifetime of the organism. We know that mutations must accumulate in neurons over their long lifespan, but until now we had no clear picture of the frequency or pattern of neuronal mutations.
Using the seemingly impossible approach of sequencing genomes of individual human brain cells, a group of researchers have now shown that the average human neuron carries a whopping 1700 mutations per neuron. These neuroscientists, based at Harvard and MIT, sorted individual neurons from the postmortem brains of three individuals, amplified their genomic DNA, and were able to use whole-genome sequencing to obtain individual cell genome sequences at ~40x coverage.
In addition to the sheer number of neuronal mutations, there are some other surprises in the data. Most of the mutations were unique to single neurons, rather than present in all cells, suggesting that most brain mutations arise after the brain has fully formed. Also, mutations were enriched in genes involved in neural function and development, and most likely happened during gene transcription, which hints that the most important genes are also the ones most likely to mutate.
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Lodato MA, Woodworth MB, Lee S, Evrony GD, Mehta BK, Karger A, Lee S, Chittenden TW, D'Gama AM, Cai X, Luquette LJ, Lee E, Park PJ, Walsh CA. (2015) Somatic mutation in single human neurons tracks developmental and transcriptional history. Science 350:94-8